Re: [PATCH] mm/hwpoison: Check the subpage, not the head page

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Mon, Jan 31, 2022 at 11:24:45AM -0800, Mike Kravetz wrote:
> On 1/31/22 05:24, Matthew Wilcox wrote:
> > On Mon, Jan 31, 2022 at 05:44:35AM +0000, HORIGUCHI NAOYA(堀口 直也) wrote:
> >> On Sun, Jan 30, 2022 at 09:14:21PM +0000, Matthew Wilcox wrote:
> >>> On Sun, Jan 30, 2022 at 12:58:17PM -0800, David Rientjes wrote:
> >>>> On Sun, 30 Jan 2022, Matthew Wilcox (Oracle) wrote:
> >>>>
> >>>>> Hardware poison is tracked on a per-page basis, not on the head page.
> >>>>>
> >>>>> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> >>>>> ---
> >>>>>  mm/rmap.c | 4 ++--
> >>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>>>>
> >>>>> diff --git a/mm/rmap.c b/mm/rmap.c
> >>>>> index 6a1e8c7f6213..09b08888120e 100644
> >>>>> --- a/mm/rmap.c
> >>>>> +++ b/mm/rmap.c
> >>>>> @@ -1553,7 +1553,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> >>>>>  		/* Update high watermark before we lower rss */
> >>>>>  		update_hiwater_rss(mm);
> >>>>>  
> >>>>> -		if (PageHWPoison(page) && !(flags & TTU_IGNORE_HWPOISON)) {
> >>>>> +		if (PageHWPoison(subpage) && !(flags & TTU_IGNORE_HWPOISON)) {
> >>>>>  			pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
> >>>>>  			if (PageHuge(page)) {
> >>>>>  				hugetlb_count_sub(compound_nr(page), mm);
> >>>>> @@ -1873,7 +1873,7 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
> >>>>>  			 * memory are supported.
> >>>>>  			 */
> >>>>>  			subpage = page;
> >>>>> -		} else if (PageHWPoison(page)) {
> >>>>> +		} else if (PageHWPoison(subpage)) {
> >>>>>  			pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
> >>>>>  			if (PageHuge(page)) {
> >>>>>  				hugetlb_count_sub(compound_nr(page), mm);
> >>>>
> >>>> This looks correct.  Correct me if I'm wrong that this is for consistency 
> >>>> and cleanup and that there is no bug being fixed by this, however.
> >>>
> >>> Oh, no, I think there's a real bug here.  It's just that we're looking
> >>> at an uncommon & hence rarely-tested scenario -- a memory fault in the
> >>> middle of a THP (in mainline; obviously it'll be a little more common
> >>> with arbitrary sized folios).  I don't do HWPoison testing myself, so
> >>> this was by inspection and not from testing.  A scenario where things
> >>> would go wrong is a memory error on a non-head-page would go unnoticed
> >>> when migrating or unmapping.  Contrariwise, if there's a hardware error
> >>> on a head page, all the subpages get treated as poisoned, even though
> >>> they shouldn't be.
> >>
> >> Thank you for reporting.  As you point out, the current check does not
> >> handle thp properly.  The reason of checking head page here is to handle
> >> hwpoisoned hugetlb (which has PG_hwpoison on the head page even if the error
> >> is on any of tail page).  So I think that the proper fix is to add a helper
> >> function to check page type (normal, thp, or hugetlb) as well as PageHWPoison.
> > 
> > I think this handles HugeTLB pages correctly:
> > 
> >                 subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
> > 
> > As I understand the HugeTLB code, pvmw.pte refers to the head page, not
> > the subpage (unlike a PTE-mapped THP page, where it would refer to the
> > precise page).  But I cheerfully admit that the intricacies of the
> > HugeTLB code are not something I'm an expert on.

Sorry, you're right.

> 
> Your understanding is correct.  Here is the comment for the routine which
> sets pvmw.pte.
> 
>  * For HugeTLB pages, @pvmw->pte is set to the relevant page table entry
>  * regardless of which page table level the page is mapped at. @pvmw->pmd is
>  * NULL.

Thank you for the clarification.

> 
> Just another thought.  With new hugetlb vmmemmap optimizations, it is not
> possible to set poison on hugetlb tail pages until after allocating struct
> pages.

So the proposed patch is fine to me.

Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>