Add a generic associative array implementation.

Add a generic associative array implementation that can be used as the
container for keyrings, thereby massively increasing the capacity available
whilst also speeding up searching in keyrings that contain a lot of keys.

This may also be useful in FS-Cache for tracking cookies.

Documentation is added into Documentation/associative_array.txt

Some of the properties of the implementation are:

 (1) Objects are opaque pointers.  The implementation does not care where they
     point (if anywhere) or what they point to (if anything).

     [!] NOTE: Pointers to objects _must_ be zero in the two least significant
     	       bits.

 (2) Objects do not need to contain linkage blocks for use by the array.  This
     permits an object to be located in multiple arrays simultaneously.
     Rather, the array is made up of metadata blocks that point to objects.

 (3) Objects are labelled as being one of two types (the type is a bool value).
     This information is stored in the array, but has no consequence to the
     array itself or its algorithms.

 (4) Objects require index keys to locate them within the array.

 (5) Index keys must be unique.  Inserting an object with the same key as one
     already in the array will replace the old object.

 (6) Index keys can be of any length and can be of different lengths.

 (7) Index keys should encode the length early on, before any variation due to
     length is seen.

 (8) Index keys can include a hash to scatter objects throughout the array.

 (9) The array can iterated over.  The objects will not necessarily come out in
     key order.

(10) The array can be iterated whilst it is being modified, provided the RCU
     readlock is being held by the iterator.  Note, however, under these
     circumstances, some objects may be seen more than once.  If this is a
     problem, the iterator should lock against modification.  Objects will not
     be missed, however, unless deleted.

(11) Objects in the array can be looked up by means of their index key.

(12) Objects can be looked up whilst the array is being modified, provided the
     RCU readlock is being held by the thread doing the look up.

The implementation uses a tree of 16-pointer nodes internally that are indexed
on each level by nibbles from the index key.  To improve memory efficiency,
shortcuts can be emplaced to skip over what would otherwise be a series of
single-occupancy nodes.  Further, nodes pack leaf object pointers into spare
space in the node rather than making an extra branch until as such time an
object needs to be added to a full node.

Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 1745 insertions, 0 deletions

