mm: keep page cache radix tree nodes in check

Previously, page cache radix tree nodes were freed after reclaim emptied
out their page pointers. But now reclaim stores shadow entries in their
place, which are only reclaimed when the inodes themselves are
reclaimed. This is problematic for bigger files that are still in use
after they have a significant amount of their cache reclaimed, without
any of those pages actually refaulting. The shadow entries will just
sit there and waste memory. In the worst case, the shadow entries will
accumulate until the machine runs out of memory.

To get this under control, the VM will track radix tree nodes
exclusively containing shadow entries on a per-NUMA node list. Per-NUMA
rather than global because we expect the radix tree nodes themselves to
be allocated node-locally and we want to reduce cross-node references of
otherwise independent cache workloads. A simple shrinker will then
reclaim these nodes on memory pressure.

A few things need to be stored in the radix tree node to implement the
shadow node LRU and allow tree deletions coming from the list:

1. There is no index available that would describe the reverse path
from the node up to the tree root, which is needed to perform a
deletion. To solve this, encode in each node its offset inside the
parent. This can be stored in the unused upper bits of the same
member that stores the node's height at no extra space cost.

2. The number of shadow entries needs to be counted in addition to the
regular entries, to quickly detect when the node is ready to go to
the shadow node LRU list. The current entry count is an unsigned
int but the maximum number of entries is 64, so a shadow counter
can easily be stored in the unused upper bits.

3. Tree modification needs tree lock and tree root, which are located
in the address space, so store an address_space backpointer in the
node. The parent pointer of the node is in a union with the 2-word
rcu_head, so the backpointer comes at no extra cost as well.

4. The node needs to be linked to an LRU list, which requires a list
head inside the node. This does increase the size of the node, but
it does not change the number of objects that fit into a slab page.

[[e-mail]: export the right function]
Signed-off-by: Johannes Weiner <[e-mail]>
Reviewed-by: Rik van Riel <[e-mail]>
Reviewed-by: Minchan Kim <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Bob Liu <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Dave Chinner <[e-mail]>
Cc: Greg Thelen <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: KOSAKI Motohiro <[e-mail]>
Cc: Luigi Semenzato <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Metin Doslu <[e-mail]>
Cc: Michel Lespinasse <[e-mail]>
Cc: Ozgun Erdogan <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Ryan Mallon <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/list_lru.h

···
 /* list_lru_walk_cb has to always return one of those */
 enum lru_status {
 	LRU_REMOVED,		/* item removed from list */
+	LRU_REMOVED_RETRY,	/* item removed, but lock has been
+				   dropped and reacquired */
 	LRU_ROTATE,		/* item referenced, give another pass */
 	LRU_SKIP,		/* item cannot be locked, skip */
 	LRU_RETRY,		/* item not freeable. May drop the lock
···
 };
 
 void list_lru_destroy(struct list_lru *lru);
-int list_lru_init(struct list_lru *lru);
+int list_lru_init_key(struct list_lru *lru, struct lock_class_key *key);
+static inline int list_lru_init(struct list_lru *lru)
+{
+	return list_lru_init_key(lru, NULL);
+}
 
 /**
  * list_lru_add: add an element to the lru list's tail

include/linux/mmzone.h

···
 #endif
 	WORKINGSET_REFAULT,
 	WORKINGSET_ACTIVATE,
+	WORKINGSET_NODERECLAIM,
 	NR_ANON_TRANSPARENT_HUGEPAGES,
 	NR_FREE_CMA_PAGES,
 	NR_VM_ZONE_STAT_ITEMS };

include/linux/radix-tree.h

···
 #define RADIX_TREE_TAG_LONGS	\
 	((RADIX_TREE_MAP_SIZE + BITS_PER_LONG - 1) / BITS_PER_LONG)
 
