gro: Avoid unnecessary comparison after skb_gro_header

For the overwhelming majority of cases, skb_gro_header's return
value cannot be NULL.  Yet we must check it because of its current
form.  This patch splits it up into multiple functions in order
to avoid this.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1132,18 +1132,23 @@ static inline void skb_gro_pull(struct sk_buff *skb, unsigned int len)
 	NAPI_GRO_CB(skb)->data_offset += len;
 }
 
-static inline void *skb_gro_header(struct sk_buff *skb, unsigned int hlen)
+static inline void *skb_gro_header_fast(struct sk_buff *skb,
+					unsigned int offset)
 {
-	unsigned int offset = skb_gro_offset(skb);
+	return NAPI_GRO_CB(skb)->frag0 + offset;
+}
+
+static inline int skb_gro_header_hard(struct sk_buff *skb, unsigned int hlen)
+{
+	return NAPI_GRO_CB(skb)->frag0_len < hlen;
+}
 
-	hlen += offset;
-	if (NAPI_GRO_CB(skb)->frag0_len < hlen) {
+static inline void *skb_gro_header_slow(struct sk_buff *skb, unsigned int hlen,
+					unsigned int offset)
+{
 	NAPI_GRO_CB(skb)->frag0 = NULL;
 	NAPI_GRO_CB(skb)->frag0_len = 0;
 	return pskb_may_pull(skb, hlen) ? skb->data + offset : NULL;
-	}
-
-	return NAPI_GRO_CB(skb)->frag0 + offset;
 }
 
 static inline void *skb_gro_mac_header(struct sk_buff *skb)

net/core/dev.c

@@ -2590,18 +2590,25 @@ struct sk_buff *napi_frags_skb(struct napi_struct *napi)
 {
 	struct sk_buff *skb = napi->skb;
 	struct ethhdr *eth;
+	unsigned int hlen;
+	unsigned int off;
 
 	napi->skb = NULL;
 
 	skb_reset_mac_header(skb);
 	skb_gro_reset_offset(skb);
 
-	eth = skb_gro_header(skb, sizeof(*eth));
-	if (!eth) {
+	off = skb_gro_offset(skb);
+	hlen = off + sizeof(*eth);
+	eth = skb_gro_header_fast(skb, off);
+	if (skb_gro_header_hard(skb, hlen)) {
+		eth = skb_gro_header_slow(skb, hlen, off);
+		if (unlikely(!eth)) {
 			napi_reuse_skb(napi, skb);
 			skb = NULL;
 			goto out;
 		}
+	}
 
 	skb_gro_pull(skb, sizeof(*eth));
 

net/ipv4/af_inet.c

@@ -1246,13 +1246,20 @@ static struct sk_buff **inet_gro_receive(struct sk_buff **head,
 	struct sk_buff **pp = NULL;
 	struct sk_buff *p;
 	struct iphdr *iph;
+	unsigned int hlen;
+	unsigned int off;
 	int flush = 1;
 	int proto;
 	int id;
 
-	iph = skb_gro_header(skb, sizeof(*iph));
+	off = skb_gro_offset(skb);
+	hlen = off + sizeof(*iph);
+	iph = skb_gro_header_fast(skb, off);
+	if (skb_gro_header_hard(skb, hlen)) {
+		iph = skb_gro_header_slow(skb, hlen, off);
 		if (unlikely(!iph))
 			goto out;
+	}
 
 	proto = iph->protocol & (MAX_INET_PROTOS - 1);
 

net/ipv4/tcp.c

@@ -2518,20 +2518,30 @@ struct sk_buff **tcp_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 	unsigned int thlen;
 	unsigned int flags;
 	unsigned int mss = 1;
+	unsigned int hlen;
+	unsigned int off;
 	int flush = 1;
 	int i;
 
-	th = skb_gro_header(skb, sizeof(*th));
+	off = skb_gro_offset(skb);
+	hlen = off + sizeof(*th);
+	th = skb_gro_header_fast(skb, off);
+	if (skb_gro_header_hard(skb, hlen)) {
+		th = skb_gro_header_slow(skb, hlen, off);
 		if (unlikely(!th))
 			goto out;
+	}
 
 	thlen = th->doff * 4;
 	if (thlen < sizeof(*th))
 		goto out;
 
-	th = skb_gro_header(skb, thlen);
+	hlen = off + thlen;
+	if (skb_gro_header_hard(skb, hlen)) {
+		th = skb_gro_header_slow(skb, hlen, off);
 		if (unlikely(!th))
 			goto out;
+	}
 
 	skb_gro_pull(skb, thlen);
 

net/ipv6/af_inet6.c

@@ -817,13 +817,20 @@ static struct sk_buff **ipv6_gro_receive(struct sk_buff **head,
 	struct sk_buff *p;
 	struct ipv6hdr *iph;
 	unsigned int nlen;
+	unsigned int hlen;
+	unsigned int off;
 	int flush = 1;
 	int proto;
 	__wsum csum;
 
-	iph = skb_gro_header(skb, sizeof(*iph));
+	off = skb_gro_offset(skb);
+	hlen = off + sizeof(*iph);
+	iph = skb_gro_header_fast(skb, off);
+	if (skb_gro_header_hard(skb, hlen)) {
+		iph = skb_gro_header_slow(skb, hlen, off);
 		if (unlikely(!iph))
 			goto out;
+	}
 
 	skb_gro_pull(skb, sizeof(*iph));
 	skb_set_transport_header(skb, skb_gro_offset(skb));