Re: [PATCH net] tcp: do not shrink window clamp when SO_RCVBUF is locked

Eric Dumazet
2026-04-27T15:38:58Z

On Mon, Apr 27, 2026 at 8:32 AM Ankit Jain <ankit-aj.jain@broadcom.com> wrote:
>
> 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 <karen.badiryan@broadcom.com>
> Signed-off-by: Ankit Jain <ankit-aj.jain@broadcom.com>

Make sure to add a selftests (in ./tools/testing/selftests/net/packetdrill/ )

Thanks.