I think I am still on thin ice with this patchset but its reasonably
simple now and I hope that the simplification by removing the
unwind code and the migration of swapcache pages is something that
turns out to be workable. I have tested with some simple
test programs here that definitely are not able to produce all
potential troublesome scenarios.

This patchset depends on V4 of the Swap Migration patchset.

It implements direct page migration on top of that patchset
with the same APIs supported by the swap migration patchset.

The patchset consists of three patches:

1. SWAP_REFERENCE

try_to_unmap pages currently does not distinguish between a failure
due to a really unswappable page and a failure due to the page having
just been referenced. The patch adds another exit code allowing
the detection of failure due to a recently referenced page. In that
case the page migration code may want to immediately retry again.

2. Direct page migration

This patch extends the migrate_pages() function to support direct
page migration. Limitation: No support for migrating dirty pages
backed by files. SwapCache pages are swapped out. Generally fall
back to swap if necessary.

3. Policy layer update

This adds support for direct page migration to the memory policy
layer. It adds the capability to sys_migrate to do a staggered
move of pages.