linux-kernel - Re: [PATCH v7 3/6] zswap: make shrinking memcg-aware

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Date:   Wed, 29 Nov 2023 16:21:26 -0800
From:   Nhat Pham <nphamcs@...il.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     akpm@...ux-foundation.org, cerasuolodomenico@...il.com,
        yosryahmed@...gle.com, sjenning@...hat.com, ddstreet@...e.org,
        vitaly.wool@...sulko.com, mhocko@...nel.org,
        roman.gushchin@...ux.dev, shakeelb@...gle.com,
        muchun.song@...ux.dev, chrisl@...nel.org, linux-mm@...ck.org,
        kernel-team@...a.com, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org, shuah@...nel.org
Subject: Re: [PATCH v7 3/6] zswap: make shrinking memcg-aware

On Wed, Nov 29, 2023 at 7:17 AM Johannes Weiner <hannes@...xchg.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 = container_of(w, typeof(*pool),
> >                                               shrink_work);
> > +     struct mem_cgroup *memcg;
> >       int ret, failures = 0;
> >
> > +     /* global reclaim will select cgroup in a round-robin fashion. */
> >       do {
> > -             ret = zswap_reclaim_entry(pool);
> > -             if (ret) {
> > -                     zswap_reject_reclaim_fail++;
> > -                     if (ret != -EAGAIN)
> > -                             break;
> > +             spin_lock(&zswap_pools_lock);
> > +             memcg = pool->next_shrink =
> > +                     mem_cgroup_iter_online(NULL, pool->next_shrink, NULL, true);
> > +
> > +             /* full round trip */
> > +             if (!memcg) {
> > +                     spin_unlock(&zswap_pools_lock);
> >                       if (++failures == 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.

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

Thanks for the review, Johannes!