linux-kernel - Re: [PATCH] zswap: do not shrink if cgroup may not zswap

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Date:   Tue, 30 May 2023 16:37:23 -0700
From:   Nhat Pham <nphamcs@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     hannes@...xchg.org, cerasuolodomenico@...il.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        sjenning@...hat.com, ddstreet@...e.org, vitaly.wool@...sulko.com,
        yosryahmed@...gle.com, kernel-team@...a.com
Subject: Re: [PATCH] zswap: do not shrink if cgroup may not zswap

On Tue, May 30, 2023 at 3:30 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Tue, 30 May 2023 15:24:40 -0700 Nhat Pham <nphamcs@...il.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 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.
> >
> > 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.
>