-struct radix_tree_node {
-	unsigned int	height;		/* Height from the bottom */
-	unsigned int	count;
-	union {
-		struct radix_tree_node *parent;	/* Used when ascending tree */
-		struct rcu_head	rcu_head;	/* Used when freeing node */
-	};
-	void __rcu	*slots[RADIX_TREE_MAP_SIZE];
-	unsigned long	tags[RADIX_TREE_MAX_TAGS][RADIX_TREE_TAG_LONGS];
-};
-
 #define RADIX_TREE_INDEX_BITS  (8 /* CHAR_BIT */ * sizeof(unsigned long))
 #define RADIX_TREE_MAX_PATH (DIV_ROUND_UP(RADIX_TREE_INDEX_BITS, \
 					  RADIX_TREE_MAP_SHIFT))
+
+/* Height component in node->path */
+#define RADIX_TREE_HEIGHT_SHIFT	(RADIX_TREE_MAX_PATH + 1)
+#define RADIX_TREE_HEIGHT_MASK	((1UL << RADIX_TREE_HEIGHT_SHIFT) - 1)
+
+/* Internally used bits of node->count */
+#define RADIX_TREE_COUNT_SHIFT	(RADIX_TREE_MAP_SHIFT + 1)
+#define RADIX_TREE_COUNT_MASK	((1UL << RADIX_TREE_COUNT_SHIFT) - 1)
+
+struct radix_tree_node {
+	unsigned int	path;	/* Offset in parent & height from the bottom */
+	unsigned int	count;
+	union {
+		struct {
+			/* Used when ascending tree */
+			struct radix_tree_node *parent;
+			/* For tree user */
+			void *private_data;
+		};
+		/* Used when freeing node */
+		struct rcu_head	rcu_head;
+	};
+	/* For tree user */
+	struct list_head private_list;
+	void __rcu	*slots[RADIX_TREE_MAP_SIZE];
+	unsigned long	tags[RADIX_TREE_MAX_TAGS][RADIX_TREE_TAG_LONGS];
+};
 
 /* root tags are stored in gfp_mask, shifted by __GFP_BITS_SHIFT */
 struct radix_tree_root {
···
 			  struct radix_tree_node **nodep, void ***slotp);
 void *radix_tree_lookup(struct radix_tree_root *, unsigned long);
 void **radix_tree_lookup_slot(struct radix_tree_root *, unsigned long);
-bool __radix_tree_delete_node(struct radix_tree_root *root, unsigned long index,
+bool __radix_tree_delete_node(struct radix_tree_root *root,
 			      struct radix_tree_node *node);
 void *radix_tree_delete_item(struct radix_tree_root *, unsigned long, void *);
 void *radix_tree_delete(struct radix_tree_root *, unsigned long);

include/linux/swap.h

···
 void *workingset_eviction(struct address_space *mapping, struct page *page);
 bool workingset_refault(void *shadow);
 void workingset_activation(struct page *page);
+extern struct list_lru workingset_shadow_nodes;
+
+static inline unsigned int workingset_node_pages(struct radix_tree_node *node)
+{
+	return node->count & RADIX_TREE_COUNT_MASK;
+}
+
+static inline void workingset_node_pages_inc(struct radix_tree_node *node)
+{
+	node->count++;
+}
+
+static inline void workingset_node_pages_dec(struct radix_tree_node *node)
+{
+	node->count--;
+}
+
+static inline unsigned int workingset_node_shadows(struct radix_tree_node *node)
+{
+	return node->count >> RADIX_TREE_COUNT_SHIFT;
+}
+
+static inline void workingset_node_shadows_inc(struct radix_tree_node *node)
+{
+	node->count += 1U << RADIX_TREE_COUNT_SHIFT;
+}
+
+static inline void workingset_node_shadows_dec(struct radix_tree_node *node)
+{
+	node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
+}
 
 /* linux/mm/page_alloc.c */
 extern unsigned long totalram_pages;

lib/radix-tree.c

···
 
 		/* Increase the height.  */
 		newheight = root->height+1;
-		node->height = newheight;
+		BUG_ON(newheight & ~RADIX_TREE_HEIGHT_MASK);
+		node->path = newheight;
 		node->count = 1;
 		node->parent = NULL;
 		slot = root->rnode;
···
 			/* Have to add a child node.  */
 			if (!(slot = radix_tree_node_alloc(root)))
 				return -ENOMEM;
-			slot->height = height;
+			slot->path = height;
 			slot->parent = node;
 			if (node) {
 				rcu_assign_pointer(node->slots[offset], slot);
 				node->count++;
+				slot->path |= offset << RADIX_TREE_HEIGHT_SHIFT;
 			} else
 				rcu_assign_pointer(root->rnode, ptr_to_indirect(slot));
 		}
···
 	}
 	node = indirect_to_ptr(node);
 
