ipv4: Fix ARP behavior with many mac-vlans

Ben Greear wrote:
> I have 500 mac-vlans on a system talking to 500 other
> mac-vlans.  My problem is that the arp-table gets extremely
> huge because every time an arp-request comes in on all mac-vlans,
> a stale arp entry is added for each mac-vlan.  I have filtering
> turned on, but that doesn't help because the neigh_event_ns call
> below will cause a stale neighbor entry to be created regardless
> of whether a replay will be sent or not.
> Maybe the neigh_event code should be below the checks for dont_send,
> and only create check neigh_event_ns if we are !dont_send?

The attached patch makes it work much better for me.  The patch
will cause the code to NOT create a stale neighbor entry if we
are not going to respond to the ARP request.  The old code
*would* create a stale entry even if we are not going to respond.
Signed-off-by: NBen Greear <greearb@candelatech.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/arp.c

@@ -818,19 +818,19 @@ static int arp_process(struct sk_buff *skb)
 		addr_type = rt->rt_type;
 
 		if (addr_type == RTN_LOCAL) {
-			n = neigh_event_ns(&arp_tbl, sha, &sip, dev);
-			if (n) {
 			int dont_send = 0;
 
 			if (!dont_send)
-					dont_send |= arp_ignore(in_dev, sip, tip);
+				dont_send |= arp_ignore(in_dev,sip,tip);
 			if (!dont_send && IN_DEV_ARPFILTER(in_dev))
-					dont_send |= arp_filter(sip, tip, dev);
-				if (!dont_send)
+				dont_send |= arp_filter(sip,tip,dev);
+			if (!dont_send) {
+				n = neigh_event_ns(&arp_tbl, sha, &sip, dev);
+				if (n) {
 					arp_send(ARPOP_REPLY,ETH_P_ARP,sip,dev,tip,sha,dev->dev_addr,sha);
-
 					neigh_release(n);
 				}
+			}
 			goto out;
 		} else if (IN_DEV_FORWARD(in_dev)) {
 			    if (addr_type == RTN_UNICAST  && rt->u.dst.dev != dev &&