From: Vlastimil Babka <vbabka@suse.cz>
Date: Sat, 10 Sep 2016 10:34:06 +0000 (+1000)
Subject: mm-compaction-require-only-min-watermarks-for-non-costly-orders-fix orders-fix
X-Git-Url: http://drtracing.org/?a=commitdiff_plain;h=b5a7064c45a5c9703395cc6ac60dc29139326143;p=deliverable%2Flinux.git

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 8df46ee472f9..6d3504749af4 100644
--- 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;