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

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Date:	Fri, 20 Jun 2008 09:53:25 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	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

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> 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.

The device ns could be a mount of an fs with the devices created in it,
while mknod becomes a symlink from that fs.  And once a network
namespace is a filesystem, we can aim for the plan9 NAT solution of
mounting a remote /net onto ours.  Neat.

But bye-bye posix?

-serge
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