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

[Vlastimil Babka <vbabka@xxxxxxx>]

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@xxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
---
 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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