Date:	Tue, 06 Jan 2009 15:04:17 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	Trond Myklebust <trond.myklebust@....uio.no>,
	Matt Helsley <matthltc@...ibm.com>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	linux-nfs@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Chuck Lever <chuck.lever@...cle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linux Containers <containers@...ts.osdl.org>,
	Cedric Le Goater <clg@...ibm.com>
Subject: Re: [RFC][PATCH 2/4] sunrpc: Use utsnamespaces

"Serge E. Hallyn" <serue@...ibm.com> writes:

> Quoting Trond Myklebust (trond.myklebust@....uio.no):
>> On Tue, 2009-01-06 at 14:02 -0600, Serge E. Hallyn wrote:
>> > Quoting Matt Helsley (matthltc@...ibm.com):
>> > > We can often specify the UTS namespace to use when starting an RPC client.
>> > > However sometimes no UTS namespace is available (specifically during
> system
>> > > shutdown as the last NFS mount in a container is unmounted) so fall
>> > > back to the initial UTS namespace.
>> > 
>> > So what happens if we take this patch and do nothing else?
>> > 
>> > The only potential problem situation will be rpc requests
>> > made on behalf of a container in which the last task has
>> > exited, right?  So let's say a container did an nfs mount
>> > and then exits, causing an nfs umount request.
>> > 
>> > That umount request will now be sent with the wrong nodename.
>> > Does that actually cause problems, will the server use the
>> > nodename to try and determine the client sending the request?
>> 
>> The NFSv2/v3 umount rpc call will be sent by the 'umount' program from
>> userspace, not the kernel. The problem here is that because lazy mounts
>> exist, the lifetime of the RPC client may be longer than that of the
>
> Right that was what i was referring to.
>
>> container. In addition, it may be shared among more than 1 container,
>> because superblocks can be shared.
>
> Good point.  And in that case what do we care about (even though
> apparently we just might not care at all :) - who did the mount,
> or who is using it?
>
> In fact one thing I noticed in Matt's patch 3 was that he copied
> in the nodename verbatim, so a future hostname() by the container
> wouldn't be reflected, again not sure if that would matter.
>
>> One thing you need to be aware of here is that inode dirty data
>> writebacks may be initiated by completely different processes than the
>> one that dirtied the inode.
>
> Right, but I *was* thinking that we wanted to associate the nodename
> on the rpc calls with the hostname of the mounter, not the actor.  Maybe
> you'll tell me above that that is bogus.
>
>> IOW: Aside from being extremely ugly, approaches like [PATCH 4/4] which
>> rely on being able to determine the container-specific node name at RPC
>> generation time are therefore going to return incorrect values.
>
> So should we use patch 2/4, plus (as someone - was it you? - suggested)
> using a DEFAULT instead of init_utsname()->nodename when
> current->utsname() == NULL?

Is there any reason to believe that the kernel helper threads will ever
have a useful namespace value?  I don't think so.

That implies to me you want to capture the value at mount time, and to
pass it in to the rpc_call creation, and only at very specific well
defined points where we interact with user space should we examine
current->utsname().  At which point there should be no question
of current->utsname() is valid as the user space process is alive.

Eric
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