[PATCH 16/17] [LogFS] btree.c

Joern Engel
2009-11-20T20:22:59Z

---
lib/btree.c | 797 +++++++++++++

diff --git a/lib/btree.c b/lib/btree.c
new file mode 100644
index 0000000..41859a8
--- /dev/null
+++ b/lib/btree.c
@@ -0,0 +1,797 @@
+/*
+ * lib/btree.c - Simple In-memory B+Tree
+ *
+ * As should be obvious for Linux kernel code, license is GPLv2
+ *
+ * Copyright (c) 2007-2008 Joern Engel <joern@logfs.org>
+ * Bits and pieces stolen from Peter Zijlstra's code, which is
+ * Copyright 2007, Red Hat Inc. Peter Zijlstra <pzijlstr@redhat.com>
+ * GPLv2
+ *
+ * see http://programming.kicks-ass.net/kernel-patches/vma_lookup/btree.patch
+ *
+ * A relatively simple B+Tree implementation. I have written it as a learning
+ * excercise to understand how B+Trees work. Turned out to be useful as well.
+ *
+ * B+Trees can be used similar to Linux radix trees (which don't have anything
+ * in common with textbook radix trees, beware). Prerequisite for them working
+ * well is that access to a random tree node is much faster than a large number
+ * of operations within each node.
+ *
+ * Disks have fulfilled the prerequisite for a long time. More recently DRAM
+ * has gained similar properties, as memory access times, when measured in cpu
+ * cycles, have increased. Cacheline sizes have increased as well, which also
+ * helps B+Trees.
+ *
+ * Compared to radix trees, B+Trees are more efficient when dealing with a
+ * sparsely populated address space. Between 25% and 50% of the memory is
+ * occupied with valid pointers. When densely populated, radix trees contain
+ * ~98% pointers - hard to beat. Very sparse radix trees contain only ~2%
+ * pointers.
+ *
+ * This particular implementation stores pointers identified by a long value.
+ * Storing NULL pointers is illegal, lookup will return NULL when no entry
+ * was found.
+ *
+ * A tricks was used that is not commonly found in textbooks. The lowest
+ * values are to the right, not to the left. All used slots within a node
+ * are on the left, all unused slots contain NUL values. Most operations
+ * simply loop once over all slots and terminate on the first NUL.
+ */
+
+#include <linux/btree.h>
+#include <linux/cache.h>
+#include <linux/kernel.h>
+#include <linux/slab.h>
+#include <linux/module.h>
+
+#define MAX(a, b) ((a) > (b) ? (a) : (b))
+#define NODESIZE MAX(L1_CACHE_BYTES, 128)
+
+struct btree_geo {
+ int keylen;
+ int no_pairs;
+ int no_longs;
+};
+
+struct btree_geo btree_geo32 = {
+ .keylen = 1,
+ .no_pairs = NODESIZE / sizeof(long) / 2,
+ .no_longs = NODESIZE / sizeof(long) / 2,
+};
+EXPORT_SYMBOL_GPL(btree_geo32);
+
+#define LONG_PER_U64 (64 / BITS_PER_LONG)
+struct btree_geo btree_geo64 = {
+ .keylen = LONG_PER_U64,
+ .no_pairs = NODESIZE / sizeof(long) / (1 + LONG_PER_U64),
+ .no_longs = LONG_PER_U64 * (NODESIZE / sizeof(long) / (1 + LONG_PER_U64)),
+};
+EXPORT_SYMBOL_GPL(btree_geo64);
+
+struct btree_geo btree_geo128 = {
+ .keylen = 2 * LONG_PER_U64,
+ .no_pairs = NODESIZE / sizeof(long) / (1 + 2 * LONG_PER_U64),
+ .no_longs = 2 * LONG_PER_U64 * (NODESIZE / sizeof(long) / (1 + 2 * LONG_PER_U64)),
+};
+EXPORT_SYMBOL_GPL(btree_geo128);
+
+static struct kmem_cache *btree_cachep;
+
+void *btree_alloc(gfp_t gfp_mask, void *pool_data)
+{
+ return kmem_cache_alloc(btree_cachep, gfp_mask);
+}
+EXPORT_SYMBOL_GPL(btree_alloc);
+
+void btree_free(void *element, void *pool_data)
+{
+ kmem_cache_free(btree_cachep, element);
+}
+EXPORT_SYMBOL_GPL(btree_free);
+
