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

[Shakeel Butt]

On Mon, Apr 20, 2020 at 3:11 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> The move_lock is a per-memcg lock, but the VM accounting code that
> needs to acquire it comes from the page and follows page->mem_cgroup
> under RCU protection. That means that the page becomes unlocked not
> when we drop the move_lock, but when we update page->mem_cgroup. And
> that assignment doesn't imply any memory ordering. If that pointer
> write gets reordered against the reads of the page state -
> page_mapped, PageDirty etc. the state may change while we rely on it
> being stable and we can end up corrupting the counters.
>
> Place an SMP memory barrier to make sure we're done with all page
> state by the time the new page->mem_cgroup becomes visible.
>
> Also replace the open-coded move_lock with a lock_page_memcg() to make
> it more obvious what we're serializing against.
>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>