xfs: don't allocate into the data fork for an unshare request

For an unshare request, we only have to take action if the data fork has
a shared mapping.  We don't care if someone else set up a cow operation.
If we find nothing in the data fork, return a hole to avoid allocating
space.

Note that fallocate will replace the delalloc reservation with an
unwritten extent anyway, so this has no user-visible effects outside of
avoiding unnecessary updates.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 285885c308bd7db943ca58357305a52a70cdcb6f..18c8f168b1532d043b76d201fff199a936c0cf0d 100644 (file)
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1006,8 +1006,9 @@ xfs_buffered_write_iomap_begin(
        if (eof)
                imap.br_startoff = end_fsb; /* fake hole until the end */
 
-       /* We never need to allocate blocks for zeroing a hole. */
-       if ((flags & IOMAP_ZERO) && imap.br_startoff > offset_fsb) {
+       /* We never need to allocate blocks for zeroing or unsharing a hole. */
+       if ((flags & (IOMAP_UNSHARE | IOMAP_ZERO)) &&
+           imap.br_startoff > offset_fsb) {
                xfs_hole_to_iomap(ip, iomap, offset_fsb, imap.br_startoff);
                goto out_unlock;
        }