Re: [PATCH] mm: memcontrol: reclaim and OOM kill when shrinking memory.max below usage

From: Vladimir Davydov <vdavydov@virtuozzo.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
	<cgroups@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<kernel-team@fb.com>
Subject: Re: [PATCH] mm: memcontrol: reclaim and OOM kill when shrinking memory.max below usage
Date: Thu, 17 Mar 2016 11:23:45 +0300	[thread overview]
Message-ID: <20160317082345.GF18142@esperanza> (raw)
In-Reply-To: <20160316201329.GA15498@cmpxchg.org>

On Wed, Mar 16, 2016 at 01:13:29PM -0700, Johannes Weiner wrote:
> On Wed, Mar 16, 2016 at 06:15:09PM +0300, Vladimir Davydov wrote:
> > On Tue, Mar 15, 2016 at 10:18:48PM -0700, Johannes Weiner wrote:
> > > On Fri, Mar 11, 2016 at 12:19:31PM +0300, Vladimir Davydov wrote:
> > ...
> > > > Come to think of it, shouldn't we restore the old limit and return EBUSY
> > > > if we failed to reclaim enough memory?
> > > 
> > > I suspect it's very rare that it would fail. But even in that case
> > > it's probably better to at least not allow new charges past what the
> > > user requested, even if we can't push the level back far enough.
> > 
> > It's of course good to set the limit before trying to reclaim memory,
> > but isn't it strange that even if the cgroup's memory can't be reclaimed
> > to meet the new limit (tmpfs files or tasks protected from oom), the
> > write will still succeed? It's a rare use case, but still.
> 
> It's not optimal, but there is nothing we can do about it, is there? I
> don't want to go back to the racy semantics that allow the application
> to balloon up again after the limit restriction fails.
> 
> > I've one more concern regarding this patch. It's about calling OOM while
> > reclaiming cgroup memory. AFAIU OOM killer can be quite disruptive for a
> > workload, so is it really good to call it when normal reclaim fails?
> > 
> > W/o OOM killer you can optimistically try to adjust memory.max and if it
> > fails you can manually kill some processes in the container or restart
> > it or cancel the limit update. With your patch adjusting memory.max
> > never fails, but OOM might kill vital processes rendering the whole
> > container useless. Wouldn't it be better to let the user decide if
> > processes should be killed or not rather than calling OOM forcefully?
> 
> Those are the memory.max semantics, though. Why should there be a
> difference between the container growing beyond the limit and the
> limit cutting into the container?
> 
> If you don't want OOM kills, set memory.high instead. This way you get
> the memory pressure *and* the chance to do your own killing.

Fair enough.

Thanks,
Vladimir