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

Johannes Weiner
2020-04-22T17:42:33Z

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@cmpxchg.org> 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.