mm/mpol: Use special PROT_NONE to migrate pages

mm/mpol: Use special PROT_NONE to migrate pages

Combine our previous PROT_NONE, mpol_misplaced and
migrate_misplaced_page() pieces into an effective migrate on fault
scheme.

Note that (on x86) we rely on PROT_NONE pages being !present and avoid
the TLB flush from try_to_unmap(TTU_MIGRATION). This greatly improves
the page-migration performance.

Suggested-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Paul Turner <pjt@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Link: http://lkml.kernel.org/n/tip-e98gyl8kr9jzooh2s4piuils@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>