[PATCH mptcp-net v3 0/2] mptcp: sync the msk->sndbuf at accept() time

[PATCH mptcp-net v3 0/2] mptcp: sync the msk->sndbuf at accept() time

[Gang Yan]

From: Gang Yan <yangang@kylinos.cn>

Changelog:
v3:
  - Add a check in diag.sh to check the sndbuf of the server side.

Gang Yan (2):
  mptcp: sync the msk->sndbuf at accept() time
  selftests: mptcp: add a check for sndbuf of S/C

 net/mptcp/protocol.c                      |  1 +
 tools/testing/selftests/net/mptcp/diag.sh | 26 +++++++++++++++++++++++
 2 files changed, 27 insertions(+)

-- 
2.43.0

[PATCH mptcp-net v3 1/2] mptcp: sync the msk->sndbuf at accept() time

[Gang Yan]

From: Gang Yan <yangang@kylinos.cn>

After an MPTCP connection is established, the sk_sndbuf of client's msk
can be updated through 'subflow_finish_connect'. However, the newly
accepted msk on the server side has a small sk_sndbuf than
msk->first->sk_sndbuf:

'''
MPTCP: msk:00000000e55b09db, msk->sndbuf:20480, msk->first->sndbuf:2626560
'''

This patch synchronizes the sk_sndbuf by triggering its update during
accept.

Fixes: 8005184fd1ca ("mptcp: refactor sndbuf auto-tuning")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/602
Signed-off-by: Gang Yan <yangang@kylinos.cn>
---
 net/mptcp/protocol.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 17b9a8c13ebf..6f6c8cabf74f 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -4252,6 +4252,7 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
 
 		mptcp_graft_subflows(newsk);
 		mptcp_rps_record_subflows(msk);
+		__mptcp_propagate_sndbuf(newsk, mptcp_subflow_tcp_sock(subflow));
 
 		/* Do late cleanup for the first subflow as necessary. Also
 		 * deal with bad peers not doing a complete shutdown.
-- 
2.43.0

Re: [PATCH mptcp-net v3 1/2] mptcp: sync the msk->sndbuf at accept() time

[Paolo Abeni]

On 4/7/26 10:45 AM, Gang Yan wrote:
> From: Gang Yan <yangang@kylinos.cn>
> 
> After an MPTCP connection is established, the sk_sndbuf of client's msk
> can be updated through 'subflow_finish_connect'. However, the newly
> accepted msk on the server side has a small sk_sndbuf than
> msk->first->sk_sndbuf:
> 
> '''
> MPTCP: msk:00000000e55b09db, msk->sndbuf:20480, msk->first->sndbuf:2626560
> '''
> 
> This patch synchronizes the sk_sndbuf by triggering its update during
> accept.

I finally could wrap my head around the root cause: tcp_child_process()
-> tcp_init_transfer() -> tcp_sndbuf_expand() can and will grow the ssk
sndbuf _after_ tcp_check_req() -> subflow_syn_recv_sock() ->
mptcp_sk_clone_init() where __mptcp_propagate_sndbuf() is currently hooked.

I think the latter __mptcp_propagate_sndbuf() invocation can be removed,
and the commit message expanded to capture some of the above.

With the above included feel free to add:

Acked-by: Paolo Abeni <pabeni@redhat.com>

Re: [PATCH mptcp-net v3 1/2] mptcp: sync the msk->sndbuf at accept() time

[Matthieu Baerts]

Hi Paolo,

On 10/04/2026 11:04, Paolo Abeni wrote:
> On 4/7/26 10:45 AM, Gang Yan wrote:
>> From: Gang Yan <yangang@kylinos.cn>
>>
>> After an MPTCP connection is established, the sk_sndbuf of client's msk
>> can be updated through 'subflow_finish_connect'. However, the newly
>> accepted msk on the server side has a small sk_sndbuf than
>> msk->first->sk_sndbuf:
>>
>> '''
>> MPTCP: msk:00000000e55b09db, msk->sndbuf:20480, msk->first->sndbuf:2626560
>> '''
>>
>> This patch synchronizes the sk_sndbuf by triggering its update during
>> accept.
> 
> I finally could wrap my head around the root cause: tcp_child_process()
> -> tcp_init_transfer() -> tcp_sndbuf_expand() can and will grow the ssk
> sndbuf _after_ tcp_check_req() -> subflow_syn_recv_sock() ->
> mptcp_sk_clone_init() where __mptcp_propagate_sndbuf() is currently hooked.
> 
> I think the latter __mptcp_propagate_sndbuf() invocation can be removed,
> and the commit message expanded to capture some of the above.
> 
> With the above included feel free to add:
> 
> Acked-by: Paolo Abeni <pabeni@redhat.com>
Thank you for this review!

