Re: mem_cgroup_page_lruvec: BUG: unable to handle kernel NULL pointerdereference at 00000000000001a8

From: Richard Weinberger
Date: Thu Jun 13 2013 - 10:46:06 EST


Am 13.06.2013 16:39, schrieb Michal Hocko:
On Thu 13-06-13 15:34:59, Richard Weinberger wrote:
Am 13.06.2013 15:32, schrieb Michal Hocko:
Ohh and could you post the config please? Sorry should have asked
earlier.

See attachment.

Nothing unusual there. Could you enable CONFIG_DEBUG_VM maybe it will
help too catch the problem earlier.

OK

On Thu 13-06-13 15:29:08, Michal Hocko wrote:

On Thu 13-06-13 14:06:20, Richard Weinberger wrote:
[...]
All code
========
0: 89 50 08 mov %edx,0x8(%rax)
3: 48 89 d1 mov %rdx,%rcx
6: 0f 1f 40 00 nopl 0x0(%rax)
a: 49 8b 04 24 mov (%r12),%rax
e: 48 89 c2 mov %rax,%rdx
11: 48 c1 e8 38 shr $0x38,%rax
15: 83 e0 03 and $0x3,%eax
nid = page_to_nid
18: 48 c1 ea 3a shr $0x3a,%rdx
zid = page_zonenum

Ohh, I am wrong here. rdx should be nid and eax the zid.


1c: 48 69 c0 38 01 00 00 imul $0x138,%rax,%rax
23: 48 03 84 d1 e0 02 00 add 0x2e0(%rcx,%rdx,8),%rax
&memcg->nodeinfo[nid]->zoneinfo[zid]

2a: 00
2b:* 48 3b 58 70 cmp 0x70(%rax),%rbx <-- trapping instruction

OK, so this maps to:
if (unlikely(lruvec->zone != zone)) <<<
lruvec->zone = zone;

[35355.883056] RSP: 0000:ffff88003d523aa8 EFLAGS: 00010002
[35355.883056] RAX: 0000000000000138 RBX: ffff88003fffa600 RCX: ffff88003e04a800
[35355.883056] RDX: 0000000000000020 RSI: 0000000000000000 RDI: 0000000000028500
[35355.883056] RBP: ffff88003d523ab8 R08: 0000000000000000 R09: 0000000000000000
[35355.883056] R10: 0000000000000000 R11: dead000000100100 R12: ffffea0000a14000
[35355.883056] R13: ffff88003e04b138 R14: ffff88003d523bb8 R15: ffffea0000a14020
[35355.883056] FS: 0000000000000000(0000) GS:ffff88003fd80000(0000)

RAX (lruvec) is obviously incorrect and it doesn't make any sense. rax should
contain an address at an offset from ffff88003e04a800 But there is 0x138 there
instead.

Hmm, now that I am looking at the registers again. RDX which should be
nid seems to be quite big. It says this is node 32. Does the machine
have really so many NUMA nodes?

No. It's a KVM guest with two CPUs. Nothing special.
qemu command line:
qemu-kvm -m 1G -drive file=lxc_host.qcow2,if=virtio -nographic -kernel linux/arch/x86/boot/bzImage -append console=ttyS0 root=/dev/vda2 -net user,hostfwd=tcp::5555-:22 -net nic,model=e1000 -smp 4

Thanks,
//richard
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