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

["Cédric Le Goater" <clg@redhat.com>]

On 3/12/24 13:32, Cédric Le Goater wrote:
> On 3/11/24 20:03, Fabiano Rosas wrote:
>> Cédric Le Goater <clg@redhat.com> writes:
>>
>>> On 3/8/24 15:36, Fabiano Rosas wrote:
>>>> 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.
>>>
>>> This hack makes things work :
>>>
>>> @@ -3452,6 +3452,9 @@ static void *migration_thread(void *opaq
>>>            qemu_savevm_send_colo_enable(s->to_dst_file);
>>>        }
>>> +    qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
>>> +                            MIGRATION_STATUS_SETUP);
>>> +
>>
>> Why move it all the way up here? Has moving the wait_unplug before the
>> 'if (ret)' block not worked for you?
> 
> We could be sleeping while holding the BQL. It looked wrong.

Sorry wrong answer. Yes I can try moving it before the 'if (ret)' block.
I can reproduce easily with an x86 guest running on PPC64.

Thanks,

C.