[Bug 1939179] Re: qemu-ga fsfreeze crashes the kernel

[Thomas Huth]

https://gitlab.com/qemu-project/qemu/-/issues/520
... thanks!

** Bug watch added: gitlab.com/qemu-project/qemu/-/issues #520
   https://gitlab.com/qemu-project/qemu/-/issues/520

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

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https://bugs.launchpad.net/bugs/1939179

Title:
  qemu-ga fsfreeze crashes the kernel

Status in QEMU:
  Invalid

Bug description:
  Hello,

  Still required your attention, duplicate from:
  https://bugs.launchpad.net/bugs/1807073
  https://bugs.launchpad.net/bugs/1813045

  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.

  Moreover, a lot of Proxmox users are complaining about the issue as well:
  
https://forum.proxmox.com/threads/error-vm-100-qmp-command-guest-fsfreeze-thaw-failed-got-timeout.68082/
  
https://forum.proxmox.com/threads/problem-with-fsfreeze-freeze-and-qemu-guest-agent.65707/

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