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: 5684ab1a0eff ("mptcp: give rcvlowat some love")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/protocol.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index a93e661ef5c4..73526f1d768f 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -760,10 +760,15 @@ 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);
-- 
2.47.1