net: gro: enable fast path for more cases

Currently the so-called GRO fast path is only enabled for
napi_frags_skb() callers.

After the prior patch, we no longer have to clear frag0 whenever
we pulled bytes to skb->head.

We therefore can initialize frag0 to skb->data so that GRO
fast path can be used in the following additional cases:

- Drivers using header split (populating skb->data with headers,
  and having payload in one or more page fragments).

- Drivers not using any page frag (entire packet is in skb->data)

Add a likely() in skb_gro_may_pull() to help the compiler
to generate better code if possible.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/net/gro.h

@@ -148,7 +148,7 @@ static inline void *skb_gro_header_fast(const struct sk_buff *skb,
 static inline bool skb_gro_may_pull(const struct sk_buff *skb,
 				    unsigned int hlen)
 {
-	return hlen <= NAPI_GRO_CB(skb)->frag0_len;
+	return likely(hlen <= NAPI_GRO_CB(skb)->frag0_len);
 }
 
 static inline void *skb_gro_header_slow(struct sk_buff *skb, unsigned int hlen,

net/core/gro.c

@@ -369,15 +369,21 @@ static void gro_list_prepare(const struct list_head *head,
 
 static inline void skb_gro_reset_offset(struct sk_buff *skb, u32 nhoff)
 {
-	const struct skb_shared_info *pinfo = skb_shinfo(skb);
-	const skb_frag_t *frag0 = &pinfo->frags[0];
+	const struct skb_shared_info *pinfo;
+	const skb_frag_t *frag0;
+	unsigned int headlen;
 
 	NAPI_GRO_CB(skb)->data_offset = 0;
-	NAPI_GRO_CB(skb)->frag0 = NULL;
-	NAPI_GRO_CB(skb)->frag0_len = 0;
+	headlen = skb_headlen(skb);
+	NAPI_GRO_CB(skb)->frag0 = skb->data;
+	NAPI_GRO_CB(skb)->frag0_len = headlen;
+	if (headlen)
+		return;
+
+	pinfo = skb_shinfo(skb);
+	frag0 = &pinfo->frags[0];
 
-	if (!skb_headlen(skb) && pinfo->nr_frags &&
-	    !PageHighMem(skb_frag_page(frag0)) &&
+	if (pinfo->nr_frags && !PageHighMem(skb_frag_page(frag0)) &&
 	    (!NET_IP_ALIGN || !((skb_frag_off(frag0) + nhoff) & 3))) {
 		NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);
 		NAPI_GRO_CB(skb)->frag0_len = min_t(unsigned int,
@@ -710,7 +716,10 @@ static struct sk_buff *napi_frags_skb(struct napi_struct *napi)
 		}
 	} else {
 		eth = (const struct ethhdr *)skb->data;
-		gro_pull_from_frag0(skb, hlen);
+
+		if (NAPI_GRO_CB(skb)->frag0 != skb->data)
+			gro_pull_from_frag0(skb, hlen);
+
 		NAPI_GRO_CB(skb)->frag0 += hlen;
 		NAPI_GRO_CB(skb)->frag0_len -= hlen;
 	}