Merge tag 'md-3.3' of git://neil.brown.name/md

md update for 3.3

Big change is new hot-replacement.
A slot in an array can hold 2 devices - one that
wants-replacement and one that is the replacement.
Once the replacement is built - either from the
original or (in the case of errors) from elsewhere,
the wants-replacement device will be removed.

* tag 'md-3.3' of git://neil.brown.name/md: (36 commits)
  md/raid1: Mark device want_replacement when we see a write error.
  md/raid1: If there is a spare and a want_replacement device, start replacement.
  md/raid1: recognise replacements when assembling arrays.
  md/raid1: handle activation of replacement device when recovery completes.
  md/raid1: Allow a failed replacement device to be removed.
  md/raid1: Allocate spare to store replacement devices and their bios.
  md/raid1:  Replace use of mddev->raid_disks with conf->raid_disks.
  md/raid10: If there is a spare and a want_replacement device, start replacement.
  md/raid10: recognise replacements when assembling array.
  md/raid10: Allow replacement device to be replace old drive.
  md/raid10: handle recovery of replacement devices.
  md/raid10:  Handle replacement devices during resync.
  md/raid10: writes should get directed to replacement as well as original.
  md/raid10: allow removal of failed replacement devices.
  md/raid10: preferentially read from replacement device if possible.
  md/raid10:  change read_balance to return an rdev
  md/raid10: prepare data structures for handling replacement.
  md/raid5: Mark device want_replacement when we see a write error.
  md/raid5: If there is a spare and a want_replacement device, start replacement.
  md/raid5: recognise replacements when assembling array.
  ...

13 files changed, 1280 insertions, 438 deletions

diff --git a/Documentation/md.txt b/Documentation/md.txt
index fc94770f44ab..993fba37b7d1 100644
--- a/Documentation/md.txt
+++ b/Documentation/md.txt
@@ -357,14 +357,14 @@ Each directory contains:
         written to, that device.
 
       state
-        A file recording the current state of the device in the array
+        A file recording the current state of the device in the array
         which can be a comma separated list of
               faulty   - device has been kicked from active use due to
-                         a detected fault or it has unacknowledged bad
-                         blocks
+                         a detected fault, or it has unacknowledged bad
+                         blocks
               in_sync  - device is a fully in-sync member of the array
               writemostly - device will only be subject to read
-                         requests if there are no other options.
+                         requests if there are no other options.
                          This applies only to raid1 arrays.
               blocked  - device has failed, and the failure hasn't been
                          acknowledged yet by the metadata handler.
@@ -374,6 +374,13 @@ Each directory contains:
                          This includes spares that are in the process
                          of being recovered to
               write_error - device has ever seen a write error.
+              want_replacement - device is (mostly) working but probably
+                         should be replaced, either due to errors or
+                         due to user request.
+              replacement - device is a replacement for another active
+                         device with same raid_disk.
+
+
         This list may grow in future.
         This can be written to.
         Writing "faulty"  simulates a failure on the device.
@@ -386,6 +393,13 @@ Each directory contains:
         Writing "in_sync" sets the in_sync flag.
         Writing "write_error" sets writeerrorseen flag.
         Writing "-write_error" clears writeerrorseen flag.
+        Writing "want_replacement" is allowed at any time except to a
+                replacement device or a spare.  It sets the flag.
+        Writing "-want_replacement" is allowed at any time.  It clears
+                the flag.
+        Writing "replacement" or "-replacement" is only allowed before
+                starting the array.  It sets or clears the flag.
+
 
         This file responds to select/poll. Any change to 'faulty'
         or 'blocked' causes an event.
diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
index 6d03774b176e..cdf36b1e9aa6 100644
--- a/drivers/md/bitmap.c
+++ b/drivers/md/bitmap.c
@@ -1149,12 +1149,12 @@ void bitmap_daemon_work(struct mddev *mddev)
                 return;
         }
         if (time_before(jiffies, bitmap->daemon_lastrun
-                        + bitmap->mddev->bitmap_info.daemon_sleep))
+                        + mddev->bitmap_info.daemon_sleep))
                 goto done;
 
         bitmap->daemon_lastrun = jiffies;
         if (bitmap->allclean) {
-                bitmap->mddev->thread->timeout = MAX_SCHEDULE_TIMEOUT;
+                mddev->thread->timeout = MAX_SCHEDULE_TIMEOUT;
                 goto done;
         }
         bitmap->allclean = 1;
@@ -1206,7 +1206,7 @@ void bitmap_daemon_work(struct mddev *mddev)
                          * sure that events_cleared is up-to-date.
                          */
                         if (bitmap->need_sync &&
-                            bitmap->mddev->bitmap_info.external == 0) {
+                            mddev->bitmap_info.external == 0) {
                                 bitmap_super_t *sb;
                                 bitmap->need_sync = 0;
                                 sb = kmap_atomic(bitmap->sb_page, KM_USER0);
@@ -1270,8 +1270,8 @@ void bitmap_daemon_work(struct mddev *mddev)
 
  done:
         if (bitmap->allclean == 0)
-                bitmap->mddev->thread->timeout =
-                        bitmap->mddev->bitmap_info.daemon_sleep;
+                mddev->thread->timeout =
+                        mddev->bitmap_info.daemon_sleep;
         mutex_unlock(&mddev->bitmap_info.mutex);
 }
 
@@ -1587,7 +1587,7 @@ static void bitmap_set_memory_bits(struct bitmap *bitmap, sector_t offset, int n
         }
         if (!*bmc) {
                 struct page *page;
-                *bmc = 1 | (needed ? NEEDED_MASK : 0);
+                *bmc = 2 | (needed ? NEEDED_MASK : 0);
                 bitmap_count_page(bitmap, offset, 1);
                 page = filemap_get_page(bitmap, offset >> CHUNK_BLOCK_SHIFT(bitmap));
                 set_page_attr(bitmap, page, BITMAP_PAGE_PENDING);
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 5d1b6762f108..ca8527fe77eb 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -1713,6 +1713,8 @@ static int super_1_validate(struct mddev *mddev, struct md_rdev *rdev)
                 }
                 if (sb->devflags & WriteMostly1)
                         set_bit(WriteMostly, &rdev->flags);
+                if (le32_to_cpu(sb->feature_map) & MD_FEATURE_REPLACEMENT)
+                        set_bit(Replacement, &rdev->flags);
         } else /* MULTIPATH are always insync */
                 set_bit(In_sync, &rdev->flags);
 
@@ -1766,6 +1768,9 @@ static void super_1_sync(struct mddev *mddev, struct md_rdev *rdev)
                 sb->recovery_offset =
                         cpu_to_le64(rdev->recovery_offset);
         }
+        if (test_bit(Replacement, &rdev->flags))
+                sb->feature_map |=
+                        cpu_to_le32(MD_FEATURE_REPLACEMENT);
 
         if (mddev->reshape_position != MaxSector) {
                 sb->feature_map |= cpu_to_le32(MD_FEATURE_RESHAPE_ACTIVE);
@@ -2559,6 +2564,15 @@ state_show(struct md_rdev *rdev, char *page)
                 len += sprintf(page+len, "%swrite_error", sep);
                 sep = ",";
         }
+        if (test_bit(WantReplacement, &rdev->flags)) {
+                len += sprintf(page+len, "%swant_replacement", sep);
+                sep = ",";
+        }
+        if (test_bit(Replacement, &rdev->flags)) {
+                len += sprintf(page+len, "%sreplacement", sep);
+                sep = ",";
+        }
+
         return len+sprintf(page+len, "\n");
 }
 
@@ -2627,6 +2641,42 @@ state_store(struct md_rdev *rdev, const char *buf, size_t len)
         } else if (cmd_match(buf, "-write_error")) {
                 clear_bit(WriteErrorSeen, &rdev->flags);
                 err = 0;
+        } else if (cmd_match(buf, "want_replacement")) {
+                /* Any non-spare device that is not a replacement can
+                 * become want_replacement at any time, but we then need to
+                 * check if recovery is needed.
+                 */
+                if (rdev->raid_disk >= 0 &&
+                    !test_bit(Replacement, &rdev->flags))
+                        set_bit(WantReplacement, &rdev->flags);
+                set_bit(MD_RECOVERY_NEEDED, &rdev->mddev->recovery);
+                md_wakeup_thread(rdev->mddev->thread);
+                err = 0;
+        } else if (cmd_match(buf, "-want_replacement")) {
+                /* Clearing 'want_replacement' is always allowed.
+                 * Once replacements starts it is too late though.
+                 */
+                err = 0;
+                clear_bit(WantReplacement, &rdev->flags);
+        } else if (cmd_match(buf, "replacement")) {
+                /* Can only set a device as a replacement when array has not
+                 * yet been started.  Once running, replacement is automatic
+                 * from spares, or by assigning 'slot'.
+                 */
+                if (rdev->mddev->pers)
+                        err = -EBUSY;
+                else {
+                        set_bit(Replacement, &rdev->flags);
+                        err = 0;
+                }
+        } else if (cmd_match(buf, "-replacement")) {
+                /* Similarly, can only clear Replacement before start */
+                if (rdev->mddev->pers)
+                        err = -EBUSY;
+                else {
+                        clear_bit(Replacement, &rdev->flags);
+                        err = 0;
+                }
         }
         if (!err)
                 sysfs_notify_dirent_safe(rdev->sysfs_state);
@@ -2688,7 +2738,7 @@ slot_store(struct md_rdev *rdev, const char *buf, size_t len)
                 if (rdev->mddev->pers->hot_remove_disk == NULL)
                         return -EINVAL;
                 err = rdev->mddev->pers->
-                        hot_remove_disk(rdev->mddev, rdev->raid_disk);
+                        hot_remove_disk(rdev->mddev, rdev);
                 if (err)
                         return err;
                 sysfs_unlink_rdev(rdev->mddev, rdev);
@@ -2696,7 +2746,6 @@ slot_store(struct md_rdev *rdev, const char *buf, size_t len)
                 set_bit(MD_RECOVERY_NEEDED, &rdev->mddev->recovery);
                 md_wakeup_thread(rdev->mddev->thread);
         } else if (rdev->mddev->pers) {
-                struct md_rdev *rdev2;
                 /* Activating a spare .. or possibly reactivating
                  * if we ever get bitmaps working here.
                  */
@@ -2710,10 +2759,6 @@ slot_store(struct md_rdev *rdev, const char *buf, size_t len)
                 if (rdev->mddev->pers->hot_add_disk == NULL)
                         return -EINVAL;
 
-                list_for_each_entry(rdev2, &rdev->mddev->disks, same_set)
-                        if (rdev2->raid_disk == slot)
-                                return -EEXIST;
-
                 if (slot >= rdev->mddev->raid_disks &&
                     slot >= rdev->mddev->raid_disks + rdev->mddev->delta_disks)
                         return -ENOSPC;
@@ -6053,8 +6098,15 @@ static int md_ioctl(struct block_device *bdev, fmode_t mode,
         struct mddev *mddev = NULL;
         int ro;
 
-        if (!capable(CAP_SYS_ADMIN))
-                return -EACCES;
+        switch (cmd) {
+        case RAID_VERSION:
+        case GET_ARRAY_INFO:
+        case GET_DISK_INFO:
+                break;
+        default:
+                if (!capable(CAP_SYS_ADMIN))
+                        return -EACCES;
+        }
 
         /*
          * Commands dealing with the RAID driver but not any
@@ -6714,8 +6766,11 @@ static int md_seq_show(struct seq_file *seq, void *v)
                         if (test_bit(Faulty, &rdev->flags)) {
                                 seq_printf(seq, "(F)");
                                 continue;
-                        } else if (rdev->raid_disk < 0)
+                        }
+                        if (rdev->raid_disk < 0)
                                 seq_printf(seq, "(S)"); /* spare */
+                        if (test_bit(Replacement, &rdev->flags))
+                                seq_printf(seq, "(R)");
                         sectors += rdev->sectors;
                 }
 
@@ -7337,29 +7392,27 @@ static int remove_and_add_spares(struct mddev *mddev)
                      ! test_bit(In_sync, &rdev->flags)) &&
                     atomic_read(&rdev->nr_pending)==0) {
                         if (mddev->pers->hot_remove_disk(
-                                    mddev, rdev->raid_disk)==0) {
+                                    mddev, rdev) == 0) {
                                 sysfs_unlink_rdev(mddev, rdev);
                                 rdev->raid_disk = -1;
                         }
                 }
 
-        if (mddev->degraded) {
-                list_for_each_entry(rdev, &mddev->disks, same_set) {
-                        if (rdev->raid_disk >= 0 &&
-                            !test_bit(In_sync, &rdev->flags) &&
-                            !test_bit(Faulty, &rdev->flags))
+        list_for_each_entry(rdev, &mddev->disks, same_set) {
+                if (rdev->raid_disk >= 0 &&
+                    !test_bit(In_sync, &rdev->flags) &&
+                    !test_bit(Faulty, &rdev->flags))
+                        spares++;
+                if (rdev->raid_disk < 0
+                    && !test_bit(Faulty, &rdev->flags)) {
+                        rdev->recovery_offset = 0;
+                        if (mddev->pers->
+                            hot_add_disk(mddev, rdev) == 0) {
+                                if (sysfs_link_rdev(mddev, rdev))
+                                        /* failure here is OK */;
                                 spares++;
-                        if (rdev->raid_disk < 0
-                            && !test_bit(Faulty, &rdev->flags)) {
-                                rdev->recovery_offset = 0;
-                                if (mddev->pers->
-                                    hot_add_disk(mddev, rdev) == 0) {
-                                        if (sysfs_link_rdev(mddev, rdev))
-                                                /* failure here is OK */;
-                                        spares++;
-                                        md_new_event(mddev);
-                                        set_bit(MD_CHANGE_DEVS, &mddev->flags);
-                                }
+                                md_new_event(mddev);
+                                set_bit(MD_CHANGE_DEVS, &mddev->flags);
                         }
                 }
         }
@@ -7474,7 +7527,7 @@ void md_check_recovery(struct mddev *mddev)
                                     test_bit(Faulty, &rdev->flags) &&
                                     atomic_read(&rdev->nr_pending)==0) {
                                         if (mddev->pers->hot_remove_disk(
-                                                    mddev, rdev->raid_disk)==0) {
+                                                    mddev, rdev) == 0) {
                                                 sysfs_unlink_rdev(mddev, rdev);
                                                 rdev->raid_disk = -1;
                                         }
diff --git a/drivers/md/md.h b/drivers/md/md.h
index cf742d9306ec..44c63dfeeb2b 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -72,34 +72,7 @@ struct md_rdev {
          * This reduces the burden of testing multiple flags in many cases
          */
 
-        unsigned long   flags;
-#define Faulty          1               /* device is known to have a fault */
-#define In_sync         2               /* device is in_sync with rest of array */
-#define WriteMostly     4               /* Avoid reading if at all possible */
-#define AutoDetected    7               /* added by auto-detect */
-#define Blocked         8               /* An error occurred but has not yet
-                                         * been acknowledged by the metadata
-                                         * handler, so don't allow writes
-                                         * until it is cleared */
-#define WriteErrorSeen  9               /* A write error has been seen on this
-                                         * device
-                                         */
-#define FaultRecorded   10              /* Intermediate state for clearing
-                                         * Blocked.  The Fault is/will-be
-                                         * recorded in the metadata, but that
-                                         * metadata hasn't been stored safely
-                                         * on disk yet.
-                                         */
-#define BlockedBadBlocks 11             /* A writer is blocked because they
-                                         * found an unacknowledged bad-block.
-                                         * This can safely be cleared at any
-                                         * time, and the writer will re-check.
-                                         * It may be set at any time, and at
-                                         * worst the writer will timeout and
-                                         * re-check.  So setting it as
-                                         * accurately as possible is good, but
-                                         * not absolutely critical.
-                                         */
+        unsigned long   flags;  /* bit set of 'enum flag_bits' bits. */
         wait_queue_head_t blocked_wait;
 
         int desc_nr;                    /* descriptor index in the superblock */
@@ -152,6 +125,44 @@ struct md_rdev {
                 sector_t size;          /* in sectors */
         } badblocks;
 };
+enum flag_bits {
+        Faulty,                 /* device is known to have a fault */
+        In_sync,                /* device is in_sync with rest of array */
+        WriteMostly,            /* Avoid reading if at all possible */
+        AutoDetected,           /* added by auto-detect */
+        Blocked,                /* An error occurred but has not yet
+                                 * been acknowledged by the metadata
+                                 * handler, so don't allow writes
+                                 * until it is cleared */
+        WriteErrorSeen,         /* A write error has been seen on this
+                                 * device
+                                 */
+        FaultRecorded,          /* Intermediate state for clearing
+                                 * Blocked.  The Fault is/will-be
+                                 * recorded in the metadata, but that
+                                 * metadata hasn't been stored safely
+                                 * on disk yet.
+                                 */
+        BlockedBadBlocks,       /* A writer is blocked because they
+                                 * found an unacknowledged bad-block.
+                                 * This can safely be cleared at any
+                                 * time, and the writer will re-check.
+                                 * It may be set at any time, and at
+                                 * worst the writer will timeout and
+                                 * re-check.  So setting it as
+                                 * accurately as possible is good, but
+                                 * not absolutely critical.
+                                 */
+        WantReplacement,        /* This device is a candidate to be
+                                 * hot-replaced, either because it has
+                                 * reported some faults, or because
+                                 * of explicit request.
+                                 */
+        Replacement,            /* This device is a replacement for
+                                 * a want_replacement device with same
+                                 * raid_disk number.
+                                 */
+};
 
