Re: [PATCH] virtio_balloon: fix towards_target when deflating balloon

Re: [PATCH] virtio_balloon: fix towards_target when deflating balloon

[Linus Torvalds]

On Mon, 18 Aug 2008, Anthony Liguori wrote: <
> 
> This handles the case where v < vb->num_pages and ensures we get a 
> small, negative, s64 as the result.

That's just horrible code.

Maybe the compiler notices that you're doing something stupid, but 
basically, please don't do this.

> -	return v - vb->num_pages;
> +	if (v < vb->num_pages)
> +		return -(s64)(vb->num_pages - v);
> +	else
> +		return v - vb->num_pages;

What's wrong with just doing

	return (s64)v - vb->num_pages;

instead?

Casting 'v' to s64 guarantees that the subtraction will eb done in 64 
bits, and the compiler can just generate the trivial non-conditional code.

		Linus

Re: [PATCH] virtio_balloon: fix towards_target when deflating balloon

[Anthony Liguori]

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)

Re: [PATCH] virtio_balloon: fix towards_target when deflating balloon

[Linus Torvalds]

On Mon, 18 Aug 2008, Anthony Liguori wrote:
> 
> 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

Well, we could perhaps add a sparse warning that makes noise when a 
unsigned subtraction is cast to a wider signed field. I dunno if it would 
catch anything interesting, or just cause a ton of irritating noise.

> 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.

Ugly and inefficient is not acceptable, even for these kinds of reasons. 
So yes, the simpler version is much better.

		Linus

Re: [PATCH] virtio_balloon: fix towards_target when deflating balloon

[Rusty Russell]

On Tuesday 19 August 2008 08:15:31 Anthony Liguori wrote:
> -	return v - vb->num_pages;
> +	if (v < vb->num_pages)
> +		return -(s64)(vb->num_pages - v);
> +	else
> +		return v - vb->num_pages;

With all due respect, WTF?

Did you mean:

	return (s64)v - vb->num_pages;

I'm really amazed this bug got this far though...
Rusty.

[PATCH] virtio_balloon: fix towards_target when deflating balloon

[Anthony Liguori]

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
same negative number we desire, we get a very large positive number.

This handles the case where v < vb->num_pages and ensures we get a small,
negative, s64 as the result.

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..bd3c384 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -158,7 +158,10 @@ 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;
+	if (v < vb->num_pages)
+		return -(s64)(vb->num_pages - v);
+	else
+		return v - vb->num_pages;
 }
 
 static void update_balloon_size(struct virtio_balloon *vb)