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Date:   Mon, 3 Aug 2020 11:03:49 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Roman Gushchin <guro@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Shakeel Butt <shakeelb@google.com>, linux-mm@kvack.org,
        Vlastimil Babka <vbabka@suse.cz>, kernel-team@fb.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 05/19] mm: memcontrol: decouple reference counting
 from page accounting
Message-ID: <20200803150349.GA502356@cmpxchg.org>
References: <20200623174037.3951353-1-guro@fb.com>
 <20200623174037.3951353-6-guro@fb.com>
 <20200803090033.GE5174@dhcp22.suse.cz>
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On Mon, Aug 03, 2020 at 11:00:33AM +0200, Michal Hocko wrote:
> On Tue 23-06-20 10:40:23, Roman Gushchin wrote:
> > @@ -5456,7 +5460,10 @@ static int mem_cgroup_move_account(struct page *page,
> >  	 */
> >  	smp_mb();
> >  
> > -	page->mem_cgroup = to; 	/* caller should have done css_get */
> > +	css_get(&to->css);
> > +	css_put(&from->css);
> > +
> > +	page->mem_cgroup = to;
> >  
> >  	__unlock_page_memcg(from);
> 
> What prevents from memcg to be released here?

->attach_task() and kill_css() are exclusive through the cgroup_mutex,
so the base ref cannot disappear from under us during this operation.