linux-kernel - Re: Grace period

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Message-ID: <20120410133913.GF18465@fieldses.org>
Date:	Tue, 10 Apr 2012 09:39:13 -0400
From:	"bfields@...ldses.org" <bfields@...ldses.org>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Jeff Layton <jlayton@...hat.com>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Grace period

On Tue, Apr 10, 2012 at 02:56:12PM +0400, Stanislav Kinsbursky wrote:
> 09.04.2012 22:11, bfields@...ldses.org пишет:
> >Since NFSv4 doesn't have a separate MOUNT protocol, clients need to be
> >able to do readdir's and lookups to get to exported filesystems.  We
> >support this in the Linux server by exporting all the filesystems from
> >"/" on down that must be traversed to reach a given filesystem.  These
> >exports are very restricted (e.g. only parents of exports are visible).
> >
> 
> Ok, thanks for explanation.
> So, this pseudoroot looks like a part of NFS server internal
> implementation, but not a part of a standard. That's good.
> 
> >>Why does it prevents implementing of check for "superblock-network
> >>namespace" pair on NFS server start and forbid (?) it in case of
> >>this pair is shared already in other namespace? I.e. maybe this
> >>pseudoroot can be an exclusion from this rule?
> >
> >That might work.  It's read-only and consists only of directories, so
> >the grace period doesn't affect it.
> >
> 
> I've just realized, that this per-sb grace period won't work.
> I.e., it's a valid situation, when two or more containers located on
> the same filesystem, but shares different parts of it. And there is
> not conflict here at all.

Well, there may be some conflict in that a file could be hardlinked into
both subtrees, and that file could be locked from users of either
export.

--b.

> I don't see any clear and simple way how to handle such races,
> because otherwise we have to tie network namespace and filesystem
> namespace.
> I.e. there will be required some way to define, was passed export
> directory shared already somewhere else or not.
> 
> Realistic solution - since export check should be done in initial
> file system environment (most probably container will have it's own
> root), then we to pass this data to some kernel thread/userspace
> daemon in initial file system environment somehow (sockets doesn't
> suits here... Shared memory?).
> 
> Improbable solution - patching VFS layer...
> 
> -- 
> Best regards,
> Stanislav Kinsbursky
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