Keep track of allocated minor numbers with an IDR instead of a bit-set.
(dm-use-idr.patch from 2.6.8-rc4-mm1)

--- diff/drivers/md/dm.c	2004-08-14 05:55:59.000000000 -0500
+++ source/drivers/md/dm.c	2004-08-16 10:33:57.671460168 -0500
@@ -15,15 +15,13 @@
 #include <linux/buffer_head.h>
 #include <linux/mempool.h>
 #include <linux/slab.h>
+#include <linux/idr.h>
 
 static const char *_name = DM_NAME;
 
 static unsigned int major = 0;
 static unsigned int _major = 0;
 
-static int realloc_minor_bits(unsigned long requested_minor);
-static void free_minor_bits(void);
-
 /*
  * One of these is allocated per bio.
  */
@@ -113,19 +111,11 @@
 		return -ENOMEM;
 	}
 
-	r = realloc_minor_bits(1024);
-	if (r < 0) {
-		kmem_cache_destroy(_tio_cache);
-		kmem_cache_destroy(_io_cache);
-		return r;
-	}
-
 	_major = major;
 	r = register_blkdev(_major, _name);
 	if (r < 0) {
 		kmem_cache_destroy(_tio_cache);
 		kmem_cache_destroy(_io_cache);
-		free_minor_bits();
 		return r;
 	}
 
@@ -139,7 +129,6 @@
 {
 	kmem_cache_destroy(_tio_cache);
 	kmem_cache_destroy(_io_cache);
-	free_minor_bits();
 
 	if (unregister_blkdev(_major, _name) < 0)
 		DMERR("devfs_unregister_blkdev failed");
@@ -624,59 +613,15 @@
 }
 
 /*-----------------------------------------------------------------
- * A bitset is used to keep track of allocated minor numbers.
+ * An IDR is used to keep track of allocated minor numbers.
  *---------------------------------------------------------------*/
 static DECLARE_MUTEX(_minor_lock);
-static unsigned long *_minor_bits = NULL;
-static unsigned long _max_minors = 0;
-
-#define MINORS_SIZE(minors) ((minors / BITS_PER_LONG) * sizeof(unsigned long))
-
-static int realloc_minor_bits(unsigned long requested_minor)
-{
-	unsigned long max_minors;
-	unsigned long *minor_bits, *tmp;
-
-	if (requested_minor < _max_minors)
-		return -EINVAL;
-
-	/* Round up the requested minor to the next power-of-2. */
-	max_minors = 1 << fls(requested_minor - 1);
-	if (max_minors > (1 << MINORBITS))
-		return -EINVAL;
-
-	minor_bits = kmalloc(MINORS_SIZE(max_minors), GFP_KERNEL);
-	if (!minor_bits)
-		return -ENOMEM;
-	memset(minor_bits, 0, MINORS_SIZE(max_minors));
-
-	/* Copy the existing bit-set to the new one. */
-	if (_minor_bits)
-		memcpy(minor_bits, _minor_bits, MINORS_SIZE(_max_minors));
-
-	tmp = _minor_bits;
-	_minor_bits = minor_bits;
-	_max_minors = max_minors;
-	if (tmp)
-		kfree(tmp);
-
-	return 0;
-}
-
-static void free_minor_bits(void)
-{
-	down(&_minor_lock);
-	kfree(_minor_bits);
-	_minor_bits = NULL;
-	_max_minors = 0;
-	up(&_minor_lock);
-}
+static DEFINE_IDR(_minor_idr);
 
 static void free_minor(unsigned int minor)
 {
 	down(&_minor_lock);
-	if (minor < _max_minors)
-		clear_bit(minor, _minor_bits);
+	idr_remove(&_minor_idr, minor);
 	up(&_minor_lock);
 }
 
@@ -685,24 +630,37 @@
  */
 static int specific_minor(unsigned int minor)
 {
-	int r = 0;
+	int r, m;
 
 	if (minor > (1 << MINORBITS))
 		return -EINVAL;
 
 	down(&_minor_lock);
-	if (minor >= _max_minors) {
-		r = realloc_minor_bits(minor);
-		if (r) {
-			up(&_minor_lock);
-			return r;
-		}
+
+	if (idr_find(&_minor_idr, minor)) {
+		r = -EBUSY;
+		goto out;
 	}
 
-	if (test_and_set_bit(minor, _minor_bits))
+	r = idr_pre_get(&_minor_idr, GFP_KERNEL);
+	if (!r) {
+		r = -ENOMEM;
+		goto out;
+	}
+
+	r = idr_get_new_above(&_minor_idr, specific_minor, minor, &m);
+	if (r) {
+		goto out;
+	}
+
+	if (m != minor) {
+		idr_remove(&_minor_idr, m);
 		r = -EBUSY;
-	up(&_minor_lock);
+		goto out;
+	}
 
+out:
+	up(&_minor_lock);
 	return r;
 }
 
@@ -712,21 +670,29 @@
 	unsigned int m;
 
 	down(&_minor_lock);
-	m = find_first_zero_bit(_minor_bits, _max_minors);
-	if (m >= _max_minors) {
-		r = realloc_minor_bits(_max_minors * 2);
-		if (r) {
-			up(&_minor_lock);
-			return r;
-		}
-		m = find_first_zero_bit(_minor_bits, _max_minors);
+
+	r = idr_pre_get(&_minor_idr, GFP_KERNEL);
+	if (!r) {
+		r = -ENOMEM;
+		goto out;
+	}
+
+	r = idr_get_new(&_minor_idr, next_free_minor, &m);
+	if (r) {
+		goto out;
+	}
+
+	if (m > (1 << MINORBITS)) {
+		idr_remove(&_minor_idr, m);
+		r = -ENOSPC;
+		goto out;
 	}
 
-	set_bit(m, _minor_bits);
 	*minor = m;
-	up(&_minor_lock);
 
-	return 0;
+out:
+	up(&_minor_lock);
+	return r;
 }
 
 static struct block_device_operations dm_blk_dops;