* [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error
@ 2026-10-28 5:41 Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 1/6] selftests: mptcp: connect: retry write() on EAGAIN Anil Kaushik
` (6 more replies)
0 siblings, 7 replies; 9+ messages in thread
From: Anil Kaushik @ 2026-10-28 5:41 UTC (permalink / raw)
To: mptcp; +Cc: Anil Kaushik
A subflow closed after an error is never re-created, even once the path
is back (#440).
When TCP gives up on an established subflow we created, after a
retransmission or keepalive timeout, the in-kernel PM now creates a new
one to the same addresses, while another subflow can still send data.
Its SYN retries space out the attempts. Their number is set by a new
limit, subflow-retries, like TCP_SYNCNT: the maximum, 127, by default,
about 4 hours, and 0 disables it.
Patch 1 fixes a selftest tool issue seen with the new test. Patches 2
and 3 add the new limit, patch 4 the retry, patches 5 and 6 the
selftests.
I used an LLM to help write and review the code, the tests and the
commit messages of this series. I checked and tested every change
myself.
Known issue: in about 1 run in 20, more often on debug kernels, the
first new test fails. Once the path is back, the re-created subflow
joins, but the server resets it right away with an MP_RST, after
DSSNotMatching, and the PAWS counters are high on both sides. It looks
like an existing issue on the receive path, triggered by the two-way
transfer with losses rather than by the retry itself, but I have not
found the cause yet: it does not show up with tcpdump or tracepoints
running. Any hint is welcome.
Changes in v12, after the netdev Sashiko review of v11: when no other
subflow can send data, only the re-created subflows still connecting are
closed, a joined one is fully established soon; a subflow in CLOSE_WAIT
no longer keeps them open either; the doc says the initial subflow is
not re-created if the peer does not accept joins to its initial address;
small test fixes; and the patches are rebased on the current export
branch.
Changes in v11, after the Sashiko review of v10: a subflow is only
re-created while another subflow can send data, so the close timeout
works as before, and all the code re-arming or stopping it is gone; the
re-created subflows not established yet are closed from the worker, or
when the MPTCP socket is shut down or closed, never while iterating on
the subflows; the path-back test is gone, and the third test checks the
only subflow is not re-created.
Changes in v10, after the Sashiko review of v9: when the close timeout
expires, the connection is kept if a re-created subflow connected in the
meantime, since an active join does not stop the timer; the re-created
subflows still connecting are closed once the msk is no longer
established and no other subflow is left; a new test lets the path come
back before the close timeout; and the patches are rebased on the
current export branch, without the Assisted-by tag, removed upstream.
Changes in v9, after the Sashiko reviews of v8 and more checks: every
timeout is retried, whatever ICMP error TCP reports for it, and only
resets, local aborts and MPTCP protocol errors are excluded; if no other
subflow can send data, the close timeout still applies, the new subflow
no longer keeps the connection open for all its SYN retries; the
re-created subflows closed with the msk have their data in the msk
backlog cleaned, since they can get established in between; the doc says
this is a single connection attempt, ended by an ICMP error, and that 0
disables it; the close test blocks both paths, and its server reads all
the data first; the tests read the counters before blocking a path; and
a new test checks the close timeout with only one subflow.
Changes in v8, after the Sashiko reviews of v7 and Matthieu's comments:
the default number of SYN retries is the maximum, 127; the re-created
subflows still connecting are closed before the write shutdown
disconnects them; the doc is more precise; and a separate test closes
the connection while a subflow is being re-created.
Changes in v7, after the netdev Sashiko reviews of v5 and v6, and
Abdul's tests: only a path error, a retransmission or keepalive
timeout, makes the PM re-create a subflow, not a reset, so a peer
cannot make it loop; the flags are bools under the msk socket lock;
mptcp.rst documents tcp_retries2 and the keepalive needed by a subflow
that only receives data; and small fixes in the tests.
Changes in v6, after the Sashiko reviews (netdev Sashiko on v4, which
also applied to v5, and sashiko.dev on v5): only subflows that were
established are re-created, so the PM still moves on to the next
endpoints as before; removed remote addresses and the local ID 0 are
handled too; the policy checks the maximum value, so the parse_limit()
patch is gone; and the test uses the net/lib.sh helpers. Details in
each patch.
v11: https://lore.kernel.org/mptcp/20261026125345.3019372-1-anilkaushikwireless@gmail.com/
v10: https://lore.kernel.org/mptcp/20261026063657.2955783-1-anilkaushikwireless@gmail.com/
v9: https://lore.kernel.org/mptcp/20261025134909.2794368-1-anilkaushikwireless@gmail.com/
v8: https://lore.kernel.org/mptcp/20261022151130.2285198-1-anilkaushikwireless@gmail.com/
v7: https://lore.kernel.org/mptcp/20261020031711.1896660-1-anilkaushikwireless@gmail.com/
v6: https://lore.kernel.org/mptcp/20261018041748.1655711-1-anilkaushikwireless@gmail.com/
v5: https://lore.kernel.org/mptcp/20261016160926.1155262-1-anilkaushikwireless@gmail.com/
v4: https://lore.kernel.org/mptcp/20261016055010.980819-1-anilkaushikwireless@gmail.com/
v3: https://lore.kernel.org/mptcp/20261014143349.768138-1-anilkaushikwireless@gmail.com/
v2: https://lore.kernel.org/mptcp/20261013025653.523755-1-anilkaushikwireless@gmail.com/
v1: https://lore.kernel.org/mptcp/20261012171740.464517-1-anilkaushikwireless@gmail.com/
Anil Kaushik (6):
selftests: mptcp: connect: retry write() on EAGAIN
netlink: specs: mptcp: add the subflow-retries attribute
mptcp: pm: in-kernel: add the subflow-retries limit
mptcp: pm: in-kernel: retry subflows closed after an error
selftests: mptcp: pm_nl_ctl: support the subflow-retries limit
selftests: mptcp: join: validate subflow retry after link failure
Documentation/netlink/specs/mptcp_pm.yaml | 13 +
Documentation/networking/mptcp.rst | 19 +
include/uapi/linux/mptcp.h | 2 +-
include/uapi/linux/mptcp_pm.h | 1 +
net/mptcp/mptcp_pm_gen.c | 10 +-
net/mptcp/mptcp_pm_gen.h | 4 +-
net/mptcp/pm.c | 39 ++
net/mptcp/pm_kernel.c | 160 +++++++-
net/mptcp/protocol.c | 78 +++-
net/mptcp/protocol.h | 51 +++
net/mptcp/sockopt.c | 2 +
.../selftests/net/mptcp/mptcp_connect.c | 18 +
.../testing/selftests/net/mptcp/mptcp_join.sh | 342 +++++++++++++++++-
.../testing/selftests/net/mptcp/pm_netlink.sh | 53 ++-
tools/testing/selftests/net/mptcp/pm_nl_ctl.c | 34 +-
15 files changed, 798 insertions(+), 28 deletions(-)
--
2.25.1
^ permalink raw reply [flat|nested] 9+ messages in thread
* [PATCH mptcp-next v12 1/6] selftests: mptcp: connect: retry write() on EAGAIN
2026-10-28 5:41 [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
@ 2026-10-28 5:41 ` Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 2/6] netlink: specs: mptcp: add the subflow-retries attribute Anil Kaushik
` (5 subsequent siblings)
6 siblings, 0 replies; 9+ messages in thread
From: Anil Kaushik @ 2026-10-28 5:41 UTC (permalink / raw)
To: mptcp; +Cc: Anil Kaushik
mptcp_connect treats any write() error as fatal. But write() can fail
with EAGAIN even after poll() said the socket was writable: the MPTCP
send buffer shrinks when a subflow is closed.
The tool then exits and the transfer fails on both sides. Instead, go
back to poll() and wait for more space. If a poll timeout is set, still
fail after as many retries, 1ms apart: poll() does not check every
cause of EAGAIN.
Fixes: 048d19d444be ("mptcp: add basic kselftest for mptcp")
Signed-off-by: Anil Kaushik <anilkaushikwireless@gmail.com>
---
Notes:
v6:
- the comment says the limit only applies with a poll timeout (netdev
Sashiko)
v5:
- no change
v4:
- count the EAGAIN retries, not ms (Sashiko)
- no limit when there is no poll timeout, -t 0 (Sashiko)
v3:
- do not wait forever on EAGAIN (Sashiko)
- wrap lines over 80 columns
v2:
- Fixes tag
.../selftests/net/mptcp/mptcp_connect.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff --git a/tools/testing/selftests/net/mptcp/mptcp_connect.c b/tools/testing/selftests/net/mptcp/mptcp_connect.c
index 178d98d91fea..49017d00ab69 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_connect.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_connect.c
@@ -638,6 +638,7 @@ static int copyfd_io_poll(int infd, int peerfd, int outfd,
.events = POLLIN | POLLOUT,
};
unsigned int total_wlen = 0, total_rlen = 0;
+ int eagain_cnt = 0;
set_nonblock(peerfd, true);
@@ -711,6 +712,22 @@ static int copyfd_io_poll(int infd, int peerfd, int outfd,
bw = do_rnd_write(peerfd, winfo->buf + winfo->off, winfo->len);
if (bw < 0) {
+ /* POLLOUT does not guarantee that
+ * the next write will not block:
+ * the MPTCP send buffer can shrink
+ * in between, e.g. when a subflow
+ * is closed. Wait for more space,
+ * but not forever if a poll timeout
+ * is set: poll() does not check
+ * every cause of EAGAIN.
+ */
+ if (errno == EAGAIN &&
+ (poll_timeout < 0 ||
+ ++eagain_cnt <= poll_timeout)) {
+ usleep(1000);
+ continue;
+ }
+
/* expected reset, continue to read */
if (cfg_rcv_trunc &&
(errno == ECONNRESET ||
@@ -723,6 +740,7 @@ static int copyfd_io_poll(int infd, int peerfd, int outfd,
return 111;
}
+ eagain_cnt = 0;
winfo->off += bw;
winfo->len -= bw;
total_wlen += bw;
--
2.25.1
^ permalink raw reply related [flat|nested] 9+ messages in thread
* [PATCH mptcp-next v12 2/6] netlink: specs: mptcp: add the subflow-retries attribute
2026-10-28 5:41 [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 1/6] selftests: mptcp: connect: retry write() on EAGAIN Anil Kaushik
@ 2026-10-28 5:41 ` Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 3/6] mptcp: pm: in-kernel: add the subflow-retries limit Anil Kaushik
` (4 subsequent siblings)
6 siblings, 0 replies; 9+ messages in thread
From: Anil Kaushik @ 2026-10-28 5:41 UTC (permalink / raw)
To: mptcp; +Cc: Anil Kaushik
It will be used by the in-kernel PM to set the number of SYN
retransmissions of a subflow re-created after a timeout, with the other
limits: add it to the set-limits and get-limits commands.
The uapi header and the generated files are updated with ynl-regen.
