umount_tree(): take all victims out of propagation graph at once

For each removed mount we need to calculate where the slaves will end up.
To avoid duplicating that work, do it for all mounts to be removed
at once, taking the mounts themselves out of propagation graph as
we go, then do all transfers; the duplicate work on finding destinations
is avoided since if we run into a mount that already had destination found,
we don't need to trace the rest of the way. That's guaranteed
O(removed mounts) for finding destinations and removing from propagation
graph and O(surviving mounts that have master removed) for transfers.

Reviewed-by: Christian Brauner <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/namespace.c

···
 	if (how & UMOUNT_PROPAGATE)
 		propagate_umount(&tmp_list);
 
+	bulk_make_private(&tmp_list);
+
 	while (!list_empty(&tmp_list)) {
 		struct mnt_namespace *ns;
 		bool disconnect;
···
 				umount_mnt(p);
 			}
 		}
-		change_mnt_propagation(p, MS_PRIVATE);
 		if (disconnect)
 			hlist_add_head(&p->mnt_umount, &unmounted);
 

fs/pnode.c

···
 	return m->mnt.mnt_flags & MNT_UMOUNT;
 }
 
-static struct mount *propagation_source(struct mount *mnt)
-{
-	do {
-		struct mount *m;
-		for (m = next_peer(mnt); m != mnt; m = next_peer(m)) {
-			if (!will_be_unmounted(m))
-				return m;
-		}
-		mnt = mnt->mnt_master;
-	} while (mnt && will_be_unmounted(mnt));
-	return mnt;
-}
-
 static void transfer_propagation(struct mount *mnt, struct mount *to)
 {
 	struct hlist_node *p = NULL, *n;
···
 		return;
 	}
 	if (IS_MNT_SHARED(mnt)) {
-		if (type == MS_SLAVE || !hlist_empty(&mnt->mnt_slave_list))
-			m = propagation_source(mnt);
 		if (list_empty(&mnt->mnt_share)) {
 			mnt_release_group_id(mnt);
 		} else {
+			m = next_peer(mnt);
 			list_del_init(&mnt->mnt_share);
 			mnt->mnt_group_id = 0;
 		}
···
 			mnt->mnt_t_flags |= T_UNBINDABLE;
 		else
 			mnt->mnt_t_flags &= ~T_UNBINDABLE;
+	}
+}
+
+static struct mount *trace_transfers(struct mount *m)
+{
+	while (1) {
+		struct mount *next = next_peer(m);
+
+		if (next != m) {
+			list_del_init(&m->mnt_share);
+			m->mnt_group_id = 0;
+			m->mnt_master = next;
+		} else {
+			if (IS_MNT_SHARED(m))
+				mnt_release_group_id(m);
+			next = m->mnt_master;
+		}
+		hlist_del_init(&m->mnt_slave);
+		CLEAR_MNT_SHARED(m);
+		SET_MNT_MARK(m);
+
+		if (!next || !will_be_unmounted(next))
+			return next;
+		if (IS_MNT_MARKED(next))
+			return next->mnt_master;
+		m = next;
+	}
+}
+
+static void set_destinations(struct mount *m, struct mount *master)
+{
+	struct mount *next;
+
+	while ((next = m->mnt_master) != master) {
+		m->mnt_master = master;
+		m = next;
+	}
+}
+
+void bulk_make_private(struct list_head *set)
+{
+	struct mount *m;
+
+	list_for_each_entry(m, set, mnt_list)
+		if (!IS_MNT_MARKED(m))
+			set_destinations(m, trace_transfers(m));
+
+	list_for_each_entry(m, set, mnt_list) {
+		transfer_propagation(m, m->mnt_master);
+		m->mnt_master = NULL;
+		CLEAR_MNT_MARK(m);
 	}
 }
 

fs/pnode.h

···
 }
 
 void change_mnt_propagation(struct mount *, int);
+void bulk_make_private(struct list_head *);
 int propagate_mnt(struct mount *, struct mountpoint *, struct mount *,
 		struct hlist_head *);
 void propagate_umount(struct list_head *);