[NET]: Set default socket rmem/wmem values more sanely and consistently.

1) Take into account the fact that struct sk_buff, which counts towards
   socket buffer limits, changes across different platforms.
2) Give ipv4/ipv6 ICMP sockets more accurately the wmem limits they
   want.

include/linux/skbuff.h

@@ -274,9 +274,6 @@ struct sk_buff {
 				*end;
 };
 
-#define SK_WMEM_MAX	65535
-#define SK_RMEM_MAX	65535
-
 #ifdef __KERNEL__
 /*
  *	Handling routines are only of interest to the kernel

net/core/sock.c

@@ -126,6 +126,16 @@
 #include <net/tcp.h>
 #endif
 
+/* Take into consideration the size of the struct sk_buff overhead in the
+ * determination of these values, since that is non-constant across
+ * platforms.  This makes socket queueing behavior and performance
+ * not depend upon such differences.
+ */
+#define _SK_MEM_PACKETS		256
+#define _SK_MEM_OVERHEAD	(sizeof(struct sk_buff) + 256)
+#define SK_WMEM_MAX		(_SK_MEM_OVERHEAD * _SK_MEM_PACKETS)
+#define SK_RMEM_MAX		(_SK_MEM_OVERHEAD * _SK_MEM_PACKETS)
+
 /* Run time adjustable parameters. */
 __u32 sysctl_wmem_max = SK_WMEM_MAX;
 __u32 sysctl_rmem_max = SK_RMEM_MAX;

net/ipv4/icmp.c

@@ -1115,7 +1115,13 @@ void __init icmp_init(struct net_proto_family *ops)
 			panic("Failed to create the ICMP control socket.\n");
 
 		per_cpu(__icmp_socket, i)->sk->sk_allocation = GFP_ATOMIC;
-		per_cpu(__icmp_socket, i)->sk->sk_sndbuf = SK_WMEM_MAX * 2;
+
+		/* Enough space for 2 64K ICMP packets, including
+		 * sk_buff struct overhead.
+		 */
+		per_cpu(__icmp_socket, i)->sk->sk_sndbuf =
+			(2 * ((64 * 1024) + sizeof(struct sk_buff)));
+
 		inet = inet_sk(per_cpu(__icmp_socket, i)->sk);
 		inet->uc_ttl = -1;
 		inet->pmtudisc = IP_PMTUDISC_DONT;

net/ipv6/icmp.c

@@ -693,7 +693,13 @@ int __init icmpv6_init(struct net_proto_family *ops)
 
 		sk = per_cpu(__icmpv6_socket, i)->sk;
 		sk->sk_allocation = GFP_ATOMIC;
-		sk->sk_sndbuf = SK_WMEM_MAX * 2;
+
+		/* Enough space for 2 64K ICMP packets, including
+		 * sk_buff struct overhead.
+		 */
+		sk->sk_sndbuf =
+			(2 * ((64 * 1024) + sizeof(struct sk_buff)));
+
 		sk->sk_prot->unhash(sk);
 	}