Re: [PATCH] mm: hugetlb: fix a race between memory-failure/soft_offline and gather_surplus_pages

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

On Wed, Apr 21, 2021 at 10:03:34AM +0200, Michal Hocko wrote:
> [Cc Naoya]
> 
> On Wed 21-04-21 14:02:59, Muchun Song wrote:
> > The possible bad scenario:
> > 
> > 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)
> > 
> > The refcount can possibly be increased by memory-failure or soft_offline
> > handlers, we can trigger VM_BUG_ON_PAGE and wrongly add the page to the
> > hugetlb pool list.
> 
> The hwpoison side of this looks really suspicious to me. It shouldn't
> really touch the reference count of hugetlb pages without being very
> careful (and having hugetlb_lock held).

I have the same feeling, there is a window where a hugepage is refcounted
during converting from buddy free pages into free hugepage, so refcount
alone is not enough to prevent the race.  hugetlb_lock is retaken after
alloc_surplus_huge_page returns, so simply holding hugetlb_lock in
get_hwpoison_page() seems not work.  Is there any status bit to show that a
hugepage is just being initialized (not in free hugepage pool or in use)?

> What would happen if the
> reference count was increased after the page has been enqueed into the
> pool? This can just blow up later.

Yes, this is another concern.

Thanks,
Naoya Horiguchi