[Cluster-devel] [PATCHv3 dlm/next 7/8] fs: dlm: add reliable connection if reconnect

[Alexander Ahring Oder Aring <aahringo@redhat.com>]

Hi,

On Sat, Apr 3, 2021 at 11:34 AM Alexander Ahring Oder Aring
<aahringo@redhat.com> wrote:
>
...
>
> > It seems to me that the only time DLM might need to retransmit data, is
> > when recovering from a connection failure. So why can't we just resend
> > unacknowledged data at reconnection time? That'd probably simplify the
> > code a lot (no need to maintain a retransmission timeout on TX, no need
> > to handle sequence numbers that are in the future on RX).
> >
>
> I can try to remove the timer, timeout and do the above approach to
> retransmit at reconnect. Then I test it again and I will report back
> to see if it works or why we have other problems.
>

I have an implementation of this running and so far I don't see any problems.

> > Also, couldn't we set the DLM sequence numbers in
> > dlm_midcomms_commit_buffer_3_2() rather than using a callback function
> > in dlm_lowcomms_new_buffer()?
> >
...
>
> Yes, I looked into TCP_REPAIR at first and I agree it can be used to
> solve this problem. However TCP_REPAIR can be used as a part of a more
> generic solution, there needs to be something "additional handling"
> done e.g. additional socket options to let the application layer save
> states before receiving errors. I am also concerned how it would work

The code [0] is what I meant above. It will call
tcp_write_queue_purge(); before reporting the error over error
queue/callback. That need to be handled differently to allow dumping
the actual TCP state and restore at reconnect, at least that is what I
have in my mind.

- Alex

[0] https://elixir.bootlin.com/linux/v5.12-rc6/source/net/ipv4/tcp_input.c#L4239