net: pskb_expand_head() optimization

Move frags[] at the end of struct skb_shared_info, and make
pskb_expand_head() copy only the used part of it instead of whole array.

This should avoid kmemcheck warnings and speedup pskb_expand_head() as
well, avoiding a lot of cache misses.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -202,10 +202,11 @@ struct skb_shared_info {
 	 */
 	atomic_t	dataref;
 
-	skb_frag_t	frags[MAX_SKB_FRAGS];
 	/* Intermediate layers must ensure that destructor_arg
 	 * remains valid until skb destructor */
 	void *		destructor_arg;
+	/* must be last field, see pskb_expand_head() */
+	skb_frag_t	frags[MAX_SKB_FRAGS];
 };
 
 /* We divide dataref into two halves.  The higher 16 bits hold references

net/core/skbuff.c

@@ -817,7 +817,7 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	memcpy(data + nhead, skb->head, skb->tail - skb->head);
 #endif
 	memcpy(data + size, skb_end_pointer(skb),
-	       sizeof(struct skb_shared_info));
+	       offsetof(struct skb_shared_info, frags[skb_shinfo(skb)->nr_frags]));
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
 		get_page(skb_shinfo(skb)->frags[i].page);