-	height = node->height;
+	height = node->path & RADIX_TREE_HEIGHT_MASK;
 	if (index > radix_tree_maxindex(height))
 		return NULL;
 
···
 		return (index == 0);
 	node = indirect_to_ptr(node);
 
-	height = node->height;
+	height = node->path & RADIX_TREE_HEIGHT_MASK;
 	if (index > radix_tree_maxindex(height))
 		return 0;
 
···
 {
 	unsigned shift, tag = flags & RADIX_TREE_ITER_TAG_MASK;
 	struct radix_tree_node *rnode, *node;
-	unsigned long index, offset;
+	unsigned long index, offset, height;
 
 	if ((flags & RADIX_TREE_ITER_TAGGED) && !root_tag_get(root, tag))
 		return NULL;
···
 		return NULL;
 
 restart:
-	shift = (rnode->height - 1) * RADIX_TREE_MAP_SHIFT;
+	height = rnode->path & RADIX_TREE_HEIGHT_MASK;
+	shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
 	offset = index >> shift;
 
 	/* Index outside of the tree */
···
 	unsigned int shift, height;
 	unsigned long i;
 
-	height = slot->height;
+	height = slot->path & RADIX_TREE_HEIGHT_MASK;
 	shift = (height-1) * RADIX_TREE_MAP_SHIFT;
 
 	for ( ; height > 1; height--) {
···
 		}
 
 		node = indirect_to_ptr(node);
-		max_index = radix_tree_maxindex(node->height);
+		max_index = radix_tree_maxindex(node->path &
+						RADIX_TREE_HEIGHT_MASK);
 		if (cur_index > max_index) {
 			rcu_read_unlock();
 			break;
···
  *
  *	Returns %true if @node was freed, %false otherwise.
  */
-bool __radix_tree_delete_node(struct radix_tree_root *root, unsigned long index,
+bool __radix_tree_delete_node(struct radix_tree_root *root,
 			      struct radix_tree_node *node)
 {
 	bool deleted = false;
···
 
 		parent = node->parent;
 		if (parent) {
-			index >>= RADIX_TREE_MAP_SHIFT;
+			unsigned int offset;
 
-			parent->slots[index & RADIX_TREE_MAP_MASK] = NULL;
+			offset = node->path >> RADIX_TREE_HEIGHT_SHIFT;
+			parent->slots[offset] = NULL;
 			parent->count--;
 		} else {
 			root_tag_clear_all(root);
···
 	node->slots[offset] = NULL;
 	node->count--;
 
-	__radix_tree_delete_node(root, index, node);
+	__radix_tree_delete_node(root, node);
 
 	return entry;
 }
···
 EXPORT_SYMBOL(radix_tree_tagged);
 
 static void
-radix_tree_node_ctor(void *node)
+radix_tree_node_ctor(void *arg)
 {
-	memset(node, 0, sizeof(struct radix_tree_node));
+	struct radix_tree_node *node = arg;
+
+	memset(node, 0, sizeof(*node));
+	INIT_LIST_HEAD(&node->private_list);
 }
 
 static __init unsigned long __maxindex(unsigned int height)

mm/filemap.c

···
 static void page_cache_tree_delete(struct address_space *mapping,
 				   struct page *page, void *shadow)
 {
-	if (shadow) {
-		void **slot;
+	struct radix_tree_node *node;
+	unsigned long index;
+	unsigned int offset;
+	unsigned int tag;
+	void **slot;
 
