[Bug 1846427] [NEW] 4.1.0: qcow2 corruption on savevm/quit/loadvm cycle

* [Bug 1846427] [NEW] 4.1.0: qcow2 corruption on savevm/quit/loadvm cycle
@ 2019-10-02 22:43 Michael Weiser
  2019-10-03 10:08 ` [Bug 1846427] " Dr. David Alan Gilbert
                   ` (36 more replies)
  0 siblings, 37 replies; 38+ messages in thread
From: Michael Weiser @ 2019-10-02 22:43 UTC (permalink / raw)
  To: qemu-devel

Public bug reported:

I'm seeing massive corruption of qcow2 images with qemu 4.1.0 and git
master as of 7f21573c822805a8e6be379d9bcf3ad9effef3dc after a few
savevm/quit/loadvm cycles. I've narrowed it down to the following
reproducer (further notes below):

# qemu-img check debian.qcow2
No errors were found on the image.
251601/327680 = 76.78% allocated, 1.63% fragmented, 0.00% compressed clusters
Image end offset: 18340446208
# bin/qemu/bin/qemu-system-x86_64 -machine pc-q35-4.0.1,accel=kvm -m 4096 -chardev stdio,id=charmonitor -mon chardev=charmonitor -drive file=debian.qcow2,id=d -S
qemu-system-x86_64: warning: dbind: Couldn't register with accessibility bus: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
QEMU 4.1.50 monitor - type 'help' for more information
(qemu) loadvm foo
(qemu) c
(qemu) qcow2_free_clusters failed: Invalid argument
qcow2_free_clusters failed: Invalid argument
qcow2_free_clusters failed: Invalid argument
qcow2_free_clusters failed: Invalid argument
quit
[m@nargothrond:~] qemu-img check debian.qcow2
Leaked cluster 85179 refcount=2 reference=1
Leaked cluster 85180 refcount=2 reference=1
ERROR cluster 266150 refcount=0 reference=2
[...]
ERROR OFLAG_COPIED data cluster: l2_entry=422840000 refcount=1

9493 errors were found on the image.
Data may be corrupted, or further writes to the image may corrupt it.

2 leaked clusters were found on the image.
This means waste of disk space, but no harm to data.
259266/327680 = 79.12% allocated, 1.67% fragmented, 0.00% compressed clusters
Image end offset: 18340446208

This is on a x86_64 Linux 5.3.1 Gentoo host with qemu-system-x86_64 and
accel=kvm. The compiler is gcc-9.2.0 with the rest of the system
similarly current.

Reproduced with qemu-4.1.0 from distribution package as well as vanilla
git checkout of tag v4.1.0 and commit
7f21573c822805a8e6be379d9bcf3ad9effef3dc (today's master). Does not
happen with qemu compiled from vanilla checkout of tag v4.0.0. Build
sequence:

./configure --prefix=$HOME/bin/qemu-bisect --target-list=x86_64-softmmu --disable-werror --disable-docs
[...]
CFLAGS            -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -g
[...] (can provide full configure output if helpful)
make -j8 install

The kind of guest OS does not matter: seen with Debian testing 64bit,
Windows 7 x86/x64 BIOS and Windows 7 x64 EFI.

The virtual storage controller does not seem to matter: seen with VirtIO
SCSI, emulated SCSI and emulated SATA AHCI.

Caching modes (none, directsync, writeback), aio mode (threads, native)
or discard (ignore, unmap) or detect-zeroes (off, unmap) does not
influence occurence either.

Having more RAM in the guest seems to increase odds of corruption: With
512MB to the Debian guest problem hardly occurs at all, with 4GB RAM it
happens almost instantly.

An automated reproducer works as follows:

- the guest *does* mount its root fs and swap with option discard and my
testing leaves me with the impression that file deletion rather than
reading is causing the issue

- foo is a snapshot of the running Debian VM which is already running
command

# while true ; do dd if=/dev/zero of=foo bs=10240k count=400 ; done

to produce some I/O to the disk (4GB file with 4GB of RAM).

- on the host a loop continuously resumes and saves the guest state and
quits qemu inbetween:

# while true ; do (echo loadvm foo ; echo c ; sleep 10 ; echo stop ;
echo savevm foo ; echo quit ) | bin/qemu-bisect/bin/qemu-system-x86_64
-machine pc-q35-3.1,accel=kvm -m 4096 -chardev stdio,id=charmonitor -mon
chardev=charmonitor -drive file=debian.qcow2,id=d -S -display none ;
done

- quitting qemu inbetween saves and loads seems to be necessary for the
problem to occur. Just continusouly in one session saving and loading
guest state does not trigger it.

- For me, after about 2 to 6 iterations of above loop the image is
corrupted.

- corruption manifests with other messages from qemu as well, e.g.:

(qemu) loadvm foo
Error: Device 'd' does not have the requested snapshot 'foo'

Using above reproducer I have to the be best of my ability bisected the
introduction of the problem to commit
69f47505ee66afaa513305de0c1895a224e52c45 (block: avoid recursive
block_status call if possible). qemu compiled from the commit before
does not exhibit the issue, from that commit on it does and reverting
the commit off of current master makes it disappear.

