toaruos/kernel/core/alloc.c

/*
 * Kevin Lange's Slab Allocator
 *
 * Implemented for CS241, Fall 2010, machine problem 7
 * at the University of Illinois, Urbana-Champaign.
 *
 * Overall competition winner for speed.
 * Well ranked in memory usage.
 *
 * Copyright (c) 2010 Kevin Lange.  All rights reserved.
 *
 * Developed by: Kevin Lange <lange7@acm.uiuc.edu>
 *               Dave Majnemer <dmajnem2@acm.uiuc.edu>
 *               Assocation for Computing Machinery
 *               University of Illinois, Urbana-Champaign
 *               http://acm.uiuc.edu
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to
 * deal with the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *   1. Redistributions of source code must retain the above copyright notice,
 *      this list of conditions and the following disclaimers.
 *   2. Redistributions in binary form must reproduce the above copyright
 *      notice, this list of conditions and the following disclaimers in the
 *      documentation and/or other materials provided with the distribution.
 *   3. Neither the names of the Association for Computing Machinery, the
 *      University of Illinois, nor the names of its contributors may be used
 *      to endorse or promote products derived from this Software without
 *      specific prior written permission.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
 * CONTRIBUTORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * WITH THE SOFTWARE.
 *
 * ##########
 * # README #
 * ##########
 *
 * About the slab allocator
 * """"""""""""""""""""""""
 *
 * This is a simple implementation of a "slab" allocator. It works by operating
 * on "bins" of items of predefined sizes and a set of pseudo-bins of any size.
 * When a new allocation request is made, the allocator determines if it will
 * fit in an existing bin. If there are no bins of the correct size for a given
 * allocation request, the allocator will make a bin and add it to a(n empty)
 * list of available bins of that size. In this implementation, we use sizes
 * from 4 bytes (32 bit) or 8 bytes (64-bit) to 2KB for bins, fitting a 4K page
 * size. The implementation allows the number of pages in a single bin to be
 * increased, as well as allowing for changing the size of page (though this
 * should, for the most part, remain 4KB under any modern system).
 *
 * Special thanks
 * """"""""""""""
 *
 * I would like to thank Dave Majnemer, who I have credited above as a
 * contributor, for his assistance. Without Dave, klmalloc would be a mash
 * up of bits of forward movement in no discernible pattern. Dave helped
 * me ensure that I could build a proper slab allocator and has consantly
 * derided me for not fixing the bugs and to-do items listed in the last
 * section of this readme.
 *
 * GCC Function Attributes
 * """""""""""""""""""""""
 *
 * A couple of GCC function attributes, designated by the __attribute__
 * directive, are used in this code to streamline optimization.
 * I've chosen to include a brief overview of the particular attributes
 * I am making use of:
 *
 * - malloc:
 *   Tells gcc that a given function is a memory allocator
 *   and that non-NULL values it returns should never be
 *   associated with other chunks of memory. We use this for
 *   alloc, realloc and calloc, as is requested in the gcc
 *   documentation for the attribute.
 *
 * - always_inline:
 *   Tells gcc to always inline the given code, regardless of the
 *   optmization level. Small functions that would be noticeably
 *   slower with the overhead of paramter handling are given
 *   this attribute.
 *
 * - pure:
 *   Tells gcc that a function only uses inputs and its output.
 *
 * Things to work on
 * """""""""""""""""
 *
 * TODO: Try to be more consistent on comment widths...
 * FIXME: Make thread safe! Not necessary for competition, but would be nice.
 * FIXME: Splitting/coalescing is broken. Fix this ASAP!
 *
**/

/*
 * Disable assertions when not in debug mode,
 * as this saves a lot of time.
 */
#if !defined(DEBUG)
#define NDEBUG
#endif

#define _XOPEN_SOURCE 700

/* Includes {{{ */
#include <system.h>
/* }}} */
/* Definitions {{{ */

/*
 * Defines for often-used integral values
 * related to our binning and paging strategy.
 */
#define NUM_BINS 11U								/* Number of bins, total, under 32-bit. */
#define SMALLEST_BIN_LOG 2U							/* Logarithm base two of the smallest bin: log_2(sizeof(int32)). */
#define BIG_BIN (NUM_BINS - 1)						/* Index for the big bin, (NUM_BINS - 1) */
#define SMALLEST_BIN (1UL << SMALLEST_BIN_LOG)		/* Size of the smallest bin. */

