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Patch series "Migrate Pages in lieu of discard", v11.

We're starting to see systems with more and more kinds of memory such as
Intel's implementation of persistent memory.

Let's say you have a system with some DRAM and some persistent memory.
Today, once DRAM fills up, reclaim will start and some of the DRAM
contents will be thrown out.  Allocations will, at some point, start
falling over to the slower persistent memory.

That has two nasty properties.  First, the newer allocations can end up in
the slower persistent memory.  Second, reclaimed data in DRAM are just
discarded even if there are gobs of space in persistent memory that could
be used.

This patchset implements a solution to these problems.  At the end of the
reclaim process in shrink_page_list() just before the last page refcount
is dropped, the page is migrated to persistent memory instead of being
dropped.

While I've talked about a DRAM/PMEM pairing, this approach would function
in any ...