netdev - Re: [PATCH net-next-2.6] net: pskb_expand

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Message-ID: <20100912104534.GA2056@del.dom.local>
Date:	Sun, 12 Sep 2010 12:45:34 +0200
From:	Jarek Poplawski <jarkao2@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next-2.6] net: pskb_expand_head() optimization

On Sat, Sep 11, 2010 at 08:30:02PM -0700, David Miller wrote:
> From: Jarek Poplawski <jarkao2@...il.com>
> Date: Sat, 11 Sep 2010 14:31:40 +0200
> 
> > Otherwise seems OK, but I still would like to know the scenario
> > demanding this change.
> 
> I want to make sk_buff use list_head, including all uses such as
> frag_list et al.
> 
> If the frag_list chain can be shared, a doubly linked list cannot be
> used.
> 
> This is someting I've been gradually working on now for more than 2
> years :)

Hmm... Then the first message/changelog in this thread seems to
describe the future bug, only with doubly linked lists. If so, it was
a bit misleading to me ;-)

Then a few more questions:
1) if doubly linked lists really require such pskb_copying, isn't it
   all too costly?
2) why skb_clone isn't enough instead of pskb_copy?
3) since skb_clone has some cost too, why e.g. saving only the pointer
   to the tail of the list in skb_shared_info isn't enough?

Jarek P.
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