linux-kernel - Re: [PATCH 2/3] memcg: Limit the number of events registered on oom

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Date:	Wed, 7 Aug 2013 15:57:34 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Anton Vorontsov <anton.vorontsov@...aro.org>
Subject: Re: [PATCH 2/3] memcg: Limit the number of events registered on
 oom_control

On Wed 07-08-13 09:47:41, Tejun Heo wrote:
> Hello,
> 
> On Wed, Aug 07, 2013 at 03:37:46PM +0200, Michal Hocko wrote:
> > > It isn't different from listening from epoll, for example.
> > 
> > epoll limits the number of watchers, no?
> 
> Not that I know of.  It'll be limited by max open fds but I don't
> think there are other limits. 

max_user_watches seems to be a limit (4% of lowmem in maximum).

> Why would there be?

Because userspace should hog kernel resources without any limit.

> > > If there needs to be kernel memory limit, shouldn't that be handled by
> > > kmemcg?
> > 
> > kmemcg would surely help but turning it on just because of potential
> > abuse of the event registration API sounds like an overkill.
> > 
> > I think having a cap for user trigable kernel resources is a good thing
> > in general.
> 
> I don't know.  It's just very arbitrary because listening to events
> itself isn't (and shouldn't) be something which consumes resource
> which isn't attributed to the listener and this artificially creates a
> global resource.  The problem with memory usage event is breaching
> that rule with shared kmalloc() so putting well-defined limit on it is
> fine but the latter two create additional artificial restrictions
> which are both unnecessary and unconventional.  No?

Hmm, OK so you think that the fd limit is sufficient already?
-- 
Michal Hocko
SUSE Labs
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