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

[David Hildenbrand]

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)

-- 

Thanks,

David / dhildenb