radix-tree: tidy up __radix_tree_create()

[deliverable/linux.git] / lib / radix-tree.c

/*
 * Copyright (C) 2001 Momchil Velikov
 * Portions Copyright (C) 2001 Christoph Hellwig
 * Copyright (C) 2005 SGI, Christoph Lameter
 * Copyright (C) 2006 Nick Piggin
 * Copyright (C) 2012 Konstantin Khlebnikov
 * Copyright (C) 2016 Intel, Matthew Wilcox
 * Copyright (C) 2016 Intel, Ross Zwisler
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2, or (at
 * your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

#include <linux/errno.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/radix-tree.h>
#include <linux/percpu.h>
#include <linux/slab.h>
#include <linux/kmemleak.h>
#include <linux/notifier.h>
#include <linux/cpu.h>
#include <linux/string.h>
#include <linux/bitops.h>
#include <linux/rcupdate.h>
#include <linux/preempt.h>              /* in_interrupt() */


/*
 * Radix tree node cache.
 */
static struct kmem_cache *radix_tree_node_cachep;

/*
 * The radix tree is variable-height, so an insert operation not only has
 * to build the branch to its corresponding item, it also has to build the
 * branch to existing items if the size has to be increased (by
 * radix_tree_extend).
 *
 * The worst case is a zero height tree with just a single item at index 0,
 * and then inserting an item at index ULONG_MAX. This requires 2 new branches
 * of RADIX_TREE_MAX_PATH size to be created, with only the root node shared.
 * Hence:
 */
#define RADIX_TREE_PRELOAD_SIZE (RADIX_TREE_MAX_PATH * 2 - 1)

/*
 * Per-cpu pool of preloaded nodes
 */
struct radix_tree_preload {
        unsigned nr;
        /* nodes->private_data points to next preallocated node */
        struct radix_tree_node *nodes;
};
static DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) = { 0, };

static inline void *node_to_entry(void *ptr)
{
        return (void *)((unsigned long)ptr | RADIX_TREE_INTERNAL_NODE);
}

#define RADIX_TREE_RETRY        node_to_entry(NULL)

#ifdef CONFIG_RADIX_TREE_MULTIORDER
/* Sibling slots point directly to another slot in the same node */
static inline bool is_sibling_entry(struct radix_tree_node *parent, void *node)
{
        void **ptr = node;
        return (parent->slots <= ptr) &&
                        (ptr < parent->slots + RADIX_TREE_MAP_SIZE);
}
#else
static inline bool is_sibling_entry(struct radix_tree_node *parent, void *node)
{
        return false;
}
#endif

static inline unsigned long get_slot_offset(struct radix_tree_node *parent,
                                                 void **slot)
{
        return slot - parent->slots;
}

static unsigned radix_tree_descend(struct radix_tree_node *parent,
                                struct radix_tree_node **nodep, unsigned offset)
{
        void **entry = rcu_dereference_raw(parent->slots[offset]);

#ifdef CONFIG_RADIX_TREE_MULTIORDER
        if (radix_tree_is_internal_node(entry)) {
                unsigned long siboff = get_slot_offset(parent, entry);
                if (siboff < RADIX_TREE_MAP_SIZE) {
                        offset = siboff;
                        entry = rcu_dereference_raw(parent->slots[offset]);
                }
        }
#endif

        *nodep = (void *)entry;
        return offset;
}

static inline gfp_t root_gfp_mask(struct radix_tree_root *root)
{
        return root->gfp_mask & __GFP_BITS_MASK;
}

static inline void tag_set(struct radix_tree_node *node, unsigned int tag,
                int offset)
{
        __set_bit(offset, node->tags[tag]);
}

static inline void tag_clear(struct radix_tree_node *node, unsigned int tag,
                int offset)
{
        __clear_bit(offset, node->tags[tag]);
}

static inline int tag_get(struct radix_tree_node *node, unsigned int tag,
                int offset)
{
        return test_bit(offset, node->tags[tag]);
}

static inline void root_tag_set(struct radix_tree_root *root, unsigned int tag)
{
        root->gfp_mask |= (__force gfp_t)(1 << (tag + __GFP_BITS_SHIFT));
}

static inline void root_tag_clear(struct radix_tree_root *root, unsigned tag)
{
        root->gfp_mask &= (__force gfp_t)~(1 << (tag + __GFP_BITS_SHIFT));
}

static inline void root_tag_clear_all(struct radix_tree_root *root)
{
        root->gfp_mask &= __GFP_BITS_MASK;
}

static inline int root_tag_get(struct radix_tree_root *root, unsigned int tag)
{
        return (__force int)root->gfp_mask & (1 << (tag + __GFP_BITS_SHIFT));
}

static inline unsigned root_tags_get(struct radix_tree_root *root)
{
        return (__force unsigned)root->gfp_mask >> __GFP_BITS_SHIFT;
}

/*
 * Returns 1 if any slot in the node has this tag set.
 * Otherwise returns 0.
 */
static inline int any_tag_set(struct radix_tree_node *node, unsigned int tag)
{
        unsigned idx;
        for (idx = 0; idx < RADIX_TREE_TAG_LONGS; idx++) {
                if (node->tags[tag][idx])
                        return 1;
        }
        return 0;
}

/**
 * radix_tree_find_next_bit - find the next set bit in a memory region
 *
 * @addr: The address to base the search on
 * @size: The bitmap size in bits
 * @offset: The bitnumber to start searching at
 *
 * Unrollable variant of find_next_bit() for constant size arrays.
 * Tail bits starting from size to roundup(size, BITS_PER_LONG) must be zero.
 * Returns next bit offset, or size if nothing found.
 */
static __always_inline unsigned long
radix_tree_find_next_bit(const unsigned long *addr,
                         unsigned long size, unsigned long offset)
{
        if (!__builtin_constant_p(size))
                return find_next_bit(addr, size, offset);

        if (offset < size) {
                unsigned long tmp;

                addr += offset / BITS_PER_LONG;
                tmp = *addr >> (offset % BITS_PER_LONG);
                if (tmp)
                        return __ffs(tmp) + offset;
                offset = (offset + BITS_PER_LONG) & ~(BITS_PER_LONG - 1);
                while (offset < size) {
                        tmp = *++addr;
                        if (tmp)
                                return __ffs(tmp) + offset;
                        offset += BITS_PER_LONG;
                }
        }
        return size;
}

