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

[Wei Yang <richardw.yang@linux.intel.com>]

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

-- 
Wei Yang
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