7 files changed, 270 insertions, 175 deletions

diff --git a/fs/nilfs2/nilfs.h b/fs/nilfs2/nilfs.h
index a7f5bc724e33..19af5ab86275 100644
--- a/fs/nilfs2/nilfs.h
+++ b/fs/nilfs2/nilfs.h
@@ -275,13 +275,10 @@ extern void nilfs_error(struct super_block *, const char *, const char *, ...)
 extern void nilfs_warning(struct super_block *, const char *, const char *, ...)
        __attribute__ ((format (printf, 3, 4)));
 extern struct nilfs_super_block *
-nilfs_load_super_block(struct super_block *, struct buffer_head **);
-extern struct nilfs_super_block *
-nilfs_reload_super_block(struct super_block *, struct buffer_head **, int);
+nilfs_read_super_block(struct super_block *, u64, int, struct buffer_head **);
 extern int nilfs_store_magic_and_option(struct super_block *,
                                         struct nilfs_super_block *, char *);
-extern void nilfs_update_last_segment(struct nilfs_sb_info *, int);
-extern int nilfs_commit_super(struct nilfs_sb_info *);
+extern int nilfs_commit_super(struct nilfs_sb_info *, int);
 extern int nilfs_attach_checkpoint(struct nilfs_sb_info *, __u64);
 extern void nilfs_detach_checkpoint(struct nilfs_sb_info *);
 
diff --git a/fs/nilfs2/recovery.c b/fs/nilfs2/recovery.c
index 6ab4c8fc5e9f..6ade0963fc1d 100644
--- a/fs/nilfs2/recovery.c
+++ b/fs/nilfs2/recovery.c
@@ -870,7 +870,6 @@ int nilfs_search_super_root(struct the_nilfs *nilfs, struct nilfs_sb_info *sbi,
                 if (scan_newer)
                         ri->ri_need_recovery = NILFS_RECOVERY_SR_UPDATED;
                 else {
-                        nilfs->ns_prot_seq = ssi.seg_seq;
                         if (nilfs->ns_mount_state & NILFS_VALID_FS)
                                 goto super_root_found;
                         scan_newer = 1;
diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index e43558d50e78..fb70ec3be20e 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -2068,7 +2068,8 @@ static void nilfs_segctor_complete_write(struct nilfs_sc_info *sci)
 
         if (update_sr) {
                 nilfs_set_last_segment(nilfs, segbuf->sb_pseg_start,
-                                       segbuf->sb_sum.seg_seq, nilfs->ns_cno);
+                                       segbuf->sb_sum.seg_seq, nilfs->ns_cno++);
+                sbi->s_super->s_dirt = 1;
 
                 clear_bit(NILFS_SC_HAVE_DELTA, &sci->sc_flags);
                 clear_bit(NILFS_SC_DIRTY, &sci->sc_flags);
@@ -2224,9 +2225,6 @@ static int nilfs_segctor_do_construct(struct nilfs_sc_info *sci, int mode)
 
                 /* Commit segments */
                 if (has_sr) {
-                        down_write(&nilfs->ns_sem);
-                        nilfs_update_last_segment(sbi, 1);
-                        up_write(&nilfs->ns_sem);
                         nilfs_segctor_commit_free_segments(sci);
                         nilfs_segctor_clear_metadata_dirty(sci);
                 }
@@ -2564,7 +2562,7 @@ static int nilfs_segctor_construct(struct nilfs_sc_info *sci,
                 if (test_bit(NILFS_SC_SUPER_ROOT, &sci->sc_flags) &&
                     nilfs_discontinued(nilfs)) {
                         down_write(&nilfs->ns_sem);
-                        req->sb_err = nilfs_commit_super(sbi);
+                        req->sb_err = nilfs_commit_super(sbi, 0);
                         up_write(&nilfs->ns_sem);
                 }
         }
diff --git a/fs/nilfs2/segment.h b/fs/nilfs2/segment.h
index 4a64eb82f1f5..a98fc1ed0bbb 100644
--- a/fs/nilfs2/segment.h
+++ b/fs/nilfs2/segment.h
@@ -206,8 +206,6 @@ enum {
                                            logical segment with a super root */
 #define NILFS_SC_DEFAULT_SR_FREQ    30  /* Maximum frequency of super root
                                            creation */
-#define NILFS_SC_DEFAULT_SB_FREQ    30  /* Minimum interval of periodical
-                                           update of superblock (reserved) */
 
