Re: [PATCH net] tcp: update window_clamp when SO_RCVBUF is set

Eric Dumazet
2026-04-08T19:27:32Z

On Tue, Apr 7, 2026 at 5:14 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> Commit under Fixes moved recomputing the window clamp to
> tcp_measure_rcv_mss() (when scaling_ratio changes).
> I suspect it missed the fact that we don't recompute the clamp
> when rcvbuf is set. Until scaling_ratio changes we are
> stuck with the old window clamp which may be based on
> the small initial buffer. scaling_ratio may never change.
>
> Inspired by Eric's recent commit d1361840f8c5 ("tcp: fix
> SO_RCVLOWAT and RCVBUF autotuning") plumb the user action
> thru to TCP and have it update the clamp.
>
> A smaller fix would be to just have tcp_rcvbuf_grow()
> adjust the clamp even if SOCK_RCVBUF_LOCK is set.
> But IIUC this is what we were trying to get away from
> in the first place.
>
> Fixes: a2cbb1603943 ("tcp: Update window clamping condition")
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Reviewed-by: Eric Dumazet <edumaze@google.com>