Re: [PATCH v5 1/2] mm,hwpoison: fix race with hugetlb page allocation

[Oscar Salvador <osalvador@suse.de>]

On Tue, May 25, 2021 at 08:07:07AM +0000, HORIGUCHI NAOYA(堀口 直也) wrote:
> OK, here's the current draft.
> 
> Thanks,
> Naoya Horiguchi
> 
> ---
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> Date: Tue, 18 May 2021 23:49:18 +0900
> Subject: [PATCH] mm,hwpoison: fix race with hugetlb page allocation
> 
> When hugetlb page fault (under overcommitting situation) and
> memory_failure() race, VM_BUG_ON_PAGE() is triggered by the following race:
> 
>     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)
> 
> __get_hwpoison_page() only checks the page refcount before taking an
> additional one for memory error handling, which is wrong because there's
> a time window where compound pages have non-zero refcount during
> initialization.  So make __get_hwpoison_page() check page status a bit
> more for hugetlb pages.

I think that this changelog would benefit from some information about the new
!PageLRU && !__PageMovable check.

>  static int __get_hwpoison_page(struct page *page)
>  {
>  	struct page *head = compound_head(page);
> +	int ret = 0;
> +	bool hugetlb = false;
> +
> +	ret = get_hwpoison_huge_page(head, &hugetlb);
> +	if (hugetlb)
> +		return ret;
> +
> +	if (!PageLRU(head) && !__PageMovable(head))
> +		return 0;

This definitely needs a comment hinting the reader why we need to check for this.
AFAICS, this is to close the race where a page is about to be a hugetlb page soon,
so we do not go for get_page_unless_zero(), right?

From soft_offline_page's POV I __guess__ that's fine because we only deal with
pages we know about.
But what about memory_failure()? I think memory_failure() is less picky about that,
so it is okay to not take a refcount on that case?

-- 
Oscar Salvador
SUSE L3