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

[Michal Hocko <mhocko@suse.com>]

On Wed 21-04-21 17:50:06, Muchun Song wrote:
> On Wed, Apr 21, 2021 at 3:34 PM Michal Hocko <mhocko@suse.com> 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?

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).
-- 
Michal Hocko
SUSE Labs