mptcp: fix spurious wake-up on under memory pressure

JIRA: https://issues.redhat.com/browse/RHEL-84571
Upstream Status: net.git commit e226d92

commit e226d92
Author: Paolo Abeni <pabeni@redhat.com>
Date:   Mon Jan 13 16:44:57 2025 +0100

    mptcp: fix spurious wake-up on under memory pressure

    The wake-up condition currently implemented by mptcp_epollin_ready()
    is wrong, as it could mark the MPTCP socket as readable even when
    no data are present and the system is under memory pressure.

    Explicitly check for some data being available in the receive queue.

    Fixes: 5684ab1 ("mptcp: give rcvlowat some love")
    Cc: stable@vger.kernel.org
    Signed-off-by: Paolo Abeni <pabeni@redhat.com>
    Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
    Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
    Link: https://patch.msgid.link/20250113-net-mptcp-connect-st-flakes-v1-2-0d986ee7b1b6@kernel.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Davide Caratti <dcaratti@redhat.com>

Author: dcaratti

net/mptcp/protocol.h

@@ -739,10 +739,15 @@ static inline u64 mptcp_data_avail(const struct mptcp_sock *msk)
 
 static inline bool mptcp_epollin_ready(const struct sock *sk)
 {
+	u64 data_avail = mptcp_data_avail(mptcp_sk(sk));
+
+	if (!data_avail)
+		return false;
+
 	/* mptcp doesn't have to deal with small skbs in the receive queue,
-	 * at it can always coalesce them
+	 * as it can always coalesce them
 	 */
-	return (mptcp_data_avail(mptcp_sk(sk)) >= sk->sk_rcvlowat) ||
+	return (data_avail >= sk->sk_rcvlowat) ||
 	       (mem_cgroup_sockets_enabled && sk->sk_memcg &&
 		mem_cgroup_under_socket_pressure(sk->sk_memcg)) ||
 	       READ_ONCE(tcp_memory_pressure);