+static unsigned long *btree_node_alloc(struct btree_head *head, gfp_t gfp)
+{
+ unsigned long *node;
+
+ node = mempool_alloc(head->mempool, gfp);
+ memset(node, 0, NODESIZE);
+ return node;
+}
+
+static int longcmp(const unsigned long *l1, const unsigned long *l2, size_t n)
+{
+ size_t i;
+
+ for (i = 0; i < n; i++) {
+ if (l1[i] < l2[i])
+ return -1;
+ if (l1[i] > l2[i])
+ return 1;
+ }
+ return 0;
+}
+
+static unsigned long *longcpy(unsigned long *dest, const unsigned long *src,
+ size_t n)
+{
+ size_t i;
+
+ for (i = 0; i < n; i++)
+ dest[i] = src[i];
+ return dest;
+}
+
+static unsigned long *longset(unsigned long *s, unsigned long c, size_t n)
+{
+ size_t i;
+
+ for (i = 0; i < n; i++)
+ s[i] = c;
+ return s;
+}
+
+static void dec_key(struct btree_geo *geo, unsigned long *key)
+{
+ unsigned long val;
+ int i;
+
+ for (i = geo->keylen - 1; i >= 0; i--) {
+ val = key[i];
+ key[i] = val - 1;
+ if (val)
+ break;
+ }
+}
+
+static unsigned long *bkey(struct btree_geo *geo, unsigned long *node, int n)
+{
+ return &node[n * geo->keylen];
+}
+
+static void *bval(struct btree_geo *geo, unsigned long *node, int n)
+{
+ return (void *)node[geo->no_longs + n];
+}
+
+static void setkey(struct btree_geo *geo, unsigned long *node, int n,
+ unsigned long *key)
+{
+ longcpy(bkey(geo, node, n), key, geo->keylen);
+}
+
+static void setval(struct btree_geo *geo, unsigned long *node, int n,
+ void *val)
+{
+ node[geo->no_longs + n] = (unsigned long) val;
+}
+
+static void clearpair(struct btree_geo *geo, unsigned long *node, int n)
+{
+ longset(bkey(geo, node, n), 0, geo->keylen);
+ node[geo->no_longs + n] = 0;
+}
+
+static inline void __btree_init(struct btree_head *head)
+{
+ head->node = NULL;
+ head->height = 0;
+}
+
+void btree_init_mempool(struct btree_head *head, mempool_t *mempool)
+{
+ __btree_init(head);
+ head->mempool = mempool;
+}
+EXPORT_SYMBOL_GPL(btree_init_mempool);
+
+int btree_init(struct btree_head *head)
+{
+ __btree_init(head);
+ head->mempool = mempool_create(0, btree_alloc, btree_free, NULL);
+ if (!head->mempool)
+ return -ENOMEM;
+ return 0;
+}
+EXPORT_SYMBOL_GPL(btree_init);
+
+void btree_destroy(struct btree_head *head)
+{
+ mempool_destroy(head->mempool);
+ head->mempool = NULL;
+}
+EXPORT_SYMBOL_GPL(btree_destroy);
+
+void *btree_last(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *key)
+{
+ int height = head->height;
+ unsigned long *node = head->node;
+
+ if (height == 0)
+ return NULL;
+
+ for ( ; height > 1; height--)
+ node = bval(geo, node, 0);
+
+ longcpy(key, bkey(geo, node, 0), geo->keylen);
+ return bval(geo, node, 0);
+}
+EXPORT_SYMBOL_GPL(btree_last);
+
+static int keycmp(struct btree_geo *geo, unsigned long *node, int pos,
+ unsigned long *key)
+{
+ return longcmp(bkey(geo, node, pos), key, geo->keylen);
+}
+
+static int keyzero(struct btree_geo *geo, unsigned long *key)
+{
+ int i;
+
+ for (i = 0; i < geo->keylen; i++)
+ if (key[i])
+ return 0;
+
+ return 1;
+}
+
+void *btree_lookup(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *key)
+{
+ int i, height = head->height;
+ unsigned long *node = head->node;
+
+ if (height == 0)
+ return NULL;
+
+ for ( ; height > 1; height--) {
+ for (i = 0; i < geo->no_pairs; i++)
+ if (keycmp(geo, node, i, key) <= 0)
+ break;
+ if (i == geo->no_pairs)
+ return NULL;
+ node = bval(geo, node, i);
+ if (!node)
+ return NULL;
+ }
+
+ if (!node)
+ return NULL;
+
+ for (i = 0; i < geo->no_pairs; i++)
+ if (keycmp(geo, node, i, key) == 0)
