Re: [PATCH v2] mm: memcontrol: fix root_mem_cgroup charging

[Roman Gushchin <guro@fb.com>]

On Sun, May 09, 2021 at 05:54:03PM -0700, Andrew Morton wrote:
> On Sun, 25 Apr 2021 15:54:10 +0800 Muchun Song <songmuchun@bytedance.com> wrote:
> 
> > The below scenario can cause the page counters of the root_mem_cgroup
> > to be out of balance.
> > 
> > CPU0:                                   CPU1:
> > 
> > objcg = get_obj_cgroup_from_current()
> > obj_cgroup_charge_pages(objcg)
> >                                         memcg_reparent_objcgs()
> >                                             // reparent to root_mem_cgroup
> >                                             WRITE_ONCE(iter->memcg, parent)
> >     // memcg == root_mem_cgroup
> >     memcg = get_mem_cgroup_from_objcg(objcg)
> >     // do not charge to the root_mem_cgroup
> >     try_charge(memcg)
> > 
> > obj_cgroup_uncharge_pages(objcg)
> >     memcg = get_mem_cgroup_from_objcg(objcg)
> >     // uncharge from the root_mem_cgroup
> >     refill_stock(memcg)
> >         drain_stock(memcg)
> >             page_counter_uncharge(&memcg->memory)
> > 
> > get_obj_cgroup_from_current() never returns a root_mem_cgroup's objcg,
> > so we never explicitly charge the root_mem_cgroup. And it's not
> > going to change. It's all about a race when we got an obj_cgroup
> > pointing at some non-root memcg, but before we were able to charge it,
> > the cgroup was gone, objcg was reparented to the root and so we're
> > skipping the charging. Then we store the objcg pointer and later use
> > to uncharge the root_mem_cgroup.
> > 
> > This can cause the page counter to be less than the actual value.
> > Although we do not display the value (mem_cgroup_usage) so there
> > shouldn't be any actual problem, but there is a WARN_ON_ONCE in
> > the page_counter_cancel(). Who knows if it will trigger? So it
> > is better to fix it.
> > 
> 
> It sounds like Roman will be acking this, but some additional reviewer
> attention would be helpful, please.
> 

The patch is technically correct, so I'm ok to ack it:
Acked-by: Roman Gushchin <guro@fb.com>

I remember Michal was looking into it, so he can probably add something here.

Thanks!