[PATCH] xfs: clear delalloc and cache on buffered write failure

[Brian Foster <bfoster@redhat.com>]

commit fa7f138ac4c70dc00519c124cf7cd4862a0a5b0e upstream.

The buffered write failure handling code in
xfs_file_iomap_end_delalloc() has a couple minor problems. First, if
written == 0, start_fsb is not rounded down and it fails to kill off a
delalloc block if the start offset is block unaligned. This results in a
lingering delalloc block and broken delalloc block accounting detected
at unmount time. Fix this by rounding down start_fsb in the unlikely
event that written == 0.

Second, it is possible for a failed overwrite of a delalloc extent to
leave dirty pagecache around over a hole in the file. This is because is
possible to hit ->iomap_end() on write failure before the iomap code has
attempted to allocate pagecache, and thus has no need to clean it up. If
the targeted delalloc extent was successfully written by a previous
write, however, then it does still have dirty pages when ->iomap_end()
punches out the underlying blocks. This ultimately results in writeback
over a hole. To fix this problem, unconditionally punch out the
pagecache from XFS before the associated delalloc range.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
CC: stable@vger.kernel.org # v4.8+
---

This is for stable... it fixes a latent regression exposed by
d1908f52557b ("fs: break out of iomap_file_buffered_write on fatal
signals"), which is to be backported to 4.8+ stable kernels.

Brian

 fs/xfs/xfs_iomap.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 2af0dda..fe3562b 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1064,7 +1064,15 @@ xfs_file_iomap_end_delalloc(
 	xfs_fileoff_t		end_fsb;
 	int			error = 0;
 
-	start_fsb = XFS_B_TO_FSB(mp, offset + written);
+	/*
+	 * start_fsb refers to the first unused block after a short write. If
+	 * nothing was written, round offset down to point at the first block in
+	 * the range.
+	 */
+	if (unlikely(!written))
+		start_fsb = XFS_B_TO_FSBT(mp, offset);
+	else
+		start_fsb = XFS_B_TO_FSB(mp, offset + written);
 	end_fsb = XFS_B_TO_FSB(mp, offset + length);
 
 	/*
@@ -1076,6 +1084,9 @@ xfs_file_iomap_end_delalloc(
 	 * blocks in the range, they are ours.
 	 */
 	if (start_fsb < end_fsb) {
+		truncate_pagecache_range(VFS_I(ip), XFS_FSB_TO_B(mp, start_fsb),
+					 XFS_FSB_TO_B(mp, end_fsb) - 1);
+
 		xfs_ilock(ip, XFS_ILOCK_EXCL);
 		error = xfs_bmap_punch_delalloc_range(ip, start_fsb,
 					       end_fsb - start_fsb);
-- 
2.7.4