[PATCH v2] 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 | 53 +++++++++++++++++++------------------------------
 1 file changed, 20 insertions(+), 33 deletions(-)

I realized shortly after sending the first version that the code can
be further simplified by removing __rmqueue_cma_fallback() altogether.

Build, boot and runtime tested that CMA is indeed used up first.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7d3460c7a480..b257f9651ce9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1634,17 +1634,6 @@ static int fallbacks[MIGRATE_TYPES][MIGRATE_PCPTYPES - 1] = {
 	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE   },
 };
 
-#ifdef CONFIG_CMA
-static __always_inline struct page *__rmqueue_cma_fallback(struct zone *zone,
-					unsigned int order)
-{
-	return __rmqueue_smallest(zone, order, MIGRATE_CMA);
-}
-#else
-static inline struct page *__rmqueue_cma_fallback(struct zone *zone,
-					unsigned int order) { return NULL; }
-#endif
-
 /*
  * Move the free pages in a range to the freelist tail of the requested type.
  * Note that start_page and end_pages are not aligned on a pageblock
@@ -2124,29 +2113,27 @@ __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;
-		}
+#ifdef CONFIG_CMA
+	/*
+	 * 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 (alloc_flags & ALLOC_CMA) {
+		page = __rmqueue_smallest(zone, order, MIGRATE_CMA);
+		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;
+#endif
+
+	for (;;) {
+		page = __rmqueue_smallest(zone, order, migratetype);
+		if (page)
+			break;
+		if (!__rmqueue_fallback(zone, order, migratetype, alloc_flags))
+			break;
 	}
 	return page;
 }
-- 
2.41.0