Re: [bug report] mm, hwpoison: memory_failure races with alloc_fresh_huge_page/free_huge_page

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/8/4 15:44, Naoya Horiguchi wrote:
> On Thu, Aug 04, 2022 at 04:40:25PM +0900, Naoya Horiguchi wrote:
>> On Tue, Aug 02, 2022 at 02:27:36PM +0800, Miaohe Lin wrote:
>>> On 2022/8/2 12:07, Muchun Song wrote:
>>>> On Tue, Aug 02, 2022 at 10:00:50AM +0800, Miaohe Lin wrote:
>>>>> Hi all:
>>>>>     When I investigate the mm/memory-failure.c code again, I found there's a possible race window
>>>>> between memory_failure and alloc_fresh_huge_page/free_huge_page. Thank about the below scene:
>>>>>
>>>>> CPU 1							CPU 2
>>>>> alloc_fresh_huge_page -- page refcnt > 0		memory_failure
>>>>>   prep_new_huge_page					  get_huge_page_for_hwpoison
>>>>> 							    !PageHeadHuge -- so 2(not a hugepage) is returned
>>>>>     hugetlb_vmemmap_optimize -- subpages is read-only
>>>>>     set_compound_page_dtor -- PageHuge is true now, but too late!!!
>>>>> 							  TestSetPageHWPoison(p)
>>>>> 							    -- We might write to read-only subpages here!!!
>>>>>
>>>>> Another similar scene:
>>>>>
>>>>> CPU 1							CPU 2
>>>>> free_huge_page -- page refcnt == 0 and not PageHuge	memory_failure
>>>>> 							  get_huge_page_for_hwpoison
>>>>> 							    !PageHeadHuge -- so 2(not a hugepage) is returned
>>>>> 							  TestSetPageHWPoison(p)
>>>>> 							    -- We might write to read-only subpages here!!!
>>>>>   hugetlb_vmemmap_restore -- subpages can be written to now, but too late!!!
>>>>>
>>>>
>>>> I agree this race is possible, I have proposed this race in thread [1].
>>
>> Thank you for reminding this, and I agree that we need some solution.
>>
>>>
>>> Oh, I remember I see the race proposed in [1] but I did not look into that carefully at that time. Sorry.
>>>
>>>> But I didn't think more how to solve it.
>>> I hope this thread can find a good solution. :)
>>
>> Both of the races show that __get_huge_page_for_hwpoison() fails to
>> capture the case of generic compound page during turning into hugetlb,
> 
> I noticed my wrong description just after sending this out. The second race
> is about generic compound page turning into "buddy", sorry.

Never mind. Thanks for your thought.

> 
> - Naoya Horiguchi
> 
>> What makes things complicated is that no one knows to which state
>> such a compound page finally turns into. So I think that if the page
>> to be handled is an unknown compound page, we need to wait until it
>> becomes some known page state to avoid misjudging.
>>
>> If we need a quick small fix, we may replace the check "!PageHeadHuge()"
>> in __get_huge_page_for_hwpoison() with "!PageCompound()", and add another
>> retry path in get_huge_page_for_hwpoison() for non-hugetlb compound pages.

Sounds like a good idea. If we can figure out a stable way to tell it from other
compound page, it will be worth fixing it. Let's have a try. :)

>>
>> Thanks,
>> Naoya Horiguchi

Many thanks for your suggestion, Naoya.

> .
>