#define PAGE_SIZE 0x1000							/* Size of a page (in bytes), should be 4KB */
#define PAGE_MASK (PAGE_SIZE - 1)					/* Block mask, size of a page * number of pages - 1. */
#define SKIP_P INT32_MAX							/* INT32_MAX is half of UINT32_MAX; this gives us a 50% marker for skip lists. */
#define SKIP_MAX_LEVEL 6							/* We have a maximum of 6 levels in our skip lists. */

/* }}} */

/*
 * Internal functions.
 */
static void * __attribute__ ((malloc)) klmalloc(size_t size);
static void * __attribute__ ((malloc)) klrealloc(void * ptr, size_t size);
static void * __attribute__ ((malloc)) klcalloc(size_t nmemb, size_t size);
static void klfree(void * ptr);

void * __attribute__ ((malloc)) malloc(size_t size) {
	return klmalloc(size);
}

void * __attribute__ ((malloc)) realloc(void * ptr, size_t size) {
	return klrealloc(ptr, size);
}

void * __attribute__ ((malloc)) calloc(size_t nmemb, size_t size) {
	return klcalloc(nmemb, size);
}

void free(void * ptr) {
	klfree(ptr);
}


/* Bin management {{{ */

/*
 * Adjust bin size in bin_size call to proper bounds.
 */
static size_t  __attribute__ ((always_inline, pure)) klmalloc_adjust_bin(size_t bin)
{
	if (bin <= (size_t)SMALLEST_BIN_LOG)
	{
		return 0;
	}
	bin -= SMALLEST_BIN_LOG + 1;
	if (bin > (size_t)BIG_BIN) {
		return BIG_BIN;
	}
	return bin;
}

/*
 * Given a size value, find the correct bin
 * to place the requested allocation in.
 */
static size_t __attribute__ ((always_inline, pure)) klmalloc_bin_size(size_t size) {
	size_t bin = sizeof(size) * CHAR_BIT - __builtin_clzl(size);
	bin += !!(size & (size - 1));
	return klmalloc_adjust_bin(bin);
}

/*
 * Bin header - One page of memory.
 * Appears at the front of a bin to point to the
 * previous bin (or NULL if the first), the next bin
 * (or NULL if the last) and the head of the bin, which
 * is a stack of cells of data.
 */
typedef struct _klmalloc_bin_header {
	struct _klmalloc_bin_header *  next;	/* Pointer to the next node. */
	void * head;							/* Head of this bin. */
	size_t size;							/* Size of this bin, if big; otherwise bin index. */
} klmalloc_bin_header;

/*
 * A big bin header is basically the same as a regular bin header
 * only with a pointer to the previous (physically) instead of
 * a "next" and with a list of forward headers.
 */
typedef struct _klmalloc_big_bin_header {
	struct _klmalloc_big_bin_header * next;
	void * head;
	size_t size;
	struct _klmalloc_big_bin_header * prev;
	struct _klmalloc_big_bin_header * forward[SKIP_MAX_LEVEL+1];
} klmalloc_big_bin_header;


/*
 * List of pages in a bin.
 */
typedef struct _klmalloc_bin_header_head {
	klmalloc_bin_header * first;
} klmalloc_bin_header_head;

/*
 * Array of available bins.
 */
static klmalloc_bin_header_head klmalloc_bin_head[NUM_BINS - 1];	/* Small bins */
static struct _klmalloc_big_bins {
	klmalloc_big_bin_header head;
	int level;
} klmalloc_big_bins;
static klmalloc_big_bin_header * klmalloc_newest_big = NULL;		/* Newest big bin */

/* }}} Bin management */
/* Doubly-Linked List {{{ */

/*
 * Remove an entry from a page list.
 * Decouples the element from its
 * position in the list by linking
 * its neighbors to eachother.
 */
static void __attribute__ ((always_inline)) klmalloc_list_decouple(klmalloc_bin_header_head *head, klmalloc_bin_header *node) {
	klmalloc_bin_header *next	= node->next;
	head->first = next;
	node->next = NULL;
}