diff --git a/lib/assoc_array.c b/lib/assoc_array.c
new file mode 100644
index 000000000000..a0952818f938
--- /dev/null
+++ b/lib/assoc_array.c
@@ -0,0 +1,1745 @@
+/* Generic associative array implementation.
+ *
+ * See Documentation/assoc_array.txt for information.
+ *
+ * Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+//#define DEBUG
+#include <linux/slab.h>
+#include <linux/assoc_array_priv.h>
+
+/*
+ * Iterate over an associative array.  The caller must hold the RCU read lock
+ * or better.
+ */
+static int assoc_array_subtree_iterate(const struct assoc_array_ptr *root,
+                                       const struct assoc_array_ptr *stop,
+                                       int (*iterator)(const void *leaf,
+                                                       void *iterator_data),
+                                       void *iterator_data)
+{
+        const struct assoc_array_shortcut *shortcut;
+        const struct assoc_array_node *node;
+        const struct assoc_array_ptr *cursor, *ptr, *parent;
+        unsigned long has_meta;
+        int slot, ret;
+
+        cursor = root;
+
+begin_node:
+        if (assoc_array_ptr_is_shortcut(cursor)) {
+                /* Descend through a shortcut */
+                shortcut = assoc_array_ptr_to_shortcut(cursor);
+                smp_read_barrier_depends();
+                cursor = ACCESS_ONCE(shortcut->next_node);
+        }
+
+        node = assoc_array_ptr_to_node(cursor);
+        smp_read_barrier_depends();
+        slot = 0;
+
+        /* We perform two passes of each node.
+         *
+         * The first pass does all the leaves in this node.  This means we
+         * don't miss any leaves if the node is split up by insertion whilst
+         * we're iterating over the branches rooted here (we may, however, see
+         * some leaves twice).
+         */
+        has_meta = 0;
+        for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
+                ptr = ACCESS_ONCE(node->slots[slot]);
+                has_meta |= (unsigned long)ptr;
+                if (ptr && assoc_array_ptr_is_leaf(ptr)) {
+                        /* We need a barrier between the read of the pointer
+                         * and dereferencing the pointer - but only if we are
+                         * actually going to dereference it.
+                         */
+                        smp_read_barrier_depends();
+
+                        /* Invoke the callback */
+                        ret = iterator(assoc_array_ptr_to_leaf(ptr),
+                                       iterator_data);
+                        if (ret)
+                                return ret;
+                }
+        }
+
+        /* The second pass attends to all the metadata pointers.  If we follow
+         * one of these we may find that we don't come back here, but rather go
+         * back to a replacement node with the leaves in a different layout.
+         *
+         * We are guaranteed to make progress, however, as the slot number for
+         * a particular portion of the key space cannot change - and we
+         * continue at the back pointer + 1.
+         */
+        if (!(has_meta & ASSOC_ARRAY_PTR_META_TYPE))
+                goto finished_node;
+        slot = 0;
+
+continue_node:
+        node = assoc_array_ptr_to_node(cursor);
+        smp_read_barrier_depends();
+
+        for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
+                ptr = ACCESS_ONCE(node->slots[slot]);
+                if (assoc_array_ptr_is_meta(ptr)) {
+                        cursor = ptr;
+                        goto begin_node;
+                }
+        }
+
+finished_node:
+        /* Move up to the parent (may need to skip back over a shortcut) */
+        parent = ACCESS_ONCE(node->back_pointer);
+        slot = node->parent_slot;
+        if (parent == stop)
+                return 0;
+
+        if (assoc_array_ptr_is_shortcut(parent)) {
+                shortcut = assoc_array_ptr_to_shortcut(parent);
+                smp_read_barrier_depends();
+                cursor = parent;
+                parent = ACCESS_ONCE(shortcut->back_pointer);
+                slot = shortcut->parent_slot;
+                if (parent == stop)
+                        return 0;
+        }
+
+        /* Ascend to next slot in parent node */
+        cursor = parent;
+        slot++;
+        goto continue_node;
+}
+
+/**
+ * assoc_array_iterate - Pass all objects in the array to a callback
+ * @array: The array to iterate over.
+ * @iterator: The callback function.
+ * @iterator_data: Private data for the callback function.
+ *
+ * Iterate over all the objects in an associative array.  Each one will be
+ * presented to the iterator function.
+ *
+ * If the array is being modified concurrently with the iteration then it is
+ * possible that some objects in the array will be passed to the iterator
+ * callback more than once - though every object should be passed at least
+ * once.  If this is undesirable then the caller must lock against modification
+ * for the duration of this function.
+ *
+ * The function will return 0 if no objects were in the array or else it will
+ * return the result of the last iterator function called.  Iteration stops
+ * immediately if any call to the iteration function results in a non-zero
+ * return.
+ *
+ * The caller should hold the RCU read lock or better if concurrent
+ * modification is possible.
+ */
+int assoc_array_iterate(const struct assoc_array *array,
+                        int (*iterator)(const void *object,
+                                        void *iterator_data),
+                        void *iterator_data)
+{
+        struct assoc_array_ptr *root = ACCESS_ONCE(array->root);
+
+        if (!root)
+                return 0;
+        return assoc_array_subtree_iterate(root, NULL, iterator, iterator_data);
+}
+
+enum assoc_array_walk_status {
+        assoc_array_walk_tree_empty,
+        assoc_array_walk_found_terminal_node,
+        assoc_array_walk_found_wrong_shortcut,
+} status;
+
+struct assoc_array_walk_result {
+        struct {
+                struct assoc_array_node *node;  /* Node in which leaf might be found */
+                int             level;
+                int             slot;
+        } terminal_node;
+        struct {
+                struct assoc_array_shortcut *shortcut;
+                int             level;
+                int             sc_level;
+                unsigned long   sc_segments;
+                unsigned long   dissimilarity;
+        } wrong_shortcut;
+};
+
+/*
+ * Navigate through the internal tree looking for the closest node to the key.
+ */
+static enum assoc_array_walk_status
+assoc_array_walk(const struct assoc_array *array,
+                 const struct assoc_array_ops *ops,
+                 const void *index_key,
+                 struct assoc_array_walk_result *result)
+{
+        struct assoc_array_shortcut *shortcut;
+        struct assoc_array_node *node;
+        struct assoc_array_ptr *cursor, *ptr;
+        unsigned long sc_segments, dissimilarity;
+        unsigned long segments;
+        int level, sc_level, next_sc_level;
+        int slot;
+
+        pr_devel("-->%s()\n", __func__);
+
+        cursor = ACCESS_ONCE(array->root);
+        if (!cursor)
+                return assoc_array_walk_tree_empty;
+
+        level = 0;
+
+        /* Use segments from the key for the new leaf to navigate through the
+         * internal tree, skipping through nodes and shortcuts that are on
+         * route to the destination.  Eventually we'll come to a slot that is
+         * either empty or contains a leaf at which point we've found a node in
+         * which the leaf we're looking for might be found or into which it
+         * should be inserted.
+         */
+jumped:
+        segments = ops->get_key_chunk(index_key, level);
+        pr_devel("segments[%d]: %lx\n", level, segments);
+
+        if (assoc_array_ptr_is_shortcut(cursor))
+                goto follow_shortcut;
+
+consider_node:
+        node = assoc_array_ptr_to_node(cursor);
+        smp_read_barrier_depends();
+
+        slot = segments >> (level & ASSOC_ARRAY_KEY_CHUNK_MASK);
+        slot &= ASSOC_ARRAY_FAN_MASK;
+        ptr = ACCESS_ONCE(node->slots[slot]);
+
+        pr_devel("consider slot %x [ix=%d type=%lu]\n",
+                 slot, level, (unsigned long)ptr & 3);
+
+        if (!assoc_array_ptr_is_meta(ptr)) {
+                /* The node doesn't have a node/shortcut pointer in the slot
+                 * corresponding to the index key that we have to follow.
+                 */
+                result->terminal_node.node = node;
+                result->terminal_node.level = level;
+                result->terminal_node.slot = slot;
+                pr_devel("<--%s() = terminal_node\n", __func__);
+                return assoc_array_walk_found_terminal_node;
+        }
+
+        if (assoc_array_ptr_is_node(ptr)) {
+                /* There is a pointer to a node in the slot corresponding to
+                 * this index key segment, so we need to follow it.
+                 */
+                cursor = ptr;
+                level += ASSOC_ARRAY_LEVEL_STEP;
+                if ((level & ASSOC_ARRAY_KEY_CHUNK_MASK) != 0)
+                        goto consider_node;
+                goto jumped;
+        }
+
+        /* There is a shortcut in the slot corresponding to the index key
+         * segment.  We follow the shortcut if its partial index key matches
+         * this leaf's.  Otherwise we need to split the shortcut.
+         */
+        cursor = ptr;
+follow_shortcut:
+        shortcut = assoc_array_ptr_to_shortcut(cursor);
+        smp_read_barrier_depends();
+        pr_devel("shortcut to %d\n", shortcut->skip_to_level);
+        sc_level = level + ASSOC_ARRAY_LEVEL_STEP;
+        BUG_ON(sc_level > shortcut->skip_to_level);
+
+        do {
+                /* Check the leaf against the shortcut's index key a word at a
+                 * time, trimming the final word (the shortcut stores the index
+                 * key completely from the root to the shortcut's target).
+                 */
+                if ((sc_level & ASSOC_ARRAY_KEY_CHUNK_MASK) == 0)
