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

[Kent Overstreet]

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@xxxxxxxxxx>
> > Signed-off-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> >
> > 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++) {