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

[Michal Hocko]

[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@xxxxxxxxxxxxx>
> ---
>  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