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

[Muchun Song]

On Wed, Apr 21, 2021 at 4:33 PM HORIGUCHI NAOYA(堀口　直也)
<naoya.horiguchi@xxxxxxx> wrote:
>
> 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)?

HPageFreed() can indicate whether a page is on the
free pool list.

>
> > 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