[XFS] Ensure a frozen filesystem has a clean log before writing the dummy

record.

The current Linux XFS freeze code is a mess. We flush the metadata buffers
out while we are still allowing new transactions to start and then fail to
flush the dirty buffers back out before writing the unmount and dummy
records to the log.

This leads to problems when the frozen filesystem is used for snapshots -
we do log recovery on a readonly image and often it appears that the log
image in the snapshot is not correct. Hence we end up with hangs, oops and
mount failures when trying to mount a snapshot image that has been created
when the filesystem has not been correctly frozen.

To fix this, we need to move th metadata flush to after we wait for all
current transactions to complete in teh second stage of the freeze. This
means that when we write the final log records, the log should be clean
and recovery should never occur on a snapshot image created from a frozen
filesystem.

SGI-PV: 959267
SGI-Modid: xfs-linux-melb:xfs-kern:28010a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>

1 files changed, 22 insertions, 4 deletions

diff --git a/fs/xfs/xfs_vfsops.c b/fs/xfs/xfs_vfsops.c
index f5ea74b999b6..c2a6eab7fa2d 100644
--- a/fs/xfs/xfs_vfsops.c
+++ b/fs/xfs/xfs_vfsops.c
@@ -872,6 +872,10 @@ xfs_statvfs(
  *                     this by simply making sure the log gets flushed
  *                     if SYNC_BDFLUSH is set, and by actually writing it
  *                     out otherwise.
+ *      SYNC_DIO_WAIT - The caller wants us to wait for all direct I/Os
+ *                     as well to ensure all data I/O completes before we
+ *                     return. Forms the drain side of the write barrier needed
+ *                     to safely quiesce the filesystem.
  *
  */
 /*ARGSUSED*/
@@ -883,10 +887,7 @@ xfs_sync(
 {
         xfs_mount_t     *mp = XFS_BHVTOM(bdp);
 
-        if (unlikely(flags == SYNC_QUIESCE))
-                return xfs_quiesce_fs(mp);
-        else
-                return xfs_syncsub(mp, flags, NULL);
+        return xfs_syncsub(mp, flags, NULL);
 }
 
 /*
@@ -1172,6 +1173,12 @@ xfs_sync_inodes(
                         }
 
                 }
+                /*
+                 * When freezing, we need to wait ensure direct I/O is complete
+                 * as well to ensure all data modification is complete here
+                 */
+                if (flags & SYNC_DIO_WAIT)
+                        vn_iowait(vp);
 
                 if (flags & SYNC_BDFLUSH) {
                         if ((flags & SYNC_ATTR) &&
@@ -1950,15 +1957,26 @@ xfs_showargs(
         return 0;
 }
 
+/*
+ * Second stage of a freeze. The data is already frozen, now we have to take
+ * care of the metadata. New transactions are already blocked, so we need to
+ * wait for any remaining transactions to drain out before proceding.
+ */
 STATIC void
 xfs_freeze(
         bhv_desc_t      *bdp)
 {
         xfs_mount_t     *mp = XFS_BHVTOM(bdp);
 
+        /* wait for all modifications to complete */
         while (atomic_read(&mp->m_active_trans) > 0)
                 delay(100);
 
+        /* flush inodes and push all remaining buffers out to disk */
+        xfs_quiesce_fs(mp);
+
+        BUG_ON(atomic_read(&mp->m_active_trans) > 0);
+
         /* Push the superblock and write an unmount record */
         xfs_log_unmount_write(mp);
         xfs_unmountfs_writesb(mp);