[RFC PATCH v2 1/7] mm: page_alloc: avoid merging non-fallbackable pageblocks with others.

[Zi Yan]

From: Zi Yan <ziy at nvidia.com>

This is done in addition to MIGRATE_ISOLATE pageblock merge avoidance.
It prepares for the upcoming removal of the MAX_ORDER-1 alignment
requirement for CMA and alloc_contig_range().

MIGRARTE_HIGHATOMIC should not merge with other migratetypes like
MIGRATE_ISOLATE and MIGRARTE_CMA[1], so this commit prevents that too.
Also add MIGRARTE_HIGHATOMIC to fallbacks array for completeness.

[1] https://lore.kernel.org/linux-mm/20211130100853.GP3366@techsingularity.net/

Signed-off-by: Zi Yan <ziy at nvidia.com>
---
 include/linux/mmzone.h |  6 ++++++
 mm/page_alloc.c        | 28 ++++++++++++++++++----------
 2 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 58e744b78c2c..b925431b0123 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -83,6 +83,12 @@ static inline bool is_migrate_movable(int mt)
 	return is_migrate_cma(mt) || mt == MIGRATE_MOVABLE;
 }
 
+/* See fallbacks[MIGRATE_TYPES][3] in page_alloc.c */
+static inline bool migratetype_has_fallback(int mt)
+{
+	return mt < MIGRATE_PCPTYPES;
+}
+
 #define for_each_migratetype_order(order, type) \
 	for (order = 0; order < MAX_ORDER; order++) \
 		for (type = 0; type < MIGRATE_TYPES; type++)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index edfd6c81af82..107a5f186d3b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1041,6 +1041,12 @@ buddy_merge_likely(unsigned long pfn, unsigned long buddy_pfn,
 	return page_is_buddy(higher_page, higher_buddy, order + 1);
 }
 
+static inline bool has_non_fallback_pageblock(struct zone *zone)
+{
+	return has_isolate_pageblock(zone) || zone_cma_pages(zone) != 0 ||
+		zone->nr_reserved_highatomic != 0;
+}
+
 /*
  * Freeing function for a buddy system allocator.
  *
@@ -1116,14 +1122,15 @@ static inline void __free_one_page(struct page *page,
 	}
 	if (order < MAX_ORDER - 1) {
 		/* If we are here, it means order is >= pageblock_order.
-		 * We want to prevent merge between freepages on isolate
-		 * pageblock and normal pageblock. Without this, pageblock
-		 * isolation could cause incorrect freepage or CMA accounting.
+		 * We want to prevent merge between freepages on pageblock
+		 * without fallbacks and normal pageblock. Without this,
+		 * pageblock isolation could cause incorrect freepage or CMA
+		 * accounting or HIGHATOMIC accounting.
 		 *
 		 * We don't want to hit this code for the more frequent
 		 * low-order merging.
 		 */
-		if (unlikely(has_isolate_pageblock(zone))) {
+		if (unlikely(has_non_fallback_pageblock(zone))) {
 			int buddy_mt;
 
 			buddy_pfn = __find_buddy_pfn(pfn, order);
@@ -1131,8 +1138,8 @@ static inline void __free_one_page(struct page *page,
 			buddy_mt = get_pageblock_migratetype(buddy);
 
 			if (migratetype != buddy_mt
-					&& (is_migrate_isolate(migratetype) ||
-						is_migrate_isolate(buddy_mt)))
+					&& (!migratetype_has_fallback(migratetype) ||
+						!migratetype_has_fallback(buddy_mt)))
 				goto done_merging;
 		}
 		max_order = order + 1;
@@ -2483,6 +2490,7 @@ static int fallbacks[MIGRATE_TYPES][3] = {
 	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,   MIGRATE_TYPES },
 	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_TYPES },
 	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,   MIGRATE_TYPES },
+	[MIGRATE_HIGHATOMIC] = { MIGRATE_TYPES }, /* Never used */
 #ifdef CONFIG_CMA
 	[MIGRATE_CMA]         = { MIGRATE_TYPES }, /* Never used */
 #endif
@@ -2794,8 +2802,8 @@ static void reserve_highatomic_pageblock(struct page *page, struct zone *zone,
 
 	/* Yoink! */
 	mt = get_pageblock_migratetype(page);
-	if (!is_migrate_highatomic(mt) && !is_migrate_isolate(mt)
-	    && !is_migrate_cma(mt)) {
+	/* Only reserve normal pageblock */
+	if (migratetype_has_fallback(mt)) {
 		zone->nr_reserved_highatomic += pageblock_nr_pages;
 		set_pageblock_migratetype(page, MIGRATE_HIGHATOMIC);
 		move_freepages_block(zone, page, MIGRATE_HIGHATOMIC, NULL);
@@ -3544,8 +3552,8 @@ int __isolate_free_page(struct page *page, unsigned int order)
 		struct page *endpage = page + (1 << order) - 1;
 		for (; page < endpage; page += pageblock_nr_pages) {
 			int mt = get_pageblock_migratetype(page);
-			if (!is_migrate_isolate(mt) && !is_migrate_cma(mt)
-			    && !is_migrate_highatomic(mt))
+			/* Only change normal pageblock */
+			if (migratetype_has_fallback(mt))
 				set_pageblock_migratetype(page,
 							  MIGRATE_MOVABLE);
 		}
-- 
2.33.0