#ifndef __KERNEL__
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",
                node, node->offset,
                node->tags[0][0], node->tags[1][0], node->tags[2][0],
                node->shift, node->count, node->parent);

        for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
                unsigned long first = index | (i << node->shift);
                unsigned long last = first | ((1UL << node->shift) - 1);
                void *entry = node->slots[i];
                if (!entry)
                        continue;
                if (is_sibling_entry(node, entry)) {
                        pr_debug("radix sblng %p offset %ld val %p indices %ld-%ld\n",
                                        entry, i,
                                        *(void **)entry_to_node(entry),
                                        first, last);
                } else if (!radix_tree_is_internal_node(entry)) {
                        pr_debug("radix entry %p offset %ld indices %ld-%ld\n",
                                        entry, i, first, last);
                } else {
                        dump_node(entry_to_node(entry), first);
                }
        }
}

/* For debug */
static void radix_tree_dump(struct radix_tree_root *root)
{
        pr_debug("radix root: %p rnode %p tags %x\n",
                        root, root->rnode,
                        root->gfp_mask >> __GFP_BITS_SHIFT);
        if (!radix_tree_is_internal_node(root->rnode))
                return;
        dump_node(entry_to_node(root->rnode), 0);
}
#endif

/*
 * This assumes that the caller has performed appropriate preallocation, and
 * that the caller has pinned this thread of control to the current CPU.
 */
static struct radix_tree_node *
radix_tree_node_alloc(struct radix_tree_root *root)
{
        struct radix_tree_node *ret = NULL;
        gfp_t gfp_mask = root_gfp_mask(root);

        /*
         * Preload code isn't irq safe and it doesn't make sense to use
         * preloading during an interrupt anyway as all the allocations have
         * to be atomic. So just do normal allocation when in interrupt.
         */
        if (!gfpflags_allow_blocking(gfp_mask) && !in_interrupt()) {
                struct radix_tree_preload *rtp;

                /*
                 * Even if the caller has preloaded, try to allocate from the
                 * cache first for the new node to get accounted.
                 */
                ret = kmem_cache_alloc(radix_tree_node_cachep,
                                       gfp_mask | __GFP_ACCOUNT | __GFP_NOWARN);
                if (ret)
                        goto out;

                /*
                 * Provided the caller has preloaded here, we will always
                 * succeed in getting a node here (and never reach
                 * kmem_cache_alloc)
                 */
                rtp = this_cpu_ptr(&radix_tree_preloads);
                if (rtp->nr) {
                        ret = rtp->nodes;
                        rtp->nodes = ret->private_data;
                        ret->private_data = NULL;
                        rtp->nr--;
                }
                /*
                 * Update the allocation stack trace as this is more useful
                 * for debugging.
                 */
                kmemleak_update_trace(ret);
                goto out;
        }
        ret = kmem_cache_alloc(radix_tree_node_cachep,
                               gfp_mask | __GFP_ACCOUNT);
out:
        BUG_ON(radix_tree_is_internal_node(ret));
        return ret;
}

static void radix_tree_node_rcu_free(struct rcu_head *head)
{
        struct radix_tree_node *node =
                        container_of(head, struct radix_tree_node, rcu_head);
        int i;

        /*
         * must only free zeroed nodes into the slab. radix_tree_shrink
         * can leave us with a non-NULL entry in the first slot, so clear
         * that here to make sure.
         */
        for (i = 0; i < RADIX_TREE_MAX_TAGS; i++)
                tag_clear(node, i, 0);

        node->slots[0] = NULL;
        node->count = 0;

        kmem_cache_free(radix_tree_node_cachep, node);
}

static inline void
radix_tree_node_free(struct radix_tree_node *node)
{
        call_rcu(&node->rcu_head, radix_tree_node_rcu_free);
}

/*
 * Load up this CPU's radix_tree_node buffer with sufficient objects to
 * ensure that the addition of a single element in the tree cannot fail.  On
 * success, return zero, with preemption disabled.  On error, return -ENOMEM
 * with preemption not disabled.
 *
 * To make use of this facility, the radix tree must be initialised without
 * __GFP_DIRECT_RECLAIM being passed to INIT_RADIX_TREE().
 */
static int __radix_tree_preload(gfp_t gfp_mask)
{
        struct radix_tree_preload *rtp;
        struct radix_tree_node *node;
        int ret = -ENOMEM;

        preempt_disable();
        rtp = this_cpu_ptr(&radix_tree_preloads);
        while (rtp->nr < RADIX_TREE_PRELOAD_SIZE) {
                preempt_enable();
                node = kmem_cache_alloc(radix_tree_node_cachep, gfp_mask);
                if (node == NULL)
                        goto out;
                preempt_disable();
                rtp = this_cpu_ptr(&radix_tree_preloads);
                if (rtp->nr < RADIX_TREE_PRELOAD_SIZE) {
                        node->private_data = rtp->nodes;
                        rtp->nodes = node;
                        rtp->nr++;
                } else {
                        kmem_cache_free(radix_tree_node_cachep, node);
                }
        }
        ret = 0;
out:
        return ret;
}

/*
 * Load up this CPU's radix_tree_node buffer with sufficient objects to
 * ensure that the addition of a single element in the tree cannot fail.  On
 * success, return zero, with preemption disabled.  On error, return -ENOMEM
 * with preemption not disabled.
 *
 * To make use of this facility, the radix tree must be initialised without
 * __GFP_DIRECT_RECLAIM being passed to INIT_RADIX_TREE().
 */
int radix_tree_preload(gfp_t gfp_mask)
{
        /* Warn on non-sensical use... */
        WARN_ON_ONCE(!gfpflags_allow_blocking(gfp_mask));
        return __radix_tree_preload(gfp_mask);
}
EXPORT_SYMBOL(radix_tree_preload);

/*
 * The same as above function, except we don't guarantee preloading happens.
 * We do it, if we decide it helps. On success, return zero with preemption
 * disabled. On error, return -ENOMEM with preemption not disabled.
 */
int radix_tree_maybe_preload(gfp_t gfp_mask)
{
        if (gfpflags_allow_blocking(gfp_mask))
                return __radix_tree_preload(gfp_mask);
        /* Preloading doesn't help anything with this gfp mask, skip it */
        preempt_disable();
        return 0;
}
EXPORT_SYMBOL(radix_tree_maybe_preload);

