linux-kernel - Re: [PATCH -next] mm/filemap: fix that first page is not mark accessed in filemap

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Date:   Fri, 10 Jun 2022 15:34:11 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Yu Kuai <yukuai3@...wei.com>, kent.overstreet@...il.com
Cc:     akpm@...ux-foundation.org, axboe@...nel.dk,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH -next] mm/filemap: fix that first page is not mark
 accessed in filemap_read()

On Mon, Jun 06, 2022 at 09:10:03AM +0800, Yu Kuai wrote:
> On 2022/06/03 2:30, Matthew Wilcox wrote:
> > On Thu, Jun 02, 2022 at 04:21:29PM +0800, Yu Kuai wrote:
> > > In filemap_read(), 'ra->prev_pos' is set to 'iocb->ki_pos + copied',
> > > while it should be 'iocb->ki_ops'.
> > 
> > Can you walk me through your reasoning which leads you to believe that
> > it should be ki_pos instead of ki_pos + copied?  As I understand it,
> > prev_pos is the end of the previous read, not the beginning of the
> > previous read.
> 
> Hi, Matthew
> 
> The main reason is the following judgement in flemap_read():
> 
> if (iocb->ki_pos >> PAGE_SHIFT !=	-> current page
>     ra->prev_pos >> PAGE_SHIFT)		-> previous page
>         folio_mark_accessed(fbatch.folios[0]);
> 
> Which means if current page is the same as previous page, don't mark
> page accessed. However, prev_pos is set to 'ki_pos + copied' during last
> read, which will cause 'prev_pos >> PAGE_SHIFT' to be current page
> instead of previous page.
> 
> I was thinking that if prev_pos is set to the begining of the previous
> read, 'prev_pos >> PAGE_SHIFT' will be previous page as expected. Set to
> the end of previous read is ok, however, I think the caculation of
> previous page should be '(prev_pos - 1) >> PAGE_SHIFT' instead.

OK, I think Kent broke this in 723ef24b9b37 ("mm/filemap/c: break
generic_file_buffered_read up into multiple functions").  Before:

-       prev_index = ra->prev_pos >> PAGE_SHIFT;
-       prev_offset = ra->prev_pos & (PAGE_SIZE-1);
...
-               if (prev_index != index || offset != prev_offset)
-                       mark_page_accessed(page);

After:
+       if (iocb->ki_pos >> PAGE_SHIFT != ra->prev_pos >> PAGE_SHIFT)
+               mark_page_accessed(page);

So surely this should have been:

+       if (iocb->ki_pos != ra->prev_pos)
+               mark_page_accessed(page);

Kent, do you recall why you changed it the way you did?