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

From: Jakub Kicinski <[email-masked]>
Date: Fri, 4 Aug 2023 17:57:00 -0700

On Thu,  3 Aug 2023 12:08:09 +0200 Hannes Reinecke wrote:
> 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.

> -			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 */

> +			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).

side note - I still prefer 80 char max lines, please. It seems to result
in prettier code ovarall as it forces people to think more about code
structure.

>  		if (used < rxm->full_len) {
>  			rxm->offset += used;
>  			rxm->full_len -= used;
-- 
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