On Wed, Jun 7, 2023 at 12:09 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > It's unclear (to me) whether we should proceed with this. Thoughts, please? > > Here's what I presently have in mm-hotfixes-unstable: > > > From: Nhat Pham <nphamcs@xxxxxxxxx> > Subject: zswap: do not shrink if cgroup may not zswap > Date: Tue, 30 May 2023 15:24:40 -0700 > > 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 0 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. > > More generally, a cgroup might unnecessarily evict pages from other > cgroups before we drive the memcg back below its limit. > > This patch fixes the issue by rejecting zswap store attempt without > shrinking the pool when obj_cgroup_may_zswap() returns false. > > [akpm@xxxxxxxxxxxxxxxxxxxx: fix return of unintialized value] > [akpm@xxxxxxxxxxxxxxxxxxxx: s/ENOSPC/ENOMEM/] > Link: https://lkml.kernel.org/r/20230530222440.2777700-1-nphamcs@xxxxxxxxx > Link: https://lkml.kernel.org/r/20230530232435.3097106-1-nphamcs@xxxxxxxxx > Fixes: f4840ccfca25 ("zswap: memcg accounting") > Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx> > Cc: Dan Streetman <ddstreet@xxxxxxxx> > Cc: Domenico Cerasuolo <cerasuolodomenico@xxxxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Seth Jennings <sjenning@xxxxxxxxxx> > Cc: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx> > Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > --- > > mm/zswap.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > --- a/mm/zswap.c~zswap-do-not-shrink-if-cgroup-may-not-zswap > +++ a/mm/zswap.c > @@ -1174,9 +1174,16 @@ static int zswap_frontswap_store(unsigne > goto reject; > } > > + /* > + * XXX: zswap reclaim does not work with cgroups yet. Without a > + * cgroup-aware entry LRU, we will push out entries system-wide based on > + * local cgroup limits. > + */ > objcg = get_obj_cgroup_from_page(page); > - if (objcg && !obj_cgroup_may_zswap(objcg)) > - goto shrink; > + if (objcg && !obj_cgroup_may_zswap(objcg)) { > + ret = -ENOMEM; > + goto reject; > + } > > /* reclaim space if needed */ > if (zswap_is_full()) { > _ > Apologies for lack of clarity - yep this is the final version I had in mind too.
