linux-kernel - Re: [PATCH] IPC initialize shmmax and shmall from the current value not the default

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Message-ID: <1399166450.2573.15.camel@buesod1.americas.hpqcorp.net>
Date:	Sat, 03 May 2014 18:20:50 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Marian Marinov <mm@...com>
Cc:	akpm@...ux-foundation.org, n-horiguchi@...jp.nec.com,
	Greg KH <gregkh@...uxfoundation.org>, manfred@...orfullife.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: [PATCH] IPC initialize shmmax and shmall from the current value
 not the default

On Sun, 2014-05-04 at 03:28 +0300, Marian Marinov wrote:
> On 05/04/2014 02:53 AM, Davidlohr Bueso wrote:
> > On Sun, 2014-05-04 at 01:48 +0300, Marian Marinov wrote:
> >> When we are creating new IPC namespace that should be cloned from the current namespace it is a good idea to copy the
> >> values of the current shmmax and shmall to the new namespace.
> >
> > Why is this a good idea?
> >
> > This would break userspace that relies on the current behavior.
> > Furthermore we've recently changed the default value of both these
> > limits to be as large as you can get, thus deprecating them. I don't
> > like the idea of this being replaced by namespaces.
> >
> > Thanks,
> > Davidlohr
> >
> 
> The current behavior is create_ipc_ns()->shm_init_ns()
> 
> void shm_init_ns(struct ipc_namespace *ns)
> {
>      ns->shm_ctlmax = SHMMAX;
>      ns->shm_ctlall = SHMALL;
>      ns->shm_ctlmni = SHMMNI;
>      ns->shm_rmid_forced = 0;
>      ns->shm_tot = 0;
>      ipc_init_ids(&shm_ids(ns));
> }
> 
> This means that whenever you are creating an IPC namespace it gets its SHMMAX and SHMALL values from the defaults for 
> the kernel.

This is exactly what I meant by 'current behavior'.

> If for some reason you want to have smaller(or bigger, for older kernels) limit. This means changing the values in 
> /proc/sys/kernel/shmmax and /proc/sys/kernel/shmall. However the program that is started with the new IPC namespace may 
> lack privileges to write to these files and so it can not modify them.

I see no reason why namespaces should behave any different than the rest
of the system, wrt this. And this changes how *and* when these limits
are set, which impacts at a userspace level with no justification.

> What I'm proposing is simply to copy the current values of the host machine, as set by a privileged process before the 
> namespace creation.
> 
> Maybe a better approach would be to allow the changes to be done by processes having CAP_SYS_RESOURCE inside the new 
> namespace?

Why do you need this? Is there any real impact/issue you're seeing?

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