+ return bval(geo, node, i);
+ return NULL;
+}
+EXPORT_SYMBOL_GPL(btree_lookup);
+
+int btree_update(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *key, void *val)
+{
+ int i, height = head->height;
+ unsigned long *node = head->node;
+
+ if (height == 0)
+ return -ENOENT;
+
+ for ( ; height > 1; height--) {
+ for (i = 0; i < geo->no_pairs; i++)
+ if (keycmp(geo, node, i, key) <= 0)
+ break;
+ if (i == geo->no_pairs)
+ return -ENOENT;
+ node = bval(geo, node, i);
+ if (!node)
+ return -ENOENT;
+ }
+
+ if (!node)
+ return -ENOENT;
+
+ for (i = 0; i < geo->no_pairs; i++)
+ if (keycmp(geo, node, i, key) == 0) {
+ setval(geo, node, i, val);
+ return 0;
+ }
+ return -ENOENT;
+}
+EXPORT_SYMBOL_GPL(btree_update);
+
+/*
+ * Usually this function is quite similar to normal lookup. But the key of
+ * a parent node may be smaller than the smallest key of all its siblings.
+ * In such a case we cannot just return NULL, as we have only proven that no
+ * key smaller than __key, but larger than this parent key exists.
+ * So we set __key to the parent key and retry. We have to use the smallest
+ * such parent key, which is the last parent key we encountered.
+ */
+void *btree_get_prev(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *__key)
+{
+ int i, height;
+ unsigned long *node, *oldnode;
+ unsigned long *retry_key = NULL, key[geo->keylen];
+
+ if (keyzero(geo, __key))
+ return NULL;
+
+ if (head->height == 0)
+ return NULL;
+retry:
+ longcpy(key, __key, geo->keylen);
+ dec_key(geo, key);
+
+ node = head->node;
+ for (height = head->height ; height > 1; height--) {
+ for (i = 0; i < geo->no_pairs; i++)
+ if (keycmp(geo, node, i, key) <= 0)
+ break;
+ if (i == geo->no_pairs)
+ goto miss;
+ oldnode = node;
+ node = bval(geo, node, i);
+ if (!node)
+ goto miss;
+ retry_key = bkey(geo, oldnode, i);
+ }
+
+ if (!node)
+ goto miss;
+
+ for (i = 0; i < geo->no_pairs; i++) {
+ if (keycmp(geo, node, i, key) <= 0) {
+ if (bval(geo, node, i)) {
+ longcpy(__key, bkey(geo, node, i), geo->keylen);
+ return bval(geo, node, i);
+ } else
+ goto miss;
+ }
+ }
+miss:
+ if (retry_key) {
+ __key = retry_key;
+ retry_key = NULL;
+ goto retry;
+ }
+ return NULL;
+}
+
+static int getpos(struct btree_geo *geo, unsigned long *node,
+ unsigned long *key)
+{
+ int i;
+
+ for (i = 0; i < geo->no_pairs; i++) {
+ if (keycmp(geo, node, i, key) <= 0)
+ break;
+ }
+ return i;
+}
+
+static int getfill(struct btree_geo *geo, unsigned long *node, int start)
+{
+ int i;
+
+ for (i = start; i < geo->no_pairs; i++)
+ if (!bval(geo, node, i))
+ break;
+ return i;
+}
+
+/*
+ * locate the correct leaf node in the btree
+ */
+static unsigned long *find_level(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *key, int level)
+{
+ unsigned long *node = head->node;
+ int i, height;
+
+ for (height = head->height; height > level; height--) {
+ for (i = 0; i < geo->no_pairs; i++)
+ if (keycmp(geo, node, i, key) <= 0)
+ break;
+
+ if ((i == geo->no_pairs) || !bval(geo, node, i)) {
+ /* right-most key is too large, update it */
+ /* FIXME: If the right-most key on higher levels is
+ * always zero, this wouldn't be necessary. */
+ i--;
+ setkey(geo, node, i, key);
+ }
+ BUG_ON(i < 0);
+ node = bval(geo, node, i);
+ }
+ BUG_ON(!node);
+ return node;
+}
+
+static int btree_grow(struct btree_head *head, struct btree_geo *geo,
+ gfp_t gfp)
+{
+ unsigned long *node;
+ int fill;
+
+ node = btree_node_alloc(head, gfp);
+ if (!node)