 #define BB_LEN_MASK     (0x00000000000001FFULL)
 #define BB_OFFSET_MASK  (0x7FFFFFFFFFFFFE00ULL)
@@ -428,7 +439,7 @@ struct md_personality
          */
         void (*error_handler)(struct mddev *mddev, struct md_rdev *rdev);
         int (*hot_add_disk) (struct mddev *mddev, struct md_rdev *rdev);
-        int (*hot_remove_disk) (struct mddev *mddev, int number);
+        int (*hot_remove_disk) (struct mddev *mddev, struct md_rdev *rdev);
         int (*spare_active) (struct mddev *mddev);
         sector_t (*sync_request)(struct mddev *mddev, sector_t sector_nr, int *skipped, int go_faster);
         int (*resize) (struct mddev *mddev, sector_t sectors);
@@ -482,15 +493,20 @@ static inline char * mdname (struct mddev * mddev)
 static inline int sysfs_link_rdev(struct mddev *mddev, struct md_rdev *rdev)
 {
         char nm[20];
-        sprintf(nm, "rd%d", rdev->raid_disk);
-        return sysfs_create_link(&mddev->kobj, &rdev->kobj, nm);
+        if (!test_bit(Replacement, &rdev->flags)) {
+                sprintf(nm, "rd%d", rdev->raid_disk);
+                return sysfs_create_link(&mddev->kobj, &rdev->kobj, nm);
+        } else
+                return 0;
 }
 
 static inline void sysfs_unlink_rdev(struct mddev *mddev, struct md_rdev *rdev)
 {
         char nm[20];
-        sprintf(nm, "rd%d", rdev->raid_disk);
-        sysfs_remove_link(&mddev->kobj, nm);
+        if (!test_bit(Replacement, &rdev->flags)) {
+                sprintf(nm, "rd%d", rdev->raid_disk);
+                sysfs_remove_link(&mddev->kobj, nm);
+        }
 }
 
 /*
diff --git a/drivers/md/multipath.c b/drivers/md/multipath.c
index 5899246fa37e..a222f516660e 100644
--- a/drivers/md/multipath.c
+++ b/drivers/md/multipath.c
@@ -292,17 +292,16 @@ static int multipath_add_disk(struct mddev *mddev, struct md_rdev *rdev)
         return err;
 }
 
-static int multipath_remove_disk(struct mddev *mddev, int number)
+static int multipath_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
 {
         struct mpconf *conf = mddev->private;
         int err = 0;
-        struct md_rdev *rdev;
+        int number = rdev->raid_disk;
         struct multipath_info *p = conf->multipaths + number;
 
         print_multipath_conf(conf);
 
-        rdev = p->rdev;
-        if (rdev) {
+        if (rdev == p->rdev) {
                 if (test_bit(In_sync, &rdev->flags) ||
                     atomic_read(&rdev->nr_pending)) {
                         printk(KERN_ERR "hot-remove-disk, slot %d is identified"
diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index ede2461e79c5..cc24f0cb7ee3 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -135,7 +135,7 @@ out_free_pages:
                         put_page(r1_bio->bios[j]->bi_io_vec[i].bv_page);
         j = -1;
 out_free_bio:
-        while ( ++j < pi->raid_disks )
+        while (++j < pi->raid_disks)
                 bio_put(r1_bio->bios[j]);
         r1bio_pool_free(r1_bio, data);
         return NULL;
@@ -164,7 +164,7 @@ static void put_all_bios(struct r1conf *conf, struct r1bio *r1_bio)
 {
         int i;
 
-        for (i = 0; i < conf->raid_disks; i++) {
+        for (i = 0; i < conf->raid_disks * 2; i++) {
                 struct bio **bio = r1_bio->bios + i;
                 if (!BIO_SPECIAL(*bio))
                         bio_put(*bio);
@@ -185,7 +185,7 @@ static void put_buf(struct r1bio *r1_bio)
         struct r1conf *conf = r1_bio->mddev->private;
         int i;
 
-        for (i=0; i<conf->raid_disks; i++) {
+        for (i = 0; i < conf->raid_disks * 2; i++) {
                 struct bio *bio = r1_bio->bios[i];
                 if (bio->bi_end_io)
                         rdev_dec_pending(conf->mirrors[i].rdev, r1_bio->mddev);
@@ -277,13 +277,14 @@ static inline void update_head_pos(int disk, struct r1bio *r1_bio)
 static int find_bio_disk(struct r1bio *r1_bio, struct bio *bio)
 {
         int mirror;
-        int raid_disks = r1_bio->mddev->raid_disks;
+        struct r1conf *conf = r1_bio->mddev->private;
+        int raid_disks = conf->raid_disks;
 
-        for (mirror = 0; mirror < raid_disks; mirror++)
+        for (mirror = 0; mirror < raid_disks * 2; mirror++)
                 if (r1_bio->bios[mirror] == bio)
                         break;
 
-        BUG_ON(mirror == raid_disks);
+        BUG_ON(mirror == raid_disks * 2);
         update_head_pos(mirror, r1_bio);
 
         return mirror;
@@ -390,6 +391,11 @@ static void raid1_end_write_request(struct bio *bio, int error)
         if (!uptodate) {
                 set_bit(WriteErrorSeen,
                         &conf->mirrors[mirror].rdev->flags);
+                if (!test_and_set_bit(WantReplacement,
+                                      &conf->mirrors[mirror].rdev->flags))
+                        set_bit(MD_RECOVERY_NEEDED, &
+                                conf->mddev->recovery);
+
                 set_bit(R1BIO_WriteError, &r1_bio->state);
         } else {
                 /*
@@ -505,7 +511,7 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
                 start_disk = conf->last_used;
         }
 
-        for (i = 0 ; i < conf->raid_disks ; i++) {
+        for (i = 0 ; i < conf->raid_disks * 2 ; i++) {
                 sector_t dist;
                 sector_t first_bad;
                 int bad_sectors;
@@ -609,7 +615,7 @@ int md_raid1_congested(struct mddev *mddev, int bits)
                 return 1;
 
         rcu_read_lock();
-        for (i = 0; i < mddev->raid_disks; i++) {
+        for (i = 0; i < conf->raid_disks; i++) {
                 struct md_rdev *rdev = rcu_dereference(conf->mirrors[i].rdev);
                 if (rdev && !test_bit(Faulty, &rdev->flags)) {
                         struct request_queue *q = bdev_get_queue(rdev->bdev);
@@ -974,7 +980,7 @@ read_again:
          */
         plugged = mddev_check_plugged(mddev);
 
-        disks = conf->raid_disks;
+        disks = conf->raid_disks * 2;
  retry_write:
         blocked_rdev = NULL;
         rcu_read_lock();
@@ -988,7 +994,8 @@ read_again:
                 }
                 r1_bio->bios[i] = NULL;
                 if (!rdev || test_bit(Faulty, &rdev->flags)) {
-                        set_bit(R1BIO_Degraded, &r1_bio->state);
+                        if (i < conf->raid_disks)
+                                set_bit(R1BIO_Degraded, &r1_bio->state);
                         continue;
                 }
 
@@ -1263,6 +1270,25 @@ static int raid1_spare_active(struct mddev *mddev)
          */
         for (i = 0; i < conf->raid_disks; i++) {
                 struct md_rdev *rdev = conf->mirrors[i].rdev;
+                struct md_rdev *repl = conf->mirrors[conf->raid_disks + i].rdev;
+                if (repl
+                    && repl->recovery_offset == MaxSector
+                    && !test_bit(Faulty, &repl->flags)
+                    && !test_and_set_bit(In_sync, &repl->flags)) {
+                        /* replacement has just become active */
+                        if (!rdev ||
+                            !test_and_clear_bit(In_sync, &rdev->flags))
+                                count++;
+                        if (rdev) {
+                                /* Replaced device not technically
+                                 * faulty, but we need to be sure
+                                 * it gets removed and never re-added
+                                 */
+                                set_bit(Faulty, &rdev->flags);
+                                sysfs_notify_dirent_safe(
+                                        rdev->sysfs_state);
+                        }
+                }
                 if (rdev
                     && !test_bit(Faulty, &rdev->flags)
                     && !test_and_set_bit(In_sync, &rdev->flags)) {
@@ -1286,7 +1312,7 @@ static int raid1_add_disk(struct mddev *mddev, struct md_rdev *rdev)
         int mirror = 0;
         struct mirror_info *p;
         int first = 0;
-        int last = mddev->raid_disks - 1;
+        int last = conf->raid_disks - 1;
 
         if (mddev->recovery_disabled == conf->recovery_disabled)
                 return -EBUSY;
@@ -1294,8 +1320,9 @@ static int raid1_add_disk(struct mddev *mddev, struct md_rdev *rdev)
         if (rdev->raid_disk >= 0)
                 first = last = rdev->raid_disk;
 
-        for (mirror = first; mirror <= last; mirror++)
-                if ( !(p=conf->mirrors+mirror)->rdev) {
+        for (mirror = first; mirror <= last; mirror++) {
+                p = conf->mirrors+mirror;
+                if (!p->rdev) {
 
                         disk_stack_limits(mddev->gendisk, rdev->bdev,
                                           rdev->data_offset << 9);
@@ -1322,21 +1349,35 @@ static int raid1_add_disk(struct mddev *mddev, struct md_rdev *rdev)
                         rcu_assign_pointer(p->rdev, rdev);
                         break;
                 }
+                if (test_bit(WantReplacement, &p->rdev->flags) &&
+                    p[conf->raid_disks].rdev == NULL) {
+                        /* Add this device as a replacement */
+                        clear_bit(In_sync, &rdev->flags);
+                        set_bit(Replacement, &rdev->flags);
+                        rdev->raid_disk = mirror;
+                        err = 0;
+                        conf->fullsync = 1;
+                        rcu_assign_pointer(p[conf->raid_disks].rdev, rdev);
+                        break;
+                }
+        }
         md_integrity_add_rdev(rdev, mddev);
         print_conf(conf);
         return err;
 }
 
-static int raid1_remove_disk(struct mddev *mddev, int number)
+static int raid1_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
 {
         struct r1conf *conf = mddev->private;
         int err = 0;
-        struct md_rdev *rdev;
+        int number = rdev->raid_disk;
         struct mirror_info *p = conf->mirrors+ number;
 
+        if (rdev != p->rdev)
+                p = conf->mirrors + conf->raid_disks + number;
+
         print_conf(conf);
-        rdev = p->rdev;
-        if (rdev) {
+        if (rdev == p->rdev) {
                 if (test_bit(In_sync, &rdev->flags) ||
                     atomic_read(&rdev->nr_pending)) {
                         err = -EBUSY;
@@ -1358,7 +1399,21 @@ static int raid1_remove_disk(struct mddev *mddev, int number)
                         err = -EBUSY;
                         p->rdev = rdev;
                         goto abort;
-                }
+                } else if (conf->mirrors[conf->raid_disks + number].rdev) {
+                        /* We just removed a device that is being replaced.
+                         * Move down the replacement.  We drain all IO before
+                         * doing this to avoid confusion.
+                         */
+                        struct md_rdev *repl =
+                                conf->mirrors[conf->raid_disks + number].rdev;
+                        raise_barrier(conf);
+                        clear_bit(Replacement, &repl->flags);
+                        p->rdev = repl;
+                        conf->mirrors[conf->raid_disks + number].rdev = NULL;
+                        lower_barrier(conf);
+                        clear_bit(WantReplacement, &rdev->flags);
+                } else
+                        clear_bit(WantReplacement, &rdev->flags);
                 err = md_integrity_register(mddev);
         }
 abort:
@@ -1411,6 +1466,10 @@ static void end_sync_write(struct bio *bio, int error)
                 } while (sectors_to_go > 0);
                 set_bit(WriteErrorSeen,
                         &conf->mirrors[mirror].rdev->flags);
+                if (!test_and_set_bit(WantReplacement,
+                                      &conf->mirrors[mirror].rdev->flags))
+                        set_bit(MD_RECOVERY_NEEDED, &
+                                mddev->recovery);
                 set_bit(R1BIO_WriteError, &r1_bio->state);
         } else if (is_badblock(conf->mirrors[mirror].rdev,
                                r1_bio->sector,
@@ -1441,8 +1500,13 @@ static int r1_sync_page_io(struct md_rdev *rdev, sector_t sector,
         if (sync_page_io(rdev, sector, sectors << 9, page, rw, false))
                 /* success */
                 return 1;
-        if (rw == WRITE)
+        if (rw == WRITE) {
                 set_bit(WriteErrorSeen, &rdev->flags);
+                if (!test_and_set_bit(WantReplacement,
+                                      &rdev->flags))
+                        set_bit(MD_RECOVERY_NEEDED, &
+                                rdev->mddev->recovery);
+        }
         /* need to record an error - either for the block or the device */
         if (!rdev_set_badblocks(rdev, sector, sectors, 0))
                 md_error(rdev->mddev, rdev);
@@ -1493,7 +1557,7 @@ static int fix_sync_read_error(struct r1bio *r1_bio)
                                 }
                         }
                         d++;
-                        if (d == conf->raid_disks)
+                        if (d == conf->raid_disks * 2)
                                 d = 0;
                 } while (!success && d != r1_bio->read_disk);
 
@@ -1510,7 +1574,7 @@ static int fix_sync_read_error(struct r1bio *r1_bio)
                                mdname(mddev),
                                bdevname(bio->bi_bdev, b),
                                (unsigned long long)r1_bio->sector);
-                        for (d = 0; d < conf->raid_disks; d++) {
+                        for (d = 0; d < conf->raid_disks * 2; d++) {
                                 rdev = conf->mirrors[d].rdev;
                                 if (!rdev || test_bit(Faulty, &rdev->flags))
                                         continue;
@@ -1536,7 +1600,7 @@ static int fix_sync_read_error(struct r1bio *r1_bio)
                 /* write it back and re-read */
                 while (d != r1_bio->read_disk) {
                         if (d == 0)
-                                d = conf->raid_disks;
+                                d = conf->raid_disks * 2;
                         d--;
                         if (r1_bio->bios[d]->bi_end_io != end_sync_read)
                                 continue;
@@ -1551,7 +1615,7 @@ static int fix_sync_read_error(struct r1bio *r1_bio)
                 d = start;
                 while (d != r1_bio->read_disk) {
                         if (d == 0)
-                                d = conf->raid_disks;
+                                d = conf->raid_disks * 2;
                         d--;
                         if (r1_bio->bios[d]->bi_end_io != end_sync_read)
                                 continue;
@@ -1584,7 +1648,7 @@ static int process_checks(struct r1bio *r1_bio)
         int primary;
         int i;
 
-        for (primary = 0; primary < conf->raid_disks; primary++)
+        for (primary = 0; primary < conf->raid_disks * 2; primary++)
                 if (r1_bio->bios[primary]->bi_end_io == end_sync_read &&
                     test_bit(BIO_UPTODATE, &r1_bio->bios[primary]->bi_flags)) {
                         r1_bio->bios[primary]->bi_end_io = NULL;
@@ -1592,7 +1656,7 @@ static int process_checks(struct r1bio *r1_bio)
                         break;
                 }
         r1_bio->read_disk = primary;
-        for (i = 0; i < conf->raid_disks; i++) {
+        for (i = 0; i < conf->raid_disks * 2; i++) {
                 int j;
                 int vcnt = r1_bio->sectors >> (PAGE_SHIFT- 9);
                 struct bio *pbio = r1_bio->bios[primary];
@@ -1656,7 +1720,7 @@ static void sync_request_write(struct mddev *mddev, struct r1bio *r1_bio)
 {
         struct r1conf *conf = mddev->private;
         int i;
-        int disks = conf->raid_disks;
+        int disks = conf->raid_disks * 2;
         struct bio *bio, *wbio;
 
         bio = r1_bio->bios[r1_bio->read_disk];
@@ -1737,7 +1801,7 @@ static void fix_read_error(struct r1conf *conf, int read_disk,
                                 success = 1;
                         else {
                                 d++;
-                                if (d == conf->raid_disks)
+                                if (d == conf->raid_disks * 2)
                                         d = 0;
                         }
                 } while (!success && d != read_disk);
@@ -1753,7 +1817,7 @@ static void fix_read_error(struct r1conf *conf, int read_disk,
                 start = d;
                 while (d != read_disk) {
                         if (d==0)
-                                d = conf->raid_disks;
+                                d = conf->raid_disks * 2;
                         d--;
                         rdev = conf->mirrors[d].rdev;
                         if (rdev &&
@@ -1765,7 +1829,7 @@ static void fix_read_error(struct r1conf *conf, int read_disk,
                 while (d != read_disk) {
                         char b[BDEVNAME_SIZE];
                         if (d==0)
-                                d = conf->raid_disks;
+                                d = conf->raid_disks * 2;
                         d--;
                         rdev = conf->mirrors[d].rdev;
                         if (rdev &&
@@ -1887,7 +1951,7 @@ static void handle_sync_write_finished(struct r1conf *conf, struct r1bio *r1_bio
 {
         int m;
         int s = r1_bio->sectors;
-        for (m = 0; m < conf->raid_disks ; m++) {
+        for (m = 0; m < conf->raid_disks * 2 ; m++) {
                 struct md_rdev *rdev = conf->mirrors[m].rdev;
                 struct bio *bio = r1_bio->bios[m];
                 if (bio->bi_end_io == NULL)
@@ -1909,7 +1973,7 @@ static void handle_sync_write_finished(struct r1conf *conf, struct r1bio *r1_bio
 static void handle_write_finished(struct r1conf *conf, struct r1bio *r1_bio)
 {
         int m;
-        for (m = 0; m < conf->raid_disks ; m++)
+        for (m = 0; m < conf->raid_disks * 2 ; m++)
                 if (r1_bio->bios[m] == IO_MADE_GOOD) {
                         struct md_rdev *rdev = conf->mirrors[m].rdev;
                         rdev_clear_badblocks(rdev,
@@ -2184,7 +2248,7 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr, int *skipp
         r1_bio->state = 0;
         set_bit(R1BIO_IsSync, &r1_bio->state);
 
