Re: [PATCH] net/tls: avoid TCP window full during ->read_sock()

quoted

When flushing the backlog after decoding each record in ->read_sock()
we may end up with really long records, causing a TCP window full as
the TCP window would only be increased again after we process the
record. So we should rather process the record first to allow the
TCP window to be increased again before flushing the backlog.

quoted

-			released = tls_read_flush_backlog(sk, prot, rxm->full_len, to_decrypt,
-							  decrypted, &flushed_at);
  			skb = darg.skb;
+			/* TLS 1.3 may have updated the length by more than overhead */

quoted

+			rxm = strp_msg(skb);
+			tlm = tls_msg(skb);
  			decrypted += rxm->full_len;
  
  			tls_rx_rec_done(ctx);

@@ -2280,6 +2275,12 @@ int tls_sw_read_sock(struct sock *sk, read_descriptor_t *desc,
  			goto read_sock_requeue;
  		}
  		copied += used;
+		/*
+		 * flush backlog after processing the TLS record, otherwise we might
+		 * end up with really large records and triggering a TCP window full.
+		 */
+		released = tls_read_flush_backlog(sk, prot, decrypted - copied, decrypted,
+						  copied, &flushed_at);

I'm surprised moving the flushing out makes a difference.
rx_list should generally hold at most 1 skb (16kB) unless something
is PEEKing the data.

Looking at it closer I think the problem may be calling args to
tls_read_flush_backlog(). Since we don't know how much data
reader wants we can't sensibly evaluate the first condition,
so how would it work if instead of this patch we did:

-			released = tls_read_flush_backlog(sk, prot, rxm->full_len, to_decrypt,
+			released = tls_read_flush_backlog(sk, prot, INT_MAX, 0,
							  decrypted, &flushed_at);

That would give us a flush every 128k of data (or every record if
inq is shorter than 16kB).

What happens if the window is smaller than 128K ? isn't that what
Hannes is trying to solve for?

Hannes, do you have some absolute numbers to how the window behaves?