netdev - Re: [PATCH net-next] ndisc: Remove unused space at tail of skb for ndisc messages. (TAKE 3)

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Message-Id: <20130104.151710.601442706907153024.davem@davemloft.net>
Date:	Fri, 04 Jan 2013 15:17:10 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	yoshfuji@...ux-ipv6.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] ndisc: Remove unused space at tail of skb for
 ndisc messages. (TAKE 3)

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 04 Jan 2013 08:00:19 -0800

> On Fri, 2013-01-04 at 22:58 +0900, YOSHIFUJI Hideaki wrote:
>> Currently, the size of skb allocated for NDISC is MAX_HEADER +
>> LL_RESERVED_SPACE(dev) + packet length + dev->needed_tailroom,
>> but only LL_RESERVED_SPACE(dev) bytes is "reserved" for headers.
>> As a result, the skb looks like this (after construction of the
>> message):
>> 
>> head       data                   tail                       end
>> +--------------------------------------------------------------+
>> +           |                      |          |                |
>> +--------------------------------------------------------------+
>> |<-hlen---->|<---ipv6 packet------>|<--tlen-->|<--MAX_HEADER-->|
>>     =LL_                               = dev
>>      RESERVED_                           ->needed_
>>      SPACE(dev)                            tailroom
>> 
>> As the name implies, "MAX_HEADER" is used for headers, and should
>> be "reserved" in prior to packet construction.  Or, if some space
>> is really required at the tail of ther skb, it should be
>> explicitly documented.
>> 
>> We have several option after construction of NDISC message:
>> 
>> Option 1:
>> 
>> head       data                   tail       end
>> +---------------------------------------------+
>> +           |                      |          |
>> +---------------------------------------------+
>> |<-hlen---->|<---ipv6 packet------>|<--tlen-->|
>>    =LL_                                = dev
>>     RESERVED_                           ->needed_
>>     SPACE(dev)                            tailroom
> 
> Acked-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks for your persistence.
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