ipv4: Support multipath hashing on inner IP pkts for GRE tunnel

Multipath hash policy value of 0 isn't distributing since the outer IP
dest and src aren't varied eventhough the inner ones are. Since the flow
is on the inner ones in the case of tunneled traffic, hashing on them is
desired.

This is done mainly for IP over GRE, hence only tested for that. But
anything else supported by flow dissection should work.

v2: Use skb_flow_dissect_flow_keys() directly so that other tunneling
    can be supported through flow dissection (per Nikolay Aleksandrov).
v3: Remove accidental inclusion of ports in the hash keys and clarify
    the documentation (Nikolay Alexandrov).
Signed-off-by: Stephen Suryaputra <ssuryaextr@gmail.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -80,6 +80,7 @@ fib_multipath_hash_policy - INTEGER
 	Possible values:
 	0 - Layer 3
 	1 - Layer 4
+	2 - Layer 3 or inner Layer 3 if present
 
 fib_sync_mem - UNSIGNED INTEGER
 	Amount of dirty memory from fib entries that can be backlogged before

net/ipv4/route.c

@@ -1930,6 +1930,23 @@ int fib_multipath_hash(const struct net *net, const struct flowi4 *fl4,
 			hash_keys.basic.ip_proto = fl4->flowi4_proto;
 		}
 		break;
+	case 2:
+		memset(&hash_keys, 0, sizeof(hash_keys));
+		hash_keys.control.addr_type = FLOW_DISSECTOR_KEY_IPV4_ADDRS;
+		/* skb is currently provided only when forwarding */
+		if (skb) {
+			struct flow_keys keys;
+
+			skb_flow_dissect_flow_keys(skb, &keys, 0);
+
+			hash_keys.addrs.v4addrs.src = keys.addrs.v4addrs.src;
+			hash_keys.addrs.v4addrs.dst = keys.addrs.v4addrs.dst;
+		} else {
+			/* Same as case 0 */
+			hash_keys.addrs.v4addrs.src = fl4->saddr;
+			hash_keys.addrs.v4addrs.dst = fl4->daddr;
+		}
+		break;
 	}
 	mhash = flow_hash_from_keys(&hash_keys);
 

net/ipv4/sysctl_net_ipv4.c

@@ -1008,7 +1008,7 @@ static struct ctl_table ipv4_net_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_fib_multipath_hash_policy,
 		.extra1		= &zero,
-		.extra2		= &one,
+		.extra2		= &two,
 	},
 #endif
 	{