Re: [Qemu-devel] safety of migration_bitmap_extend

[Wen Congyang <wency@cn.fujitsu.com>]

On 11/04/2015 05:19 PM, Dr. David Alan Gilbert wrote:
> * Wen Congyang (wency@cn.fujitsu.com) wrote:
>> On 11/04/2015 05:05 PM, Dr. David Alan Gilbert wrote:
>>> * Wen Congyang (wency@cn.fujitsu.com) wrote:
>>>> On 11/03/2015 09:47 PM, Dr. David Alan Gilbert wrote:
>>>>> * Juan Quintela (quintela@redhat.com) wrote:
>>>>>> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
>>>>>>> Hi,
>>>>>>>   I'm trying to understand why migration_bitmap_extend is correct/safe;
>>>>>>> If I understand correctly, you're arguing that:
>>>>>>>
>>>>>>>   1) the migration_bitmap_mutex around the extend, stops any sync's happening
>>>>>>>      and so no new bits will be set during the extend.
>>>>>>>
>>>>>>>   2) If migration sends a page and clears a bitmap entry, it doesn't
>>>>>>>      matter if we lose the 'clear' because we're copying it as
>>>>>>>      we extend it, because losing the clear just means the page
>>>>>>>      gets resent, and so the data is OK.
>>>>>>>
>>>>>>> However, doesn't (2) mean that migration_dirty_pages might be wrong?
>>>>>>> If a page was sent, the bit cleared, and migration_dirty_pages decremented,
>>>>>>> then if we copy over that bitmap and 'set' that bit again then migration_dirty_pages
>>>>>>> is too small; that means that either migration would finish too early,
>>>>>>> or more likely, migration_dirty_pages would wrap-around -ve and
>>>>>>> never finish.
>>>>>>>
>>>>>>> Is there a reason it's really safe?
>>>>>>
>>>>>> No.  It is reasonably safe.  Various values of reasonably.
>>>>>>
>>>>>> migration_dirty_pages should never arrive at values near zero.  Because
>>>>>> we move to the completion stage way before it gets a value near zero.
>>>>>> (We could have very, very bad luck, as in it is not safe).
>>>>>
>>>>> That's only true if we hit the qemu_file_rate_limit() in ram_save_iterate;
>>>>> if we don't hit the rate limit (e.g. because we're CPU or network limited
>>>>> to slower than the set limit) then I think ram_save_iterate will go all the
>>>>> way to sending every page; if that happens it'll go once more
>>>>> around the main migration loop, and call the pending routine, and now get
>>>>> a -ve (very +ve) number of pending pages, so continuously do ram_save_iterate
>>>>> again.
>>>>>
>>>>> We've had that type of bug before when we messed up the dirty-pages calculation
>>>>> during hotplug.
>>>>
>>>> IIUC, migration_bitmap_extend() is called when migration is running, and we hotplug
>>>> a device.
>>>>
>>>> In this case, I think we hold the iothread mutex when migration_bitmap_extend() is called.
>>>>
>>>> ram_save_complete() is also protected by the iothread mutex.
>>>>
>>>> So if migration_bitmap_extend() is called, the migration thread may be blocked in
>>>> migration_completion() and wait it. qemu_savevm_state_complete() will be called after
>>>> migration_completion() returns.
>>>
>>> But I don't think ram_save_iterate is protected by that lock, and my concern
>>> is that the dirty-pages calculation is wrong during the iteration phase, and then
>>> the iteration phase will never exit and never try and get to ram_save_complete.
>>
>> Yes, the dirty-pages may be wrong. But it is smaller, not larger than the exact value.
>> Why will the iteration phase never exit?
> 
> Imagine that migration_dirty_pages is slightly too small and we enter ram_save_iterate;
> ram_save_iterate now sends *all* it's pages, it decrements migration_dirty_pages for
> every page sent.  At the end of ram_save_iterate, migration_dirty_pages would be negative.
> But migration_dirty_pages is *u*int64_t; so we exit ram_save_iterate,
> go around the main migration_thread loop again and call qemu_savevm_state_pending, and
> it returns a very large number (because it's actually a negative number), so we keep
> going around the loop, because it never gets smaller.

I don't know how to trigger the problem. I think store migration_dirty_pages in BitmapRcu
can fix this problem.

Thanks
Wen Congyang

> 
> Dave
> 
>>
>> Thanks
>> Wen Congyang
>>
>>>
>>> Dave
>>>
>>>>
>>>> Thanks
>>>> Wen Congyang
>>>>
>>>>>
>>>>>> Now, do we really care if migration_dirty_pages is exact?  Not really,
>>>>>> we just use it to calculate if we should start the throotle or not.
>>>>>> That only test that each 1 second, so if we have written a couple of
>>>>>> pages that we are not accounting for, things should be reasonably safe.
>>>>>>
>>>>>> Once told that, I don't know why we didn't catch that problem during
>>>>>> review (yes, I am guilty here).  Not sure how to really fix it,
>>>>>> thought.  I think that the problem is more theoretical than real, but
>>>>>
>>>>> Dave
>>>>>
>>>>>> ....
>>>>>>
>>>>>> Thanks, Juan.
>>>>>>
>>>>>>>
>>>>>>> Dave
>>>>>>>
>>>>>>> --
>>>>>>> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>>>>> --
>>>>> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>>>>>
>>>>> .
>>>>>
>>>>
>>> --
>>> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>>> .
>>>
>>
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> .
>