Re: [RFC PATCH] mm/filemap.c: fix the timing of asignment of prev_pos

[Matthew Wilcox <willy@infradead.org>]

On Wed, Aug 17, 2022 at 08:16:57AM -0700, Andrew Morton wrote:
> On Wed, 17 Aug 2022 21:51:57 +0800 Guixin Liu <kanie@linux.alibaba.com> wrote:
> 
> > The prev_pos should be assigned before the iocb->ki_pos is incremented,
> > so that the prev_pos is the exact location of the last visit.
> > 
> > Fixes: 06c0444290cec ("mm/filemap.c: generic_file_buffered_read() now
> > uses find_get_pages_contig")
> > Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
> > 
> > ---
> > Hi guys,
> >     When I`m running repetitive 4k read io which has same offset,
> > I find that access to folio_mark_accessed is inevitable in the
> > read process, the reason is that the prev_pos is assigned after the
> > iocb->ki_pos is incremented, so that the prev_pos is always not equal
> > to the position currently visited.
> >     Is this a bug that needs fixing?
> 
> It looks wrong to me and it does appear that 06c0444290cecf0 did this
> unintentionally.

That commit was the start of a problem, but I think I restored the
original behaviour in 5ccc944dce3d.  You were part of that discussion
back in June:
https://lore.kernel.org/linux-mm/20220602082129.2805890-1-yukuai3@huawei.com/