Re: [Qemu-devel] [PATCH] migration: invalidate cache before source start

[John Snow <jsnow@redhat.com>]

On 06/25/2018 01:50 PM, Dr. David Alan Gilbert wrote:
> * Dr. David Alan Gilbert (dgilbert@redhat.com) wrote:
>> * Vladimir Sementsov-Ogievskiy (vsementsov@virtuozzo.com) wrote:
>>> 15.06.2018 15:06, Dr. David Alan Gilbert wrote:
>>>> * Vladimir Sementsov-Ogievskiy (vsementsov@virtuozzo.com) wrote:
>>>>> Invalidate cache before source start in case of failed migration.
>>>>>
>>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>>> Why doesn't the code at the bottom of migration_completion,
>>>> fail_invalidate:   and the code in migrate_fd_cancel   handle this?
>>>>
>>>> What case did you see it in that those didn't handle?
>>>> (Also I guess it probably should set s->block_inactive = false)
>>>
>>> on source I see:
>>>
>>> 81392@1529065750.766289:migrate_set_state new state 7
>>> 81392@1529065750.766330:migration_thread_file_err
>>> 81392@1529065750.766332:migration_thread_after_loop
>>>
>>> so, we are leaving loop on
>>>         if (qemu_file_get_error(s->to_dst_file)) {
>>>             migrate_set_state(&s->state, current_active_state,
>>> MIGRATION_STATUS_FAILED);
>>> trace_migration_thread_file_err();
>>> break;
>>>         }
>>>
>>> and skip migration_completion()
>>
>> Yeh, OK; I'd seen soemthing else a few days ago, where a cancellation
>> test that had previously ended with a 'cancelled' state has now ended up
>> in 'failed' (which is the state 7 you have above).
>> I suspect there's something else going on as well; I think what is
>> supposed to happen in the case of 'cancel' is that it spins in 'cancelling' for
>> a while in migrate_fd_cancel and then at the bottom of migrate_fd_cancel
>> it does the recovery, but because it's going to failed instead, then
>> it's jumping over that recovery.
> 
> Going back and actually looking at the patch again;
> can I ask for 1 small change;
>    Can you set s->block_inactive = false   in the case where you
> don't get the local_err (Like we do at the bottom of migrate_fd_cancel)
> 
> 
> Does that make sense?
> 
> Thanks,
> 
> Dave
> 

Vladimir, one more question for you because I'm not as familiar with
this code:

In the normal case we need to invalidate the qcow2 cache as a way to
re-engage the disk (yes?) when we have failed during the late-migration
steps.

In this case, we seem to be observing a failure during the bulk transfer
loop. Why is it important to invalidate the cache at this step -- would
the disk have been inactivated yet? It shouldn't, because it's in the
bulk transfer phase -- or am I missing something?

I feel like this code is behaving in a way that's fairly surprising for
a casual reader so I was hoping you could elaborate for me.

--js