-        for (i=0; i < conf->raid_disks; i++) {
+        for (i = 0; i < conf->raid_disks * 2; i++) {
                 struct md_rdev *rdev;
                 bio = r1_bio->bios[i];
 
@@ -2203,7 +2267,8 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr, int *skipp
                 rdev = rcu_dereference(conf->mirrors[i].rdev);
                 if (rdev == NULL ||
                     test_bit(Faulty, &rdev->flags)) {
-                        still_degraded = 1;
+                        if (i < conf->raid_disks)
+                                still_degraded = 1;
                 } else if (!test_bit(In_sync, &rdev->flags)) {
                         bio->bi_rw = WRITE;
                         bio->bi_end_io = end_sync_write;
@@ -2254,7 +2319,7 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr, int *skipp
                  * need to mark them bad on all write targets
                  */
                 int ok = 1;
-                for (i = 0 ; i < conf->raid_disks ; i++)
+                for (i = 0 ; i < conf->raid_disks * 2 ; i++)
                         if (r1_bio->bios[i]->bi_end_io == end_sync_write) {
                                 struct md_rdev *rdev =
                                         rcu_dereference(conf->mirrors[i].rdev);
@@ -2323,7 +2388,7 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr, int *skipp
                                 len = sync_blocks<<9;
                 }
 
-                for (i=0 ; i < conf->raid_disks; i++) {
+                for (i = 0 ; i < conf->raid_disks * 2; i++) {
                         bio = r1_bio->bios[i];
                         if (bio->bi_end_io) {
                                 page = bio->bi_io_vec[bio->bi_vcnt].bv_page;
@@ -2356,7 +2421,7 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr, int *skipp
          */
         if (test_bit(MD_RECOVERY_REQUESTED, &mddev->recovery)) {
                 atomic_set(&r1_bio->remaining, read_targets);
-                for (i=0; i<conf->raid_disks; i++) {
+                for (i = 0; i < conf->raid_disks * 2; i++) {
                         bio = r1_bio->bios[i];
                         if (bio->bi_end_io == end_sync_read) {
                                 md_sync_acct(bio->bi_bdev, nr_sectors);
@@ -2393,7 +2458,8 @@ static struct r1conf *setup_conf(struct mddev *mddev)
         if (!conf)
                 goto abort;
 
-        conf->mirrors = kzalloc(sizeof(struct mirror_info)*mddev->raid_disks,
+        conf->mirrors = kzalloc(sizeof(struct mirror_info)
+                                * mddev->raid_disks * 2,
                                  GFP_KERNEL);
         if (!conf->mirrors)
                 goto abort;
@@ -2405,7 +2471,7 @@ static struct r1conf *setup_conf(struct mddev *mddev)
         conf->poolinfo = kzalloc(sizeof(*conf->poolinfo), GFP_KERNEL);
         if (!conf->poolinfo)
                 goto abort;
-        conf->poolinfo->raid_disks = mddev->raid_disks;
+        conf->poolinfo->raid_disks = mddev->raid_disks * 2;
         conf->r1bio_pool = mempool_create(NR_RAID1_BIOS, r1bio_pool_alloc,
                                           r1bio_pool_free,
                                           conf->poolinfo);
@@ -2414,14 +2480,20 @@ static struct r1conf *setup_conf(struct mddev *mddev)
 
         conf->poolinfo->mddev = mddev;
 
+        err = -EINVAL;
         spin_lock_init(&conf->device_lock);
         list_for_each_entry(rdev, &mddev->disks, same_set) {
                 int disk_idx = rdev->raid_disk;
                 if (disk_idx >= mddev->raid_disks
                     || disk_idx < 0)
                         continue;
-                disk = conf->mirrors + disk_idx;
+                if (test_bit(Replacement, &rdev->flags))
+                        disk = conf->mirrors + conf->raid_disks + disk_idx;
+                else
+                        disk = conf->mirrors + disk_idx;
 
+                if (disk->rdev)
+                        goto abort;
                 disk->rdev = rdev;
 
                 disk->head_position = 0;
@@ -2437,11 +2509,27 @@ static struct r1conf *setup_conf(struct mddev *mddev)
         conf->pending_count = 0;
         conf->recovery_disabled = mddev->recovery_disabled - 1;
 
+        err = -EIO;
         conf->last_used = -1;
-        for (i = 0; i < conf->raid_disks; i++) {
+        for (i = 0; i < conf->raid_disks * 2; i++) {
 
                 disk = conf->mirrors + i;
 
+                if (i < conf->raid_disks &&
+                    disk[conf->raid_disks].rdev) {
+                        /* This slot has a replacement. */
+                        if (!disk->rdev) {
+                                /* No original, just make the replacement
+                                 * a recovering spare
+                                 */
+                                disk->rdev =
+                                        disk[conf->raid_disks].rdev;
+                                disk[conf->raid_disks].rdev = NULL;
+                        } else if (!test_bit(In_sync, &disk->rdev->flags))
+                                /* Original is not in_sync - bad */
+                                goto abort;
+                }
+
                 if (!disk->rdev ||
                     !test_bit(In_sync, &disk->rdev->flags)) {
                         disk->head_position = 0;
@@ -2455,7 +2543,6 @@ static struct r1conf *setup_conf(struct mddev *mddev)
                         conf->last_used = i;
         }
 
-        err = -EIO;
         if (conf->last_used < 0) {
                 printk(KERN_ERR "md/raid1:%s: no operational mirrors\n",
                        mdname(mddev));
@@ -2665,7 +2752,7 @@ static int raid1_reshape(struct mddev *mddev)
         if (!newpoolinfo)
                 return -ENOMEM;
         newpoolinfo->mddev = mddev;
-        newpoolinfo->raid_disks = raid_disks;
+        newpoolinfo->raid_disks = raid_disks * 2;
 
         newpool = mempool_create(NR_RAID1_BIOS, r1bio_pool_alloc,
                                  r1bio_pool_free, newpoolinfo);
@@ -2673,7 +2760,8 @@ static int raid1_reshape(struct mddev *mddev)
                 kfree(newpoolinfo);
                 return -ENOMEM;
         }
-        newmirrors = kzalloc(sizeof(struct mirror_info) * raid_disks, GFP_KERNEL);
+        newmirrors = kzalloc(sizeof(struct mirror_info) * raid_disks * 2,
+                             GFP_KERNEL);
         if (!newmirrors) {
                 kfree(newpoolinfo);
                 mempool_destroy(newpool);
diff --git a/drivers/md/raid1.h b/drivers/md/raid1.h
index c732b6cce935..80ded139314c 100644
--- a/drivers/md/raid1.h
+++ b/drivers/md/raid1.h
@@ -12,6 +12,9 @@ struct mirror_info {
  * pool was allocated for, so they know how much to allocate and free.
  * mddev->raid_disks cannot be used, as it can change while a pool is active
  * These two datums are stored in a kmalloced struct.
+ * The 'raid_disks' here is twice the raid_disks in r1conf.
+ * This allows space for each 'real' device can have a replacement in the
+ * second half of the array.
  */
 
 struct pool_info {
@@ -21,7 +24,9 @@ struct pool_info {
 
 struct r1conf {
         struct mddev            *mddev;
-        struct mirror_info              *mirrors;
+        struct mirror_info      *mirrors;       /* twice 'raid_disks' to
+                                                 * allow for replacements.
+                                                 */
         int                     raid_disks;
 
         /* When choose the best device for a read (read_balance())
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 685ddf325ee4..6e8aa213f0d5 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -73,7 +73,8 @@ static void * r10bio_pool_alloc(gfp_t gfp_flags, void *data)
         struct r10conf *conf = data;
         int size = offsetof(struct r10bio, devs[conf->copies]);
 
-        /* allocate a r10bio with room for raid_disks entries in the bios array */
+        /* allocate a r10bio with room for raid_disks entries in the
+         * bios array */
         return kzalloc(size, gfp_flags);
 }
 
@@ -123,12 +124,19 @@ static void * r10buf_pool_alloc(gfp_t gfp_flags, void *data)
                 if (!bio)
                         goto out_free_bio;
                 r10_bio->devs[j].bio = bio;
+                if (!conf->have_replacement)
+                        continue;
+                bio = bio_kmalloc(gfp_flags, RESYNC_PAGES);
+                if (!bio)
+                        goto out_free_bio;
+                r10_bio->devs[j].repl_bio = bio;
         }
         /*
          * Allocate RESYNC_PAGES data pages and attach them
          * where needed.
          */
         for (j = 0 ; j < nalloc; j++) {
+                struct bio *rbio = r10_bio->devs[j].repl_bio;
                 bio = r10_bio->devs[j].bio;
                 for (i = 0; i < RESYNC_PAGES; i++) {
                         if (j == 1 && !test_bit(MD_RECOVERY_SYNC,
@@ -143,6 +151,8 @@ static void * r10buf_pool_alloc(gfp_t gfp_flags, void *data)
                                 goto out_free_pages;
 
                         bio->bi_io_vec[i].bv_page = page;
+                        if (rbio)
+                                rbio->bi_io_vec[i].bv_page = page;
                 }
         }
 
@@ -156,8 +166,11 @@ out_free_pages:
                         safe_put_page(r10_bio->devs[j].bio->bi_io_vec[i].bv_page);
         j = -1;
 out_free_bio:
-        while ( ++j < nalloc )
+        while (++j < nalloc) {
                 bio_put(r10_bio->devs[j].bio);
+                if (r10_bio->devs[j].repl_bio)
+                        bio_put(r10_bio->devs[j].repl_bio);
+        }
         r10bio_pool_free(r10_bio, conf);
         return NULL;
 }
@@ -178,6 +191,9 @@ static void r10buf_pool_free(void *__r10_bio, void *data)
                         }
                         bio_put(bio);
                 }
+                bio = r10bio->devs[j].repl_bio;
+                if (bio)
+                        bio_put(bio);
         }
         r10bio_pool_free(r10bio, conf);
 }
@@ -191,6 +207,10 @@ static void put_all_bios(struct r10conf *conf, struct r10bio *r10_bio)
                 if (!BIO_SPECIAL(*bio))
                         bio_put(*bio);
                 *bio = NULL;
+                bio = &r10_bio->devs[i].repl_bio;
+                if (r10_bio->read_slot < 0 && !BIO_SPECIAL(*bio))
+                        bio_put(*bio);
+                *bio = NULL;
         }
 }
 
@@ -275,19 +295,27 @@ static inline void update_head_pos(int slot, struct r10bio *r10_bio)
  * Find the disk number which triggered given bio
  */
 static int find_bio_disk(struct r10conf *conf, struct r10bio *r10_bio,
-                         struct bio *bio, int *slotp)
+                         struct bio *bio, int *slotp, int *replp)
 {
         int slot;
+        int repl = 0;
 
-        for (slot = 0; slot < conf->copies; slot++)
+        for (slot = 0; slot < conf->copies; slot++) {
                 if (r10_bio->devs[slot].bio == bio)
                         break;
+                if (r10_bio->devs[slot].repl_bio == bio) {
+                        repl = 1;
+                        break;
+                }
+        }
 
         BUG_ON(slot == conf->copies);
         update_head_pos(slot, r10_bio);
 
         if (slotp)
                 *slotp = slot;
+        if (replp)
+                *replp = repl;
         return r10_bio->devs[slot].devnum;
 }
 
@@ -296,11 +324,13 @@ static void raid10_end_read_request(struct bio *bio, int error)
         int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
         struct r10bio *r10_bio = bio->bi_private;
         int slot, dev;
+        struct md_rdev *rdev;
         struct r10conf *conf = r10_bio->mddev->private;
 
 
         slot = r10_bio->read_slot;
         dev = r10_bio->devs[slot].devnum;
+        rdev = r10_bio->devs[slot].rdev;
         /*
          * this branch is our 'one mirror IO has finished' event handler:
          */
@@ -318,7 +348,7 @@ static void raid10_end_read_request(struct bio *bio, int error)
                  */
                 set_bit(R10BIO_Uptodate, &r10_bio->state);
                 raid_end_bio_io(r10_bio);
-                rdev_dec_pending(conf->mirrors[dev].rdev, conf->mddev);
+                rdev_dec_pending(rdev, conf->mddev);
         } else {
                 /*
                  * oops, read error - keep the refcount on the rdev
@@ -327,7 +357,7 @@ static void raid10_end_read_request(struct bio *bio, int error)
                 printk_ratelimited(KERN_ERR
                                    "md/raid10:%s: %s: rescheduling sector %llu\n",
                                    mdname(conf->mddev),
-                                   bdevname(conf->mirrors[dev].rdev->bdev, b),
+                                   bdevname(rdev->bdev, b),
                                    (unsigned long long)r10_bio->sector);
                 set_bit(R10BIO_ReadError, &r10_bio->state);
                 reschedule_retry(r10_bio);
@@ -366,17 +396,35 @@ static void raid10_end_write_request(struct bio *bio, int error)
         int dev;
         int dec_rdev = 1;
         struct r10conf *conf = r10_bio->mddev->private;
-        int slot;
+        int slot, repl;
+        struct md_rdev *rdev = NULL;
 
-        dev = find_bio_disk(conf, r10_bio, bio, &slot);
+        dev = find_bio_disk(conf, r10_bio, bio, &slot, &repl);
 
+        if (repl)
+                rdev = conf->mirrors[dev].replacement;
+        if (!rdev) {
+                smp_rmb();
+                repl = 0;
+                rdev = conf->mirrors[dev].rdev;
+        }
         /*
          * this branch is our 'one mirror IO has finished' event handler:
          */
         if (!uptodate) {
-                set_bit(WriteErrorSeen, &conf->mirrors[dev].rdev->flags);
-                set_bit(R10BIO_WriteError, &r10_bio->state);
-                dec_rdev = 0;
+                if (repl)
+                        /* Never record new bad blocks to replacement,
+                         * just fail it.
+                         */
+                        md_error(rdev->mddev, rdev);
+                else {
+                        set_bit(WriteErrorSeen, &rdev->flags);
+                        if (!test_and_set_bit(WantReplacement, &rdev->flags))
+                                set_bit(MD_RECOVERY_NEEDED,
+                                        &rdev->mddev->recovery);
+                        set_bit(R10BIO_WriteError, &r10_bio->state);
+                        dec_rdev = 0;
+                }
         } else {
                 /*
                  * Set R10BIO_Uptodate in our master bio, so that
@@ -393,12 +441,15 @@ static void raid10_end_write_request(struct bio *bio, int error)
                 set_bit(R10BIO_Uptodate, &r10_bio->state);
 
                 /* Maybe we can clear some bad blocks. */
-                if (is_badblock(conf->mirrors[dev].rdev,
+                if (is_badblock(rdev,
                                 r10_bio->devs[slot].addr,
                                 r10_bio->sectors,
                                 &first_bad, &bad_sectors)) {
                         bio_put(bio);
-                        r10_bio->devs[slot].bio = IO_MADE_GOOD;
+                        if (repl)
+                                r10_bio->devs[slot].repl_bio = IO_MADE_GOOD;
+                        else
+                                r10_bio->devs[slot].bio = IO_MADE_GOOD;
                         dec_rdev = 0;
                         set_bit(R10BIO_MadeGood, &r10_bio->state);
                 }
@@ -414,7 +465,6 @@ static void raid10_end_write_request(struct bio *bio, int error)
                 rdev_dec_pending(conf->mirrors[dev].rdev, conf->mddev);
 }
 
-
 /*
  * RAID10 layout manager
  * As well as the chunksize and raid_disks count, there are two
@@ -562,14 +612,16 @@ static int raid10_mergeable_bvec(struct request_queue *q,
  * FIXME: possibly should rethink readbalancing and do it differently
  * depending on near_copies / far_copies geometry.
  */
-static int read_balance(struct r10conf *conf, struct r10bio *r10_bio, int *max_sectors)
+static struct md_rdev *read_balance(struct r10conf *conf,
+                                    struct r10bio *r10_bio,
+                                    int *max_sectors)
 {
         const sector_t this_sector = r10_bio->sector;
         int disk, slot;
         int sectors = r10_bio->sectors;
         int best_good_sectors;
         sector_t new_distance, best_dist;
-        struct md_rdev *rdev;
+        struct md_rdev *rdev, *best_rdev;
         int do_balance;
         int best_slot;
 
