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

["Cédric Le Goater" <clg@redhat.com>]

On 3/12/24 14:34, Cédric Le Goater wrote:
> 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.

That works just the same.

Peter, Fabiano,

What would you prefer  ?

1. move qemu_savevm_wait_unplug() before qemu_savevm_state_setup(),
    means one new patch.

2. leave qemu_savevm_wait_unplug() after qemu_savevm_state_setup()
    and handle state_setup() errors after waiting. means an update
    of this patch.


Thanks,

C.