[PATCH] dm: suspend: add noflush pushback

In device-mapper I/O is sometimes queued within targets for later processing.
For example the multipath target can be configured to store I/O when no paths
are available instead of returning it -EIO.

This patch allows the device-mapper core to instruct a target to transfer the
contents of any such in-target queue back into the core.  This frees up the
resources used by the target so the core can replace that target with an
alternative one and then resend the I/O to it.  Without this patch the only
way to change the target in such circumstances involves returning the I/O with
an error back to the filesystem/application.  In the multipath case, this
patch will let us add new paths for existing I/O to try after all the existing
paths have failed.

    DMF_NOFLUSH_SUSPENDING
    ----------------------

If the DM_NOFLUSH_FLAG ioctl option is specified at suspend time, the
DMF_NOFLUSH_SUSPENDING flag is set in md->flags during dm_suspend().  It
is always cleared before dm_suspend() returns.

The flag must be visible while the target is flushing pending I/Os so it
is set before presuspend where the flush starts and unset after the wait
for md->pending where the flush ends.

Target drivers can check this flag by calling dm_noflush_suspending().

    DM_MAPIO_REQUEUE / DM_ENDIO_REQUEUE
    -----------------------------------

A target's map() function can now return DM_MAPIO_REQUEUE to request the
device mapper core queue the bio.

Similarly, a target's end_io() function can return DM_ENDIO_REQUEUE to request
the same.  This has been labelled 'pushback'.

The __map_bio() and clone_endio() functions in the core treat these return
values as errors and call dec_pending() to end the I/O.

    dec_pending
    -----------

dec_pending() saves the pushback request in struct dm_io->error.  Once all
the split clones have ended, dec_pending() will put the original bio on
the md->pushback list.  Note that this supercedes any I/O errors.

It is possible for the suspend with DM_NOFLUSH_FLAG to be aborted while
in progress (e.g. by user interrupt).  dec_pending() checks for this and
returns -EIO if it happened.

    pushdback list and pushback_lock
    --------------------------------

The bio is queued on md->pushback temporarily in dec_pending(), and after
all pending I/Os return, md->pushback is merged into md->deferred in
dm_suspend() for re-issuing at resume time.

md->pushback_lock protects md->pushback.
The lock should be held with irq disabled because dec_pending() can be
called from interrupt context.

Queueing bios to md->pushback in dec_pending() must be done atomically
with the check for DMF_NOFLUSH_SUSPENDING flag.  So md->pushback_lock is
held when checking the flag.  Otherwise dec_pending() may queue a bio to
md->pushback after the interrupted dm_suspend() flushes md->pushback.
Then the bio would be left in md->pushback.

Flag setting in dm_suspend() can be done without md->pushback_lock because
the flag is checked only after presuspend and the set value is already
made visible via the target's presuspend function.

The flag can be checked without md->pushback_lock (e.g. the first part of
the dec_pending() or target drivers), because the flag is checked again
with md->pushback_lock held when the bio is really queued to md->pushback
as described above.  So even if the flag is cleared after the lockless
checkings, the bio isn't left in md->pushback but returned to applications
with -EIO.

    Other notes on the current patch
    --------------------------------

- md->pushback is added to the struct mapped_device instead of using
  md->deferred directly because md->io_lock which protects md->deferred is
  rw_semaphore and can't be used in interrupt context like dec_pending(),
  and md->io_lock protects the DMF_BLOCK_IO flag of md->flags too.

- Don't issue lock_fs() in dm_suspend() if the DM_NOFLUSH_FLAG
  ioctl option is specified, because I/Os generated by lock_fs() would be
  pushed back and never return if there were no valid devices.

- If an error occurs in dm_suspend() after the DMF_NOFLUSH_SUSPENDING
  flag is set, md->pushback must be flushed because I/Os may be queued to
  the list already.  (flush_and_out label in dm_suspend())

    Test results
    ------------

I have tested using multipath target with the next patch.

The following tests are for regression/compatibility:
  - I/Os succeed when valid paths exist;
  - I/Os fail when there are no valid paths and queue_if_no_path is not
    set;
  - I/Os are queued in the multipath target when there are no valid paths and
    queue_if_no_path is set;
  - The queued I/Os above fail when suspend is issued without the
    DM_NOFLUSH_FLAG ioctl option.  I/Os spanning 2 multipath targets also
    fail.

The following tests are for the normal code path of new pushback feature:
  - Queued I/Os in the multipath target are flushed from the target
    but don't return when suspend is issued with the DM_NOFLUSH_FLAG
    ioctl option;
  - The I/Os above are queued in the multipath target again when
    resume is issued without path recovery;
  - The I/Os above succeed when resume is issued after path recovery
    or table load;
  - Queued I/Os in the multipath target succeed when resume is issued
    with the DM_NOFLUSH_FLAG ioctl option after table load. I/Os
    spanning 2 multipath targets also succeed.

The following tests are for the error paths of the new pushback feature:
  - When the bdget_disk() fails in dm_suspend(), the
    DMF_NOFLUSH_SUSPENDING flag is cleared and I/Os already queued to the
    pushback list are flushed properly.
  - When suspend with the DM_NOFLUSH_FLAG ioctl option is interrupted,
      o I/Os which had already been queued to the pushback list
        at the time don't return, and are re-issued at resume time;
      o I/Os which hadn't been returned at the time return with EIO.

Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: dm-devel@redhat.com
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

3 files changed, 111 insertions, 10 deletions

diff --git a/drivers/md/dm-bio-list.h b/drivers/md/dm-bio-list.h
index bbf4615f0e30..da4349649f7f 100644
--- a/drivers/md/dm-bio-list.h
+++ b/drivers/md/dm-bio-list.h
@@ -44,6 +44,20 @@ static inline void bio_list_merge(struct bio_list *bl, struct bio_list *bl2)
         bl->tail = bl2->tail;
 }
 
+static inline void bio_list_merge_head(struct bio_list *bl,
+                                       struct bio_list *bl2)
+{
+        if (!bl2->head)
+                return;
+
+        if (bl->head)
+                bl2->tail->bi_next = bl->head;
+        else
+                bl->tail = bl2->tail;
+
+        bl->head = bl2->head;
+}
+
 static inline struct bio *bio_list_pop(struct bio_list *bl)
 {
         struct bio *bio = bl->head;
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index d8544e1a4c1f..fe7c56e10435 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -68,10 +68,12 @@ union map_info *dm_get_mapinfo(struct bio *bio)
 #define DMF_FROZEN 2
 #define DMF_FREEING 3
 #define DMF_DELETING 4
+#define DMF_NOFLUSH_SUSPENDING 5
 
 struct mapped_device {
         struct rw_semaphore io_lock;
         struct semaphore suspend_lock;
+        spinlock_t pushback_lock;
         rwlock_t map_lock;
         atomic_t holders;
         atomic_t open_count;
@@ -90,6 +92,7 @@ struct mapped_device {
         atomic_t pending;
         wait_queue_head_t wait;
         struct bio_list deferred;
+        struct bio_list pushback;
 
         /*
          * The current mapping.
@@ -444,23 +447,50 @@ int dm_set_geometry(struct mapped_device *md, struct hd_geometry *geo)
  *   you this clearly demarcated crap.
  *---------------------------------------------------------------*/
 
+static int __noflush_suspending(struct mapped_device *md)
+{
+        return test_bit(DMF_NOFLUSH_SUSPENDING, &md->flags);
+}
+
 /*
  * Decrements the number of outstanding ios that a bio has been
  * cloned into, completing the original io if necc.
  */
 static void dec_pending(struct dm_io *io, int error)
 {
-        if (error)
+        unsigned long flags;
+
+        /* Push-back supersedes any I/O errors */
+        if (error && !(io->error > 0 && __noflush_suspending(io->md)))
                 io->error = error;
 
         if (atomic_dec_and_test(&io->io_count)) {
+                if (io->error == DM_ENDIO_REQUEUE) {
+                        /*
+                         * Target requested pushing back the I/O.
+                         * This must be handled before the sleeper on
+                         * suspend queue merges the pushback list.
+                         */
+                        spin_lock_irqsave(&io->md->pushback_lock, flags);
+                        if (__noflush_suspending(io->md))
+                                bio_list_add(&io->md->pushback, io->bio);
+                        else
+                                /* noflush suspend was interrupted. */
+                                io->error = -EIO;
+                        spin_unlock_irqrestore(&io->md->pushback_lock, flags);
+                }
+
                 if (end_io_acct(io))
                         /* nudge anyone waiting on suspend queue */
                         wake_up(&io->md->wait);
 
-                blk_add_trace_bio(io->md->queue, io->bio, BLK_TA_COMPLETE);
+                if (io->error != DM_ENDIO_REQUEUE) {
+                        blk_add_trace_bio(io->md->queue, io->bio,
+                                          BLK_TA_COMPLETE);
+
+                        bio_endio(io->bio, io->bio->bi_size, io->error);
+                }
 
-                bio_endio(io->bio, io->bio->bi_size, io->error);
                 free_io(io->md, io);
         }
 }
@@ -480,7 +510,11 @@ static int clone_endio(struct bio *bio, unsigned int done, int error)
 
         if (endio) {
                 r = endio(tio->ti, bio, error, &tio->info);
-                if (r < 0)
+                if (r < 0 || r == DM_ENDIO_REQUEUE)
+                        /*
+                         * error and requeue request are handled
+                         * in dec_pending().
+                         */
                         error = r;
                 else if (r == DM_ENDIO_INCOMPLETE)
                         /* The target will handle the io */
@@ -554,8 +588,8 @@ static void __map_bio(struct dm_target *ti, struct bio *clone,
                                     clone->bi_sector);
 
                 generic_make_request(clone);
-        } else if (r < 0) {
-                /* error the io and bail out */
+        } else if (r < 0 || r == DM_MAPIO_REQUEUE) {
+                /* error the io and bail out, or requeue it if needed */
                 md = tio->io->md;
                 dec_pending(tio->io, r);
                 /*
@@ -952,6 +986,7 @@ static struct mapped_device *alloc_dev(int minor)
         memset(md, 0, sizeof(*md));
         init_rwsem(&md->io_lock);
         init_MUTEX(&md->suspend_lock);
+        spin_lock_init(&md->pushback_lock);
         rwlock_init(&md->map_lock);
         atomic_set(&md->holders, 1);
         atomic_set(&md->open_count, 0);
@@ -1282,10 +1317,12 @@ static void unlock_fs(struct mapped_device *md)
 int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
 {
         struct dm_table *map = NULL;
+        unsigned long flags;
         DECLARE_WAITQUEUE(wait, current);
         struct bio *def;
         int r = -EINVAL;
         int do_lockfs = suspend_flags & DM_SUSPEND_LOCKFS_FLAG ? 1 : 0;
+        int noflush = suspend_flags & DM_SUSPEND_NOFLUSH_FLAG ? 1 : 0;
 
         down(&md->suspend_lock);
 
@@ -1294,6 +1331,13 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
 
         map = dm_get_table(md);
 
+        /*
+         * DMF_NOFLUSH_SUSPENDING must be set before presuspend.
+         * This flag is cleared before dm_suspend returns.
+         */
+        if (noflush)
+                set_bit(DMF_NOFLUSH_SUSPENDING, &md->flags);
+
         /* This does not get reverted if there's an error later. */
         dm_table_presuspend_targets(map);
 
@@ -1301,11 +1345,14 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
         if (!md->suspended_bdev) {
                 DMWARN("bdget failed in dm_suspend");
                 r = -ENOMEM;
-                goto out;
+                goto flush_and_out;
         }
 
-        /* Flush I/O to the device. */
-        if (do_lockfs) {
+        /*
+         * Flush I/O to the device.
+         * noflush supersedes do_lockfs, because lock_fs() needs to flush I/Os.
+         */
+        if (do_lockfs && !noflush) {
                 r = lock_fs(md);
                 if (r)
                         goto out;
@@ -1341,6 +1388,14 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
         down_write(&md->io_lock);
         remove_wait_queue(&md->wait, &wait);
 
+        if (noflush) {
+                spin_lock_irqsave(&md->pushback_lock, flags);
+                clear_bit(DMF_NOFLUSH_SUSPENDING, &md->flags);
+                bio_list_merge_head(&md->deferred, &md->pushback);
+                bio_list_init(&md->pushback);
+                spin_unlock_irqrestore(&md->pushback_lock, flags);
+        }
+
         /* were we interrupted ? */
         r = -EINTR;
         if (atomic_read(&md->pending)) {
@@ -1349,7 +1404,7 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
                 __flush_deferred_io(md, def);
                 up_write(&md->io_lock);
                 unlock_fs(md);
-                goto out;
+                goto out; /* pushback list is already flushed, so skip flush */
         }
         up_write(&md->io_lock);
 
@@ -1359,6 +1414,25 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
 
         r = 0;
 
+flush_and_out:
+        if (r && noflush) {
+                /*
+                 * Because there may be already I/Os in the pushback list,
+                 * flush them before return.
+                 */
+                down_write(&md->io_lock);
+
+                spin_lock_irqsave(&md->pushback_lock, flags);
+                clear_bit(DMF_NOFLUSH_SUSPENDING, &md->flags);
+                bio_list_merge_head(&md->deferred, &md->pushback);
+                bio_list_init(&md->pushback);
+                spin_unlock_irqrestore(&md->pushback_lock, flags);
+
+                def = bio_list_get(&md->deferred);
+                __flush_deferred_io(md, def);
+                up_write(&md->io_lock);
+        }
+
 out:
         if (r && md->suspended_bdev) {
                 bdput(md->suspended_bdev);
@@ -1445,6 +1519,17 @@ int dm_suspended(struct mapped_device *md)
         return test_bit(DMF_SUSPENDED, &md->flags);
 }
 
+int dm_noflush_suspending(struct dm_target *ti)
+{
+        struct mapped_device *md = dm_table_get_md(ti->table);
+        int r = __noflush_suspending(md);
+
+        dm_put(md);
+
+        return r;
+}
+EXPORT_SYMBOL_GPL(dm_noflush_suspending);
+
 static struct block_device_operations dm_blk_dops = {
         .open = dm_blk_open,
         .release = dm_blk_close,
diff --git a/drivers/md/dm.h b/drivers/md/dm.h
index c307ca9a4c33..2f796b1436b2 100644
--- a/drivers/md/dm.h
+++ b/drivers/md/dm.h
@@ -36,12 +36,14 @@
  * Definitions of return values from target end_io function.
  */
 #define DM_ENDIO_INCOMPLETE     1
+#define DM_ENDIO_REQUEUE        2
 
 /*
  * Definitions of return values from target map function.
  */
 #define DM_MAPIO_SUBMITTED      0
 #define DM_MAPIO_REMAPPED       1
+#define DM_MAPIO_REQUEUE        DM_ENDIO_REQUEUE
 
 /*
  * Suspend feature flags