[IPV4] fib_trie: avoid rescan on dump

This converts dumping (and flushing) of large route tables form O(N^2)
to O(N). If the route dump took multiple pages then the dump routine
gets called again. The old code kept track of location by counter, the
new code instead uses the last key.

This is a really big win ( 0.3 sec vs 12 sec) for big route tables.

One side effect is that if the table changes during the dump, then the
last key will not be found, and we will return -EBUSY.
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -1917,35 +1917,43 @@ static int fn_trie_dump_leaf(struct leaf *l, struct fib_table *tb,
 	return skb->len;
 }
 
-
-
 static int fn_trie_dump(struct fib_table *tb, struct sk_buff *skb,
 			struct netlink_callback *cb)
 {
 	struct leaf *l;
 	struct trie *t = (struct trie *) tb->tb_data;
-	int h = 0;
-	int s_h = cb->args[2];
+	t_key key = cb->args[2];
 
 	rcu_read_lock();
-	for (h = 0, l = trie_firstleaf(t); l != NULL; h++, l = trie_nextleaf(l)) {
-		if (h < s_h)
-			continue;
-
-		if (h > s_h) {
-			cb->args[3] = 0;
-			cb->args[4] = 0;
+	/* Dump starting at last key.
+	 * Note: 0.0.0.0/0 (ie default) is first key.
+	 */
+	if (!key)
+		l = trie_firstleaf(t);
+	else {
+		l = fib_find_node(t, key);
+		if (!l) {
+			/* The table changed during the dump, rather than
+			 * giving partial data, just make application retry.
+			 */
+			rcu_read_unlock();
+			return -EBUSY;
 		}
+	}
 
+	while (l) {
+		cb->args[2] = l->key;
 		if (fn_trie_dump_leaf(l, tb, skb, cb) < 0) {
 			rcu_read_unlock();
-			cb->args[2] = h;
 			return -1;
 		}
+
+		l = trie_nextleaf(l);
+		memset(&cb->args[3], 0,
+		       sizeof(cb->args) - 3*sizeof(cb->args[0]));
 	}
 	rcu_read_unlock();
 
-	cb->args[2] = h;
 	return skb->len;
 }