linux-kernel - Re: [PATCH v2] mm/filemap: don't allow partially uptodate page for pipes

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Date:   Tue, 1 Nov 2016 16:37:27 +0100
From:   Jan Kara <jack@...e.cz>
To:     Eryu Guan <guaneryu@...il.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        viro@...iv.linux.org.uk, jack@...e.cz,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/filemap: don't allow partially uptodate page for
 pipes

On Tue 01-11-16 15:43:07, Eryu Guan wrote:
> Starting from 4.9-rc1 kernel, I started noticing some test failures
> of sendfile(2) and splice(2) (sendfile0N and splice01 from LTP) when
> testing on sub-page block size filesystems (tested both XFS and
> ext4), these syscalls start to return EIO in the tests. e.g.
> 
> sendfile02    1  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 26, got: -1
> sendfile02    2  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 24, got: -1
> sendfile02    3  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 22, got: -1
> sendfile02    4  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 20, got: -1
> 
> This is because that in sub-page block size cases, we don't need the
> whole page to be uptodate, only the part we care about is uptodate
> is OK (if fs has ->is_partially_uptodate defined). But
> page_cache_pipe_buf_confirm() doesn't have the ability to check the
> partially-uptodate case, it needs the whole page to be uptodate. So
> it returns EIO in this case.
> 
> This is a regression introduced by commit 82c156f85384 ("switch
> generic_file_splice_read() to use of ->read_iter()"). Prior to the
> change, generic_file_splice_read() doesn't allow partially-uptodate
> page either, so it worked fine.
> 
> Fix it by skipping the partially-uptodate check if we're working on
> a pipe in do_generic_file_read(), so we read the whole page from
> disk as long as the page is not uptodate.
> 
> Signed-off-by: Eryu Guan <guaneryu@...il.com>

The patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
> 
> I think the other way to fix it is to add the ability to check & allow
> partially-uptodate page to page_cache_pipe_buf_confirm(), but that is much
> harder to do and seems gain little.
> 
> v2:
> - Update summary a little bit
> - Update commit log
> - Add comment to the code
> - Add more people/list to cc
> 
> v1: http://marc.info/?l=linux-mm&m=147756897431777&w=2
> 
>  mm/filemap.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 849f459..670264d 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1734,6 +1734,9 @@ static ssize_t do_generic_file_read(struct file *filp, loff_t *ppos,
>  			if (inode->i_blkbits == PAGE_SHIFT ||
>  					!mapping->a_ops->is_partially_uptodate)
>  				goto page_not_up_to_date;
> +			/* pipes can't handle partially uptodate pages */
> +			if (unlikely(iter->type & ITER_PIPE))
> +				goto page_not_up_to_date;
>  			if (!trylock_page(page))
>  				goto page_not_up_to_date;
>  			/* Did it get truncated before we got the lock? */
> -- 
> 2.7.4
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR