Re: [PATCH -next] mm/filemap: fix that first page is not mark accessed in filemap_read()

[Yu Kuai <yukuai3@huawei.com>]

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.

> 
> For consequence,
>> folio_mark_accessed() will not be called for 'fbatch.folios[0]' since
>> 'iocb->ki_pos' is always equal to 'ra->prev_pos'.
> 
> I don't follow this, but maybe I'm just being slow.

I mssing a condition here:

Under small io sequential read, folio_mark_accessed() will not be called
for 'fbatch.folios[0]' since 'iocb->ki_pos' is always equal to
'ra->prev_pos'.

Thanks,
Kuai
> 
> .
>