[PATCH] fs/remap_range: avoid spurious writeback on zero length request

[Brian Foster <bfoster@redhat.com>]

generic_remap_checks() can reduce the effective request length (i.e.,
after the reflink extend to EOF case is handled) down to zero. If this
occurs, __generic_remap_file_range_prep() proceeds through dio
serialization, file mapping flush calls, and may invoke file_modified()
before returning back to the filesystem caller, all of which immediately
check for len == 0 and return.

While this is mostly harmless, it is spurious and not completely
without side effect. A filemap write call can submit I/O (but not
wait on it) when the specified end byte precedes the start but
happens to land on the same aligned page boundary, which can occur
from __generic_remap_file_range_prep() when len is 0.

The dedupe path already has a len == 0 check to break out before doing
range comparisons. Lift this check a bit earlier in the function to
cover the general case of len == 0 and avoid the unnecessary work.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/remap_range.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/remap_range.c b/fs/remap_range.c
index 654912d06862..32ea992f9acc 100644
--- a/fs/remap_range.c
+++ b/fs/remap_range.c
@@ -306,6 +306,8 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 			remap_flags);
 	if (ret)
 		return ret;
+	if (*len == 0)
+		return 0;
 
 	/* Wait for the completion of any pending IOs on both files */
 	inode_dio_wait(inode_in);
@@ -328,9 +330,6 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 	if (remap_flags & REMAP_FILE_DEDUP) {
 		bool		is_same = false;
 
-		if (*len == 0)
-			return 0;
-
 		if (!IS_DAX(inode_in))
 			ret = vfs_dedupe_file_range_compare(file_in, pos_in,
 					file_out, pos_out, *len, &is_same);
-- 
2.37.3