[PATCH RFC 07/19] mm/page_alloc: don't overload migratetype in find_suitable_fallback()

[Brendan Jackman <jackmanb@google.com>]

This function currently returns a signed integer that encodes status
in-band, as negative numbers, along with a migratetype.

This function is about to be updated to a mode where this in-band
signaling no longer makes sense. Therefore, switch to a more
explicit/verbose style that encodes the status and migratetype
separately.

In the spirit of making things more explicit, also create an enum to
avoid using magic integer literals with special meanings. This enables
documenting the values at their definition instead of in one of the
callers.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
 mm/compaction.c |  3 ++-
 mm/internal.h   | 14 +++++++++++---
 mm/page_alloc.c | 40 +++++++++++++++++++++++-----------------
 3 files changed, 36 insertions(+), 21 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 1e8f8eca318c6..cf65a3425500c 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2323,7 +2323,8 @@ static enum compact_result __compact_finished(struct compact_control *cc)
 		 * Job done if allocation would steal freepages from
 		 * other migratetype buddy lists.
 		 */
-		if (find_suitable_fallback(area, order, migratetype, true) >= 0)
+		if (find_suitable_fallback(area, order, migratetype, true, NULL)
+		    == FALLBACK_FOUND)
 			/*
 			 * Movable pages are OK in any pageblock. If we are
 			 * stealing for a non-movable allocation, make sure
diff --git a/mm/internal.h b/mm/internal.h
index cb0af847d7d99..1d88e56a9dee0 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1028,9 +1028,17 @@ static inline void init_cma_pageblock(struct page *page)
 }
 #endif
 
-
-int find_suitable_fallback(struct free_area *area, unsigned int order,
-			   int migratetype, bool claimable);
+enum fallback_result {
+	/* Found suitable migratetype, *mt_out is valid. */
+	FALLBACK_FOUND,
+	/* No fallback found in requested order. */
+	FALLBACK_EMPTY,
+	/* Passed @claimable, but claiming whole block is a bad idea. */
+	FALLBACK_NOCLAIM,
+};
+enum fallback_result
+find_suitable_fallback(struct free_area *area, unsigned int order,
+		       int migratetype, bool claimable, unsigned int *mt_out);
 
 static inline bool free_area_empty(struct free_area *area, int migratetype)
 {
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fcc32737f451e..1cd74a5901ded 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2280,25 +2280,29 @@ static bool should_try_claim_block(unsigned int order, int start_mt)
  * we would do this whole-block claiming. This would help to reduce
  * fragmentation due to mixed migratetype pages in one pageblock.
  */
-int find_suitable_fallback(struct free_area *area, unsigned int order,
-			   int migratetype, bool claimable)
+enum fallback_result
+find_suitable_fallback(struct free_area *area, unsigned int order,
+		       int migratetype, bool claimable, unsigned int *mt_out)
 {
 	int i;
 
 	if (claimable && !should_try_claim_block(order, migratetype))
-		return -2;
+		return FALLBACK_NOCLAIM;
 
 	if (area->nr_free == 0)
-		return -1;
+		return FALLBACK_EMPTY;
 
 	for (i = 0; i < MIGRATE_PCPTYPES - 1 ; i++) {
 		int fallback_mt = fallbacks[migratetype][i];
 
-		if (!free_area_empty(area, fallback_mt))
-			return fallback_mt;
+		if (!free_area_empty(area, fallback_mt)) {
+			if (mt_out)
+				*mt_out = fallback_mt;
+			return FALLBACK_FOUND;
+		}
 	}
 
-	return -1;
+	return FALLBACK_EMPTY;
 }
 
 /*
@@ -2408,16 +2412,16 @@ __rmqueue_claim(struct zone *zone, int order, int start_migratetype,
 	 */
 	for (current_order = MAX_PAGE_ORDER; current_order >= min_order;
 				--current_order) {
-		area = &(zone->free_area[current_order]);
-		fallback_mt = find_suitable_fallback(area, current_order,
-						     start_migratetype, true);
+		enum fallback_result result;
 
-		/* No block in that order */
-		if (fallback_mt == -1)
+		area = &(zone->free_area[current_order]);
+		result = find_suitable_fallback(area, current_order,
+						start_migratetype, true, &fallback_mt);
+
+		if (result == FALLBACK_EMPTY)
 			continue;
 
-		/* Advanced into orders too low to claim, abort */
-		if (fallback_mt == -2)
+		if (result == FALLBACK_NOCLAIM)
 			break;
 
 		page = get_page_from_free_area(area, fallback_mt);
@@ -2447,10 +2451,12 @@ __rmqueue_steal(struct zone *zone, int order, int start_migratetype)
 	int fallback_mt;
 
 	for (current_order = order; current_order < NR_PAGE_ORDERS; current_order++) {
+		enum fallback_result result;
+
 		area = &(zone->free_area[current_order]);
-		fallback_mt = find_suitable_fallback(area, current_order,
-						     start_migratetype, false);
-		if (fallback_mt == -1)
+		result = find_suitable_fallback(area, current_order, start_migratetype,
+						false, &fallback_mt);
+		if (result == FALLBACK_EMPTY)
 			continue;
 
 		page = get_page_from_free_area(area, fallback_mt);

-- 
2.51.2