netdev - Re: [RFC PATCH net] tcp: Update pcount after skb

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Message-ID: <20150605180254.GC2951343@devbig242.prn2.facebook.com>
Date:	Fri, 5 Jun 2015 11:02:55 -0700
From:	Martin KaFai Lau <kafai@...com>
To:	Eric Dumazet <edumazet@...gle.com>
CC:	netdev <netdev@...r.kernel.org>, Kernel Team <kernel-team@...com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [RFC PATCH net] tcp: Update pcount after skb_pull() during mtu
 probing

On Fri, Jun 05, 2015 at 09:53:51AM -0700, Eric Dumazet wrote:
> Sounds good, although I would simply get rid of all this complexity in
> this very unlikely path.
> 
> Would you instead try the following ?
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index eeb59befaf06867b00e1dd6ded7742b2f0bcd821..41ef2c96b2e810e289d6d773e4be09c493b99c09
> 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1987,16 +1987,7 @@ static int tcp_mtu_probe(struct sock *sk)
>                 } else {
>                         TCP_SKB_CB(nskb)->tcp_flags |=
> TCP_SKB_CB(skb)->tcp_flags &
>                                                    ~(TCPHDR_FIN|TCPHDR_PSH);
> -                       if (!skb_shinfo(skb)->nr_frags) {
> -                               skb_pull(skb, copy);
> -                               if (skb->ip_summed != CHECKSUM_PARTIAL)
> -                                       skb->csum = csum_partial(skb->data,
> -                                                                skb->len, 0);
> -                       } else {
> -                               __pskb_trim_head(skb, copy);
> -                               tcp_set_skb_tso_segs(sk, skb, mss_now);
> -                       }
> -                       TCP_SKB_CB(skb)->seq += copy;
> +                       tcp_trim_head(sk, skb, copy);
tcp_trim_head() does not take the mss_now.
Is it fine to have mss_now <= tcp_skb_mss(skb)? or we can depend on
the tcp_init_tso_segs() in the tcp_write_xmit() to take care of it?
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