[PATCH v3 7/8] mm, compaction: restrict async compaction to pageblocks of same migratetype

[Vlastimil Babka <vbabka@suse.cz>]

The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE
pageblocks. This is a heuristic intended to reduce latency, based on the
assumption that non-MOVABLE pageblocks are unlikely to contain movable pages.

However, with the exception of THP's, most high-order allocations are not
movable. Should the async compaction succeed, this increases the chance that
the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the
long-term fragmentation worse.

This patch attempts to help the situation by changing async direct compaction
so that the migrate scanner only scans the pageblocks of the requested
migratetype. If it's a non-MOVABLE type and there are such pageblocks that do
contain movable pages, chances are that the allocation can succeed within one
of such pageblocks, removing the need for a fallback. If that fails, the
subsequent sync attempt will ignore this restriction.

In testing based on 4.9 kernel with stress-highalloc from mmtests configured
for order-4 GFP_KERNEL allocations, this patch has reduced the number of
unmovable allocations falling back to movable pageblocks by 30%. The number
of movable allocations falling back is reduced by 12%.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/compaction.c | 11 +++++++++--
 mm/page_alloc.c | 20 +++++++++++++-------
 2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index c48da73e30a5..2c288e75840d 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -986,10 +986,17 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
 static bool suitable_migration_source(struct compact_control *cc,
 							struct page *page)
 {
-	if (cc->mode != MIGRATE_ASYNC)
+	int block_mt;
+
+	if ((cc->mode != MIGRATE_ASYNC) || !cc->direct_compaction)
 		return true;
 
-	return is_migrate_movable(get_pageblock_migratetype(page));
+	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 */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index db96d1ebbed8..01ce6d41cb20 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3665,6 +3665,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 						struct alloc_context *ac)
 {
 	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
+	const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
 	struct page *page = NULL;
 	unsigned int alloc_flags;
 	unsigned long did_some_progress;
@@ -3732,12 +3733,17 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 
 	/*
 	 * For costly allocations, try direct compaction first, as it's likely
-	 * that we have enough base pages and don't need to reclaim. Don't try
-	 * that for allocations that are allowed to ignore watermarks, as the
-	 * ALLOC_NO_WATERMARKS attempt didn't yet happen.
+	 * that we have enough base pages and don't need to reclaim. For non-
+	 * movable high-order allocations, do that as well, as compaction will
+	 * try prevent permanent fragmentation by migrating from blocks of the
+	 * same migratetype.
+	 * Don't try this for allocations that are allowed to ignore
+	 * watermarks, as the ALLOC_NO_WATERMARKS attempt didn't yet happen.
 	 */
-	if (can_direct_reclaim && order > PAGE_ALLOC_COSTLY_ORDER &&
-		!gfp_pfmemalloc_allowed(gfp_mask)) {
+	if (can_direct_reclaim &&
+			(costly_order ||
+			   (order > 0 && ac->migratetype != MIGRATE_MOVABLE))
+			&& !gfp_pfmemalloc_allowed(gfp_mask)) {
 		page = __alloc_pages_direct_compact(gfp_mask, order,
 						alloc_flags, ac,
 						INIT_COMPACT_PRIORITY,
@@ -3749,7 +3755,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		 * Checks for costly allocations with __GFP_NORETRY, which
 		 * includes THP page fault allocations
 		 */
-		if (gfp_mask & __GFP_NORETRY) {
+		if (costly_order && (gfp_mask & __GFP_NORETRY)) {
 			/*
 			 * If compaction is deferred for high-order allocations,
 			 * it is because sync compaction recently failed. If
@@ -3830,7 +3836,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	 * Do not retry costly high order allocations unless they are
 	 * __GFP_REPEAT
 	 */
-	if (order > PAGE_ALLOC_COSTLY_ORDER && !(gfp_mask & __GFP_REPEAT))
+	if (costly_order && !(gfp_mask & __GFP_REPEAT))
 		goto nopage;
 
 	if (should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
-- 
2.12.0