Re: [PATCH 2/2] mm/memory-failure.c: not necessary to recalculate hpage

[Wei Yang]

On Wed, Jan 08, 2020 at 01:20:44PM +0100, David Hildenbrand wrote:
>On 06.12.19 02:48, Wei Yang wrote:
>> On Thu, Dec 05, 2019 at 04:06:20PM +0100, David Hildenbrand wrote:
>>> On 02.12.19 23:28, Wei Yang wrote:
>>>> On Wed, Nov 20, 2019 at 04:07:38PM +0100, David Hildenbrand wrote:
>>>>> On 18.11.19 09:20, Wei Yang wrote:
>>>>>> hpage is not changed.
>>>>>>
>>>>>> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
>>>>>> ---
>>>>>>   mm/memory-failure.c | 1 -
>>>>>>   1 file changed, 1 deletion(-)
>>>>>>
>>>>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>>>>> index 392ac277b17d..9784f4339ae7 100644
>>>>>> --- a/mm/memory-failure.c
>>>>>> +++ b/mm/memory-failure.c
>>>>>> @@ -1319,7 +1319,6 @@ int memory_failure(unsigned long pfn, int flags)
>>>>>>   		}
>>>>>>   		unlock_page(p);
>>>>>>   		VM_BUG_ON_PAGE(!page_count(p), p);
>>>>>> -		hpage = compound_head(p);
>>>>>>   	}
>>>>>>   	/*
>>>>>>
>>>>>
>>>>> I am *absolutely* no transparent huge page expert (sorry :) ), but won't the
>>>>> split_huge_page(p) eventually split the compound page, such that
>>>>> compound_head(p) will return something else after that call?
>>>>>
>>>>
>>>> Hi, David
>>>>
>>>> Took sometime to look into the code and re-think about it. Found maybe we can
>>>> simplify this in another way.
>>>>
>>>> First, code touches here means split_huge_page() succeeds and "p" is now a PTE
>>>> page. So compound_head(p) == p.
>>>
>>> While this would also be my intuition, I can't state that this is
>>> guaranteed to be the case (IOW, I did not check the code/documentation) :)
>>>
>> 
>> If my understanding is correct, split_huge_page() succeeds the THP would be
>> tear down to normal page.
>> 
>>>>
>>>> Then let's look at who will use hpage in the following function. There are two
>>>> uses in current upstream:
>>>>
>>>>     * page_flags calculation
>>>>     * hwpoison_user_mappings()
>>>>
>>>> The first one would be removed in next patch since PageHuge is handled at the
>>>> beginning.
>>>>
>>>> And in the second place, comment says if split succeeds, hpage points to page
>>>> "p".
>>>>
>>>> After all, we don't need to re-calculate hpage after split, and just replace
>>>> hpage in hwpoison_user_mappings() with p is enough.
>>>
>>> That assumption would only be true in case all compound pages at this
>>> point are transparent huge pages, no? AFAIK that is not necessarily
>>> true. Or am I missing something?
>>>
>> 
>> Function hwpoison_user_mappings() just handle user space mapping. If my
>> understanding is correct, we just have three type of pages would be used in
>> user space mapping:
>> 
>>     * normal page
>>     * THP
>>     * hugetlb
>> 
>> Since THP would be split or already returned and hugetlb is handled in another
>> branch, this means for other pages hwpoison_user_mappings() would just return
>> true.
>> 
>
>Sorry for the late reply :)
>
>While I think you are correct, I am not sure if the change you are
>suggesting is a) future proof and b) worth it. IOW, the recalculation
>after the split makes it clear that something changed and that the
>compound page does no longer exist. I might be wrong of course and this
>cleanup makes perfect sense :)
>

Yep, you are welcome.

I would think about the whole picture again.

>
>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
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