diff --git a/arch/um/include/mem.h b/arch/um/include/mem.h
index 10c46c3..99d3ad4 100644
--- a/arch/um/include/mem.h
+++ b/arch/um/include/mem.h
@@ -13,6 +13,7 @@
 extern int is_remapped(void *virt);
 extern int physmem_remove_mapping(void *virt);
 extern void physmem_forget_descriptor(int fd);
+extern unsigned long to_phys(void *virt);
 
 #endif