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

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Date:	Mon, 05 May 2014 22:59:50 +0300
From:	Marian Marinov <mm@...com>
To:	Manfred Spraul <manfred@...orfullife.com>,
	Davidlohr Bueso <davidlohr@...com>
CC:	akpm@...ux-foundation.org, n-horiguchi@...jp.nec.com,
	Greg KH <gregkh@...uxfoundation.org>,
	"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 02:17 PM, Manfred Spraul wrote:
> Hi Marian,
>
> Note: The limits will soon be increased to (nearly) ULONG_MAX.
> I.e.: If you propose the patch because you are running into issues with a too small SEMMAX after an
> unshare(CLONE_NEWIPC), then this will be fixed soon.
>
>
> On 05/04/2014 01:53 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.
> The idea sounds reasonable:
> If an admin has reduced the limits, then the reduction should also apply after a unshare(CLONE_NEWIPC).
>
> But:
> Your patch doesn't use the current shmmax, it uses the shmmax from init_ipc_ns.
> Would it be possible to use the current values?

In my tests it worked exactly as expected.
Here is an example:

[root@sp2 ~]# sysctl -a|grep shmmax
kernel.shmmax = 68719476736
[root@sp2 ~]# lxc-attach -n cent_plain
[root@...alhost ~]# sysctl -a|grep shmmax
kernel.shmmax = 68719476736
[root@...alhost ~]# halt
[root@sp2 ~]# sysctl -a|grep shmmax
kernel.shmmax = 68719476736
[root@sp2 ~]# sysctl kernel.shmmax=34359738368
kernel.shmmax = 34359738368
[root@sp2 ~]# lxc-start -n cent_plain -d
[root@sp2 ~]# lxc-attach -n cent_plain
[root@...alhost ~]# sysctl -a|grep shmmax
kernel.shmmax = 34359738368
[root@...alhost ~]#

So it seams to work as expected :)

It works because wen you setup a new shmmax limit it is actually the limit in the init_ipc_ns.
So when we are creating a new ipc_ns its ok to copy the values from init_ipc_ns.

-Marian

>
>> 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.
> Davidlohr: We are not deprecating them, we make the default huge.
> The limits should stay as usable as they were.
>
> With regards to breaking user space, I must think about it a bit more.
> Right now, each new namespace starts with SEMMAX=32MB, i.e. an often unusable default.
>
> --
>      Manfred
>


-- 
Marian Marinov
Founder & CEO of 1H Ltd.
Jabber/GTalk: hackman@...ber.org
ICQ: 7556201
Mobile: +359 886 660 270
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