qemu-ga fsfreeze crashes the kernel

[Arthur Lyubochenya]

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/

[Thomas Huth (th-huth)]

We are currently in progress of retiring this bug tracker here... could you please open a new ticket on gitlab instead:

 https://gitlab.com/qemu-project/qemu/-/issues

Thanks and sorry for the inconvenience.

[Thomas Huth (th-huth)]

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