Martin Schwidefsky wrote:
> The state transitions (a diagram would be nicer but that is too hard
> to do in ascii art...):
> {Ur,Sr,Vr,Pr}: a resident page will change its block usage state if the
>      guest requests it with page_set_{unused,stable,volatile}.
> {Uz,Sz,Vz}: a logically zero page will change its block usage state if the
>      guest requests it with page_set_{unused,stable,volatile}. The
>      guest can't create the Pz state, the state will be Vz instead.
> Ur -> Uz: the host system can remove an unused, resident page from memory
> Sz -> Sr: on first access a stable, logically zero page will become resident
> Sr -> Sp: the host system can swap a stable page to disk
> Sp -> Sr: a guest access to a Sp page forces the host to retrieve it
> Vr -> Vz: the host can discard a volatile page
> Sp -> Uz: a page preserved by the host will be removed if the guest sets 
>      the block usage state to unused.
> Sp -> Vz: a page preserved by the host will be discarded if the guest sets
>      the block usage state to volatile.
> Pr -> Sp: the host can move a page from Pr to Sp if it discovers that the
>      page is dirty while trying to discard the page. The page content is
>      written to the paging device.
> Pr -> Vz: the host can discard a Pr page. The Pz state is replaced by the
>      Vz state.

I created the attached .dot graph based purely on this description.  It 
looks reasonable, but I didn't see how a page enters a Pr state.

    J