Signed-off-by: Anil Kaushik <anilkaushikwireless@gmail.com>
---
Notes:
v9:
- after a timeout, not any error, and about 4 hours with the default
RTO settings
v8:
- the default is the maximum, MAX_TCP_SYNCNT (127) (Matthieu)
v7:
- the doc gives the default value (netdev Sashiko)
v6 (netdev Sashiko):
- the policy checks the maximum value, 127 (MAX_TCP_SYNCNT)
- the doc says the value works like TCP_SYNCNT
v5:
- new, split from the retry patch (Matthieu)
Documentation/netlink/specs/mptcp_pm.yaml | 13 +++++++++++++
include/uapi/linux/mptcp_pm.h | 1 +
net/mptcp/mptcp_pm_gen.c | 10 ++++++----
net/mptcp/mptcp_pm_gen.h | 4 ++--
4 files changed, 22 insertions(+), 6 deletions(-)
diff --git a/Documentation/netlink/specs/mptcp_pm.yaml b/Documentation/netlink/specs/mptcp_pm.yaml
index 39f3facc38e5..690d3c5dfdd2 100644
--- a/Documentation/netlink/specs/mptcp_pm.yaml
+++ b/Documentation/netlink/specs/mptcp_pm.yaml
@@ -196,6 +196,16 @@ attribute-sets:
name: addr-remote
type: nest
nested-attributes: address
+ -
+ name: subflow-retries
+ type: u32
+ checks:
+ max: 127
+ doc: |
+ SYN retries of a subflow re-created after a timeout, like the
+ TCP_SYNCNT socket option, so with tcp_syn_linear_timeouts on top.
+ The default is the maximum, MAX_TCP_SYNCNT (127), about 4 hours
+ with the default RTO settings. 0 to not re-create it.
-
name: event-attr
enum-name: mptcp-event-attr
@@ -327,6 +337,7 @@ operations:
attributes:
- rcv-add-addrs
- subflows
+ - subflow-retries
-
name: get-limits
doc: Get protocol limits
@@ -337,10 +348,12 @@ operations:
attributes:
- rcv-add-addrs
- subflows
+ - subflow-retries
reply:
attributes:
- rcv-add-addrs
- subflows
+ - subflow-retries
-
name: set-flags
doc: Change endpoint flags
diff --git a/include/uapi/linux/mptcp_pm.h b/include/uapi/linux/mptcp_pm.h
index fe9863d75350..43e9643deda9 100644
--- a/include/uapi/linux/mptcp_pm.h
+++ b/include/uapi/linux/mptcp_pm.h
@@ -102,6 +102,7 @@ enum {
MPTCP_PM_ATTR_TOKEN,
MPTCP_PM_ATTR_LOC_ID,
MPTCP_PM_ATTR_ADDR_REMOTE,
+ MPTCP_PM_ATTR_SUBFLOW_RETRIES,
__MPTCP_ATTR_AFTER_LAST
};
diff --git a/net/mptcp/mptcp_pm_gen.c b/net/mptcp/mptcp_pm_gen.c
index c180930a8e0e..e2d4af1bc1d5 100644
--- a/net/mptcp/mptcp_pm_gen.c
+++ b/net/mptcp/mptcp_pm_gen.c
@@ -44,15 +44,17 @@ const struct nla_policy mptcp_pm_flush_addrs_nl_policy[MPTCP_PM_ENDPOINT_ADDR +
};
/* MPTCP_PM_CMD_SET_LIMITS - do */
-const struct nla_policy mptcp_pm_set_limits_nl_policy[MPTCP_PM_ATTR_SUBFLOWS + 1] = {
+const struct nla_policy mptcp_pm_set_limits_nl_policy[MPTCP_PM_ATTR_SUBFLOW_RETRIES + 1] = {
[MPTCP_PM_ATTR_RCV_ADD_ADDRS] = { .type = NLA_U32, },
[MPTCP_PM_ATTR_SUBFLOWS] = { .type = NLA_U32, },
+ [MPTCP_PM_ATTR_SUBFLOW_RETRIES] = NLA_POLICY_MAX(NLA_U32, 127),
};
/* MPTCP_PM_CMD_GET_LIMITS - do */
-const struct nla_policy mptcp_pm_get_limits_nl_policy[MPTCP_PM_ATTR_SUBFLOWS + 1] = {
+const struct nla_policy mptcp_pm_get_limits_nl_policy[MPTCP_PM_ATTR_SUBFLOW_RETRIES + 1] = {
[MPTCP_PM_ATTR_RCV_ADD_ADDRS] = { .type = NLA_U32, },
[MPTCP_PM_ATTR_SUBFLOWS] = { .type = NLA_U32, },
+ [MPTCP_PM_ATTR_SUBFLOW_RETRIES] = NLA_POLICY_MAX(NLA_U32, 127),
};
/* MPTCP_PM_CMD_SET_FLAGS - do */
@@ -127,7 +129,7 @@ const struct genl_ops mptcp_pm_nl_ops[11] = {
.validate = GENL_DONT_VALIDATE_STRICT,
.doit = mptcp_pm_nl_set_limits_doit,
.policy = mptcp_pm_set_limits_nl_policy,
- .maxattr = MPTCP_PM_ATTR_SUBFLOWS,
+ .maxattr = MPTCP_PM_ATTR_SUBFLOW_RETRIES,
.flags = GENL_UNS_ADMIN_PERM,
},
{
@@ -135,7 +137,7 @@ const struct genl_ops mptcp_pm_nl_ops[11] = {
.validate = GENL_DONT_VALIDATE_STRICT,
.doit = mptcp_pm_nl_get_limits_doit,
.policy = mptcp_pm_get_limits_nl_policy,
- .maxattr = MPTCP_PM_ATTR_SUBFLOWS,
+ .maxattr = MPTCP_PM_ATTR_SUBFLOW_RETRIES,
},
{
.cmd = MPTCP_PM_CMD_SET_FLAGS,
diff --git a/net/mptcp/mptcp_pm_gen.h b/net/mptcp/mptcp_pm_gen.h
index b9280bcb43f5..5b4dee8a0a34 100644
--- a/net/mptcp/mptcp_pm_gen.h
+++ b/net/mptcp/mptcp_pm_gen.h
@@ -23,9 +23,9 @@ extern const struct nla_policy mptcp_pm_get_addr_nl_policy[MPTCP_PM_ATTR_TOKEN +
extern const struct nla_policy mptcp_pm_flush_addrs_nl_policy[MPTCP_PM_ENDPOINT_ADDR + 1];
-extern const struct nla_policy mptcp_pm_set_limits_nl_policy[MPTCP_PM_ATTR_SUBFLOWS + 1];
+extern const struct nla_policy mptcp_pm_set_limits_nl_policy[MPTCP_PM_ATTR_SUBFLOW_RETRIES + 1];
-extern const struct nla_policy mptcp_pm_get_limits_nl_policy[MPTCP_PM_ATTR_SUBFLOWS + 1];
+extern const struct nla_policy mptcp_pm_get_limits_nl_policy[MPTCP_PM_ATTR_SUBFLOW_RETRIES + 1];
extern const struct nla_policy mptcp_pm_set_flags_nl_policy[MPTCP_PM_ATTR_ADDR_REMOTE + 1];
--
2.25.1
^ permalink raw reply related [flat|nested] 9+ messages in thread
* [PATCH mptcp-next v12 3/6] mptcp: pm: in-kernel: add the subflow-retries limit
2026-10-28 5:41 [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 1/6] selftests: mptcp: connect: retry write() on EAGAIN Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 2/6] netlink: specs: mptcp: add the subflow-retries attribute Anil Kaushik
@ 2026-10-28 5:41 ` Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 4/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
` (3 subsequent siblings)
6 siblings, 0 replies; 9+ messages in thread
From: Anil Kaushik @ 2026-10-28 5:41 UTC (permalink / raw)
To: mptcp; +Cc: Anil Kaushik
A new limit for the in-kernel PM, set and read with the other limits:
the number of SYN retries of a subflow re-created after a timeout, like
the TCP_SYNCNT socket option. The default is the maximum, MAX_TCP_SYNCNT
(127), about 4 hours, and 0 to not re-create it. The netlink policy
checks this maximum value.
It is also exposed in struct mptcp_info, in place of the reserved field.
The next patch uses it.
Signed-off-by: Anil Kaushik <anilkaushikwireless@gmail.com>
---
Notes:
v9:
- MAX_TCP_SYNCNT instead of 127 for the default
- after a timeout, not any error
v8:
- the default is the maximum, 127, about 4 hours (Matthieu)
v6:
- the netlink policy checks the maximum, no more parse_limit() change
(netdev Sashiko)
- the limit works like TCP_SYNCNT, say so (netdev Sashiko)
v5:
- new, split from the retry patch (Matthieu)
- the limit is also in struct mptcp_info (Matthieu)
include/uapi/linux/mptcp.h | 2 +-
net/mptcp/pm_kernel.c | 23 ++++++++++++++++++++++-
net/mptcp/protocol.h | 1 +
net/mptcp/sockopt.c | 2 ++
4 files changed, 26 insertions(+), 2 deletions(-)
diff --git a/include/uapi/linux/mptcp.h b/include/uapi/linux/mptcp.h
index 72a5d030154e..d7e3f359773d 100644
--- a/include/uapi/linux/mptcp.h
+++ b/include/uapi/linux/mptcp.h
@@ -72,7 +72,7 @@ struct mptcp_info {
__u8 mptcpi_subflows_total;
__u8 mptcpi_endp_laminar_max;
__u8 mptcpi_endp_fullmesh_max;
- __u8 reserved;
+ __u8 mptcpi_limit_subflow_retries;
__u32 mptcpi_last_data_sent;
__u32 mptcpi_last_data_recv;
__u32 mptcpi_last_ack_recv;
diff --git a/net/mptcp/pm_kernel.c b/net/mptcp/pm_kernel.c
index d3014bf57bf3..7fb4c7874340 100644
--- a/net/mptcp/pm_kernel.c
+++ b/net/mptcp/pm_kernel.c
@@ -25,6 +25,7 @@ struct pm_nl_pernet {
u8 endp_fullmesh_max;
u8 limit_add_addr_accepted;
u8 limit_extra_subflows;
+ u8 limit_subflow_retries;
u8 next_id;
DECLARE_BITMAP(id_bitmap, MPTCP_PM_MAX_ADDR_ID + 1);
};
@@ -96,6 +97,14 @@ u8 mptcp_pm_get_limit_extra_subflows(const struct mptcp_sock *msk)
}
EXPORT_SYMBOL_GPL(mptcp_pm_get_limit_extra_subflows);
+u8 mptcp_pm_get_limit_subflow_retries(const struct mptcp_sock *msk)
+{
+ struct pm_nl_pernet *pernet = pm_nl_get_pernet_from_msk(msk);
+
+ return READ_ONCE(pernet->limit_subflow_retries);
+}
+EXPORT_SYMBOL_GPL(mptcp_pm_get_limit_subflow_retries);
+
static bool has_subflow_daddr(const struct mptcp_sock *msk,
const struct mptcp_addr_info *daddr)
{
@@ -1419,7 +1428,8 @@ static int parse_limit(struct genl_info *info, int id, unsigned int *limit)
int mptcp_pm_nl_set_limits_doit(struct sk_buff *skb, struct genl_info *info)
{
struct pm_nl_pernet *pernet = genl_info_pm_nl(info);
- unsigned int rcv_addrs, subflows;
+ unsigned int rcv_addrs, subflows, retries;
+ struct nlattr *attr;
int ret;
spin_lock_bh(&pernet->lock);
@@ -1433,8 +1443,13 @@ int mptcp_pm_nl_set_limits_doit(struct sk_buff *skb, struct genl_info *info)
if (ret)
goto unlock;
+ /* the maximum, MAX_TCP_SYNCNT, is checked by the policy */
+ attr = info->attrs[MPTCP_PM_ATTR_SUBFLOW_RETRIES];
+ retries = nla_get_u32_default(attr, pernet->limit_subflow_retries);
+
WRITE_ONCE(pernet->limit_add_addr_accepted, rcv_addrs);
WRITE_ONCE(pernet->limit_extra_subflows, subflows);
+ WRITE_ONCE(pernet->limit_subflow_retries, retries);
unlock:
spin_unlock_bh(&pernet->lock);
@@ -1464,6 +1479,10 @@ int mptcp_pm_nl_get_limits_doit(struct sk_buff *skb, struct genl_info *info)
READ_ONCE(pernet->limit_extra_subflows)))
goto fail;
+ if (nla_put_u32(msg, MPTCP_PM_ATTR_SUBFLOW_RETRIES,
+ READ_ONCE(pernet->limit_subflow_retries)))
+ goto fail;
+
genlmsg_end(msg, reply);
return genlmsg_reply(msg, info);
@@ -1618,6 +1637,8 @@ static int __net_init pm_nl_init_net(struct net *net)
/* Cit. 2 subflows ought to be enough for anybody. */
pernet->limit_extra_subflows = 2;
+ /* the maximum: about 4 hours with the default RTOs */
+ pernet->limit_subflow_retries = MAX_TCP_SYNCNT;
pernet->next_id = 1;
spin_lock_init(&pernet->lock);
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 3be60da12924..4aa4aedb2ba7 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -1266,6 +1266,7 @@ u8 mptcp_pm_get_endp_laminar_max(const struct mptcp_sock *msk);
u8 mptcp_pm_get_endp_fullmesh_max(const struct mptcp_sock *msk);
u8 mptcp_pm_get_limit_add_addr_accepted(const struct mptcp_sock *msk);
u8 mptcp_pm_get_limit_extra_subflows(const struct mptcp_sock *msk);
+u8 mptcp_pm_get_limit_subflow_retries(const struct mptcp_sock *msk);
/* called under PM lock */
static inline void __mptcp_pm_close_subflow(struct mptcp_sock *msk)
diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index 922f6ae5c80c..d3c4abf59ec5 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -1075,6 +1075,8 @@ void mptcp_diag_fill_info(struct mptcp_sock *msk, struct mptcp_info *info)
mptcp_pm_get_endp_laminar_max(msk);
info->mptcpi_endp_fullmesh_max =
mptcp_pm_get_endp_fullmesh_max(msk);
+ info->mptcpi_limit_subflow_retries =
+ mptcp_pm_get_limit_subflow_retries(msk);
}
if (__mptcp_check_fallback(msk))
--
2.25.1
^ permalink raw reply related [flat|nested] 9+ messages in thread
* [PATCH mptcp-next v12 4/6] mptcp: pm: in-kernel: retry subflows closed after an error
2026-10-28 5:41 [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
` (2 preceding siblings ...)
