Re: [PATCH v2 2/2] mm/zswap: Support batch writeback in shrink_memcg()

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Jul 22, 2026 at 09:52:18PM -0700, Yosry Ahmed wrote:
> On Wed, Jul 22, 2026 at 7:27 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Fri, Jul 17, 2026 at 04:51:51PM +0800, Hao Jia wrote:
> > > @@ -1369,7 +1402,7 @@ static void shrink_worker(struct work_struct *w)
> > >                       goto resched;
> > >               }
> > >
> > > -             ret = shrink_memcg(memcg);
> > > +             ret = shrink_memcg(memcg, NR_ZSWAP_WB_BATCH);
> > >               /* drop the extra reference */
> > >               mem_cgroup_put(memcg);
> > >
> > > @@ -1493,7 +1526,7 @@ bool zswap_store(struct folio *folio)
> > >       objcg = get_obj_cgroup_from_folio(folio);
> > >       if (objcg && !obj_cgroup_may_zswap(objcg)) {
> > >               memcg = get_mem_cgroup_from_objcg(objcg);
> > > -             if (shrink_memcg(memcg)) {
> > > +             if (shrink_memcg(memcg, 1)) {
> >
> > Why 64 for the global limit but only 1 for the cgroup limit? That
> > seems arbitrary in multiple ways.
> 
> I suggested that we keep the writeback here without batching and do
> that change separately, mainly out of abundance of caution as
> writeback is done synchronously here so the extra latency could be
> problematic. I think we probably want to measure the performance
> impact of that separately.
> 
> That being said, this path is potentially too expensive anyway due to
> the flush, but I would rather we do some basic measurements before
> batching here.
> 
> What do you think?

It's not an unknown, right? We know this works for direct reclaimers,
cgroup limit reclaim e.g., and what the latency implications are.

Because of how reclaim works, we also know it'll call zswap_store() in
batches of SWAP_CLUSTER_MAX. If we don't batch here, they're likely to
each call shrink_memcg() once we're at the limit - while still risking
rejections due to compressibility differences.

My worry is that if we start with an inconsistency, we'll be stuck
with it for a long time.

I'd rather start with the clean, consistent version. Dial it back only
if we have data to justfiy the complication that we can put into a
comment and the changelog that outlines why exactly it's different.