@@ -578,6 +630,7 @@ static int read_balance(struct r10conf *conf, struct r10bio *r10_bio, int *max_s
 retry:
         sectors = r10_bio->sectors;
         best_slot = -1;
+        best_rdev = NULL;
         best_dist = MaxSector;
         best_good_sectors = 0;
         do_balance = 1;
@@ -599,10 +652,16 @@ retry:
                 if (r10_bio->devs[slot].bio == IO_BLOCKED)
                         continue;
                 disk = r10_bio->devs[slot].devnum;
-                rdev = rcu_dereference(conf->mirrors[disk].rdev);
+                rdev = rcu_dereference(conf->mirrors[disk].replacement);
+                if (rdev == NULL || test_bit(Faulty, &rdev->flags) ||
+                    r10_bio->devs[slot].addr + sectors > rdev->recovery_offset)
+                        rdev = rcu_dereference(conf->mirrors[disk].rdev);
                 if (rdev == NULL)
                         continue;
-                if (!test_bit(In_sync, &rdev->flags))
+                if (test_bit(Faulty, &rdev->flags))
+                        continue;
+                if (!test_bit(In_sync, &rdev->flags) &&
+                    r10_bio->devs[slot].addr + sectors > rdev->recovery_offset)
                         continue;
 
                 dev_sector = r10_bio->devs[slot].addr;
@@ -627,6 +686,7 @@ retry:
                                 if (good_sectors > best_good_sectors) {
                                         best_good_sectors = good_sectors;
                                         best_slot = slot;
+                                        best_rdev = rdev;
                                 }
                                 if (!do_balance)
                                         /* Must read from here */
@@ -655,16 +715,15 @@ retry:
                 if (new_distance < best_dist) {
                         best_dist = new_distance;
                         best_slot = slot;
+                        best_rdev = rdev;
                 }
         }
-        if (slot == conf->copies)
+        if (slot >= conf->copies) {
                 slot = best_slot;
+                rdev = best_rdev;
+        }
 
         if (slot >= 0) {
-                disk = r10_bio->devs[slot].devnum;
-                rdev = rcu_dereference(conf->mirrors[disk].rdev);
-                if (!rdev)
-                        goto retry;
                 atomic_inc(&rdev->nr_pending);
                 if (test_bit(Faulty, &rdev->flags)) {
                         /* Cannot risk returning a device that failed
@@ -675,11 +734,11 @@ retry:
                 }
                 r10_bio->read_slot = slot;
         } else
-                disk = -1;
+                rdev = NULL;
         rcu_read_unlock();
         *max_sectors = best_good_sectors;
 
-        return disk;
+        return rdev;
 }
 
 static int raid10_congested(void *data, int bits)
@@ -846,7 +905,6 @@ static void unfreeze_array(struct r10conf *conf)
 static void make_request(struct mddev *mddev, struct bio * bio)
 {
         struct r10conf *conf = mddev->private;
-        struct mirror_info *mirror;
         struct r10bio *r10_bio;
         struct bio *read_bio;
         int i;
@@ -945,27 +1003,27 @@ static void make_request(struct mddev *mddev, struct bio * bio)
                 /*
                  * read balancing logic:
                  */
-                int disk;
+                struct md_rdev *rdev;
                 int slot;
 
 read_again:
-                disk = read_balance(conf, r10_bio, &max_sectors);
-                slot = r10_bio->read_slot;
-                if (disk < 0) {
+                rdev = read_balance(conf, r10_bio, &max_sectors);
+                if (!rdev) {
                         raid_end_bio_io(r10_bio);
                         return;
                 }
-                mirror = conf->mirrors + disk;
+                slot = r10_bio->read_slot;
 
                 read_bio = bio_clone_mddev(bio, GFP_NOIO, mddev);
                 md_trim_bio(read_bio, r10_bio->sector - bio->bi_sector,
                             max_sectors);
 
                 r10_bio->devs[slot].bio = read_bio;
+                r10_bio->devs[slot].rdev = rdev;
 
                 read_bio->bi_sector = r10_bio->devs[slot].addr +
-                        mirror->rdev->data_offset;
-                read_bio->bi_bdev = mirror->rdev->bdev;
+                        rdev->data_offset;
+                read_bio->bi_bdev = rdev->bdev;
                 read_bio->bi_end_io = raid10_end_read_request;
                 read_bio->bi_rw = READ | do_sync;
                 read_bio->bi_private = r10_bio;
@@ -1025,6 +1083,7 @@ read_again:
          */
         plugged = mddev_check_plugged(mddev);
 
+        r10_bio->read_slot = -1; /* make sure repl_bio gets freed */
         raid10_find_phys(conf, r10_bio);
 retry_write:
         blocked_rdev = NULL;
@@ -1034,12 +1093,25 @@ retry_write:
         for (i = 0;  i < conf->copies; i++) {
                 int d = r10_bio->devs[i].devnum;
                 struct md_rdev *rdev = rcu_dereference(conf->mirrors[d].rdev);
+                struct md_rdev *rrdev = rcu_dereference(
+                        conf->mirrors[d].replacement);
+                if (rdev == rrdev)
+                        rrdev = NULL;
                 if (rdev && unlikely(test_bit(Blocked, &rdev->flags))) {
                         atomic_inc(&rdev->nr_pending);
                         blocked_rdev = rdev;
                         break;
                 }
+                if (rrdev && unlikely(test_bit(Blocked, &rrdev->flags))) {
+                        atomic_inc(&rrdev->nr_pending);
+                        blocked_rdev = rrdev;
+                        break;
+                }
+                if (rrdev && test_bit(Faulty, &rrdev->flags))
+                        rrdev = NULL;
+
                 r10_bio->devs[i].bio = NULL;
+                r10_bio->devs[i].repl_bio = NULL;
                 if (!rdev || test_bit(Faulty, &rdev->flags)) {
                         set_bit(R10BIO_Degraded, &r10_bio->state);
                         continue;
@@ -1088,6 +1160,10 @@ retry_write:
                 }
                 r10_bio->devs[i].bio = bio;
                 atomic_inc(&rdev->nr_pending);
+                if (rrdev) {
+                        r10_bio->devs[i].repl_bio = bio;
+                        atomic_inc(&rrdev->nr_pending);
+                }
         }
         rcu_read_unlock();
 
@@ -1096,11 +1172,23 @@ retry_write:
                 int j;
                 int d;
 
-                for (j = 0; j < i; j++)
+                for (j = 0; j < i; j++) {
                         if (r10_bio->devs[j].bio) {
                                 d = r10_bio->devs[j].devnum;
                                 rdev_dec_pending(conf->mirrors[d].rdev, mddev);
                         }
+                        if (r10_bio->devs[j].repl_bio) {
+                                struct md_rdev *rdev;
+                                d = r10_bio->devs[j].devnum;
+                                rdev = conf->mirrors[d].replacement;
+                                if (!rdev) {
+                                        /* Race with remove_disk */
+                                        smp_mb();
+                                        rdev = conf->mirrors[d].rdev;
+                                }
+                                rdev_dec_pending(rdev, mddev);
+                        }
+                }
                 allow_barrier(conf);
                 md_wait_for_blocked_rdev(blocked_rdev, mddev);
                 wait_barrier(conf);
@@ -1147,6 +1235,31 @@ retry_write:
                 bio_list_add(&conf->pending_bio_list, mbio);
                 conf->pending_count++;
                 spin_unlock_irqrestore(&conf->device_lock, flags);
+
+                if (!r10_bio->devs[i].repl_bio)
+                        continue;
+
+                mbio = bio_clone_mddev(bio, GFP_NOIO, mddev);
+                md_trim_bio(mbio, r10_bio->sector - bio->bi_sector,
+                            max_sectors);
+                r10_bio->devs[i].repl_bio = mbio;
+
+                /* We are actively writing to the original device
+                 * so it cannot disappear, so the replacement cannot
+                 * become NULL here
+                 */
+                mbio->bi_sector = (r10_bio->devs[i].addr+
+                                   conf->mirrors[d].replacement->data_offset);
+                mbio->bi_bdev = conf->mirrors[d].replacement->bdev;
+                mbio->bi_end_io = raid10_end_write_request;
+                mbio->bi_rw = WRITE | do_sync | do_fua;
+                mbio->bi_private = r10_bio;
+
+                atomic_inc(&r10_bio->remaining);
+                spin_lock_irqsave(&conf->device_lock, flags);
+                bio_list_add(&conf->pending_bio_list, mbio);
+                conf->pending_count++;
+                spin_unlock_irqrestore(&conf->device_lock, flags);
         }
 
         /* Don't remove the bias on 'remaining' (one_write_done) until
@@ -1309,9 +1422,27 @@ static int raid10_spare_active(struct mddev *mddev)
          */
         for (i = 0; i < conf->raid_disks; i++) {
                 tmp = conf->mirrors + i;
-                if (tmp->rdev
-                    && !test_bit(Faulty, &tmp->rdev->flags)
-                    && !test_and_set_bit(In_sync, &tmp->rdev->flags)) {
+                if (tmp->replacement
+                    && tmp->replacement->recovery_offset == MaxSector
+                    && !test_bit(Faulty, &tmp->replacement->flags)
+                    && !test_and_set_bit(In_sync, &tmp->replacement->flags)) {
+                        /* Replacement has just become active */
+                        if (!tmp->rdev
+                            || !test_and_clear_bit(In_sync, &tmp->rdev->flags))
+                                count++;
+                        if (tmp->rdev) {
+                                /* Replaced device not technically faulty,
+                                 * but we need to be sure it gets removed
+                                 * and never re-added.
+                                 */
+                                set_bit(Faulty, &tmp->rdev->flags);
+                                sysfs_notify_dirent_safe(
+                                        tmp->rdev->sysfs_state);
+                        }
+                        sysfs_notify_dirent_safe(tmp->replacement->sysfs_state);
+                } else if (tmp->rdev
+                           && !test_bit(Faulty, &tmp->rdev->flags)
+                           && !test_and_set_bit(In_sync, &tmp->rdev->flags)) {
                         count++;
                         sysfs_notify_dirent(tmp->rdev->sysfs_state);
                 }
@@ -1353,8 +1484,25 @@ static int raid10_add_disk(struct mddev *mddev, struct md_rdev *rdev)
                 struct mirror_info *p = &conf->mirrors[mirror];
                 if (p->recovery_disabled == mddev->recovery_disabled)
                         continue;
-                if (p->rdev)
-                        continue;
+                if (p->rdev) {
+                        if (!test_bit(WantReplacement, &p->rdev->flags) ||
+                            p->replacement != NULL)
+                                continue;
+                        clear_bit(In_sync, &rdev->flags);
+                        set_bit(Replacement, &rdev->flags);
+                        rdev->raid_disk = mirror;
+                        err = 0;
+                        disk_stack_limits(mddev->gendisk, rdev->bdev,
+                                          rdev->data_offset << 9);
+                        if (rdev->bdev->bd_disk->queue->merge_bvec_fn) {
+                                blk_queue_max_segments(mddev->queue, 1);
+                                blk_queue_segment_boundary(mddev->queue,
+                                                           PAGE_CACHE_SIZE - 1);
+                        }
+                        conf->fullsync = 1;
+                        rcu_assign_pointer(p->replacement, rdev);
+                        break;
+                }
 
                 disk_stack_limits(mddev->gendisk, rdev->bdev,
                                   rdev->data_offset << 9);
@@ -1385,40 +1533,61 @@ static int raid10_add_disk(struct mddev *mddev, struct md_rdev *rdev)
         return err;
 }
 
-static int raid10_remove_disk(struct mddev *mddev, int number)
+static int raid10_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
 {
         struct r10conf *conf = mddev->private;
         int err = 0;
-        struct md_rdev *rdev;
-        struct mirror_info *p = conf->mirrors+ number;
+        int number = rdev->raid_disk;
+        struct md_rdev **rdevp;
+        struct mirror_info *p = conf->mirrors + number;
 
         print_conf(conf);
-        rdev = p->rdev;
-        if (rdev) {
-                if (test_bit(In_sync, &rdev->flags) ||
-                    atomic_read(&rdev->nr_pending)) {
-                        err = -EBUSY;
-                        goto abort;
-                }
-                /* Only remove faulty devices in recovery
-                 * is not possible.
-                 */
-                if (!test_bit(Faulty, &rdev->flags) &&
-                    mddev->recovery_disabled != p->recovery_disabled &&
-                    enough(conf, -1)) {
-                        err = -EBUSY;
-                        goto abort;
-                }
-                p->rdev = NULL;
-                synchronize_rcu();
-                if (atomic_read(&rdev->nr_pending)) {
-                        /* lost the race, try later */
-                        err = -EBUSY;
-                        p->rdev = rdev;
-                        goto abort;
-                }
-                err = md_integrity_register(mddev);
+        if (rdev == p->rdev)
+                rdevp = &p->rdev;
+        else if (rdev == p->replacement)
+                rdevp = &p->replacement;
+        else
+                return 0;
+
+        if (test_bit(In_sync, &rdev->flags) ||
+            atomic_read(&rdev->nr_pending)) {
+                err = -EBUSY;
+                goto abort;
         }
+        /* Only remove faulty devices if recovery
+         * is not possible.
+         */
+        if (!test_bit(Faulty, &rdev->flags) &&
+            mddev->recovery_disabled != p->recovery_disabled &&
+            (!p->replacement || p->replacement == rdev) &&
+            enough(conf, -1)) {
+                err = -EBUSY;
+                goto abort;
+        }
+        *rdevp = NULL;
+        synchronize_rcu();
+        if (atomic_read(&rdev->nr_pending)) {
+                /* lost the race, try later */
+                err = -EBUSY;
+                *rdevp = rdev;
+                goto abort;
+        } else if (p->replacement) {
+                /* We must have just cleared 'rdev' */
+                p->rdev = p->replacement;
+                clear_bit(Replacement, &p->replacement->flags);
+                smp_mb(); /* Make sure other CPUs may see both as identical
+                           * but will never see neither -- if they are careful.
+                           */
+                p->replacement = NULL;
+                clear_bit(WantReplacement, &rdev->flags);
+        } else
+                /* We might have just remove the Replacement as faulty
+                 * Clear the flag just in case
+                 */
+                clear_bit(WantReplacement, &rdev->flags);
+
+        err = md_integrity_register(mddev);
+
 abort:
 
         print_conf(conf);
@@ -1432,7 +1601,7 @@ static void end_sync_read(struct bio *bio, int error)
         struct r10conf *conf = r10_bio->mddev->private;
         int d;
 
-        d = find_bio_disk(conf, r10_bio, bio, NULL);
+        d = find_bio_disk(conf, r10_bio, bio, NULL, NULL);
 
         if (test_bit(BIO_UPTODATE, &bio->bi_flags))
                 set_bit(R10BIO_Uptodate, &r10_bio->state);
@@ -1493,19 +1662,34 @@ static void end_sync_write(struct bio *bio, int error)
         sector_t first_bad;
         int bad_sectors;
         int slot;
-
-        d = find_bio_disk(conf, r10_bio, bio, &slot);
+        int repl;
+        struct md_rdev *rdev = NULL;
+
+        d = find_bio_disk(conf, r10_bio, bio, &slot, &repl);
+        if (repl)
+                rdev = conf->mirrors[d].replacement;
+        if (!rdev) {
+                smp_mb();
+                rdev = conf->mirrors[d].rdev;
+        }
 
         if (!uptodate) {
-                set_bit(WriteErrorSeen, &conf->mirrors[d].rdev->flags);
-                set_bit(R10BIO_WriteError, &r10_bio->state);
-        } else if (is_badblock(conf->mirrors[d].rdev,
+                if (repl)
+                        md_error(mddev, rdev);
+                else {
+                        set_bit(WriteErrorSeen, &rdev->flags);
+                        if (!test_and_set_bit(WantReplacement, &rdev->flags))
+                                set_bit(MD_RECOVERY_NEEDED,
+                                        &rdev->mddev->recovery);
+                        set_bit(R10BIO_WriteError, &r10_bio->state);
+                }
+        } else if (is_badblock(rdev,
                              r10_bio->devs[slot].addr,
                              r10_bio->sectors,
                              &first_bad, &bad_sectors))
                 set_bit(R10BIO_MadeGood, &r10_bio->state);
 
-        rdev_dec_pending(conf->mirrors[d].rdev, mddev);
+        rdev_dec_pending(rdev, mddev);
 
         end_sync_request(r10_bio);
 }
@@ -1609,6 +1793,29 @@ static void sync_request_write(struct mddev *mddev, struct r10bio *r10_bio)
                 generic_make_request(tbio);
         }
 