2026-10-28 5:41 ` [PATCH mptcp-next v12 3/6] mptcp: pm: in-kernel: add the subflow-retries limit Anil Kaushik
@ 2026-10-28 5:41 ` Anil Kaushik
2026-10-28 6:11 ` sashiko-bot
2026-10-28 5:41 ` [PATCH mptcp-next v12 5/6] selftests: mptcp: pm_nl_ctl: support the subflow-retries limit Anil Kaushik
` (2 subsequent siblings)
6 siblings, 1 reply; 9+ messages in thread
From: Anil Kaushik @ 2026-10-28 5:41 UTC (permalink / raw)
To: mptcp; +Cc: Anil Kaushik
With the in-kernel PM, a subflow closed after an error, e.g. a timeout
when a path silently drops packets, is never re-created, even once the
path works again. For example, with a main link and a backup link,
connections move to the backup after an outage but never come back.
When TCP gives up on an established subflow created by this side, after
a retransmission or keepalive timeout, whatever ICMP error was received
from the path before, the PM now creates a new one with the same
addresses. Its SYN retries, set like TCP_SYNCNT by the subflow-retries
limit, space out the attempts. This is a single connection attempt: an
ICMP error, or a local error when connecting, ends it, and it is not
retried again. It is also closed if the MPTCP socket is closed in the
meantime. This is only done while another subflow can send data:
otherwise the connection is closed after the close timeout, as before.
The re-created subflows still connecting are closed when no other
subflow can send data anymore, and the ones not established yet when the
MPTCP socket is shut down or closed. Once it is established, it is
handled like any other subflow. Re-creating the initial subflow uses one
of the additional subflows allowed by the limits.
__mptcp_close_ssk() now always cleans the references to the closed
subflow in the MPTCP backlog, not only with MPTCP_CF_PUSH: a re-created
subflow closed with the MPTCP socket can get established in between.
The other paths without MPTCP_CF_PUSH have already purged the backlog.
A subflow reset by the peer, or by anything on the path, is not
re-created: the path works, and re-creating it right away could loop.
A subflow that never got established is not re-created either: the PM
keeps trying the next endpoints, as before. Subflows aborted locally,
also by MPTCP, or using an address that has been removed are not
retried.
TCP only closes a subflow after tcp_retries2 retransmissions, about 15
minutes by default. A subflow that only receives data never gets an
error: the application has to enable TCP keepalive on the MPTCP socket,
which MPTCP applies to all the subflows. Both are documented in
mptcp.rst.
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/440
Signed-off-by: Anil Kaushik <anilkaushikwireless@gmail.com>
---
Notes:
v12:
- when no other subflow can send data, only close the re-created
subflows still connecting: a joined one is fully established
soon, and stop once one got established (netdev Sashiko)
- a subflow in CLOSE_WAIT does not keep the re-created ones open
either: one helper for both checks (netdev Sashiko)
- mptcp.rst: the initial subflow is not re-created if the peer
does not accept joins to its initial address (netdev Sashiko)
v11:
- re-create a subflow only while another subflow can send data: the
close timeout is then handled as before, no more re-arming nor
stopping it (sashiko.dev)
- close the re-created subflows not established yet from the worker,
or when the msk is shut down or closed, never while iterating on
the subflows: mptcp_close_ssk() only schedules the worker
(sashiko.dev)
- also close them once the msk is no longer established, whatever
their TCP state, also when a DATA_FIN is handled outside the worker
(sashiko.dev)
- SUB_CLOSED event for them while the msk is established, like
mptcp_close_ssk(): they can have been joined already
- another subflow in CLOSE_WAIT, being closed, does not count as one
that can send data
v10:
- keep the connection when the close timeout expires if a
re-created subflow connected in the meantime: an active join
does not stop the timer, mptcp_finish_join() returns early for
it (sashiko.dev)
- close the re-created subflows still connecting once the msk is
no longer established, after a DATA_FIN or a shutdown(), when
no other subflow is left: they cannot join anymore, and would
keep the msk around (sashiko.dev)
v9:
- every timeout is retried, whatever ICMP error TCP reports for it,
the last one received, see tcp_write_err(): resets, local aborts
and MPTCP protocol errors are excluded instead of listing the
timeout errors, so IPv4 and IPv6 behave the same (netdev Sashiko)
- mptcp.rst: 0 does not re-create the subflows (netdev Sashiko)
- the re-created subflows closed with the msk get no SUB_CLOSED
event, like the other subflows closed with it
- if no other subflow can send data, the close timeout still applies:
creating the new subflow no longer keeps the connection open for
the whole SYN retries
- doc and commit message: a single connection attempt, ended by an
ICMP error or a local connect error
- __mptcp_close_ssk() always cleans the msk backlog of the closed
subflow: a re-created subflow closed with the msk can have been
established in between, with data in the backlog (netdev Sashiko).
It is still closed without MPTCP_CF_PUSH, which would re-send all
the pending data
- tcp_sock_set_syncnt(), without the ssk lock
v8:
- mptcp.rst gives the default, 127, about 4 hours
- close the re-created subflows still connecting before
__mptcp_wr_shutdown() disconnects them, and also the already
disconnected ones, e.g. after shutdown(SHUT_WR) (sashiko.dev)
- mptcp.rst: the initial subflow on the client side, the subflows
limit, and only one attempt (netdev Sashiko)
v7:
- only after a path error, a retransmission or keepalive timeout: a
reset from the peer or the path could make it loop (netdev
Sashiko)
- retry and no_retry are bools under the msk socket lock, in the
padding after close_event_done, not bits next to the reset ones
(netdev Sashiko)
- mark all the subflows using a removed address, whatever their
state (netdev Sashiko)
- doc: tcp_retries2, and keepalive for a subflow that only receives
data (Abdul Wasey's tests)
- not retried after a reset by MPTCP itself, even with an old sk_err
left by IP_RECVERR (netdev Sashiko)
- close the re-created subflows still connecting without
MPTCP_CF_PUSH: they never carried data (netdev Sashiko)
v6 (netdev Sashiko):
- only re-create subflows that were established: the PM still tries the
next endpoints when one never gets established
- only once TCP closed the subflow, to read its reset flags safely
- removed addresses, also the remote ones and the local ID 0: the dead
subflows are marked in mptcp_pm_rm_addr_or_subflow(), instead of
looking up the endpoint
v5 (Matthieu):
- split in 4 patches
- reverse xmas tree in mptcp_pm_kernel_retry_check()
- "in-kernel" in the subject, like the other in-kernel PM patches
- close the re-created subflows still connecting when the socket is
closed (Matthieu: closing the socket during a retry)
v4 (Matthieu):
- netlink limit, on by default, instead of the sysctl
- one new subflow per failure, its SYN retries set from the limit: no
socket reuse, no attempts counter, no 60s check
- no MIB counter
- not retried if the endpoint was removed, instead of the pm.c change
- called through pm.c
- new bit in the subflow context, no new hole
- commit message: MP_RST, not any RST (Sashiko)
v3 (Sashiko):
- no retry after MP_RST without the T flag, local abort or EBADMSG
- no retry on a removed endpoint or address
- use only request_bkup for the backup flag
v2:
- use the subflow sockets, no list or timer
- code moved to pm_kernel.c
Documentation/networking/mptcp.rst | 19 ++++
net/mptcp/pm.c | 39 ++++++++
net/mptcp/pm_kernel.c | 137 +++++++++++++++++++++++++++++
net/mptcp/protocol.c | 78 +++++++++++++++-
net/mptcp/protocol.h | 50 +++++++++++
5 files changed, 320 insertions(+), 3 deletions(-)
diff --git a/Documentation/networking/mptcp.rst b/Documentation/networking/mptcp.rst
index b6753ffb9c9a..719f1fda3690 100644
--- a/Documentation/networking/mptcp.rst
+++ b/Documentation/networking/mptcp.rst
@@ -73,6 +73,25 @@ To be able to use multiple IP addresses on a host to create multiple *subflows*
addresses can be used. This can be configured with ``ip mptcp endpoint`` for
example.
