linux-kernel - Re: [PATCH 04/24] mm/swap: avoid setting page lock bit and doing extra unlock check

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Message-ID: <CAMgjq7D_OJiaNPWquFqxk5DKZXqoUMrD7Y_CRM7D0RnRnHmW7Q@mail.gmail.com>
Date:   Mon, 20 Nov 2023 19:15:25 +0800
From:   Kairui Song <ryncsn@...il.com>
To:     Chris Li <chrisl@...nel.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Huang, Ying" <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/24] mm/swap: avoid setting page lock bit and doing
 extra unlock check

Chris Li <chrisl@...nel.org> 于2023年11月20日周一 12:18写道：
>
> On Sun, Nov 19, 2023 at 11:48 AM Kairui Song <ryncsn@...il.com> wrote:
> >
> > From: Kairui Song <kasong@...cent.com>
> >
> > When swapping in a page, mem_cgroup_swapin_charge_folio is called for new
> > allocated folio, nothing else is referencing the folio so no need to set
> > the lock bit. This avoided doing unlock check on error path.
> >
> > Signed-off-by: Kairui Song <kasong@...cent.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 entry, 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 = 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_mask, 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 observed it.
> > @@ -483,13 +485,9 @@ struct page *__read_swap_cache_async(swp_entry_t entry, 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