linux-kernel - Re: [PATCH] memcg: force charge kmem counter too

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Date:   Thu, 31 May 2018 08:56:42 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Thelen <gthelen@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: force charge kmem counter too

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@...nel.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@...nel.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