Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <CAKEwX=PrqV+cCGmdTcFPMqP-48Y=abQTkSOUzJnOWWpc4rR4cA@mail.gmail.com>
 <20230530222440.2777700-1-nphamcs@gmail.com> <20230530153006.8b73c276b1aaf5c3275f0b3f@linux-foundation.org>
In-Reply-To: <20230530153006.8b73c276b1aaf5c3275f0b3f@linux-foundation.org>
From:   Nhat Pham <nphamcs@gmail.com>
Date:   Tue, 30 May 2023 16:37:23 -0700
Message-ID: <CAKEwX=OBtWfcYJkCTM0QE-P+u-AvzkSe+C9QqmLxdDMxSS_45Q@mail.gmail.com>
Subject: Re: [PATCH] zswap: do not shrink if cgroup may not zswap
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     hannes@cmpxchg.org, cerasuolodomenico@gmail.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        sjenning@redhat.com, ddstreet@ieee.org, vitaly.wool@konsulko.com,
        yosryahmed@google.com, kernel-team@meta.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Tue, May 30, 2023 at 3:30=E2=80=AFPM Andrew Morton <akpm@linux-foundatio=
n.org> wrote:
>
> On Tue, 30 May 2023 15:24:40 -0700 Nhat Pham <nphamcs@gmail.com> wrote:
>
> > Before storing a page, zswap first checks if the number of stored pages
> > exceeds the limit specified by memory.zswap.max, for each cgroup in the
> > hierarchy. If this limit is reached or exceeded, then zswap shrinking i=
s
> > triggered and short-circuits the store attempt.
> >
> > However, since the zswap's LRU is not memcg-aware, this can create the
> > following pathological behavior: the cgroup whose zswap limit is
> > reached will evict pages from other cgroups continually, without
> > lowering its own zswap usage. This means the shrinking will continue
> > until the need for swap ceases or the pool becomes empty.
> >
> > As a result of this, we observe a disproportionate amount of zswap
> > writeback and a perpetually small zswap pool in our experiments, even
> > though the pool limit is never hit.
>
> That sounds unpleasant.  Do you think the patch should be backported
> into earlier (-stable) kernels?

I think it should be, for any kernel version after f4840ccfca25.


>
> > This patch fixes the issue by rejecting zswap store attempt without
> > shrinking the pool when obj_cgroup_may_zswap() returns false.
>