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

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Date:	Wed, 18 Apr 2012 21:18:27 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	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 net-next] tcp: avoid expensive pskb_expand_head() calls

On Wed, 2012-04-18 at 14:40 -0400, Neal Cardwell wrote:

> 
> > +       prev_packets_acked = tcp_skb_pcount(skb);
> 
> FWIW, I'd find old_pcount or prev_pcount a little easier to read than
> prev_packets_acked here (I see "oldpcount" is used in tcp_output
> in a similar context).
> 

You're right, I'll change this.

> >        TCP_SKB_CB(skb)->seq += len;
> > +       TCP_SKB_CB(skb)->header.offset_ack = 0;
> 
> If the caller decides to trim a prefix of the skb that does not extend
> to snd_una, then setting offset_ack to 0 here will cause us to forget
> that some prefix is ACKed when we should have remembered this.
> However, the API for the function invites the caller to chop off an
> arbitrary amount (and there's no comment to disuade the caller from
> trying this). This seems to risk bugs in the future.
> 
> To attempt to make this API safer and simpler for future generations,
> what do you think about calculating the len inside tcp_trim_head(),
> something like:
> 
> static int tcp_trim_head(struct sock *sk, struct sk_buff *skb)
> {
>   u32 len = tp->snd_una - TCP_SKB_CB(skb)->seq;
> ...
> 
> ...
>  if (tcp_trim_head(sk, skb))
> ...
> 

Very good point, I'll use this suggestion too in v2.

Thanks !


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