mm: cma: discard clean pages during contiguous allocation instead of migration

mm: cma: discard clean pages during contiguous allocation instead of migration

Drop clean cache pages instead of migration during alloc_contig_range() to
minimise allocation latency by reducing the amount of migration that is
necessary.  It's useful for CMA because latency of migration is more
important than evicting the background process's working set.  In
addition, as pages are reclaimed then fewer free pages for migration
targets are required so it avoids memory reclaiming to get free pages,
which is a contributory factor to increased latency.

I measured elapsed time of __alloc_contig_migrate_range() which migrates
10M in 40M movable zone in QEMU machine.

Before - 146ms, After - 7ms

[[email protected]: fix nommu build]
Signed-off-by: Mel Gorman <[email protected]>
Signed-off-by: Minchan Kim <[email protected]>
Reviewed-by: Mel Gorman <[email protected]>
Cc: Marek Szyprowski <[email protected]>
Acked-by: Michal Nazarewicz <[email protected]>
Cc: Rik van Riel <[email protected]>
Tested-by: Kyungmin Park <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>