Re: [PATCH v7 05/19] mm: memcontrol: decouple reference counting from page accounting

[Johannes Weiner]

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.