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[Bug 1292234] Re: qcow2 image corruption on non-extent filesystems (ext3)


Yes, this is an upstream bug. So it affects anyone using the right filesystem and CONFIGs. Once we fix this upstream, then it will be submitted as a stable kernel update and make its way into all stable kernels as applicable.

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  qcow2 image corruption on non-extent filesystems (ext3)

Status in linux package in Ubuntu:
  In Progress
Status in qemu package in Ubuntu:

Bug description:
  Users of non-extent ext4 filesystems (ext4 ^extents, or ext3 w/ CONFIG_EXT4_USE_FOR_EXT23=y) can encounter data corruption when using fallocate with FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE flags.

  [Test Case]
  1) Setup ext4 ^extents, or ext3 filesystem with CONFIG_EXT4_USE_FOR_EXT23=y
  2) Create and install a VM using a qcow2 image and store the file on the filesystem
  3) Snapshot the image with qemu-img
  4) Boot the image and do some disk operations (fio,etc)
  5) Shutdown image and delete snapshot
  6) Repeat 3-5 until VM no longer boots due to image corruption, generally this takes a few iterations depending on disk operations.

  This is being discussed upstream here:

  A temporary fix would be to disable punch_hole for non-extent
  filesystem. This is how the normal ext3 module handles this and it is
  up to userspace to handle the failure. I've run this with the test
  case and was able to run for 600 iterations over 3 days where most
  failures occur within the first 2-20 iterations.

  diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
  index 5653fa4..e14cdfe 100644
  --- a/fs/ext4/inode.c
  +++ b/fs/ext4/inode.c
  @@ -3367,6 +3367,10 @@ int ext4_punch_hole(struct inode *inode, loff_t
  offset, loff_t length)
    unsigned int credits;
    int ret = 0;

  +	/* EXTENTS required */
  +	if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
  +		return -EOPNOTSUPP;
    if (!S_ISREG(inode->i_mode))
     return -EOPNOTSUPP;


  The security team uses a tool (http://bazaar.launchpad.net/~ubuntu-
  bugcontrol/ubuntu-qa-tools/master/view/head:/vm-tools/uvt) that uses
  libvirt snapshots quite a bit. I noticed after upgrading to trusty
  some time ago that qemu 1.7 (and the qemu 2.0 in the candidate ppa)
  has had stability problems such that the disk/partition table seems to
  be corrupted after removing a libvirt snapshot and then creating
  another with the same name. I don't have a very simple reproducer, but
  had enough that hallyn suggested I file a bug. First off:

  qemu-kvm 2.0~git-20140307.4c288ac-0ubuntu2

  $ cat /proc/version_signature
  Ubuntu 3.13.0-16.36-generic 3.13.5

  $ qemu-img info ./forhallyn-trusty-amd64.img
  image: ./forhallyn-trusty-amd64.img
  file format: qcow2
  virtual size: 8.0G (8589934592 bytes)
  disk size: 4.0G
  cluster_size: 65536
  Format specific information:
      compat: 0.10

  Steps to reproduce:
  1. create a virtual machine. For a simplified reproducer, I used virt-manager with:
    OS type: Linux
    Version: Ubuntu 14.04
    Memory: 768
    CPUs: 1

    Select managed or existing (Browse, new volume)
      Create a new storage volume:
        Max capacity: 8192
        Allocation: 0

      firmware: default

  2. install a VM. I used trusty-desktop-amd64.iso from Jan 23 since it
  seems like I can hit the bug more reliably if I have lots of updates
  in a dist-upgrade. I have seen this with lucid-trusty guests that are
  i386 and amd64. After the install, reboot and then cleanly shutdown

  3. Backup the image file somewhere since steps 1 and 2 take a while :)

  4. Execute the following commands which are based on what our uvt tool

  $ virsh snapshot-create-as forhallyn-trusty-amd64 pristine "uvt snapshot"
  $ virsh snapshot-current --name forhallyn-trusty-amd64
  $ virsh start forhallyn-trusty-amd64
  $ virsh snapshot-list forhallyn-trusty-amd64 # this is showing as shutoff after start, this might be different with qemu 1.5

  in guest:
  sudo apt-get update
  sudo apt-get dist-upgrade
  780 upgraded...
  shutdown -h now

  $ virsh snapshot-delete forhallyn-trusty-amd64 pristine --children
  $ virsh snapshot-create-as forhallyn-trusty-amd64 pristine "uvt snapshot"

  $ virsh start forhallyn-trusty-amd64 # this command works, but there
  is often disk corruption

  The idea behind the above is to create a new VM with a pristine
  snapshot that we could revert later if we wanted. Instead, we boot the
  VM, run apt-get dist-upgrade, cleanly shutdown and then remove the old
  'pristine' snapshot and create a new 'pristine' snapshot. The
  intention is to update the VM and the pristine snapshot so that when
  we boot the next time, we boot from the updated VM and can revert back
  to the updated VM.

  After running 'virsh start' after doing snapshot-delete/snapshot-
  create-as, the disk may be corrupted. This can be seen with grub
  failing to find .mod files, the kernel not booting, init failing, etc.

  This does not seem to be related to the machine type used. Ie, pc-
  i440fx-1.5, pc-i440fx-1.7 and pc-i440fx-2.0 all fail with qemu 2.0,
  pc-i440fx-1.5 and pc-i440fx-1.7 fail with qemu 1.7 and pc-i440fx-1.5
  works fine with qemu 1.5.

  Only workaround I know if is to downgrade qemu to 1.5.0+dfsg-
  3ubuntu5.4 from Ubuntu 13.10.

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