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

[David Hildenbrand]

On 28.01.19 16:04, Rafael Aquini wrote:
> On Mon, Jan 28, 2019 at 10:01:56AM -0500, Rafael Aquini wrote:
>> On Mon, Jan 28, 2019 at 03:38:38PM +0100, David Hildenbrand wrote:
>>> 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.
>>>
>>
>> Yes, balloon deflation could always race against migration
>> This race was a problem, initially, and was dealt with, via:
>>
>> commit 117aad1e9e4d97448d1df3f84b08bd65811e6d6a
>> Author: Rafael Aquini <aquini@xxxxxxxxxx>
>> Date:   Mon Sep 30 13:45:16 2013 -0700
>>
>>     mm: avoid reinserting isolated balloon pages into LRU lists
>>
>>  
>>
>> I think this upstream patch has re-introduced it, in a more subtle way,
>> as we're stumbling on it now, again:
>>
>> commit d6d86c0a7f8ddc5b38cf089222cb1d9540762dc2
>> Author: Konstantin Khlebnikov <k.khlebnikov@xxxxxxxxxxx>
>> Date:   Thu Oct 9 15:29:27 2014 -0700
>>
>>     mm/balloon_compaction: redesign ballooned pages management
>>     
>>
>>
>> On this particular race against migration case, virtio ballon deflation would 
>> not see it before
>>
>> commit b1123ea6d3b3da25af5c8a9d843bd07ab63213f4
>> Author: Minchan Kim <minchan@xxxxxxxxxx>
>> Date:   Tue Jul 26 15:23:09 2016 -0700
>>
>>     mm: balloon: use general non-lru movable page feature
>>
>> as the recently released balloon page would be post-processed 
>> without the page->lru list handling, which for migration stability
>> purposes must be done under the protection of page_lock.
>>
>>
> 
> missing part here:
> 
> I think your patch adresses this new case.
> 
> 
> Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>
> 
> 
>> get rid of balloon reference count.
> 
> ^^ this was a left over (sorry about my fat-fingers)

:D

Thanks! I'll resend with

Cc: stable@xxxxxxxxxxxxxxx # 3.12+
Fixes: d6d86c0a7f8d ("mm/balloon_compaction: redesign ballooned pages
management")

-- 

Thanks,

David / dhildenb