RE: [net] e1000: Small packets may get corrupted during padding by HW

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2012-09-17 at 07:33 +0000, Dave, Tushar N wrote:
> >-----Original Message-----
> >From: netdev-owner@vger.kernel.org [mailto:netdev-owner@vger.kernel.org]
> >On Behalf Of John Fastabend

> >Also wouldn't you want an unlikely() in your patch?
> 
> No because it is quite normal to have packet < ETH_ZLEN. e.g. ARP packets.

ARP packets ? Hardly a performance problem.

Or make sure all these packets have enough tailroom, or else you are
going to hit the cost of reallocating packets.

I would better point TCP pure ACK packets, since their size can be 54
bytes.

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index cfe6ffe..aefc681 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3083,8 +3083,9 @@ void tcp_send_ack(struct sock *sk)
 	/* We are not putting this on the write queue, so
 	 * tcp_transmit_skb() will set the ownership to this
 	 * sock.
+	 * Add 64 bytes of tailroom so that some drivers can use skb_pad()
 	 */
-	buff = alloc_skb(MAX_TCP_HEADER, sk_gfp_atomic(sk, GFP_ATOMIC));
+	buff = alloc_skb(MAX_TCP_HEADER + 64, sk_gfp_atomic(sk, GFP_ATOMIC));
 	if (buff == NULL) {
 		inet_csk_schedule_ack(sk);
 		inet_csk(sk)->icsk_ack.ato = TCP_ATO_MIN;