Re: [PATCH] Partial revert "x86/xen: fix balloon target initialization for PVH dom0"

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Jan 22, 2026 at 12:03:47PM -0500, Jason Andryuk wrote:
> On 2026-01-22 11:57, Roger Pau Monné wrote:
> > On Thu, Jan 22, 2026 at 09:40:01AM -0500, Jason Andryuk wrote:
> > > On 2026-01-21 12:49, Roger Pau Monné wrote:
> > > > I haven't tested it yet to see whether that's OK to do on PV, I would
> > > > think PV and PVH would be the same here, since the setting of the
> > > > xenstore target value is based in the return of
> > > > XENMEM_current_reservation for both.
> > > 
> > > On a system with 32GB and dom0=pvh dom0_mem=7G:
> > > 
> > > [    0.295201] xen:balloon: current_pages: 1835007 get_num_physpages 8220126
> > > xen_released_pages 6385120
> > > [    0.295201] ------------[ cut here ]------------
> > > [    0.295201] Released pages underflow current target
> > > 
> > > 8220126 - 6385120 = 1835006
> > > 
> > > And also for PV:
> > > 
> > > [    1.406923] xen:balloon: current_pages: 1835008 get_num_physpages 8220127
> > > xen_released_pages 6385120
> > > [    1.406928] ------------[ cut here ]------------
> > > [    1.406931] Released pages underflow current target
> > > 
> > > 
> > > So we don't want to subtract xen_released_pages for dom0.  Is
> > > xen_released_pages expected to be non-zero for a domU?
> > 
> > Oh, yes.  In fact I think the patch here is wrong for PV dom0, as it
> > shouldn't subtract xen_released_pages from xen_start_info->nr_pages.
> > I will need to send v2.
> 
> To be clear, the numbers and warning are from the follow on
> current_reservation patch.

Yes, but I think it's also bad to use xen_start_info->nr_pages -
xen_released_pages (my current proposal).  xen_released_pages had a
different meaning on PV which I kind of stolen for the unpopulated
alloc work.  It was originally meant to signal pages that are freed
during the boot process when it's not possible to relocate them; see
xen_set_identity_and_release_chunk.  Unpopulated alloc doesn't free
the pages it uses, because they are already free to start with.

I think I need to introduce a new counter that accounts for pages
consumed by unpopulated alloc strictly, and not re-use the
xen_released_pages counter.

All this memory accounting is far more complex than it should be
sadly.

Thanks, Roger.