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

[Andy Smith <andy@strugglers.net>]

Hi,

I'm also seeing this.

With:

GRUB_CMDLINE_XEN="dom0_mem=1024M,max:1024M …"

I see a flurry of "xen:balloon: Cannot add additional memory (-17)"
messages every time a domU is shut down.

In this configuration I note that I also see with "free -m" that
dom0 has 929M, though "xl list" shows 1024M.

I can run "xl mem-set 0 1024" without visible error but it does not
prevent the error messages being logged.

With:

GRUB_CMDLINE_XEN="dom0_mem=1024M …"

I do not see any of the balloon error messages.

In this configuration "free -m" shows 768M RAM for dom0, though "xl
list" still shows 1024M.

xen-hypervisor-4.4-amd64    4.4.1-9+deb8u3
linux-image-3.16.0-4-amd64  3.16.7-ckt20-1+deb8u2

I'm guessing that the reason I end up with 768M RAM in dom0 is
because I have not specified the *maximum* RAM for dom0, so the
kernel scales structures for the possible case of dom0 getting the
entire 128G of RAM at some point.

I added "mem=1G" to the Linux kernel command line and then dom0
boots with "free -m" showing 930M.

So, at the moment I am running with "dom0_mem=1024M" on the Xen
command line, "mem=1G" on the Linux kernel command line, and
"autoballoon=off" in /etc/xen/xl.conf.

This seems to avoid the xen:balloon error messages and provide
something more like thr expected amount of dom0 memory. Is there a
downside of not specifying the ",max:1024M" part?

Cheers,
Andy