Re: [PATCH v2] mm,hwpoison: fix race with compound page allocation

[Oscar Salvador <osalvador@suse.de>]

On Thu, May 06, 2021 at 10:31:22AM +0900, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> Date: Thu, 6 May 2021 09:54:39 +0900
> Subject: [PATCH] mm,hwpoison: fix race with compound page allocation
> 
> When hugetlb page fault (under overcommiting 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 time
> windows where compound pages have non-zero refcount during initialization.
> 
> So makes __get_hwpoison_page() check more page status 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+

Hi Naoya, 

thanks for the patch.
I have some concerns though, more below:

> ---
>  mm/memory-failure.c | 53 +++++++++++++++++++++++++++------------------
>  1 file changed, 32 insertions(+), 21 deletions(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index a3659619d293..966a1d6b0bc8 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1095,30 +1095,41 @@ static int __get_hwpoison_page(struct page *page)
>  {
>  	struct page *head = compound_head(page);
>  
> -	if (!PageHuge(head) && PageTransHuge(head)) {
> -		/*
> -		 * Non anonymous thp exists only in allocation/free time. We
> -		 * can't handle such a case correctly, so let's give it up.
> -		 * This should be better than triggering BUG_ON when kernel
> -		 * tries to touch the "partially handled" page.
> -		 */
> -		if (!PageAnon(head)) {
> -			pr_err("Memory failure: %#lx: non anonymous thp\n",
> -				page_to_pfn(page));
> -			return 0;
> +	if (PageCompound(page)) {
> +		if (PageSlab(page)) {
> +			return get_page_unless_zero(page);
> +		} else if (PageHuge(head)) {
> +			int ret = 0;
> +
> +			spin_lock(&hugetlb_lock);
> +			if (HPageFreed(head) || HPageMigratable(head))
> +				ret = get_page_unless_zero(head);
> +			spin_unlock(&hugetlb_lock);
> +			return ret;

Ok, I am probably overthinking this but should we re-check under the
lock wehther the page is a hugetlb page?
My concern is, what would happen if:

CPU0                                          CPU1
 __get_hwpoison_page                          
  PageHuge(head) == T                         
                                              dissolve hugetlb page
   hugetlb_lock                               


In that case, by the time we get to check hugetlb flags, those checks
might return false, and we do not get a refcount.
So, I guess my question is: Should we re-check under the lock, and if it
is not, do a "goto try_to_get_ref" that starts right at the beginning,
or goes directly to the get_page_unless_zero at the end (the former
probably better)?

As I said, I might be overthinking this, but well.

-- 
Oscar Salvador
SUSE L3