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Date:   Mon, 31 Jan 2022 13:24:27 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     HORIGUCHI =?utf-8?B?TkFPWUEo5aCA5Y+j44CA55u05LmfKQ==?= 
        <naoya.horiguchi@nec.com>
Cc:     David Rientjes <rientjes@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm/hwpoison: Check the subpage, not the head page
Message-ID: <YffjC9+JsrZB5ekr@casper.infradead.org>
References: <20220130013042.1906881-1-willy@infradead.org>
 <dcfa1ee5-1512-5e49-92c2-4a33ab59080@google.com>
 <Yfb/rVg1HYHkKBgO@casper.infradead.org>
 <20220131054432.GA856839@hori.linux.bs1.fc.nec.co.jp>
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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.