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

[David Miller <davem@davemloft.net>]

From: Zoltan Kiss <zoltan.kiss@citrix.com>
Date: Wed, 4 Jun 2014 19:58:51 +0100

> 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.
> 
> Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>

Applied, and:

> -		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;

I took care of adding the {} to the else block, as suggested by Wei Liu.