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

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