NFS and mount_nullfs, kernel panic?

[Christer Solskogen]

I have a machine which works as a prison for a couple of jails. That
particular machine has its /usr/ports, /usr/ports/distfiles, and
/usr/ports/packages mounted via NFS from another FreeBSD machine. The
machine that crases also nullmounts those filesystems into each jail.
I'm not completly sure that this is the reason the machine crashes,
but i suspect that is the reason.
What can I do to figure it how? What kind of debugging knobs do I need
to apply to learn more about the problem?

-- 
chs,