> - implement _xen_bus_dmamem_alloc_range(), which will request a contigous
>   set of pages to the hypervisor if the pages returned by uvm_pglistalloc()
>   don't fit the constraints.

- it seems to leak memory in the case of EINVAL.

- there is no recovery in the case of failure of MEMOP_increase_reservation.
  is this intended?  i think it's better to do MEMOP_decrease_reservation
  only when MEMOP_increase_reservation succeeded.
  MEMOP_decrease_reservation unlikely helps MEMOP_increase_reservation
  anyway.

YAMAMOTO Takashi