Re: [PATCH 1/4] VFS: Add a VFS helper function vfs_remote_path_lookup()

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, Mar 11, 2009 at 03:50:33PM -0400, Trond Myklebust wrote:
> The purpose of this patch is to improve the mount path lookup support for
> filesystems such as NFSv4, which require you to look up a mount path
> string in a remote server's namespace.
> 
> Traversing such a path is pretty much identical to walking a local path,
> in that it may involve following symlinks and even following referrals to
> volumes that reside on other servers. Since the standard VFS path lookup
> code already supports all these features (using in-kernel automounts for
> following referrals) it would be nice to be able to reuse that code rather
> than special case the mount path lookup in the NFS client.
> 
> This patch therefore defines a VFS helper function that sets up a temporary
> mount namespace to represent the server namespace, and has the current
> task pivot into that prior to doing the path lookup. Upon completion, it
> pivots back into the original namespace, and destroys the private one.

NAK.  You are relying on too many things about caller (e.g. just what happens
if you have shared fs_struct?  Does caller guarantee it won't happen?), so
that's a bloody bad interface.  Open-code that sucker, then let's see what
to do with surrounding code.  As it stands - no.