netdev - Re: TCP and reordering

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Message-ID: <1354440685.21562.399.camel@shinybook.infradead.org>
Date:	Sun, 02 Dec 2012 09:31:25 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Benjamin LaHaise <bcrl@...ck.org>,
	Vijay Subramanian <subramanian.vijay@...il.com>,
	David Miller <davem@...emloft.net>, saku@...i.fi,
	rick.jones2@...com, netdev@...r.kernel.org
Subject: Re: TCP and reordering

On Sat, 2012-12-01 at 18:40 -0800, Eric Dumazet wrote:
> > +static unsigned int skb_acct_len(struct sk_buff *skb)
> > +{
> > +	return skb_tail_pointer(skb) - skb_network_header(skb);
> > +}
> 
> Apparently this driver doesnt add any feature at alloc_netdev() time, so
> it might work. (no frags in any skb)

Well... as long as no driver appends stuff to the *tail* of the skb. As
long as they only *prepend* headers for the device to use, we're fine.
But I think that's fairly safe, for now.

For PPP I think I'll end up keeping a tuple of
	{ skb, orig_len, orig_destructor, orig_sk }
for each skb which is in-flight. Since the queues of those should be
*short*, and should generally complete in-order, that shouldn't be too
much overhead. It'll only need a small static array of them per-channel.

(Before complaining about a *static* array holding metadata about items
on a queue, stop thinking of the PPP channel being a queue and start
thinking of it more like a ring buffer.)

-- 
dwmw2



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