linux-kernel - Re: [PATCH v2 22/23] mm: rename CONFIG_MEMORY_BALLOON -> CONFIG

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Message-ID: <8dc368cf-255f-4d27-8340-9b6d12131b0a@lucifer.local>
Date: Thu, 15 Jan 2026 16:57:01 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org,
        Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
        linux-doc@...r.kernel.org, virtualization@...ts.linux.dev,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Jonathan Corbet <corbet@....net>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jerrin Shaji George <jerrin.shaji-george@...adcom.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Eugenio Pérez <eperezma@...hat.com>,
        Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH v2 22/23] mm: rename CONFIG_MEMORY_BALLOON ->
 CONFIG_BALLOON

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