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Wed, 07 Jun 2023 12:17:46 -0700 (PDT) MIME-Version: 1.0 References: <20230530162153.836565-1-nphamcs@gmail.com> <20230607120939.c1256fac4bb4e081811c02da@linux-foundation.org> In-Reply-To: <20230607120939.c1256fac4bb4e081811c02da@linux-foundation.org> From: Yosry Ahmed Date: Wed, 7 Jun 2023 12:17:09 -0700 Message-ID: Subject: Re: [PATCH] zswap: do not shrink when memory.zswap.max is 0 To: Andrew Morton Cc: Nhat Pham , 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, kernel-team@meta.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 24A211C0008 X-Stat-Signature: xtjewybwn78pcmgw8cq8tqu18xswwe1p X-HE-Tag: 1686165467-14814 X-HE-Meta: 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 G1Fu1+l2 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Jun 7, 2023 at 12:09=E2=80=AFPM Andrew Morton wrote: > > It's unclear (to me) whether we should proceed with this. Thoughts, plea= se? This version looks good to me. I added my Reviewed-by on some version, but I guess it got lost with all the versions and fixlets :) > > Here's what I presently have in mm-hotfixes-unstable: > > > From: Nhat Pham > Subject: zswap: do not shrink if cgroup may not zswap > Date: Tue, 30 May 2023 15:24:40 -0700 > > 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 is > 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 0 will > evict pages from other cgroups continually, without lowering its own zswa= p > 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. > > More generally, a cgroup might unnecessarily evict pages from other > cgroups before we drive the memcg back below its limit. > > This patch fixes the issue by rejecting zswap store attempt without > shrinking the pool when obj_cgroup_may_zswap() returns false. > > [akpm@linux-foundation.org: fix return of unintialized value] > [akpm@linux-foundation.org: s/ENOSPC/ENOMEM/] > Link: https://lkml.kernel.org/r/20230530222440.2777700-1-nphamcs@gmail.co= m > Link: https://lkml.kernel.org/r/20230530232435.3097106-1-nphamcs@gmail.co= m > Fixes: f4840ccfca25 ("zswap: memcg accounting") > Signed-off-by: Nhat Pham Reviewed-by: Yosry Ahmed > Cc: Dan Streetman > Cc: Domenico Cerasuolo > Cc: Johannes Weiner > Cc: Seth Jennings > Cc: Vitaly Wool > Cc: Yosry Ahmed > Cc: > Signed-off-by: Andrew Morton > --- > > mm/zswap.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > --- a/mm/zswap.c~zswap-do-not-shrink-if-cgroup-may-not-zswap > +++ a/mm/zswap.c > @@ -1174,9 +1174,16 @@ static int zswap_frontswap_store(unsigne > goto reject; > } > > + /* > + * XXX: zswap reclaim does not work with cgroups yet. Without a > + * cgroup-aware entry LRU, we will push out entries system-wide b= ased on > + * local cgroup limits. > + */ > objcg =3D get_obj_cgroup_from_page(page); > - if (objcg && !obj_cgroup_may_zswap(objcg)) > - goto shrink; > + if (objcg && !obj_cgroup_may_zswap(objcg)) { > + ret =3D -ENOMEM; > + goto reject; > + } > > /* reclaim space if needed */ > if (zswap_is_full()) { > _ >