[PATCH RFC 3/4] mm/page_alloc: always move pages to the tail of the freelist in unset_migratetype_isolate()

[David Hildenbrand]

Page isolation doesn't actually touch the pages, it simply isolates
pageblocks and moves all free pages to the MIGRATE_ISOLATE freelist.

We already place pages to the tail of the freelists when undoing
isolation via __putback_isolated_page(), let's do it in any case
(e.g., if order == pageblock_order) and document the behavior.

This change results in all pages getting onlined via online_pages() to
be placed to the tail of the freelist.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Scott Cheloha <cheloha@xxxxxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 include/linux/page-isolation.h |  2 ++
 mm/page_alloc.c                | 36 +++++++++++++++++++++++++++++-----
 mm/page_isolation.c            |  8 ++++++--
 3 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/include/linux/page-isolation.h b/include/linux/page-isolation.h
index 572458016331..a36be2cf4dbb 100644
--- a/include/linux/page-isolation.h
+++ b/include/linux/page-isolation.h
@@ -38,6 +38,8 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
 void set_pageblock_migratetype(struct page *page, int migratetype);
 int move_freepages_block(struct zone *zone, struct page *page,
 				int migratetype, int *num_movable);
+int move_freepages_block_tail(struct zone *zone, struct page *page,
+			      int migratetype);
 
 /*
  * Changes migrate type in [start_pfn, end_pfn) to be MIGRATE_ISOLATE.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index bba9a0f60c70..75b0f49b4022 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -899,6 +899,15 @@ static inline void move_to_free_list(struct page *page, struct zone *zone,
 	list_move(&page->lru, &area->free_list[migratetype]);
 }
 
+/* Used for pages which are on another list */
+static inline void move_to_free_list_tail(struct page *page, struct zone *zone,
+					  unsigned int order, int migratetype)
+{
+	struct free_area *area = &zone->free_area[order];
+
+	list_move_tail(&page->lru, &area->free_list[migratetype]);
+}
+
 static inline void del_page_from_free_list(struct page *page, struct zone *zone,
 					   unsigned int order)
 {
@@ -2323,7 +2332,7 @@ static inline struct page *__rmqueue_cma_fallback(struct zone *zone,
  */
 static int move_freepages(struct zone *zone,
 			  struct page *start_page, struct page *end_page,
-			  int migratetype, int *num_movable)
+			  int migratetype, int *num_movable, bool to_tail)
 {
 	struct page *page;
 	unsigned int order;
@@ -2354,7 +2363,10 @@ static int move_freepages(struct zone *zone,
 		VM_BUG_ON_PAGE(page_zone(page) != zone, page);
 
 		order = page_order(page);
-		move_to_free_list(page, zone, order, migratetype);
+		if (to_tail)
+			move_to_free_list_tail(page, zone, order, migratetype);
+		else
+			move_to_free_list(page, zone, order, migratetype);
 		page += 1 << order;
 		pages_moved += 1 << order;
 	}
@@ -2362,8 +2374,9 @@ static int move_freepages(struct zone *zone,
 	return pages_moved;
 }
 
-int move_freepages_block(struct zone *zone, struct page *page,
-				int migratetype, int *num_movable)
+static int __move_freepages_block(struct zone *zone, struct page *page,
+				  int migratetype, int *num_movable,
+				  bool to_tail)
 {
 	unsigned long start_pfn, end_pfn;
 	struct page *start_page, *end_page;
@@ -2384,7 +2397,20 @@ int move_freepages_block(struct zone *zone, struct page *page,
 		return 0;
 
 	return move_freepages(zone, start_page, end_page, migratetype,
-								num_movable);
+			      num_movable, to_tail);
+}
+
+int move_freepages_block(struct zone *zone, struct page *page,
+			 int migratetype, int *num_movable)
+{
+	return __move_freepages_block(zone, page, migratetype, num_movable,
+				      false);
+}
+
+int move_freepages_block_tail(struct zone *zone, struct page *page,
+			      int migratetype)
+{
+	return __move_freepages_block(zone, page, migratetype, NULL, true);
 }
 
 static void change_pageblock_range(struct page *pageblock_page,
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index abfe26ad59fd..84aa1d14751d 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -83,7 +83,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
 	 * Because freepage with more than pageblock_order on isolated
 	 * pageblock is restricted to merge due to freepage counting problem,
 	 * it is possible that there is free buddy page.
-	 * move_freepages_block() doesn't care of merge so we need other
+	 * move_freepages_block*() don't care about merging, so we need another
 	 * approach in order to merge them. Isolation and free will make
 	 * these pages to be merged.
 	 */
@@ -106,9 +106,13 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
 	 * If we isolate freepage with more than pageblock_order, there
 	 * should be no freepage in the range, so we could avoid costly
 	 * pageblock scanning for freepage moving.
+	 *
+	 * We didn't actually touch any of the isolated pages, so place them
+	 * to the tail of the freelists. This is especially relevant during
+	 * memory onlining.
 	 */
 	if (!isolated_page) {
-		nr_pages = move_freepages_block(zone, page, migratetype, NULL);
+		nr_pages = move_freepages_block_tail(zone, page, migratetype);
 		__mod_zone_freepage_state(zone, nr_pages, migratetype);
 	}
 	set_pageblock_migratetype(page, migratetype);
-- 
2.26.2