linux-kernel - [PATCH 06/19] mm: page_alloc: Only check the zone id check if pages are buddies

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Message-Id: <1399974350-11089-7-git-send-email-mgorman@suse.de>
Date:	Tue, 13 May 2014 10:45:37 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>
Subject: [PATCH 06/19] mm: page_alloc: Only check the zone id check if pages are buddies

A node/zone index is used to check if pages are compatible for merging
but this happens unconditionally even if the buddy page is not free. Defer
the calculation as long as possible. Ideally we would check the zone boundary
but nodes can overlap.

Signed-off-by: Mel Gorman <mgorman@...e.de>
Acked-by: Johannes Weiner <hannes@...xchg.org>
Acked-by: Rik van Riel <riel@...hat.com>
---
 mm/page_alloc.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3b6ae9d..8971953 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -508,16 +508,26 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
 	if (!pfn_valid_within(page_to_pfn(buddy)))
 		return 0;
 
-	if (page_zone_id(page) != page_zone_id(buddy))
-		return 0;
-
 	if (page_is_guard(buddy) && page_order(buddy) == order) {
 		VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
+
+		if (page_zone_id(page) != page_zone_id(buddy))
+			return 0;
+
 		return 1;
 	}
 
 	if (PageBuddy(buddy) && page_order(buddy) == order) {
 		VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
+
+		/*
+		 * zone check is done late to avoid uselessly
+		 * calculating zone/node ids for pages that could
+		 * never merge.
+		 */
+		if (page_zone_id(page) != page_zone_id(buddy))
+			return 0;
+
 		return 1;
 	}
 	return 0;
-- 
1.8.4.5

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