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

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

[gregkh]

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>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 2ca06a2f65310aeef30bb69b7405437a14766e4d
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2025012037-siesta-sulfite-8b05@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ 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.

Fixes: e3859603ba13 ("mptcp: better msk receive window updates")
Cc: stable@vger.kernel.org
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/20250107131845.5e5de3c5@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250113-net-mptcp-connect-st-flakes-v1-1-0d986ee7b1b6@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index a62bc874bf1e..123f3f297284 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -607,7 +607,6 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 	}
 	opts->ext_copy.use_ack = 1;
 	opts->suboptions = OPTION_MPTCP_DSS;
-	WRITE_ONCE(msk->old_wspace, __mptcp_space((struct sock *)msk));
 
 	/* Add kind/length/subtype/flag overhead if mapping is not populated */
 	if (dss_size == 0)
@@ -1288,7 +1287,7 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th)
 			}
 			MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_RCVWNDCONFLICT);
 		}
-		return;
+		goto update_wspace;
 	}
 
 	if (rcv_wnd_new != rcv_wnd_old) {
@@ -1313,6 +1312,9 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th)
 		th->window = htons(new_win);
 		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_RCVWNDSHARED);
 	}
+
+update_wspace:
+	WRITE_ONCE(msk->old_wspace, tp->rcv_wnd);
 }
 
 __sum16 __mptcp_make_csum(u64 data_seq, u32 subflow_seq, u16 data_len, __wsum sum)

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]

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.