linux-kernel - Re: [patch -mm 0/4] mqueue namespace

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Message-ID: <48B7C55B.4090609@fr.ibm.com>
Date:	Fri, 29 Aug 2008 11:46:03 +0200
From:	Cedric Le Goater <clg@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Containers <containers@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Pavel Emelianov <xemul@...nvz.org>,
	Serge Hallyn <serue@...ibm.com>
Subject: Re: [patch -mm 0/4] mqueue namespace

Eric W. Biederman wrote:
> ebiederm@...ssion.com (Eric W. Biederman) writes:
> 
>> One way to fix that is to add a hidden directory to the mnt namespace.
>> Where magic in kernel filesystems can be mounted.  Only visible
>> with a magic openat flag.  Then:
>>
>> fd = openat(AT_FDKERN, ".", O_DIRECTORY)
>> fchdir(fd);
>> umount("./mqueue", MNT_DETACH);
>> mount(("none", "./mqueue", "mqueue", 0, NULL);
>>
>> Would unshare the mqueue namespace.
>>
>> Implemented for plan9 this would solve a problem of how do you get
>> access to all of it's special filesystems.  As only bind mounts
>> and remote filesystem mounts are available.  For linux thinking about
>> it might shake the conversation up a bit.
> 
> Thinking about this some more.  What is especially attractive if we do
> all namespaces this way is that it solves two lurking problems.
> 1) How do you keep a namespace around without a process in it.
> 2) How do you enter a container.
> 
> If we could land the namespaces in the filesystem we could easily
> persist them past the point where a process is present in one if we so
> choose.
>
> Entering a container would be a matter of replacing your current
> namespaces mounts with namespace mounts take from the filesystem.
> 
> I expect performance would degrade in practice, but it is tempting
> to implement it and run a benchmark and see if we can measure anything.

http://wiki.openvz.org/Containers/Mini-summit_2008_notes

you seem to have talked about this idea at the summit but the notes
are a bit short on the "entering a container" topic. Have you had time to
work on the POC the notes are talking about ? 

the mqueue namespace (and sysv ipc) is typically one of these namespaces
with valid objects which can have no processes in it. 

C. 
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