mm-compaction-require-only-min-watermarks-for-non-costly-orders-fix orders-fix

mm-compaction-require-only-min-watermarks-for-non-costly-orders-fix orders-fix

Clarify why __isolate_free_page() does a order-0 watermark check with
apparent (1UL << order) gap, per Joonsoo.

Link: http://lkml.kernel.org/r/7ae4baec-4eca-e70b-2a69-94bea4fb19fa@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: David Rientjes <rientjes@google.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>