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

[Kent Overstreet]

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 :)