Re: [PATCH v1 1/3] mm: zswap: fix global shrinker memcg iteration

[Yosry Ahmed]

On Wed, Jun 12, 2024 at 7:36 PM Takero Funaki <flintglass@xxxxxxxxx> wrote:
>
> 2024年6月13日(木) 11:18 Yosry Ahmed <yosryahmed@xxxxxxxxxx>:
>
> > > The corrected version of the cleaner should be:
> > > ```c
> > > void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
> > > {
> > >         /* lock out zswap shrinker walking memcg tree */
> > >         spin_lock(&zswap_shrink_lock);
> > >         if (zswap_next_shrink == memcg) {
> > >                 do {
> > >                         zswap_next_shrink = mem_cgroup_iter(NULL,
> > >                                         zswap_next_shrink, NULL);
> > >                         spin_unlock(&zswap_shrink_lock);
> > >                         spin_lock(&zswap_shrink_lock);
> > >                         if (!zswap_next_shrink)
> > >                                 break;
> > >                 } while (!mem_cgroup_online(zswap_next_shrink));
> > >         }
> > >         spin_unlock(&zswap_shrink_lock);
> > > }
> > > ```
> >
> > Is the idea here to avoid moving the iterator to another offline memcg
> > that zswap_memcg_offline_cleanup() was already called for, to avoid
> > holding a ref on that memcg until the next run of zswap shrinking?
> >
> > If yes, I think it's probably worth doing. But why do we need to
> > release and reacquire the lock in the loop above?
>
> Yes, the existing cleaner might leave the offline, already-cleaned memcg.
>
> The reacquiring lock is to not loop inside the critical section.
> In shrink_worker of v0 patch, the loop was restarted on offline memcg
> without releasing the lock. Nhat pointed out that we should drop the
> lock after every mem_cgroup_iter() call. v1 was changed to reacquire
> once per iteration like the cleaner code above.

I am not sure how often we'll run into a situation where we'll be
holding the lock for too long tbh. It should be unlikely to keep
encountering offline memcgs for a long time.

Nhat, do you think this could cause a problem in practice?