net: more accurate skb truesize

skb truesize currently accounts for sk_buff struct and part of skb head.
kmalloc() roundings are also ignored.

Considering that skb_shared_info is larger than sk_buff, its time to
take it into account for better memory accounting.

This patch introduces SKB_TRUESIZE(X) macro to centralize various
assumptions into a single place.

At skb alloc phase, we put skb_shared_info struct at the exact end of
skb head, to allow a better use of memory (lowering number of
reallocations), since kmalloc() gives us power-of-two memory blocks.

Unless SLUB/SLUB debug is active, both skb->head and skb_shared_info are
aligned to cache lines, as before.

Note: This patch might trigger performance regressions because of
misconfigured protocol stacks, hitting per socket or global memory
limits that were previously not reached. But its a necessary step for a
more accurate memory accounting.

Signed-off-by: Eric Dumazet <[email protected]>
CC: Andi Kleen <[email protected]>
CC: Ben Hutchings <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

net/iucv/af_iucv.c

@@ -1819,7 +1819,7 @@ static void iucv_callback_rx(struct iucv_path *path, struct iucv_message *msg)
 		goto save_message;
 
 	len = atomic_read(&sk->sk_rmem_alloc);
-	len += iucv_msg_length(msg) + sizeof(struct sk_buff);
+	len += SKB_TRUESIZE(iucv_msg_length(msg));
 	if (len > sk->sk_rcvbuf)
 		goto save_message;