linux-kernel - Re: mem_cgroup_page_lruvec: BUG: unable to handle kernel NULL pointer dereference at 00000000000001a8

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Date:	Thu, 13 Jun 2013 16:45:55 +0200
From:	Richard Weinberger <richard@....at>
To:	Michal Hocko <mhocko@...e.cz>
CC:	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	cgroups mailinglist <cgroups@...r.kernel.org>,
	"kamezawa.hiroyu@...fujitsu.com" <kamezawa.hiroyu@...fujitsu.com>,
	bsingharora@...il.com, hannes@...xchg.org
Subject: Re: mem_cgroup_page_lruvec: BUG: unable to handle kernel NULL pointer
 dereference at 00000000000001a8

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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