diff --git a/arch/um/include/os.h b/arch/um/include/os.h
index 112d728..2cccfa5 100644
--- a/arch/um/include/os.h
+++ b/arch/um/include/os.h
@@ -167,7 +167,7 @@
 #endif
 
 /* mem.c */
-extern int create_mem_file(unsigned long len);
+extern int create_mem_file(unsigned long long len);
 
 /* process.c */
 extern unsigned long os_process_pc(int pid);