Re: [PATCH v2 net-next] tcp: avoid expensive pskb_expand_head() calls

Eric Dumazet
2012-04-19T11:40:21Z

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 ?