xfs: delayed alloc blocks beyond EOF are valid after writeback

There is an assumption in the parts of XFS that flushing a dirty
file will make all the delayed allocation blocks disappear from an
inode. That is, that after calling xfs_flush_pages() then
ip->i_delayed_blks will be zero.

This is an invalid assumption as we may have specualtive
preallocation beyond EOF and they are recorded in
ip->i_delayed_blks. A flush of the dirty pages of an inode will not
change the state of these blocks beyond EOF, so a non-zero
deeelalloc block count after a flush is valid.

The bmap code has an invalid ASSERT() that needs to be removed, and
the swapext code has a bug in that while it swaps the data forks
around, it fails to swap the i_delayed_blks counter associated with
the fork and hence can get the block accounting wrong.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 7 insertions, 2 deletions

diff --git a/fs/xfs/xfs_bmap.c b/fs/xfs/xfs_bmap.c
index 08b179fa9e8f..4111cd3966c7 100644
--- a/fs/xfs/xfs_bmap.c
+++ b/fs/xfs/xfs_bmap.c
@@ -5471,8 +5471,13 @@ xfs_getbmap(
 			if (error)
 				goto out_unlock_iolock;
 		}
-
-		ASSERT(ip->i_delayed_blks == 0);
+		/*
+		 * even after flushing the inode, there can still be delalloc
+		 * blocks on the inode beyond EOF due to speculative
+		 * preallocation. These are not removed until the release
+		 * function is called or the inode is inactivated. Hence we
+		 * cannot assert here that ip->i_delayed_blks == 0.
+		 */
 	}
 
 	lock = xfs_ilock_map_shared(ip);