/*
 * The maximum index which can be stored in a radix tree
 */
static inline unsigned long shift_maxindex(unsigned int shift)
{
        return (RADIX_TREE_MAP_SIZE << shift) - 1;
}

static inline unsigned long node_maxindex(struct radix_tree_node *node)
{
        return shift_maxindex(node->shift);
}

static unsigned radix_tree_load_root(struct radix_tree_root *root,
                struct radix_tree_node **nodep, unsigned long *maxindex)
{
        struct radix_tree_node *node = rcu_dereference_raw(root->rnode);

        *nodep = node;

        if (likely(radix_tree_is_internal_node(node))) {
                node = entry_to_node(node);
                *maxindex = node_maxindex(node);
                return node->shift + RADIX_TREE_MAP_SHIFT;
        }

        *maxindex = 0;
        return 0;
}

/*
 *      Extend a radix tree so it can store key @index.
 */
static int radix_tree_extend(struct radix_tree_root *root,
                                unsigned long index, unsigned int shift)
{
        struct radix_tree_node *slot;
        unsigned int maxshift;
        int tag;

        /* Figure out what the shift should be.  */
        maxshift = shift;
        while (index > shift_maxindex(maxshift))
                maxshift += RADIX_TREE_MAP_SHIFT;

        slot = root->rnode;
        if (!slot)
                goto out;

        do {
                struct radix_tree_node *node = radix_tree_node_alloc(root);

                if (!node)
                        return -ENOMEM;

                /* Propagate the aggregated tag info into the new root */
                for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
                        if (root_tag_get(root, tag))
                                tag_set(node, tag, 0);
                }

                BUG_ON(shift > BITS_PER_LONG);
                node->shift = shift;
                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;
                slot = node_to_entry(node);
                rcu_assign_pointer(root->rnode, slot);
                shift += RADIX_TREE_MAP_SHIFT;
        } while (shift <= maxshift);
out:
        return maxshift + RADIX_TREE_MAP_SHIFT;
}

/**
 *      __radix_tree_create     -       create a slot in a radix tree
 *      @root:          radix tree root
 *      @index:         index key
 *      @order:         index occupies 2^order aligned slots
 *      @nodep:         returns node
 *      @slotp:         returns slot
 *
 *      Create, if necessary, and return the node and slot for an item
 *      at position @index in the radix tree @root.
 *
 *      Until there is more than one item in the tree, no nodes are
 *      allocated and @root->rnode is used as a direct slot instead of
 *      pointing to a node, in which case *@nodep will be NULL.
 *
 *      Returns -ENOMEM, or 0 for success.
 */
int __radix_tree_create(struct radix_tree_root *root, unsigned long index,
                        unsigned order, struct radix_tree_node **nodep,
                        void ***slotp)
{
        struct radix_tree_node *node = NULL, *child;
        void **slot = (void **)&root->rnode;
        unsigned long maxindex;
        unsigned int shift, offset = 0;
        unsigned long max = index | ((1UL << order) - 1);

        shift = radix_tree_load_root(root, &child, &maxindex);

        /* Make sure the tree is high enough.  */
        if (max > maxindex) {
                int error = radix_tree_extend(root, max, shift);
                if (error < 0)
                        return error;
                shift = error;
                child = root->rnode;
                if (order == shift)
                        shift += RADIX_TREE_MAP_SHIFT;
        }

        while (shift > order) {
                shift -= RADIX_TREE_MAP_SHIFT;
                if (child == NULL) {
                        /* Have to add a child node.  */
                        child = radix_tree_node_alloc(root);
                        if (!child)
                                return -ENOMEM;
                        child->shift = shift;
                        child->offset = offset;
                        child->parent = node;
                        rcu_assign_pointer(*slot, node_to_entry(child));
                        if (node)
                                node->count++;
                } else if (!radix_tree_is_internal_node(child))
                        break;

                /* Go a level down */
                node = entry_to_node(child);
                offset = (index >> shift) & RADIX_TREE_MAP_MASK;
                offset = radix_tree_descend(node, &child, offset);
                slot = &node->slots[offset];
        }

#ifdef CONFIG_RADIX_TREE_MULTIORDER
        /* Insert pointers to the canonical entry */
        if (order > shift) {
                unsigned i, n = 1 << (order - shift);
                offset = offset & ~(n - 1);
                slot = &node->slots[offset];
                child = node_to_entry(slot);
                for (i = 0; i < n; i++) {
                        if (slot[i])
                                return -EEXIST;
                }

                for (i = 1; i < n; i++) {
                        rcu_assign_pointer(slot[i], child);
                        node->count++;
                }
        }
#endif

        if (nodep)
                *nodep = node;
        if (slotp)
                *slotp = slot;
        return 0;
}

/**
 *      __radix_tree_insert    -    insert into a radix tree
 *      @root:          radix tree root
 *      @index:         index key
 *      @order:         key covers the 2^order indices around index
 *      @item:          item to insert
 *
 *      Insert an item into the radix tree at position @index.
 */
int __radix_tree_insert(struct radix_tree_root *root, unsigned long index,
                        unsigned order, void *item)
{
        struct radix_tree_node *node;
        void **slot;
        int error;

        BUG_ON(radix_tree_is_internal_node(item));

        error = __radix_tree_create(root, index, order, &node, &slot);
        if (error)
                return error;
        if (*slot != NULL)
                return -EEXIST;
        rcu_assign_pointer(*slot, item);

        if (node) {
                unsigned offset = get_slot_offset(node, slot);
                node->count++;
                BUG_ON(tag_get(node, 0, offset));
                BUG_ON(tag_get(node, 1, offset));
                BUG_ON(tag_get(node, 2, offset));
        } else {
                BUG_ON(root_tags_get(root));
        }

        return 0;
}
EXPORT_SYMBOL(__radix_tree_insert);