+                        segments = ops->get_key_chunk(index_key, sc_level);
+
+                sc_segments = shortcut->index_key[sc_level >> ASSOC_ARRAY_KEY_CHUNK_SHIFT];
+                dissimilarity = segments ^ sc_segments;
+
+                if (round_up(sc_level, ASSOC_ARRAY_KEY_CHUNK_SIZE) > shortcut->skip_to_level) {
+                        /* Trim segments that are beyond the shortcut */
+                        int shift = shortcut->skip_to_level & ASSOC_ARRAY_KEY_CHUNK_MASK;
+                        dissimilarity &= ~(ULONG_MAX << shift);
+                        next_sc_level = shortcut->skip_to_level;
+                } else {
+                        next_sc_level = sc_level + ASSOC_ARRAY_KEY_CHUNK_SIZE;
+                        next_sc_level = round_down(next_sc_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
+                }
+
+                if (dissimilarity != 0) {
+                        /* This shortcut points elsewhere */
+                        result->wrong_shortcut.shortcut = shortcut;
+                        result->wrong_shortcut.level = level;
+                        result->wrong_shortcut.sc_level = sc_level;
+                        result->wrong_shortcut.sc_segments = sc_segments;
+                        result->wrong_shortcut.dissimilarity = dissimilarity;
+                        return assoc_array_walk_found_wrong_shortcut;
+                }
+
+                sc_level = next_sc_level;
+        } while (sc_level < shortcut->skip_to_level);
+
+        /* The shortcut matches the leaf's index to this point. */
+        cursor = ACCESS_ONCE(shortcut->next_node);
+        if (((level ^ sc_level) & ~ASSOC_ARRAY_KEY_CHUNK_MASK) != 0) {
+                level = sc_level;
+                goto jumped;
+        } else {
+                level = sc_level;
+                goto consider_node;
+        }
+}
+
+/**
+ * assoc_array_find - Find an object by index key
+ * @array: The associative array to search.
+ * @ops: The operations to use.
+ * @index_key: The key to the object.
+ *
+ * Find an object in an associative array by walking through the internal tree
+ * to the node that should contain the object and then searching the leaves
+ * there.  NULL is returned if the requested object was not found in the array.
+ *
+ * The caller must hold the RCU read lock or better.
+ */
+void *assoc_array_find(const struct assoc_array *array,
+                       const struct assoc_array_ops *ops,
+                       const void *index_key)
+{
+        struct assoc_array_walk_result result;
+        const struct assoc_array_node *node;
+        const struct assoc_array_ptr *ptr;
+        const void *leaf;
+        int slot;
+
+        if (assoc_array_walk(array, ops, index_key, &result) !=
+            assoc_array_walk_found_terminal_node)
+                return NULL;
+
+        node = result.terminal_node.node;
+        smp_read_barrier_depends();
+
+        /* If the target key is available to us, it's has to be pointed to by
+         * the terminal node.
+         */
+        for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
+                ptr = ACCESS_ONCE(node->slots[slot]);
+                if (ptr && assoc_array_ptr_is_leaf(ptr)) {
+                        /* We need a barrier between the read of the pointer
+                         * and dereferencing the pointer - but only if we are
+                         * actually going to dereference it.
+                         */
+                        leaf = assoc_array_ptr_to_leaf(ptr);
+                        smp_read_barrier_depends();
+                        if (ops->compare_object(leaf, index_key))
+                                return (void *)leaf;
+                }
+        }
+
+        return NULL;
+}
+
+/*
+ * Destructively iterate over an associative array.  The caller must prevent
+ * other simultaneous accesses.
+ */
+static void assoc_array_destroy_subtree(struct assoc_array_ptr *root,
+                                        const struct assoc_array_ops *ops)
+{
+        struct assoc_array_shortcut *shortcut;
+        struct assoc_array_node *node;
+        struct assoc_array_ptr *cursor, *parent = NULL;
+        int slot = -1;
+
+        pr_devel("-->%s()\n", __func__);
+
+        cursor = root;
+        if (!cursor) {
+                pr_devel("empty\n");
+                return;
+        }
+
+move_to_meta:
+        if (assoc_array_ptr_is_shortcut(cursor)) {
+                /* Descend through a shortcut */
+                pr_devel("[%d] shortcut\n", slot);
+                BUG_ON(!assoc_array_ptr_is_shortcut(cursor));
+                shortcut = assoc_array_ptr_to_shortcut(cursor);
+                BUG_ON(shortcut->back_pointer != parent);
+                BUG_ON(slot != -1 && shortcut->parent_slot != slot);
+                parent = cursor;
+                cursor = shortcut->next_node;
+                slot = -1;
+                BUG_ON(!assoc_array_ptr_is_node(cursor));
+        }
+
+        pr_devel("[%d] node\n", slot);
+        node = assoc_array_ptr_to_node(cursor);
+        BUG_ON(node->back_pointer != parent);
+        BUG_ON(slot != -1 && node->parent_slot != slot);
+        slot = 0;
+
+continue_node:
+        pr_devel("Node %p [back=%p]\n", node, node->back_pointer);
+        for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
+                struct assoc_array_ptr *ptr = node->slots[slot];
+                if (!ptr)
+                        continue;
+                if (assoc_array_ptr_is_meta(ptr)) {
+                        parent = cursor;
+                        cursor = ptr;
+                        goto move_to_meta;
+                }
+
+                if (ops) {
+                        pr_devel("[%d] free leaf\n", slot);
+                        ops->free_object(assoc_array_ptr_to_leaf(ptr));
+                }
+        }
+
+        parent = node->back_pointer;
+        slot = node->parent_slot;
+        pr_devel("free node\n");
+        kfree(node);
+        if (!parent)
+                return; /* Done */
+
+        /* Move back up to the parent (may need to free a shortcut on
+         * the way up) */
+        if (assoc_array_ptr_is_shortcut(parent)) {
+                shortcut = assoc_array_ptr_to_shortcut(parent);
+                BUG_ON(shortcut->next_node != cursor);
+                cursor = parent;
+                parent = shortcut->back_pointer;
+                slot = shortcut->parent_slot;
+                pr_devel("free shortcut\n");
+                kfree(shortcut);
+                if (!parent)
+                        return;
+
+                BUG_ON(!assoc_array_ptr_is_node(parent));
+        }
+
+        /* Ascend to next slot in parent node */
+        pr_devel("ascend to %p[%d]\n", parent, slot);
+        cursor = parent;
+        node = assoc_array_ptr_to_node(cursor);
+        slot++;
+        goto continue_node;
+}
+
+/**
+ * assoc_array_destroy - Destroy an associative array
+ * @array: The array to destroy.
+ * @ops: The operations to use.
+ *
+ * Discard all metadata and free all objects in an associative array.  The
+ * array will be empty and ready to use again upon completion.  This function
+ * cannot fail.
+ *
+ * The caller must prevent all other accesses whilst this takes place as no
+ * attempt is made to adjust pointers gracefully to permit RCU readlock-holding
+ * accesses to continue.  On the other hand, no memory allocation is required.
+ */
+void assoc_array_destroy(struct assoc_array *array,
+                         const struct assoc_array_ops *ops)
+{
+        assoc_array_destroy_subtree(array->root, ops);
+        array->root = NULL;
+}
+
+/*
+ * Handle insertion into an empty tree.
+ */
+static bool assoc_array_insert_in_empty_tree(struct assoc_array_edit *edit)
+{
+        struct assoc_array_node *new_n0;
+
+        pr_devel("-->%s()\n", __func__);
+
+        new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
+        if (!new_n0)
+                return false;
+
+        edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
+        edit->leaf_p = &new_n0->slots[0];
+        edit->adjust_count_on = new_n0;
+        edit->set[0].ptr = &edit->array->root;
+        edit->set[0].to = assoc_array_node_to_ptr(new_n0);
+
+        pr_devel("<--%s() = ok [no root]\n", __func__);
+        return true;
+}
+
+/*
+ * Handle insertion into a terminal node.
+ */
+static bool assoc_array_insert_into_terminal_node(struct assoc_array_edit *edit,
+                                                  const struct assoc_array_ops *ops,
+                                                  const void *index_key,
+                                                  struct assoc_array_walk_result *result)
+{
+        struct assoc_array_shortcut *shortcut, *new_s0;
+        struct assoc_array_node *node, *new_n0, *new_n1, *side;
+        struct assoc_array_ptr *ptr;
+        unsigned long dissimilarity, base_seg, blank;
+        size_t keylen;
+        bool have_meta;
+        int level, diff;
+        int slot, next_slot, free_slot, i, j;
+
+        node    = result->terminal_node.node;
+        level   = result->terminal_node.level;
+        edit->segment_cache[ASSOC_ARRAY_FAN_OUT] = result->terminal_node.slot;
+
+        pr_devel("-->%s()\n", __func__);
+
+        /* We arrived at a node which doesn't have an onward node or shortcut
+         * pointer that we have to follow.  This means that (a) the leaf we
+         * want must go here (either by insertion or replacement) or (b) we
+         * need to split this node and insert in one of the fragments.
+         */
+        free_slot = -1;
+
+        /* Firstly, we have to check the leaves in this node to see if there's
+         * a matching one we should replace in place.
+         */
+        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
+                ptr = node->slots[i];
+                if (!ptr) {
+                        free_slot = i;
+                        continue;
+                }
+                if (ops->compare_object(assoc_array_ptr_to_leaf(ptr), index_key)) {
+                        pr_devel("replace in slot %d\n", i);
+                        edit->leaf_p = &node->slots[i];
+                        edit->dead_leaf = node->slots[i];