+        /* Now write out to any replacement devices
+         * that are active
+         */
+        for (i = 0; i < conf->copies; i++) {
+                int j, d;
+                int vcnt = r10_bio->sectors >> (PAGE_SHIFT-9);
+
+                tbio = r10_bio->devs[i].repl_bio;
+                if (!tbio || !tbio->bi_end_io)
+                        continue;
+                if (r10_bio->devs[i].bio->bi_end_io != end_sync_write
+                    && r10_bio->devs[i].bio != fbio)
+                        for (j = 0; j < vcnt; j++)
+                                memcpy(page_address(tbio->bi_io_vec[j].bv_page),
+                                       page_address(fbio->bi_io_vec[j].bv_page),
+                                       PAGE_SIZE);
+                d = r10_bio->devs[i].devnum;
+                atomic_inc(&r10_bio->remaining);
+                md_sync_acct(conf->mirrors[d].replacement->bdev,
+                             tbio->bi_size >> 9);
+                generic_make_request(tbio);
+        }
+
 done:
         if (atomic_dec_and_test(&r10_bio->remaining)) {
                 md_done_sync(mddev, r10_bio->sectors, 1);
@@ -1668,8 +1875,13 @@ static void fix_recovery_read_error(struct r10bio *r10_bio)
                                           s << 9,
                                           bio->bi_io_vec[idx].bv_page,
                                           WRITE, false);
-                        if (!ok)
+                        if (!ok) {
                                 set_bit(WriteErrorSeen, &rdev->flags);
+                                if (!test_and_set_bit(WantReplacement,
+                                                      &rdev->flags))
+                                        set_bit(MD_RECOVERY_NEEDED,
+                                                &rdev->mddev->recovery);
+                        }
                 }
                 if (!ok) {
                         /* We don't worry if we cannot set a bad block -
@@ -1709,7 +1921,7 @@ static void recovery_request_write(struct mddev *mddev, struct r10bio *r10_bio)
 {
         struct r10conf *conf = mddev->private;
         int d;
-        struct bio *wbio;
+        struct bio *wbio, *wbio2;
 
         if (!test_bit(R10BIO_Uptodate, &r10_bio->state)) {
                 fix_recovery_read_error(r10_bio);
@@ -1721,12 +1933,20 @@ static void recovery_request_write(struct mddev *mddev, struct r10bio *r10_bio)
          * share the pages with the first bio
          * and submit the write request
          */
-        wbio = r10_bio->devs[1].bio;
         d = r10_bio->devs[1].devnum;
-
-        atomic_inc(&conf->mirrors[d].rdev->nr_pending);
-        md_sync_acct(conf->mirrors[d].rdev->bdev, wbio->bi_size >> 9);
-        generic_make_request(wbio);
+        wbio = r10_bio->devs[1].bio;
+        wbio2 = r10_bio->devs[1].repl_bio;
+        if (wbio->bi_end_io) {
+                atomic_inc(&conf->mirrors[d].rdev->nr_pending);
+                md_sync_acct(conf->mirrors[d].rdev->bdev, wbio->bi_size >> 9);
+                generic_make_request(wbio);
+        }
+        if (wbio2 && wbio2->bi_end_io) {
+                atomic_inc(&conf->mirrors[d].replacement->nr_pending);
+                md_sync_acct(conf->mirrors[d].replacement->bdev,
+                             wbio2->bi_size >> 9);
+                generic_make_request(wbio2);
+        }
 }
 
 
@@ -1779,8 +1999,12 @@ static int r10_sync_page_io(struct md_rdev *rdev, sector_t sector,
         if (sync_page_io(rdev, sector, sectors << 9, page, rw, false))
                 /* success */
                 return 1;
-        if (rw == WRITE)
+        if (rw == WRITE) {
                 set_bit(WriteErrorSeen, &rdev->flags);
+                if (!test_and_set_bit(WantReplacement, &rdev->flags))
+                        set_bit(MD_RECOVERY_NEEDED,
+                                &rdev->mddev->recovery);
+        }
         /* need to record an error - either for the block or the device */
         if (!rdev_set_badblocks(rdev, sector, sectors, 0))
                 md_error(rdev->mddev, rdev);
@@ -2060,10 +2284,9 @@ static int narrow_write_error(struct r10bio *r10_bio, int i)
 static void handle_read_error(struct mddev *mddev, struct r10bio *r10_bio)
 {
         int slot = r10_bio->read_slot;
-        int mirror = r10_bio->devs[slot].devnum;
         struct bio *bio;
         struct r10conf *conf = mddev->private;
-        struct md_rdev *rdev;
+        struct md_rdev *rdev = r10_bio->devs[slot].rdev;
         char b[BDEVNAME_SIZE];
         unsigned long do_sync;
         int max_sectors;
@@ -2081,15 +2304,15 @@ static void handle_read_error(struct mddev *mddev, struct r10bio *r10_bio)
                 fix_read_error(conf, mddev, r10_bio);
                 unfreeze_array(conf);
         }
-        rdev_dec_pending(conf->mirrors[mirror].rdev, mddev);
+        rdev_dec_pending(rdev, mddev);
 
         bio = r10_bio->devs[slot].bio;
         bdevname(bio->bi_bdev, b);
         r10_bio->devs[slot].bio =
                 mddev->ro ? IO_BLOCKED : NULL;
 read_more:
-        mirror = read_balance(conf, r10_bio, &max_sectors);
-        if (mirror == -1) {
+        rdev = read_balance(conf, r10_bio, &max_sectors);
+        if (rdev == NULL) {
                 printk(KERN_ALERT "md/raid10:%s: %s: unrecoverable I/O"
                        " read error for block %llu\n",
                        mdname(mddev), b,
@@ -2103,7 +2326,6 @@ read_more:
         if (bio)
                 bio_put(bio);
         slot = r10_bio->read_slot;
-        rdev = conf->mirrors[mirror].rdev;
         printk_ratelimited(
                 KERN_ERR
                 "md/raid10:%s: %s: redirecting"
@@ -2117,6 +2339,7 @@ read_more:
                     r10_bio->sector - bio->bi_sector,
                     max_sectors);
         r10_bio->devs[slot].bio = bio;
+        r10_bio->devs[slot].rdev = rdev;
         bio->bi_sector = r10_bio->devs[slot].addr
                 + rdev->data_offset;
         bio->bi_bdev = rdev->bdev;
@@ -2187,6 +2410,22 @@ static void handle_write_completed(struct r10conf *conf, struct r10bio *r10_bio)
                                             r10_bio->sectors, 0))
                                         md_error(conf->mddev, rdev);
                         }
+                        rdev = conf->mirrors[dev].replacement;
+                        if (r10_bio->devs[m].repl_bio == NULL)
+                                continue;
+                        if (test_bit(BIO_UPTODATE,
+                                     &r10_bio->devs[m].repl_bio->bi_flags)) {
+                                rdev_clear_badblocks(
+                                        rdev,
+                                        r10_bio->devs[m].addr,
+                                        r10_bio->sectors);
+                        } else {
+                                if (!rdev_set_badblocks(
+                                            rdev,
+                                            r10_bio->devs[m].addr,
+                                            r10_bio->sectors, 0))
+                                        md_error(conf->mddev, rdev);
+                        }
                 }
                 put_buf(r10_bio);
         } else {
@@ -2209,6 +2448,15 @@ static void handle_write_completed(struct r10conf *conf, struct r10bio *r10_bio)
                                 }
                                 rdev_dec_pending(rdev, conf->mddev);
                         }
+                        bio = r10_bio->devs[m].repl_bio;
+                        rdev = conf->mirrors[dev].replacement;
+                        if (rdev && bio == IO_MADE_GOOD) {
+                                rdev_clear_badblocks(
+                                        rdev,
+                                        r10_bio->devs[m].addr,
+                                        r10_bio->sectors);
+                                rdev_dec_pending(rdev, conf->mddev);
+                        }
                 }
                 if (test_bit(R10BIO_WriteError,
                              &r10_bio->state))
@@ -2272,9 +2520,14 @@ static void raid10d(struct mddev *mddev)
 static int init_resync(struct r10conf *conf)
 {
         int buffs;
+        int i;
 
         buffs = RESYNC_WINDOW / RESYNC_BLOCK_SIZE;
         BUG_ON(conf->r10buf_pool);
+        conf->have_replacement = 0;
+        for (i = 0; i < conf->raid_disks; i++)
+                if (conf->mirrors[i].replacement)
+                        conf->have_replacement = 1;
         conf->r10buf_pool = mempool_create(buffs, r10buf_pool_alloc, r10buf_pool_free, conf);
         if (!conf->r10buf_pool)
                 return -ENOMEM;
@@ -2355,9 +2608,22 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr,
                                 bitmap_end_sync(mddev->bitmap, sect,
                                                 &sync_blocks, 1);
                         }
-                } else /* completed sync */
+                } else {
+                        /* completed sync */
+                        if ((!mddev->bitmap || conf->fullsync)
+                            && conf->have_replacement
+                            && test_bit(MD_RECOVERY_SYNC, &mddev->recovery)) {
+                                /* Completed a full sync so the replacements
+                                 * are now fully recovered.
+                                 */
+                                for (i = 0; i < conf->raid_disks; i++)
+                                        if (conf->mirrors[i].replacement)
+                                                conf->mirrors[i].replacement
+                                                        ->recovery_offset
+                                                        = MaxSector;
+                        }
                         conf->fullsync = 0;
-
+                }
                 bitmap_close_sync(mddev->bitmap);
                 close_sync(conf);
                 *skipped = 1;
@@ -2414,23 +2680,30 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr,
                         sector_t sect;
                         int must_sync;
                         int any_working;
-
-                        if (conf->mirrors[i].rdev == NULL ||
-                            test_bit(In_sync, &conf->mirrors[i].rdev->flags)) 
+                        struct mirror_info *mirror = &conf->mirrors[i];
+
+                        if ((mirror->rdev == NULL ||
+                             test_bit(In_sync, &mirror->rdev->flags))
+                            &&
+                            (mirror->replacement == NULL ||
+                             test_bit(Faulty,
+                                      &mirror->replacement->flags)))
                                 continue;
 
                         still_degraded = 0;
                         /* want to reconstruct this device */
                         rb2 = r10_bio;
                         sect = raid10_find_virt(conf, sector_nr, i);
-                        /* Unless we are doing a full sync, we only need
-                         * to recover the block if it is set in the bitmap
+                        /* Unless we are doing a full sync, or a replacement
+                         * we only need to recover the block if it is set in
+                         * the bitmap
                          */
                         must_sync = bitmap_start_sync(mddev->bitmap, sect,
                                                       &sync_blocks, 1);
                         if (sync_blocks < max_sync)
                                 max_sync = sync_blocks;
                         if (!must_sync &&
+                            mirror->replacement == NULL &&
                             !conf->fullsync) {
                                 /* yep, skip the sync_blocks here, but don't assume
                                  * that there will never be anything to do here
@@ -2500,33 +2773,60 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr,
                                 bio->bi_end_io = end_sync_read;
                                 bio->bi_rw = READ;
                                 from_addr = r10_bio->devs[j].addr;
-                                bio->bi_sector = from_addr +
-                                        conf->mirrors[d].rdev->data_offset;
-                                bio->bi_bdev = conf->mirrors[d].rdev->bdev;
-                                atomic_inc(&conf->mirrors[d].rdev->nr_pending);
-                                atomic_inc(&r10_bio->remaining);
-                                /* and we write to 'i' */
+                                bio->bi_sector = from_addr + rdev->data_offset;
+                                bio->bi_bdev = rdev->bdev;
+                                atomic_inc(&rdev->nr_pending);
+                                /* and we write to 'i' (if not in_sync) */
 
                                 for (k=0; k<conf->copies; k++)
                                         if (r10_bio->devs[k].devnum == i)
                                                 break;
                                 BUG_ON(k == conf->copies);
-                                bio = r10_bio->devs[1].bio;
-                                bio->bi_next = biolist;
-                                biolist = bio;
-                                bio->bi_private = r10_bio;
-                                bio->bi_end_io = end_sync_write;
-                                bio->bi_rw = WRITE;
                                 to_addr = r10_bio->devs[k].addr;
-                                bio->bi_sector = to_addr +
-                                        conf->mirrors[i].rdev->data_offset;
-                                bio->bi_bdev = conf->mirrors[i].rdev->bdev;
-
                                 r10_bio->devs[0].devnum = d;
                                 r10_bio->devs[0].addr = from_addr;
                                 r10_bio->devs[1].devnum = i;
                                 r10_bio->devs[1].addr = to_addr;
 
+                                rdev = mirror->rdev;
+                                if (!test_bit(In_sync, &rdev->flags)) {
+                                        bio = r10_bio->devs[1].bio;
+                                        bio->bi_next = biolist;
+                                        biolist = bio;
+                                        bio->bi_private = r10_bio;
+                                        bio->bi_end_io = end_sync_write;
+                                        bio->bi_rw = WRITE;
+                                        bio->bi_sector = to_addr
+                                                + rdev->data_offset;
+                                        bio->bi_bdev = rdev->bdev;
+                                        atomic_inc(&r10_bio->remaining);
+                                } else
+                                        r10_bio->devs[1].bio->bi_end_io = NULL;
+
+                                /* and maybe write to replacement */
+                                bio = r10_bio->devs[1].repl_bio;
+                                if (bio)
+                                        bio->bi_end_io = NULL;
+                                rdev = mirror->replacement;
+                                /* Note: if rdev != NULL, then bio
+                                 * cannot be NULL as r10buf_pool_alloc will
+                                 * have allocated it.
+                                 * So the second test here is pointless.
+                                 * But it keeps semantic-checkers happy, and
+                                 * this comment keeps human reviewers
+                                 * happy.
+                                 */
+                                if (rdev == NULL || bio == NULL ||
+                                    test_bit(Faulty, &rdev->flags))
+                                        break;
+                                bio->bi_next = biolist;
+                                biolist = bio;
+                                bio->bi_private = r10_bio;
+                                bio->bi_end_io = end_sync_write;
+                                bio->bi_rw = WRITE;
+                                bio->bi_sector = to_addr + rdev->data_offset;
+                                bio->bi_bdev = rdev->bdev;
+                                atomic_inc(&r10_bio->remaining);
                                 break;
                         }
                         if (j == conf->copies) {
@@ -2544,8 +2844,16 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr,
                                         for (k = 0; k < conf->copies; k++)
                                                 if (r10_bio->devs[k].devnum == i)
                                                         break;
-                                        if (!rdev_set_badblocks(
-                                                    conf->mirrors[i].rdev,
+                                        if (!test_bit(In_sync,
+                                                      &mirror->rdev->flags)
+                                            && !rdev_set_badblocks(
+                                                    mirror->rdev,
+                                                    r10_bio->devs[k].addr,
+                                                    max_sync, 0))
+                                                any_working = 0;
+                                        if (mirror->replacement &&
+                                            !rdev_set_badblocks(
+                                                    mirror->replacement,
                                                     r10_bio->devs[k].addr,
                                                     max_sync, 0))
                                                 any_working = 0;
@@ -2556,7 +2864,7 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr,
                                                 printk(KERN_INFO "md/raid10:%s: insufficient "
                                                        "working devices for recovery.\n",
                                                        mdname(mddev));
-                                        conf->mirrors[i].recovery_disabled
+                                        mirror->recovery_disabled
                                                 = mddev->recovery_disabled;
                                 }
                                 break;
@@ -2605,6 +2913,9 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr,
                         sector_t first_bad, sector;
                         int bad_sectors;
 
+                        if (r10_bio->devs[i].repl_bio)
+                                r10_bio->devs[i].repl_bio->bi_end_io = NULL;
+
                         bio = r10_bio->devs[i].bio;
                         bio->bi_end_io = NULL;
                         clear_bit(BIO_UPTODATE, &bio->bi_flags);
@@ -2635,6 +2946,27 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr,
                                 conf->mirrors[d].rdev->data_offset;
                         bio->bi_bdev = conf->mirrors[d].rdev->bdev;
                         count++;
+
+                        if (conf->mirrors[d].replacement == NULL ||
+                            test_bit(Faulty,
+                                     &conf->mirrors[d].replacement->flags))
+                                continue;
+
+                        /* Need to set up for writing to the replacement */
+                        bio = r10_bio->devs[i].repl_bio;
+                        clear_bit(BIO_UPTODATE, &bio->bi_flags);
+
+                        sector = r10_bio->devs[i].addr;
+                        atomic_inc(&conf->mirrors[d].rdev->nr_pending);
+                        bio->bi_next = biolist;
+                        biolist = bio;
+                        bio->bi_private = r10_bio;
+                        bio->bi_end_io = end_sync_write;
+                        bio->bi_rw = WRITE;
+                        bio->bi_sector = sector +
+                                conf->mirrors[d].replacement->data_offset;
+                        bio->bi_bdev = conf->mirrors[d].replacement->bdev;
+                        count++;
                 }
 
                 if (count < 2) {
@@ -2643,6 +2975,11 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr,
                                 if (r10_bio->devs[i].bio->bi_end_io)
                                         rdev_dec_pending(conf->mirrors[d].rdev,
                                                          mddev);
+                                if (r10_bio->devs[i].repl_bio &&
+                                    r10_bio->devs[i].repl_bio->bi_end_io)
+                                        rdev_dec_pending(
+                                                conf->mirrors[d].replacement,
+                                                mddev);
                         }
                         put_buf(r10_bio);
                         biolist = NULL;
@@ -2896,6 +3233,16 @@ static int run(struct mddev *mddev)
                         continue;
                 disk = conf->mirrors + disk_idx;
 
+                if (test_bit(Replacement, &rdev->flags)) {
+                        if (disk->replacement)
+                                goto out_free_conf;
+                        disk->replacement = rdev;
+                } else {
+                        if (disk->rdev)
+                                goto out_free_conf;
+                        disk->rdev = rdev;
+                }
+
                 disk->rdev = rdev;
                 disk_stack_limits(mddev->gendisk, rdev->bdev,
                                   rdev->data_offset << 9);
@@ -2923,6 +3270,13 @@ static int run(struct mddev *mddev)
 
                 disk = conf->mirrors + i;
 
+                if (!disk->rdev && disk->replacement) {
+                        /* The replacement is all we have - use it */
+                        disk->rdev = disk->replacement;
+                        disk->replacement = NULL;
+                        clear_bit(Replacement, &disk->rdev->flags);
+                }
+
                 if (!disk->rdev ||
                     !test_bit(In_sync, &disk->rdev->flags)) {
                         disk->head_position = 0;
diff --git a/drivers/md/raid10.h b/drivers/md/raid10.h
index 7facfdf841f4..7c615613c381 100644
--- a/drivers/md/raid10.h
+++ b/drivers/md/raid10.h
@@ -2,7 +2,7 @@
 #define _RAID10_H
 
 struct mirror_info {
-        struct md_rdev  *rdev;
+        struct md_rdev  *rdev, *replacement;
         sector_t        head_position;
         int             recovery_disabled;      /* matches
                                                  * mddev->recovery_disabled
@@ -18,12 +18,13 @@ struct r10conf {
         spinlock_t              device_lock;
 
