On Fri 15-01-21 20:49:40, 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 dissolve_free_huge_page().
>
> We should make sure that the page is already on the free list
> when it is dissolved.
>
> As a result __free_huge_page would corrupt page(s) already in the buddy
> allocator.
>
> Fixes: c8721bbbdd36 ("mm: memory-hotplug: enable memory hotplug to handle hugepage")
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!
> ---
> mm/hugetlb.c | 39 +++++++++++++++++++++++++++++++++++++++
> 1 file changed, 39 insertions(+)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4741d60f8955..b99fe4a2b435 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 + 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);
> +}
> +
> /* 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);
> }
>
> @@ -1754,6 +1772,7 @@ int dissolve_free_huge_page(struct page *page)
> {
> int rc = -EBUSY;
>
> +retry:
> /* Not to disrupt normal path by vainly holding hugetlb_lock */
> if (!PageHuge(page))
> return 0;
> @@ -1770,6 +1789,26 @@ 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;
> +
> + /*
> + * We should make sure that the page is already on the free list
> + * when it is dissolved.
> + */
> + if (unlikely(!PageHugeFreed(head))) {
> + spin_unlock(&hugetlb_lock);
> + cond_resched();
> +
> + /*
> + * Theoretically, we should return -EBUSY when we
> + * encounter this race. In fact, we have a chance
> + * to successfully dissolve the page if we do a
> + * retry. Because the race window is quite small.
> + * If we seize this opportunity, it is an optimization
> + * for increasing the success rate of dissolving page.
> + */
> + goto retry;
> + }
> +
> /*
> * Move PageHWPoison flag from head page to the raw error page,
> * which makes any subpages rather than the error page reusable.
> --
> 2.11.0
--
Michal Hocko
SUSE Labs