+                        pr_devel("<--%s() = ok [replace]\n", __func__);
+                        return true;
+                }
+        }
+
+        /* If there is a free slot in this node then we can just insert the
+         * leaf here.
+         */
+        if (free_slot >= 0) {
+                pr_devel("insert in free slot %d\n", free_slot);
+                edit->leaf_p = &node->slots[free_slot];
+                edit->adjust_count_on = node;
+                pr_devel("<--%s() = ok [insert]\n", __func__);
+                return true;
+        }
+
+        /* The node has no spare slots - so we're either going to have to split
+         * it or insert another node before it.
+         *
+         * Whatever, we're going to need at least two new nodes - so allocate
+         * those now.  We may also need a new shortcut, but we deal with that
+         * when we need it.
+         */
+        new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
+        if (!new_n0)
+                return false;
+        edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
+        new_n1 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
+        if (!new_n1)
+                return false;
+        edit->new_meta[1] = assoc_array_node_to_ptr(new_n1);
+
+        /* We need to find out how similar the leaves are. */
+        pr_devel("no spare slots\n");
+        have_meta = false;
+        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
+                ptr = node->slots[i];
+                if (assoc_array_ptr_is_meta(ptr)) {
+                        edit->segment_cache[i] = 0xff;
+                        have_meta = true;
+                        continue;
+                }
+                base_seg = ops->get_object_key_chunk(
+                        assoc_array_ptr_to_leaf(ptr), level);
+                base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
+                edit->segment_cache[i] = base_seg & ASSOC_ARRAY_FAN_MASK;
+        }
+
+        if (have_meta) {
+                pr_devel("have meta\n");
+                goto split_node;
+        }
+
+        /* The node contains only leaves */
+        dissimilarity = 0;
+        base_seg = edit->segment_cache[0];
+        for (i = 1; i < ASSOC_ARRAY_FAN_OUT; i++)
+                dissimilarity |= edit->segment_cache[i] ^ base_seg;
+
+        pr_devel("only leaves; dissimilarity=%lx\n", dissimilarity);
+
+        if ((dissimilarity & ASSOC_ARRAY_FAN_MASK) == 0) {
+                /* The old leaves all cluster in the same slot.  We will need
+                 * to insert a shortcut if the new node wants to cluster with them.
+                 */
+                if ((edit->segment_cache[ASSOC_ARRAY_FAN_OUT] ^ base_seg) == 0)
+                        goto all_leaves_cluster_together;
+
+                /* Otherwise we can just insert a new node ahead of the old
+                 * one.
+                 */
+                goto present_leaves_cluster_but_not_new_leaf;
+        }
+
+split_node:
+        pr_devel("split node\n");
+
+        /* We need to split the current node; we know that the node doesn't
+         * simply contain a full set of leaves that cluster together (it
+         * contains meta pointers and/or non-clustering leaves).
+         *
+         * We need to expel at least two leaves out of a set consisting of the
+         * leaves in the node and the new leaf.
+         *
+         * We need a new node (n0) to replace the current one and a new node to
+         * take the expelled nodes (n1).
+         */
+        edit->set[0].to = assoc_array_node_to_ptr(new_n0);
+        new_n0->back_pointer = node->back_pointer;
+        new_n0->parent_slot = node->parent_slot;
+        new_n1->back_pointer = assoc_array_node_to_ptr(new_n0);
+        new_n1->parent_slot = -1; /* Need to calculate this */
+
+do_split_node:
+        pr_devel("do_split_node\n");
+
+        new_n0->nr_leaves_on_branch = node->nr_leaves_on_branch;
+        new_n1->nr_leaves_on_branch = 0;
+
+        /* Begin by finding two matching leaves.  There have to be at least two
+         * that match - even if there are meta pointers - because any leaf that
+         * would match a slot with a meta pointer in it must be somewhere
+         * behind that meta pointer and cannot be here.  Further, given N
+         * remaining leaf slots, we now have N+1 leaves to go in them.
+         */
+        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
+                slot = edit->segment_cache[i];
+                if (slot != 0xff)
+                        for (j = i + 1; j < ASSOC_ARRAY_FAN_OUT + 1; j++)
+                                if (edit->segment_cache[j] == slot)
+                                        goto found_slot_for_multiple_occupancy;
+        }
+found_slot_for_multiple_occupancy:
+        pr_devel("same slot: %x %x [%02x]\n", i, j, slot);
+        BUG_ON(i >= ASSOC_ARRAY_FAN_OUT);
+        BUG_ON(j >= ASSOC_ARRAY_FAN_OUT + 1);
+        BUG_ON(slot >= ASSOC_ARRAY_FAN_OUT);
+
+        new_n1->parent_slot = slot;
+
+        /* Metadata pointers cannot change slot */
+        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++)
+                if (assoc_array_ptr_is_meta(node->slots[i]))
+                        new_n0->slots[i] = node->slots[i];
+                else
+                        new_n0->slots[i] = NULL;
+        BUG_ON(new_n0->slots[slot] != NULL);
+        new_n0->slots[slot] = assoc_array_node_to_ptr(new_n1);
+
+        /* Filter the leaf pointers between the new nodes */
+        free_slot = -1;
+        next_slot = 0;
+        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
+                if (assoc_array_ptr_is_meta(node->slots[i]))
+                        continue;
+                if (edit->segment_cache[i] == slot) {
+                        new_n1->slots[next_slot++] = node->slots[i];
+                        new_n1->nr_leaves_on_branch++;
+                } else {
+                        do {
+                                free_slot++;
+                        } while (new_n0->slots[free_slot] != NULL);
+                        new_n0->slots[free_slot] = node->slots[i];
+                }
+        }
+
+        pr_devel("filtered: f=%x n=%x\n", free_slot, next_slot);
+
+        if (edit->segment_cache[ASSOC_ARRAY_FAN_OUT] != slot) {
+                do {
+                        free_slot++;
+                } while (new_n0->slots[free_slot] != NULL);
+                edit->leaf_p = &new_n0->slots[free_slot];
+                edit->adjust_count_on = new_n0;
+        } else {
+                edit->leaf_p = &new_n1->slots[next_slot++];
+                edit->adjust_count_on = new_n1;
+        }
+
+        BUG_ON(next_slot <= 1);
+
+        edit->set_backpointers_to = assoc_array_node_to_ptr(new_n0);
+        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
+                if (edit->segment_cache[i] == 0xff) {
+                        ptr = node->slots[i];
+                        BUG_ON(assoc_array_ptr_is_leaf(ptr));
+                        if (assoc_array_ptr_is_node(ptr)) {
+                                side = assoc_array_ptr_to_node(ptr);
+                                edit->set_backpointers[i] = &side->back_pointer;
+                        } else {
+                                shortcut = assoc_array_ptr_to_shortcut(ptr);
+                                edit->set_backpointers[i] = &shortcut->back_pointer;
+                        }
+                }
+        }
+
+        ptr = node->back_pointer;
+        if (!ptr)
+                edit->set[0].ptr = &edit->array->root;
+        else if (assoc_array_ptr_is_node(ptr))
+                edit->set[0].ptr = &assoc_array_ptr_to_node(ptr)->slots[node->parent_slot];
+        else
+                edit->set[0].ptr = &assoc_array_ptr_to_shortcut(ptr)->next_node;
+        edit->excised_meta[0] = assoc_array_node_to_ptr(node);
+        pr_devel("<--%s() = ok [split node]\n", __func__);
+        return true;
+
+present_leaves_cluster_but_not_new_leaf:
+        /* All the old leaves cluster in the same slot, but the new leaf wants
+         * to go into a different slot, so we create a new node to hold the new
+         * leaf and a pointer to a new node holding all the old leaves.
+         */
+        pr_devel("present leaves cluster but not new leaf\n");
+
+        new_n0->back_pointer = node->back_pointer;
+        new_n0->parent_slot = node->parent_slot;
+        new_n0->nr_leaves_on_branch = node->nr_leaves_on_branch;
+        new_n1->back_pointer = assoc_array_node_to_ptr(new_n0);
+        new_n1->parent_slot = edit->segment_cache[0];
+        new_n1->nr_leaves_on_branch = node->nr_leaves_on_branch;
+        edit->adjust_count_on = new_n0;
+
+        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++)
+                new_n1->slots[i] = node->slots[i];
+
+        new_n0->slots[edit->segment_cache[0]] = assoc_array_node_to_ptr(new_n0);
+        edit->leaf_p = &new_n0->slots[edit->segment_cache[ASSOC_ARRAY_FAN_OUT]];
+
+        edit->set[0].ptr = &assoc_array_ptr_to_node(node->back_pointer)->slots[node->parent_slot];
+        edit->set[0].to = assoc_array_node_to_ptr(new_n0);
+        edit->excised_meta[0] = assoc_array_node_to_ptr(node);
+        pr_devel("<--%s() = ok [insert node before]\n", __func__);
+        return true;
+
+all_leaves_cluster_together:
+        /* All the leaves, new and old, want to cluster together in this node
+         * in the same slot, so we have to replace this node with a shortcut to
+         * skip over the identical parts of the key and then place a pair of
+         * nodes, one inside the other, at the end of the shortcut and
+         * distribute the keys between them.
+         *
+         * Firstly we need to work out where the leaves start diverging as a
+         * bit position into their keys so that we know how big the shortcut
+         * needs to be.
+         *
+         * We only need to make a single pass of N of the N+1 leaves because if
+         * any keys differ between themselves at bit X then at least one of
+         * them must also differ with the base key at bit X or before.
+         */
+        pr_devel("all leaves cluster together\n");
