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?
> 
> Oh, and if this is the right diagnosis, then this is the fix for the
> current tree:
> 
> +++ b/mm/filemap.c
> @@ -2673,8 +2673,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
>                   * When a sequential read accesses a page several times, only
>                   * mark it as accessed the first time.
>                   */
> -               if (iocb->ki_pos >> PAGE_SHIFT !=
> -                   ra->prev_pos >> PAGE_SHIFT)
> +               if (iocb->ki_pos != ra->prev_pos)
>                          folio_mark_accessed(fbatch.folios[0]);
> 
>                  for (i = 0; i < folio_batch_count(&fbatch); i++) {
> 
> 

I think this is the fix we want - I think Yu basically had the right 
idea and had the off by one fix, this should be clearer though:

Yu, can you confirm the fix?

-- >8 --
Subject: [PATCH] filemap: Fix off by one error when marking folios accessed

In filemap_read() we mark pages accessed as we read them - but we don't
want to do so redundantly, if the previous read already did so.

But there was an off by one error: we want to check if the current page
was the same as the last page we read from, but the last page we read
from was (ra->prev_pos - 1) >> PAGE_SHIFT.

Reported-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

diff --git a/mm/filemap.c b/mm/filemap.c
index 9daeaab360..8d5c8043cb 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2704,7 +2704,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct 
iov_iter *iter,
                  * mark it as accessed the first time.
                  */
                 if (iocb->ki_pos >> PAGE_SHIFT !=
-                   ra->prev_pos >> PAGE_SHIFT)
+                   (ra->prev_pos - 1) >> PAGE_SHIFT)
                         folio_mark_accessed(fbatch.folios[0]);

                 for (i = 0; i < folio_batch_count(&fbatch); i++) {