ipv6: speedup inet6_dump_addr()

When handling large number of netdevices, inet6_dump_addr()
is very slow because it has O(N^2) complexity.

Instead of scanning one single list, we can use the NETDEV_HASHENTRIES
sub lists of the dev_index hash table, and RCU lookups.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -3481,31 +3481,17 @@ enum addr_type_t
 	ANYCAST_ADDR,
 };
 
-static int inet6_dump_addr(struct sk_buff *skb, struct netlink_callback *cb,
-			   enum addr_type_t type)
+/* called with rcu_read_lock() */
+static int in6_dump_addrs(struct inet6_dev *idev, struct sk_buff *skb,
+			  struct netlink_callback *cb, enum addr_type_t type,
+			  int s_ip_idx, int *p_ip_idx)
 {
-	int idx, ip_idx;
-	int s_idx, s_ip_idx;
-	int err = 1;
-	struct net_device *dev;
-	struct inet6_dev *idev = NULL;
 	struct inet6_ifaddr *ifa;
 	struct ifmcaddr6 *ifmca;
 	struct ifacaddr6 *ifaca;
-	struct net *net = sock_net(skb->sk);
-
-	s_idx = cb->args[0];
-	s_ip_idx = ip_idx = cb->args[1];
+	int err = 1;
+	int ip_idx = *p_ip_idx;
 
-	idx = 0;
-	for_each_netdev(net, dev) {
-		if (idx < s_idx)
-			goto cont;
-		if (idx > s_idx)
-			s_ip_idx = 0;
-		ip_idx = 0;
-		if ((idev = in6_dev_get(dev)) == NULL)
-			goto cont;
 	read_lock_bh(&idev->lock);
 	switch (type) {
 	case UNICAST_ADDR:
@@ -3557,15 +3543,52 @@ static int inet6_dump_addr(struct sk_buff *skb, struct netlink_callback *cb,
 		break;
 	}
 	read_unlock_bh(&idev->lock);
-		in6_dev_put(idev);
+	*p_ip_idx = ip_idx;
+	return err;
+}
 
-		if (err <= 0)
-			break;
+static int inet6_dump_addr(struct sk_buff *skb, struct netlink_callback *cb,
+			   enum addr_type_t type)
+{
+	struct net *net = sock_net(skb->sk);
+	int h, s_h;
+	int idx, ip_idx;
+	int s_idx, s_ip_idx;
+	struct net_device *dev;
+	struct inet6_dev *idev;
+	struct hlist_head *head;
+	struct hlist_node *node;
+
+	s_h = cb->args[0];
+	s_idx = idx = cb->args[1];
+	s_ip_idx = ip_idx = cb->args[2];
+
+	rcu_read_lock();
+	for (h = s_h; h < NETDEV_HASHENTRIES; h++, s_idx = 0) {
+		idx = 0;
+		head = &net->dev_index_head[h];
+		hlist_for_each_entry_rcu(dev, node, head, index_hlist) {
+			if (idx < s_idx)
+				goto cont;
+			if (idx > s_idx)
+				s_ip_idx = 0;
+			ip_idx = 0;
+			if ((idev = __in6_dev_get(dev)) == NULL)
+				goto cont;
+
+			if (in6_dump_addrs(idev, skb, cb, type,
+					   s_ip_idx, &ip_idx) <= 0)
+				goto done;
 cont:
 			idx++;
 		}
-	cb->args[0] = idx;
-	cb->args[1] = ip_idx;
+	}
+done:
+	rcu_read_unlock();
+	cb->args[0] = h;
+	cb->args[1] = idx;
+	cb->args[2] = ip_idx;
+
 	return skb->len;
 }