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