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

[Kent Overstreet <kent.overstreet@gmail.com>]

On 6/10/22 10:36, Matthew Wilcox wrote:
> On Fri, Jun 10, 2022 at 03:34:11PM +0100, Matthew Wilcox wrote:
>> 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?

So the idea was that if we're reading from a different _page_ that we 
read from previously, we should be marking it accessed. But there's an 
off by one error, it should have been

if (iocb->ki_pos >> PAGE_SHIFT != (ra->prev_pos - 1) >> PAGE_SHIFT)
	folio_mark_accessed(fbatch.folios[0])

It looks like this is what Yukai was arriving at too when he was saying 
ki_pos + copied - 1, this is just a cleaner way of writing it :)