+ return -ENOMEM;
+ if (head->node) {
+ fill = getfill(geo, head->node, 0);
+ setkey(geo, node, 0, bkey(geo, head->node, fill - 1));
+ setval(geo, node, 0, head->node);
+ }
+ head->node = node;
+ head->height++;
+ return 0;
+}
+
+static void btree_shrink(struct btree_head *head, struct btree_geo *geo)
+{
+ unsigned long *node;
+ int fill;
+
+ if (head->height <= 1)
+ return;
+
+ node = head->node;
+ fill = getfill(geo, node, 0);
+ BUG_ON(fill > 1);
+ head->node = bval(geo, node, 0);
+ head->height--;
+ mempool_free(node, head->mempool);
+}
+
+static int btree_insert_level(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *key, void *val, int level,
+ gfp_t gfp)
+{
+ unsigned long *node;
+ int i, pos, fill, err;
+
+ BUG_ON(!val);
+ if (head->height < level) {
+ err = btree_grow(head, geo, gfp);
+ if (err)
+ return err;
+ }
+
+retry:
+ node = find_level(head, geo, key, level);
+ pos = getpos(geo, node, key);
+ fill = getfill(geo, node, pos);
+ /* two identical keys are not allowed */
+ BUG_ON(pos < fill && keycmp(geo, node, pos, key) == 0);
+
+ if (fill == geo->no_pairs) {
+ /* need to split node */
+ unsigned long *new;
+
+ new = btree_node_alloc(head, gfp);
+ if (!new)
+ return -ENOMEM;
+ err = btree_insert_level(head, geo,
+ bkey(geo, node, fill / 2 - 1),
+ new, level + 1, gfp);
+ if (err) {
+ mempool_free(new, head->mempool);
+ return err;
+ }
+ for (i = 0; i < fill / 2; i++) {
+ setkey(geo, new, i, bkey(geo, node, i));
+ setval(geo, new, i, bval(geo, node, i));
+ setkey(geo, node, i, bkey(geo, node, i + fill / 2));
+ setval(geo, node, i, bval(geo, node, i + fill / 2));
+ clearpair(geo, node, i + fill / 2);
+ }
+ if (fill & 1) {
+ setkey(geo, node, i, bkey(geo, node, fill - 1));
+ setval(geo, node, i, bval(geo, node, fill - 1));
+ clearpair(geo, node, fill - 1);
+ }
+ goto retry;
+ }
+ BUG_ON(fill >= geo->no_pairs);
+
+ /* shift and insert */
+ for (i = fill; i > pos; i--) {
+ setkey(geo, node, i, bkey(geo, node, i - 1));
+ setval(geo, node, i, bval(geo, node, i - 1));
+ }
+ setkey(geo, node, pos, key);
+ setval(geo, node, pos, val);
+
+ return 0;
+}
+
+int btree_insert(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *key, void *val, gfp_t gfp)
+{
+ return btree_insert_level(head, geo, key, val, 1, gfp);
+}
+EXPORT_SYMBOL_GPL(btree_insert);
+
+static void *btree_remove_level(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *key, int level);
+static void merge(struct btree_head *head, struct btree_geo *geo, int level,
+ unsigned long *left, int lfill,
+ unsigned long *right, int rfill,
+ unsigned long *parent, int lpos)
+{
+ int i;
+
+ for (i = 0; i < rfill; i++) {
+ /* Move all keys to the left */
+ setkey(geo, left, lfill + i, bkey(geo, right, i));
+ setval(geo, left, lfill + i, bval(geo, right, i));
+ }
+ /* Exchange left and right child in parent */
+ setval(geo, parent, lpos, right);
+ setval(geo, parent, lpos + 1, left);
+ /* Remove left (formerly right) child from parent */
+ btree_remove_level(head, geo, bkey(geo, parent, lpos), level + 1);
+ mempool_free(right, head->mempool);
+}
+
+static void rebalance(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *key, int level, unsigned long *child, int fill)
+{
+ unsigned long *parent, *left = NULL, *right = NULL;
+ int i, no_left, no_right;
+
+ if (fill == 0) {
+ /* Because we don't steal entries from a neigbour, this case
+ * can happen. Parent node contains a single child, this
+ * node, so merging with a sibling never happens.
