AW: hv_balloon: Only works in ubuntu

From: Florian M?ller <max06.net@outlook.com>
To: "Michael Kelley (LINUX)" <mikelley@microsoft.com>,
	vkuznets <vkuznets@redhat.com>
Cc: "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	David Hildenbrand <dhildenb@redhat.com>
Subject: AW: hv_balloon: Only works in ubuntu
Date: Mon, 13 Jun 2022 14:45:56 +0000	[thread overview]
Message-ID: <DB3PR0602MB36748AD171E0F1501C7D1898FFAB9@DB3PR0602MB3674.eurprd06.prod.outlook.com> (raw)
In-Reply-To: <PH0PR21MB302540EAB2DF0A368AF4BC57D7AB9@PH0PR21MB3025.namprd21.prod.outlook.com>

> Von: Michael Kelley (LINUX) <mikelley@microsoft.com>
> > > >> > > > >
> > > >> > > > > Issues showed up when I set up a Kali Linux Guest. I
> > > >> > > > > missed the memory configuration before booting up the
> > > >> > > > > instance, so it started with 1GB of memory, and
> > > >> > > > > ballooning active between 512MB and several TB of memory.
> > > >> > > > > Hyper-V started to allocate more and more memory to this
> > > >> > > > > guest since the reported memory requirements also
> > > >> > > > > increased. The guest kernel didn't see any of that allocated
> memory, as far as I can tell.
> >
> > Please do not forget about this: (emoji-pointing-up)
> >
> 
> Hmmm.  Right off the bat, I don't know how to fix this.  Hyper-V tells the
> guest "Here is more memory".  The hv_balloon driver adds the memory (but
> doesn't mark it "online"), and sends a positive ACK to Hyper-V.
> From Hyper-V's standpoint, it has successfully given the memory to the
> guest. But if the guest hasn't onlined the memory and isn't using it, the guest
> continues to report high memory pressure.  Hyper-V assigns yet more
> memory to the guest, still to no effect.  Having the hv_balloon driver delay
> the ACK until the memory comes online is fraught with problems, and of
> course Hyper-V has no visibility into whether the guest has onlined the
> memory.
> 
> This may be one where the guest configuration really must be
> correct.   But I'm open to other suggestions for a possible solution.
> 
> Michael

From checking the drivers code, it looks like the guest tells only the free and committed memory, not the total. I can also see considerations about num_pages_onlined in the committed-calculation.

I see 2 possible options at the moment: Adding num_total to the message (changing the protocol), or stop reporting if the guest fails to online memory after the first increase. A third, more complicated option would be checking for not onlined pages (I've seen functions for that in the code) and adding them to the free value in the report.

I'd love to write a patch for this if I had a clue how to test and debug it without rebuilding my kernel all the time. 

Flo