[2/2] mm/memory-failure.c: not necessary to recalculate hpage
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Message ID 20191118082003.26240-2-richardw.yang@linux.intel.com
State New
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Series
  • [1/2] mm/memory-failure.c: PageHuge is handled at the beginning of memory_failure
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Commit Message

Wei Yang Nov. 18, 2019, 8:20 a.m. UTC
hpage is not changed.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
---
 mm/memory-failure.c | 1 -
 1 file changed, 1 deletion(-)

Comments

David Hildenbrand Nov. 20, 2019, 3:07 p.m. UTC | #1
On 18.11.19 09:20, Wei Yang wrote:
> hpage is not changed.
> 
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> ---
>   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?
Wei Yang Nov. 21, 2019, 1:05 a.m. UTC | #2
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@linux.intel.com>
>> ---
>>   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?
>

ok, you may get some point. I lost some concentration here.

Thanks

>-- 
>
>Thanks,
>
>David / dhildenb
Wei Yang Dec. 2, 2019, 10:28 p.m. UTC | #3
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@linux.intel.com>
>> ---
>>   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.

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.

>-- 
>
>Thanks,
>
>David / dhildenb
David Hildenbrand Dec. 5, 2019, 3:06 p.m. UTC | #4
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@linux.intel.com>
>>> ---
>>>   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) :)

> 
> 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?
Wei Yang Dec. 6, 2019, 1:48 a.m. UTC | #5
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@linux.intel.com>
>>>> ---
>>>>   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

Patch
diff mbox series

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);
 	}
 
 	/*