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

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Message-ID: <538EF1B3.40904@citrix.com>
Date:	Wed, 4 Jun 2014 11:15:15 +0100
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	David Laight <David.Laight@...LAB.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

On 04/06/14 09:50, David Laight wrote:
> 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 ...
Can you elaborate a bit please on "above"?
>
> ...
>>   	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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