/**
 *      __radix_tree_lookup     -       lookup an item in a radix tree
 *      @root:          radix tree root
 *      @index:         index key
 *      @nodep:         returns node
 *      @slotp:         returns slot
 *
 *      Lookup and return the item at position @index in the radix
 *      tree @root.
 *
 *      Until there is more than one item in the tree, no nodes are
 *      allocated and @root->rnode is used as a direct slot instead of
 *      pointing to a node, in which case *@nodep will be NULL.
 */
void *__radix_tree_lookup(struct radix_tree_root *root, unsigned long index,
                          struct radix_tree_node **nodep, void ***slotp)
{
        struct radix_tree_node *node, *parent;
        unsigned long maxindex;
        unsigned int shift;
        void **slot;

 restart:
        parent = NULL;
        slot = (void **)&root->rnode;
        shift = radix_tree_load_root(root, &node, &maxindex);
        if (index > maxindex)
                return NULL;

        while (radix_tree_is_internal_node(node)) {
                unsigned offset;

                if (node == RADIX_TREE_RETRY)
                        goto restart;
                parent = entry_to_node(node);
                shift -= RADIX_TREE_MAP_SHIFT;
                offset = (index >> shift) & RADIX_TREE_MAP_MASK;
                offset = radix_tree_descend(parent, &node, offset);
                slot = parent->slots + offset;
        }

        if (nodep)
                *nodep = parent;
        if (slotp)
                *slotp = slot;
        return node;
}

/**
 *      radix_tree_lookup_slot    -    lookup a slot in a radix tree
 *      @root:          radix tree root
 *      @index:         index key
 *
 *      Returns:  the slot corresponding to the position @index in the
 *      radix tree @root. This is useful for update-if-exists operations.
 *
 *      This function can be called under rcu_read_lock iff the slot is not
 *      modified by radix_tree_replace_slot, otherwise it must be called
 *      exclusive from other writers. Any dereference of the slot must be done
 *      using radix_tree_deref_slot.
 */
void **radix_tree_lookup_slot(struct radix_tree_root *root, unsigned long index)
{
        void **slot;

        if (!__radix_tree_lookup(root, index, NULL, &slot))
                return NULL;
        return slot;
}
EXPORT_SYMBOL(radix_tree_lookup_slot);

/**
 *      radix_tree_lookup    -    perform lookup operation on a radix tree
 *      @root:          radix tree root
 *      @index:         index key
 *
 *      Lookup the item at the position @index in the radix tree @root.
 *
 *      This function can be called under rcu_read_lock, however the caller
 *      must manage lifetimes of leaf nodes (eg. RCU may also be used to free
 *      them safely). No RCU barriers are required to access or modify the
 *      returned item, however.
 */
void *radix_tree_lookup(struct radix_tree_root *root, unsigned long index)
{
        return __radix_tree_lookup(root, index, NULL, NULL);
}
EXPORT_SYMBOL(radix_tree_lookup);

/**
 *      radix_tree_tag_set - set a tag on a radix tree node
 *      @root:          radix tree root
 *      @index:         index key
 *      @tag:           tag index
 *
 *      Set the search tag (which must be < RADIX_TREE_MAX_TAGS)
 *      corresponding to @index in the radix tree.  From
 *      the root all the way down to the leaf node.
 *
 *      Returns the address of the tagged item.  Setting a tag on a not-present
 *      item is a bug.
 */
void *radix_tree_tag_set(struct radix_tree_root *root,
                        unsigned long index, unsigned int tag)
{
        struct radix_tree_node *node, *parent;
        unsigned long maxindex;
        unsigned int shift;

        shift = radix_tree_load_root(root, &node, &maxindex);
        BUG_ON(index > maxindex);

        while (radix_tree_is_internal_node(node)) {
                unsigned offset;

                shift -= RADIX_TREE_MAP_SHIFT;
                offset = (index >> shift) & RADIX_TREE_MAP_MASK;

                parent = entry_to_node(node);
                offset = radix_tree_descend(parent, &node, offset);
                BUG_ON(!node);

                if (!tag_get(parent, tag, offset))
                        tag_set(parent, tag, offset);
        }

        /* set the root's tag bit */
        if (!root_tag_get(root, tag))
                root_tag_set(root, tag);

        return node;
}
EXPORT_SYMBOL(radix_tree_tag_set);

/**
 *      radix_tree_tag_clear - clear a tag on a radix tree node
 *      @root:          radix tree root
 *      @index:         index key
 *      @tag:           tag index
 *
 *      Clear the search tag (which must be < RADIX_TREE_MAX_TAGS)
 *      corresponding to @index in the radix tree.  If this causes
 *      the leaf node to have no tags set then clear the tag in the
 *      next-to-leaf node, etc.
 *
 *      Returns the address of the tagged item on success, else NULL.  ie:
 *      has the same return value and semantics as radix_tree_lookup().
 */
void *radix_tree_tag_clear(struct radix_tree_root *root,
                        unsigned long index, unsigned int tag)
{
        struct radix_tree_node *node, *parent;
        unsigned long maxindex;
        unsigned int shift;
        int uninitialized_var(offset);

        shift = radix_tree_load_root(root, &node, &maxindex);
        if (index > maxindex)
                return NULL;

        parent = NULL;

        while (radix_tree_is_internal_node(node)) {
                shift -= RADIX_TREE_MAP_SHIFT;
                offset = (index >> shift) & RADIX_TREE_MAP_MASK;

                parent = entry_to_node(node);
                offset = radix_tree_descend(parent, &node, offset);
        }

        if (node == NULL)
                goto out;

        index >>= shift;

        while (parent) {
                if (!tag_get(parent, tag, offset))
                        goto out;
                tag_clear(parent, tag, offset);
                if (any_tag_set(parent, tag))
                        goto out;

                index >>= RADIX_TREE_MAP_SHIFT;
                offset = index & RADIX_TREE_MAP_MASK;
                parent = parent->parent;
        }

        /* clear the root's tag bit */
        if (root_tag_get(root, tag))
                root_tag_clear(root, tag);

out:
        return node;
}
EXPORT_SYMBOL(radix_tree_tag_clear);

