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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>
From:   David Hildenbrand <david@redhat.com>
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Message-ID: <b03cae19-d02a-0ba2-69a1-010ee76748e7@redhat.com>
Date:   Mon, 28 Jan 2019 14:14:28 +0100
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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

balloon_page_insert() and balloon_page_delete() modify PageMovable and
are only called with both, the page locked and b_dev_info->pages_lock
locked.

Especially:
1. balloon_page_dequeue() which tries to find an unlocked page that is
not isolated. and deletes it via balloon_page_delete().

2. balloon_page_migrate() will effectively call balloon_page_delete() on
old page and balloon_page_insert() newpage. Both pages are locked before
calling balloon_page_migrate() and the b_dev_info->pages_lock is taken
internally.


So what's left is a very small race window when migrating to a new page,
after dropping the lock of the newpage.

-- 

Thanks,

David / dhildenb