Merge branch 'mptcp-features-and-fixes-for-v6-7'

Mat Martineau says:

====================
mptcp: Features and fixes for v6.7

Patch 1 adds a configurable timeout for the MPTCP connection when all
subflows are closed, to support break-before-make use cases.

Patch 2 is a fix for a 1-byte error in rx data counters with MPTCP
fastopen connections.

Patch 3 is a minor code cleanup.

Patches 4 & 5 add handling of rcvlowat for MPTCP sockets, with a
prerequisite patch to use a common scaling ratio between TCP and MPTCP.

Patch 6 improves efficiency of memory copying in MPTCP transmit code.

Patch 7 refactors syncing of socket options from the MPTCP socket to
its subflows.

Patches 8 & 9 help the MPTCP packet scheduler perform well by changing
the handling of notsent_lowat in subflows and how available buffer space
is calculated for MPTCP-level sends.
====================

Link: https://lore.kernel.org/r/20231023-send-net-next-20231023-2-v1-0-9dc60939d371@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

8 files changed, 224 insertions, 77 deletions

diff --git a/Documentation/networking/mptcp-sysctl.rst b/Documentation/networking/mptcp-sysctl.rst
index 15f1919d640c..69975ce25a02 100644
--- a/Documentation/networking/mptcp-sysctl.rst
+++ b/Documentation/networking/mptcp-sysctl.rst
@@ -25,6 +25,17 @@ add_addr_timeout - INTEGER (seconds)
 
 	Default: 120
 
+close_timeout - INTEGER (seconds)
+	Set the make-after-break timeout: in absence of any close or
+	shutdown syscall, MPTCP sockets will maintain the status
+	unchanged for such time, after the last subflow removal, before
+	moving to TCP_CLOSE.
+
+	The default value matches TCP_TIMEWAIT_LEN. This is a per-namespace
+	sysctl.
+
+	Default: 60
+
 checksum_enabled - BOOLEAN
 	Control whether DSS checksum can be enabled.
 
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 39b731c900dd..993b7fcd4e46 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1489,13 +1489,15 @@ static inline int tcp_space_from_win(const struct sock *sk, int win)
 	return __tcp_space_from_win(tcp_sk(sk)->scaling_ratio, win);
 }
 
+/* Assume a conservative default of 1200 bytes of payload per 4K page.
+ * This may be adjusted later in tcp_measure_rcv_mss().
+ */
+#define TCP_DEFAULT_SCALING_RATIO ((1200 << TCP_RMEM_TO_WIN_SCALE) / \
+				   SKB_TRUESIZE(4096))
+
 static inline void tcp_scaling_ratio_init(struct sock *sk)
 {
-	/* Assume a conservative default of 1200 bytes of payload per 4K page.
-	 * This may be adjusted later in tcp_measure_rcv_mss().
-	 */
-	tcp_sk(sk)->scaling_ratio = (1200 << TCP_RMEM_TO_WIN_SCALE) /
-				    SKB_TRUESIZE(4096);
+	tcp_sk(sk)->scaling_ratio = TCP_DEFAULT_SCALING_RATIO;
 }
 
 /* Note: caller must be prepared to deal with negative returns */
diff --git a/net/mptcp/ctrl.c b/net/mptcp/ctrl.c
index e72b518c5d02..13fe0748dde8 100644
--- a/net/mptcp/ctrl.c
+++ b/net/mptcp/ctrl.c
@@ -27,6 +27,7 @@ struct mptcp_pernet {
 #endif
 
 	unsigned int add_addr_timeout;
+	unsigned int close_timeout;
 	unsigned int stale_loss_cnt;
 	u8 mptcp_enabled;
 	u8 checksum_enabled;
@@ -65,6 +66,13 @@ unsigned int mptcp_stale_loss_cnt(const struct net *net)
 	return mptcp_get_pernet(net)->stale_loss_cnt;
 }
 
+unsigned int mptcp_close_timeout(const struct sock *sk)
+{
+	if (sock_flag(sk, SOCK_DEAD))
+		return TCP_TIMEWAIT_LEN;
+	return mptcp_get_pernet(sock_net(sk))->close_timeout;
+}
+
 int mptcp_get_pm_type(const struct net *net)
 {
 	return mptcp_get_pernet(net)->pm_type;
@@ -79,6 +87,7 @@ static void mptcp_pernet_set_defaults(struct mptcp_pernet *pernet)
 {
 	pernet->mptcp_enabled = 1;
 	pernet->add_addr_timeout = TCP_RTO_MAX;
+	pernet->close_timeout = TCP_TIMEWAIT_LEN;
 	pernet->checksum_enabled = 0;
 	pernet->allow_join_initial_addr_port = 1;
 	pernet->stale_loss_cnt = 4;
@@ -141,6 +150,12 @@ static struct ctl_table mptcp_sysctl_table[] = {
 		.mode = 0644,
 		.proc_handler = proc_dostring,
 	},
+	{
+		.procname = "close_timeout",
+		.maxlen = sizeof(unsigned int),
+		.mode = 0644,
+		.proc_handler = proc_dointvec_jiffies,
+	},
 	{}
 };
 