-		slot = radix_tree_lookup_slot(&mapping->page_tree, page->index);
-		radix_tree_replace_slot(slot, shadow);
+	VM_BUG_ON(!PageLocked(page));
+
+	__radix_tree_lookup(&mapping->page_tree, page->index, &node, &slot);
+
+	if (shadow) {
 		mapping->nrshadows++;
 		/*
 		 * Make sure the nrshadows update is committed before
···
 		 * same time and miss a shadow entry.
 		 */
 		smp_wmb();
-	} else
-		radix_tree_delete(&mapping->page_tree, page->index);
+	}
 	mapping->nrpages--;
+
+	if (!node) {
+		/* Clear direct pointer tags in root node */
+		mapping->page_tree.gfp_mask &= __GFP_BITS_MASK;
+		radix_tree_replace_slot(slot, shadow);
+		return;
+	}
+
+	/* Clear tree tags for the removed page */
+	index = page->index;
+	offset = index & RADIX_TREE_MAP_MASK;
+	for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
+		if (test_bit(offset, node->tags[tag]))
+			radix_tree_tag_clear(&mapping->page_tree, index, tag);
+	}
+
+	/* Delete page, swap shadow entry */
+	radix_tree_replace_slot(slot, shadow);
+	workingset_node_pages_dec(node);
+	if (shadow)
+		workingset_node_shadows_inc(node);
+	else
+		if (__radix_tree_delete_node(&mapping->page_tree, node))
+			return;
+
+	/*
+	 * Track node that only contains shadow entries.
+	 *
+	 * Avoid acquiring the list_lru lock if already tracked.  The
+	 * list_empty() test is safe as node->private_list is
+	 * protected by mapping->tree_lock.
+	 */
+	if (!workingset_node_pages(node) &&
+	    list_empty(&node->private_list)) {
+		node->private_data = mapping;
+		list_lru_add(&workingset_shadow_nodes, &node->private_list);
+	}
 }
 
 /*
···
 static int page_cache_tree_insert(struct address_space *mapping,
 				  struct page *page, void **shadowp)
 {
+	struct radix_tree_node *node;
 	void **slot;
 	int error;
 
-	slot = radix_tree_lookup_slot(&mapping->page_tree, page->index);
-	if (slot) {
+	error = __radix_tree_create(&mapping->page_tree, page->index,
+				    &node, &slot);
+	if (error)
+		return error;
+	if (*slot) {
 		void *p;
 
 		p = radix_tree_deref_slot_protected(slot, &mapping->tree_lock);
 		if (!radix_tree_exceptional_entry(p))
 			return -EEXIST;
-		radix_tree_replace_slot(slot, page);
-		mapping->nrshadows--;
-		mapping->nrpages++;
 		if (shadowp)
 			*shadowp = p;
-		return 0;
+		mapping->nrshadows--;
+		if (node)
+			workingset_node_shadows_dec(node);
 	}
-	error = radix_tree_insert(&mapping->page_tree, page->index, page);
-	if (!error)
-		mapping->nrpages++;
-	return error;
+	radix_tree_replace_slot(slot, page);
+	mapping->nrpages++;
+	if (node) {
+		workingset_node_pages_inc(node);
+		/*
+		 * Don't track node that contains actual pages.
+		 *
+		 * Avoid acquiring the list_lru lock if already
+		 * untracked.  The list_empty() test is safe as
+		 * node->private_list is protected by
+		 * mapping->tree_lock.
+		 */
+		if (!list_empty(&node->private_list))
+			list_lru_del(&workingset_shadow_nodes,
+				     &node->private_list);
+	}
+	return 0;
 }
 
 static int __add_to_page_cache_locked(struct page *page,

mm/list_lru.c

···
 
 		ret = isolate(item, &nlru->lock, cb_arg);
 		switch (ret) {
+		case LRU_REMOVED_RETRY:
+			assert_spin_locked(&nlru->lock);
 		case LRU_REMOVED:
 			if (--nlru->nr_items == 0)
 				node_clear(nid, lru->active_nodes);
 			WARN_ON_ONCE(nlru->nr_items < 0);
 			isolated++;
+			/*
+			 * If the lru lock has been dropped, our list
+			 * traversal is now invalid and so we have to
+			 * restart from scratch.
+			 */
+			if (ret == LRU_REMOVED_RETRY)
+				goto restart;
 			break;
 		case LRU_ROTATE:
 			list_move_tail(item, &nlru->list);
···
 			 * The lru lock has been dropped, our list traversal is
 			 * now invalid and so we have to restart from scratch.
 			 */
+			assert_spin_locked(&nlru->lock);
 			goto restart;
 		default:
 			BUG();
···
 }
 EXPORT_SYMBOL_GPL(list_lru_walk_node);
 
-int list_lru_init(struct list_lru *lru)
+int list_lru_init_key(struct list_lru *lru, struct lock_class_key *key)
 {
 	int i;
 	size_t size = sizeof(*lru->node) * nr_node_ids;
···
 	nodes_clear(lru->active_nodes);
 	for (i = 0; i < nr_node_ids; i++) {
 		spin_lock_init(&lru->node[i].lock);
+		if (key)
+			lockdep_set_class(&lru->node[i].lock, key);
 		INIT_LIST_HEAD(&lru->node[i].list);
 		lru->node[i].nr_items = 0;
 	}
 	return 0;
 }
-EXPORT_SYMBOL_GPL(list_lru_init);
+EXPORT_SYMBOL_GPL(list_lru_init_key);
 
 void list_lru_destroy(struct list_lru *lru)
 {

mm/truncate.c

···
 static void clear_exceptional_entry(struct address_space *mapping,
 				    pgoff_t index, void *entry)
 {
+	struct radix_tree_node *node;
+	void **slot;
+
 	/* Handled by shmem itself */
 	if (shmem_mapping(mapping))
 		return;
···
 	 * without the tree itself locked.  These unlocked entries
 	 * need verification under the tree lock.
 	 */
-	if (radix_tree_delete_item(&mapping->page_tree, index, entry) == entry)
-		mapping->nrshadows--;
+	if (!__radix_tree_lookup(&mapping->page_tree, index, &node, &slot))
+		goto unlock;
+	if (*slot != entry)
+		goto unlock;
+	radix_tree_replace_slot(slot, NULL);
+	mapping->nrshadows--;
+	if (!node)
+		goto unlock;
+	workingset_node_shadows_dec(node);
+	/*
+	 * Don't track node without shadow entries.
+	 *
+	 * Avoid acquiring the list_lru lock if already untracked.
+	 * The list_empty() test is safe as node->private_list is
+	 * protected by mapping->tree_lock.
+	 */
+	if (!workingset_node_shadows(node) &&
+	    !list_empty(&node->private_list))
+		list_lru_del(&workingset_shadow_nodes, &node->private_list);
+	__radix_tree_delete_node(&mapping->page_tree, node);
+unlock:
 	spin_unlock_irq(&mapping->tree_lock);
 }
 

mm/vmstat.c

···
 #endif
 	"workingset_refault",
 	"workingset_activate",
+	"workingset_nodereclaim",
 	"nr_anon_transparent_hugepages",
 	"nr_free_cma",
 	"nr_dirty_threshold",

