Isnt this more a job for paravirt ops if it is so tightly bound to page
tables? Are we not adding another similar API?
API still has rcu issues and the example given for making things sleepable
is only working for the aging callback. The most important callback is for
try_to_unmao and page_mkclean. This means the API is still not generic
enough and likely not extendable as needed in its present form.
--