tls: possible hang when do_tcp_sendpages hits sndbuf is full case

Currently, the lower protocols sk_write_space handler is not called if
TLS is sending a scatterlist via  tls_push_sg. However, normally
tls_push_sg calls do_tcp_sendpage, which may be under memory pressure,
that in turn may trigger a wait via sk_wait_event. Typically, this
happens when the in-flight bytes exceed the sdnbuf size. In the normal
case when enough ACKs are received sk_write_space() will be called and
the sk_wait_event will be woken up allowing it to send more data
and/or return to the user.

But, in the TLS case because the sk_write_space() handler does not
wake up the events the above send will wait until the sndtimeo is
exceeded. By default this is MAX_SCHEDULE_TIMEOUT so it look like a
hang to the user (especially this impatient user). To fix this pass
the sk_write_space event to the lower layers sk_write_space event
which in the TCP case will wake any pending events.

I observed the above while integrating sockmap and ktls. It
initially appeared as test_sockmap (modified to use ktls) occasionally
hanging. To reliably reproduce this reduce the sndbuf size and stress
the tls layer by sending many 1B sends. This results in every byte
needing a header and each byte individually being sent to the crypto
layer.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Dave Watson <davejwatson@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/tls/tls_main.c

@@ -213,9 +213,14 @@ static void tls_write_space(struct sock *sk)
 {
 	struct tls_context *ctx = tls_get_ctx(sk);
 
-	/* We are already sending pages, ignore notification */
-	if (ctx->in_tcp_sendpages)
+	/* If in_tcp_sendpages call lower protocol write space handler
+	 * to ensure we wake up any waiting operations there. For example
+	 * if do_tcp_sendpages where to call sk_wait_event.
+	 */
+	if (ctx->in_tcp_sendpages) {
+		ctx->sk_write_space(sk);
 		return;
+	}
 
 	if (!sk->sk_write_pending && tls_is_pending_closed_record(ctx)) {
 		gfp_t sk_allocation = sk->sk_allocation;