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

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Date:	Mon, 17 Sep 2012 09:58:22 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Dave, Tushar N" <tushar.n.dave@...el.com>
Cc:	"Fastabend, John R" <john.r.fastabend@...el.com>,
	Michal Miroslaw <mirqus@...il.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"gospo@...hat.com" <gospo@...hat.com>,
	"sassmann@...hat.com" <sassmann@...hat.com>
Subject: RE: [net] e1000: Small packets may get corrupted during padding by
 HW

On Mon, 2012-09-17 at 07:33 +0000, Dave, Tushar N wrote:
> >-----Original Message-----
> >From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.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;


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