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

[Michal Hocko <mhocko@kernel.org>]

On Mon 25-11-19 22:11:15, Yafang Shao wrote:
> On Mon, Nov 25, 2019 at 8:45 PM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > 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:
[...]
> > > > 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.
> >
> 
> Yes, we always restart it automatically if these processes are exit
> (no matter because of OOM or some other reason).
> It is safe to do that if OOM happens, because OOM is always because of
> anon pages leaked and the restart can free these anon pages.

No this is an incorrect assumption. The OOM might happen for many
different reasons.

> But there may be some cases that we can't success to restart it, while
> if that happens the protected pages will be never be reclaimed until
> the admin reset it or make this memcg offline.

If the workload cannot be restarted for whatever reason then you need an
admin intervention and a proper cleanup. That would include resetting
reclaim protection when in use.

> When there're no processes, we don't need to protect the pages. You
> can consider it as 'fault tolerance' .

I have already tried to explain why this is a bold statement that
doesn't really hold universally and that the kernel doesn't really have
enough information to make an educated guess.

> > 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.
> 
> Actually what I concern is the  case that there's no process running
> but memory protection coninues protecting the file pages.
> OOM is just one case of them.

This sounds like a misconfiguration which should be handled by an admin.
-- 
Michal Hocko
SUSE Labs