Re: FAILED: patch "[PATCH] mptcp: be sure to send ack when mptcp-level window re-opens" failed to apply to 5.15-stable tree

[Matthieu Baerts <matttbe@kernel.org>]

Hi Greg,

On 20/01/2025 14:38, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 5.15-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.

Thank you for the notification!

> ------------------ original commit in Linus's tree ------------------
> 
> From 2ca06a2f65310aeef30bb69b7405437a14766e4d Mon Sep 17 00:00:00 2001
> From: Paolo Abeni <pabeni@redhat.com>
> Date: Mon, 13 Jan 2025 16:44:56 +0100
> Subject: [PATCH] mptcp: be sure to send ack when mptcp-level window re-opens
> 
> mptcp_cleanup_rbuf() is responsible to send acks when the user-space
> reads enough data to update the receive windows significantly.
> 
> It tries hard to avoid acquiring the subflow sockets locks by checking
> conditions similar to the ones implemented at the TCP level.
> 
> To avoid too much code duplication - the MPTCP protocol can't reuse the
> TCP helpers as part of the relevant status is maintained into the msk
> socket - and multiple costly window size computation, mptcp_cleanup_rbuf
> uses a rough estimate for the most recently advertised window size:
> the MPTCP receive free space, as recorded as at last-ack time.
> 
> Unfortunately the above does not allow mptcp_cleanup_rbuf() to detect
> a zero to non-zero win change in some corner cases, skipping the
> tcp_cleanup_rbuf call and leaving the peer stuck.
> 
> After commit ea66758c1795 ("tcp: allow MPTCP to update the announced
> window"), MPTCP has actually cheap access to the announced window value.
> Use it in mptcp_cleanup_rbuf() for a more accurate ack generation.

FYI, we are currently not planning to backport this patch: it depends on
commits ea66758c1795 ("tcp: allow MPTCP to update the announced window")
-- mentioned above -- and f3589be0c420 ("mptcp: never shrink offered
window") which are not in this v5.15 version, and linked to new features.

The issue fixed by the patch here has not been seen on a v5.15 kernel so
far, and probably only visible with other changes that have been done
later on. In other words, it looks OK not to backport this patch.

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.