fs/remap_range: avoid spurious writeback on zero length request

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. While here, account for the case where
generic_remap_check_len() may also reduce length to zero.
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/remap_range.c

@@ -304,7 +304,7 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 	/* Check that we don't violate system file offset limits. */
 	ret = generic_remap_checks(file_in, pos_in, file_out, pos_out, len,
 			remap_flags);
-	if (ret)
+	if (ret || *len == 0)
 		return ret;
 
 	/* Wait for the completion of any pending IOs on both files */
@@ -328,9 +328,6 @@ __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);
@@ -348,7 +345,7 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 
 	ret = generic_remap_check_len(inode_in, inode_out, pos_out, len,
 			remap_flags);
-	if (ret)
+	if (ret || *len == 0)
 		return ret;
 
 	/* If can't alter the file contents, we're done. */