Date:   Wed, 19 May 2021 15:32:17 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Naoya Horiguchi <nao.horiguchi@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Muchun Song <songmuchun@...edance.com>, linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Tony Luck <tony.luck@...el.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/2] mm,hwpoison: fix race with hugetlb page allocation

On 5/18/21 4:12 PM, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@....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 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.
> 
> Fixes: ead07f6a867b ("mm/memory-failure: introduce get_hwpoison_page() for consistent refcount handling")
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@....com>
> Reported-by: Muchun Song <songmuchun@...edance.com>
> Cc: stable@...r.kernel.org # 5.12+
> ---
>  include/linux/hugetlb.h |  6 ++++++
>  mm/hugetlb.c            | 15 +++++++++++++++
>  mm/memory-failure.c     |  8 +++++++-
>  3 files changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git v5.13-rc2/include/linux/hugetlb.h v5.13-rc2_patched/include/linux/hugetlb.h
> index b92f25ccef58..790ae618548d 100644
> --- v5.13-rc2/include/linux/hugetlb.h
> +++ v5.13-rc2_patched/include/linux/hugetlb.h
> @@ -149,6 +149,7 @@ bool hugetlb_reserve_pages(struct inode *inode, long from, long to,
>  long hugetlb_unreserve_pages(struct inode *inode, long start, long end,
>  						long freed);
>  bool isolate_huge_page(struct page *page, struct list_head *list);
> +int get_hwpoison_huge_page(struct page *page, bool *hugetlb);
>  void putback_active_hugepage(struct page *page);
>  void move_hugetlb_state(struct page *oldpage, struct page *newpage, int reason);
>  void free_huge_page(struct page *page);
> @@ -339,6 +340,11 @@ static inline bool isolate_huge_page(struct page *page, struct list_head *list)
>  	return false;
>  }
>  
> +static inline int get_hwpoison_huge_page(struct page *page, bool *hugetlb)
> +{
> +	return 0;
> +}
> +
>  static inline void putback_active_hugepage(struct page *page)
>  {
>  }
> diff --git v5.13-rc2/mm/hugetlb.c v5.13-rc2_patched/mm/hugetlb.c
> index 95918f410c0f..f138bae3e302 100644
> --- v5.13-rc2/mm/hugetlb.c
> +++ v5.13-rc2_patched/mm/hugetlb.c
> @@ -5847,6 +5847,21 @@ bool isolate_huge_page(struct page *page, struct list_head *list)
>  	return ret;
>  }
>  
> +int get_hwpoison_huge_page(struct page *page, bool *hugetlb)
> +{
> +	int ret = 0;
> +
> +	*hugetlb = false;
> +	spin_lock_irq(&hugetlb_lock);
> +	if (PageHeadHuge(page)) {
> +		*hugetlb = true;
> +		if (HPageFreed(page) || HPageMigratable(page))
> +			ret = get_page_unless_zero(page);
> +	}
> +	spin_unlock_irq(&hugetlb_lock);
> +	return ret;
> +}
> +
>  void putback_active_hugepage(struct page *page)
>  {
>  	spin_lock_irq(&hugetlb_lock);
> diff --git v5.13-rc2/mm/memory-failure.c v5.13-rc2_patched/mm/memory-failure.c
> index 85ad98c00fd9..353c6177e489 100644
> --- v5.13-rc2/mm/memory-failure.c
> +++ v5.13-rc2_patched/mm/memory-failure.c
> @@ -959,8 +959,14 @@ 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;
> +	bool hugetlb = false;
> +
> +	ret = get_hwpoison_huge_page(head, &hugetlb);
> +	if (hugetlb)
> +		return ret;
>  

Hello Naoya,

Thanks for your continued efforts.  However, I believe the race still
exists.  Unless I am mistaken, it is possible that page is in the hugetlb
allocation patch and racing with __get_hwpoison_page() as follows:

    CPU0:                           CPU1:

                                    gather_surplus_pages()
                                      page = alloc_surplus_huge_page()
				      	page = alloc_fresh_huge_page()
				      	  page = alloc_buddy_huge_page()
    memory_failure_hugetlb()
      get_hwpoison_page(page)
        __get_hwpoison_page(page)
	  get_hwpoison_huge_page()
	    /* Note that PageHuge()
	       is false, so hugetlb
	       not set */
	  PageTransHuge(head) false
					  prep_new_huge_page(page)
					  /* Now PageHuge() becomes true */
          get_page_unless_zero(page)

I am not sure if it is possible to handle this race in the memory error
code.  I can not think of a way to avoid potentially incrementing the
ref count on a hugetlb page as it is being created.  There is nothing
synchronizing this in the hugetlb code.

When Muchun first proposed a fix to the race, the idea was to catch the
race in the hugetlb code.  Michal suggested that the memory error code
be more careful in modifying ref counts.  I would wait a bit to see if
someone has a good idea how this can be done.  We 'may' need to revisit
the approach suggested by Muchun.
-- 
Mike Kravetz

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