[PATCH] mm: page_alloc: consume available CMA space first

[Johannes Weiner <hannes@cmpxchg.org>]

On a memcache setup with heavy anon usage and no swap, we routinely
see premature OOM kills with multiple gigabytes of free space left:

    Node 0 Normal free:4978632kB [...] free_cma:4893276kB

This free space turns out to be CMA. We set CMA regions aside for
potential hugetlb users on all of our machines, figuring that even if
there aren't any, the memory is available to userspace allocations.

When the OOMs trigger, it's from unmovable and reclaimable allocations
that aren't allowed to dip into CMA. The non-CMA regions meanwhile are
dominated by the anon pages.

Movable pages can be migrated out of CMA when necessary, but we don't
have a mechanism to migrate them *into* CMA to make room for unmovable
allocations. The only recourse we have for these pages is reclaim,
which due to a lack of swap is unavailable in our case.

Because we have more options for CMA pages, change the policy to
always fill up CMA first. This reduces the risk of premature OOMs.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/page_alloc.c | 44 ++++++++++++++++++++------------------------
 1 file changed, 20 insertions(+), 24 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7d3460c7a480..24b9102cd4f6 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1635,13 +1635,13 @@ static int fallbacks[MIGRATE_TYPES][MIGRATE_PCPTYPES - 1] = {
 };
 
 #ifdef CONFIG_CMA
-static __always_inline struct page *__rmqueue_cma_fallback(struct zone *zone,
+static __always_inline struct page *__rmqueue_cma(struct zone *zone,
 					unsigned int order)
 {
 	return __rmqueue_smallest(zone, order, MIGRATE_CMA);
 }
 #else
-static inline struct page *__rmqueue_cma_fallback(struct zone *zone,
+static inline struct page *__rmqueue_cma(struct zone *zone,
 					unsigned int order) { return NULL; }
 #endif
 
@@ -2124,29 +2124,25 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
 {
 	struct page *page;
 
-	if (IS_ENABLED(CONFIG_CMA)) {
-		/*
-		 * Balance movable allocations between regular and CMA areas by
-		 * allocating from CMA when over half of the zone's free memory
-		 * is in the CMA area.
-		 */
-		if (alloc_flags & ALLOC_CMA &&
-		    zone_page_state(zone, NR_FREE_CMA_PAGES) >
-		    zone_page_state(zone, NR_FREE_PAGES) / 2) {
-			page = __rmqueue_cma_fallback(zone, order);
-			if (page)
-				return page;
-		}
+	/*
+	 * Use up CMA first. Movable pages can be migrated out of CMA
+	 * if necessary, but they cannot migrate into it to make room
+	 * for unmovables elsewhere. The only recourse for them is
+	 * then reclaim, which might be unavailable without swap. We
+	 * want to reduce the risk of OOM with free CMA space left.
+	 */
+	if (IS_ENABLED(CONFIG_CMA) && (alloc_flags & ALLOC_CMA)) {
+		page = __rmqueue_cma(zone, order);
+		if (page)
+			return page;
 	}
-retry:
-	page = __rmqueue_smallest(zone, order, migratetype);
-	if (unlikely(!page)) {
-		if (alloc_flags & ALLOC_CMA)
-			page = __rmqueue_cma_fallback(zone, order);
-
-		if (!page && __rmqueue_fallback(zone, order, migratetype,
-								alloc_flags))
-			goto retry;
+
+	for (;;) {
+		page = __rmqueue_smallest(zone, order, migratetype);
+		if (page)
+			break;
+		if (!__rmqueue_fallback(zone, order, migratetype, alloc_flags))
+			break;
 	}
 	return page;
 }
-- 
2.41.0