[PATCH 3/5] lib: radix-tree: native accounting and tracking of special entries

[Johannes Weiner]

Add an internal tag to identify special entries that are accounted in
node->special in addition to node->count.

With this in place, the next patch can restore refault detection in
single-page files. It will also move the shadow count from the upper
bits of count to the new special counter, and then shrink count to a
char as well; the growth of struct radix_tree_node is temporary.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 include/linux/radix-tree.h | 10 ++++++----
 lib/radix-tree.c           | 14 ++++++++++----
 2 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 756b2909467e..2e1c9added23 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -68,7 +68,8 @@ enum radix_tree_tags {
 	/* Freely allocatable radix tree user tags */
 	RADIX_TREE_NR_USER_TAGS = 3,
 	/* Radix tree internal tags */
-	RADIX_TREE_NR_TAGS = RADIX_TREE_NR_USER_TAGS,
+	RADIX_TREE_TAG_SPECIAL = RADIX_TREE_NR_USER_TAGS,
+	RADIX_TREE_NR_TAGS,
 };
 
 #ifndef RADIX_TREE_MAP_SHIFT
@@ -90,9 +91,10 @@ enum radix_tree_tags {
 #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	special;	/* Special entry count */
 	union {
 		struct {
 			/* Used when ascending tree */
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index bb6ddfb60557..e58cff1d97ed 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 special %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->special, node->parent);
 
 	for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
 		unsigned long first = index | (i << node->shift);
@@ -522,9 +522,15 @@ 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))
-			entry_to_node(slot)->parent = node;
 		node->slots[0] = slot;
+		/* Extending an existing node or root->rnode? */
+		if (radix_tree_is_internal_node(slot)) {
+			entry_to_node(slot)->parent = node;
+		} else {
+			/* Moving a special root->rnode to a node */
+			if (root_tag_get(root, RADIX_TREE_TAG_SPECIAL))
+				node->special = 1;
+		}
 		slot = node_to_entry(node);
 		rcu_assign_pointer(root->rnode, slot);
 		shift += RADIX_TREE_MAP_SHIFT;
-- 
2.10.0