IMO any memory that has been registered for a P2P transaction should be
locked from being evicted. So if there's a get_user_pages call it needs
to be pinned until the put_page. The main issue being with the RDMA
case: handling an eviction when a chunk of memory has been registered as
an MR would be very tricky. The MR may be relied upon by another host
and the kernel would have to inform user-space the MR was invalid then
user-space would have to tell the remote application.