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

[David Hildenbrand]

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@xxxxxxxxxxxxxxx>
> > > ---
> > >    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@xxxxxxxxxxxxx>
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@xxxxxxx>
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@xxxxxxxxxx>

--

Thanks,

David / dhildenb