Bug#784688: Thousands of "xen:balloon: Cannot add additional memory (-17) messages" despite dom0 ballooning disabled

[Ian Campbell <ijc@debian.org>]

On Fri, 2016-01-29 at 15:59 +0000, Andy Smith wrote:
> Hi Ian,
> 
> On Fri, Jan 29, 2016 at 02:57:23PM +0000, Ian Campbell wrote:
> > I spent a bit of time investigating this, but sadly I'm not able to
> > reproduce the basis failure.
> 
> FWIW it was me who reported this with the packages in Debian stable
> (linux-image-3.16.0-4-amd64 3.16.7-ckt20-1+deb8u3,
> xen-hypervisor-4.4-amd64 4.4.1-9+deb8u3) when using
> "dom0_mem=1024M,max:1024M" on the hypervisor command line.
> 
> I must admit I found this bug when searching for the error message,
> and have only been seeing it printed a couple of times at guest
> shutdown, not thousands of times.
> 
> So if having it printed a couple of times isn't considered a bug,
> I'm sorry if I've led you astray here.

No worries, thanks for letting me know.

>  Might be worth finding a way
> to remove it anyway though; anyone having a problem is going to keep
> searching for it thinking it is relevant to their case.

Indeed. That might be tricky to arrange 100% reliably due to the way these
things work out in practice wrt ballooning and maxes and dynamic changes
due to the use by backends, an upstream thing in any case.

> At the moment I am avoiding seeing the message at all by running with
> only "dom0_mem=1024M" on the command line. What's the disadvantage
> of not having the "max:1024M" there?

I'm not 100% sure. It looks like it causes no max to be set (LONG_MAX is
the default), which I suppose would allow dom0 (from Xen's PoV, the kernel
might have its own limitations) to balloon to more than 1024M if it tried
to (which would explain it working around this issue without).

Ian.