mount: don't open the same root twise

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -1776,10 +1776,15 @@ out:
 
 int mntns_collect_root(pid_t pid)
 {
+	static int mntns_root_pid = -1;
+
 	int fd, pfd;
 	int ret;
 	char path[PATH_MAX + 1];
 
+	if (mntns_root_pid == pid) /* The required root is already opened */
+		return get_service_fd(ROOT_FD_OFF);
+
 	close_service_fd(ROOT_FD_OFF);
 
 	if (!(root_ns_mask & CLONE_NEWNS)) {
@@ -1828,6 +1833,8 @@ int mntns_collect_root(pid_t pid)
 
 set_root:
 	ret = install_service_fd(ROOT_FD_OFF, fd);
+	if (ret >= 0)
+		mntns_root_pid = pid;
 	close(fd);
 	return ret;
 }