[PATCH] ext2: preserve completed DIO bytes when buffered fallback fails

[Hungyu Lin <dennylin0707@gmail.com>]

ext2_dio_write_iter() can complete part of a direct write before
falling back to buffered I/O for a hole inside i_size. If
generic_perform_write() then fails, its error overwrites the completed
direct-I/O byte count.

For example, on a full filesystem, an 8 KiB write to a file with an
allocated 4 KiB prefix and a 4 KiB hole overwrites the prefix but returns
ENOSPC and leaves the file position unchanged. GNU dd with oflag=direct
consequently reports zero bytes written.

Preserve a positive direct-I/O result and return the buffered error
when no bytes were direct-written. This restores the fallback policy of
generic_file_write_iter(), which ext2 used before its iomap conversion.

Fixes: fb5de4358e1a ("ext2: Move direct-io to use iomap")
Cc: stable@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
 fs/ext2/file.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/ext2/file.c b/fs/ext2/file.c
index 67fe423c3828..8ddc149d4884 100644
--- a/fs/ext2/file.c
+++ b/fs/ext2/file.c
@@ -145,7 +145,8 @@ static ssize_t ext2_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		pos = iocb->ki_pos;
 		status = generic_perform_write(iocb, from);
 		if (unlikely(status < 0)) {
-			ret = status;
+			if (!ret)
+				ret = status;
 			goto out_unlock;
 		}
 
-- 
2.43.0