1/*
2 * lib/btree.c	- Simple In-memory B+Tree
3 *
4 * As should be obvious for Linux kernel code, license is GPLv2
5 *
6 * Copyright (c) 2007-2008 Joern Engel <joern@logfs.org>
7 * Bits and pieces stolen from Peter Zijlstra's code, which is
8 * Copyright 2007, Red Hat Inc. Peter Zijlstra <pzijlstr@redhat.com>
9 * GPLv2
10 *
11 * see http://programming.kicks-ass.net/kernel-patches/vma_lookup/btree.patch
12 *
13 * A relatively simple B+Tree implementation.  I have written it as a learning
14 * exercise to understand how B+Trees work.  Turned out to be useful as well.
15 *
16 * B+Trees can be used similar to Linux radix trees (which don't have anything
17 * in common with textbook radix trees, beware).  Prerequisite for them working
18 * well is that access to a random tree node is much faster than a large number
19 * of operations within each node.
20 *
21 * Disks have fulfilled the prerequisite for a long time.  More recently DRAM
22 * has gained similar properties, as memory access times, when measured in cpu
23 * cycles, have increased.  Cacheline sizes have increased as well, which also
24 * helps B+Trees.
25 *
26 * Compared to radix trees, B+Trees are more efficient when dealing with a
27 * sparsely populated address space.  Between 25% and 50% of the memory is
28 * occupied with valid pointers.  When densely populated, radix trees contain
29 * ~98% pointers - hard to beat.  Very sparse radix trees contain only ~2%
30 * pointers.
31 *
32 * This particular implementation stores pointers identified by a long value.
33 * Storing NULL pointers is illegal, lookup will return NULL when no entry
34 * was found.
35 *
36 * A tricks was used that is not commonly found in textbooks.  The lowest
37 * values are to the right, not to the left.  All used slots within a node
38 * are on the left, all unused slots contain NUL values.  Most operations
39 * simply loop once over all slots and terminate on the first NUL.
40 */
41
42#include <linux/btree.h>
43#include <linux/cache.h>
44#include <linux/kernel.h>
45#include <linux/slab.h>
46#include <linux/module.h>
47
48#define MAX(a, b) ((a) > (b) ? (a) : (b))
49#define NODESIZE MAX(L1_CACHE_BYTES, 128)
50
51struct btree_geo {
52	int keylen;
53	int no_pairs;
54	int no_longs;
55};
56
57struct btree_geo btree_geo32 = {
58	.keylen = 1,
59	.no_pairs = NODESIZE / sizeof(long) / 2,
60	.no_longs = NODESIZE / sizeof(long) / 2,
61};
62EXPORT_SYMBOL_GPL(btree_geo32);
63
64#define LONG_PER_U64 (64 / BITS_PER_LONG)
65struct btree_geo btree_geo64 = {
66	.keylen = LONG_PER_U64,
67	.no_pairs = NODESIZE / sizeof(long) / (1 + LONG_PER_U64),
68	.no_longs = LONG_PER_U64 * (NODESIZE / sizeof(long) / (1 + LONG_PER_U64)),
69};
70EXPORT_SYMBOL_GPL(btree_geo64);
71
72struct btree_geo btree_geo128 = {
73	.keylen = 2 * LONG_PER_U64,
74	.no_pairs = NODESIZE / sizeof(long) / (1 + 2 * LONG_PER_U64),
75	.no_longs = 2 * LONG_PER_U64 * (NODESIZE / sizeof(long) / (1 + 2 * LONG_PER_U64)),
76};
77EXPORT_SYMBOL_GPL(btree_geo128);
78
79static struct kmem_cache *btree_cachep;
80
81void *btree_alloc(gfp_t gfp_mask, void *pool_data)
82{
83	return kmem_cache_alloc(btree_cachep, gfp_mask);
84}
85EXPORT_SYMBOL_GPL(btree_alloc);
86
87void btree_free(void *element, void *pool_data)
88{
89	kmem_cache_free(btree_cachep, element);
90}
91EXPORT_SYMBOL_GPL(btree_free);
92
93static unsigned long *btree_node_alloc(struct btree_head *head, gfp_t gfp)
94{
95	unsigned long *node;
96
97	node = mempool_alloc(head->mempool, gfp);
98	if (likely(node))
99		memset(node, 0, NODESIZE);
100	return node;
101}
102
103static int longcmp(const unsigned long *l1, const unsigned long *l2, size_t n)
104{
105	size_t i;
106
107	for (i = 0; i < n; i++) {
108		if (l1[i] < l2[i])
109			return -1;
110		if (l1[i] > l2[i])
111			return 1;
112	}
