diff --git a/fs/namespace.c b/fs/namespace.c
index d1e8dd2e1e8e..6d4ecde682df 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1799,6 +1799,8 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
 
 	/* Gather the mounts to umount */
 	for (p = mnt; p; p = next_mnt(p, mnt)) {
+		/* A mount is unmounted once. */
+		VFS_WARN_ON_ONCE(p->mnt.mnt_flags & MNT_UMOUNT);
 		p->mnt.mnt_flags |= MNT_UMOUNT;
 		if (mnt_ns_attached(p))
 			move_from_ns(p);
diff --git a/include/linux/mount.h b/include/linux/mount.h
index acfe7ef86a1b..b16ba5d45605 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -52,7 +52,7 @@ enum mount_flags {
 	MNT_ATIME_MASK = MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME,
 
 	MNT_INTERNAL_FLAGS = MNT_INTERNAL | MNT_DOOMED |
-			     MNT_SYNC_UMOUNT | MNT_LOCKED
+			     MNT_SYNC_UMOUNT | MNT_LOCKED | MNT_UMOUNT
 };
 
 struct vfsmount {