netdev - Re: [bpf PATCH 1/2] tls: possible hang when do_tcp

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Date:   Wed, 22 Aug 2018 11:22:25 -0700
From:   Dave Watson <davejwatson@...com>
To:     John Fastabend <john.fastabend@...il.com>
CC:     <ast@...nel.org>, <daniel@...earbox.net>, <netdev@...r.kernel.org>,
        <davem@...emloft.net>
Subject: Re: [bpf PATCH 1/2] tls: possible hang when do_tcp_sendpages hits
 sndbuf is full case

On 08/22/18 08:37 AM, John Fastabend wrote:
> 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@...il.com>

Super, thanks!

Acked-by: Dave Watson <davejwatson@...com>