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Date: Wed, 18 Jul 2012 15:49:08 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Rafael Aquini <aquini@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        Rusty Russell <rusty@rustcorp.com.au>,
        "Michael S. Tsirkin" <mst@redhat.com>, Rik van Riel <riel@redhat.com>,
        Mel Gorman <mel@csn.ul.ie>, Andi Kleen <andi@firstfloor.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Minchan Kim <minchan@kernel.org>, Rafael Aquini <aquini@linux.com>
Subject: Re: [PATCH v4 2/3] virtio_balloon: introduce migration primitives
 to balloon pages
Message-Id: <20120718154908.14704344.akpm@linux-foundation.org>
In-Reply-To: <050e06731e0489867ed804387509e36d072507ec.1342485774.git.aquini@redhat.com>
References: <cover.1342485774.git.aquini@redhat.com>
	<050e06731e0489867ed804387509e36d072507ec.1342485774.git.aquini@redhat.com>
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On Tue, 17 Jul 2012 13:50:42 -0300
Rafael Aquini <aquini@redhat.com> wrote:

> Besides making balloon pages movable at allocation time and introducing
> the necessary primitives to perform balloon page migration/compaction,
> this patch also introduces the following locking scheme to provide the
> proper synchronization and protection for struct virtio_balloon elements
> against concurrent accesses due to parallel operations introduced by
> memory compaction / page migration.
>  - balloon_lock (mutex) : synchronizes the access demand to elements of
> 			  struct virtio_balloon and its queue operations;
>  - pages_lock (spinlock): special protection to balloon pages list against
> 			  concurrent list handling operations;
> 
> ...
>
> +	balloon_mapping->a_ops = &virtio_balloon_aops;
> +	balloon_mapping->backing_dev_info = (void *)vb;

hoo boy.  We're making page->mapping->backing_dev_info point at a
struct which does not have type `struct backing_dev_info'.  And then we
are exposing that page to core MM functions.  So we're hoping that core
MM will never walk down page->mapping->backing_dev_info and explode.

That's nasty, hacky and fragile.