Re: [PATCH] mm,hwpoison: fix race with compound page allocation

[HORIGUCHI NAOYA(堀口 直也)]

On Wed, Apr 28, 2021 at 10:23:49AM +0200, Oscar Salvador wrote:
> On Wed, Apr 28, 2021 at 04:46:54PM +0900, Naoya Horiguchi wrote:
> > ---
> > From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> > Date: Wed, 28 Apr 2021 15:55:47 +0900
> > Subject: [PATCH] mm,hwpoison: fix race with compound page allocation
> > 
> > When hugetlb page fault (under overcommiting situation) and memory_failure()
> > race, VM_BUG_ON_PAGE() is triggered by the following race:
> > 
> >     CPU0:                           CPU1:
> > 
> >                                     gather_surplus_pages()
> >                                       page = alloc_surplus_huge_page()
> >     memory_failure_hugetlb()
> >       get_hwpoison_page(page)
> >         __get_hwpoison_page(page)
> >           get_page_unless_zero(page)
> >                                       zero = put_page_testzero(page)
> >                                       VM_BUG_ON_PAGE(!zero, page)
> >                                       enqueue_huge_page(h, page)
> >       put_page(page)
> > 
> > __get_hwpoison_page() only checks page refcount before taking additional
> > one for memory error handling, which is wrong because there's time
> > windows where compound pages have non-zero refcount during initialization.
> > 
> > So makes __get_hwpoison_page() check more page status for a few types
> > of compound pages. PageSlab() check is added because otherwise
> > "non anonymous thp" path is wrongly chosen for slab pages.
> 
> Was it wrongly chosen even before? If so, maybe a Fix tag is warranted.

OK, I'll check when this was introduced.

> 
> > 
> > Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> > Reported-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >  mm/memory-failure.c | 48 +++++++++++++++++++++++++--------------------
> >  1 file changed, 27 insertions(+), 21 deletions(-)
> > 
> > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > index a3659619d293..61988e332712 100644
> > --- a/mm/memory-failure.c
> > +++ b/mm/memory-failure.c
> > @@ -1095,30 +1095,36 @@ static int __get_hwpoison_page(struct page *page)
> 
> > +	if (PageCompound(page)) {
> > +		if (PageSlab(page)) {
> > +			return get_page_unless_zero(page);
> > +		} else if (PageHuge(head)) {
> > +			if (HPageFreed(head) || HPageMigratable(head))
> > +				return get_page_unless_zero(head);
> 
> There were concerns raised wrt. memory-failure should not be fiddling with page's
> refcount without holding a hugetlb lock.
> So, if we really want to make this more stable, we might want to hold the lock
> here.
> 
> The clearing and setting of HPageFreed happens under the lock, and for HPageMigratable
> that is also true for the clearing part, so I think it would be more sane to do
> this under the lock to close any possible race.
> 
> Does it make sense?

Thanks, I'll update to do the check under hugetlb_lock.

- Naoya Horiguchi