[NET] skbuff: Add skb_cow_head

This patch adds an optimised version of skb_cow that avoids the copy if
the header can be modified even if the rest of the payload is cloned.

This can be used in encapsulating paths where we only need to modify the
header. As it is, this can be used in PPPOE and bridging.

Signed-off-by: Herbert Xu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/pppoe.c

···
 	/* Copy the data if there is no space for the header or if it's
 	 * read-only.
 	 */
-	if (skb_cow(skb, sizeof(*ph) + dev->hard_header_len))
+	if (skb_cow_head(skb, sizeof(*ph) + dev->hard_header_len))
 		goto abort;
 
 	__skb_push(skb, sizeof(*ph));

include/linux/skbuff.h

···
 	       skb_headroom(skb) + len <= skb->hdr_len;
 }
 
+static inline int __skb_cow(struct sk_buff *skb, unsigned int headroom,
+			    int cloned)
+{
+	int delta = 0;
+
+	if (headroom < NET_SKB_PAD)
+		headroom = NET_SKB_PAD;
+	if (headroom > skb_headroom(skb))
+		delta = headroom - skb_headroom(skb);
+
+	if (delta || cloned)
+		return pskb_expand_head(skb, ALIGN(delta, NET_SKB_PAD), 0,
+					GFP_ATOMIC);
+	return 0;
+}
+
 /**
  *	skb_cow - copy header of skb when it is required
  *	@skb: buffer to cow
···
  */
 static inline int skb_cow(struct sk_buff *skb, unsigned int headroom)
 {
-	int delta = (headroom > NET_SKB_PAD ? headroom : NET_SKB_PAD) -
-			skb_headroom(skb);
+	return __skb_cow(skb, headroom, skb_cloned(skb));
+}
 
-	if (delta < 0)
-		delta = 0;
-
-	if (delta || skb_cloned(skb))
-		return pskb_expand_head(skb, (delta + (NET_SKB_PAD-1)) &
-				~(NET_SKB_PAD-1), 0, GFP_ATOMIC);
-	return 0;
+/**
+ *	skb_cow_head - skb_cow but only making the head writable
+ *	@skb: buffer to cow
+ *	@headroom: needed headroom
+ *
+ *	This function is identical to skb_cow except that we replace the
+ *	skb_cloned check by skb_header_cloned.  It should be used when
+ *	you only need to push on some header and do not need to modify
+ *	the data.
+ */
+static inline int skb_cow_head(struct sk_buff *skb, unsigned int headroom)
+{
+	return __skb_cow(skb, headroom, skb_header_cloned(skb));
 }
 
 /**

net/bridge/br_netfilter.c

···
 	int err;
 	int header_size = ETH_HLEN + nf_bridge_encap_header_len(skb);
 
-	err = skb_cow(skb, header_size);
+	err = skb_cow_head(skb, header_size);
 	if (err)
 		return err;