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Subject: Re: [PATCH RFC] mm: migrate: don't rely on PageMovable() of newpage
 after unlocking it
To:     Michal Hocko <mhocko@kernel.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Jan Kara <jack@suse.cz>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Matthew Wilcox <willy@infradead.org>,
        Vratislav Bendel <vbendel@redhat.com>,
        Rafael Aquini <aquini@redhat.com>
References: <20190128121609.9528-1-david@redhat.com>
 <20190128130709.GJ18811@dhcp22.suse.cz>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <17e7d7e4-f4ca-a681-93e5-92a0c285be14@redhat.com>
Date:   Mon, 28 Jan 2019 14:22:52 +0100
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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