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

Re: [PATCH mptcp-net v3 1/2] mptcp: sync the msk->sndbuf at accept() time

[gang.yan]

April 10, 2026 at 5:04 PM, "Paolo Abeni" <pabeni@redhat.com mailto:pabeni@redhat.com?to=%22Paolo%20Abeni%22%20%3Cpabeni%40redhat.com%3E > wrote:


> 
> On 4/7/26 10:45 AM, Gang Yan wrote:
> 
> > 
> > From: Gang Yan <yangang@kylinos.cn>
> >  
> >  After an MPTCP connection is established, the sk_sndbuf of client's msk
> >  can be updated through 'subflow_finish_connect'. However, the newly
> >  accepted msk on the server side has a small sk_sndbuf than
> >  msk->first->sk_sndbuf:
> >  
> >  '''
> >  MPTCP: msk:00000000e55b09db, msk->sndbuf:20480, msk->first->sndbuf:2626560
> >  '''
> >  
> >  This patch synchronizes the sk_sndbuf by triggering its update during
> >  accept.
> > 
> I finally could wrap my head around the root cause: tcp_child_process()
> -> tcp_init_transfer() -> tcp_sndbuf_expand() can and will grow the ssk
> sndbuf _after_ tcp_check_req() -> subflow_syn_recv_sock() ->
> mptcp_sk_clone_init() where __mptcp_propagate_sndbuf() is currently hooked.
> 
> I think the latter __mptcp_propagate_sndbuf() invocation can be removed,
> and the commit message expanded to capture some of the above.
> 
> With the above included feel free to add:
> 
> Acked-by: Paolo Abeni <pabeni@redhat.com>
Hi Paolo,

Thanks for your suggestions. The v4 code will be sent soon.

Cherrs,
Gang
>

[PATCH mptcp-net v3 2/2] selftests: mptcp: add a check for sndbuf of S/C

[Gang Yan]

From: Gang Yan <yangang@kylinos.cn>

Add a new chk_sndbuf() helper to diag.sh that extracts the sndbuf (the
'tb' field from 'ss -m' skmem output) for both server and client MPTCP
sockets, and verifies they are equal.

Without the previous patch, it will fails:

'''
05 all listen sockets                                [ OK ]
06 after MPC handshake                               [ OK ]
07 ....chk sndbuf server/client                      [FAIL] server sndbuf=20480 != client sndbuf=2630656
'''

Signed-off-by: Gang Yan <yangang@kylinos.cn>
---
 tools/testing/selftests/net/mptcp/diag.sh | 26 +++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/tools/testing/selftests/net/mptcp/diag.sh b/tools/testing/selftests/net/mptcp/diag.sh
index d847ff1737c3..aa01a1dd0bdd 100755
--- a/tools/testing/selftests/net/mptcp/diag.sh
+++ b/tools/testing/selftests/net/mptcp/diag.sh
@@ -322,6 +322,31 @@ wait_connected()
 	done
 }
 
+chk_sndbuf()
+{
+	local server_sndbuf client_sndbuf msg
+	local port=${1}
+
+	msg="....chk sndbuf server/client"
+	server_sndbuf=$(ss -N "${ns}" -inmHM "sport" "${port}" | grep -oP 'tb\K\d+')
+	client_sndbuf=$(ss -N "${ns}" -inmHM "dport" "${port}" | grep -oP 'tb\K\d+')
+
+	mptcp_lib_print_title "${msg}"
+	if [ -z "${server_sndbuf}" ] || [ -z "${client_sndbuf}" ]; then
+		mptcp_lib_pr_fail "server sndbuf=${server_sndbuf} client sndbuf=${client_sndbuf}"
+		mptcp_lib_result_fail "${msg}"
+		ret=${KSFT_FAIL}
+	elif [ "${server_sndbuf}" != "${client_sndbuf}" ]; then
+		mptcp_lib_pr_fail "server sndbuf=${server_sndbuf} != client sndbuf=${client_sndbuf}"
+		mptcp_lib_result_fail "${msg}"
+		ret=${KSFT_FAIL}
+	else
+		mptcp_lib_pr_ok
+		mptcp_lib_result_pass "${msg}"
+	fi
+}
+
+
 trap cleanup EXIT
 mptcp_lib_ns_init ns
 
@@ -341,6 +366,7 @@ echo "b" | \
 				127.0.0.1 >/dev/null &
 wait_connected $ns 10000
 chk_msk_nr 2 "after MPC handshake"
+chk_sndbuf 10000
 chk_last_time_info 10000
 chk_msk_remote_key_nr 2 "....chk remote_key"
 chk_msk_fallback_nr 0 "....chk no fallback"
-- 
2.43.0

Re: [PATCH mptcp-net v3 0/2] mptcp: sync the msk->sndbuf at accept() time

[MPTCP CI]

Hi Gang,

Thank you for your modifications, that's great!

Our CI did some validations and here is its report:

- KVM Validation: normal (except selftest_mptcp_join): Success! ✅
- KVM Validation: normal (only selftest_mptcp_join): Success! ✅
- KVM Validation: debug (except selftest_mptcp_join): Unstable: 1 failed test(s): packetdrill_sockopts ⚠️ 
- KVM Validation: debug (only selftest_mptcp_join): Success! ✅
- KVM Validation: btf-normal (only bpftest_all): Success! ✅
- KVM Validation: btf-debug (only bpftest_all): Success! ✅
- Task: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/24181789538

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/7359a54dfdbd
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1078012


If there are some issues, you can reproduce them using the same environment as
the one used by the CI thanks to a docker image, e.g.:

    $ cd [kernel source code]
    $ docker run -v "${PWD}:${PWD}:rw" -w "${PWD}" --privileged --rm -it \
        --pull always mptcp/mptcp-upstream-virtme-docker:latest \
        auto-normal

For more details:

    https://github.com/multipath-tcp/mptcp-upstream-virtme-docker


Please note that despite all the efforts that have been already done to have a
stable tests suite when executed on a public CI like here, it is possible some
reported issues are not due to your modifications. Still, do not hesitate to
help us improve that ;-)

Cheers,
MPTCP GH Action bot
Bot operated by Matthieu Baerts (NGI0 Core)