[Bug 1813045] Re: qemu-ga fsfreeze crashes the kernel

[Thomas Huth]

The QEMU project is currently considering to move its bug tracking to another 
system. For this we need to know which bugs are still valid and which could be 
closed already. Thus we are setting older bugs to "Incomplete" now.
If you still think this bug report here is valid, then please switch the state 
back to "New" within the next 60 days, otherwise this report will be marked as 
"Expired". Or mark it as "Fix Released" if the problem has been solved with a 
newer version of QEMU already. Thank you and sorry for the inconvenience.

** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1813045

Title:
  qemu-ga fsfreeze crashes the kernel

Status in QEMU:
  Incomplete

Bug description:
  We use mainly Cloudlinux, Debian and Centos.
  We experienced many crashes on our qemu instances based on Cloudlinux during 
a snapshot.
  The issue is not related to CloudLinux directly, but to Qemu agent, which 
does not freeze the file system(s) correctly. What is actually happening:

  When VM backup is invoked, Qemu agent freezes the file systems, so no single 
change will be made during the backup. But Qemu agent does not respect the 
loop* devices in freezing order (we have checked its sources), which leads to 
the next situation: 
  1) freeze loopback fs
                ---> send async reqs to loopback thread
  2) freeze main fs
  3) loopback thread wakes up and trying to write data to the main fs, which is 
still frozen, and this finally leads to the hung task and kernel crash. 

  I believe this is the culprit:

  /dev/loop0 /tmp ext3 rw,nosuid,noexec,relatime,data=ordered 0 0
  /dev/loop0 /var/tmp ext3 rw,nosuid,noexec,relatime,data=ordered 0 0

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