Message-ID: <d8416690-072c-7d06-0aaf-3a726ce68a47@linaro.org>
Date:	Tue, 24 May 2016 09:37:21 -0700
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH] mm: page_is_guard return false when page_ext arrays are
 not allocated yet

On 5/23/2016 10:37 PM, Joonsoo Kim wrote:
> On Fri, May 20, 2016 at 10:00:06AM -0700, Shi, Yang wrote:
>> On 5/19/2016 7:40 PM, Joonsoo Kim wrote:
>>> 2016-05-20 2:18 GMT+09:00 Shi, Yang <yang.shi@...aro.org>:
>>>> On 5/18/2016 5:28 PM, Joonsoo Kim wrote:
>>>>>
>>>>> Vlastiml, thanks for ccing me on original bug report.
>>>>>
>>>>> On Wed, May 18, 2016 at 03:23:45PM -0700, Yang Shi wrote:
>>>>>>
>>>>>> When enabling the below kernel configs:
>>>>>>
>>>>>> CONFIG_DEFERRED_STRUCT_PAGE_INIT
>>>>>> CONFIG_DEBUG_PAGEALLOC
>>>>>> CONFIG_PAGE_EXTENSION
>>>>>> CONFIG_DEBUG_VM
>>>>>>
>>>>>> kernel bootup may fail due to the following oops:
>>>>>>
>>>>>> BUG: unable to handle kernel NULL pointer dereference at           (null)
>>>>>> IP: [<ffffffff8118d982>] free_pcppages_bulk+0x2d2/0x8d0
>>>>>> PGD 0
>>>>>> Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
>>>>>> Modules linked in:
>>>>>> CPU: 11 PID: 106 Comm: pgdatinit1 Not tainted 4.6.0-rc5-next-20160427 #26
>>>>>> Hardware name: Intel Corporation S5520HC/S5520HC, BIOS
>>>>>> S5500.86B.01.10.0025.030220091519 03/02/2009
>>>>>> task: ffff88017c080040 ti: ffff88017c084000 task.ti: ffff88017c084000
>>>>>> RIP: 0010:[<ffffffff8118d982>]  [<ffffffff8118d982>]
>>>>>> free_pcppages_bulk+0x2d2/0x8d0
>>>>>> RSP: 0000:ffff88017c087c48  EFLAGS: 00010046
>>>>>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000001
>>>>>> RDX: 0000000000000980 RSI: 0000000000000080 RDI: 0000000000660401
>>>>>> RBP: ffff88017c087cd0 R08: 0000000000000401 R09: 0000000000000009
>>>>>> R10: ffff88017c080040 R11: 000000000000000a R12: 0000000000000400
>>>>>> R13: ffffea0019810000 R14: ffffea0019810040 R15: ffff88066cfe6080
>>>>>> FS:  0000000000000000(0000) GS:ffff88066cd40000(0000)
>>>>>> knlGS:0000000000000000
>>>>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>>> CR2: 0000000000000000 CR3: 0000000002406000 CR4: 00000000000006e0
>>>>>> Stack:
>>>>>> ffff88066cd5bbd8 ffff88066cfe6640 0000000000000000 0000000000000000
>>>>>> 0000001f0000001f ffff88066cd5bbe8 ffffea0019810000 000000008118f53e
>>>>>> 0000000000000009 0000000000000401 ffffffff0000000a 0000000000000001
>>>>>> Call Trace:
>>>>>> [<ffffffff8118f602>] free_hot_cold_page+0x192/0x1d0
>>>>>> [<ffffffff8118f69c>] __free_pages+0x5c/0x90
>>>>>> [<ffffffff8262a676>] __free_pages_boot_core+0x11a/0x14e
>>>>>> [<ffffffff8262a6fa>] deferred_free_range+0x50/0x62
>>>>>> [<ffffffff8262aa46>] deferred_init_memmap+0x220/0x3c3
>>>>>> [<ffffffff8262a826>] ? setup_per_cpu_pageset+0x35/0x35
>>>>>> [<ffffffff8108b1f8>] kthread+0xf8/0x110
>>>>>> [<ffffffff81c1b732>] ret_from_fork+0x22/0x40
>>>>>> [<ffffffff8108b100>] ? kthread_create_on_node+0x200/0x200
>>>>>> Code: 49 89 d4 48 c1 e0 06 49 01 c5 e9 de fe ff ff 4c 89 f7 44 89 4d b8
>>>>>> 4c 89 45 c0 44 89 5d c8 48 89 4d d0 e8 62 c7 07 00 48 8b 4d d0 <48> 8b 00 44
>>>>>> 8b 5d c8 4c 8b 45 c0 44 8b 4d b8 a8 02 0f 84 05 ff
>>>>>> RIP  [<ffffffff8118d982>] free_pcppages_bulk+0x2d2/0x8d0
>>>>>> RSP <ffff88017c087c48>
>>>>>> CR2: 0000000000000000
>>>>>>
>>>>>> The problem is lookup_page_ext() returns NULL then page_is_guard() tried
>>>>>> to
>>>>>> access it in page freeing.
>>>>>>
>>>>>> page_is_guard() depends on PAGE_EXT_DEBUG_GUARD bit of page extension
>>>>>> flag, but
>>>>>> freeing page might reach here before the page_ext arrays are allocated
