[PATCH] radix-tree: reduce tree height upon partial truncation

Shrink the height of a radix tree when it is partially truncated - we only do
shrinkage of full truncation at present.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 336852f23592c8446d36b883230f755a9a9b1c61..c0bd4a91480387e0c22d43b65d7f75fc2a522188 100644 (file)
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -253,7 +253,7 @@ int radix_tree_insert(struct radix_tree_root *root,
        shift = (height-1) * RADIX_TREE_MAP_SHIFT;
 
        offset = 0;                     /* uninitialised var warning */
-       while (height > 0) {
+       do {
                if (slot == NULL) {
                        /* Have to add a child node.  */
                        if (!(slot = radix_tree_node_alloc(root)))
@@ -271,18 +271,16 @@ int radix_tree_insert(struct radix_tree_root *root,
                slot = node->slots[offset];
                shift -= RADIX_TREE_MAP_SHIFT;
                height--;
-       }
+       } while (height > 0);
 
        if (slot != NULL)
                return -EEXIST;
 
-       if (node) {
-               node->count++;
-               node->slots[offset] = item;
-               BUG_ON(tag_get(node, 0, offset));
-               BUG_ON(tag_get(node, 1, offset));
-       } else
-               root->rnode = item;
+       BUG_ON(!node);
+       node->count++;
+       node->slots[offset] = item;
+       BUG_ON(tag_get(node, 0, offset));
+       BUG_ON(tag_get(node, 1, offset));
 
        return 0;
 }
@@ -679,6 +677,29 @@ radix_tree_gang_lookup_tag(struct radix_tree_root *root, void **results,
 }
 EXPORT_SYMBOL(radix_tree_gang_lookup_tag);
 
+/**
+ *     radix_tree_shrink    -    shrink height of a radix tree to minimal
+ *     @root           radix tree root
+ */
+static inline void radix_tree_shrink(struct radix_tree_root *root)
+{
+       /* try to shrink tree height */
+       while (root->height > 1 &&
+                       root->rnode->count == 1 &&
+                       root->rnode->slots[0]) {
+               struct radix_tree_node *to_free = root->rnode;
+
+               root->rnode = to_free->slots[0];
+               root->height--;
+               /* must only free zeroed nodes into the slab */
+               tag_clear(to_free, 0, 0);
+               tag_clear(to_free, 1, 0);
+               to_free->slots[0] = NULL;
+               to_free->count = 0;
+               radix_tree_node_free(to_free);
+       }
+}
+
 /**
  *     radix_tree_delete    -    delete an item from a radix tree
  *     @root:          radix tree root
@@ -755,8 +776,13 @@ void *radix_tree_delete(struct radix_tree_root *root, unsigned long index)
        /* Now free the nodes we do not need anymore */
        for (pathp = orig_pathp; pathp->node; pathp--) {
                pathp->node->slots[pathp->offset] = NULL;
-               if (--pathp->node->count)
+               pathp->node->count--;
+
+               if (pathp->node->count) {
+                       if (pathp->node == root->rnode)
+                               radix_tree_shrink(root);
                        goto out;
+               }
 
                /* Node with zero slots in use so free it */
                radix_tree_node_free(pathp->node);