ext4: disambiguate the return value of ext4_dio_write_end_io()

The commit 9156289 ("ext4: properly sync file size update after O_SYNC
direct IO") causes confusion about the meaning of the return value of
ext4_dio_write_end_io().

Specifically, when the ext4_handle_inode_extension() operation succeeds,
ext4_dio_write_end_io() directly returns count instead of 0.

This does not cause a bug in the current kernel, but the semantics of the
return value of the ext4_dio_write_end_io() function are wrong, which is
likely to introduce bugs in the future code evolution.

Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20240919082539.381626-1-alexjlzheng@tencent.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: fatherMatrix

fs/ext4/file.c

@@ -392,8 +392,9 @@ static int ext4_dio_write_end_io(struct kiocb *iocb, ssize_t size,
 	 */
 	if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize) &&
 	    pos + size <= i_size_read(inode))
-		return size;
-	return ext4_handle_inode_extension(inode, pos, size, size);
+		return 0;
+	error = ext4_handle_inode_extension(inode, pos, size, size);
+	return error < 0 ? error : 0;
 }
 
 static const struct iomap_dio_ops ext4_dio_write_ops = {