Guest get permitted access to certain Xen-owned pages of memory.  The
majority of such pages remain allocated / associated with a guest for
its entire lifetime.  Grant table v2 status pages, however, get
de-allocated when a guest switched (back) from v2 to v1.  The freeing
of such pages requires that the hypervisor know where in the guest
these pages were mapped.  The hypervisor tracks only one use within
guest space, but racing requests from the guest to insert mappings of
these pages may result in any of them to become mapped in multiple
locations.  Upon switching back from v2 to v1, the guest would then
retain access to a page that was freed and perhaps re-used for other
purposes.