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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 30 May 2023 15:24:40 -0700 Nhat Pham <nphamcs@gmail.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.