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

[Wei Yang]

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.

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

-- 
Wei Yang
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