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From:   Nhat Pham <nphamcs@gmail.com>
Date:   Wed, 29 Nov 2023 17:17:10 -0800
Message-ID: <CAKEwX=MPkFfVp6vupje0cjePa9Uxh3orPubiDkrtewtj3N=RXA@mail.gmail.com>
Subject: Re: [PATCH v7 3/6] zswap: make shrinking memcg-aware
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     akpm@linux-foundation.org, cerasuolodomenico@gmail.com,
        yosryahmed@google.com, sjenning@redhat.com, ddstreet@ieee.org,
        vitaly.wool@konsulko.com, mhocko@kernel.org,
        roman.gushchin@linux.dev, shakeelb@google.com,
        muchun.song@linux.dev, chrisl@kernel.org, linux-mm@kvack.org,
        kernel-team@meta.com, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kselftest@vger.kernel.org, shuah@kernel.org
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On Wed, Nov 29, 2023 at 4:21=E2=80=AFPM Nhat Pham <nphamcs@gmail.com> wrote=
:
>
> On Wed, Nov 29, 2023 at 7:17=E2=80=AFAM Johannes Weiner <hannes@cmpxchg.o=
rg> wrote:
> >
> > On Mon, Nov 27, 2023 at 03:45:57PM -0800, Nhat Pham wrote:
> > >  static void shrink_worker(struct work_struct *w)
> > >  {
> > >       struct zswap_pool *pool =3D container_of(w, typeof(*pool),
> > >                                               shrink_work);
> > > +     struct mem_cgroup *memcg;
> > >       int ret, failures =3D 0;
> > >
> > > +     /* global reclaim will select cgroup in a round-robin fashion. =
*/
> > >       do {
> > > -             ret =3D zswap_reclaim_entry(pool);
> > > -             if (ret) {
> > > -                     zswap_reject_reclaim_fail++;
> > > -                     if (ret !=3D -EAGAIN)
> > > -                             break;
> > > +             spin_lock(&zswap_pools_lock);
> > > +             memcg =3D pool->next_shrink =3D
> > > +                     mem_cgroup_iter_online(NULL, pool->next_shrink,=
 NULL, true);
> > > +
> > > +             /* full round trip */
> > > +             if (!memcg) {
> > > +                     spin_unlock(&zswap_pools_lock);
> > >                       if (++failures =3D=3D MAX_RECLAIM_RETRIES)
> > >                               break;
> > > +
> > > +                     goto resched;
> > >               }
> > > +
> > > +             /*
> > > +              * Acquire an extra reference to the iterated memcg in =
case the
> > > +              * original reference is dropped by the zswap offlining=
 callback.
> > > +              */
> > > +             css_get(&memcg->css);
> >
> > struct mem_cgroup isn't defined when !CONFIG_MEMCG. This needs a
> > mem_cgroup_get() wrapper and a dummy function for no-memcg builds.
>
> I got this exact same issue a couple of versions ago, but it was
> hidden behind another helper function which can be implemented as a
> no-op in the case of !CONFIG_MEMCG, so I forgot about it until now. It
> always strikes me a bit weird that we have mem_cgroup_put() but not an
> equivalent get - let me correct that.

Actually, I'll instead implement mem_cgroup_tryget_online(), as we
have to check for the cgroup's onlineness as well anyway! If it's
online, then keep the extra reference - all good. If it's not, then
drop the original reference before releasing the lock.


>
> >
> > With that fixed, though, everything else looks good to me:
> >
> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>
> Thanks for the review, Johannes!