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From:   Kairui Song <ryncsn@gmail.com>
Date:   Mon, 20 Nov 2023 19:15:25 +0800
Message-ID: <CAMgjq7D_OJiaNPWquFqxk5DKZXqoUMrD7Y_CRM7D0RnRnHmW7Q@mail.gmail.com>
Subject: Re: [PATCH 04/24] mm/swap: avoid setting page lock bit and doing
 extra unlock check
To:     Chris Li <chrisl@kernel.org>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        "Huang, Ying" <ying.huang@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org
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Chris Li <chrisl@kernel.org> =E4=BA=8E2023=E5=B9=B411=E6=9C=8820=E6=97=A5=
=E5=91=A8=E4=B8=80 12:18=E5=86=99=E9=81=93=EF=BC=9A
>
> On Sun, Nov 19, 2023 at 11:48=E2=80=AFAM Kairui Song <ryncsn@gmail.com> w=
rote:
> >
> > From: Kairui Song <kasong@tencent.com>
> >
> > When swapping in a page, mem_cgroup_swapin_charge_folio is called for n=
ew
> > allocated folio, nothing else is referencing the folio so no need to se=
t
> > the lock bit. This avoided doing unlock check on error path.
> >
> > Signed-off-by: Kairui Song <kasong@tencent.com>
> > ---
> >  mm/swap_state.c | 20 +++++++++-----------
> >  1 file changed, 9 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index ac4fa404eaa7..45dd8b7c195d 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -458,6 +458,8 @@ struct page *__read_swap_cache_async(swp_entry_t en=
try, gfp_t gfp_mask,
>
> You move the mem_cgroup_swapin_charge_folio() inside the for loop:
>
>
>         for (;;) {
>                 int err;
>                 /*
>                  * First check the swap cache.  Since this is normally
>                  * called after swap_cache_get_folio() failed, re-calling
>                  * that would confuse statistics.
>                  */
>                 folio =3D filemap_get_folio(swap_address_space(entry),
>                                                 swp_offset(entry));
>
>
> >                                                 mpol, ilx, numa_node_id=
());
> >                 if (!folio)
> >                          goto fail_put_swap;
> > +               if (mem_cgroup_swapin_charge_folio(folio, NULL, gfp_mas=
k, entry))
> > +                       goto fail_put_folio;
>
> Wouldn't it cause repeat charging of the folio when it is racing
> against others in the for loop?

The race loser will call folio_put and discharge it?

>
> >
> >                 /*
> >                  * Swap entry may have been freed since our caller obse=
rved it.
> > @@ -483,13 +485,9 @@ struct page *__read_swap_cache_async(swp_entry_t e=
ntry, gfp_t gfp_mask,
> >         /*
> >          * The swap entry is ours to swap in. Prepare the new page.
> >          */
> > -
> >         __folio_set_locked(folio);
> >         __folio_set_swapbacked(folio);
> >
> > -       if (mem_cgroup_swapin_charge_folio(folio, NULL, gfp_mask, entry=
))
> > -               goto fail_unlock;
> > -
>
> The original code makes the charge outside of the for loop. Only the
> winner can charge once.

Right, this patch may make the charge/dis-charge path more complex for
race swapin, I'll re-check this part.

>
> Chris