[1/2] mm/memory-failure.c: PageHuge is handled at the beginning of memory_failure
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Message ID 20191118082003.26240-1-richardw.yang@linux.intel.com
State New
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  • [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
PageHuge is handled by memory_failure_hugetlb(), so this case could be
removed.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
---
 mm/memory-failure.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

Comments

David Hildenbrand Nov. 19, 2019, 12:23 p.m. UTC | #1
On 18.11.19 09:20, Wei Yang wrote:
> PageHuge is handled by memory_failure_hugetlb(), so this case could be
> removed.
> 
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> ---
>   mm/memory-failure.c | 5 +----
>   1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 3151c87dff73..392ac277b17d 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1359,10 +1359,7 @@ int memory_failure(unsigned long pfn, int flags)
>   	 * page_remove_rmap() in try_to_unmap_one(). So to determine page status
>   	 * correctly, we save a copy of the page flags at this time.
>   	 */
> -	if (PageHuge(p))
> -		page_flags = hpage->flags;
> -	else
> -		page_flags = p->flags;
> +	page_flags = p->flags;
>   
>   	/*
>   	 * unpoison always clear PG_hwpoison inside page lock
> 

I somewhat miss a proper explanation why this is safe to do. We access 
page flags here, so why is it safe to refer to the ones of the sub-page?
Wei Yang Nov. 20, 2019, 12:46 a.m. UTC | #2
On Tue, Nov 19, 2019 at 01:23:54PM +0100, David Hildenbrand wrote:
>On 18.11.19 09:20, Wei Yang wrote:
>> PageHuge is handled by memory_failure_hugetlb(), so this case could be
>> removed.
>> 
>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>> ---
>>   mm/memory-failure.c | 5 +----
>>   1 file changed, 1 insertion(+), 4 deletions(-)
>> 
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 3151c87dff73..392ac277b17d 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -1359,10 +1359,7 @@ int memory_failure(unsigned long pfn, int flags)
>>   	 * page_remove_rmap() in try_to_unmap_one(). So to determine page status
>>   	 * correctly, we save a copy of the page flags at this time.
>>   	 */
>> -	if (PageHuge(p))
>> -		page_flags = hpage->flags;
>> -	else
>> -		page_flags = p->flags;
>> +	page_flags = p->flags;
>>   	/*
>>   	 * unpoison always clear PG_hwpoison inside page lock
>> 
>
>I somewhat miss a proper explanation why this is safe to do. We access page
>flags here, so why is it safe to refer to the ones of the sub-page?
>

Hi, David

I think your comment is on this line:

	page_flags = p->flags;

Maybe we need to use this:

	page_flags = hpage->flags;

And use hpage in the following or even the whole function?

While one thing interesting is not all "compound page" is PageCompound. For
some sub-page, we can't get the correct head. This means we may just check on
the sub-page.

>-- 
>
>Thanks,
>
>David / dhildenb
David Hildenbrand Nov. 20, 2019, 3:04 p.m. UTC | #3
On 20.11.19 01:46, Wei Yang wrote:
> On Tue, Nov 19, 2019 at 01:23:54PM +0100, David Hildenbrand wrote:
>> On 18.11.19 09:20, Wei Yang wrote:
>>> PageHuge is handled by memory_failure_hugetlb(), so this case could be
>>> removed.
>>>
>>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>>> ---
>>>    mm/memory-failure.c | 5 +----
>>>    1 file changed, 1 insertion(+), 4 deletions(-)
>>>
>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>> index 3151c87dff73..392ac277b17d 100644
>>> --- a/mm/memory-failure.c
>>> +++ b/mm/memory-failure.c
>>> @@ -1359,10 +1359,7 @@ int memory_failure(unsigned long pfn, int flags)
>>>    	 * page_remove_rmap() in try_to_unmap_one(). So to determine page status
>>>    	 * correctly, we save a copy of the page flags at this time.
>>>    	 */
>>> -	if (PageHuge(p))
>>> -		page_flags = hpage->flags;
>>> -	else
>>> -		page_flags = p->flags;
>>> +	page_flags = p->flags;
>>>    	/*
>>>    	 * unpoison always clear PG_hwpoison inside page lock
>>>
>>
>> I somewhat miss a proper explanation why this is safe to do. We access page
>> flags here, so why is it safe to refer to the ones of the sub-page?
>>
> 
> Hi, David
> 
> I think your comment is on this line:
> 
> 	page_flags = p->flags;
> 
> Maybe we need to use this:
> 
> 	page_flags = hpage->flags;
> 
> And use hpage in the following or even the whole function?
> 
> While one thing interesting is not all "compound page" is PageCompound. For
> some sub-page, we can't get the correct head. This means we may just check on
> the sub-page.