113	return 0;
114}
115
116static unsigned long *longcpy(unsigned long *dest, const unsigned long *src,
117		size_t n)
118{
119	size_t i;
120
121	for (i = 0; i < n; i++)
122		dest[i] = src[i];
123	return dest;
124}
125
126static unsigned long *longset(unsigned long *s, unsigned long c, size_t n)
127{
128	size_t i;
129
130	for (i = 0; i < n; i++)
131		s[i] = c;
132	return s;
133}
134
135static void dec_key(struct btree_geo *geo, unsigned long *key)
136{
137	unsigned long val;
138	int i;
139
140	for (i = geo->keylen - 1; i >= 0; i--) {
141		val = key[i];
142		key[i] = val - 1;
143		if (val)
144			break;
145	}
146}
147
148static unsigned long *bkey(struct btree_geo *geo, unsigned long *node, int n)
149{
150	return &node[n * geo->keylen];
151}
152
153static void *bval(struct btree_geo *geo, unsigned long *node, int n)
154{
155	return (void *)node[geo->no_longs + n];
156}
157
158static void setkey(struct btree_geo *geo, unsigned long *node, int n,
159		   unsigned long *key)
160{
161	longcpy(bkey(geo, node, n), key, geo->keylen);
162}
163
164static void setval(struct btree_geo *geo, unsigned long *node, int n,
165		   void *val)
166{
167	node[geo->no_longs + n] = (unsigned long) val;
168}
169
170static void clearpair(struct btree_geo *geo, unsigned long *node, int n)
171{
172	longset(bkey(geo, node, n), 0, geo->keylen);
173	node[geo->no_longs + n] = 0;
174}
175
176static inline void __btree_init(struct btree_head *head)
177{
178	head->node = NULL;
179	head->height = 0;
180}
181
182void btree_init_mempool(struct btree_head *head, mempool_t *mempool)
183{
184	__btree_init(head);
185	head->mempool = mempool;
186}
187EXPORT_SYMBOL_GPL(btree_init_mempool);
188
189int btree_init(struct btree_head *head)
190{
191	__btree_init(head);
192	head->mempool = mempool_create(0, btree_alloc, btree_free, NULL);
193	if (!head->mempool)
194		return -ENOMEM;
195	return 0;
196}
197EXPORT_SYMBOL_GPL(btree_init);
198
199void btree_destroy(struct btree_head *head)
200{
201	mempool_free(head->node, head->mempool);
202	mempool_destroy(head->mempool);
203	head->mempool = NULL;
204}
205EXPORT_SYMBOL_GPL(btree_destroy);
206
207void *btree_last(struct btree_head *head, struct btree_geo *geo,
208		 unsigned long *key)
209{
210	int height = head->height;
211	unsigned long *node = head->node;
212
213	if (height == 0)
214		return NULL;
215
216	for ( ; height > 1; height--)
217		node = bval(geo, node, 0);
218
219	longcpy(key, bkey(geo, node, 0), geo->keylen);
220	return bval(geo, node, 0);
221}
222EXPORT_SYMBOL_GPL(btree_last);
223
224static int keycmp(struct btree_geo *geo, unsigned long *node, int pos,
225		  unsigned long *key)
226{
227	return longcmp(bkey(geo, node, pos), key, geo->keylen);
228}
229
230static int keyzero(struct btree_geo *geo, unsigned long *key)
231{
232	int i;
233
234	for (i = 0; i < geo->keylen; i++)
235		if (key[i])
236			return 0;
237
238	return 1;
239}
240
241void *btree_lookup(struct btree_head *head, struct btree_geo *geo,
242		unsigned long *key)
243{
244	int i, height = head->height;
245	unsigned long *node = head->node;
246
247	if (height == 0)
248		return NULL;
249
250	for ( ; height > 1; height--) {
251		for (i = 0; i < geo->no_pairs; i++)
252			if (keycmp(geo, node, i, key) <= 0)
253				break;
254		if (i == geo->no_pairs)
255			return NULL;
256		node = bval(geo, node, i);
257		if (!node)
258			return NULL;
259	}
260
261	if (!node)
262		return NULL;
263
264	for (i = 0; i < geo->no_pairs; i++)
265		if (keycmp(geo, node, i, key) == 0)
266			return bval(geo, node, i);
267	return NULL;
268}
269EXPORT_SYMBOL_GPL(btree_lookup);
270
271int btree_update(struct btree_head *head, struct btree_geo *geo,
272		 unsigned long *key, void *val)
273{
274	int i, height = head->height;
275	unsigned long *node = head->node;