+        diff = INT_MAX;
+        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
+                int x = ops->diff_objects(assoc_array_ptr_to_leaf(edit->leaf),
+                                          assoc_array_ptr_to_leaf(node->slots[i]));
+                if (x < diff) {
+                        BUG_ON(x < 0);
+                        diff = x;
+                }
+        }
+        BUG_ON(diff == INT_MAX);
+        BUG_ON(diff < level + ASSOC_ARRAY_LEVEL_STEP);
+
+        keylen = round_up(diff, ASSOC_ARRAY_KEY_CHUNK_SIZE);
+        keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
+
+        new_s0 = kzalloc(sizeof(struct assoc_array_shortcut) +
+                         keylen * sizeof(unsigned long), GFP_KERNEL);
+        if (!new_s0)
+                return false;
+        edit->new_meta[2] = assoc_array_shortcut_to_ptr(new_s0);
+
+        edit->set[0].to = assoc_array_shortcut_to_ptr(new_s0);
+        new_s0->back_pointer = node->back_pointer;
+        new_s0->parent_slot = node->parent_slot;
+        new_s0->next_node = assoc_array_node_to_ptr(new_n0);
+        new_n0->back_pointer = assoc_array_shortcut_to_ptr(new_s0);
+        new_n0->parent_slot = 0;
+        new_n1->back_pointer = assoc_array_node_to_ptr(new_n0);
+        new_n1->parent_slot = -1; /* Need to calculate this */
+
+        new_s0->skip_to_level = level = diff & ~ASSOC_ARRAY_LEVEL_STEP_MASK;
+        pr_devel("skip_to_level = %d [diff %d]\n", level, diff);
+        BUG_ON(level <= 0);
+
+        for (i = 0; i < keylen; i++)
+                new_s0->index_key[i] =
+                        ops->get_key_chunk(index_key, i * ASSOC_ARRAY_KEY_CHUNK_SIZE);
+
+        blank = ULONG_MAX << (level & ASSOC_ARRAY_KEY_CHUNK_MASK);
+        pr_devel("blank off [%zu] %d: %lx\n", keylen - 1, level, blank);
+        new_s0->index_key[keylen - 1] &= ~blank;
+
+        /* This now reduces to a node splitting exercise for which we'll need
+         * to regenerate the disparity table.
+         */
+        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
+                ptr = node->slots[i];
+                base_seg = ops->get_object_key_chunk(assoc_array_ptr_to_leaf(ptr),
+                                                     level);
+                base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
+                edit->segment_cache[i] = base_seg & ASSOC_ARRAY_FAN_MASK;
+        }
+
+        base_seg = ops->get_key_chunk(index_key, level);
+        base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
+        edit->segment_cache[ASSOC_ARRAY_FAN_OUT] = base_seg & ASSOC_ARRAY_FAN_MASK;
+        goto do_split_node;
+}
+
+/*
+ * Handle insertion into the middle of a shortcut.
+ */
+static bool assoc_array_insert_mid_shortcut(struct assoc_array_edit *edit,
+                                            const struct assoc_array_ops *ops,
+                                            struct assoc_array_walk_result *result)
+{
+        struct assoc_array_shortcut *shortcut, *new_s0, *new_s1;
+        struct assoc_array_node *node, *new_n0, *side;
+        unsigned long sc_segments, dissimilarity, blank;
+        size_t keylen;
+        int level, sc_level, diff;
+        int sc_slot;
+
+        shortcut        = result->wrong_shortcut.shortcut;
+        level           = result->wrong_shortcut.level;
+        sc_level        = result->wrong_shortcut.sc_level;
+        sc_segments     = result->wrong_shortcut.sc_segments;
+        dissimilarity   = result->wrong_shortcut.dissimilarity;
+
+        pr_devel("-->%s(ix=%d dis=%lx scix=%d)\n",
+                 __func__, level, dissimilarity, sc_level);
+
+        /* We need to split a shortcut and insert a node between the two
+         * pieces.  Zero-length pieces will be dispensed with entirely.
+         *
+         * First of all, we need to find out in which level the first
+         * difference was.
+         */
+        diff = __ffs(dissimilarity);
+        diff &= ~ASSOC_ARRAY_LEVEL_STEP_MASK;
+        diff += sc_level & ~ASSOC_ARRAY_KEY_CHUNK_MASK;
+        pr_devel("diff=%d\n", diff);
+
+        if (!shortcut->back_pointer) {
+                edit->set[0].ptr = &edit->array->root;
+        } else if (assoc_array_ptr_is_node(shortcut->back_pointer)) {
+                node = assoc_array_ptr_to_node(shortcut->back_pointer);
+                edit->set[0].ptr = &node->slots[shortcut->parent_slot];
+        } else {
+                BUG();
+        }
+
+        edit->excised_meta[0] = assoc_array_shortcut_to_ptr(shortcut);
+
+        /* Create a new node now since we're going to need it anyway */
+        new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
+        if (!new_n0)
+                return false;
+        edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
+        edit->adjust_count_on = new_n0;
+
+        /* Insert a new shortcut before the new node if this segment isn't of
+         * zero length - otherwise we just connect the new node directly to the
+         * parent.
+         */
+        level += ASSOC_ARRAY_LEVEL_STEP;
+        if (diff > level) {
+                pr_devel("pre-shortcut %d...%d\n", level, diff);
+                keylen = round_up(diff, ASSOC_ARRAY_KEY_CHUNK_SIZE);
+                keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
+
+                new_s0 = kzalloc(sizeof(struct assoc_array_shortcut) +
+                                 keylen * sizeof(unsigned long), GFP_KERNEL);
+                if (!new_s0)
+                        return false;
+                edit->new_meta[1] = assoc_array_shortcut_to_ptr(new_s0);
+                edit->set[0].to = assoc_array_shortcut_to_ptr(new_s0);
+                new_s0->back_pointer = shortcut->back_pointer;
+                new_s0->parent_slot = shortcut->parent_slot;
+                new_s0->next_node = assoc_array_node_to_ptr(new_n0);
+                new_s0->skip_to_level = diff;
+
+                new_n0->back_pointer = assoc_array_shortcut_to_ptr(new_s0);
+                new_n0->parent_slot = 0;
+
+                memcpy(new_s0->index_key, shortcut->index_key,
+                       keylen * sizeof(unsigned long));
+
+                blank = ULONG_MAX << (diff & ASSOC_ARRAY_KEY_CHUNK_MASK);
+                pr_devel("blank off [%zu] %d: %lx\n", keylen - 1, diff, blank);
+                new_s0->index_key[keylen - 1] &= ~blank;
+        } else {
+                pr_devel("no pre-shortcut\n");
+                edit->set[0].to = assoc_array_node_to_ptr(new_n0);
+                new_n0->back_pointer = shortcut->back_pointer;
+                new_n0->parent_slot = shortcut->parent_slot;
+        }
+
+        side = assoc_array_ptr_to_node(shortcut->next_node);
+        new_n0->nr_leaves_on_branch = side->nr_leaves_on_branch;
+
+        /* We need to know which slot in the new node is going to take a
+         * metadata pointer.
+         */
+        sc_slot = sc_segments >> (diff & ASSOC_ARRAY_KEY_CHUNK_MASK);
+        sc_slot &= ASSOC_ARRAY_FAN_MASK;
+
+        pr_devel("new slot %lx >> %d -> %d\n",
+                 sc_segments, diff & ASSOC_ARRAY_KEY_CHUNK_MASK, sc_slot);
+
+        /* Determine whether we need to follow the new node with a replacement
+         * for the current shortcut.  We could in theory reuse the current
+         * shortcut if its parent slot number doesn't change - but that's a
+         * 1-in-16 chance so not worth expending the code upon.
+         */
+        level = diff + ASSOC_ARRAY_LEVEL_STEP;
+        if (level < shortcut->skip_to_level) {
+                pr_devel("post-shortcut %d...%d\n", level, shortcut->skip_to_level);
+                keylen = round_up(shortcut->skip_to_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
+                keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
+
+                new_s1 = kzalloc(sizeof(struct assoc_array_shortcut) +
+                                 keylen * sizeof(unsigned long), GFP_KERNEL);
+                if (!new_s1)
+                        return false;
+                edit->new_meta[2] = assoc_array_shortcut_to_ptr(new_s1);
+
+                new_s1->back_pointer = assoc_array_node_to_ptr(new_n0);
+                new_s1->parent_slot = sc_slot;
+                new_s1->next_node = shortcut->next_node;
+                new_s1->skip_to_level = shortcut->skip_to_level;
+
+                new_n0->slots[sc_slot] = assoc_array_shortcut_to_ptr(new_s1);
+
+                memcpy(new_s1->index_key, shortcut->index_key,
+                       keylen * sizeof(unsigned long));
+
+                edit->set[1].ptr = &side->back_pointer;
+                edit->set[1].to = assoc_array_shortcut_to_ptr(new_s1);
+        } else {
+                pr_devel("no post-shortcut\n");
+
+                /* We don't have to replace the pointed-to node as long as we
+                 * use memory barriers to make sure the parent slot number is
+                 * changed before the back pointer (the parent slot number is
+                 * irrelevant to the old parent shortcut).
+                 */
+                new_n0->slots[sc_slot] = shortcut->next_node;
+                edit->set_parent_slot[0].p = &side->parent_slot;
+                edit->set_parent_slot[0].to = sc_slot;
+                edit->set[1].ptr = &side->back_pointer;
+                edit->set[1].to = assoc_array_node_to_ptr(new_n0);
+        }
+
+        /* Install the new leaf in a spare slot in the new node. */
+        if (sc_slot == 0)
+                edit->leaf_p = &new_n0->slots[1];
+        else
+                edit->leaf_p = &new_n0->slots[0];
+
+        pr_devel("<--%s() = ok [split shortcut]\n", __func__);
+        return edit;
+}
+
+/**
+ * assoc_array_insert - Script insertion of an object into an associative array
+ * @array: The array to insert into.
+ * @ops: The operations to use.
+ * @index_key: The key to insert at.
+ * @object: The object to insert.
+ *
+ * Precalculate and preallocate a script for the insertion or replacement of an
+ * object in an associative array.  This results in an edit script that can
+ * either be applied or cancelled.
+ *
+ * The function returns a pointer to an edit script or -ENOMEM.
+ *
+ * The caller should lock against other modifications and must continue to hold
