[PATCH 4/9] lib: radix-tree: native accounting of exceptional entries

[Johannes Weiner]

The way the page cache is sneaking shadow entries of evicted pages
into the radix tree past the node entry accounting and tracking them
manually in the upper bits of node->count is fraught with problems.

These shadow entries are marked in the tree as exceptional entries,
which are a native concept to the radix tree. Maintain an explicit
counter of exceptional entries in the radix tree node. Subsequent
patches will switch shadow entry tracking over to that counter.

DAX and shmem are the other users of exceptional entries. Since slot
replacements that change the entry type from regular to exceptional
must now be accounted, introduce a __radix_tree_replace() function
that does replacement and accounting, and switch DAX and shmem over.

The increase in radix tree node size is temporary. A followup patch
switches the shadow tracking to this new scheme and we'll no longer
need the upper bits in node->count and shrink that back to one byte.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 fs/dax.c                   |  5 +++--
 include/linux/radix-tree.h | 10 +++++++---
 lib/radix-tree.c           | 46 +++++++++++++++++++++++++++++++++++++++++++---
 mm/shmem.c                 |  8 ++++----
 4 files changed, 57 insertions(+), 12 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 014defd2e744..db78bae0dc0f 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -643,12 +643,13 @@ static void *dax_insert_mapping_entry(struct address_space *mapping,
 		}
 		mapping->nrexceptional++;
 	} else {
+		struct radix_tree_node *node;
 		void **slot;
 		void *ret;
 
-		ret = __radix_tree_lookup(page_tree, index, NULL, &slot);
+		ret = __radix_tree_lookup(page_tree, index, &node, &slot);
 		WARN_ON_ONCE(ret != entry);
-		radix_tree_replace_slot(slot, new_entry);
+		__radix_tree_replace(page_tree, node, slot, new_entry);
 	}
 	if (vmf->flags & FAULT_FLAG_WRITE)
 		radix_tree_tag_set(page_tree, index, PAGECACHE_TAG_DIRTY);
diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index af3581b8a451..7ced8a70cc8b 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -85,9 +85,10 @@ static inline bool radix_tree_is_internal_node(void *ptr)
 #define RADIX_TREE_COUNT_MASK	((1UL << RADIX_TREE_COUNT_SHIFT) - 1)
 
 struct radix_tree_node {
-	unsigned char	shift;	/* Bits remaining in each slot */
-	unsigned char	offset;	/* Slot offset in parent */
-	unsigned int	count;
+	unsigned char	shift;		/* Bits remaining in each slot */
+	unsigned char	offset;		/* Slot offset in parent */
+	unsigned int	count;		/* Total entry count */
+	unsigned char	exceptional;	/* Exceptional entry count */
 	union {
 		struct {
 			/* Used when ascending tree */
@@ -276,6 +277,9 @@ void *__radix_tree_lookup(struct radix_tree_root *root, unsigned long index,
 			  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);
+void __radix_tree_replace(struct radix_tree_root *root,
+			  struct radix_tree_node *node,
+			  void **slot, void *item);
 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 *);
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 8e6d552c40dd..7885796d35ae 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -220,10 +220,10 @@ static void dump_node(struct radix_tree_node *node, unsigned long index)
 {
 	unsigned long i;
 
-	pr_debug("radix node: %p offset %d tags %lx %lx %lx shift %d count %d parent %p\n",
+	pr_debug("radix node: %p offset %d tags %lx %lx %lx shift %d count %d exceptional %d parent %p\n",
 		node, node->offset,
 		node->tags[0][0], node->tags[1][0], node->tags[2][0],
-		node->shift, node->count, node->parent);
+		node->shift, node->count, node->exceptional, node->parent);
 
