netdev - Re: [RFC] allow skb->head to point/alias to first skb frag

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Date:	Thu, 26 Apr 2012 11:10:02 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	ilpo.jarvinen@...sinki.fi, rick.jones2@...com,
	netdev@...r.kernel.org, therbert@...gle.com, ncardwell@...gle.com,
	maze@...gle.com, ycheng@...gle.com
Subject: Re: [RFC] allow skb->head to point/alias to first skb frag

On Thu, 2012-04-26 at 04:36 -0400, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Thu, 26 Apr 2012 10:10:54 +0200
> 
> > Idea would have :
> > 
> > 1) struct sk_buff
> > 2) skb->head points to frag (aliasing, no memory allocation)
> > 3) frag of 2048 (or PAGE_SIZE/2 or PAGE_SIZE)
> 
> What would you set skb->data_len and skb->len to?
> 
> How would you handle tailroom?  We have the rule that you can't append
> to the tail of a fragmented SKB (I mean append, the way that IPSEC
> wants to write data to the end of an SKB when encrypting, for
> example).  This is only allowed on fully linear SKBs.
> 
> So, for this reason and others, you can't pretend that this new
> construction is linear in any way.  It would break a bunch of things.


The 'frag' would have a known size : 2048 bytes

But the end of it would be used by struct skb_shared_info

so data_len would be 0 in fact.

This would look like a regular linear skb.

Just a bit set in skb to say : Warning, skb->head was not kmalloced :
replace kfree(head) by put_page(...)

And this bit would be tested in GRO or tcp merge to 'upgrade' this
skb->head to proper page/frag


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