[PATCH] mm, compaction: require only min watermarks for non-costly orders-fix

[Vlastimil Babka]

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

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/page_alloc.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a5c0f914ec00..216715504fb4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2491,7 +2491,12 @@ 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;
-- 
2.9.2