276
277	if (height == 0)
278		return -ENOENT;
279
280	for ( ; height > 1; height--) {
281		for (i = 0; i < geo->no_pairs; i++)
282			if (keycmp(geo, node, i, key) <= 0)
283				break;
284		if (i == geo->no_pairs)
285			return -ENOENT;
286		node = bval(geo, node, i);
287		if (!node)
288			return -ENOENT;
289	}
290
291	if (!node)
292		return -ENOENT;
293
294	for (i = 0; i < geo->no_pairs; i++)
295		if (keycmp(geo, node, i, key) == 0) {
296			setval(geo, node, i, val);
297			return 0;
298		}
299	return -ENOENT;
300}
301EXPORT_SYMBOL_GPL(btree_update);
302
303/*
304 * Usually this function is quite similar to normal lookup.  But the key of
305 * a parent node may be smaller than the smallest key of all its siblings.
306 * In such a case we cannot just return NULL, as we have only proven that no
307 * key smaller than __key, but larger than this parent key exists.
308 * So we set __key to the parent key and retry.  We have to use the smallest
309 * such parent key, which is the last parent key we encountered.
310 */
311void *btree_get_prev(struct btree_head *head, struct btree_geo *geo,
312		     unsigned long *__key)
313{
314	int i, height;
315	unsigned long *node, *oldnode;
316	unsigned long *retry_key = NULL, key[geo->keylen];
317
318	if (keyzero(geo, __key))
319		return NULL;
320
321	if (head->height == 0)
322		return NULL;
323	longcpy(key, __key, geo->keylen);
324retry:
325	dec_key(geo, key);
326
327	node = head->node;
328	for (height = head->height ; height > 1; height--) {
329		for (i = 0; i < geo->no_pairs; i++)
330			if (keycmp(geo, node, i, key) <= 0)
331				break;
332		if (i == geo->no_pairs)
333			goto miss;
334		oldnode = node;
335		node = bval(geo, node, i);
336		if (!node)
337			goto miss;
338		retry_key = bkey(geo, oldnode, i);
339	}
340
341	if (!node)
342		goto miss;
343
344	for (i = 0; i < geo->no_pairs; i++) {
345		if (keycmp(geo, node, i, key) <= 0) {
346			if (bval(geo, node, i)) {
347				longcpy(__key, bkey(geo, node, i), geo->keylen);
348				return bval(geo, node, i);
349			} else
350				goto miss;
351		}
352	}
353miss:
354	if (retry_key) {
355		longcpy(key, retry_key, geo->keylen);
356		retry_key = NULL;
357		goto retry;
358	}
359	return NULL;
360}
361EXPORT_SYMBOL_GPL(btree_get_prev);
362
363static int getpos(struct btree_geo *geo, unsigned long *node,
364		unsigned long *key)
365{
366	int i;
367
368	for (i = 0; i < geo->no_pairs; i++) {
369		if (keycmp(geo, node, i, key) <= 0)
370			break;
371	}
372	return i;
373}
374
375static int getfill(struct btree_geo *geo, unsigned long *node, int start)
376{
377	int i;
378
379	for (i = start; i < geo->no_pairs; i++)
380		if (!bval(geo, node, i))
381			break;
382	return i;
383}
384
385/*
386 * locate the correct leaf node in the btree
387 */
388static unsigned long *find_level(struct btree_head *head, struct btree_geo *geo,
389		unsigned long *key, int level)
390{
391	unsigned long *node = head->node;
392	int i, height;
393
394	for (height = head->height; height > level; height--) {
395		for (i = 0; i < geo->no_pairs; i++)
396			if (keycmp(geo, node, i, key) <= 0)
397				break;
398
399		if ((i == geo->no_pairs) || !bval(geo, node, i)) {
400			/* right-most key is too large, update it */
401			/* FIXME: If the right-most key on higher levels is
402			 * always zero, this wouldn't be necessary. */
403			i--;
404			setkey(geo, node, i, key);
405		}
406		BUG_ON(i < 0);
407		node = bval(geo, node, i);
408	}
409	BUG_ON(!node);
410	return node;
411}
412
413static int btree_grow(struct btree_head *head, struct btree_geo *geo,
414		      gfp_t gfp)
415{
416	unsigned long *node;
417	int fill;
418
419	node = btree_node_alloc(head, gfp);
420	if (!node)
421		return -ENOMEM;
422	if (head->node) {
423		fill = getfill(geo, head->node, 0);
