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Date:   Mon, 3 Aug 2020 17:08:58 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Johannes Weiner <hannes@cmpxchg.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: <20200803150858.GW5174@dhcp22.suse.cz>
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 <20200803090033.GE5174@dhcp22.suse.cz>
 <20200803150349.GA502356@cmpxchg.org>
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On Mon 03-08-20 11:03:49, Johannes Weiner wrote:
> 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.

OK, is this worth a comment? Reference counting before other operation
on the object always makes me worried and those details are hidden
elsewhere.

Btw. with the follow up fix from Hugh
Acked-by: Michal Hocko <mhocko@suse.com>
-- 
Michal Hocko
SUSE Labs