[PATCH v2 3/6] mm: hugetlb: fix a race between freeing and dissolving the page

@@ -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 (unsigned long)head[3].mapping == -1U;
+}
+
+static inline void SetPageHugeFreed(struct page *head)
+{
+	head[3].mapping = (void *)-1U;
+}
+
+static inline void ClearPageHugeFreed(struct page *head)
+{
+	head[3].mapping = NULL;
+}
+
 /* 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;

@@ -1291,6 +1308,17 @@ static inline void destroy_compound_gigantic_page(struct page *page,
 						unsigned int order) { }
 #endif
 
+/*
+ * Because we reuse the mapping field of some tail page structs, we should
+ * reset those mapping to initial value before @head is freed to the buddy
+ * allocator. The invalid value will be checked in the free_tail_pages_check().
+ */
+static inline void reset_tail_page_mapping(struct hstate *h, struct page *head)
+{
+	if (!hstate_is_gigantic(h))
+		head[3].mapping = TAIL_MAPPING;
+}
+
 static void update_and_free_page(struct hstate *h, struct page *page)
 {
 	int i;

@@ -1298,6 +1326,7 @@ static void update_and_free_page(struct hstate *h, struct page *page)
 	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
 		return;
 
+	reset_tail_page_mapping(h, page);
 	h->nr_huge_pages--;
 	h->nr_huge_pages_node[page_to_nid(page)]--;
 	for (i = 0; i < pages_per_huge_page(h); i++) {

@@ -1504,6 +1533,7 @@ 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 +1800,14 @@ 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)))
+			goto out;
+
 		/*
 		 * Move PageHWPoison flag from head page to the raw error page,
 		 * which makes any subpages rather than the error page reusable.