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Tue, 30 May 2023 15:37:31 -0700 (PDT) MIME-Version: 1.0 References: <20230530222440.2777700-1-nphamcs@gmail.com> In-Reply-To: <20230530222440.2777700-1-nphamcs@gmail.com> From: Yosry Ahmed Date: Tue, 30 May 2023 15:36:55 -0700 Message-ID: Subject: Re: [PATCH] zswap: do not shrink if cgroup may not zswap To: Nhat Pham Cc: akpm@linux-foundation.org, 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-Queue-Id: 8244C180013 X-Rspam-User: X-Stat-Signature: omg8mpwg5kye8msoj8rwya5rswjhcpbi X-Rspamd-Server: rspam03 X-HE-Tag: 1685486253-337510 X-HE-Meta: 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 i0eEjxWk RTZK+oruJVtGSchxdxPxwOSOQl96tSz/NcVi3WBvc3Oqx6+4p8p/8+15CvtA0K/QNELc/5XOdjf6cXT1/ZlBse3jYyeu89Gb59Kcw1KfaetXI6Xii22I9dNXzH/V8hQWi75wYcEvahc/m0TRmfpZhbHzWzLAJiOtcec91iU7rOc3WPYnfsmqqxOw4QcP9xbPn2GAryp0d2f+x3W6Pxdg3GC9Ffna5vVXT5H+Y0HAS3vRGw/ecfMGYU9zIFZdpX5N26eU+n2/ucE0lQtJG+vCGtgRaRQ== 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 Tue, May 30, 2023 at 3:24=E2=80=AFPM Nhat Pham 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 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 > 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. This pathological behavior will only happen if the zswap limit is 0. Otherwise, we will see a different pathological behavior where we unnecessarily evict X pages from other cgroups before we drive the memcg back below its limit. Perhaps we should clarify this? > > 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. I am guessing this is also related to the case where the limit is 0. It would be useful to clarify this. > > This patch fixes the issue by rejecting zswap store attempt without > shrinking the pool when obj_cgroup_may_zswap() returns false. > > Fixes: f4840ccfca25 ("zswap: memcg accounting") > Signed-off-by: Nhat Pham > --- > mm/zswap.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/mm/zswap.c b/mm/zswap.c > index 59da2a415fbb..cff93643a6ab 100644 > --- a/mm/zswap.c > +++ b/mm/zswap.c > @@ -1174,9 +1174,14 @@ static int zswap_frontswap_store(unsigned type, pg= off_t offset, > 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; > + goto reject; > > /* reclaim space if needed */ > if (zswap_is_full()) { > -- > 2.34.1 > With commit log nits above: Reviewed-by: Yosry Ahmed