mm/workingset.c

···
 {
 	atomic_long_inc(&page_zone(page)->inactive_age);
 }
+
+/*
+ * Shadow entries reflect the share of the working set that does not
+ * fit into memory, so their number depends on the access pattern of
+ * the workload.  In most cases, they will refault or get reclaimed
+ * along with the inode, but a (malicious) workload that streams
+ * through files with a total size several times that of available
+ * memory, while preventing the inodes from being reclaimed, can
+ * create excessive amounts of shadow nodes.  To keep a lid on this,
+ * track shadow nodes and reclaim them when they grow way past the
+ * point where they would still be useful.
+ */
+
+struct list_lru workingset_shadow_nodes;
+
+static unsigned long count_shadow_nodes(struct shrinker *shrinker,
+					struct shrink_control *sc)
+{
+	unsigned long shadow_nodes;
+	unsigned long max_nodes;
+	unsigned long pages;
+
+	/* list_lru lock nests inside IRQ-safe mapping->tree_lock */
+	local_irq_disable();
+	shadow_nodes = list_lru_count_node(&workingset_shadow_nodes, sc->nid);
+	local_irq_enable();
+
+	pages = node_present_pages(sc->nid);
+	/*
+	 * Active cache pages are limited to 50% of memory, and shadow
+	 * entries that represent a refault distance bigger than that
+	 * do not have any effect.  Limit the number of shadow nodes
+	 * such that shadow entries do not exceed the number of active
+	 * cache pages, assuming a worst-case node population density
+	 * of 1/8th on average.
+	 *
+	 * On 64-bit with 7 radix_tree_nodes per page and 64 slots
+	 * each, this will reclaim shadow entries when they consume
+	 * ~2% of available memory:
+	 *
+	 * PAGE_SIZE / radix_tree_nodes / node_entries / PAGE_SIZE
+	 */
+	max_nodes = pages >> (1 + RADIX_TREE_MAP_SHIFT - 3);
+
+	if (shadow_nodes <= max_nodes)
+		return 0;
+
+	return shadow_nodes - max_nodes;
+}
+
+static enum lru_status shadow_lru_isolate(struct list_head *item,
+					  spinlock_t *lru_lock,
+					  void *arg)
+{
+	struct address_space *mapping;
+	struct radix_tree_node *node;
+	unsigned int i;
+	int ret;
+
+	/*
+	 * Page cache insertions and deletions synchroneously maintain
+	 * the shadow node LRU under the mapping->tree_lock and the
+	 * lru_lock.  Because the page cache tree is emptied before
+	 * the inode can be destroyed, holding the lru_lock pins any
+	 * address_space that has radix tree nodes on the LRU.
+	 *
+	 * We can then safely transition to the mapping->tree_lock to
+	 * pin only the address_space of the particular node we want
+	 * to reclaim, take the node off-LRU, and drop the lru_lock.
+	 */
+
+	node = container_of(item, struct radix_tree_node, private_list);
+	mapping = node->private_data;
+
+	/* Coming from the list, invert the lock order */
+	if (!spin_trylock(&mapping->tree_lock)) {
+		spin_unlock(lru_lock);
+		ret = LRU_RETRY;
+		goto out;
+	}
+
+	list_del_init(item);
+	spin_unlock(lru_lock);
+
+	/*
+	 * The nodes should only contain one or more shadow entries,
+	 * no pages, so we expect to be able to remove them all and
+	 * delete and free the empty node afterwards.
+	 */
+
+	BUG_ON(!node->count);
+	BUG_ON(node->count & RADIX_TREE_COUNT_MASK);
+
+	for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
+		if (node->slots[i]) {
+			BUG_ON(!radix_tree_exceptional_entry(node->slots[i]));
+			node->slots[i] = NULL;
+			BUG_ON(node->count < (1U << RADIX_TREE_COUNT_SHIFT));
+			node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
+			BUG_ON(!mapping->nrshadows);
+			mapping->nrshadows--;
+		}
+	}
+	BUG_ON(node->count);
+	inc_zone_state(page_zone(virt_to_page(node)), WORKINGSET_NODERECLAIM);
+	if (!__radix_tree_delete_node(&mapping->page_tree, node))
+		BUG();
+
+	spin_unlock(&mapping->tree_lock);
+	ret = LRU_REMOVED_RETRY;
+out:
+	local_irq_enable();
+	cond_resched();
+	local_irq_disable();
+	spin_lock(lru_lock);
+	return ret;
+}
+
+static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
+				       struct shrink_control *sc)
+{
+	unsigned long ret;
+
+	/* list_lru lock nests inside IRQ-safe mapping->tree_lock */
+	local_irq_disable();
+	ret =  list_lru_walk_node(&workingset_shadow_nodes, sc->nid,
+				  shadow_lru_isolate, NULL, &sc->nr_to_scan);
+	local_irq_enable();
+	return ret;
+}
+
+static struct shrinker workingset_shadow_shrinker = {
+	.count_objects = count_shadow_nodes,
+	.scan_objects = scan_shadow_nodes,
+	.seeks = DEFAULT_SEEKS,
+	.flags = SHRINKER_NUMA_AWARE,
+};
+
+/*
+ * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
+ * mapping->tree_lock.
+ */
+static struct lock_class_key shadow_nodes_key;
+
+static int __init workingset_init(void)
+{
+	int ret;
+
+	ret = list_lru_init_key(&workingset_shadow_nodes, &shadow_nodes_key);
+	if (ret)
+		goto err;
+	ret = register_shrinker(&workingset_shadow_shrinker);
+	if (ret)
+		goto err_list_lru;
+	return 0;
+err_list_lru:
+	list_lru_destroy(&workingset_shadow_nodes);
+err:
+	return ret;
+}
+module_init(workingset_init);