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

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Message-Id: <20230530153006.8b73c276b1aaf5c3275f0b3f@linux-foundation.org>
Date:   Tue, 30 May 2023 15:30:06 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Nhat Pham <nphamcs@...il.com>
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, 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?

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