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

[Michal Hocko <mhocko@suse.com>]

On Wed 21-04-21 16:15:00, Muchun Song wrote:
> On Wed, Apr 21, 2021 at 4:03 PM Michal Hocko <mhocko@suse.com> 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). What would happen if the
> > reference count was increased after the page has been enqueed into the
> > pool? This can just blow up later.
> 
> If the page has been enqueued into the pool, then the page can be
> allocated to other users. The page reference count will be reset to
> 1 in the dequeue_huge_page_node_exact(). Then memory-failure
> will free the page because of put_page(). This is wrong. Because
> there is another user.

Yes that is one of the scenarios but I suspect there are more lurking
there. That was my point that this should be addressed at the hwpoison
side.

-- 
Michal Hocko
SUSE Labs