netdev - Re: [Xen-devel] [PATCH v2 1/1] xen/netback: correctly calculate required slots of skb.

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Date:	Thu, 11 Jul 2013 21:35:40 +0800
From:	annie li <annie.li@...cle.com>
To:	Ian Campbell <ian.campbell@...rix.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"wei.liu2@...rix.com" <wei.liu2@...rix.com>,
	"msw@...zon.com" <msw@...zon.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
Subject: Re: [Xen-devel] [PATCH v2 1/1] xen/netback: correctly calculate required
 slots of skb.


On 2013-7-11 19:12, Ian Campbell wrote:
>>>>> The code here only
>>>>> increase count for the first time. I thought to initialize the
>>>>> count in
>>>>> xen_netbk_count_skb_slots with 1 to avoid this. But thinking of
>>>>> the
>>>>> extreme case when the header size is zero(not sure whether this
>>>>> case
>>>>> could be true), I increase the count here to keep safe in case
>>>>> header
>>>>> size is zero.
>>>> netfront requires that the first slot always contains some data,
>>>> gop_frag_copy will BUG if that's not the case.
>>>>
>>
>> In gop_frag_copy, we can not go into the while if the size is 0. Which
>> BUG_ON do you mean here?
> in gop_frag_copy:
> 		if (start_new_rx_buffer(npo->copy_off, bytes, *head)) {
> 			/*
> 			 * Netfront requires there to be some data in the head
> 			 * buffer.
> 			 */
> 			BUG_ON(*head);
>
But If there is SKB with zero header size and zero offset, the code will 
not run into the while loop in gop_frag_copy and this if condition. 
BUG_ON will not happen in such situation.

Thanks
Annie
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