linux-kernel - Re: [PATCH] mm/vmscan: drop checking if _deferred_list is empty before using TTU

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Date: Mon, 1 Jul 2024 21:35:41 +1200
From: Barry Song <21cnbao@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>, 
	David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH] mm/vmscan: drop checking if _deferred_list is empty
 before using TTU_SYNC

On Mon, Jul 1, 2024 at 8:48 PM Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 30/06/2024 00:41, Barry Song wrote:
> > From: Barry Song <v-songbaohua@...o.com>
> >
> > The optimization of list_empty(&folio->_deferred_list) aimed to prevent
> > increasing the PTL duration when a large folio is partially unmapped,
> > for example, from subpage 0 to subpage (nr - 2).
> >
> > But Ryan's commit 5ed890ce5147 ("mm: vmscan: avoid split during
> > shrink_folio_list()") actually splits this kind of large folios. This
> > makes the "optimization" useless.
> >
> > Signed-off-by: Barry Song <v-songbaohua@...o.com>
> > Cc: David Hildenbrand <david@...hat.com>
> > Cc: Ryan Roberts <ryan.roberts@....com>
>
> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
>
> > ---
> >  -v1
> >  * I remember David and Ryan once suggested that this check could be dropped
> >    while the patch was being pulled into mm-unstable. However, for some reason,
> >    I forgot to request squashing this change.
> >
> >  mm/vmscan.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 3d4c681c6d40..0761f91b407f 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1291,7 +1291,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >                        * try_to_unmap acquire PTL from the first PTE,
> >                        * eliminating the influence of temporary PTE values.
> >                        */
> > -                     if (folio_test_large(folio) && list_empty(&folio->_deferred_list))
> > +                     if (folio_test_large(folio))
>
> As it stands, the list_empty() technically needs a data_race() annotation. I
> think your original patch went into v6.10-rc1? If so, perhaps it makes sense to
> try to get this into the next rc?

Either way is fine. Hi Andrew, if you include it for the next RC, could you
please add this tag?

Fixes: 73bc32875ee9 ("mm: hold PTL from the first PTE while reclaiming
a large folio")

And an additional changelog:

Additionally, the list_empty() technically requires a data_race() annotation.

>
> >                               flags |= TTU_SYNC;
> >
> >                       try_to_unmap(folio, flags);
>

Thanks
Barry