[uml-user] BTRFS : starting an uml using BTRFS gives

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I don't get it in moment, at a 64 bit Gentoo Linux (host is Linux t44 4.0.4-hardened-r3) I get :

$ /home/tfoerste/devel/linux/linux earlyprintk ubda=t44uml ubdb=/mnt/ramdisk/t44uml_swap eth0=tuntap,tap0,72:ef:3d:b6:67:0c,192.168.1.254 mem=2000M con0=fd:0,fd:1 con=pts umid=uml_t44uml softlockup_all_cpu_backtrace=1
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
bootconsole [earlycon0] enabled
PID hash table entries: 4096 (order: 3, 32768 bytes)
Dentry cache hash table entries: 262144 (order: 9, 2097152 bytes)
Inode-cache hash table entries: 131072 (order: 8, 1048576 bytes)
Memory: 2009296K/2048000K available (3008K kernel code, 673K rwdata, 892K rodata, 113K init, 173K bss, 38704K reserved, 0K cma-reserved)
NR_IRQS:15
clocksource itimer: mask: 0xffffffffffffffff max_cycles: 0x1d854df40, max_idle_ns: 3526361616960 ns
Calibrating delay loop... 5746.68 BogoMIPS (lpj=28733440)
pid_max: default: 32768 minimum: 301
Mount-cache hash table entries: 4096 (order: 3, 32768 bytes)
Mountpoint-cache hash table entries: 4096 (order: 3, 32768 bytes)
Initializing cgroup subsys blkio
Initializing cgroup subsys devices
Initializing cgroup subsys freezer
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
xor: measuring software checksum speed
   8regs     : 20528.400 MB/sec
   8regs_prefetch: 17211.600 MB/sec
   32regs    : 19893.600 MB/sec
   32regs_prefetch: 16321.600 MB/sec
xor: using function: 8regs (20528.400 MB/sec)
NET: Registered protocol family 16
raid6: int64x1  gen()  3012 MB/s
raid6: int64x1  xor()  1664 MB/s
raid6: int64x2  gen()  3137 MB/s
raid6: int64x2  xor()  2110 MB/s
raid6: int64x4  gen()  2882 MB/s
raid6: int64x4  xor()  1801 MB/s
raid6: int64x8  gen()  2464 MB/s
raid6: int64x8  xor()  1423 MB/s
raid6: using algorithm int64x2 gen() 3137 MB/s
raid6: .... xor() 2110 MB/s, rmw enabled
raid6: using intx1 recovery algorithm
Switched to clocksource itimer
NET: Registered protocol family 2
TCP established hash table entries: 16384 (order: 5, 131072 bytes)
TCP bind hash table entries: 16384 (order: 5, 131072 bytes)
TCP: Hash tables configured (established 16384 bind 16384)
UDP hash table entries: 1024 (order: 3, 32768 bytes)
UDP-Lite hash table entries: 1024 (order: 3, 32768 bytes)
NET: Registered protocol family 1
console [stderr0] disabled
mconsole (version 2) initialized on /home/tfoerste/.uml/uml_t44uml/mconsole
Checking host MADV_REMOVE support...OK
futex hash table entries: 256 (order: 0, 6144 bytes)
VFS: Disk quotas dquot_6.6.0
VFS: Dquot-cache hash table entries: 512 (order 0, 4096 bytes)
io scheduler noop registered
io scheduler deadline registered (default)
NET: Registered protocol family 17
Initialized stdio console driver
Console initialized on /dev/tty0
                                console [tty0] enabled
console [tty0] enabled
                      bootconsole [earlycon0] disabled
bootconsole [earlycon0] disabled
                                Initializing software serial port version 1
console [mc-1] enabled
Failed to initialize ubd device 1 :Couldn't determine size of device's file
Netdevice 0 (72:ef:3d:b6:67:0c) :
TUN/TAP backend - IP = 192.168.1.254
Btrfs loaded
BTRFS: device fsid 3342a6f8-d944-44df-bffb-f64ac9f0bdc8 devid 1 transid 51 /dev/root
BTRFS info (device ubda): disk space caching is enabled
BTRFS: has skinny extents
VFS: Mounted root (btrfs filesystem) readonly on device 0:13.
devtmpfs: mounted

Modules linked in:
Pid: 1, comm: swapper Not tainted 4.1.0-rc5-00161-gaaa20fc
RIP: 0033:[<00000000602a150d>]
RSP: 0000032dff262ef8  EFLAGS: 00010212
RAX: 00000000602a14f9 RBX: 0000000060706da8 RCX: 0000032dfeb94f19
RDX: 0000000060286650 RSI: 0000032dff263fe8 RDI: 00000000dcede000
RBP: 0000032dff263fe8 R08: 0000000000000000 R09: 0000000000000000
R10: 00000000dc81fbd0 R11: 0000000000000246 R12: 00000000dcede000
R13: 0000032dff263000 R14: 0000000060286b60 R15: 00000000603277f8
Kernel panic - not syncing: Segfault with no mm
CPU: 0 PID: 1 Comm: swapper Not tainted 4.1.0-rc5-00161-gaaa20fc #2
Stack:

Modules linked in:
Pid: 1, comm: swapper Not tainted 4.1.0-rc5-00161-gaaa20fc
RIP: 0033:[<00000000602898cc>]
RSP: 0000000060665450  EFLAGS: 00010202
RAX: 0000000000000006 RBX: 000000006057088f RCX: 0000000060706da8
RDX: 0000000000000000 RSI: 0000000060715af8 RDI: 0000000000000000
RBP: 0000000060665480 R08: 0000000060665059 R09: 00000000602cf270
R10: 0000000000000008 R11: 00000000004685dd R12: 0000000000000001
R13: 0000032dff262eff R14: 00000000606656f0 R15: 0000000000000001
Kernel panic - not syncing: Segfault with no mm

$ addr2line -e ~/devel/linux/linux 00000000602a150d
/home/tfoerste/devel/linux/arch/um/os-Linux/skas/process.c:180

$ addr2line -e ~/devel/linux/linux 00000000602898cc
/home/tfoerste/devel/linux/arch/um/kernel/sysrq.c:52

The /etc/fstab of the 64 bit Gentoo Linux UML guest is :

/dev/ubda               /               btrfs           noatime         0 1
/dev/ubdb               none            swap            sw              0 0

shm                     /dev/shm        tmpfs           nodev,nosuid,noexec     0 0

What do I miss ?

-- 
Toralf
pgp key: 7B1A 07F4 EC82 0F90 D4C2  8936 872A E508 0076 E94E