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

[Fabiano Rosas <farosas@suse.de>]

Cédric Le Goater <clg@redhat.com> writes:

> This prepares ground for the changes coming next which add an Error**
> argument to the .save_setup() handler. Callers of qemu_savevm_state_setup()
> now handle the error and fail earlier setting the migration state from
> MIGRATION_STATUS_SETUP to MIGRATION_STATUS_FAILED.
>
> In qemu_savevm_state(), move the cleanup to preserve the error
> reported by .save_setup() handlers.
>
> Since the previous behavior was to ignore errors at this step of
> migration, this change should be examined closely to check that
> cleanups are still correctly done.
>
> Signed-off-by: Cédric Le Goater <clg@redhat.com>
> ---
>
>  Changes in v4:
>  
>  - Merged cleanup change in qemu_savevm_state()
>    
>  Changes in v3:
>  
>  - Set migration state to MIGRATION_STATUS_FAILED 
>  - Fixed error handling to be done under lock in bg_migration_thread()
>  - Made sure an error is always set in case of failure in
>    qemu_savevm_state_setup()
>    
>  migration/savevm.h    |  2 +-
>  migration/migration.c | 27 ++++++++++++++++++++++++---
>  migration/savevm.c    | 26 +++++++++++++++-----------
>  3 files changed, 40 insertions(+), 15 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 a49fcd53ee19df1ce0182bc99d7e064968f0317b..6d1544224e96f5edfe56939a9c8395d88ef29581 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -3408,6 +3408,8 @@ 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());
>  
> @@ -3451,9 +3453,17 @@ 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();
>  
> +    if (ret) {
> +        migrate_set_error(s, local_err);
> +        error_free(local_err);
> +        migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
> +                          MIGRATION_STATUS_FAILED);
> +        goto out;
> +     }
> +
>      qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
>                                 MIGRATION_STATUS_ACTIVE);

This^ should be before the new block it seems:

GOOD:
migrate_set_state new state setup
migrate_set_state new state wait-unplug
migrate_fd_cancel 
migrate_set_state new state cancelling
migrate_fd_cleanup 
migrate_set_state new state cancelled
migrate_fd_cancel 
ok 1 /x86_64/failover-virtio-net/migrate/abort/wait-unplug

BAD:
migrate_set_state new state setup
migrate_fd_cancel 
migrate_set_state new state cancelling
migrate_fd_cleanup 
migrate_set_state new state cancelled
qemu-system-x86_64: ram_save_setup failed: Input/output error
**
ERROR:../tests/qtest/virtio-net-failover.c:1203:test_migrate_abort_wait_unplug:
assertion failed (status == "cancelling"): ("cancelled" == "cancelling")

Otherwise migration_iteration_finish() will schedule the cleanup BH and
that will run concurrently with migrate_fd_cancel() issued by the test
and bad things happens.

=====
PS: I guess the next level in our Freestyle Concurrency video-game is to
make migrate_fd_cancel() stop setting state and poking files and only
set a flag that's tested in the other parts of the code.

>  
> @@ -3530,6 +3540,9 @@ static void *bg_migration_thread(void *opaque)
>      MigThrError thr_error;
>      QEMUFile *fb;
>      bool early_fail = true;
> +    bool setup_fail = true;
> +    Error *local_err = NULL;
> +    int ret;
>  
>      rcu_register_thread();
>      object_ref(OBJECT(s));
> @@ -3563,9 +3576,16 @@ 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);
> +    if (ret) {
> +        migrate_set_error(s, local_err);
> +        error_free(local_err);
> +        goto fail;
> +    }
>      bql_unlock();
>  
> +    setup_fail = false;
> +
>      qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
>                                 MIGRATION_STATUS_ACTIVE);
>  
> @@ -3632,7 +3652,8 @@ static void *bg_migration_thread(void *opaque)
>  
>  fail:
>      if (early_fail) {
> -        migrate_set_state(&s->state, MIGRATION_STATUS_ACTIVE,
> +        migrate_set_state(&s->state,
> +                setup_fail ? MIGRATION_STATUS_SETUP : MIGRATION_STATUS_ACTIVE,
>                  MIGRATION_STATUS_FAILED);
>          bql_unlock();
>      }
> diff --git a/migration/savevm.c b/migration/savevm.c
> index ee31ffb5e88cea723039c754c30ce2c8a0ef35f3..63fdbb5ad7d4dbfaef1d2094350bf302cc677602 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)
> @@ -1728,7 +1728,10 @@ 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) {
> @@ -1741,10 +1744,11 @@ 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;