[PATCH RFC 1/2] iomap: prioritize iter.status error over ->iomap_end()

[Brian Foster <bfoster@redhat.com>]

Jan Kara reports that commit bc264fea0f6f subtly changed error
handling behavior in iomap_iter() in the case where both iter.status
and ->iomap_end() return error codes. Previously, iter.status had
priority and would return to the caller regardless of the
->iomap_end() result. After the change, an ->iomap_end() error
returns immediately.

This had the unexpected side effect of enabling a DIO fallback to
buffered write on ext4 because ->iomap_end() could return -ENOTBLK
and overload an -EINVAL error from the core iomap direct I/O code.

This has been fixed independently in ext4, but nonetheless the
change in iomap was unintentional. Since other filesystems may use
this in similar ways, restore long standing behavior and always
return the value of iter.status if it happens to contain an error
code.

Fixes: bc264fea0f6f ("iomap: support incremental iomap_iter advances")
Diagnosed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/iomap/iter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
index cef77ca0c20b..7cc4599b9c9b 100644
--- a/fs/iomap/iter.c
+++ b/fs/iomap/iter.c
@@ -80,7 +80,7 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
 				iomap_length_trim(iter, iter->iter_start_pos,
 						  olen),
 				advanced, iter->flags, &iter->iomap);
-		if (ret < 0 && !advanced)
+		if (ret < 0 && !advanced && !iter->status)
 			return ret;
 	}
 
-- 
2.51.0