Getting segfault when trying to use MemoryBalloonDevices

[afbjorklund]

Describe the bug
Getting segfault when trying to use MemoryBalloonDevices

To Reproduce
Steps to reproduce the behavior:

1. The command runs OK, and size is changed.
2. But when the go finalizer runs, it segfaults.

func (d *LimaVzDriver) SetTargetMemory(memory int64) error {
        if d.machine == nil {
                return fmt.Errorf("no machine!")
        }
        balloons := d.machine.MemoryBalloonDevices()
        if len(balloons) != 1 {
                return fmt.Errorf("unexpected number of devices: %d", len(balloons))
        }
        balloon := vz.AsVirtioTraditionalMemoryBalloonDevice(balloons[0])
        if balloon == nil {
                return errors.New("unexpected type of balloon")
        }
        balloon.SetTargetVirtualMachineMemorySize(uint64(memory))
        return nil
}

runtime.cgocall(0x10394cb0c, 0x14000076508)
        /opt/homebrew/Cellar/go/1.25.1/libexec/src/runtime/cgocall.go:167 +0x44 fp=0x140000764d0 sp=0x14000076490 pc=0x102df3bf4
github.com/Code-Hex/vz/v3/internal/objc._Cfunc_releaseNSObject(0x600003cd1350)
        _cgo_gotypes.go:204 +0x2c fp=0x14000076500 sp=0x140000764d0 pc=0x10389749c
github.com/Code-Hex/vz/v3/internal/objc.(*Pointer).release.func1(...)
        /Users/afb/go/pkg/mod/github.com/!code-!hex/vz/v3@v3.7.1/internal/objc/objc.go:85
github.com/Code-Hex/vz/v3/internal/objc.(*Pointer).release(0x1400011c500)
        /Users/afb/go/pkg/mod/github.com/!code-!hex/vz/v3@v3.7.1/internal/objc/objc.go:85 +0x48 fp=0x14000076540 sp=0x14000076500 pc=0x1038975f8
github.com/Code-Hex/vz/v3/internal/objc.Release(...)
        /Users/afb/go/pkg/mod/github.com/!code-!hex/vz/v3@v3.7.1/internal/objc/objc.go:112
github.com/Code-Hex/vz/v3.newVirtioTraditionalMemoryBalloonDevice.func1(0x0?)
        /Users/afb/go/pkg/mod/github.com/!code-!hex/vz/v3@v3.7.1/memory_balloon.go:124 +0x24 fp=0x14000076560 sp=0x14000076540 pc=0x1038a73a4
runtime.call16(0x0, 0x103ecec40, 0x1400015e180, 0x10, 0x10, 0x10, 0x14000076600)
        /opt/homebrew/Cellar/go/1.25.1/libexec/src/runtime/asm_arm64.s:554 +0x78 fp=0x14000076580 sp=0x14000076560 pc=0x102dfd598
runtime.runFinalizers()
        /opt/homebrew/Cellar/go/1.25.1/libexec/src/runtime/mfinal.go:272 +0x3fc fp=0x140000767d0 sp=0x14000076580 pc=0x102da070c
runtime.goexit({})
        /opt/homebrew/Cellar/go/1.25.1/libexec/src/runtime/asm_arm64.s:1268 +0x4 fp=0x140000767d0 sp=0x140000767d0 pc=0x102dff584

Expected behavior
I'm not sure if the devices are supposed to be Released?

func newVirtioTraditionalMemoryBalloonDevice(pointer unsafe.Pointer, vm *VirtualMachine) *VirtioTraditionalMemoryBalloonDevice {
        device := &VirtioTraditionalMemoryBalloonDevice{
                pointer: objc.NewPointer(pointer),
                vm:      vm,
        }
        objc.SetFinalizer(device, func(self *VirtioTraditionalMemoryBalloonDevice) {
                objc.Release(self)
        })
        return device
}

Since we didn't allocate the memory, it was given to us.

https://developer.apple.com/documentation/virtualization/vzvirtualmachine/memoryballoondevices

Screenshots
If applicable, add screenshots to help explain your problem.

Environment that you use to compile (please complete the following information):

• Xcode version: 16.4
• macOS Version: 15.7.1
• mac architecture: arm64
• Go Version: 1.25.3

Additional context
Add any other context about the problem here.

[afbjorklund]

I can't see that any memory is ever returned to the host, but did a workaround for the crash:

runtime.SetFinalizer(balloon, nil)

Not sure if there is anything needed in the Linux guest, or if it just not implemented in macOS?

The memory balloon is inflated in the guest, that is the memory does disappear from the guest.

---

$ _output/bin/limactl memory set default 8192
WARN[0000] `limactl memory` is experimental             
$ lima free -m
               total        used        free      shared  buff/cache   available
Mem:            7919         382        7274           1         424        7536
Swap:              0           0           0
$ _output/bin/limactl memory set default 1024
WARN[0000] `limactl memory` is experimental             
$ lima free -m
               total        used        free      shared  buff/cache   available
Mem:            7919        7545         177           1         358         373
Swap:              0           0           0

CONFIG_MEMORY_BALLOON=y
CONFIG_BALLOON_COMPACTION=y
CONFIG_VIRTIO_BALLOON=y
CONFIG_HYPERV_BALLOON=m

But the host still uses the full 8GiB, instead of just 1GiB

PID COMMAND %CPU TIME #TH #WQ #PORTS MEM PURG CMPRS PGRP PPID STATE BOOSTS %CPU_ME %CPU_OTHRS UID FAULTS COW MSGSENT MSGRECV SYSBSD SYSMACH CSW PAGEINS IDLEW POWER INSTRS CYCLES JETPRI USER #MREGS RPRVT VPRVT VSIZE KPRVT KSHRD
94918 com.apple.Virtua 0.0 00:22.23 18 1 169 8196M 0B 1039M 94918 1 sleeping *15[3] 0.00000 0.00000 501 911622 163 1006 255 1185977 1435 249192 17 2607 0.0 0 0 40 afb N/A N/A N/A N/A N/A N/A