linux-kernel - Re: mm: BUG in pgtable_pmd_page

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Message-ID: <ba397708-953b-d24f-21e9-32c9925a2f76@virtuozzo.com>
Date:   Fri, 25 Nov 2016 19:03:06 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>, Ingo Molnar <mingo@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: mm: BUG in pgtable_pmd_page_dtor



On 11/25/2016 05:08 PM, Vlastimil Babka wrote:
> On 11/25/2016 02:07 PM, Kirill A. Shutemov wrote:
>>> --- a/mm/debug.c
>>> +++ b/mm/debug.c
>>> @@ -59,6 +59,10 @@ void __dump_page(struct page *page, const char *reason)
>>>  
>>>  	pr_emerg("flags: %#lx(%pGp)\n", page->flags, &page->flags);
>>>  
>>> +	print_hex_dump(KERN_ALERT, "raw: ", DUMP_PREFIX_NONE,
>>> +			32, (sizeof(unsigned long) == 8) ? 8 : 4,
>>
>> That's a very fancy way to write sizeof(unsigned long) ;)
>  
> Ah, damnit, thanks.
> 
> ----8<----
> From 08d2ee803567c13e3de7ce7e19338fe5286cc6b8 Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@...e.cz>
> Date: Fri, 25 Nov 2016 09:08:05 +0100
> Subject: [PATCH v3] mm, debug: print raw struct page data in __dump_page()
> 
> The __dump_page() function is used when a page metadata inconsistency is
> detected, either by standard runtime checks, or extra checks in CONFIG_DEBUG_VM
> builds. It prints some of the relevant metadata, but not the whole struct page,
> which is based on unions and interpretation is dependent on the context.
> 
> This means that sometimes e.g. a VM_BUG_ON_PAGE() checks certain field, which
> is however not printed by __dump_page() and the resulting bug report may then
> lack clues that could help in determining the root cause. This patch solves
> the problem by simply printing the whole struct page word by word, so no part
> is missing, but the interpretation of the data is left to developers. This is
> similar to e.g. x86_64 raw stack dumps.
> 
> Example output:
> 
>  page:ffffea00000475c0 count:1 mapcount:0 mapping:          (null) index:0x0
>  flags: 0x100000000000400(reserved)
>  raw: 0100000000000400 0000000000000000 0000000000000000 00000001ffffffff
>  raw: ffffea00000475e0 ffffea00000475e0 0000000000000000 0000000000000000
>  page dumped because: VM_BUG_ON_PAGE(1)
> 
> [aryabinin@...tuozzo.com: suggested print_hex_dump()]
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Andrey Ryabinin <aryabinin@...tuozzo.com>