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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 25 Jun 2019 17:15:12 -0700
Message-ID: <CALvZod7AuMLQP32P=aRJqwLMeGVUx3G86ANoM_f1Eii9f6EqbQ@mail.gmail.com>
Subject: Re: [PATCH v7 10/10] mm: reparent memcg kmem_caches on cgroup removal
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Waiman Long <longman@redhat.com>
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On Tue, Jun 11, 2019 at 4:18 PM Roman Gushchin <guro@fb.com> wrote:
>
> Let's reparent non-root kmem_caches on memcg offlining. This allows us
> to release the memory cgroup without waiting for the last outstanding
> kernel object (e.g. dentry used by another application).
>
> Since the parent cgroup is already charged, everything we need to do
> is to splice the list of kmem_caches to the parent's kmem_caches list,
> swap the memcg pointer, drop the css refcounter for each kmem_cache
> and adjust the parent's css refcounter.
>
> Please, note that kmem_cache->memcg_params.memcg isn't a stable
> pointer anymore. It's safe to read it under rcu_read_lock(),
> cgroup_mutex held, or any other way that protects the memory cgroup
> from being released.
>
> We can race with the slab allocation and deallocation paths. It's not
> a big problem: parent's charge and slab global stats are always
> correct, and we don't care anymore about the child usage and global
> stats. The child cgroup is already offline, so we don't use or show it
> anywhere.
>
> Local slab stats (NR_SLAB_RECLAIMABLE and NR_SLAB_UNRECLAIMABLE)
> aren't used anywhere except count_shadow_nodes(). But even there it
> won't break anything: after reparenting "nodes" will be 0 on child
> level (because we're already reparenting shrinker lists), and on
> parent level page stats always were 0, and this patch won't change
> anything.
>
> Signed-off-by: Roman Gushchin <guro@fb.com>

The reparenting of top level memcg and "return true" is fixed in the
later patch.

Reviewed-by: Shakeel Butt <shakeelb@google.com>