netdev - Re: [PATCH net] tcp: inherit timestamp on mtu probe

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Date:   Tue, 27 Aug 2019 22:07:01 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] tcp: inherit timestamp on mtu probe

On Tue, Aug 27, 2019 at 9:09 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> From: Willem de Bruijn <willemb@...gle.com>
>
> TCP associates tx timestamp requests with a byte in the bytestream.
> If merging skbs in tcp_mtu_probe, migrate the tstamp request.
>
> Similar to MSG_EOR, do not allow moving a timestamp from any segment
> in the probe but the last. This to avoid merging multiple timestamps.
>
> Tested with the packetdrill script at
> https://github.com/wdebruij/packetdrill/commits/mtu_probe-1
>
> Link: http://patchwork.ozlabs.org/patch/1143278/#2232897
> Fixes: 4ed2d765dfac ("net-timestamp: TCP timestamping")
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> ---
>  net/ipv4/tcp_output.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 5c46bc4c7e8d..42abc9bd687a 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2053,7 +2053,7 @@ static bool tcp_can_coalesce_send_queue_head(struct sock *sk, int len)
>                 if (len <= skb->len)
>                         break;
>
> -               if (unlikely(TCP_SKB_CB(skb)->eor))
> +               if (unlikely(TCP_SKB_CB(skb)->eor) || tcp_has_tx_tstamp(skb))
>                         return false;
>
>                 len -= skb->len;
> @@ -2170,6 +2170,7 @@ static int tcp_mtu_probe(struct sock *sk)
>                          * we need to propagate it to the new skb.
>                          */
>                         TCP_SKB_CB(nskb)->eor = TCP_SKB_CB(skb)->eor;
> +                       tcp_skb_collapse_tstamp(nskb, skb);

nit: maybe rename tcp_skb_collapse_tstamp() to tcp_skb_tstamp_copy()
or something ?

Its name came from the fact that it was only used from
tcp_collapse_retrans(), but it will no
longer be the case after your fix.

>                         tcp_unlink_write_queue(skb, sk);
>                         sk_wmem_free_skb(sk, skb);
>                 } else {
> --
> 2.23.0.187.g17f5b7556c-goog
>