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

From: Alexander Duyck <[email-masked]>
Date: Mon, 17 Sep 2012 13:53:56 -0700

On 09/17/2012 12:58 AM, Eric Dumazet wrote:
> On Mon, 2012-09-17 at 07:33 +0000, Dave, Tushar N wrote:
>>> -----Original Message-----
>>> From: [email-masked] [mailto:[email-masked]]
>>> 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;
For most systems that extra padding should already be added since
alloc_skb will cache line align the buffer anyway.

A more general fix might be to make it so that alloc_skb cannot allocate
less than 60 byte buffers on systems with a cache line size smaller than
64 bytes.

Thanks,

Alex