/**
 * radix_tree_tag_get - get a tag on a radix tree node
 * @root:               radix tree root
 * @index:              index key
 * @tag:                tag index (< RADIX_TREE_MAX_TAGS)
 *
 * Return values:
 *
 *  0: tag not present or not set
 *  1: tag set
 *
 * Note that the return value of this function may not be relied on, even if
 * the RCU lock is held, unless tag modification and node deletion are excluded
 * from concurrency.
 */
int radix_tree_tag_get(struct radix_tree_root *root,
                        unsigned long index, unsigned int tag)
{
        struct radix_tree_node *node, *parent;
        unsigned long maxindex;
        unsigned int shift;

        if (!root_tag_get(root, tag))
                return 0;

        shift = radix_tree_load_root(root, &node, &maxindex);
        if (index > maxindex)
                return 0;
        if (node == NULL)
                return 0;

        while (radix_tree_is_internal_node(node)) {
                int offset;

                shift -= RADIX_TREE_MAP_SHIFT;
                offset = (index >> shift) & RADIX_TREE_MAP_MASK;

                parent = entry_to_node(node);
                offset = radix_tree_descend(parent, &node, offset);

                if (!node)
                        return 0;
                if (!tag_get(parent, tag, offset))
                        return 0;
                if (node == RADIX_TREE_RETRY)
                        break;
        }

        return 1;
}
EXPORT_SYMBOL(radix_tree_tag_get);

static inline void __set_iter_shift(struct radix_tree_iter *iter,
                                        unsigned int shift)
{
#ifdef CONFIG_RADIX_TREE_MULTIORDER
        iter->shift = shift;
#endif
}

/**
 * radix_tree_next_chunk - find next chunk of slots for iteration
 *
 * @root:       radix tree root
 * @iter:       iterator state
 * @flags:      RADIX_TREE_ITER_* flags and tag index
 * Returns:     pointer to chunk first slot, or NULL if iteration is over
 */
void **radix_tree_next_chunk(struct radix_tree_root *root,
                             struct radix_tree_iter *iter, unsigned flags)
{
        unsigned shift, tag = flags & RADIX_TREE_ITER_TAG_MASK;
        struct radix_tree_node *node, *child;
        unsigned long index, offset, maxindex;

        if ((flags & RADIX_TREE_ITER_TAGGED) && !root_tag_get(root, tag))
                return NULL;

        /*
         * Catch next_index overflow after ~0UL. iter->index never overflows
         * during iterating; it can be zero only at the beginning.
         * And we cannot overflow iter->next_index in a single step,
         * because RADIX_TREE_MAP_SHIFT < BITS_PER_LONG.
         *
         * This condition also used by radix_tree_next_slot() to stop
         * contiguous iterating, and forbid swithing to the next chunk.
         */
        index = iter->next_index;
        if (!index && iter->index)
                return NULL;

 restart:
        shift = radix_tree_load_root(root, &child, &maxindex);
        if (index > maxindex)
                return NULL;
        if (!child)
                return NULL;

        if (!radix_tree_is_internal_node(child)) {
                /* Single-slot tree */
                iter->index = index;
                iter->next_index = maxindex + 1;
                iter->tags = 1;
                __set_iter_shift(iter, 0);
                return (void **)&root->rnode;
        }

        do {
                node = entry_to_node(child);
                shift -= RADIX_TREE_MAP_SHIFT;
                offset = (index >> shift) & RADIX_TREE_MAP_MASK;
                offset = radix_tree_descend(node, &child, offset);

                if ((flags & RADIX_TREE_ITER_TAGGED) ?
                                !tag_get(node, tag, offset) : !child) {
                        /* Hole detected */
                        if (flags & RADIX_TREE_ITER_CONTIG)
                                return NULL;

                        if (flags & RADIX_TREE_ITER_TAGGED)
                                offset = radix_tree_find_next_bit(
                                                node->tags[tag],
                                                RADIX_TREE_MAP_SIZE,
                                                offset + 1);
                        else
                                while (++offset < RADIX_TREE_MAP_SIZE) {
                                        void *slot = node->slots[offset];
                                        if (is_sibling_entry(node, slot))
                                                continue;
                                        if (slot)
                                                break;
                                }
                        index &= ~node_maxindex(node);
                        index += offset << shift;
                        /* Overflow after ~0UL */
                        if (!index)
                                return NULL;
                        if (offset == RADIX_TREE_MAP_SIZE)
                                goto restart;
                        child = rcu_dereference_raw(node->slots[offset]);
                }

                if ((child == NULL) || (child == RADIX_TREE_RETRY))
                        goto restart;
        } while (radix_tree_is_internal_node(child));

        /* Update the iterator state */
        iter->index = (index &~ node_maxindex(node)) | (offset << node->shift);
        iter->next_index = (index | node_maxindex(node)) + 1;
        __set_iter_shift(iter, shift);

        /* Construct iter->tags bit-mask from node->tags[tag] array */
        if (flags & RADIX_TREE_ITER_TAGGED) {
                unsigned tag_long, tag_bit;

                tag_long = offset / BITS_PER_LONG;
                tag_bit  = offset % BITS_PER_LONG;
                iter->tags = node->tags[tag][tag_long] >> tag_bit;
                /* This never happens if RADIX_TREE_TAG_LONGS == 1 */
                if (tag_long < RADIX_TREE_TAG_LONGS - 1) {
                        /* Pick tags from next element */
                        if (tag_bit)
                                iter->tags |= node->tags[tag][tag_long + 1] <<
                                                (BITS_PER_LONG - tag_bit);
                        /* Clip chunk size, here only BITS_PER_LONG tags */
                        iter->next_index = index + BITS_PER_LONG;
                }
        }

        return node->slots + offset;
}
EXPORT_SYMBOL(radix_tree_next_chunk);

