Re: [PATCH v4 1/2] mm,hwpoison: fix race with compound page allocation

[Oscar Salvador <osalvador@suse.de>]

On Mon, May 17, 2021 at 01:54:00PM +0900, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> 
> 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 page refcount before taking additional
                                  ^^ the?                      ^^ an
> one for memory error handling, which is wrong because there's a time
> window where compound pages have non-zero refcount during initialization.
> 
> So makes __get_hwpoison_page() check page status a bit more for a few
     ^^ make
> types of compound pages. PageSlab() check is added because otherwise
> "non anonymous thp" path is wrongly chosen.

This is no longer true with this patch, is it? What happened here?

>  static int __get_hwpoison_page(struct page *page)
>  {
>  	struct page *head = compound_head(page);
> +	int ret = 0;
> +
> +#ifdef CONFIG_HUGETLB_PAGE
> +	spin_lock(&hugetlb_lock);
> +	if (PageHuge(head) && (HPageFreed(head) || HPageMigratable(head)))
> +		ret = get_page_unless_zero(head);
> +	spin_unlock(&hugetlb_lock);
> +	if (ret > 0)
> +		return ret;
> +#endif

I am kind of fine with this, but I wonder whether it makes sense to hide this
details into helper (with an empty stub for non-hugetlb pages)?

>  	if (!PageHuge(head) && PageTransHuge(head)) {
This !PageHuge could go?


-- 
Oscar Salvador
SUSE L3