** Affects: qemu
     Importance: Undecided
         Status: New

-- 
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https://bugs.launchpad.net/bugs/1846427

Title:
  4.1.0: qcow2 corruption on savevm/quit/loadvm cycle

Status in QEMU:
  New

Bug description:
  I'm seeing massive corruption of qcow2 images with qemu 4.1.0 and git
  master as of 7f21573c822805a8e6be379d9bcf3ad9effef3dc after a few
  savevm/quit/loadvm cycles. I've narrowed it down to the following
  reproducer (further notes below):

  # qemu-img check debian.qcow2
  No errors were found on the image.
  251601/327680 = 76.78% allocated, 1.63% fragmented, 0.00% compressed clusters
  Image end offset: 18340446208
  # bin/qemu/bin/qemu-system-x86_64 -machine pc-q35-4.0.1,accel=kvm -m 4096 -chardev stdio,id=charmonitor -mon chardev=charmonitor -drive file=debian.qcow2,id=d -S
  qemu-system-x86_64: warning: dbind: Couldn't register with accessibility bus: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
  QEMU 4.1.50 monitor - type 'help' for more information
  (qemu) loadvm foo
  (qemu) c
  (qemu) qcow2_free_clusters failed: Invalid argument
  qcow2_free_clusters failed: Invalid argument
  qcow2_free_clusters failed: Invalid argument
  qcow2_free_clusters failed: Invalid argument
  quit
  [m@nargothrond:~] qemu-img check debian.qcow2
  Leaked cluster 85179 refcount=2 reference=1
  Leaked cluster 85180 refcount=2 reference=1
  ERROR cluster 266150 refcount=0 reference=2
  [...]
  ERROR OFLAG_COPIED data cluster: l2_entry=422840000 refcount=1

  9493 errors were found on the image.
  Data may be corrupted, or further writes to the image may corrupt it.

  2 leaked clusters were found on the image.
  This means waste of disk space, but no harm to data.
  259266/327680 = 79.12% allocated, 1.67% fragmented, 0.00% compressed clusters
  Image end offset: 18340446208

  This is on a x86_64 Linux 5.3.1 Gentoo host with qemu-system-x86_64
  and accel=kvm. The compiler is gcc-9.2.0 with the rest of the system
  similarly current.

  Reproduced with qemu-4.1.0 from distribution package as well as
  vanilla git checkout of tag v4.1.0 and commit
  7f21573c822805a8e6be379d9bcf3ad9effef3dc (today's master). Does not
  happen with qemu compiled from vanilla checkout of tag v4.0.0. Build
  sequence:

  ./configure --prefix=$HOME/bin/qemu-bisect --target-list=x86_64-softmmu --disable-werror --disable-docs
  [...]
  CFLAGS            -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -g
  [...] (can provide full configure output if helpful)
  make -j8 install

  The kind of guest OS does not matter: seen with Debian testing 64bit,
  Windows 7 x86/x64 BIOS and Windows 7 x64 EFI.

  The virtual storage controller does not seem to matter: seen with
  VirtIO SCSI, emulated SCSI and emulated SATA AHCI.

  Caching modes (none, directsync, writeback), aio mode (threads,
  native) or discard (ignore, unmap) or detect-zeroes (off, unmap) does
  not influence occurence either.

  Having more RAM in the guest seems to increase odds of corruption:
  With 512MB to the Debian guest problem hardly occurs at all, with 4GB
  RAM it happens almost instantly.

  An automated reproducer works as follows:

  - the guest *does* mount its root fs and swap with option discard and
  my testing leaves me with the impression that file deletion rather
  than reading is causing the issue

  - foo is a snapshot of the running Debian VM which is already running
  command

  # while true ; do dd if=/dev/zero of=foo bs=10240k count=400 ; done

  to produce some I/O to the disk (4GB file with 4GB of RAM).

  - on the host a loop continuously resumes and saves the guest state
  and quits qemu inbetween:

  # while true ; do (echo loadvm foo ; echo c ; sleep 10 ; echo stop ;
  echo savevm foo ; echo quit ) | bin/qemu-bisect/bin/qemu-system-x86_64
  -machine pc-q35-3.1,accel=kvm -m 4096 -chardev stdio,id=charmonitor
  -mon chardev=charmonitor -drive file=debian.qcow2,id=d -S -display
  none ; done

  - quitting qemu inbetween saves and loads seems to be necessary for
  the problem to occur. Just continusouly in one session saving and
  loading guest state does not trigger it.

  - For me, after about 2 to 6 iterations of above loop the image is
  corrupted.

  - corruption manifests with other messages from qemu as well, e.g.:

  (qemu) loadvm foo
  Error: Device 'd' does not have the requested snapshot 'foo'

  Using above reproducer I have to the be best of my ability bisected
  the introduction of the problem to commit
  69f47505ee66afaa513305de0c1895a224e52c45 (block: avoid recursive
  block_status call if possible). qemu compiled from the commit before
  does not exhibit the issue, from that commit on it does and reverting
  the commit off of current master makes it disappear.

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