[PATCHv6 1/1] mm: optimization on page allocation when CMA enabled

[zhaoyang.huang]

From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>

According to current CMA utilization policy, an alloc_pages(GFP_USER)
could 'steal' UNMOVABLE & RECLAIMABLE page blocks via the help of
CMA(pass zone_watermark_ok by counting CMA in but use U&R in rmqueue),
which could lead to following alloc_pages(GFP_KERNEL) fail.
Solving this by introducing second watermark checking for GFP_MOVABLE,
which could have the allocation use CMA when proper.

-- Free_pages(30MB)
|
|
-- WMARK_LOW(25MB)
|
-- Free_CMA(12MB)
|
|
--

Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
---
v6: update comments
---
---
 mm/page_alloc.c | 44 ++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 40 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 452459836b71..5a146aa7c0aa 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2078,6 +2078,43 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype,
 
 }
 
+#ifdef CONFIG_CMA
+/*
+ * GFP_MOVABLE allocation could drain UNMOVABLE & RECLAIMABLE page blocks via
+ * the help of CMA which makes GFP_KERNEL failed. Checking if zone_watermark_ok
+ * again without ALLOC_CMA to see if to use CMA first.
+ */
+static bool use_cma_first(struct zone *zone, unsigned int order, unsigned int alloc_flags)
+{
+	unsigned long watermark;
+	bool cma_first = false;
+
+	watermark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK);
+	/* check if GFP_MOVABLE pass previous zone_watermark_ok via the help of CMA */
+	if (zone_watermark_ok(zone, order, watermark, 0, alloc_flags & (~ALLOC_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.
+		 */
+		cma_first = (zone_page_state(zone, NR_FREE_CMA_PAGES) >
+				zone_page_state(zone, NR_FREE_PAGES) / 2);
+	} else {
+		/*
+		 * watermark failed means UNMOVABLE & RECLAIMBLE is not enough
+		 * now, we should use cma first to keep them stay around the
+		 * corresponding watermark
+		 */
+		cma_first = true;
+	}
+	return cma_first;
+}
+#else
+static bool use_cma_first(struct zone *zone, unsigned int order, unsigned int alloc_flags)
+{
+	return false;
+}
+#endif
 /*
  * Do the hard work of removing an element from the buddy allocator.
  * Call me with the zone->lock already held.
@@ -2091,12 +2128,11 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
 	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.
+		 * allocating from CMA base on judging zone_watermark_ok again
+		 * to see if the latest check got pass via the help of CMA
 		 */
 		if (alloc_flags & ALLOC_CMA &&
-		    zone_page_state(zone, NR_FREE_CMA_PAGES) >
-		    zone_page_state(zone, NR_FREE_PAGES) / 2) {
+			use_cma_first(zone, order, alloc_flags)) {
 			page = __rmqueue_cma_fallback(zone, order);
 			if (page)
 				return page;
-- 
2.25.1