[ISSUE] Hugetlbfs allocations not completely freed by Qemu on kernels 6.6 and 6.7

[abhocetabhac <abhocetabhac@users.noreply.github.com>]

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New issue by abhocetabhac on void-packages repository

https://github.com/void-linux/void-packages/issues/49269

Description:
### Is this a new report?

Yes

### System Info

Void 6.6.x and 6.7.x, musl, AMD Ryzen 5 3600, 32 GB RAM

### Package(s) Affected

linux6.6 and linux6.7

### Does a report exist for this bug with the project's home (upstream) and/or another distro?

I was unable to find anything in the Web about the issue.

### Expected behaviour

All allocated hugetlbfs memory should be freed upon the exit of the virtual machines, similar to the behavior observed with kernels up to 6.5.


### Actual behaviour

With kernels 6.6 and 6.7, 1/4 of the memory allocated to Qemu virtual machines from the hugetlbfs filesystem is not freed upon their exit.


### Steps to reproduce

I use hugetlbfs with 1G huge pages as the memory backend of Qemu virtual machines. Everything used to work fine up to kernel 6.5. But, with kernels 6.6 and 6.7, 1/4 of the memory allocated to virtual machines is not freed upon exit.

My kernel command line is:

linux /boot/vmlinuz root=LABEL=0_voidlinux resume=LABEL=0_swap add_efi_memmap loglevel=7 rootfstype=ext4 rootflags=ro,noatime,data=ordered transparent_hugepage=never hugepagesz=1G hugepages=16 default_hugepagesz=2M transparent_hugepage=always hugepagesz=2M hugepages=2048 psi=1

Setting 'transparent_hugepage=always' or 'madvise' for 1G does not affect the issue.

The hugetlbfs filesystem is mounted with commands:

sysctl -w vm.hugetlb_shm_group=$(getent group kvm | cut -d: -f3)
mkdir -p -m 1777 /var/tmp/hugepages
mount -t hugetlbfs hugetlbfs /var/tmp/hugepages -o mode=1770,gid=$(getent group kvm | cut -d: -f3),pagesize=1g,size=100%

The virtual machines are launched with commands like:

qemu-system-x86_64 -snapshot -drive file=vm.qcow2,if=virtio,index=0,media=disk,cache=directsync,discard=unmap,detect-zeroes=unmap -boot c -device virtio-net-pci,netdev=tap0,mac=52:66:66:00:00:30 -netdev tap,ifname=tap0,id=tap0,script=no,downscript=no -enable-kvm -machine type=q35,accel=kvm -cpu host,kvm=off -smp 4 -m 4G -object memory-backend-file,id=mem,size=4G,mem-path=/var/tmp/hugepages,share=on -numa node,memdev=mem -machine acpi=off -rtc clock=host -device virtio-vga,max_outputs=1 -device ich9-intel-hda -device hda-duplex -usb -device qemu-xhci,id=xhci -device virtio-tablet,wheel-axis=true -display default,show-cursor=on -device virtio-serial-pci -spice unix=on,addr=/tmp/qemu-spice-bb.sock,disable-ticketing=on -device virtserialport,chardev=spicechannel0,name=com.redhat.spice.0 -chardev spicevmc,id=spicechannel0,name=vdagent -monitor unix:/tmp/qemu-monitor-bb.sock,server,nowait -fsdev local,id=share0,path=/tmp/mnt-qemu,security_model=none,writeout=immediate -device virtio-9p-pci,fsdev=share0,mount_tag=share-mtt0 -daemonize

Or, for short,

qemu-system-x86_64 ... -enable-kvm -machine type=q35,accel=kvm -cpu host,kvm=off -smp 4 -m 4G -object memory-backend-file,id=mem,size=4G,mem-path=/var/tmp/hugepages,share=on -numa node,memdev=mem ...


After exiting a virtual machine with 4G of memory, 1G remains allocated. For a VM with 8G, 2G remain allocated. That can be observed with command 'df -h'.

Attempting to forcefully free the memory with the following commands does not effectively resolve the issue:

sudo umount /var/tmp/hugepages
sudo bash -c 'echo 0 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages'
sudo bash -c 'echo 3 > /proc/sys/vm/drop_caches'
sudo bash -c 'echo 1 > /proc/sys/vm/compact_memory'

After exiting virtual machines, the command

grep -R "" /sys/kernel/mm/hugepages/ /proc/sys/vm/*huge*

returns, for exemple,

/sys/kernel/mm/hugepages/hugepages-2048kB/free_hugepages:2048
/sys/kernel/mm/hugepages/hugepages-2048kB/resv_hugepages:0
/sys/kernel/mm/hugepages/hugepages-2048kB/surplus_hugepages:0
/sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages_mempolicy:2048
/sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages:2048
/sys/kernel/mm/hugepages/hugepages-2048kB/nr_overcommit_hugepages:0
/sys/kernel/mm/hugepages/hugepages-1048576kB/demote_size:2048kB
/sys/kernel/mm/hugepages/hugepages-1048576kB/free_hugepages:14
/sys/kernel/mm/hugepages/hugepages-1048576kB/resv_hugepages:0
/sys/kernel/mm/hugepages/hugepages-1048576kB/surplus_hugepages:0
/sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages_mempolicy:16
grep: /sys/kernel/mm/hugepages/hugepages-1048576kB/demote: Permission denied
/sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages:16
/sys/kernel/mm/hugepages/hugepages-1048576kB/nr_overcommit_hugepages:0
/proc/sys/vm/hugetlb_optimize_vmemmap:0
/proc/sys/vm/hugetlb_shm_group:24
/proc/sys/vm/nr_hugepages:2048
/proc/sys/vm/nr_hugepages_mempolicy:2048
/proc/sys/vm/nr_overcommit_hugepages:0