 /*
  * The default threshold amount of data, in block counts.
diff --git a/fs/nilfs2/super.c b/fs/nilfs2/super.c
index ef31e9a51c84..e2ced824c624 100644
--- a/fs/nilfs2/super.c
+++ b/fs/nilfs2/super.c
@@ -103,8 +103,9 @@ void nilfs_error(struct super_block *sb, const char *function,
                 down_write(&nilfs->ns_sem);
                 if (!(nilfs->ns_mount_state & NILFS_ERROR_FS)) {
                         nilfs->ns_mount_state |= NILFS_ERROR_FS;
-                        nilfs->ns_sbp->s_state |= cpu_to_le16(NILFS_ERROR_FS);
-                        nilfs_commit_super(sbi);
+                        nilfs->ns_sbp[0]->s_state |=
+                                cpu_to_le16(NILFS_ERROR_FS);
+                        nilfs_commit_super(sbi, 1);
                 }
                 up_write(&nilfs->ns_sem);
 
@@ -208,90 +209,106 @@ static void nilfs_clear_inode(struct inode *inode)
         nilfs_btnode_cache_clear(&ii->i_btnode_cache);
 }
 
-/**
- * nilfs_update_last_segment - change pointer to the latest segment
- * @sbi: nilfs_sb_info
- * @update_cno: flag whether to update checkpoint number.
- *
- * nilfs_update_last_segment() changes information in the super block
- * after a partial segment is written out successfully. The super
- * block is marked dirty. It will be written out at the next VFS sync
- * operations such as sync_supers() and generic_shutdown_super().
- */
-void nilfs_update_last_segment(struct nilfs_sb_info *sbi, int update_cno)
-{
-        struct the_nilfs *nilfs = sbi->s_nilfs;
-        struct nilfs_super_block *sbp = nilfs->ns_sbp;
-
-        /* nilfs->sem must be locked by the caller. */
-        spin_lock(&nilfs->ns_last_segment_lock);
-        if (update_cno)
-                nilfs->ns_last_cno = nilfs->ns_cno++;
-        sbp->s_last_seq = cpu_to_le64(nilfs->ns_last_seq);
-        sbp->s_last_pseg = cpu_to_le64(nilfs->ns_last_pseg);
-        sbp->s_last_cno = cpu_to_le64(nilfs->ns_last_cno);
-        spin_unlock(&nilfs->ns_last_segment_lock);
-
-        sbi->s_super->s_dirt = 1; /* must be set if delaying the call of
-                                     nilfs_commit_super() */
-}
-
-static int nilfs_sync_super(struct nilfs_sb_info *sbi)
+static int nilfs_sync_super(struct nilfs_sb_info *sbi, int dupsb)
 {
         struct the_nilfs *nilfs = sbi->s_nilfs;
         int err;
         int barrier_done = 0;
 
         if (nilfs_test_opt(sbi, BARRIER)) {
-                set_buffer_ordered(nilfs->ns_sbh);
+                set_buffer_ordered(nilfs->ns_sbh[0]);
                 barrier_done = 1;
         }
  retry:
-        set_buffer_dirty(nilfs->ns_sbh);
-        err = sync_dirty_buffer(nilfs->ns_sbh);
+        set_buffer_dirty(nilfs->ns_sbh[0]);
+        err = sync_dirty_buffer(nilfs->ns_sbh[0]);
         if (err == -EOPNOTSUPP && barrier_done) {
                 nilfs_warning(sbi->s_super, __func__,
                               "barrier-based sync failed. "
                               "disabling barriers\n");
                 nilfs_clear_opt(sbi, BARRIER);
                 barrier_done = 0;
-                clear_buffer_ordered(nilfs->ns_sbh);
+                clear_buffer_ordered(nilfs->ns_sbh[0]);
                 goto retry;
         }
-        if (unlikely(err))
+        if (unlikely(err)) {
                 printk(KERN_ERR
                        "NILFS: unable to write superblock (err=%d)\n", err);
-        else {
+                if (err == -EIO && nilfs->ns_sbh[1]) {
+                        nilfs_fall_back_super_block(nilfs);
+                        goto retry;
+                }
+        } else {
+                struct nilfs_super_block *sbp = nilfs->ns_sbp[0];
+
+                /*
+                 * The latest segment becomes trailable from the position
+                 * written in superblock.
+                 */
                 clear_nilfs_discontinued(nilfs);
-                spin_lock(&nilfs->ns_last_segment_lock);
-                nilfs->ns_prot_seq = le64_to_cpu(nilfs->ns_sbp->s_last_seq);
-                spin_unlock(&nilfs->ns_last_segment_lock);
+
+                /* update GC protection for recent segments */
+                if (nilfs->ns_sbh[1]) {
+                        sbp = NULL;
+                        if (dupsb) {
+                                set_buffer_dirty(nilfs->ns_sbh[1]);
+                                if (!sync_dirty_buffer(nilfs->ns_sbh[1]))
+                                        sbp = nilfs->ns_sbp[1];
+                        }
+                }
+                if (sbp) {
+                        spin_lock(&nilfs->ns_last_segment_lock);
+                        nilfs->ns_prot_seq = le64_to_cpu(sbp->s_last_seq);
+                        spin_unlock(&nilfs->ns_last_segment_lock);
+                }
         }
 
