From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH 1/4] VFS: Add a VFS helper function
	vfs_remote_path_lookup()
Date: Fri, 3 Apr 2009 02:52:20 +0100
Message-ID: <20090403015220.GS28946@ZenIV.linux.org.uk>
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org
To: Trond Myklebust <Trond.Myklebust@netapp.com>
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On Fri, Apr 03, 2009 at 02:09:54AM +0100, Al Viro wrote:
> On Fri, Apr 03, 2009 at 12:18:59AM +0100, Al Viro wrote:
> 
> > Eww...  Let's not.  First of all, the really useful part of nd->root for
> > normal case is that it allows to get ...->fs->root once.  So it's better
> > to *cache* ->fs->root in there.  At which point the flag disappears,
> > since it becomes simply nd->root.mnt != NULL.
> > 
> > The interesting part is keeping refcounting happy.  I'll see how to do
> > that - it's clearly useful on its own.  Hopefully will post later tonight...
> 
> ... and the ugly part is the check in follow_dotdot() - one about crossing
> out of chroot.  Joy...

OK, that's actually doable.
See git.kernel.org/pub/scm/linux/kernel/git/viro/vfs-2.6.git/ cache-root
for current variant; it's for comments *only* - not for merge.  After that
one we should be able to do a modified variant of e.g. vfs_path_lookup()
that would do
	get_path(root);
	nd->root = root;
instead of
	nd->root.mnt = NULL;
and have lookup with root set to given place without touching current->fs
at all.  Warning: this is completely untested.  Not even "does it boot?",
just a code dump.