Re: [PATCH v3] treewide: By default, don't quit source after migration, keep sockets open

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 25 Jul 2025 14:04:17 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Thu, Jul 24, 2025 at 07:28:58PM +0200, Stefano Brivio wrote:
> > We are hitting an issue in the KubeVirt integration where some data is
> > still sent to the source instance even after migration is complete. As
> > we exit, the kernel closes our sockets and resets connections. The
> > resulting RST segments are sent to peers, effectively terminating
> > connections that were meanwhile migrated.
> > 
> > At the moment, this is not done intentionally, but in the future
> > KubeVirt might enable OVN-Kubernetes features where source and
> > destination nodes are explicitly getting mirrored traffic for a while,
> > in order to decrease migration downtime.
> > 
> > By default, don't quit after migration is completed on the source: the
> > previous behaviour can be enabled with the new, but deprecated,
> > --migrate-exit option. After migration (as source), the -1 / --one-off
> > option has no effect.
> > 
> > Also, by default, keep migrated TCP sockets open (in repair mode) as
> > long as we're running, and ignore events on any epoll descriptor
> > representing data channels. The previous behaviour can be enabled with
> > the new, equally deprecated, --migrate-no-linger option.
> > 
> > By keeping sockets open, and not exiting, we prevent the kernel
> > running on the source node to send out RST segments if further data
> > reaches us.
> > 
> > Reported-by: Nir Dothan <ndothan@redhat.com>
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> > ---
> > v2:
> >   - assorted changes in commit message
> >   - context variable ignore_linger becomes ignore_no_linger
> >   - new options are deprecated
> >   - don't ignore events on some descriptors, drop them from epoll
> > 
> > v3:
> >   - Nir reported occasional failures (connections being reset)
> >     with both v1 and v2, because, in KubeVirt's usage, we quit as
> >     QEMU exits. Disable --one-off after migration as source, and
> >     document this exception  
> 
> This seems like an awful, awful hack.

Well, of course, it is, and long term it should be fixed in
either KubeVirt or libvirt (even though I'm not sure how, see below)
instead.

> We're abandoning consistent
> semantics on a wild guess as to what the layers above us need.

No, not really, we tested this and tested the alternative.

> Specifically, --once-off used to mean that the layer above us didn't

--one-off

> need to manage passt's lifetime; it was tied to qemu's.  Now it still
> needs to manually manage passt's lifetime, so what's the point.  So,
> if it needs passt to outlive qemu it should actually manage that and
> not use --once-off.

The main point is that it does *not* manually manage passt's lifetime
if there's no migration (which is the general case for libvirt and all
other users).

We don't have any other user with an implementation of the migration
workflow anyway (libvirt itself doesn't do that, yet). It's otherwise
unusable for KubeVirt. So I'd say let's fix it for the only user we
have.

> Requring passt to outlive qemu already seems pretty dubious to me:
> having the source still connected when passt was quitting is one thing
> - indeed it's arguably hard to avoid.  Having it still connected when
> *qemu* quits is much less defensible.

The fundamental problem here is that there's an issue in KubeVirt
(and working around it is the whole point of this patch) which implies
that packets are sent to the source pod *for a while* after migration.

We found out that the guest is generally suspended during that while,
but sometimes it might even have already exited. The pod remains,
though, as long as it's needed. That's the only certainty we have.

So, do we want to drop --one-off from the libvirt integration, and have
libvirt manage passt's lifecycle entirely (note that all users outside
KubeVirt don't use migration, so we would make the general case vastly
more complicated for the sake of correctness for a single usage...)?

Well, we can try to do that. Except that libvirt doesn't know either
for how long this traffic will reach the source pod (that's a KubeVirt
concept). So it should implement the same hack: let it outlive QEMU on
migration... as long as we have that issue in KubeVirt.

But I asked KubeVirt people, and it turns out that it's extremely
complicated to fix this in KubeVirt. So, actually, I don't see another
way to fix this in the short term. And without KubeVirt using this we
could also drop the whole feature...

-- 
Stefano