+When TCP gives up on an established subflow created by this host, after a
+retransmission or keepalive timeout, the in-kernel path manager creates it
+again, with the ``subflow-retries`` limit as its number of SYN retries, like
+``TCP_SYNCNT``: 127 by default, about 4 hours, and 0 to not re-create it. This
+is done for the additional subflows, and for the initial one on the client side:
+the new subflow then takes one of the additional subflows allowed by the
+``subflows`` limit. The initial one is not created again if the peer does not
+accept joins to its initial address. This is a single connection attempt: an
+ICMP error, or a local error when connecting, ends it, and it is not created
+again. This is only done while another subflow can send data: otherwise the
+connection is closed after ``close_timeout``, as before, and the subflows being
+re-created are closed too. A subflow reset by the peer, or by anything on the
+path, is not created again either. TCP only gives up on a subflow after
+``tcp_retries2`` retransmissions, about 15 minutes by default: shorter outages
+are handled by TCP itself. A subflow that only receives data does not notice
+that its path stopped working: the application should then enable TCP keepalive
+on the MPTCP socket (``SO_KEEPALIVE``, ``TCP_KEEPIDLE``, ``TCP_KEEPINTVL`` and
+``TCP_KEEPCNT``), which MPTCP applies to all its subflows.
+
Packet Scheduler
----------------
diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
index d7c5b50b34cc..2bb08be95cd1 100644
--- a/net/mptcp/pm.c
+++ b/net/mptcp/pm.c
@@ -694,6 +694,26 @@ void mptcp_pm_subflow_check_next(struct mptcp_sock *msk,
spin_unlock_bh(&pm->lock);
}
+/* Called with the msk socket lock held, before closing a subflow: true if
+ * it has to be re-created once closed, see mptcp_pm_subflow_retry().
+ */
+bool mptcp_pm_subflow_retry_check(struct mptcp_sock *msk,
+ const struct mptcp_subflow_context *subflow,
+ struct mptcp_pm_retry *retry)
+{
+ if (!mptcp_pm_is_kernel(msk))
+ return false;
+
+ return mptcp_pm_kernel_retry_check(msk, subflow, retry);
+}
+
+void mptcp_pm_subflow_retry(struct mptcp_sock *msk,
+ const struct mptcp_pm_retry *retry)
+{
+ if (mptcp_pm_is_kernel(msk))
+ mptcp_pm_kernel_retry(msk, retry);
+}
+
void mptcp_pm_add_addr_received(const struct sock *ssk,
const struct mptcp_addr_info *addr)
{
@@ -762,6 +782,24 @@ static void mptcp_pm_add_addr_send_ack(struct mptcp_sock *msk)
mptcp_pm_schedule_work(msk, MPTCP_PM_ADD_ADDR_SEND_ACK);
}
+/* A subflow using a removed address must not be re-created, whatever its
+ * state, e.g. closed by TCP but not collected yet, see
+ * mptcp_pm_kernel_retry_check().
+ */
+static void mptcp_pm_rm_no_retry(struct mptcp_subflow_context *subflow,
+ enum linux_mptcp_mib_field rm_type, u8 rm_id)
+{
+ u8 id;
+
+ if (rm_type == MPTCP_MIB_RMADDR)
+ id = READ_ONCE(subflow->remote_id);
+ else
+ id = subflow_get_local_id(subflow);
+
+ if (id == rm_id)
+ subflow->no_retry = true;
+}
+
static void mptcp_pm_rm_addr_or_subflow(struct mptcp_sock *msk,
const struct mptcp_rm_list *rm_list,
enum linux_mptcp_mib_field rm_type)
@@ -794,6 +832,7 @@ static void mptcp_pm_rm_addr_or_subflow(struct mptcp_sock *msk,
int how = RCV_SHUTDOWN | SEND_SHUTDOWN;
u8 id = subflow_get_local_id(subflow);
+ mptcp_pm_rm_no_retry(subflow, rm_type, rm_id);
if ((1 << inet_sk_state_load(ssk)) &
(TCPF_FIN_WAIT1 | TCPF_FIN_WAIT2 | TCPF_CLOSING | TCPF_CLOSE))
continue;
diff --git a/net/mptcp/pm_kernel.c b/net/mptcp/pm_kernel.c
index 7fb4c7874340..f06b24a9cd53 100644
--- a/net/mptcp/pm_kernel.c
+++ b/net/mptcp/pm_kernel.c
@@ -1610,6 +1610,143 @@ bool mptcp_pm_nl_check_work_pending(struct mptcp_sock *msk)
return true;
}
+/* TCP gave up on the path: after a retransmission, keepalive or user
+ * timeout, sk_err is ETIMEDOUT, or the last ICMP error received from the
+ * path, see tcp_write_err(). The other errors on an established subflow
+ * are not path failures: a reset, from the peer or from anything on the
+ * path (ECONNRESET, or EPIPE in CLOSE_WAIT, see tcp_reset()), a local
+ * abort (ECONNABORTED), or an MPTCP protocol error (EBADMSG).
+ */
+static bool mptcp_pm_retry_path_error(int err)
+{
+ switch (err) {
+ case ECONNRESET:
+ case EPIPE:
+ case ECONNABORTED:
+ case EBADMSG:
+ return false;
+ }
+
+ return true;
+}
+
+static bool mptcp_pm_retry_other_subflow(struct mptcp_sock *msk)
+{
+ struct mptcp_subflow_context *subflow;
+
+ mptcp_for_each_subflow(msk, subflow) {
+ if (mptcp_subflow_can_send(subflow))
+ return true;
+ }
+
+ return false;
+}
+
+/* Called with the msk socket lock held, before the subflow is closed */
+bool mptcp_pm_kernel_retry_check(struct mptcp_sock *msk,
+ const struct mptcp_subflow_context *subflow,
+ struct mptcp_pm_retry *retry)
+{
+ struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+ struct sock *sk = (struct sock *)msk;
+ int err;
+
+ /* Only once TCP closed it: the acquire in inet_sk_state_load() makes
+ * sure sk_err and the resetting flag checked below are the ones of
+ * this close.
+ */
+ if (inet_sk_state_load(ssk) != TCP_CLOSE)
+ return false;
+
+ err = READ_ONCE(ssk->sk_err);
+ retry->syn_retries = mptcp_pm_get_limit_subflow_retries(msk);
+ if (!retry->syn_retries || !err || sk->sk_state != TCP_ESTABLISHED ||
+ __mptcp_check_fallback(msk))
+ return false;
+
+ /* mptcp_close_ssk() will not handle it again, e.g. the initial
+ * subflow, kept in the list once closed
+ */
+ if (subflow->close_event_done || READ_ONCE(subflow->local_id) < 0)
+ return false;
+
+ /* Only a path that worked is tried again. If a subflow never got
+ * established, the PM moves on to the next endpoints instead, and a
+ * re-created subflow that cannot connect is not retried again: its SYN
+ * retransmissions already spaced out the tries.
+ */
+ if (!READ_ONCE(subflow->fully_established))
+ return false;
+
+ /* its address has been removed, see mptcp_pm_rm_addr_or_subflow() */
+ if (subflow->no_retry)
+ return false;
+
+ /* Only while another subflow can send data: without one, the
+ * connection is closed after the close timeout, as before, see
+ * __mptcp_close_ssk().
+ */
+ if (!mptcp_pm_retry_other_subflow(msk))
+ return false;
+
+ /* Only when the path stopped working. A reset, from the peer or from
+ * anything on the path, means the path works: re-creating the subflow
+ * right away could loop. Not after a reset by MPTCP itself either, with
+ * mptcp_subflow_reset(), whatever sk_err says.
+ */
+ if (!mptcp_pm_retry_path_error(err) || subflow->resetting)
+ return false;
+
+ /* Only re-create subflows initiated by this side: MP_JOIN ones, or the
+ * initial one on the client side.
+ */
+ if (!subflow->request_join &&
+ (subflow->mp_join || READ_ONCE(msk->pm.server_side)))
+ return false;
+
+ mptcp_remote_address((struct sock_common *)ssk, &retry->remote);
+ retry->remote.id = READ_ONCE(subflow->remote_id);
+ if (!retry->remote.id && READ_ONCE(msk->pm.remote_deny_join_id0))
+ return false;
+
+ mptcp_local_address((struct sock_common *)ssk, &retry->local.addr);
+ retry->local.addr.port = 0;
+ retry->local.addr.id = subflow_get_local_id(subflow);
+ retry->local.flags = subflow->request_bkup ?
+ MPTCP_PM_ADDR_FLAG_BACKUP : 0;
+ retry->local.ifindex = ssk->sk_bound_dev_if;
+
+ return true;
+}
+
+/* Called with the msk socket lock held, after the subflow has been closed */
+void mptcp_pm_kernel_retry(struct mptcp_sock *msk,
+ const struct mptcp_pm_retry *retry)
+{
+ struct mptcp_subflow_context *subflow;
+ struct sock *sk = (struct sock *)msk;
+ bool allowed;
+
+ spin_lock_bh(&msk->pm.lock);
+ allowed = msk->pm.extra_subflows <
+ mptcp_pm_get_limit_extra_subflows(msk);
+ if (allowed)
+ msk->pm.extra_subflows++;
+ spin_unlock_bh(&msk->pm.lock);
+
+ if (!allowed ||
+ __mptcp_subflow_connect(sk, &retry->local, &retry->remote))
+ return;
+
+ /* the new subflow is the last one */
+ subflow = list_last_entry(&msk->conn_list, struct mptcp_subflow_context,
+ node);
+ /* from 1 to MAX_TCP_SYNCNT, see mptcp_pm_kernel_retry_check() */
+ tcp_sock_set_syncnt(mptcp_subflow_tcp_sock(subflow),
+ retry->syn_retries);
+ subflow->retry = true;
+}
+
/* Called under PM lock */
void __mptcp_pm_kernel_worker(struct mptcp_sock *msk)
{
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 02a4f479d9d8..257059ffdc7e 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -736,6 +736,10 @@ static void mptcp_check_data_fin(struct sock *sk)
if (!__mptcp_check_fallback(msk))
mptcp_send_ack(msk);
mptcp_close_wake_up(sk);
+
+ /* not always called from the worker, which closes them */
+ if (mptcp_retry_subflows_stale(msk))
+ mptcp_schedule_work(sk);
}
}
@@ -2697,8 +2701,11 @@ static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
lock_sock_nested(ssk, SINGLE_DEPTH_NESTING);
subflow->closing = 1;
- if (flags & MPTCP_CF_PUSH)
- mptcp_cleanup_ssk_backlog(sk, ssk);
+ /* also without MPTCP_CF_PUSH: a re-created subflow closed with the msk
+ * can have been established in between, see
+ * mptcp_close_retry_subflows()
+ */
+ mptcp_cleanup_ssk_backlog(sk, ssk);
/* Borrow the fwd allocated page left-over; fwd memory for the subflow
* could be negative at this point, but will be reach zero soon - when
@@ -2788,6 +2795,49 @@ static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
}
}
+/* Re-created subflows not established yet are useless once the msk is
+ * closed or shut down. When no other subflow can send data, see
+ * mptcp_retry_subflows_stale(), only the ones still connecting are: a joined
+ * one is fully established soon. Not called while iterating on the subflows.
