mptcp: implement TCP_NOTSENT_LOWAT support

Add support for such socket option storing the user-space provided
value in a new msk field, and using such data to implement the
_mptcp_stream_memory_free() helper, similar to the TCP one.

To avoid adding more indirect calls in the fast path, open-code
a variant of sk_stream_memory_free() in mptcp_sendmsg() and add
direct calls to the mptcp stream memory free helper where possible.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/464
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -1762,6 +1762,30 @@ static int do_copy_data_nocache(struct sock *sk, int copy,
 	return 0;
 }
 
+/* open-code sk_stream_memory_free() plus sent limit computation to
+ * avoid indirect calls in fast-path.
+ * Called under the msk socket lock, so we can avoid a bunch of ONCE
+ * annotations.
+ */
+static u32 mptcp_send_limit(const struct sock *sk)
+{
+	const struct mptcp_sock *msk = mptcp_sk(sk);
+	u32 limit, not_sent;
+
+	if (sk->sk_wmem_queued >= READ_ONCE(sk->sk_sndbuf))
+		return 0;
+
+	limit = mptcp_notsent_lowat(sk);
+	if (limit == UINT_MAX)
+		return UINT_MAX;
+
+	not_sent = msk->write_seq - msk->snd_nxt;
+	if (not_sent >= limit)
+		return 0;
+
+	return limit - not_sent;
+}
+
 static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 {
 	struct mptcp_sock *msk = mptcp_sk(sk);
@@ -1806,16 +1830,19 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		struct mptcp_data_frag *dfrag;
 		bool dfrag_collapsed;
 		size_t psize, offset;
+		u32 copy_limit;
+
+		/* ensure fitting the notsent_lowat() constraint */
+		copy_limit = mptcp_send_limit(sk);
+		if (!copy_limit)
+			goto wait_for_memory;
 
 		/* reuse tail pfrag, if possible, or carve a new one from the
 		 * page allocator
 		 */
 		dfrag = mptcp_pending_tail(sk);
 		dfrag_collapsed = mptcp_frag_can_collapse_to(msk, pfrag, dfrag);
 		if (!dfrag_collapsed) {
-			if (!sk_stream_memory_free(sk))
-				goto wait_for_memory;
-
 			if (!mptcp_page_frag_refill(sk, pfrag))
 				goto wait_for_memory;
 
@@ -1830,6 +1857,7 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		offset = dfrag->offset + dfrag->data_len;
 		psize = pfrag->size - offset;
 		psize = min_t(size_t, psize, msg_data_left(msg));
+		psize = min_t(size_t, psize, copy_limit);
 		total_ts = psize + frag_truesize;
 
 		if (!sk_wmem_schedule(sk, total_ts))
@@ -3760,6 +3788,7 @@ static struct proto mptcp_prot = {
 	.unhash		= mptcp_unhash,
 	.get_port	= mptcp_get_port,
 	.forward_alloc_get	= mptcp_forward_alloc_get,
+	.stream_memory_free	= mptcp_stream_memory_free,
 	.sockets_allocated	= &mptcp_sockets_allocated,
 
 	.memory_allocated	= &tcp_memory_allocated,
@@ -3933,12 +3962,12 @@ static __poll_t mptcp_check_writeable(struct mptcp_sock *msk)
 {
 	struct sock *sk = (struct sock *)msk;
 
-	if (sk_stream_is_writeable(sk))
+	if (__mptcp_stream_is_writeable(sk, 1))
 		return EPOLLOUT | EPOLLWRNORM;
 
 	set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
 	smp_mb__after_atomic(); /* NOSPACE is changed by mptcp_write_space() */
-	if (sk_stream_is_writeable(sk))
+	if (__mptcp_stream_is_writeable(sk, 1))
 		return EPOLLOUT | EPOLLWRNORM;
 
 	return 0;

net/mptcp/protocol.h

@@ -307,6 +307,7 @@ struct mptcp_sock {
 			in_accept_queue:1,
 			free_first:1,
 			rcvspace_init:1;
+	u32		notsent_lowat;
 	struct work_struct work;
 	struct sk_buff  *ooo_last_skb;
 	struct rb_root  out_of_order_queue;
@@ -807,11 +808,36 @@ static inline bool mptcp_data_fin_enabled(const struct mptcp_sock *msk)
 	       READ_ONCE(msk->write_seq) == READ_ONCE(msk->snd_nxt);
 }
 
+static inline u32 mptcp_notsent_lowat(const struct sock *sk)
+{
+	struct net *net = sock_net(sk);
+	u32 val;
+
+	val = READ_ONCE(mptcp_sk(sk)->notsent_lowat);
+	return val ?: READ_ONCE(net->ipv4.sysctl_tcp_notsent_lowat);
+}
+
+static inline bool mptcp_stream_memory_free(const struct sock *sk, int wake)
+{
+	const struct mptcp_sock *msk = mptcp_sk(sk);
+	u32 notsent_bytes;
+
+	notsent_bytes = READ_ONCE(msk->write_seq) - READ_ONCE(msk->snd_nxt);
+	return (notsent_bytes << wake) < mptcp_notsent_lowat(sk);
+}
+
+static inline bool __mptcp_stream_is_writeable(const struct sock *sk, int wake)
+{
+	return mptcp_stream_memory_free(sk, wake) &&
+	       __sk_stream_is_writeable(sk, wake);
+}
+
 static inline void mptcp_write_space(struct sock *sk)
 {
 	/* pairs with memory barrier in mptcp_poll */
 	smp_mb();
-	sk_stream_write_space(sk);
+	if (mptcp_stream_memory_free(sk, 1))
+		sk_stream_write_space(sk);
 }
 
 static inline void __mptcp_sync_sndbuf(struct sock *sk)

net/mptcp/sockopt.c

@@ -812,6 +812,16 @@ static int mptcp_setsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 		return 0;
 	case TCP_ULP:
 		return -EOPNOTSUPP;
+	case TCP_NOTSENT_LOWAT:
+		ret = mptcp_get_int_option(msk, optval, optlen, &val);
+		if (ret)
+			return ret;
+
+		lock_sock(sk);
+		WRITE_ONCE(msk->notsent_lowat, val);
+		mptcp_write_space(sk);
+		release_sock(sk);
+		return 0;
 	case TCP_CONGESTION:
 		return mptcp_setsockopt_sol_tcp_congestion(msk, optval, optlen);
 	case TCP_CORK:
@@ -1345,6 +1355,8 @@ static int mptcp_getsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 		return mptcp_put_int_option(msk, optval, optlen, msk->cork);
 	case TCP_NODELAY:
 		return mptcp_put_int_option(msk, optval, optlen, msk->nodelay);
+	case TCP_NOTSENT_LOWAT:
+		return mptcp_put_int_option(msk, optval, optlen, msk->notsent_lowat);
 	}
 	return -EOPNOTSUPP;
 }