+ * the lock until assoc_array_apply_edit() has been called.
+ *
+ * Accesses to the tree may take place concurrently with this function,
+ * provided they hold the RCU read lock.
+ */
+struct assoc_array_edit *assoc_array_insert(struct assoc_array *array,
+                                            const struct assoc_array_ops *ops,
+                                            const void *index_key,
+                                            void *object)
+{
+        struct assoc_array_walk_result result;
+        struct assoc_array_edit *edit;
+
+        pr_devel("-->%s()\n", __func__);
+
+        /* The leaf pointer we're given must not have the bottom bit set as we
+         * use those for type-marking the pointer.  NULL pointers are also not
+         * allowed as they indicate an empty slot but we have to allow them
+         * here as they can be updated later.
+         */
+        BUG_ON(assoc_array_ptr_is_meta(object));
+
+        edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
+        if (!edit)
+                return ERR_PTR(-ENOMEM);
+        edit->array = array;
+        edit->ops = ops;
+        edit->leaf = assoc_array_leaf_to_ptr(object);
+        edit->adjust_count_by = 1;
+
+        switch (assoc_array_walk(array, ops, index_key, &result)) {
+        case assoc_array_walk_tree_empty:
+                /* Allocate a root node if there isn't one yet */
+                if (!assoc_array_insert_in_empty_tree(edit))
+                        goto enomem;
+                return edit;
+
+        case assoc_array_walk_found_terminal_node:
+                /* We found a node that doesn't have a node/shortcut pointer in
+                 * the slot corresponding to the index key that we have to
+                 * follow.
+                 */
+                if (!assoc_array_insert_into_terminal_node(edit, ops, index_key,
+                                                           &result))
+                        goto enomem;
+                return edit;
+
+        case assoc_array_walk_found_wrong_shortcut:
+                /* We found a shortcut that didn't match our key in a slot we
+                 * needed to follow.
+                 */
+                if (!assoc_array_insert_mid_shortcut(edit, ops, &result))
+                        goto enomem;
+                return edit;
+        }
+
+enomem:
+        /* Clean up after an out of memory error */
+        pr_devel("enomem\n");
+        assoc_array_cancel_edit(edit);
+        return ERR_PTR(-ENOMEM);
+}
+
+/**
+ * assoc_array_insert_set_object - Set the new object pointer in an edit script
+ * @edit: The edit script to modify.
+ * @object: The object pointer to set.
+ *
+ * Change the object to be inserted in an edit script.  The object pointed to
+ * by the old object is not freed.  This must be done prior to applying the
+ * script.
+ */
+void assoc_array_insert_set_object(struct assoc_array_edit *edit, void *object)
+{
+        BUG_ON(!object);
+        edit->leaf = assoc_array_leaf_to_ptr(object);
+}
+
+struct assoc_array_delete_collapse_context {
+        struct assoc_array_node *node;
+        const void              *skip_leaf;
+        int                     slot;
+};
+
+/*
+ * Subtree collapse to node iterator.
+ */
+static int assoc_array_delete_collapse_iterator(const void *leaf,
+                                                void *iterator_data)
+{
+        struct assoc_array_delete_collapse_context *collapse = iterator_data;
+
+        if (leaf == collapse->skip_leaf)
+                return 0;
+
+        BUG_ON(collapse->slot >= ASSOC_ARRAY_FAN_OUT);
+
+        collapse->node->slots[collapse->slot++] = assoc_array_leaf_to_ptr(leaf);
+        return 0;
+}
+
+/**
+ * assoc_array_delete - Script deletion of an object from an associative array
+ * @array: The array to search.
+ * @ops: The operations to use.
+ * @index_key: The key to the object.
+ *
+ * Precalculate and preallocate a script for the deletion of an object from an
+ * associative array.  This results in an edit script that can either be
+ * applied or cancelled.
+ *
+ * The function returns a pointer to an edit script if the object was found,
+ * NULL if the object was not found or -ENOMEM.
+ *
+ * The caller should lock against other modifications and must continue to hold
+ * the lock until assoc_array_apply_edit() has been called.
+ *
+ * Accesses to the tree may take place concurrently with this function,
+ * provided they hold the RCU read lock.
+ */
+struct assoc_array_edit *assoc_array_delete(struct assoc_array *array,
+                                            const struct assoc_array_ops *ops,
+                                            const void *index_key)
+{
+        struct assoc_array_delete_collapse_context collapse;
+        struct assoc_array_walk_result result;
+        struct assoc_array_node *node, *new_n0;
+        struct assoc_array_edit *edit;
+        struct assoc_array_ptr *ptr;
+        bool has_meta;
+        int slot, i;
+
+        pr_devel("-->%s()\n", __func__);
+
+        edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
+        if (!edit)
+                return ERR_PTR(-ENOMEM);
+        edit->array = array;
+        edit->ops = ops;
+        edit->adjust_count_by = -1;
+
+        switch (assoc_array_walk(array, ops, index_key, &result)) {
+        case assoc_array_walk_found_terminal_node:
+                /* We found a node that should contain the leaf we've been
+                 * asked to remove - *if* it's in the tree.
+                 */
+                pr_devel("terminal_node\n");
+                node = result.terminal_node.node;
+
+                for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
+                        ptr = node->slots[slot];
+                        if (ptr &&
+                            assoc_array_ptr_is_leaf(ptr) &&
+                            ops->compare_object(assoc_array_ptr_to_leaf(ptr),
+                                                index_key))
+                                goto found_leaf;
+                }
+        case assoc_array_walk_tree_empty:
+        case assoc_array_walk_found_wrong_shortcut:
+        default:
+                assoc_array_cancel_edit(edit);
+                pr_devel("not found\n");
+                return NULL;
+        }
+
+found_leaf:
+        BUG_ON(array->nr_leaves_on_tree <= 0);
+
+        /* In the simplest form of deletion we just clear the slot and release
+         * the leaf after a suitable interval.
+         */
+        edit->dead_leaf = node->slots[slot];
+        edit->set[0].ptr = &node->slots[slot];
+        edit->set[0].to = NULL;
+        edit->adjust_count_on = node;
+
+        /* If that concludes erasure of the last leaf, then delete the entire
+         * internal array.
+         */
+        if (array->nr_leaves_on_tree == 1) {
+                edit->set[1].ptr = &array->root;
+                edit->set[1].to = NULL;
+                edit->adjust_count_on = NULL;
+                edit->excised_subtree = array->root;
+                pr_devel("all gone\n");
+                return edit;
+        }
+
+        /* However, we'd also like to clear up some metadata blocks if we
+         * possibly can.
+         *
+         * We go for a simple algorithm of: if this node has FAN_OUT or fewer
+         * leaves in it, then attempt to collapse it - and attempt to
+         * recursively collapse up the tree.
+         *
+         * We could also try and collapse in partially filled subtrees to take
+         * up space in this node.
+         */
+        if (node->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT + 1) {
+                struct assoc_array_node *parent, *grandparent;
+                struct assoc_array_ptr *ptr;
+
+                /* First of all, we need to know if this node has metadata so
+                 * that we don't try collapsing if all the leaves are already
+                 * here.
+                 */
+                has_meta = false;
+                for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
+                        ptr = node->slots[i];
+                        if (assoc_array_ptr_is_meta(ptr)) {
+                                has_meta = true;
+                                break;
+                        }
+                }
+
+                pr_devel("leaves: %ld [m=%d]\n",
+                         node->nr_leaves_on_branch - 1, has_meta);
+
+                /* Look further up the tree to see if we can collapse this node
+                 * into a more proximal node too.
+                 */
+                parent = node;
+        collapse_up:
+                pr_devel("collapse subtree: %ld\n", parent->nr_leaves_on_branch);
+
+                ptr = parent->back_pointer;
+                if (!ptr)
+                        goto do_collapse;
+                if (assoc_array_ptr_is_shortcut(ptr)) {
+                        struct assoc_array_shortcut *s = assoc_array_ptr_to_shortcut(ptr);
+                        ptr = s->back_pointer;
+                        if (!ptr)
+                                goto do_collapse;
+                }
+
+                grandparent = assoc_array_ptr_to_node(ptr);
+                if (grandparent->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT + 1) {
+                        parent = grandparent;
+                        goto collapse_up;
+                }
+
+        do_collapse:
+                /* There's no point collapsing if the original node has no meta
+                 * pointers to discard and if we didn't merge into one of that
+                 * node's ancestry.
+                 */
+                if (has_meta || parent != node) {
+                        node = parent;
+
+                        /* Create a new node to collapse into */
+                        new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
+                        if (!new_n0)
+                                goto enomem;
+                        edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
+
+                        new_n0->back_pointer = node->back_pointer;
+                        new_n0->parent_slot = node->parent_slot;
+                        new_n0->nr_leaves_on_branch = node->nr_leaves_on_branch;
