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

["Cédric Le Goater" <clg@redhat.com>]

On 3/12/24 19:28, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
>> On Tue, Mar 12, 2024 at 11:24:39AM -0300, Fabiano Rosas wrote:
>>> Cédric Le Goater <clg@redhat.com> writes:
>>>
>>>> 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.
>>>
>>> Is there a point to this except "because we can"? Honest question, I
>>> might have missed the motivation.
>>
>> My previous point was, it avoids holding the resources (that will be
>> allocated in setup() routines) while we know we can wait for a long time.
>>
>> But then I found that the ordering is indeed needed at least if we don't
>> change migrate_set_state() first - it is the only place we set the status
>> to START (which I overlooked, sorry)...
>>
>> IMHO the function is not well designed; the state update of the next stage
>> should not reside in a function to wait for failover primary devices
>> conditionally. It's a bit of a mess.
>>
> 
> I agree. We can clean that up in 9.1.
> 
> 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 ?

Thanks,

C.