diff --git a/kernel/pid.c b/kernel/pid.c
index 19645b25b77c..647b4bb457b5 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -247,6 +247,14 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 		tmp = tmp->parent;
 	}
 
+	/*
+	 * ENOMEM is not the most obvious choice especially for the case
+	 * where the child subreaper has already exited and the pid
+	 * namespace denies the creation of any new processes. But ENOMEM
+	 * is what we have exposed to userspace for a long time and it is
+	 * documented behavior for pid namespaces. So we can't easily
+	 * change it even if there were an error code better suited.
+	 */
 	retval = -ENOMEM;
 
 	if (unlikely(is_child_reaper(pid))) {