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Date:   Wed, 27 Sep 2023 16:51:53 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Nhat Pham <nphamcs@gmail.com>, akpm@linux-foundation.org,
        cerasuolodomenico@gmail.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, linux-mm@kvack.org, kernel-team@meta.com,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Chris Li <chrisl@kernel.org>
Subject: Re: [PATCH v2 1/2] zswap: make shrinking memcg-aware
Message-ID: <20230927205153.GB399644@cmpxchg.org>
References: <20230919171447.2712746-1-nphamcs@gmail.com>
 <20230919171447.2712746-2-nphamcs@gmail.com>
 <CAJD7tkZqm9ZsAL0triwJPLYuN02jMMS-5Y8DE7TuDJVnOCm_7Q@mail.gmail.com>
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On Mon, Sep 25, 2023 at 01:17:04PM -0700, Yosry Ahmed wrote:
> On Tue, Sep 19, 2023 at 10:14 AM Nhat Pham <nphamcs@gmail.com> wrote:
> > +                       is_empty = false;
> > +       }
> > +       zswap_pool_put(pool);
> > +
> > +       if (is_empty)
> > +               return -EINVAL;
> > +       if (shrunk)
> > +               return 0;
> > +       return -EAGAIN;
> >  }
> >
> >  static void shrink_worker(struct work_struct *w)
> >  {
> >         struct zswap_pool *pool = container_of(w, typeof(*pool),
> >                                                 shrink_work);
> > -       int ret, failures = 0;
> > +       int ret, failures = 0, memcg_selection_failures = 0;
> >
> > +       /* global reclaim will select cgroup in a round-robin fashion. */
> >         do {
> > -               ret = zswap_reclaim_entry(pool);
> > +               /* previous next_shrink has become a zombie - restart from the top */
> 
> Do we skip zombies because all zswap entries are reparented with the objcg?
> 
> If yes, why do we restart from the top instead of just skipping them?
> memcgs after a zombie will not be reachable now IIUC.
> 
> Also, why explicitly check for zombies instead of having
> shrink_memcg() just skip memcgs with no zswap entries? The logic is
> slightly complicated.

I think this might actually be a leftover from the initial plan to do
partial walks without holding on to a reference to the last scanned
css. Similar to mem_cgroup_iter() does with the reclaim cookie - if a
dead cgroup is encountered and we lose the tree position, restart.

But now the code actually holds a reference, so I agree the zombie
thing should just be removed.