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

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Date:	Wed, 4 Jun 2014 08:50:36 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Zoltan Kiss' <zoltan.kiss@...rix.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"ian.campbell@...rix.com" <ian.campbell@...rix.com>,
	"wei.liu2@...rix.com" <wei.liu2@...rix.com>,
	"paul.durrant@...rix.com" <paul.durrant@...rix.com>,
	"linux@...elenboom.it" <linux@...elenboom.it>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"david.vrabel@...rix.com" <david.vrabel@...rix.com>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [PATCH net] xen-netback: Fix handling of skbs requiring too
 many slots

From: netdev-owner@...r.kernel.org
> 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 / PAGE_SIZE data slots, as the provided buffers are fully
> utilized.
...
> -static bool start_new_rx_buffer(int offset, unsigned long size, int head)
> +static bool start_new_rx_buffer(int offset,
> +				unsigned long size,
> +				int head,
> +				bool full_coalesce)

The above is completely incorrect layout ...

...
>  	BUG_ON(size > MAX_BUFFER_OFFSET);
> -	if ((offset + size > MAX_BUFFER_OFFSET) && offset && !head)
> +	if ((offset + size > MAX_BUFFER_OFFSET) &&
> +	    offset &&
> +	    !head &&
> +	    !full_coalesce)
>  		return true;

As is that.

	David



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