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References: <20240205110959.4021-1-ryncsn@gmail.com> <20240206022409.202536-1-sj@kernel.org>
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From: Kairui Song <ryncsn@gmail.com>
Date: Tue, 6 Feb 2024 11:13:01 +0800
Message-ID: <CAMgjq7BAOohAYpS3n3NFHcji-=oiydJJSO5oqerLqYJSVfCQ9w@mail.gmail.com>
Subject: Re: [PATCH] mm/swap: fix race condition in direct swapin path
To: SeongJae Park <sj@kernel.org>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, 
	"Huang, Ying" <ying.huang@intel.com>, Chris Li <chrisl@kernel.org>, 
	Minchan Kim <minchan@kernel.org>, Hugh Dickins <hughd@google.com>, 
	Johannes Weiner <hannes@cmpxchg.org>, Matthew Wilcox <willy@infradead.org>, Michal Hocko <mhocko@suse.com>, 
	Yosry Ahmed <yosryahmed@google.com>, David Hildenbrand <david@redhat.com>, Yu Zhao <yuzhao@google.com>, 
	linux-kernel@vger.kernel.org
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On Tue, Feb 6, 2024 at 10:24=E2=80=AFAM SeongJae Park <sj@kernel.org> wrote=
:
>
> Hi Kairui,
>
> On Mon,  5 Feb 2024 19:09:59 +0800 Kairui Song <ryncsn@gmail.com> wrote:
>
> [...]
> >  mm/memory.c   | 19 +++++++++++++++++++
> >  mm/swap.h     |  5 +++++
> >  mm/swapfile.c | 16 ++++++++++++++++
> >  3 files changed, 40 insertions(+)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 7e1f4849463a..fd7c55a292f1 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3867,6 +3867,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       if (!folio) {
> >               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> >                   __swap_count(entry) =3D=3D 1) {
> > +                     /*
> > +                      * With swap count =3D=3D 1, after we read the en=
try,
> > +                      * other threads could finish swapin first, free
> > +                      * the entry, then swapout the modified page usin=
g
> > +                      * the same entry. Now the content we just read i=
s
> > +                      * stalled, and it's undetectable as pte_same()
> > +                      * returns true due to entry reuse.
> > +                      *
> > +                      * So pin the swap entry using the cache flag eve=
n
> > +                      * cache is not used.
> > +                      */
> > +                     if (swapcache_prepare(entry))
> > +                             goto out;
> > +
>
> I'm getting below build error after this patch.  I guess maybe the code n=
eed to
> take care of CONFIG_SWAP unset case?
>
>     .../mm/memory.c: In function 'do_swap_page':
>     .../mm/memory.c:4004:8: error: implicit declaration of function 'swap=
cache_prepare'; did you mean 'swapcache_clear'? [-Werror=3Dimplicit-functio=
n-declaration]
>      4004 |    if (swapcache_prepare(entry))
>           |        ^~~~~~~~~~~~~~~~~
>           |        swapcache_clear
>
>

Ah, right. Thanks for the feedback.

For the CONFIG_SWAP unset case I added an empty function for
swapcache_clear, but the original swapcache_prepare also needs an
empty placeholder now. Will add that in V2.