Skip to contentSkip to conversation

FAILED: patch "[PATCH] mptcp: fix ack generation for fallback msk" failed to apply to 5.15-stable tree

PATCH New [email protected] 2 participants 2 messages Nov 24, 2025

Subsystems (3)
Networking Stable branch Networking MPTCP
lore
Share & tools
lore

[email protected]

author maintainer
#0 lore raw

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 <[email protected]>.

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 5e15395f6d9ec07395866c5511f4b4ac566c0c9b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025112448-naming-survey-424d@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 5e15395f6d9ec07395866c5511f4b4ac566c0c9b Mon Sep 17 00:00:00 2001
From: Paolo Abeni <[email protected]>
Date: Tue, 18 Nov 2025 08:20:19 +0100
Subject: [PATCH] mptcp: fix ack generation for fallback msk

mptcp_cleanup_rbuf() needs to know the last most recent, mptcp-level rcv_wnd sent, and such information is tracked into the msk->old_wspace field, updated at ack transmission time by mptcp_write_options().

Fallback socket do not add any mptcp options, such helper is never invoked, and msk->old_wspace value remain stale. That in turn makes ack generation at recvmsg() time quite random.

Address the issue ensuring mptcp_write_options() is invoked even for fallback sockets, and just update the needed info in such a case.

The issue went unnoticed for a long time, as mptcp currently overshots the fallback socket receive buffer autotune significantly. It is going to change in the near future.

Fixes e3859603ba13 ("mptcp: better msk receive window updates")
Cc [email protected]
Closes https://github.com/multipath-tcp/mptcp_net-next/issues/594
Signed-off-by Paolo Abeni <[email protected]>
Reviewed-by Geliang Tang <[email protected]>
Reviewed-by Matthieu Baerts (NGI0) <[email protected]>
Signed-off-by Matthieu Baerts (NGI0) <[email protected]>
Link https://patch.msgid.link/20251118-net-mptcp-misc-fixes-6-18-rc6-v1-1-806d3781c95f@kernel.org open here
Signed-off-by Jakub Kicinski <[email protected]>
net/mptcp/options.c +22 -1
index 1103b3341a70..8a63bd00807d 100644
@@ -838,8 +838,11 @@ bool mptcp_established_options(struct sock *sk, struct sk_buff *skb,
 
 	opts->suboptions = 0;
 
+	/* Force later mptcp_write_options(), but do not use any actual
+	 * option space.
+	 */
 	if (unlikely(__mptcp_check_fallback(msk) && !mptcp_check_infinite_map(skb)))
-		return false;
+		return true;
 
 	if (unlikely(skb && TCP_SKB_CB(skb)->tcp_flags & TCPHDR_RST)) {
 		if (mptcp_established_options_fastclose(sk, &opt_size, remaining, opts) ||

@@ -1319,6 +1322,20 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th)
 	WRITE_ONCE(msk->old_wspace, tp->rcv_wnd);
 }
 
+static void mptcp_track_rwin(struct tcp_sock *tp)
+{
+	const struct sock *ssk = (const struct sock *)tp;
+	struct mptcp_subflow_context *subflow;
+	struct mptcp_sock *msk;
+
+	if (!ssk)
+		return;
+
+	subflow = mptcp_subflow_ctx(ssk);
+	msk = mptcp_sk(subflow->conn);
+	WRITE_ONCE(msk->old_wspace, tp->rcv_wnd);
+}
+
 __sum16 __mptcp_make_csum(u64 data_seq, u32 subflow_seq, u16 data_len, __wsum sum)
 {
 	struct csum_pseudo_header header;

@@ -1611,6 +1628,10 @@ void mptcp_write_options(struct tcphdr *th, __be32 *ptr, struct tcp_sock *tp,
 				      opts->reset_transient,
 				      opts->reset_reason);
 		return;
+	} else if (unlikely(!opts->suboptions)) {
+		/* Fallback to TCP */
+		mptcp_track_rwin(tp);
+		return;
 	}
 
 	if (OPTION_MPTCP_PRIO & opts->suboptions) {
maintainer
#1 lore raw

PATCH 1/1 mptcp: fix ack generation for fallback msk

From: Paolo Abeni <[email protected]>

[ Upstream commit 5e15395f6d9ec07395866c5511f4b4ac566c0c9b ]

mptcp_cleanup_rbuf() needs to know the last most recent, mptcp-level rcv_wnd sent, and such information is tracked into the msk->old_wspace field, updated at ack transmission time by mptcp_write_options().

Fallback socket do not add any mptcp options, such helper is never invoked, and msk->old_wspace value remain stale. That in turn makes ack generation at recvmsg() time quite random.

Address the issue ensuring mptcp_write_options() is invoked even for fallback sockets, and just update the needed info in such a case.

The issue went unnoticed for a long time, as mptcp currently overshots the fallback socket receive buffer autotune significantly. It is going to change in the near future.

Fixes e3859603ba13 ("mptcp: better msk receive window updates")
Cc [email protected]
Closes https://github.com/multipath-tcp/mptcp_net-next/issues/594
Signed-off-by Paolo Abeni <[email protected]>
Reviewed-by Geliang Tang <[email protected]>
Reviewed-by Matthieu Baerts (NGI0) <[email protected]>
Signed-off-by Matthieu Baerts (NGI0) <[email protected]>
Link https://patch.msgid.link/20251118-net-mptcp-misc-fixes-6-18-rc6-v1-1-806d3781c95f@kernel.org open here
Signed-off-by Jakub Kicinski <[email protected]>
[ Adjust context ]
Signed-off-by Sasha Levin <[email protected]>
---
 net/mptcp/options.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)
net/mptcp/options.c +24 -1
index a71ef6de8c682..b4237e50e23fd 100644
@@ -810,8 +810,11 @@ bool mptcp_established_options(struct sock *sk, struct sk_buff *skb,
 
 	opts->suboptions = 0;
 
+	/* Force later mptcp_write_options(), but do not use any actual
+	 * option space.
+	 */
 	if (unlikely(__mptcp_check_fallback(msk)))
-		return false;
+		return true;
 
 	if (unlikely(skb && TCP_SKB_CB(skb)->tcp_flags & TCPHDR_RST)) {
 		if (mptcp_established_options_mp_fail(sk, &opt_size, remaining, opts)) {

@@ -1222,6 +1225,20 @@ static void mptcp_set_rwin(const struct tcp_sock *tp)
 		WRITE_ONCE(msk->rcv_wnd_sent, ack_seq);
 }
 
+static void mptcp_track_rwin(const struct tcp_sock *tp)
+{
+	const struct sock *ssk = (const struct sock *)tp;
+	struct mptcp_subflow_context *subflow;
+	struct mptcp_sock *msk;
+
+	if (!ssk)
+		return;
+
+	subflow = mptcp_subflow_ctx(ssk);
+	msk = mptcp_sk(subflow->conn);
+	WRITE_ONCE(msk->old_wspace, tp->rcv_wnd);
+}
+
 __sum16 __mptcp_make_csum(u64 data_seq, u32 subflow_seq, u16 data_len, __wsum sum)
 {
 	struct csum_pseudo_header header;

@@ -1283,6 +1300,12 @@ void mptcp_write_options(__be32 *ptr, const struct tcp_sock *tp,
 		return;
 	}
 
+	/* Fallback to TCP */
+	if (unlikely(!opts->suboptions)) {
+		mptcp_track_rwin(tp);
+		return;
+	}
+
 	/* DSS, MPC, MPJ and ADD_ADDR are mutually exclusive, see
 	 * mptcp_established_options*()
 	 */
Signature · 1 line
2.51.0