linux-kernel - Re: [PATCH v2 00/12] make rpc_pipefs be mountable multiple time

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Message-ID: <20101230104416.GA31824@shutemov.name>
Date:	Thu, 30 Dec 2010 12:44:16 +0200
From:	"Kirill A. Shutemov" <kas@...nvz.org>
To:	Rob Landley <rlandley@...allels.com>
Cc:	"Kirill A. Shutemov" <kas@...nvz.org>,
	Rob Landley <rob@...dley.net>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Neil Brown <neilb@...e.de>,
	Pavel Emelyanov <xemul@...allels.com>,
	linux-nfs@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 00/12] make rpc_pipefs be mountable multiple time

On Thu, Dec 30, 2010 at 04:05:07AM -0600, Rob Landley wrote:
> On 12/30/2010 03:44 AM, Kirill A. Shutemov wrote:
> >>> If no rpcmount mountoption, no rpc_pipefs was found at
> >>> '/var/lib/nfs/rpc_pipefs' and we are in init's mount namespace, we use
> >>> init_rpc_pipefs.
> >>
> >> It's the "we are in init's mount namespace" that I was wondering about.
> >>
> >> So if I naievely chroot, nfs mount stops working the way it did before I
> >> chrooted unless I do an extra setup step?
> >
> > No. It will work as before since you are still in init's mount namespace.
> > Creating new mount namespace changes rules.
> 
> Ah, CLONE_NEWNS and then you need /var/lib/nfs/rpc_pipefs.  Got it.
> 
> I'm kind of surprised that the kernel cares about a specific path under 
> /var/lib.  (Seems like policy in the kernel somehow.)

Yep. It's bad, but there is way to overwrite the default.

Other way is to leave 'rpcmount' mountoption without default.
get_rpc_pipefs(NULL) in init's mount namespace will always return
init_rpc_pipefs, without filesystem lookup.
get_rpc_pipefs(NULL) in non-init's mount namespace will always return
error.

So you will have to specify 'rpcmount' mountoption for every nfs mount in
container. Hmm, I guess, it may confuse user.

Or we can try to move the default to userspace. /sbin/mount.nfs?

> Can't it just 
> check the current process's mount list to see if an instance of 
> rpc_pipefs is mounted in the current namespace the way lxc looks for 
> cgroups?  Or are there potential performance/scalability issues with that?

What should we do if we have several rpc_pipefs mounts in the namespace?

-- 
 Kirill A. Shutemov
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