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@xxxxxxxxxxxxx>
> > > > ---
> > > > 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.
