Re: [PATCH 00/13] virtio_balloon, efi/unaccepted: support memory ballooning and free page reporting in CoCo guests

[Michael S. Tsirkin]

On Wed, Oct 28, 2026 at 11:12:47AM -0700, Kevin Loughlin wrote:
> On Wed, Oct 28, 2026 at 2:01 AM David Hildenbrand (Arm)
> <david@xxxxxxxxxx> wrote:
> >
> > On 10/27/26 03:56, Kevin Loughlin wrote:
> > >
> > [...]
> >
> > >  mm/page_alloc.c                               | 248 ++++++++++++-----
> >
> > This diff stat should give you a good indication why this series is not acceptable.
> 
> Got it. I will keep that in mind going forward, especially if I end up
> attempting a v2 that simplifies things.
> 
> On that note, I believe the majority of the bloat in mm/page_alloc.c
> comes from...
> 1. supporting the legacy inflate/deflate option
> 2. introducing support for various page sizes to be unaccepted
> 
> (1) I can get rid of by dropping the inflate/deflate option, which I
> think makes sense given free page reporting should be sufficient to
> enable dynamic guest downsizing in cooperative scenarios.
> 
> (2) becomes simpler in part just by doing (1). For further
> simplification, we could potentially restrict free page reporting for
> CoCo guests to the current 4 MB MAX_PAGE_ORDER instead of supporting
> any order 2 MB and up. There's a trade-off here for reporting
> effectiveness under fragmentation, but perhaps support for 4 MB only
> (and its relative simplicity) could be acceptable? Especially since we
> would then maintain the invariant that all pages on the unaccepted
> list are MAX_PAGE_ORDER?

I'm not sure what is meant here.
But if a lot of memory is free and it gets merged beyond MAX_PAGE_ORDER
and then it's not reported, that's counterintuitive.

> If I can sufficiently simplify the changes (especially in mm/), do you
> potentially see a path forward purely for the free page reporting
> aspect of this (dropping inflate/deflate)?
> 
> Thank you for taking the time to review, David!