Re: [pve-devel] [PATCH V2 qemu-server 1/2] enable balloon free-page-reporting

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

On 16.03.22 20:32, DERUMIER, Alexandre wrote:
> From my tests:
> a already booted vm without the balloon freepage option enabled --->
> migrating to new vm with balloon freepage option enabled : works
> 
> Then failback it to previous node : works
> 
> 
> But starting a new vm with the option enabled then migrate it to a new
> vm withtout the option:
> 
> migration die on resume.
> 
> 2022-03-16 20:28:30 average migration speed: 1.5 GiB/s - downtime 30 ms
> 2022-03-16 20:28:30 migration status: completed
> 2022-03-16 20:28:30 ERROR: tunnel replied 'ERR: resume failed - VM 104
> not running' to command 'resume 104'
> 2022-03-16 20:28:39 ERROR: migration finished with problems (duration
> 00:00:18)
> TASK ERROR: migration problems
> 

I now tested with a Debian Testing (bookworm) VM and ensured it actually logs
"Free page reporting enabled" in the kernel log and you're right it fails there.

But that's only relevant for migrating a VM that got freshly started (!) on
a new (upgraded) host and then gets migrated to an old one, and that's something
we never supported anyway (besides some very light best-effort).
As the VM doesn't even crashes then, and just fails to migrate (source VM stays
running), it's not even problematic for that case; the admin can just upgrade the
qemu-server on the other node already and continue.

Same holds for live-snapshots, only a snasphot from a freshly started VM on an
already upgraded host would cause issues on rollback, but *only* on an old host,
so the same as any QEMU update would cause anyway.

So, it seems we can just enable it with 6.2 as machine guard and be done.

> 
> If think this is because guest kernel balloon driver enabled it at boot
> only.
> 

yeah seems so.

> 
> Note that I don't think that current windows drivers already support it
> (I have looked at the source code, so even if the option is enabled at
> qemu level, it don't do nothing inside windows.
> So I think that migration will works in both direction with windows
> vms.