Linus Torvalds wrote:
> What's wrong with just doing
>
> return (s64)v - vb->num_pages;
>
> instead?
>
Nothing. It works just fine. However, I implemented it more verbosely
because this is the second time we've "fixed" this problem. See
commit bdc1681cdf1ab6a65fa935a2b3f8fc63b20c54ea
Author: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Date: Mon Mar 17 22:58:15 2008 -0500
virtio: handle > 2 billion page balloon targets
So I thought I'd rely a little less on the subtleties of promotion and
make things a bit more clear. However, I don't feel that strongly about
it so here you go.
Both v and vb->num_pages are u32 and unsigned int respectively. If v is
less
than vb->num_pages (and it is, when deflating the balloon), the result is a
very large 32-bit number. Since we're returning a s64, instead of
getting the
small negative number we desire, we get a very large positive number.
This patch explicitly casts v to a s64 in which will cause the whole
expression
to be promoted resulting in the proper results.
Rusty: please push this for 2.6.27-rc4. It's probably appropriate for the
stable tree too as it will cause an unexpected OOM when ballooning.
Signed-off-by: Anthony Liguori <aliguori@xxxxxxxxxx>
diff --git a/drivers/virtio/virtio_balloon.c
b/drivers/virtio/virtio_balloon.c
index bfef604..62eab43 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -158,7 +158,7 @@ static inline s64 towards_target(struct
virtio_balloon *vb)
vb->vdev->config->get(vb->vdev,
offsetof(struct virtio_balloon_config, num_pages),
&v, sizeof(v));
- return v - vb->num_pages;
+ return (s64)v - vb->num_pages;
}
static void update_balloon_size(struct virtio_balloon *vb)
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