>>>>>> when
>>>>>> feeding a range of pages to the allocator for the first time during
>>>>>> bootup or
>>>>>> memory hotplug.
>>>>>
>>>>>
>>>>> Patch itself looks find to me because I also found that this kind of
>>>>> problem happens during memory hotplug. So, we need to fix more sites,
>>>>> all callers of lookup_page_ext().
>>>>
>>>>
>>>> Yes, I agree. I will come up with a patch or a couple of patches to check
>>>> the return value of lookup_page_ext().
>>>>
>>>>>
>>>>> But, I'd like to know how your problem occurs during bootup.
>>>>> debug_guardpage_enabled() is turned to 'enable' after page_ext is
>>>>> initialized. Before that, page_is_guard() unconditionally returns
>>>>> false so I think that the problem what you mentioned can't happen.
>>>>>
>>>>> Could you check that when debug_guardpage_enabled() returns 'enable'
>>>>> and init_section_page_ext() is called?
>>>>
>>>>
>>>> I think the problem is I have CONFIG_DEFERRED_STRUCT_PAGE_INIT enabled,
>>>> which will defer some struct pages initialization to "pgdatinitX" kernel
>>>> thread in page_alloc_init_late(). But, page_ext_init() is called before it.
>>>> So, it leads debug_guardpage_enabled() return true, but page extension is
>>>> not allocated yet for the struct pages initialized by "pgdatinitX".
>>>
>>> No. After page_ext_init(), it is ensured that all page extension is initialized.
>>>
>>>> It sounds page_ext_init() should be called after page_alloc_init_late(). Or
>>>> it should be just incompatible with CONFIG_DEFERRED_STRUCT_PAGE_INIT.
>>>>
>>>> I will try to move the init call around.
>>>
>>> We need to investigate more. I guess that problem is introduced by
>>> CONFIG_DEFERRED_STRUCT_PAGE_INIT. It makes pfn_to_nid() invalid
>>> until page_alloc_init_late() is done. That is a big side-effect. If
>>> there is pfn walker
>>> and it uses pfn_to_nid() between memmap_init_zone() and page_alloc_init_late(),
>>> it also has same problem. So, we need to think how to fix it more
>>> carefully.
>>
>> Thanks for the analysis. I think you are correct. Since pfn_to_nid()
>> depends on memmap which has not been fully setup yet until
>> page_alloc_init_late() is done.
>>
>> So, for such usecase early_pfn_to_nid() should be used.
>>
>>>
>>> Anyway, to make sure that my assumption is true, could you confirm that
>>> below change fix your problem?
>>
>> Yes, it does.
>>
>>>
>>> Thanks.
>>>
>>> ----->8----------
>>> diff --git a/mm/page_ext.c b/mm/page_ext.c
>>> index 2d864e6..cac5dc9 100644
>>> --- a/mm/page_ext.c
>>> +++ b/mm/page_ext.c
>>> @@ -391,7 +391,7 @@ void __init page_ext_init(void)
>>>                         * -------------pfn-------------->
>>>                         * N0 | N1 | N2 | N0 | N1 | N2|....
>>>                         */
>>> -                       if (pfn_to_nid(pfn) != nid)
>>> +                       if (!early_pfn_in_nid(pfn nid))
>>
>> early_pfn_in_nid() is static function in page_alloc.c. I'm supposed
>> early_pfn_to_nid() should be used.
>
> Thanks for checking. Then, please revert your patch "mm: call
> page_ext_init() after all struct pages are initialized" and apply this
> change, because deferring page_ext_init() would make page owner which
> uses page_ext miss some early page allocation callsites. Although it
> already miss some early page allocation callsites, we don't need to
> miss more.

Definitely agree. I will come up with an incremental patch to adopt the 
new solution.

Thanks,
Yang

>
> Thanks.
>

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