Re: [PATCH 02/18] mm: memcontrol: fix theoretical race in charge moving

[Johannes Weiner]

On Wed, Apr 22, 2020 at 09:51:20AM -0700, Shakeel Butt wrote:
> On Mon, Apr 20, 2020 at 3:11 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> > @@ -5426,15 +5420,23 @@ static int mem_cgroup_move_account(struct page *page,
> >         }
> >
> >         /*
> > +        * All state has been migrated, let's switch to the new memcg.
> > +        *
> >          * It is safe to change page->mem_cgroup here because the page
> > -        * is referenced, charged, and isolated - we can't race with
> > -        * uncharging, charging, migration, or LRU putback.
> > +        * is referenced, charged, isolated, and locked: we can't race
> > +        * with (un)charging, migration, LRU putback, or anything else
> > +        * that would rely on a stable page->mem_cgroup.
> > +        *
> > +        * Note that lock_page_memcg is a memcg lock, not a page lock,
> > +        * to save space. As soon as we switch page->mem_cgroup to a
> > +        * new memcg that isn't locked, the above state can change
> > +        * concurrently again. Make sure we're truly done with it.
> >          */
> > +       smp_mb();
> 
> You said theoretical race in the subject but the above comment
> convinced me that smp_mb() is required. So, why is the race still
> theoretical?

Sorry about the confusion.

I said theoretical because I spotted it while thinking about the
code. I'm not aware of any real users that suffered the consequences
of this race condition. But they could exist in theory :-)

I think it's a real bug that needs fixing.