Re: [PATCH 9/44] new iov_iter flavour - ITER_UBUF

[Alexander Gordeev <agordeev@linux.ibm.com>]

On Fri, Jul 29, 2022 at 06:21:23PM +0100, Al Viro wrote:
> > Hi Al,
> > 
> > This changes causes sendfile09 LTP testcase fail in linux-next
> > (up to next-20220727) on s390. In fact, not this change exactly,
> > but rather 92d4d18eecb9 ("new iov_iter flavour - ITER_UBUF") -
> > which differs from what is posted here.
> > 
> > AFAICT page_cache_pipe_buf_confirm() encounters !PageUptodate()
> > and !page->mapping page and returns -ENODATA.
> > 
> > I am going to narrow the testcase and get more details, but please
> > let me know if I am missing something.
> 
> Grrr....
> 
> -               } else if (iter_is_iovec(to)) {
> +               } else if (!user_backed_iter(to)) {
> 
> in mm/shmem.c.  Spot the typo...
> 
> Could you check if replacing that line with
> 		} else if (user_backed_iter(to)) {
> 
> fixes the breakage?

Yes, it does! So just to be sure - this is the fix:

diff --git a/mm/shmem.c b/mm/shmem.c
index 8baf26eda989..5783f11351bb 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2626,7 +2626,7 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 			ret = copy_page_to_iter(page, offset, nr, to);
 			put_page(page);
 
-		} else if (!user_backed_iter(to)) {
+		} else if (user_backed_iter(to)) {
 			/*
 			 * Copy to user tends to be so well optimized, but
 			 * clear_user() not so much, that it is noticeably

Thanks!