CentOS virt - Jan 2014 - xen_balloon: reserve_additional_memory: add_memory() Errors

Hi All

Back in August there was a report of these errors popping up in dmesg, and as
far as I can tell it's still unresolved even in the latest kernel.

System RAM resource [mem 0xc8000000-0xcfffffff] cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17

On this example its running 3.10.20-11.el6.centos.alt.x86_64

To replicate this just disable Dom0 memory ballooning in
/etc/xen/xend-config.sxp and these errors will over time completely fill up
dmesg.

This is because the amount of memory within these two sys files is different:

/sys/devices/system/xen_memory/xen_memory0/info/current_kb

/sys/devices/system/xen_memory/xen_memory0/target_kb

We can manually edit to work around the problem and stop the error but its not
ideal, I would help with making a patch but don't really know where to
start!

Thanks
Chris
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On Fri, 3 Jan 2014, Chris Elliott wrote:
> Hi All
> 
> Back in August there was a report of these errors popping up in dmesg, and
as far as I can tell it?s still unresolved even in the
> latest kernel.
> ?
> 
> System RAM resource [mem 0xc8000000-0xcfffffff] cannot be added
> 
> xen_balloon: reserve_additional_memory: add_memory() failed: -17
> 
> ?
> 
> On this example its running 3.10.20-11.el6.centos.alt.x86_64
> 
> ?
> 
> To replicate this just disable Dom0 memory ballooning in
/etc/xen/xend-config.sxp and these errors will over time completely fill
> up dmesg.
When you disable dom0 memory ballooning, do you also reboot passing
dom0_mem=something to the Xen command line (for example dom0_mem=4G)?
If you do so, do you still see those errors?


 ?
> This is because the amount of memory within these two sys files is
different:
> 
> ?
> 
> /sys/devices/system/xen_memory/xen_memory0/info/current_kb
> 
> /sys/devices/system/xen_memory/xen_memory0/target_kb
> 
> ?
> 
> We can manually edit to work around the problem and stop the error but its
not ideal, I would help with making a patch but don?t
> really know where to start!

On 03/01/14 19:58, Chris Elliott wrote:
> Hi All
> 
>  
> 
> Back in August there was a report of these errors popping up in dmesg,
> and as far as I can tell it?s still unresolved even in the latest kernel.
> 
> System RAM resource [mem 0xc8000000-0xcfffffff] cannot be added
> xen_balloon: reserve_additional_memory: add_memory() failed: -17
Hmmm. I definitely thought this was fixed.  Can you provide your Xen
command line and the kernel dmesg after boot?

David

On 03/01/14 19:58, Chris Elliott wrote:
> Hi All
> 
>  
> 
> Back in August there was a report of these errors popping up in dmesg,
> and as far as I can tell it?s still unresolved even in the latest kernel.
> 
> System RAM resource [mem 0xc8000000-0xcfffffff] cannot be added
> xen_balloon: reserve_additional_memory: add_memory() failed: -17
Drivers (e.g., blkback) use ballooned pages for use for foreign
mappings.  Depending on dom0_mem options there may be no ballooned
memory available at boot and dom0's balloon is increased by allocating
pages.  This decreases current_kb.

The balloon driver then tries to decrease the balloon to match the
target but it can only do this by hotplugging new memory.  A bug in the
balloon driver means the location of this memory overlaps with an
address range that is already in use and the hotplug will fail.

As a workaround you can boot dom0 with a suitable amount of
pre-ballooned memory with:

   dom0_mem=3G,max:4G

You may need to experiment with the max: value.

David