424		setkey(geo, node, 0, bkey(geo, head->node, fill - 1));
425		setval(geo, node, 0, head->node);
426	}
427	head->node = node;
428	head->height++;
429	return 0;
430}
431
432static void btree_shrink(struct btree_head *head, struct btree_geo *geo)
433{
434	unsigned long *node;
435	int fill;
436
437	if (head->height <= 1)
438		return;
439
440	node = head->node;
441	fill = getfill(geo, node, 0);
442	BUG_ON(fill > 1);
443	head->node = bval(geo, node, 0);
444	head->height--;
445	mempool_free(node, head->mempool);
446}
447
448static int btree_insert_level(struct btree_head *head, struct btree_geo *geo,
449			      unsigned long *key, void *val, int level,
450			      gfp_t gfp)
451{
452	unsigned long *node;
453	int i, pos, fill, err;
454
455	BUG_ON(!val);
456	if (head->height < level) {
457		err = btree_grow(head, geo, gfp);
458		if (err)
459			return err;
460	}
461
462retry:
463	node = find_level(head, geo, key, level);
464	pos = getpos(geo, node, key);
465	fill = getfill(geo, node, pos);
466	/* two identical keys are not allowed */
467	BUG_ON(pos < fill && keycmp(geo, node, pos, key) == 0);
468
469	if (fill == geo->no_pairs) {
470		/* need to split node */
471		unsigned long *new;
472
473		new = btree_node_alloc(head, gfp);
474		if (!new)
475			return -ENOMEM;
476		err = btree_insert_level(head, geo,
477				bkey(geo, node, fill / 2 - 1),
478				new, level + 1, gfp);
479		if (err) {
480			mempool_free(new, head->mempool);
481			return err;
482		}
483		for (i = 0; i < fill / 2; i++) {
484			setkey(geo, new, i, bkey(geo, node, i));
485			setval(geo, new, i, bval(geo, node, i));
486			setkey(geo, node, i, bkey(geo, node, i + fill / 2));
487			setval(geo, node, i, bval(geo, node, i + fill / 2));
488			clearpair(geo, node, i + fill / 2);
489		}
490		if (fill & 1) {
491			setkey(geo, node, i, bkey(geo, node, fill - 1));
492			setval(geo, node, i, bval(geo, node, fill - 1));
493			clearpair(geo, node, fill - 1);
494		}
495		goto retry;
496	}
497	BUG_ON(fill >= geo->no_pairs);
498
499	/* shift and insert */
500	for (i = fill; i > pos; i--) {
501		setkey(geo, node, i, bkey(geo, node, i - 1));
502		setval(geo, node, i, bval(geo, node, i - 1));
503	}
504	setkey(geo, node, pos, key);
505	setval(geo, node, pos, val);
506
507	return 0;
508}
509
510int btree_insert(struct btree_head *head, struct btree_geo *geo,
511		unsigned long *key, void *val, gfp_t gfp)
512{
513	BUG_ON(!val);
514	return btree_insert_level(head, geo, key, val, 1, gfp);
515}
516EXPORT_SYMBOL_GPL(btree_insert);
517
518static void *btree_remove_level(struct btree_head *head, struct btree_geo *geo,
519		unsigned long *key, int level);
520static void merge(struct btree_head *head, struct btree_geo *geo, int level,
521		unsigned long *left, int lfill,
522		unsigned long *right, int rfill,
523		unsigned long *parent, int lpos)
524{
525	int i;
526
527	for (i = 0; i < rfill; i++) {
528		/* Move all keys to the left */
529		setkey(geo, left, lfill + i, bkey(geo, right, i));
530		setval(geo, left, lfill + i, bval(geo, right, i));
531	}
532	/* Exchange left and right child in parent */
533	setval(geo, parent, lpos, right);
534	setval(geo, parent, lpos + 1, left);
535	/* Remove left (formerly right) child from parent */
536	btree_remove_level(head, geo, bkey(geo, parent, lpos), level + 1);
537	mempool_free(right, head->mempool);
538}
539
540static void rebalance(struct btree_head *head, struct btree_geo *geo,
541		unsigned long *key, int level, unsigned long *child, int fill)
542{
543	unsigned long *parent, *left = NULL, *right = NULL;
544	int i, no_left, no_right;
545
546	if (fill == 0) {
547		/* Because we don't steal entries from a neighbour, this case
548		 * can happen.  Parent node contains a single child, this
549		 * node, so merging with a sibling never happens.
