Re: [PATCH v3 17/20] vhost_user: Make source quit after reporting migration state

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 3 Feb 2025 12:55:47 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Fri, Jan 31, 2025 at 08:39:50PM +0100, Stefano Brivio wrote:
> > On migration, the source process asks passt-helper to set TCP sockets
> > in repair mode, dumps the information we need to migrate connections,
> > and closes them.
> > 
> > At this point, we can't pass them back to passt-helper using
> > SCM_RIGHTS, because they are closed, from that perspective, and
> > sendmsg() will give us EBADF. But if we don't clear repair mode, the
> > port they are bound to will not be available for binding in the
> > target.
> > 
> > Terminate once we're done with the migration and we reported the
> > state. This is equivalent to clearing repair mode on the sockets we
> > just closed.  
> 
> As noted on the passt-repair patch, I think this is based on a
> misinterpreation of the situation.  I think the problem is that the
> sockets aren't closed in passt-repair, so the additional handle copy
> is keeping the underlying socket open.  This appears to work, because
> it is causing passt-repair to also terminate.

Right, exactly that.

> That said, we probably want to terminate on the source side after a
> succesful migrate anyway.  At the very least we need to close() all
> our sockets, and delete the corresponding flows, because we don't own
> them any more.  Quitting is probably the simplest way to do that.

I'm not sure if there's an established behaviour for helpers supporting
state migration.

We could probably close sockets, delete flows, and keep things up and
running for the rest (restart from a clean situation), but at that
point we already the guest networking is already broken in a number of
ways. So, yeah, maybe let's keep this instead.

> Which also makes me realise, on a *failed* outbound migration, we _do_
> need to turn repair mode off on everything again.  Is that implemented
> yet?

No, sorry, that's the "/* TODO: rollback */" comments in
flow_migrate_source_pre(), flow.c. But other than that, it should be
pretty much implemented, at a migrate.c level.

-- 
Stefano