@@ -163,6 +178,7 @@ static int mptcp_pernet_new_table(struct net *net, struct mptcp_pernet *pernet)
 	table[4].data = &pernet->stale_loss_cnt;
 	table[5].data = &pernet->pm_type;
 	table[6].data = &pernet->scheduler;
+	table[7].data = &pernet->close_timeout;
 
 	hdr = register_net_sysctl_sz(net, MPTCP_SYSCTL_PATH, table,
 				     ARRAY_SIZE(mptcp_sysctl_table));
diff --git a/net/mptcp/fastopen.c b/net/mptcp/fastopen.c
index bceaab8dd8e4..74698582a285 100644
--- a/net/mptcp/fastopen.c
+++ b/net/mptcp/fastopen.c
@@ -52,6 +52,7 @@ void mptcp_fastopen_subflow_synack_set_params(struct mptcp_subflow_context *subf
 
 	mptcp_set_owner_r(skb, sk);
 	__skb_queue_tail(&sk->sk_receive_queue, skb);
+	mptcp_sk(sk)->bytes_received += skb->len;
 
 	sk->sk_data_ready(sk);
 
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 886ab689a8ae..1dacc072dcca 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -121,8 +121,6 @@ struct sock *__mptcp_nmpc_sk(struct mptcp_sock *msk)
 		ret = __mptcp_socket_create(msk);
 		if (ret)
 			return ERR_PTR(ret);
-
-		mptcp_sockopt_sync(msk, msk->first);
 	}
 
 	return msk->first;
@@ -863,9 +861,8 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
 
 	/* Wake-up the reader only for in-sequence data */
 	mptcp_data_lock(sk);
-	if (move_skbs_to_msk(msk, ssk))
+	if (move_skbs_to_msk(msk, ssk) && mptcp_epollin_ready(sk))
 		sk->sk_data_ready(sk);
-
 	mptcp_data_unlock(sk);
 }
 
@@ -893,6 +890,7 @@ static bool __mptcp_finish_join(struct mptcp_sock *msk, struct sock *ssk)
 	mptcp_sockopt_sync_locked(msk, ssk);
 	mptcp_subflow_joined(msk, ssk);
 	mptcp_stop_tout_timer(sk);
+	__mptcp_propagate_sndbuf(sk, ssk);
 	return true;
 }
 
@@ -1079,15 +1077,16 @@ static void mptcp_enter_memory_pressure(struct sock *sk)
 	struct mptcp_sock *msk = mptcp_sk(sk);
 	bool first = true;
 
-	sk_stream_moderate_sndbuf(sk);
 	mptcp_for_each_subflow(msk, subflow) {
 		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
 
 		if (first)
 			tcp_enter_memory_pressure(ssk);
 		sk_stream_moderate_sndbuf(ssk);
+
 		first = false;
 	}
+	__mptcp_sync_sndbuf(sk);
 }
 
 /* ensure we get enough memory for the frag hdr, beyond some minimal amount of
@@ -1761,6 +1760,18 @@ static int mptcp_sendmsg_fastopen(struct sock *sk, struct msghdr *msg,
 	return ret;
 }
 
+static int do_copy_data_nocache(struct sock *sk, int copy,
+				struct iov_iter *from, char *to)
+{
+	if (sk->sk_route_caps & NETIF_F_NOCACHE_COPY) {
+		if (!copy_from_iter_full_nocache(to, copy, from))
+			return -EFAULT;
+	} else if (!copy_from_iter_full(to, copy, from)) {
+		return -EFAULT;
+	}
+	return 0;
+}
+
 static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 {
 	struct mptcp_sock *msk = mptcp_sk(sk);
@@ -1834,11 +1845,10 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		if (!sk_wmem_schedule(sk, total_ts))
 			goto wait_for_memory;
 
-		if (copy_page_from_iter(dfrag->page, offset, psize,
-					&msg->msg_iter) != psize) {
-			ret = -EFAULT;
+		ret = do_copy_data_nocache(sk, psize, &msg->msg_iter,
+					   page_address(dfrag->page) + offset);
+		if (ret)
 			goto do_error;
-		}
 
 		/* data successfully copied into the write queue */
 		sk_forward_alloc_add(sk, -total_ts);
@@ -1922,6 +1932,7 @@ static int __mptcp_recvmsg_mskq(struct mptcp_sock *msk,
 			if (!(flags & MSG_PEEK)) {
 				MPTCP_SKB_CB(skb)->offset += count;
 				MPTCP_SKB_CB(skb)->map_seq += count;
+				msk->bytes_consumed += count;
 			}
 			break;
 		}
@@ -1932,6 +1943,7 @@ static int __mptcp_recvmsg_mskq(struct mptcp_sock *msk,
 			WRITE_ONCE(msk->rmem_released, msk->rmem_released + skb->truesize);
 			__skb_unlink(skb, &msk->receive_queue);
 			__kfree_skb(skb);
+			msk->bytes_consumed += count;
 		}
 
 		if (copied >= len)
@@ -2391,8 +2403,8 @@ static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 	if (msk->in_accept_queue && msk->first == ssk &&
 	    (sock_flag(sk, SOCK_DEAD) || sock_flag(ssk, SOCK_DEAD))) {
 		/* ensure later check in mptcp_worker() will dispose the msk */
-		mptcp_set_close_tout(sk, tcp_jiffies32 - (TCP_TIMEWAIT_LEN + 1));
 		sock_set_flag(sk, SOCK_DEAD);
+		mptcp_set_close_tout(sk, tcp_jiffies32 - (mptcp_close_timeout(sk) + 1));
 		lock_sock_nested(ssk, SINGLE_DEPTH_NESTING);
 		mptcp_subflow_drop_ctx(ssk);
 		goto out_release;
@@ -2448,6 +2460,7 @@ out_release:
 		WRITE_ONCE(msk->first, NULL);
 
 out:
+	__mptcp_sync_sndbuf(sk);
 	if (need_push)
 		__mptcp_push_pending(sk, 0);
 
@@ -2516,7 +2529,7 @@ static bool mptcp_close_tout_expired(const struct sock *sk)
 		return false;
 
 	return time_after32(tcp_jiffies32,
-		  inet_csk(sk)->icsk_mtup.probe_timestamp + TCP_TIMEWAIT_LEN);
+		  inet_csk(sk)->icsk_mtup.probe_timestamp + mptcp_close_timeout(sk));
 }
 
 static void mptcp_check_fastclose(struct mptcp_sock *msk)
@@ -2659,7 +2672,7 @@ void mptcp_reset_tout_timer(struct mptcp_sock *msk, unsigned long fail_tout)
 		return;
 
 	close_timeout = inet_csk(sk)->icsk_mtup.probe_timestamp - tcp_jiffies32 + jiffies +
-			TCP_TIMEWAIT_LEN;
+			mptcp_close_timeout(sk);
 
 	/* the close timeout takes precedence on the fail one, and here at least one of
 	 * them is active
@@ -2755,6 +2768,7 @@ static void __mptcp_init_sock(struct sock *sk)
 	msk->rmem_fwd_alloc = 0;
 	WRITE_ONCE(msk->rmem_released, 0);
 	msk->timer_ival = TCP_RTO_MIN;
+	msk->scaling_ratio = TCP_DEFAULT_SCALING_RATIO;
 
 	WRITE_ONCE(msk->first, NULL);
 	inet_csk(sk)->icsk_sync_mss = mptcp_sync_mss;
@@ -2964,16 +2978,9 @@ void __mptcp_unaccepted_force_close(struct sock *sk)
 	__mptcp_destroy_sock(sk);
 }
 
-static __poll_t mptcp_check_readable(struct mptcp_sock *msk)
+static __poll_t mptcp_check_readable(struct sock *sk)
 {
-	/* Concurrent splices from sk_receive_queue into receive_queue will
-	 * always show at least one non-empty queue when checked in this order.
-	 */
-	if (skb_queue_empty_lockless(&((struct sock *)msk)->sk_receive_queue) &&
-	    skb_queue_empty_lockless(&msk->receive_queue))
-		return 0;
-
-	return EPOLLIN | EPOLLRDNORM;
+	return mptcp_epollin_ready(sk) ? EPOLLIN | EPOLLRDNORM : 0;
 }
 
 static void mptcp_check_listen_stop(struct sock *sk)
@@ -3011,7 +3018,7 @@ bool __mptcp_close(struct sock *sk, long timeout)
 		goto cleanup;
 	}
 
-	if (mptcp_check_readable(msk) || timeout < 0) {
+	if (mptcp_data_avail(msk) || timeout < 0) {
 		/* If the msk has read data, or the caller explicitly ask it,
 		 * do the MPTCP equivalent of TCP reset, aka MPTCP fastclose
 		 */
@@ -3138,6 +3145,7 @@ static int mptcp_disconnect(struct sock *sk, int flags)
 	msk->snd_data_fin_enable = false;
 	msk->rcv_fastclose = false;
 	msk->use_64bit_ack = false;
+	msk->bytes_consumed = 0;
 	WRITE_ONCE(msk->csum_enabled, mptcp_is_checksum_enabled(sock_net(sk)));
 	mptcp_pm_data_reset(msk);
 	mptcp_ca_reset(sk);
@@ -3219,7 +3227,7 @@ struct sock *mptcp_sk_clone_init(const struct sock *sk,
 	 * uses the correct data
 	 */
 	mptcp_copy_inaddrs(nsk, ssk);
-	mptcp_propagate_sndbuf(nsk, ssk);
+	__mptcp_propagate_sndbuf(nsk, ssk);
 
 	mptcp_rcv_space_init(msk, ssk);
 	bh_unlock_sock(nsk);
@@ -3397,6 +3405,8 @@ static void mptcp_release_cb(struct sock *sk)
 			__mptcp_set_connected(sk);
 		if (__test_and_clear_bit(MPTCP_ERROR_REPORT, &msk->cb_flags))
 			__mptcp_error_report(sk);
+		if (__test_and_clear_bit(MPTCP_SYNC_SNDBUF, &msk->cb_flags))
+			__mptcp_sync_sndbuf(sk);
 	}
 
 	__mptcp_update_rmem(sk);
@@ -3441,6 +3451,14 @@ void mptcp_subflow_process_delegated(struct sock *ssk, long status)
 			__set_bit(MPTCP_PUSH_PENDING, &mptcp_sk(sk)->cb_flags);
 		mptcp_data_unlock(sk);
 	}
+	if (status & BIT(MPTCP_DELEGATE_SNDBUF)) {
+		mptcp_data_lock(sk);
+		if (!sock_owned_by_user(sk))
+			__mptcp_sync_sndbuf(sk);
+		else
+			__set_bit(MPTCP_SYNC_SNDBUF, &mptcp_sk(sk)->cb_flags);
+		mptcp_data_unlock(sk);
+	}
 	if (status & BIT(MPTCP_DELEGATE_ACK))
 		schedule_3rdack_retransmission(ssk);
 }
@@ -3525,6 +3543,7 @@ bool mptcp_finish_join(struct sock *ssk)
 	/* active subflow, already present inside the conn_list */
 	if (!list_empty(&subflow->node)) {
 		mptcp_subflow_joined(msk, ssk);
+		mptcp_propagate_sndbuf(parent, ssk);
 		return true;
 	}
 
@@ -3909,7 +3928,7 @@ static __poll_t mptcp_poll(struct file *file, struct socket *sock,
 		mask |= EPOLLIN | EPOLLRDNORM | EPOLLRDHUP;
 
 	if (state != TCP_SYN_SENT && state != TCP_SYN_RECV) {
-		mask |= mptcp_check_readable(msk);
+		mask |= mptcp_check_readable(sk);
 		if (shutdown & SEND_SHUTDOWN)
 			mask |= EPOLLOUT | EPOLLWRNORM;
 		else
@@ -3947,6 +3966,7 @@ static const struct proto_ops mptcp_stream_ops = {
 	.sendmsg	   = inet_sendmsg,
 	.recvmsg	   = inet_recvmsg,
 	.mmap		   = sock_no_mmap,
+	.set_rcvlowat	   = mptcp_set_rcvlowat,
 };
 
 static struct inet_protosw mptcp_protosw = {
@@ -4048,6 +4068,7 @@ static const struct proto_ops mptcp_v6_stream_ops = {
 #ifdef CONFIG_COMPAT
 	.compat_ioctl	   = inet6_compat_ioctl,
 #endif
+	.set_rcvlowat	   = mptcp_set_rcvlowat,
 };
 
 static struct proto mptcp_v6_prot;
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index c4c05afdc48c..9092fcf18798 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -125,6 +125,7 @@
 #define MPTCP_RETRANSMIT	4
 #define MPTCP_FLUSH_JOIN_LIST	5
 #define MPTCP_CONNECTED		6
+#define MPTCP_SYNC_SNDBUF	7
 
 struct mptcp_skb_cb {
 	u64 map_seq;
@@ -269,6 +270,7 @@ struct mptcp_sock {
 	atomic64_t	rcv_wnd_sent;
 	u64		rcv_data_fin_seq;
 	u64		bytes_retrans;
+	u64		bytes_consumed;
 	int		rmem_fwd_alloc;
 	int		snd_burst;
 	int		old_wspace;
@@ -434,11 +436,6 @@ mptcp_subflow_rsk(const struct request_sock *rsk)
 	return (struct mptcp_subflow_request_sock *)rsk;
 }
 
-enum mptcp_data_avail {
-	MPTCP_SUBFLOW_NODATA,
-	MPTCP_SUBFLOW_DATA_AVAIL,
-};
-
 struct mptcp_delegated_action {
 	struct napi_struct napi;
 	struct list_head head;
@@ -449,6 +446,7 @@ DECLARE_PER_CPU(struct mptcp_delegated_action, mptcp_delegated_actions);
 #define MPTCP_DELEGATE_SCHEDULED	0
 #define MPTCP_DELEGATE_SEND		1
 #define MPTCP_DELEGATE_ACK		2
+#define MPTCP_DELEGATE_SNDBUF		3
 
 #define MPTCP_DELEGATE_ACTIONS_MASK	(~BIT(MPTCP_DELEGATE_SCHEDULED))
 /* MPTCP subflow context */
@@ -494,7 +492,7 @@ struct mptcp_subflow_context {
 		valid_csum_seen : 1,        /* at least one csum validated */
 		is_mptfo : 1,	    /* subflow is doing TFO */
 		__unused : 9;
-	enum mptcp_data_avail data_avail;
+	bool	data_avail;
 	bool	scheduled;
 	u32	remote_nonce;
 	u64	thmac;
@@ -522,6 +520,9 @@ struct mptcp_subflow_context {
 
 	u32	setsockopt_seq;
 	u32	stale_rcv_tstamp;
+	int     cached_sndbuf;	    /* sndbuf size when last synced with the msk sndbuf,
+				     * protected by the msk socket lock
+				     */
 
 	struct	sock *tcp_sock;	    /* tcp sk backpointer */
 	struct	sock *conn;	    /* parent mptcp_sock */
@@ -615,6 +616,7 @@ unsigned int mptcp_get_add_addr_timeout(const struct net *net);
 int mptcp_is_checksum_enabled(const struct net *net);
 int mptcp_allow_join_id0(const struct net *net);
 unsigned int mptcp_stale_loss_cnt(const struct net *net);
+unsigned int mptcp_close_timeout(const struct sock *sk);
 int mptcp_get_pm_type(const struct net *net);
 const char *mptcp_get_scheduler(const struct net *net);
 void mptcp_subflow_fully_established(struct mptcp_subflow_context *subflow,
@@ -663,6 +665,24 @@ struct sock *mptcp_subflow_get_retrans(struct mptcp_sock *msk);
 int mptcp_sched_get_send(struct mptcp_sock *msk);
 int mptcp_sched_get_retrans(struct mptcp_sock *msk);
 
+static inline u64 mptcp_data_avail(const struct mptcp_sock *msk)
+{
+	return READ_ONCE(msk->bytes_received) - READ_ONCE(msk->bytes_consumed);
+}
+
+static inline bool mptcp_epollin_ready(const struct sock *sk)
+{
+	/* mptcp doesn't have to deal with small skbs in the receive queue,
+	 * at it can always coalesce them
+	 */
+	return (mptcp_data_avail(mptcp_sk(sk)) >= sk->sk_rcvlowat) ||
+	       (mem_cgroup_sockets_enabled && sk->sk_memcg &&
+		mem_cgroup_under_socket_pressure(sk->sk_memcg)) ||
+	       READ_ONCE(tcp_memory_pressure);
+}
+
+int mptcp_set_rcvlowat(struct sock *sk, int val);
+
 static inline bool __tcp_can_send(const struct sock *ssk)
 {
 	/* only send if our side has not closed yet */
@@ -737,6 +757,7 @@ static inline bool mptcp_is_fully_established(struct sock *sk)
 	return inet_sk_state_load(sk) == TCP_ESTABLISHED &&
 	       READ_ONCE(mptcp_sk(sk)->fully_established);
 }
+
 void mptcp_rcv_space_init(struct mptcp_sock *msk, const struct sock *ssk);
 void mptcp_data_ready(struct sock *sk, struct sock *ssk);
 bool mptcp_finish_join(struct sock *sk);
@@ -764,13 +785,52 @@ static inline bool mptcp_data_fin_enabled(const struct mptcp_sock *msk)
 	       READ_ONCE(msk->write_seq) == READ_ONCE(msk->snd_nxt);
 }
 
-static inline bool mptcp_propagate_sndbuf(struct sock *sk, struct sock *ssk)
+static inline void __mptcp_sync_sndbuf(struct sock *sk)
 {
-	if ((sk->sk_userlocks & SOCK_SNDBUF_LOCK) || ssk->sk_sndbuf <= READ_ONCE(sk->sk_sndbuf))
-		return false;
+	struct mptcp_subflow_context *subflow;
+	int ssk_sndbuf, new_sndbuf;
+
+	if (sk->sk_userlocks & SOCK_SNDBUF_LOCK)
+		return;
+
+	new_sndbuf = sock_net(sk)->ipv4.sysctl_tcp_wmem[0];
+	mptcp_for_each_subflow(mptcp_sk(sk), subflow) {
+		ssk_sndbuf =  READ_ONCE(mptcp_subflow_tcp_sock(subflow)->sk_sndbuf);
+
+		subflow->cached_sndbuf = ssk_sndbuf;
+		new_sndbuf += ssk_sndbuf;
+	}
+
+	/* the msk max wmem limit is <nr_subflows> * tcp wmem[2] */
+	WRITE_ONCE(sk->sk_sndbuf, new_sndbuf);
+}
+
+/* The called held both the msk socket and the subflow socket locks,
+ * possibly under BH
+ */
+static inline void __mptcp_propagate_sndbuf(struct sock *sk, struct sock *ssk)
+{
+	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
+
+	if (READ_ONCE(ssk->sk_sndbuf) != subflow->cached_sndbuf)
+		__mptcp_sync_sndbuf(sk);
+}
+
+/* the caller held only the subflow socket lock, either in process or
+ * BH context. Additionally this can be called under the msk data lock,
+ * so we can't acquire such lock here: let the delegate action acquires
+ * the needed locks in suitable order.
+ */
+static inline void mptcp_propagate_sndbuf(struct sock *sk, struct sock *ssk)
+{
+	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
+
+	if (likely(READ_ONCE(ssk->sk_sndbuf) == subflow->cached_sndbuf))
+		return;
 
-	WRITE_ONCE(sk->sk_sndbuf, ssk->sk_sndbuf);
-	return true;
+	local_bh_disable();
+	mptcp_subflow_delegate(subflow, MPTCP_DELEGATE_SNDBUF);
+	local_bh_enable();
 }
 
 static inline void mptcp_write_space(struct sock *sk)
diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index 59bd5e114392..574e221bb765 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -95,6 +95,7 @@ static void mptcp_sol_socket_sync_intval(struct mptcp_sock *msk, int optname, in
 		case SO_SNDBUFFORCE:
 			ssk->sk_userlocks |= SOCK_SNDBUF_LOCK;
 			WRITE_ONCE(ssk->sk_sndbuf, sk->sk_sndbuf);
+			mptcp_subflow_ctx(ssk)->cached_sndbuf = sk->sk_sndbuf;
 			break;
 		case SO_RCVBUF:
 		case SO_RCVBUFFORCE:
@@ -1415,8 +1416,10 @@ static void sync_socket_options(struct mptcp_sock *msk, struct sock *ssk)
 
 	if (sk->sk_userlocks & tx_rx_locks) {
 		ssk->sk_userlocks |= sk->sk_userlocks & tx_rx_locks;
-		if (sk->sk_userlocks & SOCK_SNDBUF_LOCK)
+		if (sk->sk_userlocks & SOCK_SNDBUF_LOCK) {
 			WRITE_ONCE(ssk->sk_sndbuf, sk->sk_sndbuf);
+			mptcp_subflow_ctx(ssk)->cached_sndbuf = sk->sk_sndbuf;
+		}
 		if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
 			WRITE_ONCE(ssk->sk_rcvbuf, sk->sk_rcvbuf);
 	}
@@ -1444,37 +1447,63 @@ static void sync_socket_options(struct mptcp_sock *msk, struct sock *ssk)
 	inet_assign_bit(FREEBIND, ssk, inet_test_bit(FREEBIND, sk));
 }
 
-static void __mptcp_sockopt_sync(struct mptcp_sock *msk, struct sock *ssk)
-{
-	bool slow = lock_sock_fast(ssk);
-
-	sync_socket_options(msk, ssk);
-
-	unlock_sock_fast(ssk, slow);
-}
-
-void mptcp_sockopt_sync(struct mptcp_sock *msk, struct sock *ssk)
+void mptcp_sockopt_sync_locked(struct mptcp_sock *msk, struct sock *ssk)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
 
 	msk_owned_by_me(msk);
 
