Re: [PATCH v4 10/25] migration: Add Error** argument to qemu_savevm_state_setup()

[Peter Xu]

On Fri, Mar 15, 2024 at 03:21:27PM +0100, Cédric Le Goater wrote:
> On 3/15/24 13:20, Cédric Le Goater wrote:
> > On 3/15/24 12:01, Peter Xu wrote:
> > > On Fri, Mar 15, 2024 at 11:17:45AM +0100, Cédric Le Goater wrote:
> > > > > migrate_set_state is also unintuitive because it ignores invalid state
> > > > > transitions and we've been using that property to deal with special
> > > > > states such as POSTCOPY_PAUSED and FAILED:
> > > > > 
> > > > > - After the migration goes into POSTCOPY_PAUSED, the resumed 
> > > > > migration's
> > > > >     migrate_init() will try to set the state NONE->SETUP, which is not
> > > > >     valid.
> > > > > 
> > > > > - After save_setup fails, the migration goes into FAILED, but 
> > > > > wait_unplug
> > > > >     will try to transition SETUP->ACTIVE, which is also not valid.
> > > > > 
> > > > 
> > > > I am not sure I understand what the plan is. Both solutions are 
> > > > problematic
> > > > regarding the state transitions.
> > > > 
> > > > Should we consider that waiting for failover devices to unplug is an 
> > > > internal
> > > > step of the SETUP phase not transitioning to ACTIVE ?
> > > 
> > > If to unblock this series, IIUC the simplest solution is to do what 
> > > Fabiano
> > > suggested, that we move qemu_savevm_wait_unplug() to be before the check 
> > > of
> > > setup() ret.
> > 
> > The simplest is IMHO moving qemu_savevm_wait_unplug() before
> > qemu_savevm_state_setup() and leave patch 10 is unchanged. See
> > below the extra patch. It looks much cleaner than what we have
> > today.
> > 
> > > In that case, the state change in qemu_savevm_wait_unplug()
> > > should be benign and we should see a super small window it became ACTIVE
> > > but then it should be FAILED (and IIUC the patch itself will need to use
> > > ACTIVE as "old_state", not SETUP anymore).
> > 
> > OK. I will give it a try to compare.
> 
> Here's the alternative solution. SETUP state failures are handled after
> transitioning to ACTIVE state, which is unfortunate but probably harmless.
> I guess it's OK.

This also looks good to me, thanks.

One trivial early comment is in this case we can introduce a helper to
cover both setup() calls and UNPLUG waits and dedup the two paths.

