[PATCH] mm, compaction: extend pageblock_skip_persistent() to all compound pages

[Vlastimil Babka]

The pageblock_skip_persistent() function checks for HugeTLB pages of pageblock
order. When clearing pageblock skip bits for compaction, the bits are not
cleared for such pageblocks, because they cannot contain base pages suitable
for migration, nor free pages to use as migration targets.

This optimization can be simply extended to all compound pages of order equal
or larger than pageblock order, because migrating such pages (if they support
it) cannot help sub-pageblock fragmentation. This includes THP's and also
gigantic HugeTLB pages, which the current implementation doesn't persistently
skip due to a strict pageblock_order equality check and not recognizing tail
pages.

Additionally, this patch removes the pageblock_skip_persistent() calls from
migration and free scanner, since the generic compound page treatment together
with update_pageblock_skip() call will also lead to pageblocks starting with a
large enough compound page being immediately marked for skipping, which then
becomes persistent.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/compaction.c | 43 +++++++++++++++++--------------------------
 1 file changed, 17 insertions(+), 26 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index f91d64cb94ac..11be786af74f 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -218,17 +218,21 @@ static void reset_cached_positions(struct zone *zone)
 }
 
 /*
- * Hugetlbfs pages should consistenly be skipped until updated by the hugetlb
- * subsystem.  It is always pointless to compact pages of pageblock_order and
- * the free scanner can reconsider when no longer huge.
+ * Compound pages of >= pageblock_order should consistenly be skipped until
+ * released. It is always pointless to compact pages of such order (if they are
+ * migratable), and the pageblocks they occupy cannot contain any free pages.
  */
-static bool pageblock_skip_persistent(struct page *page, unsigned int order)
+static bool pageblock_skip_persistent(struct page *page)
 {
-	if (!PageHuge(page))
+	if (!PageCompound(page))
 		return false;
-	if (order != pageblock_order)
-		return false;
-	return true;
+
+	page = compound_head(page);
+
+	if (compound_order(page) >= pageblock_order)
+		return true;
+
+	return false;
 }
 
 /*
@@ -255,7 +259,7 @@ static void __reset_isolation_suitable(struct zone *zone)
 			continue;
 		if (zone != page_zone(page))
 			continue;
-		if (pageblock_skip_persistent(page, compound_order(page)))
+		if (pageblock_skip_persistent(page))
 			continue;
 
 		clear_pageblock_skip(page);
@@ -322,8 +326,7 @@ static inline bool isolation_suitable(struct compact_control *cc,
 	return true;
 }
 
-static inline bool pageblock_skip_persistent(struct page *page,
-					     unsigned int order)
+static inline bool pageblock_skip_persistent(struct page *page)
 {
 	return false;
 }
@@ -472,10 +475,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
 		if (PageCompound(page)) {
 			const unsigned int order = compound_order(page);
 
-			if (pageblock_skip_persistent(page, order)) {
-				set_pageblock_skip(page);
-				blockpfn = end_pfn;
-			} else if (likely(order < MAX_ORDER)) {
+			if (likely(order < MAX_ORDER)) {
 				blockpfn += (1UL << order) - 1;
 				cursor += (1UL << order) - 1;
 			}
@@ -797,10 +797,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		if (PageCompound(page)) {
 			const unsigned int order = compound_order(page);
 
-			if (pageblock_skip_persistent(page, order)) {
-				set_pageblock_skip(page);
-				low_pfn = end_pfn;
-			} else if (likely(order < MAX_ORDER))
+			if (likely(order < MAX_ORDER))
 				low_pfn += (1UL << order) - 1;
 			goto isolate_fail;
 		}
@@ -863,13 +860,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 			 * is safe to read and it's 0 for tail pages.
 			 */
 			if (unlikely(PageCompound(page))) {
-				const unsigned int order = compound_order(page);
-
-				if (pageblock_skip_persistent(page, order)) {
-					set_pageblock_skip(page);
-					low_pfn = end_pfn;
-				} else
-					low_pfn += (1UL << order) - 1;
+				low_pfn += (1UL << compound_order(page)) - 1;
 				goto isolate_fail;
 			}
 		}
-- 
2.14.1