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From: Takero Funaki <flintglass@gmail.com>
Date: Thu, 13 Jun 2024 11:35:56 +0900
Message-ID: <CAPpoddcp9rVvg77WapsuiMdMzFrV0UioJ+VbQuJbKNY7-=nvVw@mail.gmail.com>
Subject: Re: [PATCH v1 1/3] mm: zswap: fix global shrinker memcg iteration
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Nhat Pham <nphamcs@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, 
	Chengming Zhou <chengming.zhou@linux.dev>, Jonathan Corbet <corbet@lwn.net>, 
	Andrew Morton <akpm@linux-foundation.org>, 
	Domenico Cerasuolo <cerasuolodomenico@gmail.com>, linux-mm@kvack.org, linux-doc@vger.kernel.org, 
	linux-kernel@vger.kernel.org, Shakeel Butt <shakeel.butt@linux.dev>
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2024=E5=B9=B46=E6=9C=8813=E6=97=A5(=E6=9C=A8) 11:18 Yosry Ahmed <yosryahmed=
@google.com>:

> > 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 =3D=3D memcg) {
> >                 do {
> >                         zswap_next_shrink =3D 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.