         return err;
 }
 
-int nilfs_commit_super(struct nilfs_sb_info *sbi)
+int nilfs_commit_super(struct nilfs_sb_info *sbi, int dupsb)
 {
         struct the_nilfs *nilfs = sbi->s_nilfs;
-        struct nilfs_super_block *sbp = nilfs->ns_sbp;
+        struct nilfs_super_block **sbp = nilfs->ns_sbp;
         sector_t nfreeblocks;
+        time_t t;
         int err;
 
         /* nilfs->sem must be locked by the caller. */
+        if (sbp[0]->s_magic != NILFS_SUPER_MAGIC) {
+                if (sbp[1] && sbp[1]->s_magic == NILFS_SUPER_MAGIC)
+                        nilfs_swap_super_block(nilfs);
+                else {
+                        printk(KERN_CRIT "NILFS: superblock broke on dev %s\n",
+                               sbi->s_super->s_id);
+                        return -EIO;
+                }
+        }
         err = nilfs_count_free_blocks(nilfs, &nfreeblocks);
         if (unlikely(err)) {
                 printk(KERN_ERR "NILFS: failed to count free blocks\n");
                 return err;
         }
-        sbp->s_free_blocks_count = cpu_to_le64(nfreeblocks);
-        sbp->s_wtime = cpu_to_le64(get_seconds());
-        sbp->s_sum = 0;
-        sbp->s_sum = cpu_to_le32(crc32_le(nilfs->ns_crc_seed,
-                                          (unsigned char *)sbp,
-                                          le16_to_cpu(sbp->s_bytes)));
+        spin_lock(&nilfs->ns_last_segment_lock);
+        sbp[0]->s_last_seq = cpu_to_le64(nilfs->ns_last_seq);
+        sbp[0]->s_last_pseg = cpu_to_le64(nilfs->ns_last_pseg);
+        sbp[0]->s_last_cno = cpu_to_le64(nilfs->ns_last_cno);
+        spin_unlock(&nilfs->ns_last_segment_lock);
+
+        t = get_seconds();
+        nilfs->ns_sbwtime[0] = t;
+        sbp[0]->s_free_blocks_count = cpu_to_le64(nfreeblocks);
+        sbp[0]->s_wtime = cpu_to_le64(t);
+        sbp[0]->s_sum = 0;
+        sbp[0]->s_sum = cpu_to_le32(crc32_le(nilfs->ns_crc_seed,
+                                             (unsigned char *)sbp[0],
+                                             nilfs->ns_sbsize));
+        if (dupsb && sbp[1]) {
+                memcpy(sbp[1], sbp[0], nilfs->ns_sbsize);
+                nilfs->ns_sbwtime[1] = t;
+        }
         sbi->s_super->s_dirt = 0;
-        return nilfs_sync_super(sbi);
+        return nilfs_sync_super(sbi, dupsb);
 }
 
 static void nilfs_put_super(struct super_block *sb)
@@ -303,8 +320,8 @@ static void nilfs_put_super(struct super_block *sb)
 
         if (!(sb->s_flags & MS_RDONLY)) {
                 down_write(&nilfs->ns_sem);
-                nilfs->ns_sbp->s_state = cpu_to_le16(nilfs->ns_mount_state);
-                nilfs_commit_super(sbi);
+                nilfs->ns_sbp[0]->s_state = cpu_to_le16(nilfs->ns_mount_state);
+                nilfs_commit_super(sbi, 1);
                 up_write(&nilfs->ns_sem);
         }
 
@@ -330,7 +347,7 @@ static void nilfs_put_super(struct super_block *sb)
  *   2.    down_write(&nilfs->ns_sem)
  *
  * Inside NILFS, locking ns_sem is enough to protect s_dirt and the buffer
- * of the super block (nilfs->ns_sbp).
+ * of the super block (nilfs->ns_sbp[]).
  *
  * In most cases, VFS functions call lock_super() before calling these
  * methods.  So we must be careful not to bring on deadlocks when using
@@ -346,8 +363,19 @@ static void nilfs_write_super(struct super_block *sb)
         struct the_nilfs *nilfs = sbi->s_nilfs;
 
