When an application explicitly sets SO_RCVBUF, the window clamp should not be dynamically recalculated based on the memory scaling_ratio. Currently, tcp_measure_rcv_mss() aggressively crushes the window clamp down when it sees a poor skb->len to skb->truesize ratio. If the application explicitly locked the buffer via SO_RCVBUF, this recalculation causes the advertised window to drop severely. If the window drops below the interface MSS, it triggers Silly Window Syndrome (SWS) avoidance on the sender. The sender defers transmission and drops the connection into a perpetual 200ms PROBE0 timer loop, drastically reducing throughput. This is highly reproducible on loopback interfaces (MTU 65536) using Java-based workloads (like Tomcat/GemFire) where the JVM sets SO_RCVBUF to 32K or 64K. The bloated loopback truesize forces the scaling ratio to drop, crushing the window clamp to ~26K, instantly triggering SWS stalls and causing gigabyte transfers to take minutes instead of milliseconds. Since the application locked the buffer, the kernel should respect the clamp boundary and not dynamically crush it based on runtime ratios. Fixes: a2cbb1603943 ("tcp: Update window clamping condition") Cc: stable@vger.kernel.org Reported-by: Karen Badiryan Signed-off-by: Ankit Jain --- Note to reviewers: Testing Context: - The SWS deadlock was successfully reproduced on the latest netdev/net tree (v7.1-rc1) using the actual enterprise Java workload. - Applying this patch completely resolves the 504 Timeouts and restores loopback throughput. - Baseline iperf3 auto-tuning remains unaffected by this patch. For context, here is the exact sequence of events that triggers the recalculation flaw, illustrated in a packetdrill-style flow. Unpatched kernels aggressively crush the window at step 3, triggering SWS. // 1. Tomcat creates socket and hardcodes the buffer to 32K 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [32768]) = 0 +0 bind(3, ..., ...) = 0 +0 listen(3, 1) = 0 // 2. GemFire connects over loopback (simulating Jumbo MSS of 65496) +0 < S 0:0(0) win 65535 +0 > S. 0:0(0) ack 1 <...> +0 < . 1:1(0) ack 1 win 65535 +0 accept(3, ..., ...) = 4 // 3. GemFire sends a 20KB packet, dropping the scaling_ratio. // Without the patch, tcp_measure_rcv_mss() crushes the window_clamp here. +0.1 < . 1:20001(20000) ack 1 win 65535 +0.1 read(4, ..., 20000) = 20000 // 4. Assert window did not crush // WITH the patch, the kernel respects the SOCK_RCVBUF_LOCK. +0 > . 1:1(0) ack 20001 win 65535 --- net/ipv4/tcp_input.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index d5c9e65d9..c1cb9d3ed 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -248,7 +248,8 @@ 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) { + if (old_ratio != tcp_sk(sk)->scaling_ratio && + !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) { struct tcp_sock *tp = tcp_sk(sk); val = tcp_win_from_space(sk, sk->sk_rcvbuf); -- 2.53.0