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Wed, 07 Jun 2023 12:31:38 -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: Nhat Pham Date: Wed, 7 Jun 2023 12:31:27 -0700 Message-ID: Subject: Re: [PATCH] zswap: do not shrink when memory.zswap.max is 0 To: Andrew Morton Cc: Yosry Ahmed , 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-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: C6320140005 X-Stat-Signature: p5i77jjcxnberuh5k3yczu1gis9f5snb X-Rspam-User: X-HE-Tag: 1686166299-698682 X-HE-Meta: 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 50GR498g 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000013, 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? > > 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 > 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()) { > _ > Apologies for lack of clarity - yep this is the final version I had in mind too.