mptcp: allocate fwd memory separately on the rx and tx path

All the mptcp receive path is protected by the msk socket
spinlock. As consequences, the tx path has to play a few tricks to
allocate the forward memory without acquiring the spinlock multiple
times, making the overall TX path quite complex.

This patch tries to clean-up a bit the tx path, using completely
separated fwd memory allocation, for the rx and the tx path.

The forward memory allocated in the rx path is now accounted in
msk->rmem_fwd_alloc and is (still) protected by the msk socket spinlock.

To cope with the above we provide a few MPTCP-specific variants for
the helpers to charge, uncharge, reclaim and free the forward memory
in the receive path.

msk->sk_forward_alloc now accounts only the forward memory for the tx
path, we can use the plain core sock helper to manipulate it and drop
quite a bit of complexity.

On memory pressure, both rx and tx fwd memories are reclaimed.
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.h

@@ -227,7 +227,7 @@ struct mptcp_sock {
 	u64		ack_seq;
 	u64		rcv_wnd_sent;
 	u64		rcv_data_fin_seq;
-	int		wmem_reserved;
+	int		rmem_fwd_alloc;
 	struct sock	*last_snd;
 	int		snd_burst;
 	int		old_wspace;
@@ -272,19 +272,6 @@ struct mptcp_sock {
 	char		ca_name[TCP_CA_NAME_MAX];
 };
 
-#define mptcp_lock_sock(___sk, cb) do {					\
-	struct sock *__sk = (___sk); /* silence macro reuse warning */	\
-	might_sleep();							\
-	spin_lock_bh(&__sk->sk_lock.slock);				\
-	if (__sk->sk_lock.owned)					\
-		__lock_sock(__sk);					\
-	cb;								\
-	__sk->sk_lock.owned = 1;					\
-	spin_unlock(&__sk->sk_lock.slock);				\
-	mutex_acquire(&__sk->sk_lock.dep_map, 0, 0, _RET_IP_);		\
-	local_bh_enable();						\
-} while (0)
-
 #define mptcp_data_lock(sk) spin_lock_bh(&(sk)->sk_lock.slock)
 #define mptcp_data_unlock(sk) spin_unlock_bh(&(sk)->sk_lock.slock)