netdev - [PATCH net 2/3] mptcp: fix spurious wake-up on under memory pressure

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Message-Id: <20250113-net-mptcp-connect-st-flakes-v1-2-0d986ee7b1b6@kernel.org>
Date: Mon, 13 Jan 2025 16:44:57 +0100
From: "Matthieu Baerts (NGI0)" <matttbe@...nel.org>
To: mptcp@...ts.linux.dev, Mat Martineau <martineau@...nel.org>, 
 Geliang Tang <geliang@...nel.org>, "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
 Shuah Khan <shuah@...nel.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, 
 "Matthieu Baerts (NGI0)" <matttbe@...nel.org>, stable@...r.kernel.org
Subject: [PATCH net 2/3] mptcp: fix spurious wake-up on under memory
 pressure

From: Paolo Abeni <pabeni@...hat.com>

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")
Cc: stable@...r.kernel.org
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>
---
 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 a93e661ef5c435155066ce9cc109092661f0711c..73526f1d768fcb6ba5bcf1a43eb09ed5ff9d67bf 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