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

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Date:	Sun, 04 May 2014 12:29:08 +0300
From:	Marian Marinov <mm@...com>
To:	Davidlohr Bueso <davidlohr@...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 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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