Fastmap kernel panic

Hi,
I am trying to use Fastmap in combination with libvmmalloc or vmem to offload memory for quantum chemistry applications to many nvme-drives. I have gotten libvmmalloc and pmem to work. However, Fastmap gives me some trouble by causing kernel panics.
Could you please have a look an give or do you have any idea what the reason could be? Or which additional information would be helpful.

Steps to reproduce:
- Debian oldstable with Mainline Kernel 4.14.123 running in qemu 7.0.0
- fastmap commit 1af50c1
- swap off, transparent huge pages disabled
- modprobe dmap perma_mmap_buf_size=16
- /dev/dmap/dmap1 created using scripts/load-it-blkdev.sh
- rand_threads_read /dev/dmap/dmap1 2
- seems to work in the sense that is read from the device
- rand_threads_write /dev/dmap/dmap1 2
- starts to fill up the dram memory and finally leads to a kernel panic:


Things I have tried so far:
- reducing the size of the cache (perma_mmap_buf_size=16)