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

[Marian Marinov]

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@localhost ~]# sysctl -a|grep shmmax
kernel.shmmax = 68719476736
[root@localhost ~]# 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@localhost ~]# sysctl -a|grep shmmax
kernel.shmmax = 34359738368
[root@localhost ~]#

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


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Marian Marinov
Founder & CEO of 1H Ltd.
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