+ */
+static void mptcp_close_retry_subflows(struct sock *sk, bool connecting)
+{
+ struct mptcp_subflow_context *subflow, *tmp;
+ struct mptcp_sock *msk = mptcp_sk(sk);
+
+ mptcp_for_each_subflow_safe(msk, subflow, tmp) {
+ struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+
+ if (!subflow->retry || READ_ONCE(subflow->fully_established))
+ continue;
+
+ if (connecting) {
+ if (inet_sk_state_load(ssk) != TCP_SYN_SENT)
+ continue;
+ /* another one got established in the meantime */
+ if (!mptcp_retry_subflows_stale(msk))
+ break;
+ }
+
+ /* like mptcp_close_ssk(): it can have joined already */
+ if (sk->sk_state == TCP_ESTABLISHED &&
+ !subflow->close_event_done) {
+ subflow->close_event_done = true;
+ mptcp_event(MPTCP_EVENT_SUB_CLOSED, msk, ssk,
+ GFP_KERNEL);
+ }
+
+ /* Checked without the ssk lock: it can have joined, or been
+ * established in between, and is then closed anyway.
+ * __mptcp_close_ssk() cleans its data in the msk backlog, and
+ * the MPTCP retransmissions cover what it sent. No need to push
+ * the pending data again.
+ */
+ mptcp_pm_subflow_check_next(msk, subflow);
+ __mptcp_close_ssk(sk, ssk, subflow, 0);
+ }
+}
+
void mptcp_close_ssk(struct sock *sk, struct sock *ssk,
struct mptcp_subflow_context *subflow)
{
@@ -2806,6 +2856,12 @@ void mptcp_close_ssk(struct sock *sk, struct sock *ssk,
mptcp_pm_subflow_check_next(mptcp_sk(sk), subflow);
__mptcp_close_ssk(sk, ssk, subflow, MPTCP_CF_PUSH);
+
+ /* the caller can iterate on the subflows: the worker closes the
+ * re-created ones
+ */
+ if (mptcp_retry_subflows_stale(mptcp_sk(sk)))
+ mptcp_schedule_work(sk);
}
static unsigned int mptcp_sync_mss(struct sock *sk, u32 pmtu)
@@ -2823,6 +2879,8 @@ static void __mptcp_close_subflow(struct sock *sk)
mptcp_for_each_subflow_safe(msk, subflow, tmp) {
struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
int ssk_state = inet_sk_state_load(ssk);
+ struct mptcp_pm_retry retry;
+ bool do_retry;
if (ssk_state != TCP_CLOSE &&
(ssk_state != TCP_CLOSE_WAIT ||
@@ -2834,7 +2892,10 @@ static void __mptcp_close_subflow(struct sock *sk)
if (!skb_queue_empty_lockless(&ssk->sk_receive_queue))
continue;
+ do_retry = mptcp_pm_subflow_retry_check(msk, subflow, &retry);
mptcp_close_ssk(sk, ssk, subflow);
+ if (do_retry)
+ mptcp_pm_subflow_retry(msk, &retry);
}
}
@@ -3185,6 +3246,9 @@ static void mptcp_worker(struct work_struct *work)
if (test_and_clear_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags))
__mptcp_close_subflow(sk);
+ if (mptcp_retry_subflows_stale(msk))
+ mptcp_close_retry_subflows(sk, true);
+
if (mptcp_close_tout_expired(sk)) {
struct mptcp_subflow_context *subflow, *tmp;
@@ -3516,6 +3580,9 @@ bool __mptcp_close(struct sock *sk, long timeout)
goto cleanup;
}
+ /* before __mptcp_wr_shutdown() disconnects them */
+ mptcp_close_retry_subflows(sk, false);
+
if (mptcp_data_avail(msk) || timeout < 0 ||
(sock_flag(sk, SOCK_LINGER) && !sk->sk_lingertime)) {
/* If the msk has read data, or the caller explicitly ask it,
@@ -4140,7 +4207,12 @@ static void mptcp_shutdown(struct sock *sk, int how)
{
pr_debug("sk=%p, how=%d\n", sk, how);
- if ((how & SEND_SHUTDOWN) && mptcp_close_state(sk))
+ if (!(how & SEND_SHUTDOWN))
+ return;
+
+ /* before the state change and __mptcp_wr_shutdown(), as on close */
+ mptcp_close_retry_subflows(sk, false);
+ if (mptcp_close_state(sk))
__mptcp_wr_shutdown(sk);
}
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 4aa4aedb2ba7..f160e9ac7892 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -610,6 +610,8 @@ struct mptcp_subflow_context {
bool close_event_done; /* has done the post-closed part,
* protected by msk socket lock
*/
+ bool retry; /* re-created after an error, msk lock */
+ bool no_retry; /* uses a removed address, msk lock */
u32 lent_mem_frag;
u32 remote_nonce;
u32 local_nonce;
@@ -919,6 +921,37 @@ static inline bool __mptcp_subflow_active(struct mptcp_subflow_context *subflow)
return __tcp_can_send(mptcp_subflow_tcp_sock(subflow));
}
+/* Can send data, and is not being closed, e.g. in CLOSE_WAIT */
+static inline bool mptcp_subflow_can_send(struct mptcp_subflow_context *subflow)
+{
+ struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+
+ return __mptcp_subflow_active(subflow) &&
+ inet_sk_state_load(ssk) == TCP_ESTABLISHED;
+}
+
+/* Re-created subflows not established yet, while no other subflow can send
+ * data, or once the msk is no longer established: they cannot join anymore,
+ * see mptcp_finish_join(), or must not keep the connection open.
+ */
+static inline bool mptcp_retry_subflows_stale(struct mptcp_sock *msk)
+{
+ struct mptcp_subflow_context *subflow;
+ struct sock *sk = (struct sock *)msk;
+ bool retry = false;
+
+ mptcp_for_each_subflow(msk, subflow) {
+ if (sk->sk_state == TCP_ESTABLISHED &&
+ mptcp_subflow_can_send(subflow))
+ return false;
+
+ retry |= subflow->retry &&
+ !READ_ONCE(subflow->fully_established);
+ }
+
+ return retry;
+}
+
void mptcp_subflow_set_active(struct mptcp_subflow_context *subflow);
bool mptcp_subflow_active(struct mptcp_subflow_context *subflow);
@@ -1139,6 +1172,18 @@ void mptcp_pm_subflow_established(struct mptcp_sock *msk);
bool mptcp_pm_nl_check_work_pending(struct mptcp_sock *msk);
void mptcp_pm_subflow_check_next(struct mptcp_sock *msk,
const struct mptcp_subflow_context *subflow);
+
+struct mptcp_pm_retry {
+ struct mptcp_pm_local local;
+ struct mptcp_addr_info remote;
+ u8 syn_retries;
+};
+
+bool mptcp_pm_subflow_retry_check(struct mptcp_sock *msk,
+ const struct mptcp_subflow_context *subflow,
+ struct mptcp_pm_retry *retry);
+void mptcp_pm_subflow_retry(struct mptcp_sock *msk,
+ const struct mptcp_pm_retry *retry);
void mptcp_pm_add_addr_received(const struct sock *ssk,
const struct mptcp_addr_info *addr);
void mptcp_pm_add_addr_echoed(struct mptcp_sock *msk,
@@ -1260,6 +1305,11 @@ void __init mptcp_pm_userspace_register(void);
void __init mptcp_pm_nl_init(void);
void mptcp_pm_worker(struct mptcp_sock *msk);
void __mptcp_pm_kernel_worker(struct mptcp_sock *msk);
+bool mptcp_pm_kernel_retry_check(struct mptcp_sock *msk,
+ const struct mptcp_subflow_context *subflow,
+ struct mptcp_pm_retry *retry);
+void mptcp_pm_kernel_retry(struct mptcp_sock *msk,
+ const struct mptcp_pm_retry *retry);
u8 mptcp_pm_get_endp_signal_max(const struct mptcp_sock *msk);
u8 mptcp_pm_get_endp_subflow_max(const struct mptcp_sock *msk);
u8 mptcp_pm_get_endp_laminar_max(const struct mptcp_sock *msk);
--
2.25.1
^ permalink raw reply related [flat|nested] 9+ messages in thread
* [PATCH mptcp-next v12 5/6] selftests: mptcp: pm_nl_ctl: support the subflow-retries limit
2026-10-28 5:41 [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
` (3 preceding siblings ...)
2026-10-28 5:41 ` [PATCH mptcp-next v12 4/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
@ 2026-10-28 5:41 ` Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 6/6] selftests: mptcp: join: validate subflow retry after link failure Anil Kaushik
2026-10-28 6:57 ` [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error MPTCP CI
6 siblings, 0 replies; 9+ messages in thread
From: Anil Kaushik @ 2026-10-28 5:41 UTC (permalink / raw)
To: mptcp; +Cc: Anil Kaushik
'pm_nl_ctl limits' can now set the subflow-retries limit with a third
value, and shows it when the kernel supports it.
pm_netlink.sh checks its default value, that a value above the maximum
is refused, that it can be set, and that it is kept when only the other
limits are set. This is only done with pm_nl_ctl for the moment:
iproute2 does not support this limit yet.
Signed-off-by: Anil Kaushik <anilkaushikwireless@gmail.com>
---
Notes:
v12:
- check the limit is kept when only the other limits are set
(netdev Sashiko)
v9:
- try the value above the maximum after setting 0, not the default
value, which is also the maximum: a refused value is then told
apart from a clamped one (netdev Sashiko)
v8:
- the default value is now 127
v7:
- also check that the maximum value, 127, is accepted (netdev
Sashiko)
v5:
- new (Matthieu)
.../testing/selftests/net/mptcp/pm_netlink.sh | 53 ++++++++++++++++++-
tools/testing/selftests/net/mptcp/pm_nl_ctl.c | 34 +++++++++---
2 files changed, 78 insertions(+), 9 deletions(-)
diff --git a/tools/testing/selftests/net/mptcp/pm_netlink.sh b/tools/testing/selftests/net/mptcp/pm_netlink.sh
index 21bfe1311f11..5c27836f527c 100755
--- a/tools/testing/selftests/net/mptcp/pm_netlink.sh
+++ b/tools/testing/selftests/net/mptcp/pm_netlink.sh
@@ -46,15 +46,20 @@ trap cleanup EXIT
mptcp_lib_ns_init ns1
+# $3: subflow retries, only shown by pm_nl_ctl, if supported
format_limits() {
local accept="${1}"
local subflows="${2}"
+ local retries="${3}"
if mptcp_lib_is_ip_mptcp; then
# with a space at the end
printf "add_addr_accepted %d subflows %d \n" "${accept}" "${subflows}"
else
printf "accept %d\nsubflows %d\n" "${accept}" "${subflows}"
+ if [ -n "${retries}" ]; then
+ printf "subflow_retries %d\n" "${retries}"
+ fi
fi
}
@@ -75,6 +80,22 @@ get_limits_nb() {
fi
}
+# The subflow retries limit is only set and shown with pm_nl_ctl for now.
+# Prints its default value if it is supported.
+get_default_retries() {
+ if mptcp_lib_is_ip_mptcp; then
+ return
+ fi
+
+ if ip netns exec "${ns1}" ./pm_nl_ctl limits |
+ grep -q "^subflow_retries"; then
+ echo 127
+ elif mptcp_lib_expect_all_features; then
+ # missing feature: the defaults limits check will fail
+ echo 127
+ fi
+}
+
format_endpoints() {
mptcp_lib_pm_nl_format_endpoints "${@}"
}
@@ -154,8 +175,10 @@ mptcp_lib_subtests_last_ts_reset
check "show_endpoints" "" "defaults addr list"
default_limits="$(get_limits)"
+default_retries="$(get_default_retries)"
if mptcp_lib_expect_all_features; then
- check "get_limits" "$(format_limits 0 2)" "defaults limits"
+ check "get_limits" "$(format_limits 0 2 "${default_retries}")" \
+ "defaults limits"
fi
add_endpoint 10.0.1.1
@@ -224,7 +247,33 @@ check "get_limits" "${default_limits}" "subflows above hard limit"
set_limits ${max_limits} ${max_limits}
flush_endpoint ## to make sure it doesn't affect the limits
-check "get_limits" "$(format_limits ${max_limits} ${max_limits})" "set limits"
+max_limits_out="$(format_limits ${max_limits} ${max_limits} \
+ "${default_retries}")"
+check "get_limits" "${max_limits_out}" "set limits"
+
+if [ -n "${default_retries}" ]; then
+ ip netns exec "${ns1}" ./pm_nl_ctl limits ${max_limits} ${max_limits} 0
+ check "get_limits" "$(format_limits ${max_limits} ${max_limits} 0)" \
+ "set subflow retries"
+
+ # not given: unchanged
+ set_limits ${max_limits} ${max_limits}
+ check "get_limits" "$(format_limits ${max_limits} ${max_limits} 0)" \
+ "subflow retries kept"
+
+ # not from the default: a refused value differs from a clamped one
+ ip netns exec "${ns1}" ./pm_nl_ctl limits ${max_limits} ${max_limits} \
+ 128 2>/dev/null
+ check "get_limits" "$(format_limits ${max_limits} ${max_limits} 0)" \
+ "subflow retries above hard limit"
+
+ ip netns exec "${ns1}" ./pm_nl_ctl limits ${max_limits} ${max_limits} \
+ 127
+ check "get_limits" "$(format_limits ${max_limits} ${max_limits} 127)" \
+ "set max subflow retries"
+ ip netns exec "${ns1}" ./pm_nl_ctl limits ${max_limits} ${max_limits} \
+ "${default_retries}"
+fi
flush_endpoint
add_endpoint 10.0.1.1
diff --git a/tools/testing/selftests/net/mptcp/pm_nl_ctl.c b/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
index 78180da1efcc..41467b997018 100644
--- a/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
+++ b/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
@@ -39,7 +39,7 @@ static void syntax(char *argv[])
fprintf(stderr, "\tset [<ip>] [id <nr>] flags [no]backup|[no]fullmesh [port <nr>] [token <token>] [rip <ip>] [rport <port>]\n");
fprintf(stderr, "\tflush\n");
fprintf(stderr, "\tdump\n");
- fprintf(stderr, "\tlimits [<rcv addr max> <subflow max>]\n");
+ fprintf(stderr, "\tlimits [<rcv addr max> <subflow max> [<retry>]]\n");
fprintf(stderr, "\tevents\n");
fprintf(stderr, "\tlisten <local-ip> <local-port>\n");
exit(0);
@@ -1217,6 +1217,7 @@ int flush_addrs(int fd, int pm_family, int argc, char *argv[])
static void print_limits(struct nlmsghdr *nh, int pm_family, int total_len)
{
struct rtattr *attrs;
+ const char *name;
uint32_t max;
for (; NLMSG_OK(nh, total_len); nh = NLMSG_NEXT(nh, total_len)) {
@@ -1235,13 +1236,22 @@ static void print_limits(struct nlmsghdr *nh, int pm_family, int total_len)
while (RTA_OK(attrs, len)) {
int type = attrs->rta_type;
- if (type != MPTCP_PM_ATTR_RCV_ADD_ADDRS &&
- type != MPTCP_PM_ATTR_SUBFLOWS)
+ switch (type) {
+ case MPTCP_PM_ATTR_RCV_ADD_ADDRS:
+ name = "accept";
+ break;
+ case MPTCP_PM_ATTR_SUBFLOWS:
+ name = "subflows";
+ break;
+ case MPTCP_PM_ATTR_SUBFLOW_RETRIES:
+ name = "subflow_retries";
+ break;
+ default:
goto next;
+ }
memcpy(&max, RTA_DATA(attrs), 4);
- printf("%s %u\n", type == MPTCP_PM_ATTR_SUBFLOWS ?
- "subflows" : "accept", max);
+ printf("%s %u\n", name, max);
next:
attrs = RTA_NEXT(attrs, len);
@@ -1254,15 +1264,17 @@ int get_set_limits(int fd, int pm_family, int argc, char *argv[])
char data[NLMSG_ALIGN(sizeof(struct nlmsghdr)) +
NLMSG_ALIGN(sizeof(struct genlmsghdr)) +
1024];
- uint32_t rcv_addr = 0, subflows = 0;
+ uint32_t rcv_addr = 0, subflows = 0, retries = 0;
int cmd, len = sizeof(data);
struct nlmsghdr *nh;
int off = 0;
/* limit */
- if (argc == 4) {
+ if (argc == 4 || argc == 5) {
rcv_addr = atoi(argv[2]);
subflows = atoi(argv[3]);
+ if (argc == 5)
+ retries = atoi(argv[4]);
cmd = MPTCP_PM_CMD_SET_LIMITS;
} else {
cmd = MPTCP_PM_CMD_GET_LIMITS;
@@ -1287,6 +1299,14 @@ int get_set_limits(int fd, int pm_family, int argc, char *argv[])
memcpy(RTA_DATA(rta), &subflows, 4);
off += NLMSG_ALIGN(rta->rta_len);
+ if (argc == 5) {
+ rta = (void *)(data + off);
+ rta->rta_type = MPTCP_PM_ATTR_SUBFLOW_RETRIES;
+ rta->rta_len = RTA_LENGTH(4);
+ memcpy(RTA_DATA(rta), &retries, 4);
+ off += NLMSG_ALIGN(rta->rta_len);
+ }
+
/* do not expect a reply */
len = 0;
}
--
2.25.1
^ permalink raw reply related [flat|nested] 9+ messages in thread
* [PATCH mptcp-next v12 6/6] selftests: mptcp: join: validate subflow retry after link failure
2026-10-28 5:41 [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
` (4 preceding siblings ...)
2026-10-28 5:41 ` [PATCH mptcp-next v12 5/6] selftests: mptcp: pm_nl_ctl: support the subflow-retries limit Anil Kaushik
@ 2026-10-28 5:41 ` Anil Kaushik
2026-10-28 6:57 ` [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error MPTCP CI
6 siblings, 0 replies; 9+ messages in thread
From: Anil Kaushik @ 2026-10-28 5:41 UTC (permalink / raw)
To: mptcp; +Cc: Anil Kaushik
Test the subflow-retries limit, with its default value, and a long
connection in the background: drop the packets the client sends from
the additional subflow address with netfilter, without bringing the link
down, until the subflow is closed and re-created. Let them through again
once the SYN of the new subflow has been retransmitted more times than
tcp_syn_retries allows, and check the subflow comes back from the same
address. Then do the same with the initial subflow.
A second test closes the connection while a subflow is being re-created,
and checks the netlink events. Only the client sends data: the server
reads it all first, in sendfile mode, and keeps its side open. The
client gets a big MPTCP send buffer compared to its subflows, and both
paths are blocked: the data left in it is never sent, nor the DATA_FIN,
so only the close of the MPTCP socket can stop the subflow being
re-created. Only the client is closed: it has nothing left to read, so
the connection is not closed with a fastclose, which would reset all the
subflows anyway. The check is skipped if a fastclose happens. The big
files used for the long connection are emptied at the end.
A third test checks the only subflow is not re-created: there is no
other subflow to send data.
The tests are skipped if pm_nl_ctl does not show the new limit, or if the
filter rules cannot be set.
Signed-off-by: Anil Kaushik <anilkaushikwireless@gmail.com>
---
Notes:
v12:
- wait for the client side of a re-created subflow to be fully
established before going on: the close would close it too
otherwise (netdev Sashiko)
- the third test only checks the subflow is not re-created, say
only that (netdev Sashiko)
v11:
- the third test checks the only subflow is not re-created, and the
path-back test is gone: a subflow is only re-created while another
one can send data
- the close test keeps the initial subflow while checking: without the
fix, it would no longer pass when the initial subflow closes after
a few retransmissions; a SUB_CLOSED event for the re-created subflow
v10:
- new test: the path of the only subflow comes back before the
close timeout, and the connection stays open (sashiko.dev)
The path comes back as soon as the subflow is re-created, with a
30s close timeout, longer than the SYN backoff, and its transfer
has a longer poll timeout, with no data at all while the only path
is blocked: both are needed on a slow debug kernel
v9:
- wait for absolute counter values, read before blocking the path,
or 0 and 1 in the new netns: the subflow can be re-created before
a later read (netdev Sashiko)
- check that the connection is established before blocking it, in
the close test and the new one (netdev Sashiko)
- the close test blocks the other path too, with a big msk send
buffer: the pending data and the DATA_FIN no longer close the
subflow being re-created, only the close of the MPTCP socket can.
