linux-kernel - Re: mm: BUG in pgtable_pmd_page

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Date:   Fri, 25 Nov 2016 13:58:57 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     "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 12:41 PM, Andrey Ryabinin wrote:
> 
> 
> On 11/25/2016 11:42 AM, Vlastimil Babka wrote:
> 
>>  	pr_emerg("page:%p count:%d mapcount:%d mapping:%p index:%#lx",
>>  		  page, page_ref_count(page), mapcount,
>> @@ -59,6 +61,21 @@ void __dump_page(struct page *page, const char *reason)
>>  
>>  	pr_emerg("flags: %#lx(%pGp)\n", page->flags, &page->flags);
>>  
>> +	pr_alert("raw struct page data:");
>> +	for (i = 0; i < sizeof(struct page) / sizeof(unsigned long); i++) {
>> +		unsigned long *word_ptr;
>> +
>> +		word_ptr = ((unsigned long *) page) + i;
>> +
>> +		if ((i % words_per_line) == 0) {
>> +			pr_cont("\n");
>> +			pr_alert(" %016lx", *word_ptr);
>> +		} else {
>> +			pr_cont(" %016lx", *word_ptr);
>> +		}
>> +	}
>> +	pr_cont("\n");
>> +
> 
> Single call to print_hex_dump() could replace this loop.

Ah, didn't know about that one, thanks!

This also addresses Kirill's comment:

-----8<-----
>From 417467521d0a68fb70dc2d5bd151524bf0c79437 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@...e.cz>
Date: Fri, 25 Nov 2016 09:08:05 +0100
Subject: [PATCH] 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>
---
 mm/debug.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/debug.c b/mm/debug.c
index 9feb699c5d25..185c19bda078 100644
--- 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,
+			page, sizeof(struct page), false);
+
 	if (reason)
 		pr_alert("page dumped because: %s\n", reason);
 
-- 
2.10.2