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

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Message-ID: <m1ve04vkov.fsf@frodo.ebiederm.org>
Date:	Thu, 19 Jun 2008 20:39:44 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Cedric Le Goater <clg@...ibm.com>,
	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

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.

Eric
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