Re: [PATCH] mm, memcg: clear page protection when memcg oom group happens

From: Yafang Shao <laoar.shao@gmail.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	 Andrew Morton <akpm@linux-foundation.org>,
	Linux MM <linux-mm@kvack.org>
Subject: Re: [PATCH] mm, memcg: clear page protection when memcg oom group happens
Date: Mon, 25 Nov 2019 20:37:52 +0800	[thread overview]
Message-ID: <CALOAHbDx05Yo1mKOOUSTe7tdoguNVS1+rZT_ocP4nEm9Vp6VKA@mail.gmail.com> (raw)
In-Reply-To: <20191125123123.GL31714@dhcp22.suse.cz>

On Mon, Nov 25, 2019 at 8:31 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Mon 25-11-19 20:17:15, Yafang Shao wrote:
> > On Mon, Nov 25, 2019 at 7:54 PM Michal Hocko <mhocko@kernel.org> wrote:
> > >
> > > On Mon 25-11-19 19:37:59, Yafang Shao wrote:
> > > > On Mon, Nov 25, 2019 at 7:08 PM Michal Hocko <mhocko@kernel.org> wrote:
> > > > >
> > > > > On Mon 25-11-19 05:14:53, Yafang Shao wrote:
> > > > > > We set memory.oom.group to make all processes in this memcg are killed by
> > > > > > OOM killer to free more pages. In this case, it doesn't make sense to
> > > > > > protect the pages with memroy.{min, low} again if they are set.
> > > > >
> > > > > I do not see why? What does group OOM killing has anything to do with
> > > > > the reclaim protection? What is the actual problem you are trying to
> > > > > solve?
> > > > >
> > > >
> > > > The cgroup is treated as a indivisible  workload when cgroup.oom.group
> > > > is set and OOM killer is trying to kill a prcess in this cgroup.
> > >
> > > Yes this is true.
> > >
> > > > We set cgroup.oom.group is to  guarantee the workload integrity, now
> > > > that processes ara all killed, why keeps the page cache here?
> > >
> > > Because an administrator has configured the reclaim protection in a
> > > certain way and hopefully had a good reason to do that. We are not going
> > > to override that configure just because there is on OOM killer invoked
> > > and killed tasks in that memcg. The workload might get restarted and it
> > > would run under a different constrains all of the sudden which is not
> > > expected.
> > >
> > > In short kernel should never silently change the configuration made by
> > > an admistrator.
> >
> > Understood.
> >
> > So what about bellow changes ? We don't override the admin setting,
> > but we reclaim the page caches from it if this memcg is oom killed.
> > Something like,
> >
> > mem_cgroup_protected
> > {
> > ...
> > +       if (!cgroup_is_populated(memcg->css.cgroup) &&
> > mem_cgroup_under_oom_group_kill(memcg))
> > +               return MEMCG_PROT_NONE;
> > +
> >         usage = page_counter_read(&memcg->memory);
> >         if (!usage)
> >                 return MEMCG_PROT_NONE;
> > }
>
> I assume that mem_cgroup_under_oom_group_kill is essentially
>         memcg->under_oom && memcg->oom_group
> But that doesn't really help much because all the reclaim attempts have
> been already attempted and failed. I do not remember exact details about
> under_oom but I have a recollection that it wouldn't really work for
> cgroup v2 because the oom_control is not in place and so the state would
> be set for only very short time period.
>
> Again, what is a problem that you are trying to fix?

When there's no processes running in a memcg, for example if they are
killed by OOM killer, we can't reclaim the file page cache protected
by memory.min of this memcg. These file page caches are useless in
this case.
That's what I'm trying to fix.

Thanks
Yafang