Re: [PATCH v3 1/2] mm/zswap: Fix global shrinker when memory cgroup is disabled

[Yosry Ahmed]

On Wed, Jul 29, 2026 at 6:17 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 29 Jul 2026 17:30:56 -0700 Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
>
> > On Wed, Jul 29, 2026 at 3:58 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Wed, 29 Jul 2026 16:42:05 +0800 Hao Jia <jiahao.kernel@xxxxxxxxx> wrote:
> > >
> > > > Zswap writeback when the global pool limit is hit fails when memory
> > > > cgroup is disabled. The pool remains full until it is organically
> > > > drained by swapins or memory freeing, leading to zswap store failures
> > > > and pages bypassing getting written directly to the backing swap device,
> > > > causing LRU inversion (hotter pages with higher fault latency).
> > > >
> > > > This happens because mem_cgroup_iter() always returns NULL when
> > > > memory cgroups are disabled. As a result, the global shrinker
> > > > shrink_worker() repeatedly takes empty walks. After MAX_RECLAIM_RETRIES
> > > > failed attempts, the worker gives up without writing back any pages.
> > > >
> > > > Therefore, when memory cgroup is disabled, fall through with the !memcg
> > > > branch and shrink the root memcg directly.
> > > >
> > > > With memcg disabled, shrink_memcg() only returns -ENOENT when the root
> > > > LRU is empty, which means the total pages are already below thr. In the
> > > > absence of heavy concurrent zswap stores, the loop then safely bails out
> > > > via the zswap_total_pages() <= thr check; otherwise, it will resume
> > > > shrinking the memcg after processing the reschedule check. For any other
> > > > return value from shrink_memcg(), the loop is guaranteed to terminate,
> > > > either after MAX_RECLAIM_RETRIES failures or once the threshold is met.
> > > >
> > > > Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
> > > > Cc: stable@xxxxxxxxxxxxxxx
> > >
> > > How does this affect users?  What behavior do they observe when it
> > > occurs?
> >
> > I think the first paragraph sums it up pretty well, especially the
> > last sentence "hotter pages with higher fault latency".
>
> How do users observe that?
>
> See, what I'm looking for here is an explanation for why we're
> proposing a backport.  How are our users harmed by the current code and
> how does this change benefit them?

It's a potential performance regression when using zswap with memcg
disabled. Sorry I just realized you were looking for a high level
description not detailed symptoms :)