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

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Message-ID: <20180531082317.GA52285@rodete-desktop-imager.corp.google.com>
Date:   Thu, 31 May 2018 17:23:17 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Michal Hocko <mhocko@...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, May 31, 2018 at 08:56:42AM +0200, Michal Hocko wrote:
> 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

Maybe, no. I didn't set anything. IOW, it was a default without any setting
and I hit this as you can remember.
http://lkml.kernel.org/r/<20180418022912.248417-1-minchan@...nel.org>
We don't need to allocate memory for stuff even maintainers discourage.

> 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