Re: [RFD] Merge task counter into memcg

From: Glauber Costa
Date: Wed Apr 18 2012 - 08:01:47 EST

On 04/18/2012 05:10 AM, Frederic Weisbecker wrote:
2012/4/18 Frederic Weisbecker<fweisbec@xxxxxxxxx>:
On Tue, Apr 17, 2012 at 08:17:53AM -0700, Tejun Heo wrote:
Hello, Frederic.

On Thu, Apr 12, 2012 at 06:59:27PM +0200, Frederic Weisbecker wrote:
I want:

a) to prevent the forkbomb from going far enough to DDOS the machine
b) to be able to kill that forkbomb once detected, in one go without race
against concurrent forks.

I think a) can work just fine with kernel stack limiting. I also need
to be notified about the fact we reached the limit. And b) should
be feasible with the help of the cgroup freezer.

kmem allocation fail after reaching the limit which in turn should
fail task creation. Isn't that the same effect as the task_counter as

That's it.

Is there anything for which you need to know exactly the number of

No that's really about prevent/kill forkbomb as far as I'm concerned.

Hmm... so, accounting overhead aside, if the only purpose is
preventing the whole machine being brought down by a fork bomb, kmem
limiting is enough, right?

I think so yeah.

But this needs to be a well defined kind of kmem I think. Relying on
kernel memory
alone is too general to just protect against forkbombs. Kernel stack,
OTOH, should be
a good criteria.

The problem is not it being too general. The problem is it is slab-based, and it takes a lot of allocations to fill a slab page. It
is a small object, but you still have one per task. If you set the limit too high, it won't help you. If you set it too low, it will harm
other object users.

But now I'm worrying, do you think this kmem.kernel_stack limitation
is going to be useful
for other kind of usecase?

Yes. Ultimately, we want to track as many kinds of kernel memory to avoid having one container harming the others. Page tables and stack were already briefly discussed, so I think we would get to that eventually anyway.
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