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[Qemu-devel] [Bug 1587065] Re: btrfs qemu-ga - multiple mounts block fsf

From:  Christian Ehrhardt 
Subject: [Qemu-devel] [Bug 1587065] Re: btrfs qemu-ga - multiple mounts block fsfreeze
Date: Thu, 19 Jul 2018 07:06:48 -0000

Prior to the update:

$ virsh domfsfreeze x-freeze; virsh domfsthaw x-freeze
error: Unable to freeze filesystems
error: internal error: unable to execute QEMU agent command 
'guest-fsfreeze-freeze': failed to freeze /: Device or resource busy

Thawed 0 filesystem(s)

$ sudo apt install qemu-guest-agent
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be upgraded:
1 upgraded, 0 newly installed, 0 to remove and 27 not upgraded.
Need to get 132 kB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu xenial-proposed/universe amd64 
qemu-guest-agent amd64 1:2.5+dfsg-5ubuntu10.31 [132 kB]
Fetched 132 kB in 0s (1.161 kB/s)    
(Reading database ... 54125 files and directories currently installed.)
Preparing to unpack .../qemu-guest-agent_1%3a2.5+dfsg-5ubuntu10.31_amd64.deb ...
Unpacking qemu-guest-agent (1:2.5+dfsg-5ubuntu10.31) over 
(1:2.5+dfsg-5ubuntu10.30) ...
Processing triggers for man-db (2.7.5-1) ...
Processing triggers for systemd (229-4ubuntu21.2) ...
Processing triggers for ureadahead (0.100.0-19) ...
Setting up qemu-guest-agent (1:2.5+dfsg-5ubuntu10.31) ...

Retesting the above:
$ virsh domfsfreeze x-freeze; virsh domfsthaw x-freeze
Froze 1 filesystem(s)

Thawed 1 filesystem(s)

Found no other hickups due to the update, setting verified.

** Tags removed: verification-needed verification-needed-xenial
** Tags added: verification-done verification-done-xenial

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  btrfs qemu-ga - multiple mounts block fsfreeze

Status in QEMU:
  Fix Released
Status in qemu package in Ubuntu:
  Fix Released
Status in qemu source package in Xenial:
  Fix Committed

Bug description:

   * backport upstream fix to avoid double unmounts (e.g. bind mounts)
     hanging the freeze action.

  [Test Case]

   * Prepare a guest
     $ uvt-simplestreams-libvirt --verbose sync --source 
http://cloud-images.ubuntu.com/daily arch=amd64 release=xenial label=daily
  uvt-kvm create --password ubuntu x-freeze arch=amd64 release=xenial 

   * Shut it down and use virsh edit to add a channel for the agent. Add this 
somewhere under <devices>:
  <channel type='unix'>
     <source mode='bind'/>
     <target type='virtio' name='org.qemu.guest_agent.0'/>

    * Start the guest again, in it install qemu-guest-agent


    * On the Host then freeze and thaw the guest
      $ virsh domfsfreeze x-freeze
      $ virsh domfsthaw x-freeze

    # The above works, then trigger the issue, in the guest do
      $ sudo mkdir /mnt/test
      $ sudo mount -o bind / /mnt/test/

    * Now the freeze/thaw will fail unless the fix is applied to the

  [Regression Potential]

   * From looking at the code alone I could think of a different cause that
     makes it return EBUSY.
     The biggest risk I could see due to that is the agent reporting a
     successful freeze due to accepting EBUSY which is not actually true.
     But we have that in Ubuntu since Artful and
     upstream since January 2017 and there are no such reports.

  [Other Info]

   * n/a


  Having two mounts of the same device makes fsfreeze through qemu-ga 
  address@hidden:/# mount -l | grep /dev/vda2
  /dev/vda2 on / type btrfs (rw,relatime,space_cache,subvolid=257,subvol=/@)
  /dev/vda2 on /home type btrfs 

  Having two mounts is rather common with btrfs, so the feature fsfreeze
  is unusable on these systems.

  Below more information about how we encountered this issue...

  Message send to address@hidden:

  Message 1:
  I use external snapshots to backup my guests. I use the 'quiesce' option to 
flush and frees the guest file system with the qemu guest agent.

  With the exeption of one guest, this procedure works fine. On the 'unwilling' 
guest, I get this error message:
  "ERROR 2016-05-25 00:51:19 | T25-bakVMSCmsrvVH2 | fout: internal error: 
unable to execute QEMU agent command 'guest-fsfreeze-freeze': failed to freeze 
/: Device or resource busy"

  I don't think this is not some sort of time-out error, because
  activation of the fsfreeze and the error message happen immediately
  after each other:

  $ grep qemu-ga syslog.1
  May 25 00:51:19 CmsrvMTA qemu-ga: info: guest-fsfreeze called

  This is the only entry of the qemu guest agent in syslog.

  $ sudo virsh version
  Compiled against library: libvirt 1.3.1
  Using library: libvirt 1.3.1
  Gebruikte API: QEMU 1.3.1
  Draaiende hypervisor: QEMU 2.5.0

  $ virsh qemu-agent-command CmsrvMTA '{"execute": "guest-info"}'
  {"return":{"version":"2.5.0", ... 
 ... }

  For making an external snapshot, I use this command:
  $ virsh snapshot-create-as --domain CmsrvMTA sn1 --disk-only --atomic 
--quiesce --no-metadata --diskspec vda,file=/srv/poolVMS/CmsrvMTA.sn1

  Piece of reply 1:
  I have encountered this before. Some operating systems
   may have bind-mounts that let a device appear multiple times in the mount 
list. Unfortunately the guest agent is not smart enough to consider a device 
that has been frozen as succesfull and keep going. This causes this specific 

  Piece of reply 2:
  Ok, that seems to be it.

  I’ve got the ‘/’ and ‘/home’ on the same device formatted as btrfs on
  two separate sub volumes.

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