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

Michal Hocko
2021-04-21T08:03:44Z

[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.

> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
> mm/hugetlb.c | 11 ++++-------
> 1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 3476aa06da70..6c96332db34b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2145,17 +2145,14 @@ static int gather_surplus_pages(struct hstate *h, long delta)
>
> /* Free the needed pages to the hugetlb pool */
> list_for_each_entry_safe(page, tmp, &surplus_list, lru) {
> - int zeroed;
> -
> if ((--needed) < 0)
> break;
> /*
> - * This page is now managed by the hugetlb allocator and has
> - * no users -- drop the buddy allocator's reference.
> + * The refcount can possibly be increased by memory-failure or
> + * soft_offline handlers.
> */
> - zeroed = put_page_testzero(page);
> - VM_BUG_ON_PAGE(!zeroed, page);
> - enqueue_huge_page(h, page);
> + if (likely(put_page_testzero(page)))
> + enqueue_huge_page(h, page);
> }
> free:
> spin_unlock_irq(&hugetlb_lock);
> --
> 2.11.0
>

--
Michal Hocko
SUSE Labs