linux-kernel - Re: [PATCH v4 2/3] virtio_balloon: introduce migration primitives to balloon pages

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Message-Id: <20120718154908.14704344.akpm@linux-foundation.org>
Date:	Wed, 18 Jul 2012 15:49:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rafael Aquini <aquini@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Andi Kleen <andi@...stfloor.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>,
	Rafael Aquini <aquini@...ux.com>
Subject: Re: [PATCH v4 2/3] virtio_balloon: introduce migration primitives
 to balloon pages

On Tue, 17 Jul 2012 13:50:42 -0300
Rafael Aquini <aquini@...hat.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.
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