Re: [PATCH v1 1/1] migration: Fix yank on postcopy multifd crashing guest after migration

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

* Leonardo Bras (leobras@redhat.com) wrote:
> When multifd and postcopy-ram capabilities are enabled, if a
> migrate-start-postcopy is attempted, the migration will finish sending the
> memory pages and then crash with the following error:

How does that happen? Isn't multifd+postcopy still disabled, I see in 
migrate_caps_check

    if (cap_list[MIGRATION_CAPABILITY_POSTCOPY_RAM]) {
    ....
        if (cap_list[MIGRATION_CAPABILITY_MULTIFD]) {
            error_setg(errp, "Postcopy is not yet compatible with multifd");
            return false;
        }
    }


Dave

> qemu-system-x86_64: ../util/yank.c:107: yank_unregister_instance: Assertion
> `QLIST_EMPTY(&entry->yankfns)' failed.
> 
> This happens because even though all multifd channels could
> yank_register_function(), none of them could unregister it before
> unregistering the MIGRATION_YANK_INSTANCE, causing the assert to fail.
> 
> Fix that by calling multifd_load_cleanup() on postcopy_ram_listen_thread()
> before MIGRATION_YANK_INSTANCE is unregistered.
> 
> Fixes: b5eea99ec2 ("migration: Add yank feature")
> Reported-by: Li Xiaohui <xiaohli@redhat.com>
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  migration/migration.h |  1 +
>  migration/migration.c | 18 +++++++++++++-----
>  migration/savevm.c    |  2 ++
>  3 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/migration/migration.h b/migration/migration.h
> index cdad8aceaa..240f64efb0 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -473,6 +473,7 @@ void migration_make_urgent_request(void);
>  void migration_consume_urgent_request(void);
>  bool migration_rate_limit(void);
>  void migration_cancel(const Error *error);
> +bool migration_load_cleanup(void);
>  
>  void populate_vfio_info(MigrationInfo *info);
>  void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page);
> diff --git a/migration/migration.c b/migration/migration.c
> index 739bb683f3..4f363b2a95 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -486,6 +486,17 @@ void migrate_add_address(SocketAddress *address)
>                        QAPI_CLONE(SocketAddress, address));
>  }
>  
> +bool migration_load_cleanup(void)
> +{
> +    Error *local_err = NULL;
> +
> +    if (multifd_load_cleanup(&local_err)) {
> +        error_report_err(local_err);
> +        return true;
> +    }
> +    return false;
> +}
> +
>  static void qemu_start_incoming_migration(const char *uri, Error **errp)
>  {
>      const char *p = NULL;
> @@ -540,8 +551,7 @@ static void process_incoming_migration_bh(void *opaque)
>       */
>      qemu_announce_self(&mis->announce_timer, migrate_announce_params());
>  
> -    if (multifd_load_cleanup(&local_err) != 0) {
> -        error_report_err(local_err);
> +    if (migration_load_cleanup()) {
>          autostart = false;
>      }
>      /* If global state section was not received or we are in running
> @@ -646,9 +656,7 @@ fail:
>      migrate_set_state(&mis->state, MIGRATION_STATUS_ACTIVE,
>                        MIGRATION_STATUS_FAILED);
>      qemu_fclose(mis->from_src_file);
> -    if (multifd_load_cleanup(&local_err) != 0) {
> -        error_report_err(local_err);
> -    }
> +    migration_load_cleanup();
>      exit(EXIT_FAILURE);
>  }
>  
> diff --git a/migration/savevm.c b/migration/savevm.c
> index a0cdb714f7..250caff7f4 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1889,6 +1889,8 @@ static void *postcopy_ram_listen_thread(void *opaque)
>          exit(EXIT_FAILURE);
>      }
>  
> +    migration_load_cleanup();
> +
>      migrate_set_state(&mis->state, MIGRATION_STATUS_POSTCOPY_ACTIVE,
>                                     MIGRATION_STATUS_COMPLETED);
>      /*
> -- 
> 2.38.1
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK