Patch "mm, swap: fix race between swap count continuation operations" has been added to the 4.13-stable tree

@@ -246,6 +246,10 @@ struct swap_info_struct {
 					 * both locks need hold, hold swap_lock
 					 * first.
 					 */
+	spinlock_t cont_lock;		/*
+					 * protect swap count continuation page
+					 * list.
+					 */
 	struct work_struct discard_work; /* discard worker */
 	struct swap_cluster_list discard_clusters; /* discard clusters list */
 };

@@ -2635,6 +2635,7 @@ static struct swap_info_struct *alloc_sw
 	p->flags = SWP_USED;
 	spin_unlock(&swap_lock);
 	spin_lock_init(&p->lock);
+	spin_lock_init(&p->cont_lock);
 
 	return p;
 }

@@ -3307,6 +3308,7 @@ int add_swap_count_continuation(swp_entr
 	head = vmalloc_to_page(si->swap_map + offset);
 	offset &= ~PAGE_MASK;
 
+	spin_lock(&si->cont_lock);
 	/*
 	 * Page allocation does not initialize the page's lru field,
 	 * but it does always reset its private field.

@@ -3326,7 +3328,7 @@ int add_swap_count_continuation(swp_entr
 		 * a continuation page, free our allocation and use this one.
 		 */
 		if (!(count & COUNT_CONTINUED))
-			goto out;
+			goto out_unlock_cont;
 
 		map = kmap_atomic(list_page) + offset;
 		count = *map;

@@ -3337,11 +3339,13 @@ int add_swap_count_continuation(swp_entr
 		 * free our allocation and use this one.
 		 */
 		if ((count & ~COUNT_CONTINUED) != SWAP_CONT_MAX)
-			goto out;
+			goto out_unlock_cont;
 	}
 
 	list_add_tail(&page->lru, &head->lru);
 	page = NULL;			/* now it's attached, don't free it */
+out_unlock_cont:
+	spin_unlock(&si->cont_lock);
 out:
 	unlock_cluster(ci);
 	spin_unlock(&si->lock);

@@ -3366,6 +3370,7 @@ static bool swap_count_continued(struct
 	struct page *head;
 	struct page *page;
 	unsigned char *map;
+	bool ret;
 
 	head = vmalloc_to_page(si->swap_map + offset);
 	if (page_private(head) != SWP_CONTINUED) {

@@ -3373,6 +3378,7 @@ static bool swap_count_continued(struct
 		return false;		/* need to add count continuation */
 	}
 
+	spin_lock(&si->cont_lock);
 	offset &= ~PAGE_MASK;
 	page = list_entry(head->lru.next, struct page, lru);
 	map = kmap_atomic(page) + offset;

@@ -3393,8 +3399,10 @@ static bool swap_count_continued(struct
 		if (*map == SWAP_CONT_MAX) {
 			kunmap_atomic(map);
 			page = list_entry(page->lru.next, struct page, lru);
-			if (page == head)
-				return false;	/* add count continuation */
+			if (page == head) {
+				ret = false;	/* add count continuation */
+				goto out;
+			}
 			map = kmap_atomic(page) + offset;
 init_map:		*map = 0;		/* we didn't zero the page */
 		}

@@ -3407,7 +3415,7 @@ init_map:		*map = 0;		/* we didn't zero
 			kunmap_atomic(map);
 			page = list_entry(page->lru.prev, struct page, lru);
 		}
-		return true;			/* incremented */
+		ret = true;			/* incremented */
 
 	} else {				/* decrementing */
 		/*

@@ -3433,8 +3441,11 @@ init_map:		*map = 0;		/* we didn't zero
 			kunmap_atomic(map);
 			page = list_entry(page->lru.prev, struct page, lru);
 		}
-		return count == COUNT_CONTINUED;
+		ret = count == COUNT_CONTINUED;
 	}
+out:
+	spin_unlock(&si->cont_lock);
+	return ret;
 }
 
 /*