How do I run the usermode linux kernel?

I followed the steps here and compiled the kernel successfully in usermode: https://btrfs.wiki.kernel.org/index.php/Debugging_Btrfs_with_GDB But when I start ./linux in various ways it always gives me a very similar error: pc@linux-94q0:~/linux-4.11-rc4> ./linux root=/mnt Core dump limits : soft - 0 hard - NONE Checking that ptrace can change system call numbers...OK Checking syscall emulation patch for ptrace...OK Checking advanced syscall emulation patch for ptrace...OK Checking environment variables for a tempdir...none found Checking if /dev/shm is on tmpfs...OK Checking PROT_EXEC mmap in /dev/shm...OK Adding 33251328 bytes to physical memory to account for exec-shield gap Linux version 4.11.0-rc4 (pc@linux-94q0) (gcc version 4.8.5 (SUSE Linux) ) #1 Fri Mar 31 12:40:07 CEST 2017 Built 1 zonelists in Zone order, mobility grouping on. Total pages: 16087 Kernel command line: root=/mnt PID hash table entries: 256 (order: -1, 2048 bytes) Dentry cache hash table entries: 8192 (order: 4, 65536 bytes) Inode-cache hash table entries: 4096 (order: 3, 32768 bytes) Memory: 26140K/65240K available (3518K kernel code, 770K rwdata, 948K rodata, 114K init, 195K bss, 39100K reserved, 0K cma-reserved) NR_IRQS:15 clocksource: timer: mask: 0xffffffffffffffff max_cycles: 0x1cd42e205, max_idle_ns: 881590404426 ns Calibrating delay loop... 6966.47 BogoMIPS (lpj=34832384) pid_max: default: 32768 minimum: 301 Mount-cache hash table entries: 512 (order: 0, 4096 bytes) Mountpoint-cache hash table entries: 512 (order: 0, 4096 bytes) Checking that host ptys support output SIGIO...Yes Checking that host ptys support SIGIO on close...No, enabling workaround devtmpfs: initialized Using 2.6 host AIO clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 19112604462750000 ns futex hash table entries: 256 (order: 0, 6144 bytes) xor: measuring software checksum speed 8regs : 19749.600 MB/sec 8regs_prefetch: 17312.000 MB/sec 32regs : 18694.400 MB/sec 32regs_prefetch: 17317.600 MB/sec xor: using function: 8regs (19749.600 MB/sec) NET: Registered protocol family 16 raid6: int64x1 gen() 4139 MB/s raid6: int64x1 xor() 2318 MB/s raid6: int64x2 gen() 3758 MB/s raid6: int64x2 xor() 2685 MB/s raid6: int64x4 gen() 3413 MB/s raid6: int64x4 xor() 2153 MB/s raid6: int64x8 gen() 2865 MB/s raid6: int64x8 xor() 1626 MB/s raid6: using algorithm int64x1 gen() 4139 MB/s raid6: .... xor() 2318 MB/s, rmw enabled raid6: using intx1 recovery algorithm clocksource: Switched to clocksource timer VFS: Disk quotas dquot_6.6.0 VFS: Dquot-cache hash table entries: 512 (order 0, 4096 bytes) NET: Registered protocol family 2 TCP established hash table entries: 512 (order: 0, 4096 bytes) TCP bind hash table entries: 512 (order: 0, 4096 bytes) TCP: Hash tables configured (established 512 bind 512) UDP hash table entries: 256 (order: 1, 8192 bytes) UDP-Lite hash table entries: 256 (order: 1, 8192 bytes) NET: Registered protocol family 1 console [stderr0] disabled mconsole (version 2) initialized on /home/pc/.uml/y33GMV/mconsole Checking host MADV_REMOVE support...OK workingset: timestamp_bits=62 max_order=13 bucket_order=0 io scheduler noop registered io scheduler deadline registered (default) io scheduler mq-deadline registered NET: Registered protocol family 17 Initialized stdio console driver Console initialized on /dev/tty0 console [tty0] enabled Initializing software serial port version 1 console [mc-1] enabled Failed to initialize ubd device 0 :Couldn't determine size of device's file Btrfs loaded, crc32c=crc32c-generic, debug=on VFS: Cannot open root device "/mnt" or unknown-block(0,0): error -6 Please append a correct "root=" boot option; here are the available partitions: Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0) CPU: 0 PID: 1 Comm: swapper Not tainted 4.11.0-rc4 #1 Stack: 6381bd80 60066344 602a250a 62cab500 602a250a 600933ba 6381bd90 60297e6f 6381beb0 60092b41 6381be30 60380ea1 Call Trace: [] ? printk+0x0/0x94 [] show_stack+0xfe/0x158 [] ? dump_stack_print_info+0xe1/0xea [] ? bust_spinlocks+0x0/0x4f [] ? bust_spinlocks+0x0/0x4f [] ? printk+0x0/0x94 [] dump_stack+0x2a/0x2c [] panic+0x173/0x322 [] ? klist_next+0x0/0xa6 [] ? panic+0x0/0x322 [] ? kfree+0x0/0x8a [] ? SyS_mount+0xae/0xc0 [] ? printk+0x0/0x94 [] ? SyS_mount+0x0/0xc0 [] mount_block_root+0x356/0x374 [] ? strcpy+0x0/0x18 [] mount_root+0x9c/0xa0 [] ? strncmp+0x0/0x25 [] prepare_namespace+0x1de/0x238 [] ? SyS_dup+0x0/0x5e [] kernel_init_freeable+0x300/0x31b [] ? printk+0x0/0x94 [] kernel_init+0x1c/0x14a [] new_thread_handler+0x81/0xa3 Aborted (core dumped) I have now tried everything I could think of to satisfy the ./linux root= option but nothing seems to work. 1. I created a root filesystem with https://buildroot.org/ , passed that as .gz, .tar, .tar.gz, uncompressed folder 2. I put the contents of the buildroot.org into the btrfs loop device, then right clicked in the disks utility and created an .img file. Tried starting with that. 3. Of course I tried all the usual options I could think of, like ./linux root=/mnt, ./linux root=/dev/loop0 I don't know what else to try. Why is this not working? I tried finding out what the -6 error code means, but it seems all the Linux kernel error codes are positive numbers. https://gist.github.com/nullunar/4553641 I really don't know what else to do, I guess I could start reading up for hours and hours about what exactly the udb stuff means, but I was really hoping somebody could just tell me what I need to pass to the command line as my interest right now is only in debugging btrfs, not Linux in general.