/**
 * radix_tree_range_tag_if_tagged - for each item in given range set given
 *                                 tag if item has another tag set
 * @root:               radix tree root
 * @first_indexp:       pointer to a starting index of a range to scan
 * @last_index:         last index of a range to scan
 * @nr_to_tag:          maximum number items to tag
 * @iftag:              tag index to test
 * @settag:             tag index to set if tested tag is set
 *
 * This function scans range of radix tree from first_index to last_index
 * (inclusive).  For each item in the range if iftag is set, the function sets
 * also settag. The function stops either after tagging nr_to_tag items or
 * after reaching last_index.
 *
 * The tags must be set from the leaf level only and propagated back up the
 * path to the root. We must do this so that we resolve the full path before
 * setting any tags on intermediate nodes. If we set tags as we descend, then
 * we can get to the leaf node and find that the index that has the iftag
 * set is outside the range we are scanning. This reults in dangling tags and
 * can lead to problems with later tag operations (e.g. livelocks on lookups).
 *
 * The function returns the number of leaves where the tag was set and sets
 * *first_indexp to the first unscanned index.
 * WARNING! *first_indexp can wrap if last_index is ULONG_MAX. Caller must
 * be prepared to handle that.
 */
unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
                unsigned long *first_indexp, unsigned long last_index,
                unsigned long nr_to_tag,
                unsigned int iftag, unsigned int settag)
{
        struct radix_tree_node *parent, *node, *child;
        unsigned long maxindex;
        unsigned int shift = radix_tree_load_root(root, &child, &maxindex);
        unsigned long tagged = 0;
        unsigned long index = *first_indexp;

        last_index = min(last_index, maxindex);
        if (index > last_index)
                return 0;
        if (!nr_to_tag)
                return 0;
        if (!root_tag_get(root, iftag)) {
                *first_indexp = last_index + 1;
                return 0;
        }
        if (!radix_tree_is_internal_node(child)) {
                *first_indexp = last_index + 1;
                root_tag_set(root, settag);
                return 1;
        }

        node = entry_to_node(child);
        shift -= RADIX_TREE_MAP_SHIFT;

        for (;;) {
                unsigned offset = (index >> shift) & RADIX_TREE_MAP_MASK;
                offset = radix_tree_descend(node, &child, offset);
                if (!child)
                        goto next;
                if (!tag_get(node, iftag, offset))
                        goto next;
                /* Sibling slots never have tags set on them */
                if (radix_tree_is_internal_node(child)) {
                        node = entry_to_node(child);
                        shift -= RADIX_TREE_MAP_SHIFT;
                        continue;
                }

                /* tag the leaf */
                tagged++;
                tag_set(node, settag, offset);

                /* walk back up the path tagging interior nodes */
                parent = node;
                for (;;) {
                        offset = parent->offset;
                        parent = parent->parent;
                        if (!parent)
                                break;
                        /* stop if we find a node with the tag already set */
                        if (tag_get(parent, settag, offset))
                                break;
                        tag_set(parent, settag, offset);
                }
 next:
                /* Go to next item at level determined by 'shift' */
                index = ((index >> shift) + 1) << shift;
                /* Overflow can happen when last_index is ~0UL... */
                if (index > last_index || !index)
                        break;
                offset = (index >> shift) & RADIX_TREE_MAP_MASK;
                while (offset == 0) {
                        /*
                         * We've fully scanned this node. Go up. Because
                         * last_index is guaranteed to be in the tree, what
                         * we do below cannot wander astray.
                         */
                        node = node->parent;
                        shift += RADIX_TREE_MAP_SHIFT;
                        offset = (index >> shift) & RADIX_TREE_MAP_MASK;
                }
                if (is_sibling_entry(node, node->slots[offset]))
                        goto next;
                if (tagged >= nr_to_tag)
                        break;
        }
        /*
         * We need not to tag the root tag if there is no tag which is set with
         * settag within the range from *first_indexp to last_index.
         */
        if (tagged > 0)
                root_tag_set(root, settag);
        *first_indexp = index;

        return tagged;
}
EXPORT_SYMBOL(radix_tree_range_tag_if_tagged);

/**
 *      radix_tree_gang_lookup - perform multiple lookup on a radix tree
 *      @root:          radix tree root
 *      @results:       where the results of the lookup are placed
 *      @first_index:   start the lookup from this key
 *      @max_items:     place up to this many items at *results
 *
 *      Performs an index-ascending scan of the tree for present items.  Places
 *      them at *@results and returns the number of items which were placed at
 *      *@results.
 *
 *      The implementation is naive.
 *
 *      Like radix_tree_lookup, radix_tree_gang_lookup may be called under
 *      rcu_read_lock. In this case, rather than the returned results being
 *      an atomic snapshot of the tree at a single point in time, the
 *      semantics of an RCU protected gang lookup are as though multiple
 *      radix_tree_lookups have been issued in individual locks, and results
 *      stored in 'results'.
 */
unsigned int
radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
                        unsigned long first_index, unsigned int max_items)
{
        struct radix_tree_iter iter;
        void **slot;
        unsigned int ret = 0;

        if (unlikely(!max_items))
                return 0;

        radix_tree_for_each_slot(slot, root, &iter, first_index) {
                results[ret] = rcu_dereference_raw(*slot);
                if (!results[ret])
                        continue;
                if (radix_tree_is_internal_node(results[ret])) {
                        slot = radix_tree_iter_retry(&iter);
                        continue;
                }
                if (++ret == max_items)
                        break;
        }

        return ret;
}
EXPORT_SYMBOL(radix_tree_gang_lookup);

/**
 *      radix_tree_gang_lookup_slot - perform multiple slot lookup on radix tree
 *      @root:          radix tree root
 *      @results:       where the results of the lookup are placed
 *      @indices:       where their indices should be placed (but usually NULL)
 *      @first_index:   start the lookup from this key
 *      @max_items:     place up to this many items at *results
 *
 *      Performs an index-ascending scan of the tree for present items.  Places
 *      their slots at *@results and returns the number of items which were
 *      placed at *@results.
 *
 *      The implementation is naive.
 *
 *      Like radix_tree_gang_lookup as far as RCU and locking goes. Slots must
 *      be dereferenced with radix_tree_deref_slot, and if using only RCU
 *      protection, radix_tree_deref_slot may fail requiring a retry.
 */
unsigned int
radix_tree_gang_lookup_slot(struct radix_tree_root *root,
                        void ***results, unsigned long *indices,
                        unsigned long first_index, unsigned int max_items)
{
        struct radix_tree_iter iter;
        void **slot;
        unsigned int ret = 0;

        if (unlikely(!max_items))
                return 0;

        radix_tree_for_each_slot(slot, root, &iter, first_index) {
                results[ret] = slot;
                if (indices)
                        indices[ret] = iter.index;
                if (++ret == max_items)
                        break;
        }

        return ret;
}
EXPORT_SYMBOL(radix_tree_gang_lookup_slot);

