A lot of the stuff described there doesn't work, e.g. the multi key 
option and the example. The kernel code only supports single keys.
When you try to use multi keys it just fails with an error message
aboput unsupported ioctls.

To reproduce: try the example at the end of the losetup manpage

I think the example should be changed to something that actually works 
with the shipped kernel. And options that don't work either should
be hidden or at least clearly marked.

Another problem is that it tries to get a lot of entropy
and may stall for a long time. It should give hints on how
to get more entry or how to use /dev/hw_random if available