xfs: track collapse via file offset rather than extent index

The collapse range implementation uses a transaction per extent shift.
The progress of the overall operation is tracked via the current extent
index of the in-core extent list. This is racy because the ilock must be
dropped and reacquired for each transaction according to locking and log
reservation rules. Therefore, writeback to prior regions of the file is
possible and can change the extent count. This changes the extent to
which the current index refers and causes the collapse to fail mid
operation. To avoid this problem, the entire file is currently written
back before the collapse operation starts.

To eliminate the need to flush the entire file, use the file offset
(fsb) to track the progress of the overall extent shift operation rather
than the extent index. Modify xfs_bmap_shift_extents() to
unconditionally convert the start_fsb parameter to an extent index and
return the file offset of the extent where the shift left off, if
further extents exist. The bulk of ths function can remain based on
extent index as ilock is held by the caller. xfs_collapse_file_space()
now uses the fsb output as the starting point for the subsequent shift.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

1 files changed, 6 insertions, 6 deletions

diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
index 1707980f9a4b..1e96d778bb9e 100644
--- a/fs/xfs/xfs_bmap_util.c
+++ b/fs/xfs/xfs_bmap_util.c
@@ -1456,18 +1456,18 @@ xfs_collapse_file_space(
 	struct xfs_mount	*mp = ip->i_mount;
 	struct xfs_trans	*tp;
 	int			error;
-	xfs_extnum_t		current_ext = 0;
 	struct xfs_bmap_free	free_list;
 	xfs_fsblock_t		first_block;
 	int			committed;
 	xfs_fileoff_t		start_fsb;
+	xfs_fileoff_t		next_fsb;
 	xfs_fileoff_t		shift_fsb;
 
 	ASSERT(xfs_isilocked(ip, XFS_IOLOCK_EXCL));
 
 	trace_xfs_collapse_file_space(ip);
 
-	start_fsb = XFS_B_TO_FSB(mp, offset + len);
+	next_fsb = XFS_B_TO_FSB(mp, offset + len);
 	shift_fsb = XFS_B_TO_FSB(mp, len);
 
 	/*
@@ -1525,10 +1525,10 @@ xfs_collapse_file_space(
 		 * We are using the write transaction in which max 2 bmbt
 		 * updates are allowed
 		 */
-		error = xfs_bmap_shift_extents(tp, ip, &done, start_fsb,
-					       shift_fsb, &current_ext,
-					       &first_block, &free_list,
-					       XFS_BMAP_MAX_SHIFT_EXTENTS);
+		start_fsb = next_fsb;
+		error = xfs_bmap_shift_extents(tp, ip, start_fsb, shift_fsb,
+				&done, &next_fsb, &first_block, &free_list,
+				XFS_BMAP_MAX_SHIFT_EXTENTS);
 		if (error)
 			goto out;