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

[Mike Kravetz <mike.kravetz@oracle.com>]

On 5/16/21 9:54 PM, 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
> 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
> types of compound pages. PageSlab() check is added because otherwise
> "non anonymous thp" path is wrongly chosen.
> 
> Fixes: ead07f6a867b ("mm/memory-failure: introduce get_hwpoison_page() for consistent refcount handling")
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> Reported-by: Muchun Song <songmuchun@bytedance.com>
> Cc: stable@vger.kernel.org # 5.12+
> ---
> ChangeLog v4:
> - all hugetlb related check in hugetlb_lock,
> - fix build error with #ifdef CONFIG_HUGETLB_PAGE
> ---
>  mm/memory-failure.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git v5.12/mm/memory-failure.c v5.12_patched/mm/memory-failure.c
> index a3659619d293..761f982b6d7b 100644
> --- v5.12/mm/memory-failure.c
> +++ v5.12_patched/mm/memory-failure.c
> @@ -1094,6 +1094,16 @@ static int page_action(struct page_state *ps, struct page *p,
>  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 must be missing something.

The above code makes sure the page is not in one of these transitive
hugetlb states as mentioned in the commit message.  It only attempts
to take a reference on the page if it is not in one of these states.

However, if it is in such a transitive state (!HPageFreed(head) &&
!HPageMigratable(head)) we will fall through and execute the code:

	if (get_page_unless_zero(head)) {
		if (head == compound_head(page))
			return 1;

So, it seems like we will always do a get_page_unless_zero() for
PageHuge() pages?

Also, due to commit db71ef79b59b ("hugetlb: make free_huge_page irq
safe") you need to make sure interrupts are disabled when taking
hugetlb_lock.
-- 
Mike Kravetz