+                        edit->adjust_count_on = new_n0;
+
+                        collapse.node = new_n0;
+                        collapse.skip_leaf = assoc_array_ptr_to_leaf(edit->dead_leaf);
+                        collapse.slot = 0;
+                        assoc_array_subtree_iterate(assoc_array_node_to_ptr(node),
+                                                    node->back_pointer,
+                                                    assoc_array_delete_collapse_iterator,
+                                                    &collapse);
+                        pr_devel("collapsed %d,%lu\n", collapse.slot, new_n0->nr_leaves_on_branch);
+                        BUG_ON(collapse.slot != new_n0->nr_leaves_on_branch - 1);
+
+                        if (!node->back_pointer) {
+                                edit->set[1].ptr = &array->root;
+                        } else if (assoc_array_ptr_is_leaf(node->back_pointer)) {
+                                BUG();
+                        } else if (assoc_array_ptr_is_node(node->back_pointer)) {
+                                struct assoc_array_node *p =
+                                        assoc_array_ptr_to_node(node->back_pointer);
+                                edit->set[1].ptr = &p->slots[node->parent_slot];
+                        } else if (assoc_array_ptr_is_shortcut(node->back_pointer)) {
+                                struct assoc_array_shortcut *s =
+                                        assoc_array_ptr_to_shortcut(node->back_pointer);
+                                edit->set[1].ptr = &s->next_node;
+                        }
+                        edit->set[1].to = assoc_array_node_to_ptr(new_n0);
+                        edit->excised_subtree = assoc_array_node_to_ptr(node);
+                }
+        }
+
+        return edit;
+
+enomem:
+        /* Clean up after an out of memory error */
+        pr_devel("enomem\n");
+        assoc_array_cancel_edit(edit);
+        return ERR_PTR(-ENOMEM);
+}
+
+/**
+ * assoc_array_clear - Script deletion of all objects from an associative array
+ * @array: The array to clear.
+ * @ops: The operations to use.
+ *
+ * Precalculate and preallocate a script for the deletion of all the objects
+ * from an associative array.  This results in an edit script that can either
+ * be applied or cancelled.
+ *
+ * The function returns a pointer to an edit script if there are objects to be
+ * deleted, NULL if there are no objects in the array or -ENOMEM.
+ *
+ * The caller should lock against other modifications and must continue to hold
+ * the lock until assoc_array_apply_edit() has been called.
+ *
+ * Accesses to the tree may take place concurrently with this function,
+ * provided they hold the RCU read lock.
+ */
+struct assoc_array_edit *assoc_array_clear(struct assoc_array *array,
+                                           const struct assoc_array_ops *ops)
+{
+        struct assoc_array_edit *edit;
+
+        pr_devel("-->%s()\n", __func__);
+
+        if (!array->root)
+                return NULL;
+
+        edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
+        if (!edit)
+                return ERR_PTR(-ENOMEM);
+        edit->array = array;
+        edit->ops = ops;
+        edit->set[1].ptr = &array->root;
+        edit->set[1].to = NULL;
+        edit->excised_subtree = array->root;
+        edit->ops_for_excised_subtree = ops;
+        pr_devel("all gone\n");
+        return edit;
+}
+
+/*
+ * Handle the deferred destruction after an applied edit.
+ */
+static void assoc_array_rcu_cleanup(struct rcu_head *head)
+{
+        struct assoc_array_edit *edit =
+                container_of(head, struct assoc_array_edit, rcu);
+        int i;
+
+        pr_devel("-->%s()\n", __func__);
+
+        if (edit->dead_leaf)
+                edit->ops->free_object(assoc_array_ptr_to_leaf(edit->dead_leaf));
+        for (i = 0; i < ARRAY_SIZE(edit->excised_meta); i++)
+                if (edit->excised_meta[i])
+                        kfree(assoc_array_ptr_to_node(edit->excised_meta[i]));
+
+        if (edit->excised_subtree) {
+                BUG_ON(assoc_array_ptr_is_leaf(edit->excised_subtree));
+                if (assoc_array_ptr_is_node(edit->excised_subtree)) {
+                        struct assoc_array_node *n =
+                                assoc_array_ptr_to_node(edit->excised_subtree);
+                        n->back_pointer = NULL;
+                } else {
+                        struct assoc_array_shortcut *s =
+                                assoc_array_ptr_to_shortcut(edit->excised_subtree);
+                        s->back_pointer = NULL;
+                }
+                assoc_array_destroy_subtree(edit->excised_subtree,
+                                            edit->ops_for_excised_subtree);
+        }
+
+        kfree(edit);
+}
+
+/**
+ * assoc_array_apply_edit - Apply an edit script to an associative array
+ * @edit: The script to apply.
+ *
+ * Apply an edit script to an associative array to effect an insertion,
+ * deletion or clearance.  As the edit script includes preallocated memory,
+ * this is guaranteed not to fail.
+ *
+ * The edit script, dead objects and dead metadata will be scheduled for
+ * destruction after an RCU grace period to permit those doing read-only
+ * accesses on the array to continue to do so under the RCU read lock whilst
+ * the edit is taking place.
+ */
+void assoc_array_apply_edit(struct assoc_array_edit *edit)
+{
+        struct assoc_array_shortcut *shortcut;
+        struct assoc_array_node *node;
+        struct assoc_array_ptr *ptr;
+        int i;
+
+        pr_devel("-->%s()\n", __func__);
+
+        smp_wmb();
+        if (edit->leaf_p)
+                *edit->leaf_p = edit->leaf;
+
+        smp_wmb();
+        for (i = 0; i < ARRAY_SIZE(edit->set_parent_slot); i++)
+                if (edit->set_parent_slot[i].p)
+                        *edit->set_parent_slot[i].p = edit->set_parent_slot[i].to;
+
+        smp_wmb();
+        for (i = 0; i < ARRAY_SIZE(edit->set_backpointers); i++)
+                if (edit->set_backpointers[i])
+                        *edit->set_backpointers[i] = edit->set_backpointers_to;
+
+        smp_wmb();
+        for (i = 0; i < ARRAY_SIZE(edit->set); i++)
+                if (edit->set[i].ptr)
+                        *edit->set[i].ptr = edit->set[i].to;
+
+        if (edit->array->root == NULL) {
+                edit->array->nr_leaves_on_tree = 0;
+        } else if (edit->adjust_count_on) {
+                node = edit->adjust_count_on;
+                for (;;) {
+                        node->nr_leaves_on_branch += edit->adjust_count_by;
+
+                        ptr = node->back_pointer;
+                        if (!ptr)
+                                break;
+                        if (assoc_array_ptr_is_shortcut(ptr)) {
+                                shortcut = assoc_array_ptr_to_shortcut(ptr);
+                                ptr = shortcut->back_pointer;
+                                if (!ptr)
+                                        break;
+                        }
+                        BUG_ON(!assoc_array_ptr_is_node(ptr));
+                        node = assoc_array_ptr_to_node(ptr);
+                }
+
+                edit->array->nr_leaves_on_tree += edit->adjust_count_by;
+        }
+
+        call_rcu(&edit->rcu, assoc_array_rcu_cleanup);
+}
+
+/**
+ * assoc_array_cancel_edit - Discard an edit script.
+ * @edit: The script to discard.
+ *
+ * Free an edit script and all the preallocated data it holds without making
+ * any changes to the associative array it was intended for.
+ *
+ * NOTE!  In the case of an insertion script, this does _not_ release the leaf
+ * that was to be inserted.  That is left to the caller.
+ */
+void assoc_array_cancel_edit(struct assoc_array_edit *edit)
+{
+        struct assoc_array_ptr *ptr;
+        int i;
+
+        pr_devel("-->%s()\n", __func__);
+
+        /* Clean up after an out of memory error */
+        for (i = 0; i < ARRAY_SIZE(edit->new_meta); i++) {
+                ptr = edit->new_meta[i];
+                if (ptr) {
+                        if (assoc_array_ptr_is_node(ptr))
+                                kfree(assoc_array_ptr_to_node(ptr));
+                        else
+                                kfree(assoc_array_ptr_to_shortcut(ptr));
+                }
+        }
+        kfree(edit);
+}
+
+/**
+ * assoc_array_gc - Garbage collect an associative array.
+ * @array: The array to clean.
+ * @ops: The operations to use.
+ * @iterator: A callback function to pass judgement on each object.
+ * @iterator_data: Private data for the callback function.
+ *
+ * Collect garbage from an associative array and pack down the internal tree to
+ * save memory.
+ *
+ * The iterator function is asked to pass judgement upon each object in the
+ * array.  If it returns false, the object is discard and if it returns true,
+ * the object is kept.  If it returns true, it must increment the object's
+ * usage count (or whatever it needs to do to retain it) before returning.
+ *
+ * This function returns 0 if successful or -ENOMEM if out of memory.  In the
+ * latter case, the array is not changed.
+ *
+ * The caller should lock against other modifications and must continue to hold
+ * the lock until assoc_array_apply_edit() has been called.
+ *
+ * Accesses to the tree may take place concurrently with this function,
+ * provided they hold the RCU read lock.
+ */
+int assoc_array_gc(struct assoc_array *array,
+                   const struct assoc_array_ops *ops,
+                   bool (*iterator)(void *object, void *iterator_data),
+                   void *iterator_data)
+{
+        struct assoc_array_shortcut *shortcut, *new_s;
+        struct assoc_array_node *node, *new_n;
+        struct assoc_array_edit *edit;
+        struct assoc_array_ptr *cursor, *ptr;
+        struct assoc_array_ptr *new_root, *new_parent, **new_ptr_pp;
+        unsigned long nr_leaves_on_tree;
+        int keylen, slot, nr_free, next_slot, i;
+
+        pr_devel("-->%s()\n", __func__);
+
+        if (!array->root)
+                return 0;
+
+        edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
