Re: [Xen-devel] null domains after xl destroy

[Juergen Gross <jgross@xxxxxxxx>]

On 11/04/17 07:25, Glenn Enright wrote:
> Hi all
> 
> We are seeing an odd issue with domu domains from xl destroy, under
> recent 4.9 kernels a (null) domain is left behind.

I guess this is the dom0 kernel version?

> This has occurred on a variety of hardware, with no obvious commonality.
> 
> 4.4.55 does not show this behavior.
> 
> On my test machine I have the following packages installed under
> centos6, from https://xen.crc.id.au/
> 
> ~]# rpm -qa | grep xen
> xen47-licenses-4.7.2-4.el6.x86_64
> xen47-4.7.2-4.el6.x86_64
> kernel-xen-4.9.21-1.el6xen.x86_64
> xen47-ocaml-4.7.2-4.el6.x86_64
> xen47-libs-4.7.2-4.el6.x86_64
> xen47-libcacard-4.7.2-4.el6.x86_64
> xen47-hypervisor-4.7.2-4.el6.x86_64
> xen47-runtime-4.7.2-4.el6.x86_64
> kernel-xen-firmware-4.9.21-1.el6xen.x86_64
> 
> I've also replicated the issue with 4.9.17 and 4.9.20
> 
> To replicate, on a cleanly booted dom0 with one pv VM, I run the
> following on the VM
> 
> {
> while true; do
>  dd bs=1M count=512 if=/dev/zero of=test conv=fdatasync
> done
> }
> 
> Then on the dom0 I do this sequence to reliably get a null domain. This
> occurs with oxenstored and xenstored both.
> 
> {
> xl sync 1
> xl destroy 1
> }
> 
> xl list then renders something like ...
> 
> (null)                                       1     4     4     --p--d   
> 9.8     0

Something is referencing the domain, e.g. some of its memory pages are
still mapped by dom0.

> From what I can see it appears to be disk related. Affected VMs all use
> lvm storage for their boot disk. lvdisplay of the affected lv shows that
> the lv has is being help open by something.

How are the disks configured? Especially the backend type is important.

> 
> ~]# lvdisplay test/test.img | grep open
>   # open                 1
> 
> I've not been able to determine what that thing is as yet. I tried lsof,
> dmsetup, various lv tools. Waiting for the disk to be released does not
> work.
> 
> ~]# xl list
> Name                                        ID   Mem VCPUs      State
> Time(s)
> Domain-0                                     0  1512     2     r-----  
> 29.0
> (null)                                       1     4     4     --p--d   
> 9.8
> 
> xenstore-ls reports nothing for the null domain id that I can see.

Any qemu process related to the domain still running?

Any dom0 kernel messages related to Xen?


Juergen

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