[LogFS] add new flash file system

This is a new flash file system. See
Documentation/filesystems/logfs.txt

Signed-off-by: Joern Engel <joern@logfs.org>

1 files changed, 797 insertions, 0 deletions

diff --git a/lib/btree.c b/lib/btree.c
new file mode 100644
index 000000000000..41859a820218
--- /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");