[PATCH] fuse: skip destination updates for zero-byte copy

__fuse_copy_file_range() may receive a successful copy reply with zero
bytes copied.  This can happen even though the VFS only calls the file
operation with a non-zero length, for example if the FUSE daemon reaches
EOF while the kernel has stale source size information.

In that case no data was written to the destination.  Do not invalidate
the destination page cache or update the destination timestamps and cached
size state for a zero-byte copy result.

Fixes: 88bc7d5097a1 ("fuse: add support for copy_file_range()")
Signed-off-by: Yichong Chen <[email protected]>
---
 fs/fuse/file.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index ceada75310b8..75eb97b26912 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -3040,6 +3040,10 @@ static ssize_t __fuse_copy_file_range(struct file *file_in, loff_t pos_in,
 		err = -EIO;
 		goto out;
 	}
+	if (!bytes_copied) {
+		err = 0;
+		goto out;
+	}
 
 	truncate_inode_pages_range(inode_out->i_mapping,
 				   ALIGN_DOWN(pos_out, PAGE_SIZE),
-- 
2.51.0