+	ssk->sk_rcvlowat = 0;
+
+	/* subflows must ignore any latency-related settings: will not affect
+	 * the user-space - only the msk is relevant - but will foul the
+	 * mptcp scheduler
+	 */
+	tcp_sk(ssk)->notsent_lowat = UINT_MAX;
+
 	if (READ_ONCE(subflow->setsockopt_seq) != msk->setsockopt_seq) {
-		__mptcp_sockopt_sync(msk, ssk);
+		sync_socket_options(msk, ssk);
 
 		subflow->setsockopt_seq = msk->setsockopt_seq;
 	}
 }
 
-void mptcp_sockopt_sync_locked(struct mptcp_sock *msk, struct sock *ssk)
+/* unfortunately this is different enough from the tcp version so
+ * that we can't factor it out
+ */
+int mptcp_set_rcvlowat(struct sock *sk, int val)
 {
-	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
+	struct mptcp_subflow_context *subflow;
+	int space, cap;
 
-	msk_owned_by_me(msk);
+	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
+		cap = sk->sk_rcvbuf >> 1;
+	else
+		cap = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_rmem[2]) >> 1;
+	val = min(val, cap);
+	WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
 
-	if (READ_ONCE(subflow->setsockopt_seq) != msk->setsockopt_seq) {
-		sync_socket_options(msk, ssk);
+	/* Check if we need to signal EPOLLIN right now */
+	if (mptcp_epollin_ready(sk))
+		sk->sk_data_ready(sk);
 
-		subflow->setsockopt_seq = msk->setsockopt_seq;
+	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
+		return 0;
+
+	space = __tcp_space_from_win(mptcp_sk(sk)->scaling_ratio, val);
+	if (space <= sk->sk_rcvbuf)
+		return 0;
+
+	/* propagate the rcvbuf changes to all the subflows */
+	WRITE_ONCE(sk->sk_rcvbuf, space);
+	mptcp_for_each_subflow(mptcp_sk(sk), subflow) {
+		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+		bool slow;
+
+		slow = lock_sock_fast(ssk);
+		WRITE_ONCE(ssk->sk_rcvbuf, space);
+		tcp_sk(ssk)->window_clamp = val;
+		unlock_sock_fast(ssk, slow);
 	}
+	return 0;
 }
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 9c1f8d1d63d2..2b43577f952e 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -421,6 +421,7 @@ static bool subflow_use_different_dport(struct mptcp_sock *msk, const struct soc
 
 void __mptcp_set_connected(struct sock *sk)
 {
+	__mptcp_propagate_sndbuf(sk, mptcp_sk(sk)->first);
 	if (sk->sk_state == TCP_SYN_SENT) {
 		inet_sk_state_store(sk, TCP_ESTABLISHED);
 		sk->sk_state_change(sk);
@@ -472,7 +473,6 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 		return;
 
 	msk = mptcp_sk(parent);
-	mptcp_propagate_sndbuf(parent, sk);
 	subflow->rel_write_seq = 1;
 	subflow->conn_finished = 1;
 	subflow->ssn_offset = TCP_SKB_CB(skb)->seq;
@@ -1237,7 +1237,7 @@ static bool subflow_check_data_avail(struct sock *ssk)
 	struct sk_buff *skb;
 
 	if (!skb_peek(&ssk->sk_receive_queue))
-		WRITE_ONCE(subflow->data_avail, MPTCP_SUBFLOW_NODATA);
+		WRITE_ONCE(subflow->data_avail, false);
 	if (subflow->data_avail)
 		return true;
 
@@ -1271,7 +1271,7 @@ static bool subflow_check_data_avail(struct sock *ssk)
 			continue;
 		}
 
-		WRITE_ONCE(subflow->data_avail, MPTCP_SUBFLOW_DATA_AVAIL);
+		WRITE_ONCE(subflow->data_avail, true);
 		break;
 	}
 	return true;
@@ -1293,7 +1293,7 @@ fallback:
 				goto reset;
 			}
 			mptcp_subflow_fail(msk, ssk);
-			WRITE_ONCE(subflow->data_avail, MPTCP_SUBFLOW_DATA_AVAIL);
+			WRITE_ONCE(subflow->data_avail, true);
 			return true;
 		}
 
@@ -1310,7 +1310,7 @@ reset:
 			while ((skb = skb_peek(&ssk->sk_receive_queue)))
 				sk_eat_skb(ssk, skb);
 			tcp_send_active_reset(ssk, GFP_ATOMIC);
-			WRITE_ONCE(subflow->data_avail, MPTCP_SUBFLOW_NODATA);
+			WRITE_ONCE(subflow->data_avail, false);
 			return false;
 		}
 
@@ -1322,7 +1322,7 @@ reset:
 	subflow->map_seq = READ_ONCE(msk->ack_seq);
 	subflow->map_data_len = skb->len;
 	subflow->map_subflow_seq = tcp_sk(ssk)->copied_seq - subflow->ssn_offset;
-	WRITE_ONCE(subflow->data_avail, MPTCP_SUBFLOW_DATA_AVAIL);
+	WRITE_ONCE(subflow->data_avail, true);
 	return true;
 }
 
@@ -1334,7 +1334,7 @@ bool mptcp_subflow_data_available(struct sock *sk)
 	if (subflow->map_valid &&
 	    mptcp_subflow_get_map_offset(subflow) >= subflow->map_data_len) {
 		subflow->map_valid = 0;
-		WRITE_ONCE(subflow->data_avail, MPTCP_SUBFLOW_NODATA);
+		WRITE_ONCE(subflow->data_avail, false);
 
 		pr_debug("Done with mapping: seq=%u data_len=%u",
 			 subflow->map_subflow_seq,
@@ -1405,10 +1405,18 @@ static void subflow_data_ready(struct sock *sk)
 	WARN_ON_ONCE(!__mptcp_check_fallback(msk) && !subflow->mp_capable &&
 		     !subflow->mp_join && !(state & TCPF_CLOSE));
 
-	if (mptcp_subflow_data_available(sk))
+	if (mptcp_subflow_data_available(sk)) {
 		mptcp_data_ready(parent, sk);
-	else if (unlikely(sk->sk_err))
+
+		/* subflow-level lowat test are not relevant.
+		 * respect the msk-level threshold eventually mandating an immediate ack
+		 */
+		if (mptcp_data_avail(msk) < parent->sk_rcvlowat &&
+		    (tcp_sk(sk)->rcv_nxt - tcp_sk(sk)->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss)
+			inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
+	} else if (unlikely(sk->sk_err)) {
 		subflow_error_report(sk);
+	}
 }
 
 static void subflow_write_space(struct sock *ssk)
@@ -1525,8 +1533,6 @@ int __mptcp_subflow_connect(struct sock *sk, const struct mptcp_addr_info *loc,
 	if (addr.ss_family == AF_INET6)
 		addrlen = sizeof(struct sockaddr_in6);
 #endif
-	mptcp_sockopt_sync(msk, ssk);
-
 	ssk->sk_bound_dev_if = ifindex;
 	err = kernel_bind(sf, (struct sockaddr *)&addr, addrlen);
 	if (err)
@@ -1637,7 +1643,7 @@ int mptcp_subflow_create_socket(struct sock *sk, unsigned short family,
 
 	err = security_mptcp_add_subflow(sk, sf->sk);
 	if (err)
-		goto release_ssk;
+		goto err_free;
 
 	/* the newly created socket has to be in the same cgroup as its parent */
 	mptcp_attach_cgroup(sk, sf->sk);
@@ -1651,15 +1657,12 @@ int mptcp_subflow_create_socket(struct sock *sk, unsigned short family,
 	get_net_track(net, &sf->sk->ns_tracker, GFP_KERNEL);
 	sock_inuse_add(net, 1);
 	err = tcp_set_ulp(sf->sk, "mptcp");
+	if (err)
+		goto err_free;
 
-release_ssk:
+	mptcp_sockopt_sync_locked(mptcp_sk(sk), sf->sk);
 	release_sock(sf->sk);
 
-	if (err) {
-		sock_release(sf);
-		return err;
-	}
-
 	/* the newly created socket really belongs to the owning MPTCP master
 	 * socket, even if for additional subflows the allocation is performed
 	 * by a kernel workqueue. Adjust inode references, so that the
@@ -1679,6 +1682,11 @@ release_ssk:
 	mptcp_subflow_ops_override(sf->sk);
 
 	return 0;
+
+err_free:
+	release_sock(sf->sk);
+	sock_release(sf);
+	return err;
 }
 
 static struct mptcp_subflow_context *subflow_create_ctx(struct sock *sk,
@@ -1728,7 +1736,6 @@ static void subflow_state_change(struct sock *sk)
 
 	msk = mptcp_sk(parent);
 	if (subflow_simultaneous_connect(sk)) {
-		mptcp_propagate_sndbuf(parent, sk);
 		mptcp_do_fallback(sk);
 		mptcp_rcv_space_init(msk, sk);
 		pr_fallback(msk);