/**
 *      radix_tree_gang_lookup_tag - perform multiple lookup on a radix tree
 *                                   based on a tag
 *      @root:          radix tree root
 *      @results:       where the results of the lookup are placed
 *      @first_index:   start the lookup from this key
 *      @max_items:     place up to this many items at *results
 *      @tag:           the tag index (< RADIX_TREE_MAX_TAGS)
 *
 *      Performs an index-ascending scan of the tree for present items which
 *      have the tag indexed by @tag set.  Places the items at *@results and
 *      returns the number of items which were placed at *@results.
 */
unsigned int
radix_tree_gang_lookup_tag(struct radix_tree_root *root, void **results,
                unsigned long first_index, unsigned int max_items,
                unsigned int tag)
{
        struct radix_tree_iter iter;
        void **slot;
        unsigned int ret = 0;

        if (unlikely(!max_items))
                return 0;

        radix_tree_for_each_tagged(slot, root, &iter, first_index, tag) {
                results[ret] = rcu_dereference_raw(*slot);
                if (!results[ret])
                        continue;
                if (radix_tree_is_internal_node(results[ret])) {
                        slot = radix_tree_iter_retry(&iter);
                        continue;
                }
                if (++ret == max_items)
                        break;
        }

        return ret;
}
EXPORT_SYMBOL(radix_tree_gang_lookup_tag);

/**
 *      radix_tree_gang_lookup_tag_slot - perform multiple slot lookup on a
 *                                        radix tree based on a tag
 *      @root:          radix tree root
 *      @results:       where the results of the lookup are placed
 *      @first_index:   start the lookup from this key
 *      @max_items:     place up to this many items at *results
 *      @tag:           the tag index (< RADIX_TREE_MAX_TAGS)
 *
 *      Performs an index-ascending scan of the tree for present items which
 *      have the tag indexed by @tag set.  Places the slots at *@results and
 *      returns the number of slots which were placed at *@results.
 */
unsigned int
radix_tree_gang_lookup_tag_slot(struct radix_tree_root *root, void ***results,
                unsigned long first_index, unsigned int max_items,
                unsigned int tag)
{
        struct radix_tree_iter iter;
        void **slot;
        unsigned int ret = 0;

        if (unlikely(!max_items))
                return 0;

        radix_tree_for_each_tagged(slot, root, &iter, first_index, tag) {
                results[ret] = slot;
                if (++ret == max_items)
                        break;
        }

        return ret;
}
EXPORT_SYMBOL(radix_tree_gang_lookup_tag_slot);

#if defined(CONFIG_SHMEM) && defined(CONFIG_SWAP)
#include <linux/sched.h> /* for cond_resched() */

struct locate_info {
        unsigned long found_index;
        bool stop;
};

/*
 * This linear search is at present only useful to shmem_unuse_inode().
 */
static unsigned long __locate(struct radix_tree_node *slot, void *item,
                              unsigned long index, struct locate_info *info)
{
        unsigned int shift;
        unsigned long i;

        shift = slot->shift + RADIX_TREE_MAP_SHIFT;

        do {
                shift -= RADIX_TREE_MAP_SHIFT;

                for (i = (index >> shift) & RADIX_TREE_MAP_MASK;
                     i < RADIX_TREE_MAP_SIZE;
                     i++, index += (1UL << shift)) {
                        struct radix_tree_node *node =
                                        rcu_dereference_raw(slot->slots[i]);
                        if (node == RADIX_TREE_RETRY)
                                goto out;
                        if (!radix_tree_is_internal_node(node)) {
                                if (node == item) {
                                        info->found_index = index;
                                        info->stop = true;
                                        goto out;
                                }
                                continue;
                        }
                        node = entry_to_node(node);
                        if (is_sibling_entry(slot, node))
                                continue;
                        slot = node;
                        break;
                }
                if (i == RADIX_TREE_MAP_SIZE)
                        break;
        } while (shift);

out:
        if ((index == 0) && (i == RADIX_TREE_MAP_SIZE))
                info->stop = true;
        return index;
}

/**
 *      radix_tree_locate_item - search through radix tree for item
 *      @root:          radix tree root
 *      @item:          item to be found
 *
 *      Returns index where item was found, or -1 if not found.
 *      Caller must hold no lock (since this time-consuming function needs
 *      to be preemptible), and must check afterwards if item is still there.
 */
unsigned long radix_tree_locate_item(struct radix_tree_root *root, void *item)
{
        struct radix_tree_node *node;
        unsigned long max_index;
        unsigned long cur_index = 0;
        struct locate_info info = {
                .found_index = -1,
                .stop = false,
        };

        do {
                rcu_read_lock();
                node = rcu_dereference_raw(root->rnode);
                if (!radix_tree_is_internal_node(node)) {
                        rcu_read_unlock();
                        if (node == item)
                                info.found_index = 0;
                        break;
                }

                node = entry_to_node(node);

                max_index = node_maxindex(node);
                if (cur_index > max_index) {
                        rcu_read_unlock();
                        break;
                }

                cur_index = __locate(node, item, cur_index, &info);
                rcu_read_unlock();
                cond_resched();
        } while (!info.stop && cur_index <= max_index);

        return info.found_index;
}
#else
unsigned long radix_tree_locate_item(struct radix_tree_root *root, void *item)
{
        return -1;
}
#endif /* CONFIG_SHMEM && CONFIG_SWAP */

/**
 *      radix_tree_shrink    -    shrink radix tree to minimum height
 *      @root           radix tree root
 */
static inline bool radix_tree_shrink(struct radix_tree_root *root)
{
        bool shrunk = false;

        for (;;) {
                struct radix_tree_node *node = root->rnode;
                struct radix_tree_node *child;

                if (!radix_tree_is_internal_node(node))
                        break;
                node = entry_to_node(node);

                /*
                 * The candidate node has more than one child, or its child
                 * is not at the leftmost slot, or the child is a multiorder
                 * entry, we cannot shrink.
                 */
                if (node->count != 1)
                        break;
                child = node->slots[0];
                if (!child)
                        break;
                if (!radix_tree_is_internal_node(child) && node->shift)
                        break;

                if (radix_tree_is_internal_node(child))
                        entry_to_node(child)->parent = NULL;

