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

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

* 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.

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