Checked: both the close test and the new test fail without their
fix
- new test: the only subflow is re-created, and the connection is
still closed after the close timeout
- $sinfail as the server input, kill errors ignored, clearer failure
message, $cout and $sout emptied too
- the server of the second test reads all the data from the client
first, in sendfile mode, instead of blocking on a FIFO in poll
mode: the client always has data in flight (netdev Sashiko)
v8:
- comment in run_tests() matching the new condition (netdev Sashiko)
- a separate test closes the connection while a subflow is
re-created: the server sends nothing but keeps its side open, and
only the client is closed, so there is no fastclose, which would
reset all the subflows anyway; the check is skipped if one happens
(netdev Sashiko)
v7 (netdev Sashiko):
- the test_linkfail=1/2 modes regenerate the shared files when they
are empty, e.g. when running link_failure_tests() twice
- read the MP_JOIN ACK counter before letting the packets through,
and do not compare the ports, they can be reused
v6 (Sashiko):
- use the slowwait helpers from net/lib.sh
- longer first wait, while the 128 MB files are created
- skip if the filter rules cannot be set, check each drop rule
(netdev Sashiko)
- empty the big files at the end, cinsent too, keep the variables
(netdev Sashiko)
v5 (Matthieu):
- skipped if pm_nl_ctl does not show the new limit
- wait for the SYN retransmissions instead of a sleep, with
tcp_syn_linear_timeouts=0
- helper for the check after the close
- slower links with short queues, so the transfer lasts the whole
test on any machine, longer timeout (killed at the end)
v4:
- default limit, no sysctl or MIB counter
- more SYN retransmissions than tcp_syn_retries
- close the connection while a subflow is re-created
- check the netlink events
- check the waits and the new subflow port, remove the big files at
the end (Sashiko)
v3:
- check the subflow address, create the files before the background job
v2:
- one test, with netfilter
.../testing/selftests/net/mptcp/mptcp_join.sh | 342 +++++++++++++++++-
1 file changed, 334 insertions(+), 8 deletions(-)
diff --git a/tools/testing/selftests/net/mptcp/mptcp_join.sh b/tools/testing/selftests/net/mptcp/mptcp_join.sh
index c3ccce96b45b..2b847d90674e 100755
--- a/tools/testing/selftests/net/mptcp/mptcp_join.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_join.sh
@@ -52,6 +52,7 @@ nr_blank=6
# These var are used only in some tests, make sure they are not already set
unset FAILING_LINKS
unset test_linkfail
+unset server_in
unset addr_nr_ns1
unset addr_nr_ns2
unset cestab_ns1
@@ -1038,7 +1039,12 @@ do_transfer()
fi
extra_srv_args="$extra_args $extra_srv_args"
- if [ "$test_linkfail" -gt 1 ];then
+ if [ -n "${server_in}" ]; then
+ # the server reads all the data from the client before sending
+ # this file: it does not block on its input
+ extra_srv_args="$extra_srv_args -m sendfile"
+ listener_in="${server_in}"
+ elif [ "$test_linkfail" -gt 1 ]; then
listener_in="${sinfail}"
fi
ip netns exec ${listener_ns} \
@@ -1135,9 +1141,9 @@ run_tests()
cinfail=$(mktemp)
fi
make_file "$cinfail" "client" $size
- # create the input file for the failure test when
- # the first failure test run
- elif [ "$test_linkfail" -ne 0 ] && [ -z "$cinfail" ]; then
+ # create the input file for the failure test, when
+ # it is missing or has been emptied
+ elif [ "$test_linkfail" -ne 0 ] && [ ! -s "$cinfail" ]; then
# the client file must be considerably larger
# of the maximum expected cwin value, or the
# link utilization will be not predicable
@@ -1146,23 +1152,27 @@ run_tests()
size=$((size*8192))
size=$((size + ( RANDOM % 8192) ))
- cinfail=$(mktemp)
+ if [ -z "$cinfail" ]; then
+ cinfail=$(mktemp)
+ fi
make_file "$cinfail" "client" $size
fi
- if [ "$test_linkfail" -gt 2 ]; then
+ if [ "$test_linkfail" -gt 2 ] && [ -z "${server_in}" ]; then
size=$test_linkfail
if [ -z "$sinfail" ]; then
sinfail=$(mktemp)
fi
make_file "$sinfail" "server" $size
- elif [ "$test_linkfail" -eq 2 ] && [ -z "$sinfail" ]; then
+ elif [ "$test_linkfail" -eq 2 ] && [ ! -s "$sinfail" ]; then
size=$((RANDOM%16))
size=$((size+1))
size=$((size*2048))
- sinfail=$(mktemp)
+ if [ -z "$sinfail" ]; then
+ sinfail=$(mktemp)
+ fi
make_file "$sinfail" "server" $size
fi
@@ -2151,6 +2161,203 @@ wait_attempt_fail()
return 1
}
+# $1: timeout in seconds ; $2: ns ; $3: counter ; $4: increment, 1 by default
+wait_counter_inc()
+{
+ slowwait_for_counter "${1}" "${4:-1}" \
+ mptcp_lib_get_counter "${2}" "${3}" >/dev/null
+}
+
+# $1: timeout ; $2: ns ; $3: counter ; $4: value to reach
+wait_counter_ge()
+{
+ slowwait "${1}" until_counter_is ">= ${4}" \
+ mptcp_lib_get_counter "${2}" "${3}" >/dev/null
+}
+
+# $1: client address ; prints the local address and port of its subflow
+sf_from()
+{
+ ip netns exec $ns2 ss -tnH state established src "${1}" |
+ awk '{ print $3; exit }'
+}
+
+# the client got the 4th ACK of the MP_JOIN
+sf_fully_established()
+{
+ ip netns exec $ns2 ss -tniH state established src "${1}" |
+ grep -q "tcp-ulp-mptcp flags:[A-Za-z]*e"
+}
+
+has_subflow_retries()
+{
+ ip netns exec $ns2 ./pm_nl_ctl limits | grep -q "^subflow_retries"
+}
+
+# the client packets are dropped with rules in this chain
+init_nft_filter()
+{
+ init_nftables
+ if ! ip netns exec $ns2 nft list chain ip filter OUTPUT \
+ &>/dev/null; then
+ mark_as_skipped "unable to set the filter rules"
+ return 1
+ fi
+}
+
+# $1: client address
+nft_drop_from()
+{
+ ip netns exec $ns2 nft add rule ip filter OUTPUT ip saddr "${1}" drop
+}
+
+# $1: client address to block
+#
+# The link stays up, but the client packets from this address are dropped:
+# the subflow using it is closed after a few retransmissions, and re-created.
+# The packets go through again once the SYN has been retransmitted more times
+# than tcp_syn_retries allows: the new subflow still connects thanks to the
+# subflow-retries limit.
+sf_retry_after_block()
+{
+ local before ack_rx syn_tx
+
+ before=$(sf_from "${1}")
+
+ print_check "subflow from ${1}"
+ if [ -z "${before}" ]; then
+ fail_test "no established subflow from ${1} before the block"
+ return
+ fi
+
+ # read before the block: the subflow can be re-created right after it
+ syn_tx=$(mptcp_lib_get_counter $ns2 "MPTcpExtMPJoinSynTx")
+ if ! nft_drop_from "${1}"; then
+ fail_test "unable to drop the packets from ${1}"
+ return
+ fi
+ if ! wait_counter_ge "${timeout_poll}" $ns2 "MPTcpExtMPJoinSynTx" \
+ $((syn_tx + 1)); then
+ ip netns exec $ns2 nft flush chain ip filter OUTPUT
+ fail_test "subflow from ${1} not re-created"
+ return
+ fi
+ # tcp_syn_retries=1: one more retransmission than allowed
+ if ! wait_counter_inc "${timeout_poll}" \
+ $ns2 "TcpExtTCPSynRetrans" 2; then
+ ip netns exec $ns2 nft flush chain ip filter OUTPUT
+ fail_test "SYN from ${1} not retransmitted enough"
+ return
+ fi
+ # read before the flush: the next SYN can be sent right after it
+ ack_rx=$(mptcp_lib_get_counter $ns1 "MPTcpExtMPJoinAckRx")
+ ip netns exec $ns2 nft flush chain ip filter OUTPUT
+
+ if ! wait_counter_ge "${timeout_poll}" $ns1 "MPTcpExtMPJoinAckRx" \
+ $((ack_rx + 1)); then
+ fail_test "re-created subflow from ${1} not established"
+ return
+ fi
+
+ # before a close, which would close it too if not fully established
+ if ! slowwait "${timeout_poll}" sf_fully_established "${1}"; then
+ fail_test "no fully established subflow from ${1}"
+ else
+ print_ok
+ fi
+}
+
+# $1: ns ; prints the number of MP_FASTCLOSE sent and received
+fastclose_nr()
+{
+ local tx rx
+
+ tx=$(mptcp_lib_get_counter "${1}" "MPTcpExtMPFastcloseTx")
+ rx=$(mptcp_lib_get_counter "${1}" "MPTcpExtMPFastcloseRx")
+ echo $((${tx:-0} + ${rx:-0}))
+}
+
+no_sf_connecting()
+{
+ ! ip netns exec $ns2 ss -tnH state syn-sent | grep -q .
+}
+
+# The client has nothing left to read: it closes the connection without a
+# fastclose, which would reset all the subflows anyway.
+kill_client()
+{
+ local pid
+
+ for pid in $(ip netns pids $ns2); do
+ if [ "$(cat "/proc/${pid}/comm" 2>/dev/null)" = mptcp_connect ]
+ then
+ kill "${pid}" 2>/dev/null
+ fi
+ done
+}
+
+# $1: why the subflow is not being re-created before the close, if it is not
+# $2: number of MP_FASTCLOSE before the close
+chk_no_sf_connecting()
+{
+ local closed
+
+ print_check "no subflow connecting after close"
+ if [ -n "${1}" ]; then
+ fail_test "${1}"
+ return
+ fi
+
+ # closed with the MPTCP socket
+ slowwait "${timeout_poll}" no_sf_connecting
+ closed=$?
+ if [ "$(fastclose_nr $ns2)" -gt "${2}" ]; then
+ print_skip "closed with a fastclose, resetting all the subflows"
+ elif [ "${closed}" -eq 0 ]; then
+ print_ok
+ else
+ fail_test "subflow still connecting after close"
+ fi
+}
+
+# Slow links with short queues: the transfer lasts the whole test on any
+# machine, and the blocked subflow always has data in flight.
+sf_retry_init()
+{
+ local i
+
+ for i in 1 3; do
+ tc -n $ns1 qdisc add dev ns1eth$i root netem \
+ rate 4mbit limit 30 delay 1ms
+ tc -n $ns2 qdisc add dev ns2eth$i root netem \
+ rate 4mbit limit 30 delay 1ms
+ done
+ ip netns exec $ns2 sysctl -q net.ipv4.tcp_retries2=3
+ ip netns exec $ns2 sysctl -q net.ipv4.tcp_syn_retries=1
+ ip netns exec $ns2 sysctl -q net.ipv4.tcp_syn_linear_timeouts=0
+ pm_nl_set_limits $ns1 0 4
+ pm_nl_set_limits $ns2 0 2
+ pm_nl_add_endpoint $ns2 10.0.3.2 dev ns2eth3 flags subflow
+ # created here, not in the background job, for cleanup()
+ if [ -z "$cinfail" ]; then
+ cinfail=$(mktemp)
+ fi
+ if [ -z "$sinfail" ]; then
+ sinfail=$(mktemp)
+ fi
+}
+
+sf_retry_cleanup()
+{
+ ip netns exec $ns2 nft flush chain ip filter OUTPUT
+ # big files: empty them, the next tests write their own content
+ : > "$cinfail"
+ : > "$sinfail"
+ : > "$cinsent"
+ : > "$cout"
+ : > "$sout"
+}
+
set_userspace_pm()
{
local ns=$1
@@ -2585,6 +2792,125 @@ link_failure_tests()
chk_stale_nr $ns2 1 -1 2
chk_link_usage $ns2 ns2eth3 $cinsent 50
fi
+
+ # subflows closed after a silent link failure, then re-created: the
+ # additional one, then the initial one
+ if reset_with_events "subflow retry after link failure" &&
+ continue_if has_subflow_retries && init_nft_filter; then
+ sf_retry_init
+ # killed at the end: the timeout only stops a stuck test
+ { timeout_test=600 test_linkfail=131072 \
+ run_tests $ns1 $ns2 10.0.1.1 & } 2>/dev/null
+ local tests_pid=$!
+
+ # the 128 MB files are created first, that can take a while
+ wait_counter_ge $((timeout_poll * 4)) \
+ $ns1 "MPTcpExtMPJoinAckRx" 1
+ sf_retry_after_block 10.0.3.2
+ sf_retry_after_block 10.0.1.2
+ mptcp_lib_kill_group_wait $tests_pid
+ kill_events_pids
+
+ join_syn_tx=3 chk_join_nr 3 3 3
+ chk_evt_nr ns2 MPTCP_LIB_EVENT_SUB_ESTABLISHED 3
+ chk_evt_nr ns2 MPTCP_LIB_EVENT_SUB_CLOSED 2 error 2
+ sf_retry_cleanup
+ fi
+
+ # close the connection while a subflow is being re-created
+ if reset_with_events "subflow retry, close while connecting" &&
+ continue_if has_subflow_retries && init_nft_filter; then
+ sf_retry_init
+ # A big msk send buffer, compared to the subflow ones: when the
+ # client is closed, some data is not given to any subflow yet,
+ # so the DATA_FIN, which would also close the subflow being
+ # re-created, is not sent.
