[PATCH 06/25] mm, compaction: Skip pageblocks with reserved pages

[Mel Gorman]

Reserved pages are set at boot time, tend to be clustered and almost never
become unreserved. When isolating pages for either migration sources or
target, skip the entire pageblock is one PageReserved page is encountered
on the grounds that it is highly probable the entire pageblock is reserved.

The performance impact is relative to the number of reserved pages in
the system and their location so it'll be variable but intuitively it
should make sense. If the memblock allocator was ever changed to spread
reserved pages throughout the address space then this patch would be
impaired but it would also be considered a bug given that such a change
would ruin fragmentation.

On both 1-socket and 2-socket machines, scan rates are reduced slightly
on workloads that intensively allocate THP while the system is fragmented.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 mm/compaction.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index 3afa4e9188b6..94d1e5b062ea 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -484,6 +484,15 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
 			goto isolate_fail;
 		}
 
+		/*
+		 * A reserved page is never freed and tend to be clustered in
+		 * the same pageblock. Skip the block.
+		 */
+		if (PageReserved(page)) {
+			blockpfn = end_pfn;
+			break;
+		}
+
 		if (!PageBuddy(page))
 			goto isolate_fail;
 
@@ -827,6 +836,13 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 					goto isolate_success;
 			}
 
+			/*
+			 * A reserved page is never freed and tend to be
+			 * clustered in the same pageblocks. Skip the block.
+			 */
+			if (PageReserved(page))
+				low_pfn = end_pfn;
+
 			goto isolate_fail;
 		}
 
-- 
2.16.4