Re: [PATCH net-next 2/3] tcp: keep scaled no-shrink window representable

[Paolo Abeni <pabeni@redhat.com>]

On 3/17/26 7:51 AM, Wesley Atwell wrote:
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 35c3b0ab5a0cb714155d5720fe56888f71aecced..bd3a43148a87e891bc632a47ffb5b82c475e8f6f 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -3375,13 +3375,19 @@ u32 __tcp_select_window(struct sock *sk)
>  	 * scaled window will not line up with the MSS boundary anyway.
>  	 */
>  	if (tp->rx_opt.rcv_wscale) {
> -		window = free_space;
> +		u32 gran = 1U << tp->rx_opt.rcv_wscale;
>  
> -		/* Advertise enough space so that it won't get scaled away.
> -		 * Import case: prevent zero window announcement if
> -		 * 1<<rcv_wscale > mss.
> +		/* Keep tp->rcv_wnd representable in scaled units so later
> +		 * no-shrink decisions reason about the same right edge we
> +		 * can advertise on the wire. Preserve only a small non-zero
> +		 * offer that would otherwise get scaled away to zero.
>  		 */
> -		window = ALIGN(window, (1 << tp->rx_opt.rcv_wscale));
> +		if (free_space >= gran)
> +			window = round_down(free_space, gran);

The receive window already has a similar rounding in the `free_space <
(full_space >> 1)` case. This is basically excluding only:

	gran > free_space >= (full_space >> 1)

which IDK if is a realistic situation, perhaps just do the scale down
unconditionally?

Also minor nit, prefer 'granularity' over 'gran'

/P