Re: Kernel log "xen:balloon: Cannot add additional memory (-22)" every 3670 seconds

[Andy Smith <andy@xxxxxxxxxxxxxx>]

On Fri, Apr 03, 2026 at 02:54:08PM +0000, Andy Smith wrote:
> I have several machines on Xen 4.20.2 / Debian 13 that were upgraded
> from Xen 4.14.5. One of these machines does the following pair of kernel
> logs every 3670 seconds:
> 
> [44081.525564] Block size [0x80000000] unaligned hotplug range: start 
> 0x1600000000, size 0x8000000
> [44081.525574] xen:balloon: Cannot add additional memory (-22)

I still don't know why this happens on this particular machine, but some
further observations:

- I think that the error message would probably go away if the
  xen_balloon driver were not present, and as I use dom0_mem I believe I
  don't need it, but Debian kernels have it compiled in, so it can't be
  disabled without a custom kernel.

- The above two log lines are emitted immediately as the balloon driver
  is initialized at boot, and every 3670 seconds after that.

- I found some references to using "xen.balloon.hotplug_unpopulated=0"
  on the dom0 kernel command line. That didn't help.

- As part of investigating why dom0_mem=8129M resulted in much less
  actual memory seen by dom0 kernel¹ I bumped that up to dom0_mem=10429M
  resulting in dom0 kernel seeing 8107M, but interestingly

  ** this has stopped the above logs from being emitted. **

  Not at boot, not any time after.

I can only think that this is some sort of bug.

This is the only machine I'm able to trigger this behaviour on but I
have to start using it for production tasks soon, so if any developer
wants me to try any further debugging, now would be the time.

Thanks,
Andy

¹ https://lists.xenproject.org/archives/html/xen-users/2026-04/msg00001.html