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Date:   Thu, 15 Apr 2021 09:09:59 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     guro@fb.com, hannes@cmpxchg.org, akpm@linux-foundation.org,
        shakeelb@google.com, vdavydov.dev@gmail.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, fam.zheng@bytedance.com
Subject: Re: [External] Re: [PATCH 2/7] mm: memcontrol: bail out early when
 !mm in get_mem_cgroup_from_mm
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On Thu 15-04-21 11:13:16, Muchun Song wrote:
> On Wed, Apr 14, 2021 at 6:15 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Wed 14-04-21 18:04:35, Muchun Song wrote:
> > > On Wed, Apr 14, 2021 at 5:24 PM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > On Tue 13-04-21 14:51:48, Muchun Song wrote:
> > > > > When mm is NULL, we do not need to hold rcu lock and call css_tryget for
> > > > > the root memcg. And we also do not need to check !mm in every loop of
> > > > > while. So bail out early when !mm.
> > > >
> > > > mem_cgroup_charge and other callers unconditionally drop the reference
> > > > so how come this does not underflow reference count?
> > >
> > > For the root memcg, the CSS_NO_REF flag is set, so css_get
> > > and css_put do not get or put reference.
> >
> > Ohh, right you are. I must have forgot about that special case. I am
> > pretty sure I (and likely few more) will stumble over that in the future
> > again. A small comment explaining that the reference can be safely
> > ignore would be helpful.
> 
> OK. Will do.

I would go with the following if that helps
	/*
	 * No need to css_get on root memcg as the reference counting is
	 * disabled on the root level in the cgroup core. See CSS_NO_REF
	 */

Thanks
-- 
Michal Hocko
SUSE Labs