xfs: trim eofblocks before collapse range

xfs_collapse_file_space() currently writes back the entire file
undergoing collapse range to settle things down for the extent shift
algorithm. While this prevents changes to the extent list during the
collapse operation, the writeback itself is not enough to prevent
unnecessary collapse failures.

The current shift algorithm uses the extent index to iterate the in-core
extent list. If a post-eof delalloc extent persists after the writeback
(e.g., a prior zero range op where the end of the range aligns with eof
can separate the post-eof blocks such that they are not written back and
converted), xfs_bmap_shift_extents() becomes confused over the encoded
br_startblock value and fails the collapse.

As with the full writeback, this is a temporary fix until the algorithm
is improved to cope with a volatile extent list and avoid attempts to
shift post-eof extents.
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_bmap_util.c

@@ -1471,8 +1471,10 @@ xfs_collapse_file_space(
 	shift_fsb = XFS_B_TO_FSB(mp, len);
 
 	/*
-	 * writeback the entire file to prevent concurrent writeback of ranges
-	 * outside the collapsing region from changing the extent list.
+	 * Writeback the entire file and force remove any post-eof blocks. The
+	 * writeback prevents changes to the extent list via concurrent
+	 * writeback and the eofblocks trim prevents the extent shift algorithm
+	 * from running into a post-eof delalloc extent.
 	 *
 	 * XXX: This is a temporary fix until the extent shift loop below is
 	 * converted to use offsets and lookups within the ILOCK rather than
@@ -1482,6 +1484,11 @@ xfs_collapse_file_space(
 	error = filemap_write_and_wait(VFS_I(ip)->i_mapping);
 	if (error)
 		return error;
+	if (xfs_can_free_eofblocks(ip, true)) {
+		error = xfs_free_eofblocks(mp, ip, false);
+		if (error)
+			return error;
+	}
 
 	error = xfs_free_file_space(ip, offset, len);
 	if (error)