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

[Muchun Song]

On Wed, Apr 21, 2021 at 9:03 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Wed 21-04-21 17:50:06, Muchun Song wrote:
> > On Wed, Apr 21, 2021 at 3:34 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Wed 21-04-21 14:26:44, Muchun Song 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
> > > >     page_counter_uncharge(&memcg->memory)
> > > >
> > > > 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.
> > >
> > > The changelog doesn't explain the fix and why you have chosen to charge
> > > kmem objects to root memcg and left all other try_charge users intact.
> >
> > The object cgroup is special (because the page can reparent). Only the
> > user of objcg APIs should be fixed.
> >
> > > The reason is likely that those are not reparented now but that just
> > > adds an inconsistency.
> > >
> > > Is there any reason you haven't simply matched obj_cgroup_uncharge_pages
> > > to check for the root memcg and bail out early?
> >
> > Because obj_cgroup_uncharge_pages() uncharges pages from the
> > root memcg unconditionally. Why? Because some pages can be
> > reparented to root memcg, in order to ensure the correctness of
> > page counter of root memcg. We have to uncharge pages from
> > root memcg. So we do not check whether the page belongs to
> > the root memcg when it uncharges.
>
> I am not sure I follow. Let me ask differently. Wouldn't you
> achieve the same if you simply didn't uncharge root memcg in
> obj_cgroup_charge_pages?

I'm afraid not. Some pages should uncharge root memcg, some
pages should not uncharge root memcg. But all those pages belong
to the root memcg. We cannot distinguish between the two.

I believe Roman is very familiar with this mechanism (objcg APIs).

Hi Roman,

Any thoughts on this?

>
> Btw. which tree is this patch based on? The current linux-next doesn't
> uncharge from memcg->memory inside obj_cgroup_uncharge_pages (nor does
> the Linus tree).

Sorry. I should expose more details.

obj_cgroup_uncharge_pages
  refill_stock->drain_stock
    page_counter_uncharge  // uncharging is here

Thanks.

> --
> Michal Hocko
> SUSE Labs