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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 15 Jan 2026 11:50:47 -0500

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.

yea Rusty wanted to do that. balloon has lots of weird bugs like it does
not really works with large guests. We really need to get around to
fixing this, and maybe it's cleaner to do that with a new ID
than trying and failing to use the old one, was the thinking.

> 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
> 
> -- 
> Cheers
> 
> David