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

[Michal Hocko <mhocko@kernel.org>]

On Mon 25-11-19 20:37:52, Yafang Shao wrote:
> 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.

Could you be more specific please? I would assume that the group oom
configured memcg would either restart its workload when killed (that is
why you want to kill the whole workload to restart it cleanly in many
case) or simply tear down the memcg altogether.

In other words why do you care about the oom killer case so much? It is
not different that handling a lingering memcg with the workload already
finished. You simply have no way to know whether the reclaim protection
is still required. Admin is supposed to either offline the memcg that is
no longer used or drop the reclaim protection once it is not needed
because that has some visible consequences on the overall system
operation.
-- 
Michal Hocko
SUSE Labs