mm: support MIGRATE_DISCARD

mm: support MIGRATE_DISCARD

Introduce MIGRATE_DISCARD mode in migration.  It drops *clean cache pages*
instead of migration so that migration latency could be reduced by
avoiding (memcpy + page remapping).  It's useful for CMA because latency
of migration is very important rather than eviction of background
processes's workingset.  In addition, it needs less free pages for
migration targets so it could avoid memory reclaiming to get free pages,
which is another factor increase latency.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>