netdev - Re: [PATCH] net/tls: avoid TCP window full during ->read

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Message-ID: <5e04afe0-7bf3-1bdf-f4f1-49b0c7bb5dba@grimberg.me>
Date: Mon, 7 Aug 2023 10:08:50 +0300
From: Sagi Grimberg <sagi@...mberg.me>
To: Jakub Kicinski <kuba@...nel.org>, Hannes Reinecke <hare@...e.de>
Cc: Christoph Hellwig <hch@....de>, Keith Busch <kbusch@...nel.org>,
 linux-nvme@...ts.infradead.org, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] net/tls: avoid TCP window full during ->read_sock()


>> 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).

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?