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

[Muchun Song]

On Thu, Apr 22, 2021 at 8:57 AM Roman Gushchin <guro@xxxxxx> wrote:
>
> On Wed, Apr 21, 2021 at 09:39:03PM +0800, Muchun Song wrote:
> > 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?
>
> First, unfortunately we do export the root's counter on cgroup v1:
> /sys/fs/cgroup/memory/memory.kmem.usage_in_bytes
> But we don't ignore these counters for the root mem cgroup, so there
> are no bugs here. (Otherwise, please, reproduce it). So it's all about
> the potential warning in page_counter_cancel().

Right.

>
> The patch looks technically correct to me. Not sure about __try_charge()
> naming, we never use "__" prefix to do something with the root_mem_cgroup.
>
> The commit message should be more clear and mention the following:
> 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.

Very clear. Thanks.

>
> But honestly I'm not sure the problem is worth the time spent on the fix
> and the discussion. It's a small race and it's generally hard to trigger
> a kernel allocation racing with a cgroup deletion and then you need *a lot*
> of such races and then maybe there will be a single warning printed without
> *any* other consequences.

I agree the race is very small. Since the fix is easy, but a little confusing
to someone. I want to hear other people's suggestions on whether to fix it.

>
> Thanks!