Subject: Re: [PATCH] tcp: Reallocate headroom if it would overflow csum_start

From: Sergei Shtylyov <[email-masked]>
Date: Thu, 11 Apr 2013 16:41:43 +0400

Hello.

On 11-04-2013 15:19, Thomas Graf wrote:

> If a TCP retransmission gets partially ACKed and collapsed multiple
> times it is possible for the headroom to grow beyond 64K which will
> overflow the 16bit skb->csum_start which is based on the start of
> the headroom. It has been observed rarely in the wild with IPoIB due
> to the 64K MTU.

> Verify if the acking and collapsing resulted in a headroom exceeding
> what csum_start can cover and reallocate the headroom if so.

> LLNL has been running the patch for a while and has not seen the
> problem occur since.

> A big thank you to Jim Foraker <[email-masked]> and the team at
> LLNL for helping out with the investigation and testing.

> Reported-by: Jim Foraker <[email-masked]>
> Signed-off-by: Thomas Graf <[email-masked]>
[...]

    Minor formatting nit.

> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index b44cf81..bf6ceb7 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2388,8 +2388,11 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
>   	 */
>   	TCP_SKB_CB(skb)->when = tcp_time_stamp;
>
> -	/* make sure skb->data is aligned on arches that require it */
> -	if (unlikely(NET_IP_ALIGN && ((unsigned long)skb->data & 3))) {
> +	/* make sure skb->data is aligned on arches that require it
> +	 * and check if ack-trimming & collapsing extended the headroom
> +	 * beyond what csum_start can cover. */

    The preferred multi-line comment style in the networking code:

/* bla
  * bla
  */

WBR, Sergei