diff --git a/tools/testing/selftests/vm/mlock2-tests.c b/tools/testing/selftests/vm/mlock2-tests.c
index 4431994aade2..cb247219f1f7 100644
--- a/tools/testing/selftests/vm/mlock2-tests.c
+++ b/tools/testing/selftests/vm/mlock2-tests.c
@@ -1,3 +1,4 @@
+#define _GNU_SOURCE
 #include <sys/mman.h>
 #include <stdint.h>
 #include <stdio.h>