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

[Michal Hocko <mhocko@kernel.org>]

On Tue 26-11-19 11:52:19, Yafang Shao wrote:
> On Mon, Nov 25, 2019 at 10:42 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Mon, Nov 25, 2019 at 03:21:50PM +0100, Michal Hocko wrote:
> > > On Mon 25-11-19 22:11:15, Yafang Shao wrote:
> > > > 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.
> >
> > I agree, this is not obviously true. And the kernel shouldn't try to
> > guess whether the explicit userspace configuration is still desirable
> > to userspace or not. Should we also delete the cgroup when it becomes
> > empty for example?
> >
> > It's better to implement these kinds of policy decisions from
> > userspace.
> >
> > There is a cgroup.events file that can be polled, and its "populated"
> > field shows conveniently whether there are tasks in a subtree or
> > not. You can use that to clear protection settings.
> 
> Why isn't force_empty supported in cgroup2 ?

There wasn't any sound usecase AFAIR.

> In this case we can free the protected file pages immdiately with force_empty.

You can do the same thing by setting the hard limit to 0.
-- 
Michal Hocko
SUSE Labs