+ ip netns exec $ns2 sysctl -q \
+ net.ipv4.tcp_wmem="1048576 65536 262144"
+ # The server reads until the client closes, and sends nothing:
+ # the client has nothing to read, and the connection stays
+ # established. The transfer is stopped on purpose, its result
+ # is not checked.
+ : > "$sinfail"
+ { timeout_test=600 test_linkfail=131072 server_in="$sinfail" \
+ run_tests $ns1 $ns2 10.0.1.1 >/dev/null & } 2>/dev/null
+ local tests_pid=$!
+
+ local not_ready="" syn_tx
+ if ! wait_counter_ge $((timeout_poll * 4)) \
+ $ns1 "MPTcpExtMPJoinAckRx" 1; then
+ not_ready="no additional subflow before the block"
+ else
+ # read before the block: the subflow can be re-created
+ # right after it
+ syn_tx=$(mptcp_lib_get_counter $ns2 \
+ "MPTcpExtMPJoinSynTx")
+ if ! nft_drop_from 10.0.3.2; then
+ not_ready="unable to drop the packets"
+ elif ! wait_counter_ge "${timeout_poll}" \
+ $ns2 "MPTcpExtMPJoinSynTx" $((syn_tx + 1)); then
+ not_ready="no subflow re-created before close"
+ fi
+ fi
+ # Block the other path too, so this data is never sent: only
+ # the close of the MPTCP socket can then stop the subflow being
+ # re-created. The initial subflow is no longer closed after a
+ # few retransmissions: that would close it too.
+ ip netns exec $ns2 sysctl -q net.ipv4.tcp_retries2=15
+ if [ -z "${not_ready}" ] && ! nft_drop_from 10.0.1.2; then
+ not_ready="unable to drop the packets"
+ fi
+ local fc_before
+ fc_before=$(fastclose_nr $ns2)
+ kill_client
+ chk_no_sf_connecting "${not_ready}" "${fc_before}"
+ mptcp_lib_kill_group_wait $tests_pid
+ kill_events_pids
+
+ join_syn_tx=2 chk_join_nr 1 1 1
+ chk_evt_nr ns2 MPTCP_LIB_EVENT_SUB_ESTABLISHED 1
+ # the re-created subflow, closed with the MPTCP socket
+ chk_evt_nr ns2 MPTCP_LIB_EVENT_SUB_CLOSED 2 error 1
+ sf_retry_cleanup
+ fi
+
+ # the only subflow is not re-created
+ if reset_with_events "subflow retry, no other subflow" &&
+ continue_if has_subflow_retries && init_nft_filter; then
+ sf_retry_init
+ pm_nl_flush_endpoint $ns2
+ { timeout_test=600 test_linkfail=131072 \
+ run_tests $ns1 $ns2 10.0.1.1 >/dev/null & } 2>/dev/null
+ local tests_pid=$!
+
+ print_check "not re-created"
+ if ! wait_counter_ge $((timeout_poll * 4)) \
+ $ns1 "MPTcpExtMPCapableACKRX" 1; then
+ fail_test "connection not established"
+ elif ! nft_drop_from 10.0.1.2; then
+ fail_test "unable to drop the packets"
+ elif ! wait_counter_ge "${timeout_poll}" \
+ $ns2 "TcpExtTCPAbortOnTimeout" 1; then
+ fail_test "subflow not closed"
+ else
+ # the worker decides right after TCP closed the subflow
+ sleep 2
+ local syn_tx
+ syn_tx=$(mptcp_lib_get_counter $ns2 \
+ "MPTcpExtMPJoinSynTx")
+ if [ "${syn_tx}" != 0 ]; then
+ fail_test "subflow re-created"
+ else
+ print_ok
+ fi
+ fi
+ mptcp_lib_kill_group_wait $tests_pid
+ kill_events_pids
+
+ join_syn_tx=0 chk_join_nr 0 0 0
+ chk_evt_nr ns2 MPTCP_LIB_EVENT_SUB_CLOSED 1 error 1
+ sf_retry_cleanup
+ fi
}
add_addr_timeout_tests()
--
2.25.1
^ permalink raw reply related [flat|nested] 9+ messages in thread
* Re: [PATCH mptcp-next v12 4/6] mptcp: pm: in-kernel: retry subflows closed after an error
2026-10-28 5:41 ` [PATCH mptcp-next v12 4/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
@ 2026-10-28 6:11 ` sashiko-bot
0 siblings, 0 replies; 9+ messages in thread
From: sashiko-bot @ 2026-10-28 6:11 UTC (permalink / raw)
To: Anil Kaushik, mptcp
Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] mptcp: unclosed non-SYN_SENT retry subflows in mptcp_close_retry_subflows()
--
commit de5632976a598bbd4a3db378be502a28582f26e1
Author: Anil Kaushik <anilkaushikwireless@gmail.com>
mptcp: pm: in-kernel: retry subflows closed after an error
This commit teaches the in-kernel path manager to re-create an established
subflow initiated by the local host after TCP closes it due to a path timeout
while another subflow can still send data. It also adds cleanup logic to
close re-created subflows that are not yet fully established when no other
subflow can send data or when the MPTCP socket is shut down or closed.
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index 02a4f479d8c..257059ffdc7e9 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -736,6 +736,10 @@ static void mptcp_check_data_fin(struct sock *sk)
> if (!__mptcp_check_fallback(msk))
> mptcp_send_ack(msk);
> mptcp_close_wake_up(sk);
> +
> + /* not always called from the worker, which closes them */
> + if (mptcp_retry_subflows_stale(msk))
> + mptcp_schedule_work(sk);
> }
> }
[ ... ]
> @@ -2788,6 +2795,49 @@ static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
> }
> }
>
> +/* Re-created subflows not established yet are useless once the msk is
> + * closed or shut down. When no other subflow can send data, see
> + * mptcp_retry_subflows_stale(), only the ones still connecting are: a joined
> + * one is fully established soon. Not called while iterating on the subflows.
> + */
> +static void mptcp_close_retry_subflows(struct sock *sk, bool connecting)
> +{
> + struct mptcp_subflow_context *subflow, *tmp;
> + struct mptcp_sock *msk = mptcp_sk(sk);
> +
> + mptcp_for_each_subflow_safe(msk, subflow, tmp) {
> + struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
> +
> + if (!subflow->retry || READ_ONCE(subflow->fully_established))
> + continue;
> +
> + if (connecting) {
> + if (inet_sk_state_load(ssk) != TCP_SYN_SENT)
> + continue;
> + /* another one got established in the meantime */
> + if (!mptcp_retry_subflows_stale(msk))
> + break;
> + }
[ ... ]
> @@ -3185,6 +3246,9 @@ static void mptcp_worker(struct work_struct *work)
> if (test_and_clear_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags))
> __mptcp_close_subflow(sk);
>
> + if (mptcp_retry_subflows_stale(msk))
> + mptcp_close_retry_subflows(sk, true);
[Severity: High]
Can this leave unestablished retry subflows open when the MPTCP socket is no
longer established or when all other subflows have closed?
When mptcp_check_data_fin() transitions sk->sk_state out of TCP_ESTABLISHED
(for example to TCP_CLOSE_WAIT or TCP_CLOSING), mptcp_retry_subflows_stale()
returns true for any retry subflow with !subflow->fully_established and
schedules mptcp_worker().
However, mptcp_worker() unconditionally calls
mptcp_close_retry_subflows(sk, true) with connecting set to true rather than
checking sk->sk_state == TCP_ESTABLISHED. Inside
mptcp_close_retry_subflows(), when connecting is true:
if (connecting) {
if (inet_sk_state_load(ssk) != TCP_SYN_SENT)
continue;
...
}
This skips any retry subflow with !subflow->fully_established that is already
in TCP_SYN_RECV or TCP_ESTABLISHED (such as one that received the MP_JOIN
SYN-ACK and is waiting for the 4th ACK).
Similarly, if sk->sk_state is TCP_ESTABLISHED and the last active subflow
closes via mptcp_close_ssk(), leaving only the closed msk->first and a
joined (!subflow->fully_established) retry subflow in TCP_ESTABLISHED or
TCP_SYN_RECV, mptcp_close_retry_subflows(sk, true) also skips that retry
subflow.
Because the unestablished retry subflow remains in msk->conn_list alongside
msk->first, list_is_singular(&msk->conn_list) at the end of
__mptcp_close_ssk() evaluates to false:
if (list_is_singular(&msk->conn_list) && msk->first &&
inet_sk_state_load(msk->first) == TCP_CLOSE) {
if (sk->sk_state != TCP_ESTABLISHED ||
msk->in_accept_queue || sock_flag(sk, SOCK_DEAD)) {
mptcp_set_state(sk, TCP_CLOSE);
mptcp_close_wake_up(sk);
} else {
mptcp_start_tout_timer(sk);
}
}
As a result, neither mptcp_set_state(sk, TCP_CLOSE) nor
mptcp_start_tout_timer(sk) is called. Meanwhile, on the joined retry subflow
with !subflow->fully_established, __mptcp_subflow_active(subflow) is false,
so no data is queued on its TCP write queue and tcp_write_err() never fires.
If the 4th ACK never arrives, schedule_3rdack_retransmission() re-arms
icsk_delack_timer to send pure 0-byte 3rd ACKs indefinitely without a retry
limit, keeping the connection hung.
--
Sashiko AI review · https://sashiko.dev/#/patchset/20261028054143.3222998-1-anilkaushikwireless@gmail.com?part=4
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error
2026-10-28 5:41 [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
` (5 preceding siblings ...)
2026-10-28 5:41 ` [PATCH mptcp-next v12 6/6] selftests: mptcp: join: validate subflow retry after link failure Anil Kaushik
@ 2026-10-28 6:57 ` MPTCP CI
6 siblings, 0 replies; 9+ messages in thread
From: MPTCP CI @ 2026-10-28 6:57 UTC (permalink / raw)
To: Anil Kaushik; +Cc: mptcp
Hi Anil,
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): Success! ✅
- KVM Validation: debug (only selftest_mptcp_join): Success! ✅
- KVM Validation: btf-normal (only bpftest_all): Success! ✅
- KVM Validation: btf-debug (only bpftest_all): Success! ✅
- Perf: Success! ✅
- Task: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/40640831760
Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/9507620eda0b
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1195165
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)
^ permalink raw reply [flat|nested] 9+ messages in thread
end of thread, other threads:[~2026-10-28 6:57 UTC | newest]
Thread overview: 9+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2026-10-28 5:41 [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 1/6] selftests: mptcp: connect: retry write() on EAGAIN Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 2/6] netlink: specs: mptcp: add the subflow-retries attribute Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 3/6] mptcp: pm: in-kernel: add the subflow-retries limit Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 4/6] mptcp: pm: in-kernel: retry subflows closed after an error Anil Kaushik
2026-10-28 6:11 ` sashiko-bot
2026-10-28 5:41 ` [PATCH mptcp-next v12 5/6] selftests: mptcp: pm_nl_ctl: support the subflow-retries limit Anil Kaushik
2026-10-28 5:41 ` [PATCH mptcp-next v12 6/6] selftests: mptcp: join: validate subflow retry after link failure Anil Kaushik
2026-10-28 6:57 ` [PATCH mptcp-next v12 0/6] mptcp: pm: in-kernel: retry subflows closed after an error MPTCP CI
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