diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 67f2e3c389396c..7522a6987595e0 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1166,8 +1166,16 @@ extern unsigned long usemap_size(void);
 
 /*
  * We use the lower bits of the mem_map pointer to store
- * a little bit of information.  There should be at least
- * 3 bits here due to 32-bit alignment.
+ * a little bit of information.  The pointer is calculated
+ * as mem_map - section_nr_to_pfn(pnum).  The result is
+ * aligned to the minimum alignment of the two values:
+ *   1. All mem_map arrays are page-aligned.
+ *   2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT
+ *      lowest bits.  PFN_SECTION_SHIFT is arch-specific
+ *      (equal SECTION_SIZE_BITS - PAGE_SHIFT), and the
+ *      worst combination is powerpc with 256k pages,
+ *      which results in PFN_SECTION_SHIFT equal 6.
+ * To sum it up, at least 6 bits are available.
  */
 #define	SECTION_MARKED_PRESENT	(1UL<<0)
 #define SECTION_HAS_MEM_MAP	(1UL<<1)
diff --git a/mm/sparse.c b/mm/sparse.c
index 2609aba121e89c..6b8b5e91ceefb1 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -264,7 +264,11 @@ unsigned long __init node_memmap_size_bytes(int nid, unsigned long start_pfn,
  */
 static unsigned long sparse_encode_mem_map(struct page *mem_map, unsigned long pnum)
 {
-	return (unsigned long)(mem_map - (section_nr_to_pfn(pnum)));
+	unsigned long coded_mem_map =
+		(unsigned long)(mem_map - (section_nr_to_pfn(pnum)));
+	BUILD_BUG_ON(SECTION_MAP_LAST_BIT > (1UL<<PFN_SECTION_SHIFT));
+	BUG_ON(coded_mem_map & ~SECTION_MAP_MASK);
+	return coded_mem_map;
 }
 
 /*