Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.31 as permitted sender) client-ip=23.128.96.31;
MIME-Version: 1.0
References: <20231127234600.2971029-1-nphamcs@gmail.com> <20231127234600.2971029-4-nphamcs@gmail.com>
 <20231129151721.GC135852@cmpxchg.org>
In-Reply-To: <20231129151721.GC135852@cmpxchg.org>
From:   Nhat Pham <nphamcs@gmail.com>
Date:   Wed, 29 Nov 2023 16:21:26 -0800
Message-ID: <CAKEwX=M=iFGS6PQyF7FiV2JDhN0uLzSiJ3TK30nGiV1mM1wZ+A@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
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Wed, Nov 29, 2023 at 7:17=E2=80=AFAM Johannes Weiner <hannes@cmpxchg.org=
> 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, N=
ULL, 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 ca=
se the
> > +              * original reference is dropped by the zswap offlining c=
allback.
> > +              */
> > +             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.

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

Thanks for the review, Johannes!