[PATCH 0/5] Dynamic Pseudo Root

[Steve Dickson]

Here is a second go around on the nfs-utils patches
the implement dynamic pseudo root for NFS4 exports. 
This series includes changes from the review comments
I received and a number of problems I found an along
the way, but the overall design and functionality
stayed the same. 

So in a nutshell here's how it works:

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.

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.

Next, all the exported directories in the /etc/export file will 
be 'mount --bind' mounted under /var/lib/nfs/v4root. For example, for 
the following export entires:

    /usr *(rw,nohide)
    /home/tmp *(rw)

will cause the following 'mount --bind's to occur.
   
   mount --bind /usr /var/lib/nfs/v4root/usr
   mount --bind /home/tmp /var/lib/nfs/v4root/home/tmp

Now when the PUTROOTFH comes in from the client mount
the /var/lib/nfs/v4root directory is returned as the 
root file handle. This means subsequent GETFHs and LOOKUPs 
will use file handle of /var/lib/nfs/v4root as the rootfh.

Now the main advantages to implementing this in userland is
supportability as well as portability. Userlevel code,
in general, is easier to support (and change) than kernel
code. Plus, back porting this code to later releases will 
be much easier.

Now the main drawback to this implementation could be scalability.
Its not clear how well having a --bind mount for every export
will scale. But only time will tell.

In the end, I'm hopeful this will be the first toward making
V4 the default version (replacing v3). 

Comments?

steved.