Re: [PATCH] treewide: By default, don't quit source after migration, keep sockets open

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Jul 23, 2025 at 11:17:08AM +0200, Stefano Brivio wrote:
> On Wed, 23 Jul 2025 10:27:30 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> > On Tue, Jul 22, 2025 at 11:12:49PM +0200, Stefano Brivio wrote:
[snip]
> > > > I think a preferable approach would be to EPOLL_CTL_DEL each socket at
> > > > the point we would close() them with --migrate-no-linger.  I'd be fine
> > > > with that as a follow up improvement, though.  
> > > 
> > > I was pondering about adding this on top of the ignore_data_events
> > > trick, but, actually, the whole thing as of this patch is somewhat
> > > bogus because
> > > 
> > > - we're ignoring events on TCP sockets (intentional),
> > > 
> > > - we're ignoring events on the tap device (who cares, migration is only
> > >   supported with vhost-user)
> > > 
> > > - but *not* ignoring events on the vhost-user kick descriptor
> > >   (oversight).
> > > 
> > > On a second thought, it doesn't look safe to ignore events on the kick
> > > descriptor, and in any case, with this change, we don't want to prevent
> > > the guest to send out further packets. It's not expected anyway.
> > > 
> > > So I just replaced the whole thing with EPOLL_CTL_DEL (epoll_del()) as
> > > we go through the sockets. It's simpler and arguably safer.  
> > 
> > Yes, that's what I had in mind as well (I thought I put that in the
> > mail, but it looks like I didn't).  Just one additional concern that I
> > don't think need hold up merge: do we also need to epoll_del() our
> > listening sockets?
> 
> Right, I had that thought as well, and this was somewhat covered by the
> first version because we'd ignore events on those. But that would still
> come with the risk of epoll_wait() loops.

Exactly.

> In the perspective of a simple implementation / fix mostly intended for
> KubeVirt such as this one, we know that the guest is suspended at that
> point, so we'll send a SYN to it, nothing comes back, we'll eventually
> time out in 10 seconds and try to reset the connection (in the unlikely
> case we're still running *and* getting traffic at that point).

Hrm.  Maybe.  Wouldn't surprise me if there are still some weird edge
cases in her, but, yes, I suspect they're not a huge deal.

> And I guess that's fine because if we're still running and getting
> traffic after that timeout something must have gone wrong, so sending a
> RST segment doesn't look that bad to me. If the connection was meanwhile
> established with the target node, I think our RST segment won't actually
> reset the connection because sequence numbers won't match.
> 
> But surely it's not elegant and I think we should eventually have an
> explicit implementation of the whole thing, perhaps with a new socket
> state ("MIGRATED"?) and going through the whole list of listening
> sockets and... closing them, I suppose?

Right.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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