diff --git a/mm/internal.h b/mm/internal.h
index 13333bc..1981bc9 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -49,6 +49,7 @@
 /*
  * in mm/page_alloc.c
  */
+extern unsigned long highest_memmap_pfn;
 extern void __free_pages_bootmem(struct page *page, unsigned int order);
 
 /*