Re: [PATCH] memcg: force charge kmem counter too

Michal Hocko
2018-05-31T06:56:42Z

On Thu 31-05-18 15:01:33, Minchan Kim wrote:
> On Wed, May 30, 2018 at 11:14:33AM -0700, Shakeel Butt wrote:
> > On Tue, May 29, 2018 at 1:31 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > > On Mon 28-05-18 10:23:07, Shakeel Butt wrote:
> > >> On Mon, May 28, 2018 at 2:11 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > >> Though is there a precedence where the broken feature is not fixed
> > >> because an alternative is available?
> > >
> > > Well, I can see how breaking GFP_NOFAIL semantic is problematic, on the
> > > other hand we keep saying that kmem accounting in v1 is hard usable and
> > > strongly discourage people from using it. Sure we can add the code which
> > > handles _this_ particular case but that wouldn't make the whole thing
> > > more usable I strongly suspect. Maybe I am wrong and you can provide
> > > some specific examples. Is GFP_NOFAIL that common to matter?
> > >
> > > In any case we should balance between the code maintainability here.
> > > Adding more cruft into the allocator path is not free.
> > >
> >
> > We do not use kmem limits internally and this is something I found
> > through code inspection. If this patch is increasing the cost of code
> > maintainability I am fine with dropping it but at least there should a
> > comment saying that kmem limits are broken and no need fix.
>
> I agree.
>
> Even, I didn't know kmem is strongly discouraged until now. Then,
> why is it enabled by default on cgroup v1?

You have to set a non-zero limit to make it active IIRC. The code is
compiled in because v2 enables it by default.

> Let's turn if off with comment "It's broken so do not use/fix. Instead,
> please move to cgroup v2".

--
Michal Hocko
SUSE Labs