linux-kernel - Re: [PATCH] mm: page_is_guard return false when page

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Message-ID: <a1c9d2e7-6fdf-593a-58b1-928a71d647ef@linaro.org>
Date:	Fri, 20 May 2016 10:00:06 -0700
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Joonsoo Kim <js1304@...il.com>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	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 5/19/2016 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,
Yang

>                                 continue;
>                         if (init_section_page_ext(pfn, nid))
>                                 goto oom;
>