550		 */
551		btree_remove_level(head, geo, key, level + 1);
552		mempool_free(child, head->mempool);
553		return;
554	}
555
556	parent = find_level(head, geo, key, level + 1);
557	i = getpos(geo, parent, key);
558	BUG_ON(bval(geo, parent, i) != child);
559
560	if (i > 0) {
561		left = bval(geo, parent, i - 1);
562		no_left = getfill(geo, left, 0);
563		if (fill + no_left <= geo->no_pairs) {
564			merge(head, geo, level,
565					left, no_left,
566					child, fill,
567					parent, i - 1);
568			return;
569		}
570	}
571	if (i + 1 < getfill(geo, parent, i)) {
572		right = bval(geo, parent, i + 1);
573		no_right = getfill(geo, right, 0);
574		if (fill + no_right <= geo->no_pairs) {
575			merge(head, geo, level,
576					child, fill,
577					right, no_right,
578					parent, i);
579			return;
580		}
581	}
582	/*
583	 * We could also try to steal one entry from the left or right
584	 * neighbor.  By not doing so we changed the invariant from
585	 * "all nodes are at least half full" to "no two neighboring
586	 * nodes can be merged".  Which means that the average fill of
587	 * all nodes is still half or better.
588	 */
589}
590
591static void *btree_remove_level(struct btree_head *head, struct btree_geo *geo,
592		unsigned long *key, int level)
593{
594	unsigned long *node;
595	int i, pos, fill;
596	void *ret;
597
598	if (level > head->height) {
599		/* we recursed all the way up */
600		head->height = 0;
601		head->node = NULL;
602		return NULL;
603	}
604
605	node = find_level(head, geo, key, level);
606	pos = getpos(geo, node, key);
607	fill = getfill(geo, node, pos);
608	if ((level == 1) && (keycmp(geo, node, pos, key) != 0))
609		return NULL;
610	ret = bval(geo, node, pos);
611
612	/* remove and shift */
613	for (i = pos; i < fill - 1; i++) {
614		setkey(geo, node, i, bkey(geo, node, i + 1));
615		setval(geo, node, i, bval(geo, node, i + 1));
616	}
617	clearpair(geo, node, fill - 1);
618
619	if (fill - 1 < geo->no_pairs / 2) {
620		if (level < head->height)
621			rebalance(head, geo, key, level, node, fill - 1);
622		else if (fill - 1 == 1)
623			btree_shrink(head, geo);
624	}
625
626	return ret;
627}
628
629void *btree_remove(struct btree_head *head, struct btree_geo *geo,
630		unsigned long *key)
631{
632	if (head->height == 0)
633		return NULL;
634
635	return btree_remove_level(head, geo, key, 1);
636}
637EXPORT_SYMBOL_GPL(btree_remove);
638
639int btree_merge(struct btree_head *target, struct btree_head *victim,
640		struct btree_geo *geo, gfp_t gfp)
641{
642	unsigned long key[geo->keylen];
643	unsigned long dup[geo->keylen];
644	void *val;
645	int err;
646
647	BUG_ON(target == victim);
648
649	if (!(target->node)) {
650		/* target is empty, just copy fields over */
651		target->node = victim->node;
652		target->height = victim->height;
653		__btree_init(victim);
654		return 0;
655	}
656
657	/* TODO: This needs some optimizations.  Currently we do three tree
658	 * walks to remove a single object from the victim.
