Re: [PATCH 7.0.y] mptcp: update window_clamp on subflows when SO_RCVBUF is set

[Greg KH <greg@kroah.com>]

On Mon, Jun 01, 2026 at 10:50:33AM +1000, Matthieu Baerts wrote:
> Hi Sasha,
> 
> On 01/06/2026 10:39, Sasha Levin wrote:
> > From: Gang Yan <yangang@kylinos.cn>
> > 
> > [ Upstream commit 3a543ae0e2092d5c2085d5f21f7a7dbafdffea3c ]
> > 
> > Add __mptcp_subflow_set_rcvbuf() helper to write the subflow sk_rcvbuf,
> > but also to call the recently added tcp_set_rcvbuf() helper to update
> > window_clamp. This is needed because the window clap is updated when
> > scaling_ratio changes, in tcp_measure_rcv_mss(). Until scaling_ratio
> > changes, the subflow is stuck with the old window clamp which may be
> > based on a small initial buffer.
> > 
> > Use this new helper in both mptcp_sol_socket_sync_intval() (setsockopt
> > path) and sync_socket_options() (new subflow creation path).
> > 
> > Note that this patch depends on commit b025461303d8 ("tcp: update
> > window_clamp when SO_RCVBUF is set"): it fixes the issue on TCP side,
> > but the same fix is needed on MPTCP side as well.
> Thank you for the backport. I guess there is a conflict, because commit
> b025461303d8 ("tcp: update window_clamp when SO_RCVBUF is set") has not
> been backported to 7.0 (nor 6.18, 6.12 and 6.6). See the note above for
> more details.
> 
> In other words, MPTCP is imitating TCP's behaviour. So no need to
> backport this patch if the fix on TCP side is not backported either.
> 
> I think it would be better to take both the TCP fix (b025461303d8), and
> this one (3a543ae0e209). But maybe the TCP fix has not been backported
> on purpose?

It was not tagged for stable inclusion, so it was not applied to any
stable trees.  As the tcp change isn't included, I'll just drop this one
too, thanks.

greg k-h