netdev - Re: [PATCH v2 net-next] tcp: avoid expensive pskb_expand

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Date:	Thu, 19 Apr 2012 13:40:13 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	Neal Cardwell <ncardwell@...gle.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>,
	Maciej Żenczykowski <maze@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH v2 net-next] tcp: avoid expensive pskb_expand_head()
 calls

On Thu, 2012-04-19 at 13:30 +0200, Eric Dumazet wrote:
> On Thu, 2012-04-19 at 14:10 +0300, Ilpo Järvinen wrote:
> 
> > Now that you have non-zero offset_ack, are the tcp_fragment() callsites 
> > safe and working? ...I'm mostly worried about tcp_mark_head_lost which 
> > does some assumptions about tp->snd_una and TCP_SKB_CB(skb)->seq, however, 
> > also other fragmenting does not preserve offset_ack properly (which might 
> > not be end of world though)?
> 
> Good point, I'll take a look.

Hmm, the only point this could matter is if a packet is retransmitted.
For other packets, offset_ack = 0 (default value on skb allocation)

And tcp_retransmit_skb() first call tcp_trim_head(sk, skb) if needed so
tcp_fragment() is called with == 0

        if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
                if (before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
                        BUG();
                if (tcp_trim_head(sk, skb))
                        return -ENOMEM;
        }

...
if (skb->len > cur_mss) {
	if (tcp_fragment(sk, skb, cur_mss, cur_mss))



I could add a BUG_ON(offset_ack == 0) to make sure this assertion is
true.

What do you think ?


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