         /* geometry */
-        int                     near_copies;  /* number of copies laid out raid0 style */
+        int                     near_copies;  /* number of copies laid out
+                                               * raid0 style */
         int                     far_copies;   /* number of copies laid out
                                                * at large strides across drives
                                                */
-        int                     far_offset;   /* far_copies are offset by 1 stripe
-                                               * instead of many
+        int                     far_offset;   /* far_copies are offset by 1
+                                               * stripe instead of many
                                                */
         int                     copies;       /* near_copies * far_copies.
                                                * must be <= raid_disks
@@ -34,10 +35,11 @@ struct r10conf {
                                                * 1 stripe.
                                                */
 
-        sector_t                dev_sectors;  /* temp copy of mddev->dev_sectors */
+        sector_t                dev_sectors;  /* temp copy of
+                                               * mddev->dev_sectors */
 
-        int chunk_shift; /* shift from chunks to sectors */
-        sector_t chunk_mask;
+        int                     chunk_shift; /* shift from chunks to sectors */
+        sector_t                chunk_mask;
 
         struct list_head        retry_list;
         /* queue pending writes and submit them on unplug */
@@ -45,20 +47,22 @@ struct r10conf {
         int                     pending_count;
 
         spinlock_t              resync_lock;
-        int nr_pending;
-        int nr_waiting;
-        int nr_queued;
-        int barrier;
+        int                     nr_pending;
+        int                     nr_waiting;
+        int                     nr_queued;
+        int                     barrier;
         sector_t                next_resync;
         int                     fullsync;  /* set to 1 if a full sync is needed,
                                             * (fresh device added).
                                             * Cleared when a sync completes.
                                             */
-
+        int                     have_replacement; /* There is at least one
+                                                   * replacement device.
+                                                   */
         wait_queue_head_t       wait_barrier;
 
-        mempool_t *r10bio_pool;
-        mempool_t *r10buf_pool;
+        mempool_t               *r10bio_pool;
+        mempool_t               *r10buf_pool;
         struct page             *tmppage;
 
         /* When taking over an array from a different personality, we store
@@ -98,11 +102,18 @@ struct r10bio {
          * When resyncing we also use one for each copy.
          * When reconstructing, we use 2 bios, one for read, one for write.
          * We choose the number when they are allocated.
+         * We sometimes need an extra bio to write to the replacement.
          */
         struct {
-                struct bio              *bio;
-                sector_t addr;
-                int devnum;
+                struct bio      *bio;
+                union {
+                        struct bio      *repl_bio; /* used for resync and
+                                                    * writes */
+                        struct md_rdev  *rdev;     /* used for reads
+                                                    * (read_slot >= 0) */
+                };
+                sector_t        addr;
+                int             devnum;
         } devs[0];
 };
 
@@ -121,17 +132,19 @@ struct r10bio {
 #define BIO_SPECIAL(bio) ((unsigned long)bio <= 2)
 
 /* bits for r10bio.state */
-#define R10BIO_Uptodate 0
-#define R10BIO_IsSync   1
-#define R10BIO_IsRecover 2
-#define R10BIO_Degraded 3
+enum r10bio_state {
+        R10BIO_Uptodate,
+        R10BIO_IsSync,
+        R10BIO_IsRecover,
+        R10BIO_Degraded,
 /* Set ReadError on bios that experience a read error
  * so that raid10d knows what to do with them.
  */
-#define R10BIO_ReadError 4
+        R10BIO_ReadError,
 /* If a write for this request means we can clear some
  * known-bad-block records, we set this flag.
  */
-#define R10BIO_MadeGood 5
-#define R10BIO_WriteError 6
+        R10BIO_MadeGood,
+        R10BIO_WriteError,
+};
 #endif
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 858fdbb7eb07..360f2b98f62b 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -370,12 +370,10 @@ static struct stripe_head *__find_stripe(struct r5conf *conf, sector_t sector,
  * of the two sections, and some non-in_sync devices may
  * be insync in the section most affected by failed devices.
  */
-static int has_failed(struct r5conf *conf)
+static int calc_degraded(struct r5conf *conf)
 {
-        int degraded;
+        int degraded, degraded2;
         int i;
-        if (conf->mddev->reshape_position == MaxSector)
-                return conf->mddev->degraded > conf->max_degraded;
 
         rcu_read_lock();
         degraded = 0;
@@ -399,14 +397,14 @@ static int has_failed(struct r5conf *conf)
                                 degraded++;
         }
         rcu_read_unlock();
-        if (degraded > conf->max_degraded)
-                return 1;
+        if (conf->raid_disks == conf->previous_raid_disks)
+                return degraded;
         rcu_read_lock();
-        degraded = 0;
+        degraded2 = 0;
         for (i = 0; i < conf->raid_disks; i++) {
                 struct md_rdev *rdev = rcu_dereference(conf->disks[i].rdev);
                 if (!rdev || test_bit(Faulty, &rdev->flags))
-                        degraded++;
+                        degraded2++;
                 else if (test_bit(In_sync, &rdev->flags))
                         ;
                 else
@@ -416,9 +414,22 @@ static int has_failed(struct r5conf *conf)
                          * almost certainly hasn't.
                          */
                         if (conf->raid_disks <= conf->previous_raid_disks)
-                                degraded++;
+                                degraded2++;
         }
         rcu_read_unlock();
+        if (degraded2 > degraded)
+                return degraded2;
+        return degraded;
+}
+
+static int has_failed(struct r5conf *conf)
+{
+        int degraded;
+
+        if (conf->mddev->reshape_position == MaxSector)
+                return conf->mddev->degraded > conf->max_degraded;
+
+        degraded = calc_degraded(conf);
         if (degraded > conf->max_degraded)
                 return 1;
         return 0;
@@ -492,8 +503,9 @@ static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
 
         for (i = disks; i--; ) {
                 int rw;
-                struct bio *bi;
-                struct md_rdev *rdev;
+                int replace_only = 0;
+                struct bio *bi, *rbi;
+                struct md_rdev *rdev, *rrdev = NULL;
                 if (test_and_clear_bit(R5_Wantwrite, &sh->dev[i].flags)) {
                         if (test_and_clear_bit(R5_WantFUA, &sh->dev[i].flags))
                                 rw = WRITE_FUA;
@@ -501,27 +513,57 @@ static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
                                 rw = WRITE;
                 } else if (test_and_clear_bit(R5_Wantread, &sh->dev[i].flags))
                         rw = READ;
-                else
+                else if (test_and_clear_bit(R5_WantReplace,
+                                            &sh->dev[i].flags)) {
+                        rw = WRITE;
+                        replace_only = 1;
+                } else
                         continue;
 
                 bi = &sh->dev[i].req;
+                rbi = &sh->dev[i].rreq; /* For writing to replacement */
 
                 bi->bi_rw = rw;
-                if (rw & WRITE)
+                rbi->bi_rw = rw;
+                if (rw & WRITE) {
                         bi->bi_end_io = raid5_end_write_request;
-                else
+                        rbi->bi_end_io = raid5_end_write_request;
+                } else
                         bi->bi_end_io = raid5_end_read_request;
 
                 rcu_read_lock();
+                rrdev = rcu_dereference(conf->disks[i].replacement);
+                smp_mb(); /* Ensure that if rrdev is NULL, rdev won't be */
                 rdev = rcu_dereference(conf->disks[i].rdev);
+                if (!rdev) {
+                        rdev = rrdev;
+                        rrdev = NULL;
+                }
+                if (rw & WRITE) {
+                        if (replace_only)
+                                rdev = NULL;
+                        if (rdev == rrdev)
+                                /* We raced and saw duplicates */
+                                rrdev = NULL;
+                } else {
+                        if (test_bit(R5_ReadRepl, &sh->dev[i].flags) && rrdev)
+                                rdev = rrdev;
+                        rrdev = NULL;
+                }
+
                 if (rdev && test_bit(Faulty, &rdev->flags))
                         rdev = NULL;
                 if (rdev)
                         atomic_inc(&rdev->nr_pending);
+                if (rrdev && test_bit(Faulty, &rrdev->flags))
+                        rrdev = NULL;
+                if (rrdev)
+                        atomic_inc(&rrdev->nr_pending);
                 rcu_read_unlock();
 
                 /* We have already checked bad blocks for reads.  Now
-                 * need to check for writes.
+                 * need to check for writes.  We never accept write errors
+                 * on the replacement, so we don't to check rrdev.
                  */
                 while ((rw & WRITE) && rdev &&
                        test_bit(WriteErrorSeen, &rdev->flags)) {
@@ -551,7 +593,8 @@ static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
                 }
 
                 if (rdev) {
-                        if (s->syncing || s->expanding || s->expanded)
+                        if (s->syncing || s->expanding || s->expanded
+                            || s->replacing)
                                 md_sync_acct(rdev->bdev, STRIPE_SECTORS);
 
                         set_bit(STRIPE_IO_STARTED, &sh->state);
@@ -563,16 +606,38 @@ static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
                         atomic_inc(&sh->count);
                         bi->bi_sector = sh->sector + rdev->data_offset;
                         bi->bi_flags = 1 << BIO_UPTODATE;
-                        bi->bi_vcnt = 1;
-                        bi->bi_max_vecs = 1;
                         bi->bi_idx = 0;
-                        bi->bi_io_vec = &sh->dev[i].vec;
                         bi->bi_io_vec[0].bv_len = STRIPE_SIZE;
                         bi->bi_io_vec[0].bv_offset = 0;
                         bi->bi_size = STRIPE_SIZE;
                         bi->bi_next = NULL;
+                        if (rrdev)
+                                set_bit(R5_DOUBLE_LOCKED, &sh->dev[i].flags);
                         generic_make_request(bi);
-                } else {
+                }
+                if (rrdev) {
+                        if (s->syncing || s->expanding || s->expanded
+                            || s->replacing)
+                                md_sync_acct(rrdev->bdev, STRIPE_SECTORS);
+
+                        set_bit(STRIPE_IO_STARTED, &sh->state);
+
+                        rbi->bi_bdev = rrdev->bdev;
+                        pr_debug("%s: for %llu schedule op %ld on "
+                                 "replacement disc %d\n",
+                                __func__, (unsigned long long)sh->sector,
+                                rbi->bi_rw, i);
+                        atomic_inc(&sh->count);
+                        rbi->bi_sector = sh->sector + rrdev->data_offset;
+                        rbi->bi_flags = 1 << BIO_UPTODATE;
+                        rbi->bi_idx = 0;
+                        rbi->bi_io_vec[0].bv_len = STRIPE_SIZE;
+                        rbi->bi_io_vec[0].bv_offset = 0;
+                        rbi->bi_size = STRIPE_SIZE;
+                        rbi->bi_next = NULL;
+                        generic_make_request(rbi);
+                }
+                if (!rdev && !rrdev) {
                         if (rw & WRITE)
                                 set_bit(STRIPE_DEGRADED, &sh->state);
                         pr_debug("skip op %ld on disc %d for sector %llu\n",
@@ -1583,7 +1648,7 @@ static void raid5_end_read_request(struct bio * bi, int error)
         int disks = sh->disks, i;
         int uptodate = test_bit(BIO_UPTODATE, &bi->bi_flags);
         char b[BDEVNAME_SIZE];
-        struct md_rdev *rdev;
+        struct md_rdev *rdev = NULL;
 
 
         for (i=0 ; i<disks; i++)
@@ -1597,11 +1662,23 @@ static void raid5_end_read_request(struct bio * bi, int error)
                 BUG();
                 return;
         }
+        if (test_bit(R5_ReadRepl, &sh->dev[i].flags))
+                /* If replacement finished while this request was outstanding,
+                 * 'replacement' might be NULL already.
+                 * In that case it moved down to 'rdev'.
+                 * rdev is not removed until all requests are finished.
+                 */
+                rdev = conf->disks[i].replacement;
+        if (!rdev)
+                rdev = conf->disks[i].rdev;
 
         if (uptodate) {
                 set_bit(R5_UPTODATE, &sh->dev[i].flags);
                 if (test_bit(R5_ReadError, &sh->dev[i].flags)) {
-                        rdev = conf->disks[i].rdev;
+                        /* Note that this cannot happen on a
+                         * replacement device.  We just fail those on
+                         * any error
+                         */
                         printk_ratelimited(
                                 KERN_INFO
                                 "md/raid:%s: read error corrected"
@@ -1614,16 +1691,24 @@ static void raid5_end_read_request(struct bio * bi, int error)
                         clear_bit(R5_ReadError, &sh->dev[i].flags);
                         clear_bit(R5_ReWrite, &sh->dev[i].flags);
                 }
-                if (atomic_read(&conf->disks[i].rdev->read_errors))
-                        atomic_set(&conf->disks[i].rdev->read_errors, 0);
+                if (atomic_read(&rdev->read_errors))
+                        atomic_set(&rdev->read_errors, 0);
         } else {
-                const char *bdn = bdevname(conf->disks[i].rdev->bdev, b);
+                const char *bdn = bdevname(rdev->bdev, b);
                 int retry = 0;
-                rdev = conf->disks[i].rdev;
 
                 clear_bit(R5_UPTODATE, &sh->dev[i].flags);
                 atomic_inc(&rdev->read_errors);
-                if (conf->mddev->degraded >= conf->max_degraded)
+                if (test_bit(R5_ReadRepl, &sh->dev[i].flags))
+                        printk_ratelimited(
+                                KERN_WARNING
+                                "md/raid:%s: read error on replacement device "
+                                "(sector %llu on %s).\n",
+                                mdname(conf->mddev),
+                                (unsigned long long)(sh->sector
+                                                     + rdev->data_offset),
+                                bdn);
+                else if (conf->mddev->degraded >= conf->max_degraded)
                         printk_ratelimited(
                                 KERN_WARNING
                                 "md/raid:%s: read error not correctable "
@@ -1657,7 +1742,7 @@ static void raid5_end_read_request(struct bio * bi, int error)
                         md_error(conf->mddev, rdev);
                 }
         }
-        rdev_dec_pending(conf->disks[i].rdev, conf->mddev);
+        rdev_dec_pending(rdev, conf->mddev);
         clear_bit(R5_LOCKED, &sh->dev[i].flags);
         set_bit(STRIPE_HANDLE, &sh->state);
         release_stripe(sh);
@@ -1668,14 +1753,30 @@ static void raid5_end_write_request(struct bio *bi, int error)
         struct stripe_head *sh = bi->bi_private;
         struct r5conf *conf = sh->raid_conf;
         int disks = sh->disks, i;
+        struct md_rdev *uninitialized_var(rdev);
         int uptodate = test_bit(BIO_UPTODATE, &bi->bi_flags);
         sector_t first_bad;
         int bad_sectors;
+        int replacement = 0;
 
-        for (i=0 ; i<disks; i++)
-                if (bi == &sh->dev[i].req)
+        for (i = 0 ; i < disks; i++) {
+                if (bi == &sh->dev[i].req) {
+                        rdev = conf->disks[i].rdev;
                         break;
-
+                }
+                if (bi == &sh->dev[i].rreq) {
+                        rdev = conf->disks[i].replacement;
+                        if (rdev)
+                                replacement = 1;
+                        else
+                                /* rdev was removed and 'replacement'
+                                 * replaced it.  rdev is not removed
+                                 * until all requests are finished.
+                                 */
+                                rdev = conf->disks[i].rdev;
+                        break;
+                }
+        }
         pr_debug("end_write_request %llu/%d, count %d, uptodate: %d.\n",
                 (unsigned long long)sh->sector, i, atomic_read(&sh->count),
                 uptodate);
@@ -1684,21 +1785,33 @@ static void raid5_end_write_request(struct bio *bi, int error)
                 return;
         }
 
-        if (!uptodate) {
-                set_bit(WriteErrorSeen, &conf->disks[i].rdev->flags);
-                set_bit(R5_WriteError, &sh->dev[i].flags);
-        } else if (is_badblock(conf->disks[i].rdev, sh->sector, STRIPE_SECTORS,
-                               &first_bad, &bad_sectors))
-                set_bit(R5_MadeGood, &sh->dev[i].flags);
+        if (replacement) {
+                if (!uptodate)
+                        md_error(conf->mddev, rdev);
+                else if (is_badblock(rdev, sh->sector,
+                                     STRIPE_SECTORS,
+                                     &first_bad, &bad_sectors))
+                        set_bit(R5_MadeGoodRepl, &sh->dev[i].flags);
+        } else {
+                if (!uptodate) {
+                        set_bit(WriteErrorSeen, &rdev->flags);
+                        set_bit(R5_WriteError, &sh->dev[i].flags);
+                        if (!test_and_set_bit(WantReplacement, &rdev->flags))
+                                set_bit(MD_RECOVERY_NEEDED,
+                                        &rdev->mddev->recovery);
+                } else if (is_badblock(rdev, sh->sector,
+                                       STRIPE_SECTORS,
+                                       &first_bad, &bad_sectors))
+                        set_bit(R5_MadeGood, &sh->dev[i].flags);
+        }
+        rdev_dec_pending(rdev, conf->mddev);
 