+ */
+ btree_remove_level(head, geo, key, level + 1);
+ mempool_free(child, head->mempool);
+ return;
+ }
+
+ parent = find_level(head, geo, key, level + 1);
+ i = getpos(geo, parent, key);
+ BUG_ON(bval(geo, parent, i) != child);
+
+ if (i > 0) {
+ left = bval(geo, parent, i - 1);
+ no_left = getfill(geo, left, 0);
+ if (fill + no_left <= geo->no_pairs) {
+ merge(head, geo, level,
+ left, no_left,
+ child, fill,
+ parent, i - 1);
+ return;
+ }
+ }
+ if (i + 1 < getfill(geo, parent, i)) {
+ right = bval(geo, parent, i + 1);
+ no_right = getfill(geo, right, 0);
+ if (fill + no_right <= geo->no_pairs) {
+ merge(head, geo, level,
+ child, fill,
+ right, no_right,
+ parent, i);
+ return;
+ }
+ }
+ /*
+ * We could also try to steal one entry from the left or right
+ * neighbor. By not doing so we changed the invariant from
+ * "all nodes are at least half full" to "no two neighboring
+ * nodes can be merged". Which means that the average fill of
+ * all nodes is still half or better.
+ */
+}
+
+static void *btree_remove_level(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *key, int level)
+{
+ unsigned long *node;
+ int i, pos, fill;
+ void *ret;
+
+ if (level > head->height) {
+ /* we recursed all the way up */
+ head->height = 0;
+ head->node = NULL;
+ return NULL;
+ }
+
+ node = find_level(head, geo, key, level);
+ pos = getpos(geo, node, key);
+ fill = getfill(geo, node, pos);
+ if ((level == 1) && (keycmp(geo, node, pos, key) != 0))
+ return NULL;
+ ret = bval(geo, node, pos);
+
+ /* remove and shift */
+ for (i = pos; i < fill - 1; i++) {
+ setkey(geo, node, i, bkey(geo, node, i + 1));
+ setval(geo, node, i, bval(geo, node, i + 1));
+ }
+ clearpair(geo, node, fill - 1);
+
+ if (fill - 1 < geo->no_pairs / 2) {
+ if (level < head->height)
+ rebalance(head, geo, key, level, node, fill - 1);
+ else if (fill - 1 == 1)
+ btree_shrink(head, geo);
+ }
+
+ return ret;
+}
+
+void *btree_remove(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *key)
+{
+ if (head->height == 0)
+ return NULL;
+
+ return btree_remove_level(head, geo, key, 1);
+}
+EXPORT_SYMBOL_GPL(btree_remove);
+
+int btree_merge(struct btree_head *target, struct btree_head *victim,
+ struct btree_geo *geo, gfp_t gfp)
+{
+ unsigned long key[geo->keylen];
+ unsigned long dup[geo->keylen];
+ void *val;
+ int err;
+
+ BUG_ON(target == victim);
+
+ if (!(target->node)) {
+ /* target is empty, just copy fields over */
+ target->node = victim->node;
+ target->height = victim->height;
+ __btree_init(victim);
+ return 0;
+ }
+
+ /* TODO: This needs some optimizations. Currently we do three tree
+ * walks to remove a single object from the victim.
