From mboxrd@z Thu Jan  1 00:00:00 1970
From: Martin KaFai Lau <kafai@fb.com>
Subject: Re: [RFC PATCH net] tcp: Update pcount after skb_pull() during mtu
 probing
Date: Fri, 5 Jun 2015 16:02:07 -0700
Message-ID: <20150605230206.GD2951343@devbig242.prn2.facebook.com>
References: <1433521014-2465771-1-git-send-email-kafai@fb.com>
 <CANn89iLd1bqCOxbALcDNzi--Hn8A970HmvM2__ryzjwSbOwZ6Q@mail.gmail.com>
 <20150605180254.GC2951343@devbig242.prn2.facebook.com>
 <1433539435.1895.60.camel@edumazet-glaptop2.roam.corp.google.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Cc: Eric Dumazet <edumazet@google.com>,
	netdev <netdev@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>,
	Neal Cardwell <ncardwell@google.com>,
	Yuchung Cheng <ycheng@google.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Return-path: <netdev-owner@vger.kernel.org>
Received: from mx0a-00082601.pphosted.com ([67.231.145.42]:55402 "EHLO
	mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK)
	by vger.kernel.org with ESMTP id S1752119AbbFEXCN (ORCPT
	<rfc822;netdev@vger.kernel.org>); Fri, 5 Jun 2015 19:02:13 -0400
Content-Disposition: inline
In-Reply-To: <1433539435.1895.60.camel@edumazet-glaptop2.roam.corp.google.com>
Sender: netdev-owner@vger.kernel.org
List-ID: <netdev.vger.kernel.org>

On Fri, Jun 05, 2015 at 02:23:55PM -0700, Eric Dumazet wrote:
> On Fri, 2015-06-05 at 11:02 -0700, Martin KaFai Lau wrote:
> 
> > 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?
> 
> It should be fine : packets not yet sent have tcp_skb_pcount()==0,
> so that tcp_init_tso_segs() can do the computation at the right time.
hmm.... From tcp_write_xmit(), tcp_init_tso_segs() makes tcp_skb_pcount(skb) > 0
but it may not be sent out immediately (like failing tcp_cwnd_test()).  Later,
this skb is considered for tcp_mtu_probe(), sliced and forgot to update the
pcount, I think.

I am probably missing something.  Hence, I have a side question
in tcp_init_tso_segs(),  should pcount be also recalculated if
(tso_segs == 1 && mss_now < skb->len)?  Like this:

diff --git i/net/ipv4/tcp_output.c w/net/ipv4/tcp_output.c
index a369e8a..15d1c44 100644
--- i/net/ipv4/tcp_output.c
+++ w/net/ipv4/tcp_output.c
@@ -1624,7 +1624,8 @@ static int tcp_init_tso_segs(const struct sock *sk, struct sk_buff *skb,
 {
 	int tso_segs = tcp_skb_pcount(skb);

-	if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now)) {
+	if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now) ||
+	    (tso_segs == 1 && mss_now < skb->len)) {
 		tcp_set_skb_tso_segs(sk, skb, mss_now);
 		tso_segs = tcp_skb_pcount(skb);
 	}