         down_write(&nilfs->ns_sem);
-        if (!(sb->s_flags & MS_RDONLY))
-                nilfs_commit_super(sbi);
+        if (!(sb->s_flags & MS_RDONLY)) {
+                struct nilfs_super_block **sbp = nilfs->ns_sbp;
+                u64 t = get_seconds();
+                int dupsb;
+
+                if (!nilfs_discontinued(nilfs) && t >= nilfs->ns_sbwtime[0] &&
+                    t < nilfs->ns_sbwtime[0] + NILFS_SB_FREQ) {
+                        up_write(&nilfs->ns_sem);
+                        return;
+                }
+                dupsb = sbp[1] && t > nilfs->ns_sbwtime[1] + NILFS_ALTSB_FREQ;
+                nilfs_commit_super(sbi, dupsb);
+        }
         sb->s_dirt = 0;
         up_write(&nilfs->ns_sem);
 }
@@ -436,7 +464,7 @@ static int nilfs_mark_recovery_complete(struct nilfs_sb_info *sbi)
         down_write(&nilfs->ns_sem);
         if (!(nilfs->ns_mount_state & NILFS_VALID_FS)) {
                 nilfs->ns_mount_state |= NILFS_VALID_FS;
-                err = nilfs_commit_super(sbi);
+                err = nilfs_commit_super(sbi, 1);
                 if (likely(!err))
                         printk(KERN_INFO "NILFS: recovery complete.\n");
         }
@@ -652,7 +680,7 @@ nilfs_set_default_options(struct nilfs_sb_info *sbi,
 static int nilfs_setup_super(struct nilfs_sb_info *sbi)
 {
         struct the_nilfs *nilfs = sbi->s_nilfs;
-        struct nilfs_super_block *sbp = nilfs->ns_sbp;
+        struct nilfs_super_block *sbp = nilfs->ns_sbp[0];
         int max_mnt_count = le16_to_cpu(sbp->s_max_mnt_count);
         int mnt_count = le16_to_cpu(sbp->s_mnt_count);
 
@@ -674,88 +702,29 @@ static int nilfs_setup_super(struct nilfs_sb_info *sbi)
         sbp->s_mnt_count = cpu_to_le16(mnt_count + 1);
         sbp->s_state = cpu_to_le16(le16_to_cpu(sbp->s_state) & ~NILFS_VALID_FS);
         sbp->s_mtime = cpu_to_le64(get_seconds());
-        return nilfs_commit_super(sbi);
+        return nilfs_commit_super(sbi, 1);
 }
 
-struct nilfs_super_block *
-nilfs_load_super_block(struct super_block *sb, struct buffer_head **pbh)
+struct nilfs_super_block *nilfs_read_super_block(struct super_block *sb,
+                                                 u64 pos, int blocksize,
+                                                 struct buffer_head **pbh)
 {
-        int blocksize;
-        unsigned long offset, sb_index;
-
-        /*
-         * Adjusting block size
-         * Blocksize will be enlarged when it is smaller than hardware
-         * sector size.
-         * Disk format of superblock does not change.
-         */
-        blocksize = sb_min_blocksize(sb, BLOCK_SIZE);
-        if (!blocksize) {
-                printk(KERN_ERR
-                       "NILFS: unable to set blocksize of superblock\n");
-                return NULL;
-        }
-        sb_index = NILFS_SB_OFFSET_BYTES / blocksize;
-        offset = NILFS_SB_OFFSET_BYTES % blocksize;
+        unsigned long long sb_index = pos;
+        unsigned long offset;
 
+        offset = do_div(sb_index, blocksize);
         *pbh = sb_bread(sb, sb_index);
-        if (!*pbh) {
-                printk(KERN_ERR "NILFS: unable to read superblock\n");
+        if (!*pbh)
                 return NULL;
-        }
         return (struct nilfs_super_block *)((char *)(*pbh)->b_data + offset);
 }
 
