Re: [PATCH 1/2] mm/memory-failure.c: PageHuge is handled at the beginning of memory_failure

[David Hildenbrand <david@redhat.com>]

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>

-- 

Thanks,

David / dhildenb