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

Yafang Shao
2019-11-26T03:52:58Z

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 ?
In this case we can free the protected file pages immdiately with force_empty.
The advantage of it is to avoid scaning all other memcgs in
kswapd/direct reclaim paths, because currently the reclaimer will
fairly scan all memcgs and reclaim pages from them.
What's the problem with force_empty ?

Thanks
Yafang