Re: mm: BUG in pgtable_pmd_page_dtor

[Vlastimil Babka <vbabka@suse.cz>]

On 11/24/2016 03:23 PM, Dmitry Vyukov wrote:
> On Thu, Nov 24, 2016 at 2:49 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
>> On 11/18/2016 11:19 AM, Dmitry Vyukov wrote:
>>>
>>> Hello,
>>>
>>> I've got the following BUG while running syzkaller on
>>> a25f0944ba9b1d8a6813fd6f1a86f1bd59ac25a6 (4.9-rc5). Unfortunately it's
>>> not reproducible.
>>>
>>> kernel BUG at ./include/linux/mm.h:1743!
>>> invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN
>>
>>
>> Shouldn't there be also dump_page() output? Since you've hit this:
>> VM_BUG_ON_PAGE(page->pmd_huge_pte, page);
> 
> Here it is:
> 
> [  250.326131] page:ffffea0000e196c0 count:1 mapcount:0 mapping:
>    (null) index:0x0
> [  250.343393] flags: 0x1fffc0000000000()
> [  250.345328] page dumped because: VM_BUG_ON_PAGE(page->pmd_huge_pte)
> [  250.346780] ------------[ cut here ]------------
> [  250.347742] kernel BUG at ./include/linux/mm.h:1743!

Yeah, as expected, not very useful for this particular BUG_ON :/

>> Anyway the output wouldn't contain the value of pmd_huge_pte or stuff that's
>> in union with it. I'd suggest adding a local patch that prints this in the
>> error case, in case the fuzzer hits it again.
>>
>> Heck, it might even make sense to print raw contents of struct page in
>> dump_page() as a catch-all solution? Should I send a patch?
> 
> Yes, please send.
> We are moving towards continuous build without local patches.

Something like this?
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