> 
> Thanks,
> 
> C.
> 
> 
> 
> Signed-off-by: Cédric Le Goater <clg@redhat.com>
> ---
>  migration/savevm.h    |  2 +-
>  migration/migration.c | 29 +++++++++++++++++++++++++++--
>  migration/savevm.c    | 26 +++++++++++++++-----------
>  3 files changed, 43 insertions(+), 14 deletions(-)
> 
> diff --git a/migration/savevm.h b/migration/savevm.h
> index 
> 74669733dd63a080b765866c703234a5c4939223..9ec96a995c93a42aad621595f0ed58596c532328
>  100644
> --- a/migration/savevm.h
> +++ b/migration/savevm.h
> @@ -32,7 +32,7 @@
>  bool qemu_savevm_state_blocked(Error **errp);
>  void qemu_savevm_non_migratable_list(strList **reasons);
>  int qemu_savevm_state_prepare(Error **errp);
> -void qemu_savevm_state_setup(QEMUFile *f);
> +int qemu_savevm_state_setup(QEMUFile *f, Error **errp);
>  bool qemu_savevm_state_guest_unplug_pending(void);
>  int qemu_savevm_state_resume_prepare(MigrationState *s);
>  void qemu_savevm_state_header(QEMUFile *f);
> diff --git a/migration/migration.c b/migration/migration.c
> index 
> 644e073b7dcc70cb2bdaa9c975ba478952465ff4..0704ad6226df61f2f15bd81a2897f9946d601ca7
>  100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -3427,6 +3427,8 @@ static void *migration_thread(void *opaque)
>      int64_t setup_start = qemu_clock_get_ms(QEMU_CLOCK_HOST);
>      MigThrError thr_error;
>      bool urgent = false;
> +    Error *local_err = NULL;
> +    int ret;
>      thread = migration_threads_add("live_migration", qemu_get_thread_id());
> @@ -3470,12 +3472,24 @@ static void *migration_thread(void *opaque)
>      }
>      bql_lock();
> -    qemu_savevm_state_setup(s->to_dst_file);
> +    ret = qemu_savevm_state_setup(s->to_dst_file, &local_err);
>      bql_unlock();
>      qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
>                                 MIGRATION_STATUS_ACTIVE);
> +    /*
> +     * Handle SETUP failures after waiting for virtio-net-failover
> +     * devices to unplug. This to preserve migration state transitions.
> +     */
> +    if (ret) {
> +        migrate_set_error(s, local_err);
> +        error_free(local_err);
> +        migrate_set_state(&s->state, MIGRATION_STATUS_ACTIVE,
> +                          MIGRATION_STATUS_FAILED);
> +        goto out;
> +    }
> +
>      s->setup_time = qemu_clock_get_ms(QEMU_CLOCK_HOST) - setup_start;
>      trace_migration_thread_setup_complete();
> @@ -3549,6 +3563,8 @@ static void *bg_migration_thread(void *opaque)
>      MigThrError thr_error;
>      QEMUFile *fb;
>      bool early_fail = true;
> +    Error *local_err = NULL;
> +    int ret;
>      rcu_register_thread();
>      object_ref(OBJECT(s));
> @@ -3582,12 +3598,20 @@ static void *bg_migration_thread(void *opaque)
>      bql_lock();
>      qemu_savevm_state_header(s->to_dst_file);
> -    qemu_savevm_state_setup(s->to_dst_file);
> +    ret = qemu_savevm_state_setup(s->to_dst_file, &local_err);
>      bql_unlock();
>      qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
>                                 MIGRATION_STATUS_ACTIVE);
> +    if (ret) {
> +        migrate_set_error(s, local_err);
> +        error_free(local_err);
> +        migrate_set_state(&s->state, MIGRATION_STATUS_ACTIVE,
> +                          MIGRATION_STATUS_FAILED);
> +        goto fail_setup;
> +    }
> +
>      s->setup_time = qemu_clock_get_ms(QEMU_CLOCK_HOST) - setup_start;
>      trace_migration_thread_setup_complete();
> @@ -3656,6 +3680,7 @@ fail:
>          bql_unlock();
>      }
> +fail_setup:
>      bg_migration_iteration_finish(s);
>      qemu_fclose(fb);
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 
> 1a7b5cb78a912c36ae16db703afc90ef2906b61f..0eb94e61f888adba2c0732c2cb701b110814c455
>  100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1310,11 +1310,11 @@ int qemu_savevm_state_prepare(Error **errp)
>      return 0;
>  }
> -void qemu_savevm_state_setup(QEMUFile *f)
> +int qemu_savevm_state_setup(QEMUFile *f, Error **errp)
>  {
> +    ERRP_GUARD();
>      MigrationState *ms = migrate_get_current();
>      SaveStateEntry *se;
> -    Error *local_err = NULL;
>      int ret = 0;
>      json_writer_int64(ms->vmdesc, "page_size", qemu_target_page_size());
> @@ -1323,10 +1323,9 @@ void qemu_savevm_state_setup(QEMUFile *f)
>      trace_savevm_state_setup();
>      QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
>          if (se->vmsd && se->vmsd->early_setup) {
> -            ret = vmstate_save(f, se, ms->vmdesc, &local_err);
> +            ret = vmstate_save(f, se, ms->vmdesc, errp);
>              if (ret) {
> -                migrate_set_error(ms, local_err);
> -                error_report_err(local_err);
> +                migrate_set_error(ms, *errp);
>                  qemu_file_set_error(f, ret);
>                  break;
>              }
> @@ -1346,18 +1345,19 @@ void qemu_savevm_state_setup(QEMUFile *f)
>          ret = se->ops->save_setup(f, se->opaque);
>          save_section_footer(f, se);
>          if (ret < 0) {
> +            error_setg(errp, "failed to setup SaveStateEntry with id(name): "
> +                       "%d(%s): %d", se->section_id, se->idstr, ret);
>              qemu_file_set_error(f, ret);
>              break;
>          }
>      }
>      if (ret) {
> -        return;
> +        return ret;
>      }
> -    if (precopy_notify(PRECOPY_NOTIFY_SETUP, &local_err)) {
> -        error_report_err(local_err);
> -    }
> +    /* TODO: Should we check that errp is set in case of failure ? */
> +    return precopy_notify(PRECOPY_NOTIFY_SETUP, errp);
>  }
>  int qemu_savevm_state_resume_prepare(MigrationState *s)
> @@ -1725,7 +1725,10 @@ static int qemu_savevm_state(QEMUFile *f, Error **errp)
>      ms->to_dst_file = f;
>      qemu_savevm_state_header(f);
> -    qemu_savevm_state_setup(f);
> +    ret = qemu_savevm_state_setup(f, errp);
> +    if (ret) {
> +        goto cleanup;
> +    }
>      while (qemu_file_get_error(f) == 0) {
>          if (qemu_savevm_state_iterate(f, false) > 0) {
> @@ -1738,10 +1741,11 @@ static int qemu_savevm_state(QEMUFile *f, Error 
> **errp)
>          qemu_savevm_state_complete_precopy(f, false, false);
>          ret = qemu_file_get_error(f);
>      }
> -    qemu_savevm_state_cleanup();
>      if (ret != 0) {
>          error_setg_errno(errp, -ret, "Error while writing VM state");
>      }
> +cleanup:
> +    qemu_savevm_state_cleanup();
>      if (ret != 0) {
>          status = MIGRATION_STATUS_FAILED;
> -- 
> 2.44.0
> 
> 

-- 
Peter Xu