AMD-64 UML kernel panic, using Gentoo guide

[pmeunier]

I tried configuring and running a 2.6.16 guest kernel, using
stage3-amd64-2006.0.tar.bz2 to generate the root_fs, on an AMD-64 but I get a kernel panic (see below), while following the instructions at http://www.gentoo.org/doc/en/uml.xml. The host OS is:
Linux version 2.6.15-gentoo-r7-skas3-v9-pre8 (gcc version 3.4.5 (Gentoo 3.4.5, ssp-3.4.5-1.0, pie-8.7.9))

Any idea what I could try to get this working, or what I could check? I have no experience with Gentoo Linux so I could very likely have done something silly. I assumed that I could start the guest kernel from my home directory instead of moving it to /usr/local/bin/linux, as the instructions suggest.

Thanks,
Pascal

$ ./linux ubd0=root_fs ubd1=swap_fs
Checking that ptrace can change system call numbers...OK
Checking syscall emulation patch for ptrace...missing
Checking PROT_EXEC mmap in /tmp...OK
Checking for the skas3 patch in the host:
- /proc/mm...found
- PTRACE_FAULTINFO...not found
- PTRACE_LDT...found
UML running in SKAS0 mode
Linux version 2.6.16 (gcc version 3.4.5 (Gentoo 3.4.5, ssp-3.4.5-1.0, pie-8.7.9)) #1 Fri May 26 11:38:56 EDT 2006
Built 1 zonelists
Kernel command line: ubd0=root_fs ubd1=swap_fs root=98:0
PID hash table entries: 256 (order: 8, 8192 bytes)
Dentry cache hash table entries: 8192 (order: 4, 65536 bytes)
Inode-cache hash table entries: 4096 (order: 3, 32768 bytes)
Memory: 29240k available
Mount-cache hash table entries: 256
Checking that host ptys support output SIGIO...Yes
Checking that host ptys support SIGIO on close...No, enabling workaround
Checking for /dev/anon on the host...Not available (open failed with errno 2)
Using 2.6 host AIO
NET: Registered protocol family 16
mconsole (version 2) initialized on /home/pmeunier/.uml/m32itj/mconsole
ubd: Synchronous mode
VFS: Disk quotas dquot_6.5.1
Dquot-cache hash table entries: 512 (order 0, 4096 bytes)
io scheduler noop registered
io scheduler anticipatory registered (default)
io scheduler deadline registered
io scheduler cfq registered
NET: Registered protocol family 2
IP route cache hash table entries: 512 (order: 0, 4096 bytes)
TCP established hash table entries: 2048 (order: 2, 16384 bytes)
TCP bind hash table entries: 2048 (order: 2, 16384 bytes)
TCP: Hash tables configured (established 2048 bind 2048)
TCP reno registered
TCP bic registered
NET: Registered protocol family 1
NET: Registered protocol family 17
Initialized stdio console driver
Console initialized on /dev/tty0
Initializing software serial port version 1
isa bounce pool size: 16 pages
ubda: unknown partition table
ubdb: unknown partition table
VFS: Mounted root (ext2 filesystem) readonly.
Kernel panic - not syncing: map_stub_pages : /proc/mm map for code failed, err = 22


Modules linked in:
Pid: 1, comm: swapper Not tainted 2.6.16
RIP: 0033:[<00002aaaaacf4b59>]
RSP: 00002aaaaaef1fc0 EFLAGS: 00000216
RAX: 0000000000000000 RBX: 0000000000004869 RCX: ffffffffffffffff
RDX: 0000000000000000 RSI: 0000000000000013 RDI: 0000000000004869
RBP: 00002aaaaaef1fc8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000216 R12: 0000000000004866
R13: 00002aaaaaef1000 R14: 00007fffffad78e8 R15: 00007fffffad7af8
Call Trace:
609238e8: [<60013b26>] panic_exit+0x2d/0x47
60923908: [<60036782>] notifier_call_chain+0x25/0x38
60923938: [<600288b8>] panic+0xe4/0x17a
60923958: [<600208f5>] set_signals+0x19/0x26
60923980: [<6001078d>] copy_to_user_proc+0x0/0xb
60923998: [<6004ac75>] rmqueue_bulk+0x2e/0x67
609239b8: [<600208f5>] set_signals+0x19/0x26
609239c8: [<6001f003>] file_io+0xac/0xcc
609239d8: [<6000d7a0>] _init+0x138/0x808
60923a28: [<60023f2b>] map_stub_pages+0xe3/0x1b4
60923ab8: [<6004b7d0>] mod_page_state_offset+0x2c/0x34
60923b38: [<600142a3>] new_mm+0x48/0x4f
60923b58: [<60013e36>] init_new_context_skas+0x118/0x18b
60923ba8: [<6006e37e>] do_execve+0xbf/0x22a
60923bd0: [<6000dfc8>] init+0x0/0x140
60923bf8: [<6000e734>] execve1+0x26/0x77
60923c38: [<6000e790>] um_execve+0xb/0x3c
60923c58: [<6000dfa1>] run_init_process+0x48/0x6f
60923c88: [<6000e0dc>] init+0x114/0x140
60923cb8: [<6002057b>] run_kernel_thread+0x59/0x66
60923cd8: [<6000dfc8>] init+0x0/0x140
60923cf0: [<6000dfc8>] init+0x0/0x140
60923d18: [<60020559>] run_kernel_thread+0x37/0x66
60923d60: [<600240be>] thread_wait+0x30/0x5a
60923d78: [<60024bc9>] schedule_tail+0x2b/0xff
60923d98: [<6000dfc8>] init+0x0/0x140
60923db0: [<6000dfc8>] init+0x0/0x140
60923dc8: [<60014084>] new_thread_handler+0xf1/0x127

[pmeunier]

I downloaded various kernels and root filesystems, and no matter the combination I end up with the same kernel panic about map_stub_pages not syncing and /proc/mm map for code failed with err = 22

I found a comment (http://lkml.org/lkml/2005/9/1/372) stating:
+/*
+ * This is used only, if proc_mm is available, while PTRACE_FAULTINFO
+ * isn't. Opening /proc/mm creates a new mm_context, which lacks the stub-pages
+ * Thus, we map them using /proc/mm-fd
+ */

+ n = os_write_file(fd, &mmop, sizeof(mmop));
+ if(n != sizeof(mmop))
+ panic("map_stub_pages : /proc/mm map for code failed, "
+ "err = %d\n", -n);

I tried to find out what error 22 meant, but looking at os_write_file, it calls another function that calls another and error codes are constant names, and I get lost. What does error 22 mean? It looks like it's a problem with my Gentoo host installation, as it happens no matter which guest UML kernel I use, but what needs fixing?
Thanks