netdev - Re: [PATCH net v2] xen-netback: Fix handling of skbs requiring too many slots

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Date:	Thu, 5 Jun 2014 11:07:34 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
CC:	<xen-devel@...ts.xenproject.org>, <ian.campbell@...rix.com>,
	<wei.liu2@...rix.com>, <paul.durrant@...rix.com>,
	<linux@...elenboom.it>, <netdev@...r.kernel.org>,
	<david.vrabel@...rix.com>, <davem@...emloft.net>
Subject: Re: [PATCH net v2] xen-netback: Fix handling of skbs requiring too
 many slots

On Wed, Jun 04, 2014 at 07:58:51PM +0100, Zoltan Kiss wrote:
> A recent commit (a02eb4 "xen-netback: worse-case estimate in xenvif_rx_action is
> underestimating") capped the slot estimation to MAX_SKB_FRAGS, but that triggers
> the next BUG_ON a few lines down, as the packet consumes more slots than
> estimated.
> This patch introduces full_coalesce on the skb callback buffer, which is used in
> start_new_rx_buffer() to decide whether netback needs coalescing more
> aggresively. By doing that, no packet should need more than
> (XEN_NETIF_MAX_TX_SIZE + 1) / PAGE_SIZE data slots (excluding the optional GSO
> slot, it doesn't carry data, therefore irrelevant in this case), as the provided
> buffers are fully utilized.
> 

Acked-by: Wei Liu <wei.liu2@...rix.com>

Minor nit below.

[...]
> @@ -602,10 +611,14 @@ static void xenvif_rx_action(struct xenvif *vif)
>  
>  		/* To avoid the estimate becoming too pessimal for some
>  		 * frontends that limit posted rx requests, cap the estimate
> -		 * at MAX_SKB_FRAGS.
> +		 * at MAX_SKB_FRAGS. In this case netback will fully coalesce
> +		 * the skb into the provided slots.
>  		 */
> -		if (max_slots_needed > MAX_SKB_FRAGS)
> +		if (max_slots_needed > MAX_SKB_FRAGS) {
>  			max_slots_needed = MAX_SKB_FRAGS;
> +			XENVIF_RX_CB(skb)->full_coalesce = true;
> +		} else
> +			XENVIF_RX_CB(skb)->full_coalesce = false;
>  

Please enclose this line with {}, which is required by kernel coding
style.

Wei.
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