linux-kernel - [RFC PATCH 2/2] mm/page_alloc: avoid splitting pages of order 2 and 3 in migration fallback

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Date:	Sat, 30 Jan 2016 03:25:51 +0800
From:	ChengYi He <chengyihetaipei@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mel Gorman <mgorman@...hsingularity.net>,
	Michal Hocko <mhocko@...e.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <js1304@...il.com>,
	Yaowei Bai <bywxiaobai@....com>,
	Xishi Qiu <qiuxishi@...wei.com>,
	Alexander Duyck <alexander.h.duyck@...hat.com>,
	"'Kirill A . Shutemov'" <kirill.shutemov@...ux.intel.com>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, chengyihetaipei@...il.com
Subject: [RFC PATCH 2/2] mm/page_alloc: avoid splitting pages of order 2 and
 3 in migration fallback

While buddy system fallbacks to allocate different migration type pages,
it prefers the largest feasible pages and might split the chosen page
into smalller ones. If the largest feasible pages are less than or equal
to orde-3 and migration fallback happens frequently, then order-2 and
order-3 pages can be exhausted easily. This patch aims to allocate the
smallest feasible pages for the fallback mechanism under this condition.

Signed-off-by: ChengYi He <chengyihetaipei@...il.com>
---
 mm/page_alloc.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 50c325a..3fcb653 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1802,9 +1802,22 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
 	struct page *page;
 
 	/* Find the largest possible block of pages in the other list */
-	for (current_order = MAX_ORDER-1;
-				current_order >= order && current_order <= MAX_ORDER-1;
-				--current_order) {
+	for (current_order = MAX_ORDER - 1;
+			current_order >= max_t(unsigned int, PAGE_ALLOC_COSTLY_ORDER + 1, order);
+			--current_order) {
+		page = __rmqueue_fallback_order(zone, order, start_migratetype,
+				current_order);
+
+		if (page)
+			return page;
+	}
+
+	/*
+	 * While current_order <= PAGE_ALLOC_COSTLY_ORDER, find the smallest
+	 * feasible pages in the other list to avoid splitting high order pages
+	 */
+	for (current_order = order; current_order <= PAGE_ALLOC_COSTLY_ORDER;
+			++current_order) {
 		page = __rmqueue_fallback_order(zone, order, start_migratetype,
 				current_order);
 
-- 
1.9.1