--- orig/include/asm-ppc/highmem.h
+++ mod/include/asm-ppc/highmem.h
@@ -30,6 +30,7 @@
 
 /* undef for production */
 #define HIGHMEM_DEBUG 1
+extern unsigned long highstart_pfn;
 
 extern pte_t *kmap_pte;
 extern pgprot_t kmap_prot;