 	for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
 		unsigned long first = index | (i << node->shift);
@@ -522,8 +522,13 @@ static int radix_tree_extend(struct radix_tree_root *root,
 		node->offset = 0;
 		node->count = 1;
 		node->parent = NULL;
-		if (radix_tree_is_internal_node(slot))
+		if (radix_tree_is_internal_node(slot)) {
 			entry_to_node(slot)->parent = node;
+		} else {
+			/* Moving an exceptional root->rnode to a node */
+			if (radix_tree_exceptional_entry(slot))
+				node->exceptional = 1;
+		}
 		node->slots[0] = slot;
 		slot = node_to_entry(node);
 		rcu_assign_pointer(root->rnode, slot);
@@ -649,6 +654,8 @@ int __radix_tree_insert(struct radix_tree_root *root, unsigned long index,
 	if (node) {
 		unsigned offset = get_slot_offset(node, slot);
 		node->count++;
+		if (radix_tree_exceptional_entry(item))
+			node->exceptional++;
 		BUG_ON(tag_get(node, 0, offset));
 		BUG_ON(tag_get(node, 1, offset));
 		BUG_ON(tag_get(node, 2, offset));
@@ -747,6 +754,37 @@ void *radix_tree_lookup(struct radix_tree_root *root, unsigned long index)
 EXPORT_SYMBOL(radix_tree_lookup);
 
 /**
+ * __radix_tree_replace		- replace item in a slot
+ * @root:	radix tree root
+ * @node:	pointer to tree node
+ * @slot:	pointer to slot in @node
+ * @item:	new item to store in the slot.
+ *
+ * For use with __radix_tree_lookup().  Caller must hold tree write locked
+ * across slot lookup and replacement.
+ */
+void __radix_tree_replace(struct radix_tree_root *root,
+			  struct radix_tree_node *node,
+			  void **slot, void *item)
+{
+	void *old = rcu_dereference_raw(*slot);
+	int exceptional;
+
+	WARN_ON_ONCE(radix_tree_is_internal_node(item));
+	WARN_ON_ONCE(!!item - !!old);
+
+	exceptional = !!radix_tree_exceptional_entry(item) -
+		      !!radix_tree_exceptional_entry(old);
+
+	WARN_ON_ONCE(exceptional && !node && slot != (void **)&root->rnode);
+
+	if (node)
+		node->exceptional += exceptional;
+
+	rcu_assign_pointer(*slot, item);
+}
+
+/**
  *	radix_tree_tag_set - set a tag on a radix tree node
  *	@root:		radix tree root
  *	@index:		index key
@@ -1561,6 +1599,8 @@ void *radix_tree_delete_item(struct radix_tree_root *root,
 	delete_sibling_entries(node, node_to_entry(slot), offset);
 	node->slots[offset] = NULL;
 	node->count--;
+	if (radix_tree_exceptional_entry(entry))
+		node->exceptional--;
 
 	__radix_tree_delete_node(root, node);
 
diff --git a/mm/shmem.c b/mm/shmem.c
index ad7813d73ea7..7f3a08df25c9 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -300,18 +300,18 @@ void shmem_uncharge(struct inode *inode, long pages)
 static int shmem_radix_tree_replace(struct address_space *mapping,
 			pgoff_t index, void *expected, void *replacement)
 {
+	struct radix_tree_node *node;
 	void **pslot;
 	void *item;
 
 	VM_BUG_ON(!expected);
 	VM_BUG_ON(!replacement);
-	pslot = radix_tree_lookup_slot(&mapping->page_tree, index);
-	if (!pslot)
+	item = __radix_tree_lookup(&mapping->page_tree, index, &node, &pslot);
+	if (!item)
 		return -ENOENT;
-	item = radix_tree_deref_slot_protected(pslot, &mapping->tree_lock);
 	if (item != expected)
 		return -ENOENT;
-	radix_tree_replace_slot(pslot, replacement);
+	__radix_tree_replace(&mapping->page_tree, node, pslot, replacement);
 	return 0;
 }
 
-- 
2.10.2