linux-kernel - [PATCH 5/6] mm, compaction: skip compound pages by order in free scanner

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Date:	Wed, 10 Jun 2015 11:32:33 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
	Minchan Kim <minchan@...nel.org>, Mel Gorman <mgorman@...e.de>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>
Subject: [PATCH 5/6] mm, compaction: skip compound pages by order in free scanner

The compaction free scanner is looking for PageBuddy() pages and skipping all
others.  For large compound pages such as THP or hugetlbfs, we can save a lot
of iterations if we skip them at once using their compound_order(). This is
generally unsafe and we can read a bogus value of order due to a race, but if
we are careful, the only danger is skipping too much.

When tested with stress-highalloc from mmtests on 4GB system with 1GB hugetlbfs
pages, the vmstat compact_free_scanned count decreased by at least 15%.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
Cc: Minchan Kim <minchan@...nel.org>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
Cc: Michal Nazarewicz <mina86@...a86.com>
Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Rik van Riel <riel@...hat.com>
Cc: David Rientjes <rientjes@...gle.com>
---
 mm/compaction.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index e37d361..4a14084 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -437,6 +437,24 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
 
 		if (!valid_page)
 			valid_page = page;
+
+		/*
+		 * For compound pages such as THP and hugetlbfs, we can save
+		 * potentially a lot of iterations if we skip them at once.
+		 * The check is racy, but we can consider only valid values
+		 * and the only danger is skipping too much.
+		 */
+		if (PageCompound(page)) {
+			unsigned int comp_order = compound_order(page);
+
+			if (comp_order > 0 && comp_order < MAX_ORDER) {
+				blockpfn += (1UL << comp_order) - 1;
+				cursor += (1UL << comp_order) - 1;
+			}
+
+			goto isolate_fail;
+		}
+
 		if (!PageBuddy(page))
 			goto isolate_fail;
 
@@ -496,6 +514,13 @@ isolate_fail:
 
 	}
 
+	/*
+	 * There is a tiny chance that we have read bogus compound_order(),
+	 * so be careful to not go outside of the pageblock.
+	 */
+	if (unlikely(blockpfn > end_pfn))
+		blockpfn = end_pfn;
+
 	trace_mm_compaction_isolate_freepages(*start_pfn, blockpfn,
 					nr_scanned, total_isolated);
 
-- 
2.1.4

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