netdev - [PATCH v4 net-next 18/19] ipv6: frags: get rid of ip6frag_skb_cb/FRAG6

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Date:   Sat, 31 Mar 2018 12:58:59 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>, Florian Westphal <fw@...len.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Thomas Graf <tgraf@...g.ch>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Alexander Aring <alex.aring@...il.com>,
        Stefan Schmidt <stefan@....samsung.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH v4 net-next 18/19] ipv6: frags: get rid of ip6frag_skb_cb/FRAG6_CB

ip6_frag_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@...gle.com>
---
 net/ipv6/reassembly.c | 30 ++++++++++++------------------
 1 file changed, 12 insertions(+), 18 deletions(-)

diff --git a/net/ipv6/reassembly.c b/net/ipv6/reassembly.c
index 2127da130dc2cee677059247d5e079a8afbca13d..7b52efc63f6adad2c242ae1171e5b7b27b40f0bf 100644
--- a/net/ipv6/reassembly.c
+++ b/net/ipv6/reassembly.c
@@ -62,13 +62,6 @@
 
 static const char ip6_frag_cache_name[] = "ip6-frags";
 
-struct ip6frag_skb_cb {
-	struct inet6_skb_parm	h;
-	int			offset;
-};
-
-#define FRAG6_CB(skb)	((struct ip6frag_skb_cb *)((skb)->cb))
-
 static u8 ip6_frag_ecn(const struct ipv6hdr *ipv6h)
 {
 	return 1 << (ipv6_get_dsfield(ipv6h) & INET_ECN_MASK);
@@ -250,13 +243,13 @@ static int ip6_frag_queue(struct frag_queue *fq, struct sk_buff *skb,
 	 * this fragment, right?
 	 */
 	prev = fq->q.fragments_tail;
-	if (!prev || 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 (FRAG6_CB(next)->offset >= offset)
+		if (next->ip_defrag_offset >= offset)
 			break;	/* bingo! */
 		prev = next;
 	}
@@ -271,14 +264,20 @@ static int ip6_frag_queue(struct frag_queue *fq, struct sk_buff *skb,
 
 	/* Check for overlap with preceding fragment. */
 	if (prev &&
-	    (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 && FRAG6_CB(next)->offset < end)
+	if (next && next->ip_defrag_offset < end)
 		goto discard_fq;
 
-	FRAG6_CB(skb)->offset = offset;
+	/* Note : skb->ip_defrag_offset and skb->dev share the same location */
+	dev = skb->dev;
+	if (dev)
+		fq->iif = 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;
@@ -289,11 +288,6 @@ static int ip6_frag_queue(struct frag_queue *fq, struct sk_buff *skb,
 	else
 		fq->q.fragments = skb;
 
-	dev = skb->dev;
-	if (dev) {
-		fq->iif = dev->ifindex;
-		skb->dev = NULL;
-	}
 	fq->q.stamp = skb->tstamp;
 	fq->q.meat += skb->len;
 	fq->ecn |= ecn;
@@ -380,7 +374,7 @@ static int ip6_frag_reasm(struct frag_queue *fq, struct sk_buff *prev,
 	}
 
 	WARN_ON(head == NULL);
-	WARN_ON(FRAG6_CB(head)->offset != 0);
+	WARN_ON(head->ip_defrag_offset != 0);
 
 	/* Unfragmented part is taken from the first segment. */
 	payload_len = ((head->data - skb_network_header(head)) -
-- 
2.17.0.rc1.321.gba9d0f2565-goog