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

[Guillaume Nault <gnault@redhat.com>]

On Mon, Apr 05, 2021 at 01:33:48PM -0400, Alexander Ahring Oder Aring wrote:
> 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.

Thanks. That's not usable as is, indeed.
Also, by retransmitting data from the previous send-queue, we risk
resending messages that the peer already received (for example because
the previous connection didn't receive the latest ACKs). I guess that
receiving the same DLM messages twice is going to confuse the peer.
So it looks like we'll need application level sequence numbers anyway.

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