Re: [PATCH v4 2/3] virtio_balloon: introduce migration primitivesto balloon pages

From: Andrew Morton
Date: Wed Jul 18 2012 - 18:49:06 EST

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