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

[Nhat Pham]

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