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

[Marian Marinov]

On 05/04/2014 04:20 AM, Davidlohr Bueso wrote:
> 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?
I'm using Linux Containers and I need to be able to either start containers with different SHMMAX or set different 
SHMMAX to already running containers without giving them full root access.

-Marian
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