[PATCHv5] mm: optimization on page allocation when CMA enabled

["zhaoyang.huang" <zhaoyang.huang@unisoc.com>]

From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>

Let us look at the timeline of scenarios below with WMARK_LOW=25MB WMARK_MIN=5MB
(managed pages 1.9GB). We can find that CMA begin to be used until 'C' under the
method of 'fixed 2 times of free cma over free pages' which could have the
scenario 'A' and 'B' into a fault state, that is, free UNMOVABLE & RECLAIMABLE
pages is lower than corresponding watermark without reclaiming which should be
deemed as against current memory policy. This commit try to solve this by
checking zone_watermark_ok again with removing CMA pages which could lead to a
proper time point of CMA's utilization.

-- Free_pages
|
|
-- WMARK_LOW
|
-- Free_CMA
|
|
--

Free_CMA/Free_pages(MB)      A(12/30) -->  B(12/25) -->  C(12/20)
fixed 1/2 ratio                 N             N           Y
this commit                     Y             Y           Y

Signed-off-by: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
---
v2: do proportion check when zone_watermark_ok, update commit message
v3: update coding style and simplify the logic when zone_watermark_ok
v4: code update according to Roman's suggest
v5: update commit message
---
---
 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 0745aed..4719800 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3071,6 +3071,43 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
 
 }
 
+#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.
@@ -3084,12 +3121,11 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
 	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;
-- 
1.9.1