gro: Only use skb_gro_header for completely non-linear packets

Currently skb_gro_header is used for packets which put the hardware
header in skb->data with the rest in frags.  Since the drivers that
need this optimisation all provide completely non-linear packets,
we can gain extra optimisations by only performing the frag0
optimisation for completely non-linear packets.

In particular, we can simply test frag0 (instead of skb_headlen)
to see whether the optimisation is in force.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1135,22 +1135,23 @@ static inline void *skb_gro_header(struct sk_buff *skb, unsigned int hlen)
 
 	hlen += offset;
 	if (!NAPI_GRO_CB(skb)->frag0 ||
-	    unlikely(skb_shinfo(skb)->frags[0].size + skb_headlen(skb) < hlen))
+	    unlikely(skb_shinfo(skb)->frags[0].size < hlen)) {
+		NAPI_GRO_CB(skb)->frag0 = NULL;
 		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)
 {
-	return skb_headlen(skb) ? skb_mac_header(skb) :
-	       NAPI_GRO_CB(skb)->frag0;
+	return NAPI_GRO_CB(skb)->frag0 ?: skb_mac_header(skb);
 }
 
 static inline void *skb_gro_network_header(struct sk_buff *skb)
 {
-	return skb_headlen(skb) ? skb_network_header(skb) :
-	       NAPI_GRO_CB(skb)->frag0 + skb_network_offset(skb);
+	return (NAPI_GRO_CB(skb)->frag0 ?: skb->data) +
+	       skb_network_offset(skb);
 }
 
 static inline int dev_hard_header(struct sk_buff *skb, struct net_device *dev,

net/core/dev.c

@@ -2510,7 +2510,8 @@ void skb_gro_reset_offset(struct sk_buff *skb)
 	NAPI_GRO_CB(skb)->data_offset = 0;
 	NAPI_GRO_CB(skb)->frag0 = NULL;
 
-	if (!skb_headlen(skb) && !PageHighMem(skb_shinfo(skb)->frags[0].page))
+	if (skb->mac_header == skb->tail &&
+	    !PageHighMem(skb_shinfo(skb)->frags[0].page))
 		NAPI_GRO_CB(skb)->frag0 =
 			page_address(skb_shinfo(skb)->frags[0].page) +
 			skb_shinfo(skb)->frags[0].page_offset;