linux-kernel - Re: [PATCH v12 8/8] cgroup: implement the PIDs subsystem

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Date:	Tue, 19 May 2015 15:11:34 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Aleksa Sarai <cyphar@...har.com>
cc:	Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>,
	lizefan@...wei.com, mingo@...hat.com, richard@....at,
	Frédéric Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH v12 8/8] cgroup: implement the PIDs subsystem

On Tue, 19 May 2015, Aleksa Sarai wrote:

> >> However, it should be noted that organisational operations (adding and
> >> removing tasks from a PIDs hierarchy) will *not* be prevented.
> >
> > This is how you spell: broken controller.
> 
> This has been discussed before. Organisational operations (i.e.
> attaching to a cgroup) are not to be blocked by a cgroup controller in
> the unified hierarchy. You simply can't escape out of a parent
> cgroup's limit through attaching to a child cgroup (because you will
> attach either before the fork checks against the cgroup [in which case
> the child's limit is followed -- which means you also follow the
> parent's limit] or after it checks [which means you'll hit the
> parent's limit and won't be able to fork]).

That's complete and utter nonsense. What has the parent limit to do
with the overflow of the child limit?

parent:	 limit 100   usecnt 80
child:	 limit 10    usecnt 10

So moving anything into child is violating the constraints and has to
be refused. Anything else is just dirty hackery.

Thanks,

	tglx




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