-struct nilfs_super_block *
-nilfs_reload_super_block(struct super_block *sb, struct buffer_head **pbh,
-                         int blocksize)
-{
-        struct nilfs_super_block *sbp;
-        unsigned long offset, sb_index;
-        int hw_blocksize = bdev_hardsect_size(sb->s_bdev);
-
-        if (blocksize < hw_blocksize) {
-                printk(KERN_ERR
-                       "NILFS: blocksize %d too small for device "
-                       "(sector-size = %d).\n",
-                       blocksize, hw_blocksize);
-                goto failed_sbh;
-        }
-        brelse(*pbh);
-        sb_set_blocksize(sb, blocksize);
-
-        sb_index = NILFS_SB_OFFSET_BYTES / blocksize;
-        offset = NILFS_SB_OFFSET_BYTES % blocksize;
-
-        *pbh = sb_bread(sb, sb_index);
-        if (!*pbh) {
-                printk(KERN_ERR
-                       "NILFS: cannot read superblock on 2nd try.\n");
-                goto failed;
-        }
-
-        sbp = (struct nilfs_super_block *)((char *)(*pbh)->b_data + offset);
-        if (sbp->s_magic != cpu_to_le16(NILFS_SUPER_MAGIC)) {
-                printk(KERN_ERR
-                       "NILFS: !? Magic mismatch on 2nd try.\n");
-                goto failed_sbh;
-        }
-        return sbp;
-
- failed_sbh:
-        brelse(*pbh);
-
- failed:
-        return NULL;
-}
-
 int nilfs_store_magic_and_option(struct super_block *sb,
                                  struct nilfs_super_block *sbp,
                                  char *data)
 {
         struct nilfs_sb_info *sbi = NILFS_SB(sb);
 
-        /* trying to fill super (1st stage) */
         sb->s_magic = le16_to_cpu(sbp->s_magic);
 
         /* FS independent flags */
@@ -763,11 +732,6 @@ int nilfs_store_magic_and_option(struct super_block *sb,
         sb->s_flags |= MS_NOATIME;
 #endif
 
-        if (sb->s_magic != NILFS_SUPER_MAGIC) {
-                printk("NILFS: Can't find nilfs on dev %s.\n", sb->s_id);
-                return -EINVAL;
-        }
-
         nilfs_set_default_options(sbi, sbp);
 
         sbi->s_resuid = le16_to_cpu(sbp->s_def_resuid);
@@ -775,10 +739,7 @@ int nilfs_store_magic_and_option(struct super_block *sb,
         sbi->s_interval = le32_to_cpu(sbp->s_c_interval);
         sbi->s_watermark = le32_to_cpu(sbp->s_c_block_max);
 
-        if (!parse_options(data, sb))
-                return -EINVAL;
-
-        return 0;
+        return !parse_options(data, sb) ? -EINVAL : 0 ;
 }
 
 /**
@@ -967,12 +928,12 @@ static int nilfs_remount(struct super_block *sb, int *flags, char *data)
                  * the RDONLY flag and then mark the partition as valid again.
                  */
                 down_write(&nilfs->ns_sem);
-                sbp = nilfs->ns_sbp;
+                sbp = nilfs->ns_sbp[0];
                 if (!(sbp->s_state & le16_to_cpu(NILFS_VALID_FS)) &&
                     (nilfs->ns_mount_state & NILFS_VALID_FS))
                         sbp->s_state = cpu_to_le16(nilfs->ns_mount_state);
                 sbp->s_mtime = cpu_to_le64(get_seconds());
-                nilfs_commit_super(sbi);
+                nilfs_commit_super(sbi, 1);
                 up_write(&nilfs->ns_sem);
         } else {
                 /*
diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
index 661ab762d765..33400cf0bbe2 100644
--- a/fs/nilfs2/the_nilfs.c
+++ b/fs/nilfs2/the_nilfs.c
@@ -25,6 +25,7 @@
 #include <linux/slab.h>
 #include <linux/blkdev.h>
 #include <linux/backing-dev.h>
+#include <linux/crc32.h>
 #include "nilfs.h"
 #include "segment.h"
 #include "alloc.h"
@@ -105,7 +106,8 @@ void put_nilfs(struct the_nilfs *nilfs)
         }
         if (nilfs_init(nilfs)) {
                 nilfs_destroy_gccache(nilfs);
-                brelse(nilfs->ns_sbh);
+                brelse(nilfs->ns_sbh[0]);
+                brelse(nilfs->ns_sbh[1]);
         }
         kfree(nilfs);
 }
@@ -115,6 +117,7 @@ static int nilfs_load_super_root(struct the_nilfs *nilfs,
 {
         struct buffer_head *bh_sr;
         struct nilfs_super_root *raw_sr;
+        struct nilfs_super_block **sbp = nilfs->ns_sbp;
         unsigned dat_entry_size, segment_usage_size, checkpoint_size;
         unsigned inode_size;
         int err;
@@ -124,9 +127,9 @@ static int nilfs_load_super_root(struct the_nilfs *nilfs,
                 return err;
 
         down_read(&nilfs->ns_sem);
-        dat_entry_size = le16_to_cpu(nilfs->ns_sbp->s_dat_entry_size);
-        checkpoint_size = le16_to_cpu(nilfs->ns_sbp->s_checkpoint_size);
-        segment_usage_size = le16_to_cpu(nilfs->ns_sbp->s_segment_usage_size);
+        dat_entry_size = le16_to_cpu(sbp[0]->s_dat_entry_size);
+        checkpoint_size = le16_to_cpu(sbp[0]->s_checkpoint_size);
+        segment_usage_size = le16_to_cpu(sbp[0]->s_segment_usage_size);
         up_read(&nilfs->ns_sem);
 
