[RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[akash.tyagi]

Currently, THP CMA pages share PCP lists with UNMOVABLE and RECLAIMABLE
pages. This may result in CMA THP pages being allocated from the PCP
list for other migratetypes. When this occurs, these pages may fail to
be isolated, leading to CMA allocation failures when drivers request
them.

This patch introduces a dedicated PCP list for the THP CMA migratetype,
ensuring that CMA THP pages are not mixed with other migratetypes and
remain available for CMA allocations as intended.

Signed-off-by: akash.tyagi <akash.tyagi at mediatek.com>
---
 include/linux/mmzone.h | 10 ++++++++--
 mm/page_alloc.c        |  5 +++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 283913d42d7b..dd93088ce851 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -696,11 +696,17 @@ enum zone_watermarks {
 
 /*
  * One per migratetype for each PAGE_ALLOC_COSTLY_ORDER. Two additional lists
- * are added for THP. One PCP list is used by GPF_MOVABLE, and the other PCP list
- * is used by GFP_UNMOVABLE and GFP_RECLAIMABLE.
+ * are added for THP: one for GFP_MOVABLE, and one for GFP_UNMOVABLE and
+ * GFP_RECLAIMABLE. With CMA enabled, an extra THP PCP list is added for
+ * MIGRATE_CMA, allowing further distinction between MIGRATE_MOVABLE and
+ * MIGRATE_CMA for THP allocations.
  */
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#ifdef CONFIG_CMA
+#define NR_PCP_THP 3
+#else
 #define NR_PCP_THP 2
+#endif
 #else
 #define NR_PCP_THP 0
 #endif
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2ef3c07266b3..35f8041afbcc 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -519,6 +519,11 @@ static inline unsigned int order_to_pindex(int migratetype, int order)
 	if (order > PAGE_ALLOC_COSTLY_ORDER) {
 		VM_BUG_ON(order != HPAGE_PMD_ORDER);
 
+#ifdef CONFIG_CMA
+		if (migratetype == MIGRATE_CMA)
+			return NR_LOWORDER_PCP_LISTS + 2;
+#endif
+
 		movable = migratetype == MIGRATE_MOVABLE;
 
 		return NR_LOWORDER_PCP_LISTS + movable;
-- 
2.18.0