From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Nathan Gao <zcgao@amazon.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.18 057/250] tcp: do not change rcv_ssthresh in tcp_measure_rcv_mss()
Date: Mon, 17 Aug 2026 15:30:18 +0200
Message-ID: <20260817132538.768439173@linuxfoundation.org>
In-Reply-To: <20260817132536.466235697@linuxfoundation.org>
References: <20260817132536.466235697@linuxfoundation.org>
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6.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Nathan Gao <zcgao@amazon.com>

[ Upstream commit 0e125ecfe20c077625cf0be8d750d5c3abc0dce9 ]

Commit f5da7c45188e ("tcp: adjust rcvq_space after updating scaling
ratio") replaced the direct window_clamp update in tcp_measure_rcv_mss()
with a call to tcp_set_window_clamp(), a helper that implements the
TCP_WINDOW_CLAMP setsockopt. As a side effect, the helper also shrinks
rcv_ssthresh via __tcp_adjust_rcv_ssthresh().

As a result, each scaling_ratio decrease detected by
tcp_measure_rcv_mss() also cuts rcv_ssthresh. Elsewhere in TCP,
rcv_ssthresh is usually cut under memory pressure and grows via
tcp_grow_window().

Flows whose segment sizes vary keep scaling_ratio oscillating, which
leads to an unstable rcv_ssthresh: a dip of rcv_ssthresh only recovers
via tcp_grow_window(), keeping the advertised window at a relatively
low level even after the ratio itself has recovered, and can even stall
the sender.

Observed on a customer's proxy gateway after upgrading from kernel 6.1
to 6.12: in the worst case, rcv_ssthresh was cut in half by a
scaling_ratio dip. P99 latency jumped from <10ms on 6.1 to ~100ms on
6.12, and almost returned to the 6.1 level with this patch applied.

Restore the plain WRITE_ONCE() update of window_clamp, as introduced
in commit a2cbb1603943 ("tcp: Update window clamping condition"), and
keep the rcvq_space.space adjustment. Now rcv_ssthresh is decoupled from
scaling_ratio changes in tcp_measure_rcv_mss().

Fixes: f5da7c45188e ("tcp: adjust rcvq_space after updating scaling ratio")
Signed-off-by: Nathan Gao <zcgao@amazon.com>
Link: https://patch.msgid.link/20260725030806.28135-1-zcgao@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/ipv4/tcp_input.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 8c74fbeb771d5..c1769031a9e93 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -252,7 +252,7 @@ static void tcp_measure_rcv_mss(struct sock *sk, const struct sk_buff *skb)
 				struct tcp_sock *tp = tcp_sk(sk);
 
 				val = tcp_win_from_space(sk, sk->sk_rcvbuf);
-				tcp_set_window_clamp(sk, val);
+				WRITE_ONCE(tp->window_clamp, val);
 
 				if (tp->window_clamp < tp->rcvq_space.space)
 					tp->rcvq_space.space = tp->window_clamp;
-- 
2.53.0