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

Mike Kravetz
2021-01-05T00:02:38Z

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@bytedance.com>
> ---
> 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.
>