linux-kernel - Re: [PATCH 2/9] mm, swap: always lock and check the swap cache folio before use

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Message-ID: <CAMgjq7AT79NfcKtsfgOi6LQJViXOz-Xs5LOUmOiJYJS-DwQg=A@mail.gmail.com>
Date: Tue, 2 Sep 2025 02:17:26 +0800
From: Kairui Song <ryncsn@...il.com>
To: Chris Li <chrisl@...nel.org>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>, Barry Song <baohua@...nel.org>, 
	Baoquan He <bhe@...hat.com>, Nhat Pham <nphamcs@...il.com>, 
	Kemeng Shi <shikemeng@...weicloud.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Ying Huang <ying.huang@...ux.alibaba.com>, Johannes Weiner <hannes@...xchg.org>, 
	David Hildenbrand <david@...hat.com>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/9] mm, swap: always lock and check the swap cache folio
 before use

On Sat, Aug 30, 2025 at 9:54 AM Chris Li <chrisl@...nel.org> wrote:
>
> On Wed, Aug 27, 2025 at 7:36 AM Kairui Song <ryncsn@...il.com> wrote:
> >
> > On Wed, Aug 27, 2025 at 4:21 PM Chris Li <chrisl@...nel.org> wrote:
> > >
> > > On Fri, Aug 22, 2025 at 12:21 PM Kairui Song <ryncsn@...il.com> wrote:
> > >
> > > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > > index e9d0d2784cd5..b4d39f2a1e0a 100644
> > > > --- a/mm/shmem.c
> > > > +++ b/mm/shmem.c
> > > > @@ -2379,8 +2379,6 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> > > >                         count_vm_event(PGMAJFAULT);
> > > >                         count_memcg_event_mm(fault_mm, PGMAJFAULT);
> > > >                 }
> > > > -       } else {
> > > > -               swap_update_readahead(folio, NULL, 0);
> > >
> > > Also this update readahead move to later might have a similar problem.
> > > All the bail out in the move will lose the readahead status update.
> > >
> > > The readahead deed is already done. Missing the status update seems
> > > incorrect.
> >
> > Thanks for the detailed review.
> >
> > The only change I wanted here is that swap readahead update should be
> > done after checking the folio still corresponds to the swap entry
> > triggering the swapin. That should have slight to none effect compared
> > to before considering the extremely tiny time window. We are only
> > following the convention more strictly.
> >
> > In theory it might even help to reduce false updates: if the folio no
> > longer corresponds to the swap entry, we are hitting an unrelated
> > folio, doing a readahead update will either mislead vma readahead's
> > address hint, or could clean up the readahead flag of an unrelated
> > folio without actually using it. If the folio does get hit in the
> > future, due to the missing readahead flag, the statistic will go
> > wrong.
>
> So the missing readahead stats update behavior is the correct and
> better behavior. I suggest you spit that out as a separate patch with
> appropriate comments about it too. It is also easier to bisect the
> commit if that kind of the subtle change which is considered safe
> turns out causing a problem. Causing problem not happen very often but
> it does happen before.
>

Hmm, after a second thought, maybe we should keep it as it is for now.

I just realized moving the swap_update_readahead after folio_lock is
more than just ensuring the folio is still valid. It will also cause
every swapin to do a readahead update. Previously, only cache hit
swapin will do a swap readahead update.

I did some tests, and didn't see any measurable performance difference
between putting it before / after the folio_lock. But changing it for
no good reason seems not a good idea after all.

So I think I'll keep it before the folio_lock. There is no evidence of
which strategy is better, just keep the current behaviour.

Calling swap_update_readahead even if the swap cache folio
is already invalidated is not really harmful, the only thing it does
that may effect the folio is the folio_test_clear_readahead call in
it, and we have been doing for years with no problem. Calling
swap_update_readahead for every folio might not be a good idea.