sub lvm_BaseDiff_Lvh_4_Base_opt_mp ($;$) {
    my $lvm = shift;
    my ($base_lvh, $lv_mp) = @_;
    my $diff_dev = pathcat("/dev", 
                  $base_lvh->vg_name, $base_lvh->lv_name . ".diff");
    #TPe "diff_dev=%s", $diff_dev;

# next is call to by_mp -- here is where I don't seem to be
#getting a reference back.


    my $bdiff_lvh = $lvm->by_mp($lv_mp.".diff") if @_;
    $bdiff_lvh = $lvm->by_devpath($diff_dev) unless ref $bdiff_lvh;
## commented version below was orig -- trying above -- 
## seems to work better, but seems a bit random...

    #my $bdiff_lvh = (@_ && $lvm->by_mp($lv_mp . ".diff") or
    #               $lvm->by_devpath($diff_dev));
    die P "Can't find \"diff\" dir for vol %s by mount or device path", $lv_mp
      unless ref $bdiff_lvh;
    $bdiff_lvh->fs_mp($lv_mp) unless $bdiff_lvh->fs_mp;
    $bdiff_lvh->fs_type_name($base_lvh->fs_type_name) 
      unless $bdiff_lvh->fs_type_name;