[NETFILTER]: nf_conntrack: early_drop improvement

When the maximum number of conntrack entries is reached and a new
one needs to be allocated, conntrack tries to drop an unassured
connection from the same hash bucket the new conntrack would hash
to. Since with a properly sized hash the average number of entries
per bucket is 1, the chances of actually finding one are not very
good. This patch makes it walk the hash until a minimum number of
8 entries are checked.

Based on patch by Vasily Averin <vvs@sw.ru>.
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netfilter/nf_conntrack_core.c

@@ -377,21 +377,30 @@ nf_conntrack_tuple_taken(const struct nf_conntrack_tuple *tuple,
 }
 EXPORT_SYMBOL_GPL(nf_conntrack_tuple_taken);
 
+#define NF_CT_EVICTION_RANGE	8
+
 /* There's a small race here where we may free a just-assured
    connection.  Too bad: we're in trouble anyway. */
-static int early_drop(struct hlist_head *chain)
+static int early_drop(unsigned int hash)
 {
 	/* Use oldest entry, which is roughly LRU */
 	struct nf_conntrack_tuple_hash *h;
 	struct nf_conn *ct = NULL, *tmp;
 	struct hlist_node *n;
+	unsigned int i, cnt = 0;
 	int dropped = 0;
 
 	read_lock_bh(&nf_conntrack_lock);
-	hlist_for_each_entry(h, n, chain, hnode) {
-		tmp = nf_ct_tuplehash_to_ctrack(h);
-		if (!test_bit(IPS_ASSURED_BIT, &tmp->status))
-			ct = tmp;
+	for (i = 0; i < nf_conntrack_htable_size; i++) {
+		hlist_for_each_entry(h, n, &nf_conntrack_hash[hash], hnode) {
+			tmp = nf_ct_tuplehash_to_ctrack(h);
+			if (!test_bit(IPS_ASSURED_BIT, &tmp->status))
+				ct = tmp;
+			cnt++;
+		}
+		if (ct || cnt >= NF_CT_EVICTION_RANGE)
+			break;
+		hash = (hash + 1) % nf_conntrack_htable_size;
 	}
 	if (ct)
 		atomic_inc(&ct->ct_general.use);
@@ -425,8 +434,7 @@ struct nf_conn *nf_conntrack_alloc(const struct nf_conntrack_tuple *orig,
 	if (nf_conntrack_max
 	    && atomic_read(&nf_conntrack_count) > nf_conntrack_max) {
 		unsigned int hash = hash_conntrack(orig);
-		/* Try dropping from this hash chain. */
-		if (!early_drop(&nf_conntrack_hash[hash])) {
+		if (!early_drop(hash)) {
 			atomic_dec(&nf_conntrack_count);
 			if (net_ratelimit())
 				printk(KERN_WARNING