Re: [PATCH 2/3] memcg: Limit the number of events registered onoom_control

From: Michal Hocko
Date: Wed Aug 07 2013 - 09:37:54 EST

On Wed 07-08-13 09:08:36, Tejun Heo wrote:
> Hello, Michal.
> On Wed, Aug 07, 2013 at 01:28:26PM +0200, Michal Hocko wrote:
> > There is no limit for the maximum number of oom_control events
> > registered per memcg. This might lead to an user triggered memory
> > depletion if a regular user is allowed to register events.
> >
> > Let's be more strict and cap the number of events that might be
> > registered. MAX_OOM_NOTIFY_EVENTS value is more or less random. The
> > expectation is that it should be high enough to cover reasonable
> > usecases while not too high to allow excessive resources consumption.
> > 1024 events consume something like 24KB which shouldn't be a big deal
> > and it should be good enough (even 1024 oom notification events sounds
> > crazy).
> I think putting restriction on usage_event makes sense as that builds
> a shared contiguous table from all events which can't be attributed
> correctly and makes it easy to trigger allocation failures due to
> large order allocation but is this necessary for oom and vmpressure,
> both of which allocate only for the listening task?

Once I was there I made them consistent in that regards.

> It isn't different from listening from epoll, for example.

epoll limits the number of watchers, no?

> 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.
Michal Hocko
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