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 14:34, Matthew Wilcox wrote:
> On Fri, Jun 10, 2022 at 01:47:02PM -0400, Kent Overstreet wrote:
>> 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++) {
>>
> 
> This is going to mark the folio as accessed multiple times if it's
> a multi-page folio.  How about this one?

I like that one - you can add my Reviewed-by

> 
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 5f227b5420d7..a30587f2e598 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2599,6 +2599,13 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
>   	return err;
>   }
>   
> +static inline bool pos_same_folio(loff_t pos1, loff_t pos2, struct folio *folio)
> +{
> +	unsigned int shift = folio_shift(folio);
> +
> +	return (pos1 >> shift == pos2 >> shift);
> +}
> +
>   /**
>    * filemap_read - Read data from the page cache.
>    * @iocb: The iocb to read.
> @@ -2670,11 +2677,11 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
>   		writably_mapped = mapping_writably_mapped(mapping);
>   
>   		/*
> -		 * When a sequential read accesses a page several times, only
> +		 * When a read accesses the same folio several times, only
>   		 * mark it as accessed the first time.
>   		 */
> -		if (iocb->ki_pos >> PAGE_SHIFT !=
> -		    ra->prev_pos >> PAGE_SHIFT)
> +		if (!pos_same_folio(iocb->ki_pos, ra->prev_pos - 1,
> +							fbatch.folios[0]))
>   			folio_mark_accessed(fbatch.folios[0]);
>   
>   		for (i = 0; i < folio_batch_count(&fbatch); i++) {