netdev - Re: [PATCH net] tcp: fix mid stream window clamp.

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Date: Thu, 23 Nov 2023 18:10:38 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>, Neal Cardwell <ncardwell@...gle.com>, 
	Wei Wang <weiwan@...gle.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, 
	David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Neil Spring <ntspring@...com>, 
	David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH net] tcp: fix mid stream window clamp.

CC Neal and Wei

On Thu, Nov 23, 2023 at 4:25 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> After the blamed commit below, if the user-space application performs
> window clamping when tp->rcv_wnd is 0, the TCP socket will never be
> able to announce a non 0 receive window, even after completely emptying
> the receive buffer and re-setting the window clamp to higher values.
>
> Refactor tcp_set_window_clamp() to address the issue: when the user
> decreases the current clamp value, set rcv_ssthresh according to the
> same logic used at buffer initialization time.
> When increasing the clamp value, give the rcv_ssthresh a chance to grow
> according to previously implemented heuristic.
>
> Fixes: 3aa7857fe1d7 ("tcp: enable mid stream window clamp")
> Reported-by: David Gibson <david@...son.dropbear.id.au>
> Reported-by: Stefano Brivio <sbrivio@...hat.com>
> Reviewed-by: Stefano Brivio <sbrivio@...hat.com>
> Tested-by: Stefano Brivio <sbrivio@...hat.com>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
>  net/ipv4/tcp.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 53bcc17c91e4..1a9b9064e080 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -3368,9 +3368,22 @@ int tcp_set_window_clamp(struct sock *sk, int val)
>                         return -EINVAL;
>                 tp->window_clamp = 0;
>         } else {
> -               tp->window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
> -                       SOCK_MIN_RCVBUF / 2 : val;
> -               tp->rcv_ssthresh = min(tp->rcv_wnd, tp->window_clamp);
> +               u32 new_rcv_ssthresh, old_window_clamp = tp->window_clamp;
> +               u32 new_window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
> +                                               SOCK_MIN_RCVBUF / 2 : val;
> +
> +               if (new_window_clamp == old_window_clamp)
> +                       return 0;
> +
> +               tp->window_clamp = new_window_clamp;
> +               if (new_window_clamp < old_window_clamp) {
> +                       tp->rcv_ssthresh = min(tp->rcv_ssthresh,
> +                                              new_window_clamp);
> +               } else {
> +                       new_rcv_ssthresh = min(tp->rcv_wnd, tp->window_clamp);
> +                       tp->rcv_ssthresh = max(new_rcv_ssthresh,
> +                                              tp->rcv_ssthresh);
> +               }
>         }
>         return 0;
>  }

It seems there is no provision for SO_RESERVE_MEM

I wonder if tcp_adjust_rcv_ssthresh()  could help here ?

Have you considered reverting  3aa7857fe1d7 ("tcp: enable mid stream
window clamp") ?

Thanks.