[Xen-devel] kernel log flooded with: xen_balloon: reserve_additional_memory: add_memory() failed: -17

[James Dingwall <james-xen@xxxxxxxxxxxxxx>]

Hi,

I have encountered an apparently benign error on two systems where the 
dom0 kernel log is flooded with messages like:
[52482.163855] System RAM resource [mem 0x1b8000000-0x1bfffffff] cannot 
be added
[52482.163860] xen_balloon: reserve_additional_memory: add_memory() 
failed: -17
The first line is from drivers/xen/xen-balloon.c, the second from 
mm/memory_hotplug.c
The trigger for the messages seems to be the first occasion that a Xen 
guest is shutdown.  I have noted this in a vanilla 3.6.7 and kernel 
3.5.0-18 built from Ubuntu sources.  Xen version is 4.2.0.  It is not 
clear why the dom0 is kernel is trying to balloon up as the Xen command 
line is specifies a fixed dom0 memory allocation and noselfballooning is 
specified for the kernel and ballooning is also disabled in the 
xend-config.sxp / xl.conf (one system using xm, another xl)
xen command line:
placeholder xsave=0 iommu=0 console=vga,com2 com2=115200,8n1 
dom0_mem=max:6144m
kernel command line:
root=/dev/loop0 ro console=tty1 console=hvc0 earlyprintk=xen nomodeset 
noselfballooning
Examining /proc/iomem does show that the dom0 memory allocation is 
actually 64kb short of 6144Mb:
cat /proc/iomem | grep System\ RAM
00010000-0009bfff : System RAM      [573440 bytes]
00100000-cb2dffff : System RAM      [3407740928 bytes]
100000000-1b4d83fff : System RAM    [3034071040 bytes]

Total system ram: 6442385408 - 6x2^30 = 65536

The memory range indicated in the log message is "Unusable memory" in 
/proc/iomem:
1b4d84000-82fffffff : Unusable memory

Another point of interest is that we have multiple "identical" hardware 
platforms (Dell T320) for the system running the 3.5.0-18 kernel but 
only see this error on a slightly more recent system.  Older systems 
show in /proc/iomem that all memory is System RAM.
100000000-82fffffff : System RAM  [older system BIOS 1.0]

100000000-1b4d83fff : System RAM  [newer system BIOS 1.3]
1b4d84000-82fffffff : Unusable memory

The BIOS revision between the old and new has changed so I was 
wondering if it is possible that there is a white list which affects the 
impact of the kernel option:
CONFIG_X86_RESERVE_LOW=64
This is only a guess since the amount of memory reserved is equivalent 
to the short fall calculated above.  If this is the right area perhaps 
the dom0 calculation for its memory entitlement needs to be taught to 
not to try and hotplug the missing 64k when it has been reserved.
If any other information would be useful then please let me know.

Thanks,
James


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