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Thu, 19 Mar 2026 03:50:04 -0700 (PDT) X-Received: by 2002:a05:600c:1d10:b0:477:7ae0:cd6e with SMTP id 5b1f17b1804b1-486f4422046mr113297875e9.5.1773917403694; Thu, 19 Mar 2026 03:50:03 -0700 (PDT) Received: from [192.168.88.32] ([216.128.11.196]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43b518a2cd3sm13432241f8f.32.2026.03.19.03.50.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 19 Mar 2026 03:50:03 -0700 (PDT) Message-ID: Date: Thu, 19 Mar 2026 11:50:01 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next 2/3] tcp: keep scaled no-shrink window representable To: Wesley Atwell , netdev@vger.kernel.org Cc: linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, davem@davemloft.net, dsahern@kernel.org, edumazet@google.com, horms@kernel.org, kuba@kernel.org, kuniyu@google.com, ncardwell@google.com, shuah@kernel.org References: <20260317065137.1533226-1-atwellwea@gmail.com> <20260317065137.1533226-3-atwellwea@gmail.com> Content-Language: en-US From: Paolo Abeni In-Reply-To: <20260317065137.1533226-3-atwellwea@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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< 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