Re: [PATCH RFC] mm: migrate: don't rely on PageMovable() of newpage after unlocking it

[David Hildenbrand]

On 28.01.19 14:35, Michal Hocko wrote:
> On Mon 28-01-19 14:22:52, David Hildenbrand wrote:
>> On 28.01.19 14:21, Michal Hocko wrote:
>>> On Mon 28-01-19 14:14:28, David Hildenbrand wrote:
>>>> On 28.01.19 14:07, Michal Hocko wrote:
>>>>> On Mon 28-01-19 13:16:09, David Hildenbrand wrote:
>>>>> [...]
>>>>>> My theory:
>>>>>>
>>>>>> In __unmap_and_move(), we lock the old and newpage and perform the
>>>>>> migration. In case of vitio-balloon, the new page will become
>>>>>> movable, the old page will no longer be movable.
>>>>>>
>>>>>> However, after unlocking newpage, I think there is nothing stopping
>>>>>> the newpage from getting dequeued and freed by virtio-balloon. This
>>>>>> will result in the newpage
>>>>>> 1. No longer having PageMovable()
>>>>>> 2. Getting moved to the local list before finally freeing it (using
>>>>>>    page->lru)
>>>>>
>>>>> Does that mean that the virtio-balloon can change the Movable state
>>>>> while there are other users of the page? Can you point to the code that
>>>>> does it? How come this can be safe at all? Or is the PageMovable stable
>>>>> only under the page lock?
>>>>>
>>>>
>>>> PageMovable is stable under the lock. The relevant instructions are in
>>>>
>>>> mm/balloon_compaction.c and include/linux/balloon_compaction.h
>>>
>>> OK, I have just checked __ClearPageMovable and it indeed requires
>>> PageLock. Then we also have to move is_lru = __PageMovable(page) after
>>> the page lock.
>>>
>>
>> I assume that is fine as is as the page is isolated? (yes, it will be
>> modified later when moving but we are interested in the original state)
> 
> OK, I've missed that the page is indeed isolated. Then the patch makes
> sense to me.
> 

Thanks Michal. I assume this has broken ever since balloon compaction
was introduced. I'll wait a little more and then resend as !RFC with a
cc-stable tag.

-- 

Thanks,

David / dhildenb