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>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8df46ee472f9fefe7a6d1036816e1e1d7aef7741..6d3504749af414eef6d12f44b6eb23100eec7dc2 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2489,7 +2489,12 @@ int __isolate_free_page(struct page *page, unsigned int order)
        mt = get_pageblock_migratetype(page);
 
        if (!is_migrate_isolate(mt)) {
-               /* Obey watermarks as if the page was being allocated */
+               /*
+                * Obey watermarks as if the page was being allocated. We can
+                * emulate a high-order watermark check with a raised order-0
+                * watermark, because we already know our high-order page
+                * exists.
+                */
                watermark = min_wmark_pages(zone) + (1UL << order);
                if (!zone_watermark_ok(zone, 0, watermark, 0, ALLOC_CMA))
                        return 0;