netdev - [PATCH net] tcp: consistently disable header prediction for mptcp

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Message-Id: <7f941e06e6434902ea4a0a572392f65cd2745447.1625053058.git.pabeni@redhat.com>
Date:   Wed, 30 Jun 2021 13:42:13 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Davide Caratti <dcaratti@...hat.com>,
        Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        mptcp@...ts.linux.dev
Subject: [PATCH net] tcp: consistently disable header prediction for mptcp

The MPTCP receive path is hooked only into the TCP slow-path.
The DSS presence allows plain MPTCP traffic to hit that
consistently.

Since commit e1ff9e82e2ea ("net: mptcp: improve fallback to TCP"),
when an MPTCP socket falls back to TCP, it can hit the TCP receive
fast-path, and delay or stop triggering the event notification.

Address the issue explicitly disabling the header prediction
for MPTCP sockets.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/200
Fixes: e1ff9e82e2ea ("net: mptcp: improve fallback to TCP")
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
Notes:
- I'm unable to disable header prediction consistently for MPTCP
  sockets touching only MPTCP code
- possible alternatives could be:
  - hook MPTCP in the TCP fastpath, too
  - try to pull again commit 45f119bf936b ("tcp: remove header prediction")
    avoiding the regression noded in 31770e34e43d ("tcp: Revert "tcp: remove
    header prediction"")
  I choose this option as the hopefully less invasive one.
---
 include/net/tcp.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index d05193cb0d99..b42b3e6731ed 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -682,6 +682,10 @@ static inline u32 __tcp_set_rto(const struct tcp_sock *tp)
 
 static inline void __tcp_fast_path_on(struct tcp_sock *tp, u32 snd_wnd)
 {
+	/* mptcp hooks are only on the slow path */
+	if (sk_is_mptcp((struct sock *)tp))
+		return;
+
 	tp->pred_flags = htonl((tp->tcp_header_len << 26) |
 			       ntohl(TCP_FLAG_ACK) |
 			       snd_wnd);
-- 
2.26.3