         inode_size = nilfs->ns_inode_size;
@@ -270,11 +273,8 @@ int load_nilfs(struct the_nilfs *nilfs, struct nilfs_sb_info *sbi)
                         nilfs_mdt_destroy(nilfs->ns_dat);
                         goto failed;
                 }
-                if (ri.ri_need_recovery == NILFS_RECOVERY_SR_UPDATED) {
-                        down_write(&nilfs->ns_sem);
-                        nilfs_update_last_segment(sbi, 0);
-                        up_write(&nilfs->ns_sem);
-                }
+                if (ri.ri_need_recovery == NILFS_RECOVERY_SR_UPDATED)
+                        sbi->s_super->s_dirt = 1;
         }
 
         set_nilfs_loaded(nilfs);
@@ -296,9 +296,8 @@ static unsigned long long nilfs_max_size(unsigned int blkbits)
         return res;
 }
 
-static int
-nilfs_store_disk_layout(struct the_nilfs *nilfs, struct super_block *sb,
-                        struct nilfs_super_block *sbp)
+static int nilfs_store_disk_layout(struct the_nilfs *nilfs,
+                                   struct nilfs_super_block *sbp)
 {
         if (le32_to_cpu(sbp->s_rev_level) != NILFS_CURRENT_REV) {
                 printk(KERN_ERR "NILFS: revision mismatch "
@@ -309,6 +308,10 @@ nilfs_store_disk_layout(struct the_nilfs *nilfs, struct super_block *sb,
                        NILFS_CURRENT_REV, NILFS_MINOR_REV);
                 return -EINVAL;
         }
+        nilfs->ns_sbsize = le16_to_cpu(sbp->s_bytes);
+        if (nilfs->ns_sbsize > BLOCK_SIZE)
+                return -EINVAL;
+
         nilfs->ns_inode_size = le16_to_cpu(sbp->s_inode_size);
         nilfs->ns_first_ino = le32_to_cpu(sbp->s_first_ino);
 
@@ -330,6 +333,122 @@ nilfs_store_disk_layout(struct the_nilfs *nilfs, struct super_block *sb,
         return 0;
 }
 
