diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 34f688bc5c7a..19fe7e9c39a6 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7176,7 +7176,8 @@ void zone_pcp_reset(struct zone *zone)
 
 #ifdef CONFIG_MEMORY_HOTREMOVE
 /*
- * All pages in the range must be isolated before calling this.
+ * All pages in the range must be in a single zone and isolated
+ * before calling this.
  */
 void
 __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 67bedd18429c..612122bf6a42 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -246,6 +246,7 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn,
 	return pfn;
 }
 
+/* Caller should ensure that requested range is in a single zone */
 int test_pages_isolated(unsigned long start_pfn, unsigned long end_pfn,
 			bool skip_hwpoisoned_pages)
 {