Oh wait, I think I missed the check right at the beginning of this 
function, sorry :/

Sooo, memory_failure_hugetlb() was introduced by

commit 761ad8d7c7b5485bb66fd5bccb58a891fe784544
Author: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Date:   Mon Jul 10 15:47:47 2017 -0700

     mm: hwpoison: introduce memory_failure_hugetlb()

and essentially ripped out all PageHuge() checks *except* this check. 
This check was introduced in

commit 7258ae5c5a2ce2f5969e8b18b881be40ab55433d
Author: James Morse <james.morse@arm.com>
Date:   Fri Jun 16 14:02:29 2017 -0700

     mm/memory-failure.c: use compound_head() flags for huge pages


Maybe that was just a merge oddity as both commits are only ~1month 
apart. IOW, I think Naoya's patch forgot to rip it out.


Can we make this clear in the patch description like "This is dead code 
that cannot be reached after commit 761ad8d7c7b5 ("mm: hwpoison: 
introduce memory_failure_hugetlb()")"

I assume Andrew can fix up when applying

Reviewed-by: David Hildenbrand <david@redhat.com>
Wei Yang Nov. 21, 2019, 1:07 a.m. UTC | #4
On Wed, Nov 20, 2019 at 04:04:44PM +0100, David Hildenbrand wrote:
>On 20.11.19 01:46, Wei Yang wrote:
>> On Tue, Nov 19, 2019 at 01:23:54PM +0100, David Hildenbrand wrote:
>> > On 18.11.19 09:20, Wei Yang wrote:
>> > > PageHuge is handled by memory_failure_hugetlb(), so this case could be
>> > > removed.
>> > > 
>> > > Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>> > > ---
>> > >    mm/memory-failure.c | 5 +----
>> > >    1 file changed, 1 insertion(+), 4 deletions(-)
>> > > 
>> > > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> > > index 3151c87dff73..392ac277b17d 100644
>> > > --- a/mm/memory-failure.c
>> > > +++ b/mm/memory-failure.c
>> > > @@ -1359,10 +1359,7 @@ int memory_failure(unsigned long pfn, int flags)
>> > >    	 * page_remove_rmap() in try_to_unmap_one(). So to determine page status
>> > >    	 * correctly, we save a copy of the page flags at this time.
>> > >    	 */
>> > > -	if (PageHuge(p))
>> > > -		page_flags = hpage->flags;
>> > > -	else
>> > > -		page_flags = p->flags;
>> > > +	page_flags = p->flags;
>> > >    	/*
>> > >    	 * unpoison always clear PG_hwpoison inside page lock
>> > > 
>> > 
>> > I somewhat miss a proper explanation why this is safe to do. We access page
>> > flags here, so why is it safe to refer to the ones of the sub-page?
>> > 
>> 
>> Hi, David
>> 
>> I think your comment is on this line:
>> 
>> 	page_flags = p->flags;
>> 
>> Maybe we need to use this:
>> 
>> 	page_flags = hpage->flags;
>> 
>> And use hpage in the following or even the whole function?
>> 
>> While one thing interesting is not all "compound page" is PageCompound. For
>> some sub-page, we can't get the correct head. This means we may just check on
>> the sub-page.
>
>Oh wait, I think I missed the check right at the beginning of this function,
>sorry :/
>
>Sooo, memory_failure_hugetlb() was introduced by
>
>commit 761ad8d7c7b5485bb66fd5bccb58a891fe784544
>Author: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>Date:   Mon Jul 10 15:47:47 2017 -0700
>
>    mm: hwpoison: introduce memory_failure_hugetlb()
>
>and essentially ripped out all PageHuge() checks *except* this check. This
>check was introduced in
>
>commit 7258ae5c5a2ce2f5969e8b18b881be40ab55433d
>Author: James Morse <james.morse@arm.com>
>Date:   Fri Jun 16 14:02:29 2017 -0700
>
>    mm/memory-failure.c: use compound_head() flags for huge pages
>
>
>Maybe that was just a merge oddity as both commits are only ~1month apart.
>IOW, I think Naoya's patch forgot to rip it out.
>
>
>Can we make this clear in the patch description like "This is dead code that
>cannot be reached after commit 761ad8d7c7b5 ("mm: hwpoison: introduce
>memory_failure_hugetlb()")"
>
>I assume Andrew can fix up when applying
>

That's very helpful. Thanks for your time sincerely.

>Reviewed-by: David Hildenbrand <david@redhat.com>
>
>-- 
>
>Thanks,
>
>David / dhildenb

Patch
diff mbox series

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 3151c87dff73..392ac277b17d 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1359,10 +1359,7 @@  int memory_failure(unsigned long pfn, int flags)
 	 * page_remove_rmap() in try_to_unmap_one(). So to determine page status
 	 * correctly, we save a copy of the page flags at this time.
 	 */
-	if (PageHuge(p))
-		page_flags = hpage->flags;
-	else
-		page_flags = p->flags;
+	page_flags = p->flags;
 
 	/*
 	 * unpoison always clear PG_hwpoison inside page lock