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

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 3 Feb 2025 19:52:37 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Mon, Feb 03, 2025 at 07:09:32AM +0100, Stefano Brivio wrote:
> > 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.  
> 
> By "helper" do you mean passt as a device helper to qemu, or
> passt-repair as a helper to passt.  For the latter I wouldn't expect
> so - it's only a weirdness of our situation that we need passt-repair
> at all.  If the former, I'm not really sure what you're after.

I meant passt and similar. Is there any convention we should adopt?

> > 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.  
> 
> So, I realised it's a bit more complicated than that.  We need to
> identify exactly where the "point of no return" is.  I'll discuss in
> our call tonight.

I think it's simply where we close sockets, by the way.

-- 
Stefano