                /*
                 * We don't need rcu_assign_pointer(), since we are simply
                 * moving the node from one part of the tree to another: if it
                 * was safe to dereference the old pointer to it
                 * (node->slots[0]), it will be safe to dereference the new
                 * one (root->rnode) as far as dependent read barriers go.
                 */
                root->rnode = child;

                /*
                 * We have a dilemma here. The node's slot[0] must not be
                 * NULLed in case there are concurrent lookups expecting to
                 * find the item. However if this was a bottom-level node,
                 * then it may be subject to the slot pointer being visible
                 * to callers dereferencing it. If item corresponding to
                 * slot[0] is subsequently deleted, these callers would expect
                 * their slot to become empty sooner or later.
                 *
                 * For example, lockless pagecache will look up a slot, deref
                 * the page pointer, and if the page has 0 refcount it means it
                 * was concurrently deleted from pagecache so try the deref
                 * again. Fortunately there is already a requirement for logic
                 * to retry the entire slot lookup -- the indirect pointer
                 * problem (replacing direct root node with an indirect pointer
                 * also results in a stale slot). So tag the slot as indirect
                 * to force callers to retry.
                 */
                if (!radix_tree_is_internal_node(child))
                        node->slots[0] = RADIX_TREE_RETRY;

                radix_tree_node_free(node);
                shrunk = true;
        }

        return shrunk;
}

/**
 *      __radix_tree_delete_node    -    try to free node after clearing a slot
 *      @root:          radix tree root
 *      @node:          node containing @index
 *
 *      After clearing the slot at @index in @node from radix tree
 *      rooted at @root, call this function to attempt freeing the
 *      node and shrinking the tree.
 *
 *      Returns %true if @node was freed, %false otherwise.
 */
bool __radix_tree_delete_node(struct radix_tree_root *root,
                              struct radix_tree_node *node)
{
        bool deleted = false;

        do {
                struct radix_tree_node *parent;

                if (node->count) {
                        if (node == entry_to_node(root->rnode))
                                deleted |= radix_tree_shrink(root);
                        return deleted;
                }

                parent = node->parent;
                if (parent) {
                        parent->slots[node->offset] = NULL;
                        parent->count--;
                } else {
                        root_tag_clear_all(root);
                        root->rnode = NULL;
                }

                radix_tree_node_free(node);
                deleted = true;

                node = parent;
        } while (node);

        return deleted;
}

static inline void delete_sibling_entries(struct radix_tree_node *node,
                                        void *ptr, unsigned offset)
{
#ifdef CONFIG_RADIX_TREE_MULTIORDER
        int i;
        for (i = 1; offset + i < RADIX_TREE_MAP_SIZE; i++) {
                if (node->slots[offset + i] != ptr)
                        break;
                node->slots[offset + i] = NULL;
                node->count--;
        }
#endif
}

/**
 *      radix_tree_delete_item    -    delete an item from a radix tree
 *      @root:          radix tree root
 *      @index:         index key
 *      @item:          expected item
 *
 *      Remove @item at @index from the radix tree rooted at @root.
 *
 *      Returns the address of the deleted item, or NULL if it was not present
 *      or the entry at the given @index was not @item.
 */
void *radix_tree_delete_item(struct radix_tree_root *root,
                             unsigned long index, void *item)
{
        struct radix_tree_node *node;
        unsigned int offset;
        void **slot;
        void *entry;
        int tag;

        entry = __radix_tree_lookup(root, index, &node, &slot);
        if (!entry)
                return NULL;

        if (item && entry != item)
                return NULL;

        if (!node) {
                root_tag_clear_all(root);
                root->rnode = NULL;
                return entry;
        }

        offset = get_slot_offset(node, slot);

        /*
         * Clear all tags associated with the item to be deleted.
         * This way of doing it would be inefficient, but seldom is any set.
         */
        for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
                if (tag_get(node, tag, offset))
                        radix_tree_tag_clear(root, index, tag);
        }

        delete_sibling_entries(node, node_to_entry(slot), offset);
        node->slots[offset] = NULL;
        node->count--;

        __radix_tree_delete_node(root, node);

        return entry;
}
EXPORT_SYMBOL(radix_tree_delete_item);

/**
 *      radix_tree_delete    -    delete an item from a radix tree
 *      @root:          radix tree root
 *      @index:         index key
 *
 *      Remove the item at @index from the radix tree rooted at @root.
 *
 *      Returns the address of the deleted item, or NULL if it was not present.
 */
void *radix_tree_delete(struct radix_tree_root *root, unsigned long index)
{
        return radix_tree_delete_item(root, index, NULL);
}
EXPORT_SYMBOL(radix_tree_delete);

/**
 *      radix_tree_tagged - test whether any items in the tree are tagged
 *      @root:          radix tree root
 *      @tag:           tag to test
 */
int radix_tree_tagged(struct radix_tree_root *root, unsigned int tag)
{
        return root_tag_get(root, tag);
}
EXPORT_SYMBOL(radix_tree_tagged);

static void
radix_tree_node_ctor(void *arg)
{
        struct radix_tree_node *node = arg;

        memset(node, 0, sizeof(*node));
        INIT_LIST_HEAD(&node->private_list);
}

static int radix_tree_callback(struct notifier_block *nfb,
                                unsigned long action, void *hcpu)
{
        int cpu = (long)hcpu;
        struct radix_tree_preload *rtp;
        struct radix_tree_node *node;

        /* Free per-cpu pool of preloaded nodes */
        if (action == CPU_DEAD || action == CPU_DEAD_FROZEN) {
                rtp = &per_cpu(radix_tree_preloads, cpu);
                while (rtp->nr) {
                        node = rtp->nodes;
                        rtp->nodes = node->private_data;
                        kmem_cache_free(radix_tree_node_cachep, node);
                        rtp->nr--;
                }
        }
        return NOTIFY_OK;
}

void __init radix_tree_init(void)
{
        radix_tree_node_cachep = kmem_cache_create("radix_tree_node",
                        sizeof(struct radix_tree_node), 0,
                        SLAB_PANIC | SLAB_RECLAIM_ACCOUNT,
                        radix_tree_node_ctor);
        hotcpu_notifier(radix_tree_callback, 0);
}