netdev - Re: [PATCH net-next v3] tcp: enable mid stream window clamp

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Date:   Thu, 26 Aug 2021 12:11:13 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Neil Spring <ntspring@...com>
Cc:     davem@...emloft.net, edumazet@...gle.com, yoshfuji@...ux-ipv6.org,
        dsahern@...nel.org, kuba@...nel.org, netdev@...r.kernel.org,
        ncardwell@...gle.com
Subject: Re: [PATCH net-next v3] tcp: enable mid stream window clamp

On Wed, Aug 25, 2021 at 2:02 PM Neil Spring <ntspring@...com> wrote:
>
> The TCP_WINDOW_CLAMP socket option is defined in tcp(7) to "Bound the size
> of the advertised window to this value."  Window clamping is distributed
> across two variables, window_clamp ("Maximal window to advertise" in
> tcp.h) and rcv_ssthresh ("Current window clamp").
>
> This patch updates the function where the window clamp is set to also
> reduce the current window clamp, rcv_sshthresh, if needed.  With this,
> setting the TCP_WINDOW_CLAMP option has the documented effect of limiting
> the window.
This patch looks like a bug-fix so it should be applied to net not net-next?

>
> Signed-off-by: Neil Spring <ntspring@...com>
> ---
> v2: - fix email formatting
>
> v3: - address comments by setting rcv_ssthresh based on prior window
>
>  net/ipv4/tcp.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index f931def6302e..e8b48df73c85 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -3338,6 +3338,7 @@ int tcp_set_window_clamp(struct sock *sk, int val)
>         } else {
>                 tp->window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
>                         SOCK_MIN_RCVBUF / 2 : val;
> +               tp->rcv_ssthresh = min(tp->rcv_wnd, tp->window_clamp);
>         }
>         return 0;
>  }
> --
> 2.30.2
>