+static int nilfs_valid_sb(struct nilfs_super_block *sbp)
+{
+        static unsigned char sum[4];
+        const int sumoff = offsetof(struct nilfs_super_block, s_sum);
+        size_t bytes;
+        u32 crc;
+
+        if (!sbp || le16_to_cpu(sbp->s_magic) != NILFS_SUPER_MAGIC)
+                return 0;
+        bytes = le16_to_cpu(sbp->s_bytes);
+        if (bytes > BLOCK_SIZE)
+                return 0;
+        crc = crc32_le(le32_to_cpu(sbp->s_crc_seed), (unsigned char *)sbp,
+                       sumoff);
+        crc = crc32_le(crc, sum, 4);
+        crc = crc32_le(crc, (unsigned char *)sbp + sumoff + 4,
+                       bytes - sumoff - 4);
+        return crc == le32_to_cpu(sbp->s_sum);
+}
+
+static int nilfs_sb2_bad_offset(struct nilfs_super_block *sbp, u64 offset)
+{
+        return offset < ((le64_to_cpu(sbp->s_nsegments) *
+                          le32_to_cpu(sbp->s_blocks_per_segment)) <<
+                         (le32_to_cpu(sbp->s_log_block_size) + 10));
+}
+
+static void nilfs_release_super_block(struct the_nilfs *nilfs)
+{
+        int i;
+
+        for (i = 0; i < 2; i++) {
+                if (nilfs->ns_sbp[i]) {
+                        brelse(nilfs->ns_sbh[i]);
+                        nilfs->ns_sbh[i] = NULL;
+                        nilfs->ns_sbp[i] = NULL;
+                }
+        }
+}
+
+void nilfs_fall_back_super_block(struct the_nilfs *nilfs)
+{
+        brelse(nilfs->ns_sbh[0]);
+        nilfs->ns_sbh[0] = nilfs->ns_sbh[1];
+        nilfs->ns_sbp[0] = nilfs->ns_sbp[1];
+        nilfs->ns_sbh[1] = NULL;
+        nilfs->ns_sbp[1] = NULL;
+}
+
+void nilfs_swap_super_block(struct the_nilfs *nilfs)
+{
+        struct buffer_head *tsbh = nilfs->ns_sbh[0];
+        struct nilfs_super_block *tsbp = nilfs->ns_sbp[0];
+
+        nilfs->ns_sbh[0] = nilfs->ns_sbh[1];
+        nilfs->ns_sbp[0] = nilfs->ns_sbp[1];
+        nilfs->ns_sbh[1] = tsbh;
+        nilfs->ns_sbp[1] = tsbp;
+}
+
+static int nilfs_load_super_block(struct the_nilfs *nilfs,
+                                  struct super_block *sb, int blocksize,
+                                  struct nilfs_super_block **sbpp)
+{
+        struct nilfs_super_block **sbp = nilfs->ns_sbp;
+        struct buffer_head **sbh = nilfs->ns_sbh;
+        u64 sb2off = NILFS_SB2_OFFSET_BYTES(nilfs->ns_bdev->bd_inode->i_size);
+        int valid[2], swp = 0;
+
+        sbp[0] = nilfs_read_super_block(sb, NILFS_SB_OFFSET_BYTES, blocksize,
+                                        &sbh[0]);
+        sbp[1] = nilfs_read_super_block(sb, sb2off, blocksize, &sbh[1]);
+
+        if (!sbp[0]) {
+                if (!sbp[1]) {
+                        printk(KERN_ERR "NILFS: unable to read superblock\n");
+                        return -EIO;
+                }
+                printk(KERN_WARNING
+                       "NILFS warning: unable to read primary superblock\n");
+        } else if (!sbp[1])
+                printk(KERN_WARNING
+                       "NILFS warning: unable to read secondary superblock\n");
+
+        valid[0] = nilfs_valid_sb(sbp[0]);
+        valid[1] = nilfs_valid_sb(sbp[1]);
+        swp = valid[1] &&
+                (!valid[0] ||
+                 le64_to_cpu(sbp[1]->s_wtime) > le64_to_cpu(sbp[0]->s_wtime));
+
+        if (valid[swp] && nilfs_sb2_bad_offset(sbp[swp], sb2off)) {
+                brelse(sbh[1]);
+                sbh[1] = NULL;
+                sbp[1] = NULL;
+                swp = 0;
+        }
+        if (!valid[swp]) {
+                nilfs_release_super_block(nilfs);
+                printk(KERN_ERR "NILFS: Can't find nilfs on dev %s.\n",
+                       sb->s_id);
+                return -EINVAL;
+        }
+
+        if (swp) {
+                printk(KERN_WARNING "NILFS warning: broken superblock. "
+                       "using spare superblock.\n");
+                nilfs_swap_super_block(nilfs);
+        }
+
+        nilfs->ns_sbwtime[0] = le64_to_cpu(sbp[0]->s_wtime);
+        nilfs->ns_sbwtime[1] = valid[!swp] ? le64_to_cpu(sbp[1]->s_wtime) : 0;
+        nilfs->ns_prot_seq = le64_to_cpu(sbp[valid[1] & !swp]->s_last_seq);
+        *sbpp = sbp[0];
+        return 0;
+}
+
 /**
  * init_nilfs - initialize a NILFS instance.
  * @nilfs: the_nilfs structure
@@ -352,16 +471,15 @@ nilfs_store_disk_layout(struct the_nilfs *nilfs, struct super_block *sb,
 int init_nilfs(struct the_nilfs *nilfs, struct nilfs_sb_info *sbi, char *data)
 {
         struct super_block *sb = sbi->s_super;
-        struct buffer_head *sbh;
         struct nilfs_super_block *sbp;
         struct backing_dev_info *bdi;
         int blocksize;
-        int err = 0;
+        int err;
 
         down_write(&nilfs->ns_sem);
         if (nilfs_init(nilfs)) {
                 /* Load values from existing the_nilfs */
-                sbp = nilfs->ns_sbp;
+                sbp = nilfs->ns_sbp[0];
                 err = nilfs_store_magic_and_option(sb, sbp, data);
                 if (err)
                         goto out;
@@ -377,36 +495,50 @@ int init_nilfs(struct the_nilfs *nilfs, struct nilfs_sb_info *sbi, char *data)
                 goto out;
         }
 
