[PATCH] xfs: fix off-by-one-block in xfs_discard_folio()

[Dave Chinner <david@fromorbit.com>]

From: Dave Chinner <dchinner@redhat.com>

The recent writeback corruption fixes changed the code in
xfs_discard_folio() to calculate a byte range to for punching
delalloc extents. A mistake was made in using round_up(pos) for the
end offset, because when pos points at the first byte of a block, it
does not get rounded up to point to the end byte of the block. hence
the punch range is short, and this leads to unexpected behaviour in
certain cases in xfs_bmap_punch_delalloc_range.

e.g. pos = 0 means we call xfs_bmap_punch_delalloc_range(0,0), so
there is no previous extent and it rounds up the punch to the end of
the delalloc extent it found at offset 0, not the end of the range
given to xfs_bmap_punch_delalloc_range().

Fix this by handling the zero block offset case correctly.

Fixes: 7348b322332d ("xfs: xfs_bmap_punch_delalloc_range() should take a byte range")
Reported-by: Pengfei Xu <pengfei.xu@intel.com>
Found-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 fs/xfs/xfs_aops.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 41734202796f..429f63cfd7d4 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -466,6 +466,7 @@ xfs_discard_folio(
 {
 	struct xfs_inode	*ip = XFS_I(folio->mapping->host);
 	struct xfs_mount	*mp = ip->i_mount;
+	xfs_off_t		end_off;
 	int			error;
 
 	if (xfs_is_shutdown(mp))
@@ -475,8 +476,17 @@ xfs_discard_folio(
 		"page discard on page "PTR_FMT", inode 0x%llx, pos %llu.",
 			folio, ip->i_ino, pos);
 
-	error = xfs_bmap_punch_delalloc_range(ip, pos,
-			round_up(pos, folio_size(folio)));
+	/*
+	 * Need to be careful with the case where the pos passed in points to
+	 * the first byte of the folio - rounding up won't change the value,
+	 * but in all cases here we need to end offset to point to the start
+	 * of the next folio.
+	 */
+	if (pos == folio_pos(folio))
+		end_off = pos + folio_size(folio);
+	else
+		end_off = round_up(pos, folio_size(folio));
+	error = xfs_bmap_punch_delalloc_range(ip, pos, end_off);
 
 	if (error && !xfs_is_shutdown(mp))
 		xfs_alert(mp, "page discard unable to remove delalloc mapping.");
-- 
2.39.2