From: Muchun Song <songmuchun@bytedance.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <guro@fb.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Andrew Morton <akpm@linux-foundation.org>,
Shakeel Butt <shakeelb@google.com>,
Vladimir Davydov <vdavydov.dev@gmail.com>,
LKML <linux-kernel@vger.kernel.org>,
Linux Memory Management List <linux-mm@kvack.org>,
Xiongchun duan <duanxiongchun@bytedance.com>,
fam.zheng@bytedance.com
Subject: Re: [External] Re: [PATCH] mm: memcontrol: fix root_mem_cgroup charging
Date: Wed, 21 Apr 2021 21:39:03 +0800 [thread overview]
Message-ID: <CAMZfGtUOXyK3RDZ+P0GaO4p-P0XatFB8ZbmXEFvfet1HSrdFog@mail.gmail.com> (raw)
In-Reply-To: <YIAimEdqpen3/38Z@dhcp22.suse.cz>
On Wed, Apr 21, 2021 at 9:03 PM Michal Hocko <mhocko@suse.com> wrote:
>
> 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?
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
next prev parent reply other threads:[~2021-04-21 13:39 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-21 6:26 [PATCH] mm: memcontrol: fix root_mem_cgroup charging Muchun Song
2021-04-21 7:34 ` Michal Hocko
2021-04-21 9:50 ` [External] " Muchun Song
2021-04-21 13:03 ` Michal Hocko
2021-04-21 13:39 ` Muchun Song [this message]
2021-04-22 0:57 ` Roman Gushchin
2021-04-22 3:47 ` Muchun Song
2021-04-22 18:53 ` Roman Gushchin
2021-04-23 8:20 ` Muchun Song
2021-04-22 8:44 ` Michal Hocko
2021-04-22 18:37 ` Roman Gushchin
-- strict thread matches above, loose matches on Subject: below --
2021-03-02 8:18 Muchun Song
[not found] ` <YD6K3HghLy5glOgi@carbon.dhcp.thefacebook.com>
2021-03-03 3:12 ` [External] " Muchun Song
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