6.12-stable review patch. If anyone has any objections, please let me know.
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From: Jakub Kicinski kuba@kernel.org
[ Upstream commit f5da7c45188eea71394bf445655cae2df88a7788 ]
Since commit under Fixes we set the window clamp in accordance to newly measured rcvbuf scaling_ratio. If the scaling_ratio decreased significantly we may put ourselves in a situation where windows become smaller than rcvq_space, preventing tcp_rcv_space_adjust() from increasing rcvbuf.
The significant decrease of scaling_ratio is far more likely since commit 697a6c8cec03 ("tcp: increase the default TCP scaling ratio"), which increased the "default" scaling ratio from ~30% to 50%.
Hitting the bad condition depends a lot on TCP tuning, and drivers at play. One of Meta's workloads hits it reliably under following conditions: - default rcvbuf of 125k - sender MTU 1500, receiver MTU 5000 - driver settles on scaling_ratio of 78 for the config above. Initial rcvq_space gets calculated as TCP_INIT_CWND * tp->advmss (10 * 5k = 50k). Once we find out the true scaling ratio and MSS we clamp the windows to 38k. Triggering the condition also depends on the message sequence of this workload. I can't repro the problem with simple iperf or TCP_RR-style tests.
Fixes: a2cbb1603943 ("tcp: Update window clamping condition") Reviewed-by: Eric Dumazet edumazet@google.com Reviewed-by: Neal Cardwell ncardwell@google.com Link: https://patch.msgid.link/20250217232905.3162187-1-kuba@kernel.org Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_input.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 2d43b29da15e2..bb17add6e4a78 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -243,9 +243,15 @@ static void tcp_measure_rcv_mss(struct sock *sk, const struct sk_buff *skb) do_div(val, skb->truesize); tcp_sk(sk)->scaling_ratio = val ? val : 1;
- if (old_ratio != tcp_sk(sk)->scaling_ratio) - WRITE_ONCE(tcp_sk(sk)->window_clamp, - tcp_win_from_space(sk, sk->sk_rcvbuf)); + if (old_ratio != tcp_sk(sk)->scaling_ratio) { + struct tcp_sock *tp = tcp_sk(sk); + + val = tcp_win_from_space(sk, sk->sk_rcvbuf); + tcp_set_window_clamp(sk, val); + + if (tp->window_clamp < tp->rcvq_space.space) + tp->rcvq_space.space = tp->window_clamp; + } } icsk->icsk_ack.rcv_mss = min_t(unsigned int, len, tcp_sk(sk)->advmss);