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

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Message-ID: <37fdb994-59d1-4196-b377-f3f749148b19@kernel.org>
Date: Thu, 15 Jan 2026 17:33:56 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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 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

-- 
Cheers

David