Previously, memory objects assumed that they remained attached to a
process until they are destroyed. In the past this was mostly true,
but worked by luck because a process could technically map the memory
and then close the file descriptor which would eventually explode. Now we
do the process related cleanup (MMU unmap, fixup statistics) when the
object is released from the process so the process can go away without
affecting the other holders of the mem object refcount.
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