Re: [Qemu-devel] [PATCH v5 26/28] migration: allow migrate_cancel to pause postcopy

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> On Tue, Dec 19, 2017 at 10:58:59AM +0000, Dr. David Alan Gilbert wrote:
> > * Peter Xu (peterx@redhat.com) wrote:
> > > It was allowed in the past to even cancel a postcopy migration, but it
> > > does not really make sense, and no one should be using it, since
> > > cancelling a migration during postcopy means crashing the VM at no time.
> > > 
> > > Let's just use re-use this command as a way to pause the postcopy
> > > migration when we detected that we are in postcopy migration.  This can
> > > be used when we want to trigger the postcopy recovery manually.
> > > 
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > 
> > Yes, OK, this is a little odd without having any flags or anything, but
> > it's essentially the saem reason that cancel exists - to stop a
> > migration we know that's broken for some reason.
> > 
> > (I could argue whether this should be special cased in migrate_fd_cancel
> > instead, but that's just a preference).
> > 
> > 
> > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> Firstly, thanks for the r-b.
> 
> Now after knowing your iptable test, I think we reached a consensus
> that we need to provide a command (for example, reuse migrate-cancel)
> to allow destination to shutdown its incoming migration port too.  At
> the same time, if we want to make sure the command can always work on
> destination, we'd better also let that command to be OOB-capable.
> 
> However I'm not sure whether I can let migrate-cancel be OOB-capable
> since recently we added bdrv_invalidate_cache_all() into
> migrate_fd_cancel().  That invalidation needs some mutex which might
> block.  I don't know whether it means migrate-cancel will no longer be
> suitable as an OOB command now.
> 
> So, maybe now I can do this: firstly, drop this patch; then add a new
> command to do the shutdown explicitly (allow either src/dst to
> shutdown its migration fd) and keep migrate-cancel untouched. In that
> case, I can make sure the new command will be OOB-compatible.
> 
> What do you think?

Yes I'm fine with that; misusing another command did feel a bit odd.

Dave

> -- 
> Peter Xu
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK