Re: [PATCH v2 22/23] mm: rename CONFIG_MEMORY_BALLOON -> CONFIG_BALLOON

Lorenzo Stoakes
2026-01-15T16:57:43Z

On Thu, Jan 15, 2026 at 05:33:56PM +0100, David Hildenbrand (Red Hat) wrote:
> On 1/15/26 14:55, Lorenzo Stoakes wrote:
> > On Thu, Jan 15, 2026 at 10:20:12AM +0100, David Hildenbrand (Red Hat) wrote:
> > > Let's make it consistent with the naming of the files but also with the
> > > naming of CONFIG_BALLOON_MIGRATION.
> > >
> > > While at it, add a "/* CONFIG_BALLOON */".
> >
> > Probably not relevant but cheap for me to share :) so grepped for
> > 'memory_balloon' and saw:
> >
> > include/uapi/linux/virtio_ids.h
> > 44:#define VIRTIO_ID_MEMORY_BALLOON 13 /* virtio memory balloon */
> >
> > This maybe relevant (I guess this isn't actually used anywhere?) though
> > interesting there is also VIRTIO_ID_BALLOON... hmm :)
>
> Yeah, we want to leave the virtio stuff alone.
>
> Now you'll learn something you probably wish you wouldn't know:
>
> As you spotted, there is
>
> #define VIRTIO_ID_BALLOON 5 /* virtio balloon */
>
> And
>
> #define VIRTIO_ID_MEMORY_BALLOON 13 /* virtio memory balloon */
>
>
> The virtio-spec [1] defines ID 5 to be the "Traditional Memory Balloon Device".
>
> And in there, we document that
>
> "This is the traditional balloon device. The device number 13 is reserved for
> a new memory balloon interface, with different semantics, which is expected
> in a future version of the standard. "
>
> That's in the spec already like, forever. Likely, at some point someone wanted to implement a
> new version (for whatever reason) and defined ID 13. But that never happened.
>
> So now we have these beautiful two device IDs.
>
> I'll note that the spec also defines a "DEVICE ID of Virtio Cpu balloon device as 47". But
> no changes really happened in the spec with that for the last two years (only the
> id is reserved).
>
>
> [1] https://docs.oasis-open.org/virtio/virtio/v1.4/virtio-v1.4.html#x1-4260001
>

Lord haha well that explains that ;)

> --
> Cheers
>
> David

Cheers, Lorenzo