-        rdev_dec_pending(conf->disks[i].rdev, conf->mddev);
-        
-        clear_bit(R5_LOCKED, &sh->dev[i].flags);
+        if (!test_and_clear_bit(R5_DOUBLE_LOCKED, &sh->dev[i].flags))
+                clear_bit(R5_LOCKED, &sh->dev[i].flags);
         set_bit(STRIPE_HANDLE, &sh->state);
         release_stripe(sh);
 }
 
-
 static sector_t compute_blocknr(struct stripe_head *sh, int i, int previous);
         
 static void raid5_build_block(struct stripe_head *sh, int i, int previous)
@@ -1709,12 +1822,15 @@ static void raid5_build_block(struct stripe_head *sh, int i, int previous)
         dev->req.bi_io_vec = &dev->vec;
         dev->req.bi_vcnt++;
         dev->req.bi_max_vecs++;
+        dev->req.bi_private = sh;
         dev->vec.bv_page = dev->page;
-        dev->vec.bv_len = STRIPE_SIZE;
-        dev->vec.bv_offset = 0;
 
-        dev->req.bi_sector = sh->sector;
-        dev->req.bi_private = sh;
+        bio_init(&dev->rreq);
+        dev->rreq.bi_io_vec = &dev->rvec;
+        dev->rreq.bi_vcnt++;
+        dev->rreq.bi_max_vecs++;
+        dev->rreq.bi_private = sh;
+        dev->rvec.bv_page = dev->page;
 
         dev->flags = 0;
         dev->sector = compute_blocknr(sh, i, previous);
@@ -1724,18 +1840,15 @@ static void error(struct mddev *mddev, struct md_rdev *rdev)
 {
         char b[BDEVNAME_SIZE];
         struct r5conf *conf = mddev->private;
+        unsigned long flags;
         pr_debug("raid456: error called\n");
 
-        if (test_and_clear_bit(In_sync, &rdev->flags)) {
-                unsigned long flags;
-                spin_lock_irqsave(&conf->device_lock, flags);
-                mddev->degraded++;
-                spin_unlock_irqrestore(&conf->device_lock, flags);
-                /*
-                 * if recovery was running, make sure it aborts.
-                 */
-                set_bit(MD_RECOVERY_INTR, &mddev->recovery);
-        }
+        spin_lock_irqsave(&conf->device_lock, flags);
+        clear_bit(In_sync, &rdev->flags);
+        mddev->degraded = calc_degraded(conf);
+        spin_unlock_irqrestore(&conf->device_lock, flags);
+        set_bit(MD_RECOVERY_INTR, &mddev->recovery);
+
         set_bit(Blocked, &rdev->flags);
         set_bit(Faulty, &rdev->flags);
         set_bit(MD_CHANGE_DEVS, &mddev->flags);
@@ -2362,8 +2475,9 @@ handle_failed_sync(struct r5conf *conf, struct stripe_head *sh,
         md_done_sync(conf->mddev, STRIPE_SECTORS, 0);
         clear_bit(STRIPE_SYNCING, &sh->state);
         s->syncing = 0;
+        s->replacing = 0;
         /* There is nothing more to do for sync/check/repair.
-         * For recover we need to record a bad block on all
+         * For recover/replace we need to record a bad block on all
          * non-sync devices, or abort the recovery
          */
         if (!test_bit(MD_RECOVERY_RECOVER, &conf->mddev->recovery))
@@ -2373,12 +2487,18 @@ handle_failed_sync(struct r5conf *conf, struct stripe_head *sh,
          */
         for (i = 0; i < conf->raid_disks; i++) {
                 struct md_rdev *rdev = conf->disks[i].rdev;
-                if (!rdev
-                    || test_bit(Faulty, &rdev->flags)
-                    || test_bit(In_sync, &rdev->flags))
-                        continue;
-                if (!rdev_set_badblocks(rdev, sh->sector,
-                                        STRIPE_SECTORS, 0))
+                if (rdev
+                    && !test_bit(Faulty, &rdev->flags)
+                    && !test_bit(In_sync, &rdev->flags)
+                    && !rdev_set_badblocks(rdev, sh->sector,
+                                           STRIPE_SECTORS, 0))
+                        abort = 1;
+                rdev = conf->disks[i].replacement;
+                if (rdev
+                    && !test_bit(Faulty, &rdev->flags)
+                    && !test_bit(In_sync, &rdev->flags)
+                    && !rdev_set_badblocks(rdev, sh->sector,
+                                           STRIPE_SECTORS, 0))
                         abort = 1;
         }
         if (abort) {
@@ -2387,6 +2507,22 @@ handle_failed_sync(struct r5conf *conf, struct stripe_head *sh,
         }
 }
 
+static int want_replace(struct stripe_head *sh, int disk_idx)
+{
+        struct md_rdev *rdev;
+        int rv = 0;
+        /* Doing recovery so rcu locking not required */
+        rdev = sh->raid_conf->disks[disk_idx].replacement;
+        if (rdev
+            && !test_bit(Faulty, &rdev->flags)
+            && !test_bit(In_sync, &rdev->flags)
+            && (rdev->recovery_offset <= sh->sector
+                || rdev->mddev->recovery_cp <= sh->sector))
+                rv = 1;
+
+        return rv;
+}
+
 /* fetch_block - checks the given member device to see if its data needs
  * to be read or computed to satisfy a request.
  *
@@ -2406,6 +2542,7 @@ static int fetch_block(struct stripe_head *sh, struct stripe_head_state *s,
             (dev->toread ||
              (dev->towrite && !test_bit(R5_OVERWRITE, &dev->flags)) ||
              s->syncing || s->expanding ||
+             (s->replacing && want_replace(sh, disk_idx)) ||
              (s->failed >= 1 && fdev[0]->toread) ||
              (s->failed >= 2 && fdev[1]->toread) ||
              (sh->raid_conf->level <= 5 && s->failed && fdev[0]->towrite &&
@@ -2959,22 +3096,18 @@ static void handle_stripe_expansion(struct r5conf *conf, struct stripe_head *sh)
         }
 }
 
-
 /*
  * handle_stripe - do things to a stripe.
  *
- * We lock the stripe and then examine the state of various bits
- * to see what needs to be done.
+ * We lock the stripe by setting STRIPE_ACTIVE and then examine the
+ * state of various bits to see what needs to be done.
  * Possible results:
- *    return some read request which now have data
- *    return some write requests which are safely on disc
+ *    return some read requests which now have data
+ *    return some write requests which are safely on storage
  *    schedule a read on some buffers
  *    schedule a write of some buffers
  *    return confirmation of parity correctness
  *
- * buffers are taken off read_list or write_list, and bh_cache buffers
- * get BH_Lock set before the stripe lock is released.
- *
  */
 
 static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
@@ -2983,10 +3116,10 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
         int disks = sh->disks;
         struct r5dev *dev;
         int i;
+        int do_recovery = 0;
 
         memset(s, 0, sizeof(*s));
 
-        s->syncing = test_bit(STRIPE_SYNCING, &sh->state);
         s->expanding = test_bit(STRIPE_EXPAND_SOURCE, &sh->state);
         s->expanded = test_bit(STRIPE_EXPAND_READY, &sh->state);
         s->failed_num[0] = -1;
@@ -3004,7 +3137,8 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
                 dev = &sh->dev[i];
 
                 pr_debug("check %d: state 0x%lx read %p write %p written %p\n",
-                        i, dev->flags, dev->toread, dev->towrite, dev->written);
+                         i, dev->flags,
+                         dev->toread, dev->towrite, dev->written);
                 /* maybe we can reply to a read
                  *
                  * new wantfill requests are only permitted while
@@ -3035,7 +3169,21 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
                 }
                 if (dev->written)
                         s->written++;
-                rdev = rcu_dereference(conf->disks[i].rdev);
+                /* Prefer to use the replacement for reads, but only
+                 * if it is recovered enough and has no bad blocks.
+                 */
+                rdev = rcu_dereference(conf->disks[i].replacement);
+                if (rdev && !test_bit(Faulty, &rdev->flags) &&
+                    rdev->recovery_offset >= sh->sector + STRIPE_SECTORS &&
+                    !is_badblock(rdev, sh->sector, STRIPE_SECTORS,
+                                 &first_bad, &bad_sectors))
+                        set_bit(R5_ReadRepl, &dev->flags);
+                else {
+                        if (rdev)
+                                set_bit(R5_NeedReplace, &dev->flags);
+                        rdev = rcu_dereference(conf->disks[i].rdev);
+                        clear_bit(R5_ReadRepl, &dev->flags);
+                }
                 if (rdev && test_bit(Faulty, &rdev->flags))
                         rdev = NULL;
                 if (rdev) {
@@ -3077,20 +3225,38 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
                         set_bit(R5_Insync, &dev->flags);
 
                 if (rdev && test_bit(R5_WriteError, &dev->flags)) {
-                        clear_bit(R5_Insync, &dev->flags);
-                        if (!test_bit(Faulty, &rdev->flags)) {
+                        /* This flag does not apply to '.replacement'
+                         * only to .rdev, so make sure to check that*/
+                        struct md_rdev *rdev2 = rcu_dereference(
+                                conf->disks[i].rdev);
+                        if (rdev2 == rdev)
+                                clear_bit(R5_Insync, &dev->flags);
+                        if (rdev2 && !test_bit(Faulty, &rdev2->flags)) {
                                 s->handle_bad_blocks = 1;
-                                atomic_inc(&rdev->nr_pending);
+                                atomic_inc(&rdev2->nr_pending);
                         } else
                                 clear_bit(R5_WriteError, &dev->flags);
                 }
                 if (rdev && test_bit(R5_MadeGood, &dev->flags)) {
-                        if (!test_bit(Faulty, &rdev->flags)) {
+                        /* This flag does not apply to '.replacement'
+                         * only to .rdev, so make sure to check that*/
+                        struct md_rdev *rdev2 = rcu_dereference(
+                                conf->disks[i].rdev);
+                        if (rdev2 && !test_bit(Faulty, &rdev2->flags)) {
                                 s->handle_bad_blocks = 1;
-                                atomic_inc(&rdev->nr_pending);
+                                atomic_inc(&rdev2->nr_pending);
                         } else
                                 clear_bit(R5_MadeGood, &dev->flags);
                 }
+                if (test_bit(R5_MadeGoodRepl, &dev->flags)) {
+                        struct md_rdev *rdev2 = rcu_dereference(
+                                conf->disks[i].replacement);
+                        if (rdev2 && !test_bit(Faulty, &rdev2->flags)) {
+                                s->handle_bad_blocks = 1;
+                                atomic_inc(&rdev2->nr_pending);
+                        } else
+                                clear_bit(R5_MadeGoodRepl, &dev->flags);
+                }
                 if (!test_bit(R5_Insync, &dev->flags)) {
                         /* The ReadError flag will just be confusing now */
                         clear_bit(R5_ReadError, &dev->flags);
@@ -3102,9 +3268,25 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
                         if (s->failed < 2)
                                 s->failed_num[s->failed] = i;
                         s->failed++;
+                        if (rdev && !test_bit(Faulty, &rdev->flags))
+                                do_recovery = 1;
                 }
         }
         spin_unlock_irq(&conf->device_lock);
+        if (test_bit(STRIPE_SYNCING, &sh->state)) {
+                /* If there is a failed device being replaced,
+                 *     we must be recovering.
+                 * else if we are after recovery_cp, we must be syncing
+                 * else we can only be replacing
+                 * sync and recovery both need to read all devices, and so
+                 * use the same flag.
+                 */
+                if (do_recovery ||
+                    sh->sector >= conf->mddev->recovery_cp)
+                        s->syncing = 1;
+                else
+                        s->replacing = 1;
+        }
         rcu_read_unlock();
 }
 
@@ -3146,7 +3328,7 @@ static void handle_stripe(struct stripe_head *sh)
 
         if (unlikely(s.blocked_rdev)) {
                 if (s.syncing || s.expanding || s.expanded ||
-                    s.to_write || s.written) {
+                    s.replacing || s.to_write || s.written) {
                         set_bit(STRIPE_HANDLE, &sh->state);
                         goto finish;
                 }
@@ -3172,7 +3354,7 @@ static void handle_stripe(struct stripe_head *sh)
                 sh->reconstruct_state = 0;
                 if (s.to_read+s.to_write+s.written)
                         handle_failed_stripe(conf, sh, &s, disks, &s.return_bi);
-                if (s.syncing)
+                if (s.syncing + s.replacing)
                         handle_failed_sync(conf, sh, &s);
         }
 
@@ -3203,7 +3385,9 @@ static void handle_stripe(struct stripe_head *sh)
          */
         if (s.to_read || s.non_overwrite
             || (conf->level == 6 && s.to_write && s.failed)
-            || (s.syncing && (s.uptodate + s.compute < disks)) || s.expanding)
+            || (s.syncing && (s.uptodate + s.compute < disks))
+            || s.replacing
+            || s.expanding)
                 handle_stripe_fill(sh, &s, disks);
 
         /* Now we check to see if any write operations have recently
@@ -3265,7 +3449,20 @@ static void handle_stripe(struct stripe_head *sh)
                         handle_parity_checks5(conf, sh, &s, disks);
         }
 
-        if (s.syncing && s.locked == 0 && test_bit(STRIPE_INSYNC, &sh->state)) {
+        if (s.replacing && s.locked == 0
+            && !test_bit(STRIPE_INSYNC, &sh->state)) {
+                /* Write out to replacement devices where possible */
+                for (i = 0; i < conf->raid_disks; i++)
+                        if (test_bit(R5_UPTODATE, &sh->dev[i].flags) &&
+                            test_bit(R5_NeedReplace, &sh->dev[i].flags)) {
+                                set_bit(R5_WantReplace, &sh->dev[i].flags);
+                                set_bit(R5_LOCKED, &sh->dev[i].flags);
+                                s.locked++;
+                        }
+                set_bit(STRIPE_INSYNC, &sh->state);
+        }
+        if ((s.syncing || s.replacing) && s.locked == 0 &&
+            test_bit(STRIPE_INSYNC, &sh->state)) {
                 md_done_sync(conf->mddev, STRIPE_SECTORS, 1);
                 clear_bit(STRIPE_SYNCING, &sh->state);
         }
@@ -3363,6 +3560,15 @@ finish:
                                                      STRIPE_SECTORS);
                                 rdev_dec_pending(rdev, conf->mddev);
                         }
+                        if (test_and_clear_bit(R5_MadeGoodRepl, &dev->flags)) {
+                                rdev = conf->disks[i].replacement;
+                                if (!rdev)
+                                        /* rdev have been moved down */
+                                        rdev = conf->disks[i].rdev;
+                                rdev_clear_badblocks(rdev, sh->sector,
+                                                     STRIPE_SECTORS);
+                                rdev_dec_pending(rdev, conf->mddev);
+                        }
                 }
 
         if (s.ops_request)
@@ -3586,6 +3792,7 @@ static int chunk_aligned_read(struct mddev *mddev, struct bio * raid_bio)
         int dd_idx;
         struct bio* align_bi;
         struct md_rdev *rdev;
+        sector_t end_sector;
 
         if (!in_chunk_boundary(mddev, raid_bio)) {
                 pr_debug("chunk_aligned_read : non aligned\n");
@@ -3610,9 +3817,19 @@ static int chunk_aligned_read(struct mddev *mddev, struct bio * raid_bio)
                                                     0,
                                                     &dd_idx, NULL);
 
+        end_sector = align_bi->bi_sector + (align_bi->bi_size >> 9);
         rcu_read_lock();
-        rdev = rcu_dereference(conf->disks[dd_idx].rdev);
-        if (rdev && test_bit(In_sync, &rdev->flags)) {
+        rdev = rcu_dereference(conf->disks[dd_idx].replacement);
+        if (!rdev || test_bit(Faulty, &rdev->flags) ||
+            rdev->recovery_offset < end_sector) {
+                rdev = rcu_dereference(conf->disks[dd_idx].rdev);
+                if (rdev &&
+                    (test_bit(Faulty, &rdev->flags) ||
+                    !(test_bit(In_sync, &rdev->flags) ||
+                      rdev->recovery_offset >= end_sector)))
+                        rdev = NULL;
+        }
+        if (rdev) {
                 sector_t first_bad;
                 int bad_sectors;
 
@@ -4137,7 +4354,6 @@ static inline sector_t sync_request(struct mddev *mddev, sector_t sector_nr, int
                 return sync_blocks * STRIPE_SECTORS; /* keep things rounded to whole stripes */
         }
 
-
         bitmap_cond_end_sync(mddev->bitmap, sector_nr);
 
         sh = get_active_stripe(conf, sector_nr, 0, 1, 0);
@@ -4208,7 +4424,6 @@ static int  retry_aligned_read(struct r5conf *conf, struct bio *raid_bio)
                         return handled;
                 }
 
-                set_bit(R5_ReadError, &sh->dev[dd_idx].flags);
                 if (!add_stripe_bio(sh, raid_bio, dd_idx, 0)) {
                         release_stripe(sh);
                         raid5_set_bi_hw_segments(raid_bio, scnt);
@@ -4635,7 +4850,15 @@ static struct r5conf *setup_conf(struct mddev *mddev)
                         continue;
                 disk = conf->disks + raid_disk;
 
