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

[Jakub Kicinski <kuba@kernel.org>]

On Tue,  7 Apr 2026 17:14:38 -0700 Jakub Kicinski 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.

Hi Eric, any thoughts?
I always assume you are displeased if you don't reply within 8 hours :)

I should say that everyone has obviously discouraged the team that run
into this from using SO_RCVBUF. I'm fascinated by how they decided that
it helps since it clearly doesn't work. AI sure makes it easy for
people to "try things". Sigh.