Re: [Qemu-devel] [RFC 19/29] migration: let dst listen on port always

[Peter Xu]

On Tue, Aug 01, 2017 at 11:56:10AM +0100, Daniel P. Berrange wrote:
> On Fri, Jul 28, 2017 at 04:06:28PM +0800, Peter Xu wrote:
> > Signed-off-by: Peter Xu <address@hidden>
> > ---
> >  migration/exec.c   | 2 +-
> >  migration/fd.c     | 2 +-
> >  migration/socket.c | 4 ++--
> >  3 files changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/migration/exec.c b/migration/exec.c
> > index 08b599e..b4412db 100644
> > --- a/migration/exec.c
> > +++ b/migration/exec.c
> > @@ -49,7 +49,7 @@ static gboolean exec_accept_incoming_migration(QIOChannel 
> > *ioc,
> >  {
> >      migration_channel_process_incoming(ioc);
> >      object_unref(OBJECT(ioc));
> > -    return FALSE; /* unregister */
> > +    return TRUE; /* keep it registered */
> >  }
> >  
> >  void exec_start_incoming_migration(const char *command, Error **errp)
> > diff --git a/migration/fd.c b/migration/fd.c
> > index 30f5258..865277a 100644
> > --- a/migration/fd.c
> > +++ b/migration/fd.c
> > @@ -49,7 +49,7 @@ static gboolean fd_accept_incoming_migration(QIOChannel 
> > *ioc,
> >  {
> >      migration_channel_process_incoming(ioc);
> >      object_unref(OBJECT(ioc));
> > -    return FALSE; /* unregister */
> > +    return TRUE; /* keep it registered */
> >  }
> >  
> >  void fd_start_incoming_migration(const char *infd, Error **errp)
> > diff --git a/migration/socket.c b/migration/socket.c
> > index 757d382..f2c2d01 100644
> > --- a/migration/socket.c
> > +++ b/migration/socket.c
> > @@ -153,8 +153,8 @@ static gboolean 
> > socket_accept_incoming_migration(QIOChannel *ioc,
> >  
> >  out:
> >      /* Close listening socket as its no longer needed */
> > -    qio_channel_close(ioc, NULL);
> > -    return FALSE; /* unregister */
> > +    // qio_channel_close(ioc, NULL);
> > +    return TRUE; /* keep it registered */
> >  }
> 
> 
> This is not a very desirable approach IMHO.
> 
> There are two separate things at play - first we have the listener socket,
> and second we have the I/O watch that monitors for incoming clients.
> 
> The current code here closes the listener, and returns FALSE to unregister
> the event loop watch.
> 
> You're reversing both of these so that we keep the listener open and we
> keep monitoring for incoming clients. Ignoring migration resume for a
> minute, this means that the destination QEMU will now accept arbitrarily
> many incoming clients and keep trying to start a new incoming migration.
> 
> The behaviour we need is diferent. We *want* to unregister the event
> loop watch once we've accepted a client. We should only keep the socket
> listener in existance, but *not* accept any more clients. Only once we
> have hit a problem and want to accept a new client to do migration
> recovery, should we be re-adding the event loop watch.

I replied with another approach in the other thread: how about we
re-enable the listen port duing "postcopy-pause" state, and disable
the listen port when get out of that migration state?

Here "listen port" I mean both the IO watch and the socket object. Now
what I can think of: we keep these objects always there, meanwhile we
introduce a new bit for migration, say, "accept_incoming", to decide
whether we will really accept one connection. Then we drop the new
connection if that bit is not set.

(Or a new QIOChannelFeature to temporarily refuse incoming
 connection? E.g., QIO_CHANNEL_FEATURE_LISTEN_REFUSE?)

Thanks,

-- 
Peter Xu