[PATCH] net/tls: avoid TCP window full during ->read_sock()
Sagi Grimberg
sagi at grimberg.me
Mon Aug 7 00:08:50 PDT 2023
>> 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?
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