[PATCH 4/4] xfs: don't allocate into the data fork for an unshare request

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From: Darrick J. Wong <djwong@xxxxxxxxxx>

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@xxxxxxxxxx>
Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 fs/xfs/xfs_iomap.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)


diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 285885c308bd..18c8f168b153 100644
--- 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;
 	}




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