-        sbp = nilfs_load_super_block(sb, &sbh);
-        if (!sbp) {
+        blocksize = sb_min_blocksize(sb, BLOCK_SIZE);
+        if (!blocksize) {
+                printk(KERN_ERR "NILFS: unable to set blocksize\n");
                 err = -EINVAL;
                 goto out;
         }
+        err = nilfs_load_super_block(nilfs, sb, blocksize, &sbp);
+        if (err)
+                goto out;
+
         err = nilfs_store_magic_and_option(sb, sbp, data);
         if (err)
                 goto failed_sbh;
 
         blocksize = BLOCK_SIZE << le32_to_cpu(sbp->s_log_block_size);
         if (sb->s_blocksize != blocksize) {
-                sbp = nilfs_reload_super_block(sb, &sbh, blocksize);
-                if (!sbp) {
+                int hw_blocksize = bdev_hardsect_size(sb->s_bdev);
+
+                if (blocksize < hw_blocksize) {
+                        printk(KERN_ERR
+                               "NILFS: blocksize %d too small for device "
+                               "(sector-size = %d).\n",
+                               blocksize, hw_blocksize);
                         err = -EINVAL;
+                        goto failed_sbh;
+                }
+                nilfs_release_super_block(nilfs);
+                sb_set_blocksize(sb, blocksize);
+
+                err = nilfs_load_super_block(nilfs, sb, blocksize, &sbp);
+                if (err)
                         goto out;
                         /* not failed_sbh; sbh is released automatically
                            when reloading fails. */
-                }
         }
         nilfs->ns_blocksize_bits = sb->s_blocksize_bits;
 
-        err = nilfs_store_disk_layout(nilfs, sb, sbp);
+        err = nilfs_store_disk_layout(nilfs, sbp);
         if (err)
                 goto failed_sbh;
 
         sb->s_maxbytes = nilfs_max_size(sb->s_blocksize_bits);
 
         nilfs->ns_mount_state = le16_to_cpu(sbp->s_state);
-        nilfs->ns_sbh = sbh;
-        nilfs->ns_sbp = sbp;
 
         bdi = nilfs->ns_bdev->bd_inode_backing_dev_info;
         if (!bdi)
@@ -443,7 +575,7 @@ int init_nilfs(struct the_nilfs *nilfs, struct nilfs_sb_info *sbi, char *data)
         return err;
 
  failed_sbh:
-        brelse(sbh);
+        nilfs_release_super_block(nilfs);
         goto out;
 }
 
diff --git a/fs/nilfs2/the_nilfs.h b/fs/nilfs2/the_nilfs.h
index d750e48257c9..30fe58778d05 100644
--- a/fs/nilfs2/the_nilfs.h
+++ b/fs/nilfs2/the_nilfs.h
@@ -49,8 +49,10 @@ enum {
  * @ns_sem: semaphore for shared states
  * @ns_writer_mutex: mutex protecting ns_writer attach/detach
  * @ns_writer_refcount: number of referrers on ns_writer
- * @ns_sbh: buffer head of the on-disk super block
- * @ns_sbp: pointer to the super block data
+ * @ns_sbh: buffer heads of on-disk super blocks
+ * @ns_sbp: pointers to super block data
+ * @ns_sbwtime: previous write time of super blocks
+ * @ns_sbsize: size of valid data in super block
  * @ns_supers: list of nilfs super block structs
  * @ns_seg_seq: segment sequence counter
  * @ns_segnum: index number of the latest full segment.
@@ -101,8 +103,10 @@ struct the_nilfs {
          * - protecting s_dirt in the super_block struct
          *   (see nilfs_write_super) and the following fields.
          */
-        struct buffer_head     *ns_sbh;
-        struct nilfs_super_block *ns_sbp;
+        struct buffer_head     *ns_sbh[2];
+        struct nilfs_super_block *ns_sbp[2];
+        time_t                  ns_sbwtime[2];
+        unsigned                ns_sbsize;
         unsigned                ns_mount_state;
         struct list_head        ns_supers;
 
@@ -182,6 +186,10 @@ THE_NILFS_FNS(INIT, init)
 THE_NILFS_FNS(LOADED, loaded)
 THE_NILFS_FNS(DISCONTINUED, discontinued)
 
+/* Minimum interval of periodical update of superblocks (in seconds) */
+#define NILFS_SB_FREQ           10
+#define NILFS_ALTSB_FREQ        60  /* spare superblock */
+
 void nilfs_set_last_segment(struct the_nilfs *, sector_t, u64, __u64);
 struct the_nilfs *alloc_nilfs(struct block_device *);
 void put_nilfs(struct the_nilfs *);
@@ -190,6 +198,8 @@ int load_nilfs(struct the_nilfs *, struct nilfs_sb_info *);
 int nilfs_count_free_blocks(struct the_nilfs *, sector_t *);
 int nilfs_checkpoint_is_mounted(struct the_nilfs *, __u64, int);
 int nilfs_near_disk_full(struct the_nilfs *);
+void nilfs_fall_back_super_block(struct the_nilfs *);
+void nilfs_swap_super_block(struct the_nilfs *);
 
 
 static inline void get_nilfs(struct the_nilfs *nilfs)