-                disk->rdev = rdev;
+                if (test_bit(Replacement, &rdev->flags)) {
+                        if (disk->replacement)
+                                goto abort;
+                        disk->replacement = rdev;
+                } else {
+                        if (disk->rdev)
+                                goto abort;
+                        disk->rdev = rdev;
+                }
 
                 if (test_bit(In_sync, &rdev->flags)) {
                         char b[BDEVNAME_SIZE];
@@ -4724,6 +4947,7 @@ static int run(struct mddev *mddev)
         int dirty_parity_disks = 0;
         struct md_rdev *rdev;
         sector_t reshape_offset = 0;
+        int i;
 
         if (mddev->recovery_cp != MaxSector)
                 printk(KERN_NOTICE "md/raid:%s: not clean"
@@ -4813,12 +5037,25 @@ static int run(struct mddev *mddev)
         conf->thread = NULL;
         mddev->private = conf;
 
-        /*
-         * 0 for a fully functional array, 1 or 2 for a degraded array.
-         */
-        list_for_each_entry(rdev, &mddev->disks, same_set) {
-                if (rdev->raid_disk < 0)
+        for (i = 0; i < conf->raid_disks && conf->previous_raid_disks;
+             i++) {
+                rdev = conf->disks[i].rdev;
+                if (!rdev && conf->disks[i].replacement) {
+                        /* The replacement is all we have yet */
+                        rdev = conf->disks[i].replacement;
+                        conf->disks[i].replacement = NULL;
+                        clear_bit(Replacement, &rdev->flags);
+                        conf->disks[i].rdev = rdev;
+                }
+                if (!rdev)
                         continue;
+                if (conf->disks[i].replacement &&
+                    conf->reshape_progress != MaxSector) {
+                        /* replacements and reshape simply do not mix. */
+                        printk(KERN_ERR "md: cannot handle concurrent "
+                               "replacement and reshape.\n");
+                        goto abort;
+                }
                 if (test_bit(In_sync, &rdev->flags)) {
                         working_disks++;
                         continue;
@@ -4852,8 +5089,10 @@ static int run(struct mddev *mddev)
                 dirty_parity_disks++;
         }
 
-        mddev->degraded = (max(conf->raid_disks, conf->previous_raid_disks)
-                           - working_disks);
+        /*
+         * 0 for a fully functional array, 1 or 2 for a degraded array.
+         */
+        mddev->degraded = calc_degraded(conf);
 
         if (has_failed(conf)) {
                 printk(KERN_ERR "md/raid:%s: not enough operational devices"
@@ -5016,7 +5255,25 @@ static int raid5_spare_active(struct mddev *mddev)
 
         for (i = 0; i < conf->raid_disks; i++) {
                 tmp = conf->disks + i;
-                if (tmp->rdev
+                if (tmp->replacement
+                    && tmp->replacement->recovery_offset == MaxSector
+                    && !test_bit(Faulty, &tmp->replacement->flags)
+                    && !test_and_set_bit(In_sync, &tmp->replacement->flags)) {
+                        /* Replacement has just become active. */
+                        if (!tmp->rdev
+                            || !test_and_clear_bit(In_sync, &tmp->rdev->flags))
+                                count++;
+                        if (tmp->rdev) {
+                                /* Replaced device not technically faulty,
+                                 * but we need to be sure it gets removed
+                                 * and never re-added.
+                                 */
+                                set_bit(Faulty, &tmp->rdev->flags);
+                                sysfs_notify_dirent_safe(
+                                        tmp->rdev->sysfs_state);
+                        }
+                        sysfs_notify_dirent_safe(tmp->replacement->sysfs_state);
+                } else if (tmp->rdev
                     && tmp->rdev->recovery_offset == MaxSector
                     && !test_bit(Faulty, &tmp->rdev->flags)
                     && !test_and_set_bit(In_sync, &tmp->rdev->flags)) {
@@ -5025,49 +5282,68 @@ static int raid5_spare_active(struct mddev *mddev)
                 }
         }
         spin_lock_irqsave(&conf->device_lock, flags);
-        mddev->degraded -= count;
+        mddev->degraded = calc_degraded(conf);
         spin_unlock_irqrestore(&conf->device_lock, flags);
         print_raid5_conf(conf);
         return count;
 }
 
-static int raid5_remove_disk(struct mddev *mddev, int number)
+static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
 {
         struct r5conf *conf = mddev->private;
         int err = 0;
-        struct md_rdev *rdev;
+        int number = rdev->raid_disk;
+        struct md_rdev **rdevp;
         struct disk_info *p = conf->disks + number;
 
         print_raid5_conf(conf);
-        rdev = p->rdev;
-        if (rdev) {
-                if (number >= conf->raid_disks &&
-                    conf->reshape_progress == MaxSector)
-                        clear_bit(In_sync, &rdev->flags);
+        if (rdev == p->rdev)
+                rdevp = &p->rdev;
+        else if (rdev == p->replacement)
+                rdevp = &p->replacement;
+        else
+                return 0;
 
-                if (test_bit(In_sync, &rdev->flags) ||
-                    atomic_read(&rdev->nr_pending)) {
-                        err = -EBUSY;
-                        goto abort;
-                }
-                /* Only remove non-faulty devices if recovery
-                 * isn't possible.
-                 */
-                if (!test_bit(Faulty, &rdev->flags) &&
-                    mddev->recovery_disabled != conf->recovery_disabled &&
-                    !has_failed(conf) &&
-                    number < conf->raid_disks) {
-                        err = -EBUSY;
-                        goto abort;
-                }
-                p->rdev = NULL;
-                synchronize_rcu();
-                if (atomic_read(&rdev->nr_pending)) {
-                        /* lost the race, try later */
-                        err = -EBUSY;
-                        p->rdev = rdev;
-                }
+        if (number >= conf->raid_disks &&
+            conf->reshape_progress == MaxSector)
+                clear_bit(In_sync, &rdev->flags);
+
+        if (test_bit(In_sync, &rdev->flags) ||
+            atomic_read(&rdev->nr_pending)) {
+                err = -EBUSY;
+                goto abort;
         }
+        /* Only remove non-faulty devices if recovery
+         * isn't possible.
+         */
+        if (!test_bit(Faulty, &rdev->flags) &&
+            mddev->recovery_disabled != conf->recovery_disabled &&
+            !has_failed(conf) &&
+            (!p->replacement || p->replacement == rdev) &&
+            number < conf->raid_disks) {
+                err = -EBUSY;
+                goto abort;
+        }
+        *rdevp = NULL;
+        synchronize_rcu();
+        if (atomic_read(&rdev->nr_pending)) {
+                /* lost the race, try later */
+                err = -EBUSY;
+                *rdevp = rdev;
+        } else if (p->replacement) {
+                /* We must have just cleared 'rdev' */
+                p->rdev = p->replacement;
+                clear_bit(Replacement, &p->replacement->flags);
+                smp_mb(); /* Make sure other CPUs may see both as identical
+                           * but will never see neither - if they are careful
+                           */
+                p->replacement = NULL;
+                clear_bit(WantReplacement, &rdev->flags);
+        } else
+                /* We might have just removed the Replacement as faulty-
+                 * clear the bit just in case
+                 */
+                clear_bit(WantReplacement, &rdev->flags);
 abort:
 
         print_raid5_conf(conf);
@@ -5103,8 +5379,9 @@ static int raid5_add_disk(struct mddev *mddev, struct md_rdev *rdev)
                 disk = rdev->saved_raid_disk;
         else
                 disk = first;
-        for ( ; disk <= last ; disk++)
-                if ((p=conf->disks + disk)->rdev == NULL) {
+        for ( ; disk <= last ; disk++) {
+                p = conf->disks + disk;
+                if (p->rdev == NULL) {
                         clear_bit(In_sync, &rdev->flags);
                         rdev->raid_disk = disk;
                         err = 0;
@@ -5113,6 +5390,17 @@ static int raid5_add_disk(struct mddev *mddev, struct md_rdev *rdev)
                         rcu_assign_pointer(p->rdev, rdev);
                         break;
                 }
+                if (test_bit(WantReplacement, &p->rdev->flags) &&
+                    p->replacement == NULL) {
+                        clear_bit(In_sync, &rdev->flags);
+                        set_bit(Replacement, &rdev->flags);
+                        rdev->raid_disk = disk;
+                        err = 0;
+                        conf->fullsync = 1;
+                        rcu_assign_pointer(p->replacement, rdev);
+                        break;
+                }
+        }
         print_raid5_conf(conf);
         return err;
 }
@@ -5286,8 +5574,7 @@ static int raid5_start_reshape(struct mddev *mddev)
                  * pre and post number of devices.
                  */
                 spin_lock_irqsave(&conf->device_lock, flags);
-                mddev->degraded += (conf->raid_disks - conf->previous_raid_disks)
-                        - added_devices;
+                mddev->degraded = calc_degraded(conf);
                 spin_unlock_irqrestore(&conf->device_lock, flags);
         }
         mddev->raid_disks = conf->raid_disks;
@@ -5356,17 +5643,15 @@ static void raid5_finish_reshape(struct mddev *mddev)
                         revalidate_disk(mddev->gendisk);
                 } else {
                         int d;
-                        mddev->degraded = conf->raid_disks;
-                        for (d = 0; d < conf->raid_disks ; d++)
-                                if (conf->disks[d].rdev &&
-                                    test_bit(In_sync,
-                                             &conf->disks[d].rdev->flags))
-                                        mddev->degraded--;
+                        spin_lock_irq(&conf->device_lock);
+                        mddev->degraded = calc_degraded(conf);
+                        spin_unlock_irq(&conf->device_lock);
                         for (d = conf->raid_disks ;
                              d < conf->raid_disks - mddev->delta_disks;
                              d++) {
                                 struct md_rdev *rdev = conf->disks[d].rdev;
-                                if (rdev && raid5_remove_disk(mddev, d) == 0) {
+                                if (rdev &&
+                                    raid5_remove_disk(mddev, rdev) == 0) {
                                         sysfs_unlink_rdev(mddev, rdev);
                                         rdev->raid_disk = -1;
                                 }
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index e10c5531f9c5..8d8e13934a48 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -27,7 +27,7 @@
  * The possible state transitions are:
  *
  *  Empty -> Want   - on read or write to get old data for  parity calc
- *  Empty -> Dirty  - on compute_parity to satisfy write/sync request.(RECONSTRUCT_WRITE)
+ *  Empty -> Dirty  - on compute_parity to satisfy write/sync request.
  *  Empty -> Clean  - on compute_block when computing a block for failed drive
  *  Want  -> Empty  - on failed read
  *  Want  -> Clean  - on successful completion of read request
@@ -226,8 +226,11 @@ struct stripe_head {
                 #endif
         } ops;
         struct r5dev {
-                struct bio      req;
-                struct bio_vec  vec;
+                /* rreq and rvec are used for the replacement device when
+                 * writing data to both devices.
+                 */
+                struct bio      req, rreq;
+                struct bio_vec  vec, rvec;
                 struct page     *page;
                 struct bio      *toread, *read, *towrite, *written;
                 sector_t        sector;                 /* sector of this page */
@@ -239,7 +242,13 @@ struct stripe_head {
  *     for handle_stripe.
  */
 struct stripe_head_state {
-        int syncing, expanding, expanded;
+        /* 'syncing' means that we need to read all devices, either
+         * to check/correct parity, or to reconstruct a missing device.
+         * 'replacing' means we are replacing one or more drives and
+         * the source is valid at this point so we don't need to
+         * read all devices, just the replacement targets.
+         */
+        int syncing, expanding, expanded, replacing;
         int locked, uptodate, to_read, to_write, failed, written;
         int to_fill, compute, req_compute, non_overwrite;
         int failed_num[2];
@@ -252,38 +261,41 @@ struct stripe_head_state {
         int handle_bad_blocks;
 };
 
-/* Flags */
-#define R5_UPTODATE     0       /* page contains current data */
-#define R5_LOCKED       1       /* IO has been submitted on "req" */
-#define R5_OVERWRITE    2       /* towrite covers whole page */
+/* Flags for struct r5dev.flags */
+enum r5dev_flags {
+        R5_UPTODATE,    /* page contains current data */
+        R5_LOCKED,      /* IO has been submitted on "req" */
+        R5_DOUBLE_LOCKED,/* Cannot clear R5_LOCKED until 2 writes complete */
+        R5_OVERWRITE,   /* towrite covers whole page */
 /* and some that are internal to handle_stripe */
-#define R5_Insync       3       /* rdev && rdev->in_sync at start */
-#define R5_Wantread     4       /* want to schedule a read */
-#define R5_Wantwrite    5
-#define R5_Overlap      7       /* There is a pending overlapping request on this block */
-#define R5_ReadError    8       /* seen a read error here recently */
-#define R5_ReWrite      9       /* have tried to over-write the readerror */
+        R5_Insync,      /* rdev && rdev->in_sync at start */
+        R5_Wantread,    /* want to schedule a read */
+        R5_Wantwrite,
+        R5_Overlap,     /* There is a pending overlapping request
+                         * on this block */
+        R5_ReadError,   /* seen a read error here recently */
+        R5_ReWrite,     /* have tried to over-write the readerror */
 
-#define R5_Expanded     10      /* This block now has post-expand data */
-#define R5_Wantcompute  11      /* compute_block in progress treat as
-                                 * uptodate
-                                 */
-#define R5_Wantfill     12      /* dev->toread contains a bio that needs
-                                 * filling
-                                 */
-#define R5_Wantdrain    13      /* dev->towrite needs to be drained */
-#define R5_WantFUA      14      /* Write should be FUA */
-#define R5_WriteError   15      /* got a write error - need to record it */
-#define R5_MadeGood     16      /* A bad block has been fixed by writing to it*/
-/*
- * Write method
- */
-#define RECONSTRUCT_WRITE       1
-#define READ_MODIFY_WRITE       2
-/* not a write method, but a compute_parity mode */
-#define CHECK_PARITY            3
-/* Additional compute_parity mode -- updates the parity w/o LOCKING */
-#define UPDATE_PARITY           4
+        R5_Expanded,    /* This block now has post-expand data */
+        R5_Wantcompute, /* compute_block in progress treat as
+                         * uptodate
+                         */
+        R5_Wantfill,    /* dev->toread contains a bio that needs
+                         * filling
+                         */
+        R5_Wantdrain,   /* dev->towrite needs to be drained */
+        R5_WantFUA,     /* Write should be FUA */
+        R5_WriteError,  /* got a write error - need to record it */
+        R5_MadeGood,    /* A bad block has been fixed by writing to it */
+        R5_ReadRepl,    /* Will/did read from replacement rather than orig */
+        R5_MadeGoodRepl,/* A bad block on the replacement device has been
+                         * fixed by writing to it */
+        R5_NeedReplace, /* This device has a replacement which is not
+                         * up-to-date at this stripe. */
+        R5_WantReplace, /* We need to update the replacement, we have read
+                         * data in, and now is a good time to write it out.
+                         */
+};
 
 /*
  * Stripe state
@@ -311,13 +323,14 @@ enum {
 /*
  * Operation request flags
  */
-#define STRIPE_OP_BIOFILL       0
-#define STRIPE_OP_COMPUTE_BLK   1
-#define STRIPE_OP_PREXOR        2
-#define STRIPE_OP_BIODRAIN      3
-#define STRIPE_OP_RECONSTRUCT   4
-#define STRIPE_OP_CHECK 5
-
+enum {
+        STRIPE_OP_BIOFILL,
+        STRIPE_OP_COMPUTE_BLK,
+        STRIPE_OP_PREXOR,
+        STRIPE_OP_BIODRAIN,
+        STRIPE_OP_RECONSTRUCT,
+        STRIPE_OP_CHECK,
+};
 /*
  * Plugging:
  *
@@ -344,13 +357,12 @@ enum {
 
 
 struct disk_info {
-        struct md_rdev  *rdev;
+        struct md_rdev  *rdev, *replacement;
 };
 
 struct r5conf {
         struct hlist_head       *stripe_hashtbl;
         struct mddev            *mddev;
-        struct disk_info        *spare;
         int                     chunk_sectors;
         int                     level, algorithm;
         int                     max_degraded;
diff --git a/include/linux/raid/md_p.h b/include/linux/raid/md_p.h
index 9e65d9e20662..6f6df86f1ae5 100644
--- a/include/linux/raid/md_p.h
+++ b/include/linux/raid/md_p.h
@@ -277,7 +277,10 @@ struct mdp_superblock_1 {
                                            */
 #define MD_FEATURE_RESHAPE_ACTIVE       4
 #define MD_FEATURE_BAD_BLOCKS           8 /* badblock list is not empty */
-
-#define MD_FEATURE_ALL                  (1|2|4|8)
+#define MD_FEATURE_REPLACEMENT          16 /* This device is replacing an
+                                            * active device with same 'role'.
+                                            * 'recovery_offset' is also set.
+                                            */
+#define MD_FEATURE_ALL                  (1|2|4|8|16)
 
 #endif 
diff --git a/include/linux/raid/pq.h b/include/linux/raid/pq.h
index 2b59cc824395..53272e9860a7 100644
--- a/include/linux/raid/pq.h
+++ b/include/linux/raid/pq.h
@@ -132,7 +132,7 @@ void raid6_dual_recov(int disks, size_t bytes, int faila, int failb,
                                                      PROT_READ|PROT_WRITE,   \
                                                      MAP_PRIVATE|MAP_ANONYMOUS,\
                                                      0, 0))
-# define free_pages(x, y)       munmap((void *)(x), (y)*PAGE_SIZE)
+# define free_pages(x, y)       munmap((void *)(x), PAGE_SIZE << (y))
 
 static inline void cpu_relax(void)
 {