[RFC PATCH 25/26] mm: page_alloc: disallow fallbacks when 2M defrag is enabled

[Johannes Weiner <hannes@cmpxchg.org>]

Fallbacks are already unlikely due to watermarks being enforced
against MIGRATE_FREE blocks. Eliminate them altogether. This allows
compaction to look exclusively at movable blocks, reducing the number
of pageblocks it needs to scan on an ongoing basis.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/compaction.c | 52 +++++--------------------------------------------
 mm/internal.h   |  2 +-
 mm/page_alloc.c |  8 ++++++++
 3 files changed, 14 insertions(+), 48 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index e33c99eb34a8..37dfd1878bef 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1258,46 +1258,6 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
 #endif /* CONFIG_COMPACTION || CONFIG_CMA */
 #ifdef CONFIG_COMPACTION
 
-static bool suitable_migration_source(struct compact_control *cc,
-							struct page *page)
-{
-	int block_mt;
-
-	if (pageblock_skip_persistent(page))
-		return false;
-
-	if ((cc->mode != MIGRATE_ASYNC) || !cc->direct_compaction)
-		return true;
-
-	block_mt = get_pageblock_migratetype(page);
-
-	if (cc->migratetype == MIGRATE_MOVABLE)
-		return is_migrate_movable(block_mt);
-	else
-		return block_mt == cc->migratetype;
-}
-
-/* Returns true if the page is within a block suitable for migration to */
-static bool suitable_migration_target(struct compact_control *cc,
-							struct page *page)
-{
-	int mt = get_pageblock_migratetype(page);
-
-	/* If the page is a large free page, then disallow migration */
-	if (mt == MIGRATE_FREE)
-		return false;
-
-	if (cc->ignore_block_suitable)
-		return true;
-
-	/* If the block is MIGRATE_MOVABLE or MIGRATE_CMA, allow migration */
-	if (is_migrate_movable(mt))
-		return true;
-
-	/* Otherwise skip the block */
-	return false;
-}
-
 static inline unsigned int
 freelist_scan_limit(struct compact_control *cc)
 {
@@ -1620,7 +1580,7 @@ static void isolate_freepages(struct compact_control *cc)
 			continue;
 
 		/* Check the block is suitable for migration */
-		if (!suitable_migration_target(cc, page))
+		if (!is_migrate_movable(get_pageblock_migratetype(page)))
 			continue;
 
 		/* If isolation recently failed, do not retry */
@@ -1927,14 +1887,12 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
 			continue;
 
 		/*
-		 * For async direct compaction, only scan the pageblocks of the
-		 * same migratetype without huge pages. Async direct compaction
-		 * is optimistic to see if the minimum amount of work satisfies
-		 * the allocation. The cached PFN is updated as it's possible
-		 * that all remaining blocks between source and target are
+		 * The cached PFN is updated as it's possible that all
+		 * remaining blocks between source and target are
 		 * unsuitable and the compaction scanners fail to meet.
 		 */
-		if (!suitable_migration_source(cc, page)) {
+		if (pageblock_skip_persistent(page) ||
+		    !is_migrate_movable(get_pageblock_migratetype(page))) {
 			update_cached_migrate(cc, block_end_pfn);
 			continue;
 		}
diff --git a/mm/internal.h b/mm/internal.h
index 24f43f5db88b..1c0886c3ce0e 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -741,7 +741,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
 #define ALLOC_HIGH		 0x20 /* __GFP_HIGH set */
 #define ALLOC_CPUSET		 0x40 /* check for correct cpuset */
 #define ALLOC_CMA		 0x80 /* allow allocations from CMA areas */
-#ifdef CONFIG_ZONE_DMA32
+#if defined(CONFIG_ZONE_DMA32) && !defined(CONFIG_COMPACTION)
 #define ALLOC_NOFRAGMENT	0x100 /* avoid mixing pageblock types */
 #else
 #define ALLOC_NOFRAGMENT	  0x0
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f835a5548164..9db588a1de3b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2622,11 +2622,19 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
  *
  * The other migratetypes do not have fallbacks.
  */
+#ifdef CONFIG_COMPACTION
+static int fallbacks[MIGRATE_TYPES][2] = {
+	[MIGRATE_UNMOVABLE]   = { MIGRATE_FREE, MIGRATE_TYPES },
+	[MIGRATE_MOVABLE]     = { MIGRATE_FREE, MIGRATE_TYPES },
+	[MIGRATE_RECLAIMABLE] = { MIGRATE_FREE, MIGRATE_TYPES },
+};
+#else
 static int fallbacks[MIGRATE_TYPES][4] = {
 	[MIGRATE_UNMOVABLE]   = { MIGRATE_FREE, MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,   MIGRATE_TYPES },
 	[MIGRATE_MOVABLE]     = { MIGRATE_FREE, MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_TYPES },
 	[MIGRATE_RECLAIMABLE] = { MIGRATE_FREE, MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,   MIGRATE_TYPES },
 };
+#endif
 
 #ifdef CONFIG_CMA
 static __always_inline struct page *__rmqueue_cma_fallback(struct zone *zone,
-- 
2.39.2