geom_linux_lvm: can't read Linux LVM volume

[Harry Newton]

Trying to read a Linux raid array ( as per
https://wiki.archlinux.org/index.php/Software_RAID_and_LVM ) which I can
read perfectly well on a couple of Linux boxes.  geom_linux_lvm.ko loaded,
but:

  geom linux_lvm list
  geom linux_lvm status

return nothing.  kern.geom.debugflags=16 and kern.geom.linux_lvm.debug=16,
but *nothing* in logs.  No /dev/linux_lvm/ entries appear, though the two
component disks appear as /dev/da0 and /dev/da1.

Suspect I maybe missing something obvious: any ideas where I can start
debugging this ?

/Harry