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

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Date:	Fri, 04 Jan 2013 08:00:19 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] ndisc: Remove unused space at tail of skb for
 ndisc messages. (TAKE 3)

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>

Thanks !


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