inet: frags: get rid of nf_ct_frag6_skb_cb/NFCT_FRAG6_CB

nf_ct_frag6_queue() uses skb->cb[] to store the fragment offset,
meaning that we could use two cache lines per skb when finding
the insertion point, if for some reason inet6_skb_parm size
is increased in the future.

By using skb->ip_defrag_offset instead of skb->cb[] we pack all the fields
in a single cache line, matching what we did for IPv4.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/netfilter/nf_conntrack_reasm.c

@@ -52,14 +52,6 @@
 
 static const char nf_frags_cache_name[] = "nf-frags";
 
-struct nf_ct_frag6_skb_cb
-{
-	struct inet6_skb_parm	h;
-	int			offset;
-};
-
-#define NFCT_FRAG6_CB(skb)	((struct nf_ct_frag6_skb_cb *)((skb)->cb))
-
 static struct inet_frags nf_frags;
 
 #ifdef CONFIG_SYSCTL
@@ -270,13 +262,13 @@ static int nf_ct_frag6_queue(struct frag_queue *fq, struct sk_buff *skb,
 	 * this fragment, right?
 	 */
 	prev = fq->q.fragments_tail;
-	if (!prev || NFCT_FRAG6_CB(prev)->offset < offset) {
+	if (!prev || prev->ip_defrag_offset < offset) {
 		next = NULL;
 		goto found;
 	}
 	prev = NULL;
 	for (next = fq->q.fragments; next != NULL; next = next->next) {
-		if (NFCT_FRAG6_CB(next)->offset >= offset)
+		if (next->ip_defrag_offset >= offset)
 			break;	/* bingo! */
 		prev = next;
 	}
@@ -292,14 +284,19 @@ static int nf_ct_frag6_queue(struct frag_queue *fq, struct sk_buff *skb,
 
 	/* Check for overlap with preceding fragment. */
 	if (prev &&
-	    (NFCT_FRAG6_CB(prev)->offset + prev->len) > offset)
+	    (prev->ip_defrag_offset + prev->len) > offset)
 		goto discard_fq;
 
 	/* Look for overlap with succeeding segment. */
-	if (next && NFCT_FRAG6_CB(next)->offset < end)
+	if (next && next->ip_defrag_offset < end)
 		goto discard_fq;
 
-	NFCT_FRAG6_CB(skb)->offset = offset;
+	/* Note : skb->ip_defrag_offset and skb->dev share the same location */
+	if (skb->dev)
+		fq->iif = skb->dev->ifindex;
+	/* Makes sure compiler wont do silly aliasing games */
+	barrier();
+	skb->ip_defrag_offset = offset;
 
 	/* Insert this fragment in the chain of fragments. */
 	skb->next = next;
@@ -310,10 +307,6 @@ static int nf_ct_frag6_queue(struct frag_queue *fq, struct sk_buff *skb,
 	else
 		fq->q.fragments = skb;
 
-	if (skb->dev) {
-		fq->iif = skb->dev->ifindex;
-		skb->dev = NULL;
-	}
 	fq->q.stamp = skb->tstamp;
 	fq->q.meat += skb->len;
 	fq->ecn |= ecn;
@@ -357,7 +350,7 @@ nf_ct_frag6_reasm(struct frag_queue *fq, struct sk_buff *prev,  struct net_devic
 	inet_frag_kill(&fq->q);
 
 	WARN_ON(head == NULL);
-	WARN_ON(NFCT_FRAG6_CB(head)->offset != 0);
+	WARN_ON(head->ip_defrag_offset != 0);
 
 	ecn = ip_frag_ecn_table[fq->ecn];
 	if (unlikely(ecn == 0xff))