+        if (!edit)
+                return -ENOMEM;
+        edit->array = array;
+        edit->ops = ops;
+        edit->ops_for_excised_subtree = ops;
+        edit->set[0].ptr = &array->root;
+        edit->excised_subtree = array->root;
+
+        new_root = new_parent = NULL;
+        new_ptr_pp = &new_root;
+        cursor = array->root;
+
+descend:
+        /* If this point is a shortcut, then we need to duplicate it and
+         * advance the target cursor.
+         */
+        if (assoc_array_ptr_is_shortcut(cursor)) {
+                shortcut = assoc_array_ptr_to_shortcut(cursor);
+                keylen = round_up(shortcut->skip_to_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
+                keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
+                new_s = kmalloc(sizeof(struct assoc_array_shortcut) +
+                                keylen * sizeof(unsigned long), GFP_KERNEL);
+                if (!new_s)
+                        goto enomem;
+                pr_devel("dup shortcut %p -> %p\n", shortcut, new_s);
+                memcpy(new_s, shortcut, (sizeof(struct assoc_array_shortcut) +
+                                         keylen * sizeof(unsigned long)));
+                new_s->back_pointer = new_parent;
+                new_s->parent_slot = shortcut->parent_slot;
+                *new_ptr_pp = new_parent = assoc_array_shortcut_to_ptr(new_s);
+                new_ptr_pp = &new_s->next_node;
+                cursor = shortcut->next_node;
+        }
+
+        /* Duplicate the node at this position */
+        node = assoc_array_ptr_to_node(cursor);
+        new_n = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
+        if (!new_n)
+                goto enomem;
+        pr_devel("dup node %p -> %p\n", node, new_n);
+        new_n->back_pointer = new_parent;
+        new_n->parent_slot = node->parent_slot;
+        *new_ptr_pp = new_parent = assoc_array_node_to_ptr(new_n);
+        new_ptr_pp = NULL;
+        slot = 0;
+
+continue_node:
+        /* Filter across any leaves and gc any subtrees */
+        for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
+                ptr = node->slots[slot];
+                if (!ptr)
+                        continue;
+
+                if (assoc_array_ptr_is_leaf(ptr)) {
+                        if (iterator(assoc_array_ptr_to_leaf(ptr),
+                                     iterator_data))
+                                /* The iterator will have done any reference
+                                 * counting on the object for us.
+                                 */
+                                new_n->slots[slot] = ptr;
+                        continue;
+                }
+
+                new_ptr_pp = &new_n->slots[slot];
+                cursor = ptr;
+                goto descend;
+        }
+
+        pr_devel("-- compress node %p --\n", new_n);
+
+        /* Count up the number of empty slots in this node and work out the
+         * subtree leaf count.
+         */
+        new_n->nr_leaves_on_branch = 0;
+        nr_free = 0;
+        for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
+                ptr = new_n->slots[slot];
+                if (!ptr)
+                        nr_free++;
+                else if (assoc_array_ptr_is_leaf(ptr))
+                        new_n->nr_leaves_on_branch++;
+        }
+        pr_devel("free=%d, leaves=%lu\n", nr_free, new_n->nr_leaves_on_branch);
+
+        /* See what we can fold in */
+        next_slot = 0;
+        for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
+                struct assoc_array_shortcut *s;
+                struct assoc_array_node *child;
+
+                ptr = new_n->slots[slot];
+                if (!ptr || assoc_array_ptr_is_leaf(ptr))
+                        continue;
+
+                s = NULL;
+                if (assoc_array_ptr_is_shortcut(ptr)) {
+                        s = assoc_array_ptr_to_shortcut(ptr);
+                        ptr = s->next_node;
+                }
+
+                child = assoc_array_ptr_to_node(ptr);
+                new_n->nr_leaves_on_branch += child->nr_leaves_on_branch;
+
+                if (child->nr_leaves_on_branch <= nr_free + 1) {
+                        /* Fold the child node into this one */
+                        pr_devel("[%d] fold node %lu/%d [nx %d]\n",
+                                 slot, child->nr_leaves_on_branch, nr_free + 1,
+                                 next_slot);
+
+                        /* We would already have reaped an intervening shortcut
+                         * on the way back up the tree.
+                         */
+                        BUG_ON(s);
+
+                        new_n->slots[slot] = NULL;
+                        nr_free++;
+                        if (slot < next_slot)
+                                next_slot = slot;
+                        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
+                                struct assoc_array_ptr *p = child->slots[i];
+                                if (!p)
+                                        continue;
+                                BUG_ON(assoc_array_ptr_is_meta(p));
+                                while (new_n->slots[next_slot])
+                                        next_slot++;
+                                BUG_ON(next_slot >= ASSOC_ARRAY_FAN_OUT);
+                                new_n->slots[next_slot++] = p;
+                                nr_free--;
+                        }
+                        kfree(child);
+                } else {
+                        pr_devel("[%d] retain node %lu/%d [nx %d]\n",
+                                 slot, child->nr_leaves_on_branch, nr_free + 1,
+                                 next_slot);
+                }
+        }
+
+        pr_devel("after: %lu\n", new_n->nr_leaves_on_branch);
+
+        nr_leaves_on_tree = new_n->nr_leaves_on_branch;
+
+        /* Excise this node if it is singly occupied by a shortcut */
+        if (nr_free == ASSOC_ARRAY_FAN_OUT - 1) {
+                for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++)
+                        if ((ptr = new_n->slots[slot]))
+                                break;
+
+                if (assoc_array_ptr_is_meta(ptr) &&
+                    assoc_array_ptr_is_shortcut(ptr)) {
+                        pr_devel("excise node %p with 1 shortcut\n", new_n);
+                        new_s = assoc_array_ptr_to_shortcut(ptr);
+                        new_parent = new_n->back_pointer;
+                        slot = new_n->parent_slot;
+                        kfree(new_n);
+                        if (!new_parent) {
+                                new_s->back_pointer = NULL;
+                                new_s->parent_slot = 0;
+                                new_root = ptr;
+                                goto gc_complete;
+                        }
+
+                        if (assoc_array_ptr_is_shortcut(new_parent)) {
+                                /* We can discard any preceding shortcut also */
+                                struct assoc_array_shortcut *s =
+                                        assoc_array_ptr_to_shortcut(new_parent);
+
+                                pr_devel("excise preceding shortcut\n");
+
+                                new_parent = new_s->back_pointer = s->back_pointer;
+                                slot = new_s->parent_slot = s->parent_slot;
+                                kfree(s);
+                                if (!new_parent) {
+                                        new_s->back_pointer = NULL;
+                                        new_s->parent_slot = 0;
+                                        new_root = ptr;
+                                        goto gc_complete;
+                                }
+                        }
+
+                        new_s->back_pointer = new_parent;
+                        new_s->parent_slot = slot;
+                        new_n = assoc_array_ptr_to_node(new_parent);
+                        new_n->slots[slot] = ptr;
+                        goto ascend_old_tree;
+                }
+        }
+
+        /* Excise any shortcuts we might encounter that point to nodes that
+         * only contain leaves.
+         */
+        ptr = new_n->back_pointer;
+        if (!ptr)
+                goto gc_complete;
+
+        if (assoc_array_ptr_is_shortcut(ptr)) {
+                new_s = assoc_array_ptr_to_shortcut(ptr);
+                new_parent = new_s->back_pointer;
+                slot = new_s->parent_slot;
+
+                if (new_n->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT) {
+                        struct assoc_array_node *n;
+
+                        pr_devel("excise shortcut\n");
+                        new_n->back_pointer = new_parent;
+                        new_n->parent_slot = slot;
+                        kfree(new_s);
+                        if (!new_parent) {
+                                new_root = assoc_array_node_to_ptr(new_n);
+                                goto gc_complete;
+                        }
+
+                        n = assoc_array_ptr_to_node(new_parent);
+                        n->slots[slot] = assoc_array_node_to_ptr(new_n);
+                }
+        } else {
+                new_parent = ptr;
+        }
+        new_n = assoc_array_ptr_to_node(new_parent);
+
+ascend_old_tree:
+        ptr = node->back_pointer;
+        if (assoc_array_ptr_is_shortcut(ptr)) {
+                shortcut = assoc_array_ptr_to_shortcut(ptr);
+                slot = shortcut->parent_slot;
+                cursor = shortcut->back_pointer;
+        } else {
+                slot = node->parent_slot;
+                cursor = ptr;
+        }
+        BUG_ON(!ptr);
+        node = assoc_array_ptr_to_node(cursor);
+        slot++;
+        goto continue_node;
+
+gc_complete:
+        edit->set[0].to = new_root;
+        assoc_array_apply_edit(edit);
+        edit->array->nr_leaves_on_tree = nr_leaves_on_tree;
+        return 0;
+
+enomem:
+        pr_devel("enomem\n");
+        assoc_array_destroy_subtree(new_root, edit->ops);
+        kfree(edit);
+        return -ENOMEM;
+}