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

[Muchun Song <songmuchun@bytedance.com>]

On Wed, Apr 21, 2021 at 4:21 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Wed, Apr 21, 2021 at 04:15:00PM +0800, Muchun Song wrote:
> > > 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.
>
> Note that dequeue_huge_page_node_exact() will not hand over any pages
> which are poisoned, so in this case it will not be allocated.

But softoffline does not set page hwpoison before
__get_hwpoison_page(). So the page still can be
allocated. Right?

> But it is true that we might need hugetlb lock, this needs some more
> thought.
>
> I will have a look.
>
> --
> Oscar Salvador
> SUSE L3