Re: [Qemu-devel] [PATCH] migration: regain control of images when migration fails to complete

[Kevin Wolf <kwolf@redhat.com>]

Am 18.05.2016 um 13:13 hat Dr. David Alan Gilbert geschrieben:
> * Greg Kurz (gkurz@linux.vnet.ibm.com) wrote:
> > We currently have an error path during migration that can cause
> > the source QEMU to abort:
> 
> Hmm, wasn't there something similar recently, sorry I can't remember the details, but
> cc'ing some block people who might remember.

Not sure, but in any case calling bdrv_invalidate_cache_all() in the
error case sounds right to me.

I think I would have tried to write the patch a bit differently so that
it uses the normal error handling patterns, but that doesn't make it
wrong. Specifically, checking the error condition twice in two different
places seems strange; normally you check it once and then goto fail,
where the cleanup is done (which could include a call to
bdrv_invalidate_cache_all()).

Kevin

> Dave
> > 
> > migration_thread()
> >   migration_completion()
> >     runstate_is_running() ----------------> true if guest is running
> >     bdrv_inactivate_all() ----------------> inactivate images
> >     qemu_savevm_state_complete_precopy()
> >      ... qemu_fflush()
> >            socket_writev_buffer() --------> error because destination fails
> >          qemu_fflush() -------------------> set error on migration stream
> >   migration_completion() -----------------> set migrate state to FAILED
> > migration_thread() -----------------------> break migration loop
> >   vm_start() -----------------------------> restart guest with inactive
> >                                             images
> > 
> > and you get:
> > 
> > qemu-system-ppc64: socket_writev_buffer: Got err=104 for (32768/18446744073709551615)
> > qemu-system-ppc64: /home/greg/Work/qemu/qemu-master/block/io.c:1342:bdrv_co_do_pwritev: Assertion `!(bs->open_flags & 0x0800)' failed.
> > Aborted (core dumped)
> > 
> > If we try postcopy with a similar scenario, we also get the writev error
> > message but QEMU leaves the guest paused because entered_postcopy is true.
> > 
> > We could possibly do the same with precopy and leave the guest paused.
> > But since the historical default for migration errors is to restart the
> > source, this patch adds a call to bdrv_invalidate_cache_all() instead.
> > 
> > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > ---
> >  migration/migration.c |    9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/migration/migration.c b/migration/migration.c
> > index 991313a8629a..5726959ddfd9 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -1568,8 +1568,17 @@ static void migration_completion(MigrationState *s, int current_active_state,
> >                  ret = bdrv_inactivate_all();
> >              }
> >              if (ret >= 0) {
> > +                Error *local_err = NULL;
> > +
> >                  qemu_file_set_rate_limit(s->to_dst_file, INT64_MAX);
> >                  qemu_savevm_state_complete_precopy(s->to_dst_file, false);
> > +
> > +                if (qemu_file_get_error(s->to_dst_file)) {
> > +                    bdrv_invalidate_cache_all(&local_err);
> > +                    if (local_err) {
> > +                        error_report_err(local_err);
> > +                    }
> > +                }
> >              }
> >          }
> >          qemu_mutex_unlock_iothread();
> > 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK