Re: [PATCH 3/6] mm: hugetlb: fix a race between freeing and dissolving the page

[Mike Kravetz]

On 1/3/21 10:58 PM, Muchun Song wrote:
> There is a race condition between __free_huge_page()
> and dissolve_free_huge_page().
> 
> CPU0:                         CPU1:
> 
> // page_count(page) == 1
> put_page(page)
>   __free_huge_page(page)
>                               dissolve_free_huge_page(page)
>                                 spin_lock(&hugetlb_lock)
>                                 // PageHuge(page) && !page_count(page)
>                                 update_and_free_page(page)
>                                 // page is freed to the buddy
>                                 spin_unlock(&hugetlb_lock)
>     spin_lock(&hugetlb_lock)
>     clear_page_huge_active(page)
>     enqueue_huge_page(page)
>     // It is wrong, the page is already freed
>     spin_unlock(&hugetlb_lock)
> 
> The race windows is between put_page() and spin_lock() which
> is in the __free_huge_page().
> 
> We should make sure that the page is already on the free list
> when it is dissolved.
> 
> Fixes: c8721bbbdd36 ("mm: memory-hotplug: enable memory hotplug to handle hugepage")
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
>  mm/hugetlb.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 48 insertions(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 1f3bf1710b66..72608008f8b4 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -79,6 +79,21 @@ DEFINE_SPINLOCK(hugetlb_lock);
>  static int num_fault_mutexes;
>  struct mutex *hugetlb_fault_mutex_table ____cacheline_aligned_in_smp;
>  
> +static inline bool PageHugeFreed(struct page *head)
> +{
> +	return page_private(head) == -1UL;

	return page_private(head + 4) == -1UL;

> +}
> +
> +static inline void SetPageHugeFreed(struct page *head)
> +{
> +	set_page_private(head + 4, -1UL);
> +}
> +
> +static inline void ClearPageHugeFreed(struct page *head)
> +{
> +	set_page_private(head + 4, 0);
> +}

It is unfortunate that we can not use some existing value like
page_huge_active() to determine if dissolve_free_huge_page() should
proceed with freeing the page to buddy.  If the existing check,

	if (!page_count(page)) {

was changed to

	if (!page_count(page) && !page_huge_active(page)) {

the race window would be shrunk.  However, the most straight forward
way to fully close the window is with the approach taken here.

> +
>  /* Forward declaration */
>  static int hugetlb_acct_memory(struct hstate *h, long delta);
>  
> @@ -1028,6 +1043,7 @@ static void enqueue_huge_page(struct hstate *h, struct page *page)
>  	list_move(&page->lru, &h->hugepage_freelists[nid]);
>  	h->free_huge_pages++;
>  	h->free_huge_pages_node[nid]++;
> +	SetPageHugeFreed(page);
>  }
>  
>  static struct page *dequeue_huge_page_node_exact(struct hstate *h, int nid)
> @@ -1044,6 +1060,7 @@ static struct page *dequeue_huge_page_node_exact(struct hstate *h, int nid)
>  
>  		list_move(&page->lru, &h->hugepage_activelist);
>  		set_page_refcounted(page);
> +		ClearPageHugeFreed(page);
>  		h->free_huge_pages--;
>  		h->free_huge_pages_node[nid]--;
>  		return page;
> @@ -1504,6 +1521,7 @@ static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
>  	spin_lock(&hugetlb_lock);
>  	h->nr_huge_pages++;
>  	h->nr_huge_pages_node[nid]++;
> +	ClearPageHugeFreed(page);
>  	spin_unlock(&hugetlb_lock);
>  }
>  
> @@ -1770,6 +1788,36 @@ int dissolve_free_huge_page(struct page *page)
>  		int nid = page_to_nid(head);
>  		if (h->free_huge_pages - h->resv_huge_pages == 0)
>  			goto out;
> +
> +		/*
> +		 * There is a race condition between __free_huge_page()
> +		 * and dissolve_free_huge_page().
> +		 *
> +		 * CPU0:                         CPU1:
> +		 *
> +		 * // page_count(page) == 1
> +		 * put_page(page)
> +		 *   __free_huge_page(page)
> +		 *                               dissolve_free_huge_page(page)
> +		 *                                 spin_lock(&hugetlb_lock)
> +		 *                                 // PageHuge(page) && !page_count(page)
> +		 *                                 update_and_free_page(page)
> +		 *                                 // page is freed to the buddy
> +		 *                                 spin_unlock(&hugetlb_lock)
> +		 *     spin_lock(&hugetlb_lock)
> +		 *     enqueue_huge_page(page)
> +		 *     // It is wrong, the page is already freed
> +		 *     spin_unlock(&hugetlb_lock)
> +		 *
> +		 * The race window is between put_page() and spin_lock() which
> +		 * is in the __free_huge_page().

IMO, the description of the race condition in the commit message is
sufficient.  It does not need to be here in the code.  The below comment
should be sufficient.

-- 
Mike Kravetz

> +		 *
> +		 * We should make sure that the page is already on the free list
> +		 * when it is dissolved.
> +		 */
> +		if (unlikely(!PageHugeFreed(head)))
> +			goto out;
> +
>  		/*
>  		 * Move PageHWPoison flag from head page to the raw error page,
>  		 * which makes any subpages rather than the error page reusable.
>