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

[Matthew Wilcox <willy@infradead.org>]

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.