[PATCH net 5/7] mptcp: refresh tcp rcv_wnd snapshot when syncing receive windows

[Wesley Atwell <atwellwea@gmail.com>]

MPTCP rewrites the TCP shadow receive window on subflows when shared
receive-window state changes.

Once tp->rcv_wnd carries paired snapshot semantics, those subflow shadow
updates have to refresh the snapshot too. Convert the MPTCP window-sync
write sites to use the helper and keep the aggregate receive-space
arithmetic using the explicit rwnd-availability helper.

Signed-off-by: Wesley Atwell <atwellwea@gmail.com>
---
 net/mptcp/options.c  | 12 ++++++++----
 net/mptcp/protocol.h | 14 +++++++++++---
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 43df4293f58b..6e6aa084cbfa 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -1073,9 +1073,12 @@ static void rwin_update(struct mptcp_sock *msk, struct sock *ssk,
 		return;
 
 	/* Some other subflow grew the mptcp-level rwin since rcv_wup,
-	 * resync.
+	 * resync. Keep the TCP shadow window in its advertised u32 domain
+	 * and refresh the advertise-time scaling snapshot while doing so.
 	 */
-	tp->rcv_wnd += mptcp_rcv_wnd - subflow->rcv_wnd_sent;
+	tcp_set_rcv_wnd(tp, min_t(u64, (u64)tp->rcv_wnd +
+				  (mptcp_rcv_wnd - subflow->rcv_wnd_sent),
+				  U32_MAX));
 	subflow->rcv_wnd_sent = mptcp_rcv_wnd;
 }
 
@@ -1334,11 +1337,12 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th)
 	if (rcv_wnd_new != rcv_wnd_old) {
 raise_win:
 		/* The msk-level rcv wnd is after the tcp level one,
-		 * sync the latter.
+		 * sync the latter and refresh its advertise-time scaling
+		 * snapshot.
 		 */
 		rcv_wnd_new = rcv_wnd_old;
 		win = rcv_wnd_old - ack_seq;
-		tp->rcv_wnd = min_t(u64, win, U32_MAX);
+		tcp_set_rcv_wnd(tp, min_t(u64, win, U32_MAX));
 		new_win = tp->rcv_wnd;
 
 		/* Make sure we do not exceed the maximum possible
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 0bd1ee860316..4ea95c9c0c7a 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -408,11 +408,19 @@ static inline int mptcp_space_from_win(const struct sock *sk, int win)
 	return __tcp_space_from_win(mptcp_sk(sk)->scaling_ratio, win);
 }
 
+/* MPTCP exposes window space from the mptcp-level receive queue, so it tracks
+ * a separate backlog counter from the subflow backlog embedded in struct sock.
+ */
+static inline int mptcp_rwnd_avail(const struct sock *sk)
+{
+	return READ_ONCE(sk->sk_rcvbuf) -
+	       READ_ONCE(mptcp_sk(sk)->backlog_len) -
+	       tcp_rmem_used(sk);
+}
+
 static inline int __mptcp_space(const struct sock *sk)
 {
-	return mptcp_win_from_space(sk, READ_ONCE(sk->sk_rcvbuf) -
-				    READ_ONCE(mptcp_sk(sk)->backlog_len) -
-				    sk_rmem_alloc_get(sk));
+	return mptcp_win_from_space(sk, mptcp_rwnd_avail(sk));
 }
 
 static inline struct mptcp_data_frag *mptcp_send_head(const struct sock *sk)
-- 
2.34.1