device: 0.196
files: 0.190
other: 0.136
semantic: 0.113
PID: 0.072
graphic: 0.049
performance: 0.049
vnc: 0.042
debug: 0.041
network: 0.026
socket: 0.024
permissions: 0.023
boot: 0.022
KVM: 0.019
debug: 0.451
files: 0.124
other: 0.101
semantic: 0.070
performance: 0.038
device: 0.038
network: 0.034
socket: 0.034
PID: 0.029
permissions: 0.019
vnc: 0.018
graphic: 0.017
boot: 0.016
KVM: 0.010

qemu-ga fsfreeze crashes the kernel

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

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.

[Expired for QEMU because there has been no activity for 60 days.]

See also https://gitlab.com/qemu-project/qemu/-/issues/520