Message-ID: <20171024201522.3z2fjnfywgx2egqx@dhcp22.suse.cz>
Date:   Tue, 24 Oct 2017 22:15:22 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Greg Thelen <gthelen@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, linux-fsdevel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs, mm: account filp and names caches to kmemcg

On Tue 24-10-17 14:58:54, Johannes Weiner wrote:
> On Tue, Oct 24, 2017 at 07:55:58PM +0200, Michal Hocko wrote:
> > On Tue 24-10-17 13:23:30, Johannes Weiner wrote:
> > > On Tue, Oct 24, 2017 at 06:22:13PM +0200, Michal Hocko wrote:
> > [...]
> > > > What would prevent a runaway in case the only process in the memcg is
> > > > oom unkillable then?
> > > 
> > > In such a scenario, the page fault handler would busy-loop right now.
> > > 
> > > Disabling oom kills is a privileged operation with dire consequences
> > > if used incorrectly. You can panic the kernel with it. Why should the
> > > cgroup OOM killer implement protective semantics around this setting?
> > > Breaching the limit in such a setup is entirely acceptable.
> > > 
> > > Really, I think it's an enormous mistake to start modeling semantics
> > > based on the most contrived and non-sensical edge case configurations.
> > > Start the discussion with what is sane and what most users should
> > > optimally experience, and keep the cornercases simple.
> > 
> > I am not really seeing your concern about the semantic. The most
> > important property of the hard limit is to protect from runaways and
> > stop them if they happen. Users can use the softer variant (high limit)
> > if they are not afraid of those scenarios. It is not so insane to
> > imagine that a master task (which I can easily imagine would be oom
> > disabled) has a leak and runaway as a result.
> 
> Then you're screwed either way. Where do you return -ENOMEM in a page
> fault path that cannot OOM kill anything? Your choice is between
> maintaining the hard limit semantics or going into an infinite loop.

in the PF path yes. And I would argue that this is a reasonable
compromise to provide the gurantee the hard limit is giving us (and
the resulting isolation which is the whole point). Btw. we are already
having that behavior. All we are talking about is the non-PF path which
ENOMEMs right now and the meta-patch tried to handle it more gracefully
and only ENOMEM when there is no other option.

> I fail to see how this setup has any impact on the semantics we pick
> here. And even if it were real, it's really not what most users do.

sure, such a scenario is really on the edge but my main point was that
the hard limit is an enforcement of an isolation guarantee (as much as
possible of course).

> > We are not talking only about the page fault path. There are other
> > allocation paths to consume a lot of memory and spill over and break
> > the isolation restriction. So it makes much more sense to me to fail
> > the allocation in such a situation rather than allow the runaway to
> > continue. Just consider that such a situation shouldn't happen in
> > the first place because there should always be an eligible task to
> > kill - who would own all the memory otherwise?
> 
> Okay, then let's just stick to the current behavior.

I am definitely not pushing that thing right now. It is good to discuss
it, though. The more kernel allocations we will track the more careful we
will have to be. So maybe we will have to reconsider the current
approach. I am not sure we need it _right now_ but I feel we will
eventually have to reconsider it.
-- 
Michal Hocko
SUSE Labs

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