+ */
+ for (;;) {
+ if (!btree_last(victim, geo, key))
+ break;
+ val = btree_lookup(victim, geo, key);
+ err = btree_insert(target, geo, key, val, gfp);
+ if (err)
+ return err;
+ /* We must make a copy of the key, as the original will get
+ * mangled inside btree_remove. */
+ longcpy(dup, key, geo->keylen);
+ btree_remove(victim, geo, dup);
+ }
+ return 0;
+}
+EXPORT_SYMBOL_GPL(btree_merge);
+
+static size_t __btree_for_each(struct btree_head *head, struct btree_geo *geo,
+ unsigned long *node, unsigned long opaque,
+ void (*func)(void *elem, unsigned long opaque,
+ unsigned long *key, size_t index,
+ void *func2),
+ void *func2, int reap, int height, size_t count)
+{
+ int i;
+ unsigned long *child;
+
+ for (i = 0; i < geo->no_pairs; i++) {
+ child = bval(geo, node, i);
+ if (!child)
+ break;
+ if (height > 1)
+ count = __btree_for_each(head, geo, child, opaque,
+ func, func2, reap, height - 1, count);
+ else
+ func(child, opaque, bkey(geo, node, i), count++,
+ func2);
+ }
+ if (reap)
+ mempool_free(node, head->mempool);
+ return count;
+}
+
+static void empty(void *elem, unsigned long opaque, unsigned long *key,
+ size_t index, void *func2)
+{
+}
+
+void visitorl(void *elem, unsigned long opaque, unsigned long *key,
+ size_t index, void *__func)
+{
+ visitorl_t func = __func;
+
+ func(elem, opaque, *key, index);
+}
+EXPORT_SYMBOL_GPL(visitorl);
+
+void visitor32(void *elem, unsigned long opaque, unsigned long *__key,
+ size_t index, void *__func)
+{
+ visitor32_t func = __func;
+ u32 *key = (void *)__key;
+
+ func(elem, opaque, *key, index);
+}
+EXPORT_SYMBOL_GPL(visitor32);
+
+void visitor64(void *elem, unsigned long opaque, unsigned long *__key,
+ size_t index, void *__func)
+{
+ visitor64_t func = __func;
+ u64 *key = (void *)__key;
+
+ func(elem, opaque, *key, index);
+}
+EXPORT_SYMBOL_GPL(visitor64);
+
+void visitor128(void *elem, unsigned long opaque, unsigned long *__key,
+ size_t index, void *__func)
+{
+ visitor128_t func = __func;
+ u64 *key = (void *)__key;
+
+ func(elem, opaque, key[0], key[1], index);
+}
+EXPORT_SYMBOL_GPL(visitor128);
+
+size_t btree_visitor(struct btree_head *head, struct btree_geo *geo,
+ unsigned long opaque,
+ void (*func)(void *elem, unsigned long opaque,
+ unsigned long *key,
+ size_t index, void *func2),
+ void *func2)
+{
+ size_t count = 0;
+
+ if (!func2)
+ func = empty;
+ if (head->node)
+ count = __btree_for_each(head, geo, head->node, opaque, func,
+ func2, 0, head->height, 0);
+ return count;
+}
+EXPORT_SYMBOL_GPL(btree_visitor);
+
+size_t btree_grim_visitor(struct btree_head *head, struct btree_geo *geo,
+ unsigned long opaque,
+ void (*func)(void *elem, unsigned long opaque,
+ unsigned long *key,
+ size_t index, void *func2),
+ void *func2)
+{
+ size_t count = 0;
+
+ if (!func2)
+ func = empty;
+ if (head->node)
+ count = __btree_for_each(head, geo, head->node, opaque, func,
+ func2, 1, head->height, 0);
+ __btree_init(head);
+ return count;
+}
+EXPORT_SYMBOL_GPL(btree_grim_visitor);
+
+static int __init btree_module_init(void)
+{
+ btree_cachep = kmem_cache_create("btree_node", NODESIZE, 0,
+ SLAB_HWCACHE_ALIGN, NULL);
+ return 0;
+}
+
+static void __exit btree_module_exit(void)
+{
+ kmem_cache_destroy(btree_cachep);
+}
+
+/* If core code starts using btree, initialization should happen even earlier */
+module_init(btree_module_init);
+module_exit(btree_module_exit);
+
+MODULE_AUTHOR("Joern Engel <joern@logfs.org>");
+MODULE_AUTHOR("Johannes Berg <johannes@sipsolutions.net>");
+MODULE_LICENSE("GPL");