[RFC,PATCH 0/4] Dynamic Pseudo Root

[J. Bruce Fields]

On Fri, Dec 07, 2007 at 02:38:15PM -0500, Chuck Lever wrote:
> On Dec 7, 2007, at 2:12 PM, Steve Dickson wrote:
> > The following patch series gives rpc.mountd the ability to allocate
> > a dynamic pseudo root, so the 'fsid=0' export option is no longer
> > required. This allows v2, v3 and v4 clients mounts without any
> > changes to the server's exports list.
> >
> > One anomaly of the Linux NFS server is that it requires a pseudo root
> > to be defined. Currently the only way a pseudo root can be defined  
> > is by
> > setting the fsid to zero (i.e. fsid=0). So if we wanted to make v4
> > the default mounting version and have things just work like v2/v3
> > all of the existing exports configurations would have to change
> > (i.e. a 'fsid=0' would have to be added) to support a v4 mounts,
> > which, imho, is unacceptable. So this patch series address
> > this problem.
> >
> > How it works:
> >
> > Working with Bruce Fields, it was decide to implement this
> > functionality in userland (i.e. in rpc.mountd) by using existing
> > utilities like tmpfs.Please remember this is an RFC, so this may not
> > be the final solution, but it is a first step...
> >
> > The new '-R' command line argument, tells rpc.mountd to create a  
> > pseudo root
> > if and only if, one is not already defined. Meaning if there is an
> > export entry in /etc/exports with an 'fsid=0' option, that entry
> > will be the pseudo root and rpc.mountd will not create a root.
> 
> Why not make that the default behavior?

Because it's experimental code for now.

> Is there a case when you  need to switch this default behavior off?
> (ie, why have a command  line option to enable behavior that you need
> to have anyway).

I agree that the goal should eventually be to have it always on.


> > Now when a pseudo root is not defined, rpc.mountd will creates a  
> > pseudo
> > root in '/var/lib/nfs/v4root'. By create I mean, the /var/lib/nfs/ 
> > v4root
> > directory will be created and then be mounted as a tmpfs filesystem:
> >
> >     mount -t tmpfs tmpfs /var/lib/nfs/v4root
> >
> > Making /var/lib/nfs/v4root a true mount point.
> 
> What happens if there are more than one rpc.mountd processes running  
> on the server?  Would having rpc.mound use a private namespace allow  
> multiple instances to avoid colliding?

We should keep rpc.mountd in a private namespace, and, yes, partly
because it will keep other processes (including other mountd's) from
stepping on each other's pseudo-filesystem.

--b.