Messages in this thread Patch in this message | | | Date | Mon, 18 Aug 2008 20:09:50 -0500 | From | Anthony Liguori <> | Subject | Re: [PATCH] virtio_balloon: fix towards_target when deflating balloon |
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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@rustcorp.com.au> 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@us.ibm.com>
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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