659	 */
660	for (;;) {
661		if (!btree_last(victim, geo, key))
662			break;
663		val = btree_lookup(victim, geo, key);
664		err = btree_insert(target, geo, key, val, gfp);
665		if (err)
666			return err;
667		/* We must make a copy of the key, as the original will get
668		 * mangled inside btree_remove. */
669		longcpy(dup, key, geo->keylen);
670		btree_remove(victim, geo, dup);
671	}
672	return 0;
673}
674EXPORT_SYMBOL_GPL(btree_merge);
675
676static size_t __btree_for_each(struct btree_head *head, struct btree_geo *geo,
677			       unsigned long *node, unsigned long opaque,
678			       void (*func)(void *elem, unsigned long opaque,
679					    unsigned long *key, size_t index,
680					    void *func2),
681			       void *func2, int reap, int height, size_t count)
682{
683	int i;
684	unsigned long *child;
685
686	for (i = 0; i < geo->no_pairs; i++) {
687		child = bval(geo, node, i);
688		if (!child)
689			break;
690		if (height > 1)
691			count = __btree_for_each(head, geo, child, opaque,
692					func, func2, reap, height - 1, count);
693		else
694			func(child, opaque, bkey(geo, node, i), count++,
695					func2);
696	}
697	if (reap)
698		mempool_free(node, head->mempool);
699	return count;
700}
701
702static void empty(void *elem, unsigned long opaque, unsigned long *key,
703		  size_t index, void *func2)
704{
705}
706
707void visitorl(void *elem, unsigned long opaque, unsigned long *key,
708	      size_t index, void *__func)
709{
710	visitorl_t func = __func;
711
712	func(elem, opaque, *key, index);
713}
714EXPORT_SYMBOL_GPL(visitorl);
715
716void visitor32(void *elem, unsigned long opaque, unsigned long *__key,
717	       size_t index, void *__func)
718{
719	visitor32_t func = __func;
720	u32 *key = (void *)__key;
721
722	func(elem, opaque, *key, index);
723}
724EXPORT_SYMBOL_GPL(visitor32);
725
726void visitor64(void *elem, unsigned long opaque, unsigned long *__key,
727	       size_t index, void *__func)
728{
729	visitor64_t func = __func;
730	u64 *key = (void *)__key;
731
732	func(elem, opaque, *key, index);
733}
734EXPORT_SYMBOL_GPL(visitor64);
735
736void visitor128(void *elem, unsigned long opaque, unsigned long *__key,
737		size_t index, void *__func)
738{
739	visitor128_t func = __func;
740	u64 *key = (void *)__key;
741
742	func(elem, opaque, key[0], key[1], index);
743}
744EXPORT_SYMBOL_GPL(visitor128);
745
746size_t btree_visitor(struct btree_head *head, struct btree_geo *geo,
747		     unsigned long opaque,
748		     void (*func)(void *elem, unsigned long opaque,
749		     		  unsigned long *key,
750		     		  size_t index, void *func2),
751		     void *func2)
752{
753	size_t count = 0;
754
755	if (!func2)
756		func = empty;
757	if (head->node)
758		count = __btree_for_each(head, geo, head->node, opaque, func,
759				func2, 0, head->height, 0);
760	return count;
761}
762EXPORT_SYMBOL_GPL(btree_visitor);
763
764size_t btree_grim_visitor(struct btree_head *head, struct btree_geo *geo,
765			  unsigned long opaque,
766			  void (*func)(void *elem, unsigned long opaque,
767				       unsigned long *key,
768				       size_t index, void *func2),
769			  void *func2)
770{
771	size_t count = 0;
772
773	if (!func2)
774		func = empty;
775	if (head->node)
776		count = __btree_for_each(head, geo, head->node, opaque, func,
777				func2, 1, head->height, 0);
778	__btree_init(head);
779	return count;
780}
781EXPORT_SYMBOL_GPL(btree_grim_visitor);
782
783static int __init btree_module_init(void)
784{
785	btree_cachep = kmem_cache_create("btree_node", NODESIZE, 0,
786			SLAB_HWCACHE_ALIGN, NULL);
787	return 0;
788}
789
790static void __exit btree_module_exit(void)
791{
792	kmem_cache_destroy(btree_cachep);
793}
794
795/* If core code starts using btree, initialization should happen even earlier */
796module_init(btree_module_init);
797module_exit(btree_module_exit);
798
799MODULE_AUTHOR("Joern Engel <joern@logfs.org>");
800MODULE_AUTHOR("Johannes Berg <johannes@sipsolutions.net>");
801MODULE_LICENSE("GPL");
802