Subject: Re: [PATCH] xen/netfront: improve truesize tracking

From: Konrad Rzeszutek Wilk <[email-masked]>
Date: Tue, 18 Dec 2012 09:10:48 -0500

On Tue, Dec 18, 2012 at 01:51:51PM +0000, Ian Campbell wrote:
> Using RX_COPY_THRESHOLD is incorrect if the SKB is actually smaller
> than that. We have already accounted for this in
> NETFRONT_SKB_CB(skb)->pull_to so use that instead.
> 
> Fixes WARN_ON from skb_try_coalesce.

This should probably be also on the stable tree for 3.7 at least?

> 
> Signed-off-by: Ian Campbell <[email-masked]>
> Cc: Sander Eikelenboom <[email-masked]>
  ^^ - Reported-by: 

> Cc: Konrad Rzeszutek Wilk <[email-masked]>
  ^^ - Acked-by:

> Cc: annie li <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Cc: [email-masked] # 3.7.x only
> ---
>  drivers/net/xen-netfront.c |   15 +++++----------
>  1 files changed, 5 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index caa0110..b06ef81 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -971,17 +971,12 @@ err:
>  		 * overheads. Here, we add the size of the data pulled
>  		 * in xennet_fill_frags().
>  		 *
> -		 * We also adjust for any unused space in the main
> -		 * data area by subtracting (RX_COPY_THRESHOLD -
> -		 * len). This is especially important with drivers
> -		 * which split incoming packets into header and data,
> -		 * using only 66 bytes of the main data area (see the
> -		 * e1000 driver for example.)  On such systems,
> -		 * without this last adjustement, our achievable
> -		 * receive throughout using the standard receive
> -		 * buffer size was cut by 25%(!!!).
> +		 * We also adjust for the __pskb_pull_tail done in
> +		 * handle_incoming_queue which pulls data from the
> +		 * frags into the head area, which is already
> +		 * accounted in RX_COPY_THRESHOLD.
>  		 */
> -		skb->truesize += skb->data_len - RX_COPY_THRESHOLD;
> +		skb->truesize += skb->data_len - NETFRONT_SKB_CB(skb)->pull_to;
>  		skb->len += skb->data_len;
>  
>  		if (rx->flags & XEN_NETRXF_csum_blank)
> -- 
> 1.7.2.5
>