Re: [RFC PATCH net] tcp: Update pcount after skb_pull() during mtu probing

[Martin KaFai Lau <kafai@fb.com>]

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);
 	}