diff --git a/mm/cma.c b/mm/cma.c
index 75016fd..68ecb7a 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -64,15 +64,17 @@
 	return (1UL << (align_order - cma->order_per_bit)) - 1;
 }
 
+/*
+ * Find a PFN aligned to the specified order and return an offset represented in
+ * order_per_bits.
+ */
 static unsigned long cma_bitmap_aligned_offset(struct cma *cma, int align_order)
 {
-	unsigned int alignment;
-
 	if (align_order <= cma->order_per_bit)
 		return 0;
-	alignment = 1UL << (align_order - cma->order_per_bit);
-	return ALIGN(cma->base_pfn, alignment) -
-		(cma->base_pfn >> cma->order_per_bit);
+
+	return (ALIGN(cma->base_pfn, (1UL << align_order))
+		- cma->base_pfn) >> cma->order_per_bit;
 }
 
 static unsigned long cma_bitmap_maxno(struct cma *cma)