[PATCH mptcp-next v7 3/6] mptcp: pm: in-kernel: add the subflow-retries limit

[Anil Kaushik <anilkaushikwireless@gmail.com>]

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 an error, like
the TCP_SYNCNT socket option. 15 by default, about 20 minutes, and 0
to not re-create it. The netlink policy checks the maximum value,
MAX_TCP_SYNCNT.

It is also exposed in struct mptcp_info, in place of the reserved field.

The next patch uses it.

Assisted-by: LLM
Signed-off-by: Anil Kaushik <anilkaushikwireless@gmail.com>
---

Notes:
    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..0379e1495cf4 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;
+	/* about 20 minutes with the default RTO values */
+	pernet->limit_subflow_retries = 15;
 	pernet->next_id = 1;
 	spin_lock_init(&pernet->lock);
 
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index ed2f363bb915..6d571328aca9 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -1264,6 +1264,7 @@ 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