/*
 * Insert an entry into a page list.
 * The new entry is placed at the front
 * of the list and the existing border
 * elements are updated to point back
 * to it (our list is doubly linked).
 */
static void __attribute__ ((always_inline)) klmalloc_list_insert(klmalloc_bin_header_head *head, klmalloc_bin_header *node) {
	node->next = head->first;
	head->first = node;
}

/*
 * Get the head of a page list.
 * Because redundant function calls
 * are really great, and just in case
 * we change the list implementation.
 */
static klmalloc_bin_header * __attribute__ ((always_inline)) klmalloc_list_head(klmalloc_bin_header_head *head) {
	return head->first;
}

/* }}} Lists */
/* Skip List {{{ */

/*
 * Skip lists are efficient
 * data structures for storing
 * and searching ordered data.
 *
 * Here, the skip lists are used
 * to keep track of big bins.
 */

/*
 * Generate a random value in an appropriate range.
 * This is a xor-shift RNG.
 */
static uint32_t __attribute__ ((pure)) klmalloc_skip_rand() {
	static uint32_t x = 123456789;
	static uint32_t y = 362436069;
	static uint32_t z = 521288629;
	static uint32_t w = 88675123;

	uint32_t t;

	t = x ^ (x << 11);
	x = y; y = z; z = w;
	return w = w ^ (w >> 19) ^ t ^ (t >> 8);
}

/*
 * Generate a random level for a skip node
 */
static int __attribute__ ((pure, always_inline)) klmalloc_random_level() {
	int level = 0;
	/*
	 * Keep trying to check rand() against 50% of its maximum.
	 * This provides 50%, 25%, 12.5%, etc. chance for each level.
	 */
	while (klmalloc_skip_rand() < SKIP_P && level < SKIP_MAX_LEVEL) {
		++level;
	}
	return level;
}

/*
 * Find best fit for a given value.
 */
static klmalloc_big_bin_header * klmalloc_skip_list_findbest(size_t search_size) {
	klmalloc_big_bin_header * node = &klmalloc_big_bins.head;
	/*
	 * Loop through the skip list until we hit something > our search value.
	 */
	int i;
	for (i = klmalloc_big_bins.level; i >= 0; --i) {
		while (node->forward[i] && (node->forward[i]->size < search_size)) {
			node = node->forward[i];
			if (node)
				assert((node->size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
		}
	}
	/*
	 * This value will either be NULL (we found nothing)
	 * or a node (we found a minimum fit).
	 */
	node = node->forward[0];
	if (node) {
		assert((uintptr_t)node % PAGE_SIZE == 0);
		assert((node->size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
	}
	return node;
}

/*
 * Insert a header into the skip list.
 */
static void klmalloc_skip_list_insert(klmalloc_big_bin_header * value) {
	/*
	 * You better be giving me something valid to insert,
	 * or I will slit your ****ing throat.
	 */
	assert(value != NULL);
	assert(value->head != NULL);
	assert((uintptr_t)value % PAGE_SIZE == 0);
	assert((value->size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
	assert(value->size != 0);

	/*
	 * Starting from the head node of the bin locator...
	 */
	klmalloc_big_bin_header * node = &klmalloc_big_bins.head;
	klmalloc_big_bin_header * update[SKIP_MAX_LEVEL + 1];

	/*
	 * Loop through the skiplist to find the right place
	 * to insert the node (where ->forward[] > value)
	 */
	int i;
	for (i = klmalloc_big_bins.level; i >= 0; --i) {
		while (node->forward[i] && node->forward[i]->size < value->size) {
			node = node->forward[i];
			if (node)
				assert((node->size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
		}
		update[i] = node;
	}
	node = node->forward[0];

	/*
	 * Make the new skip node and update
	 * the forward values.
	 */
	if (node != value) {
		int level = klmalloc_random_level();
		/*
		 * Get all of the nodes before this.
		 */
		if (level > klmalloc_big_bins.level) {
			for (i = klmalloc_big_bins.level + 1; i <= level; ++i) {
				update[i] = &klmalloc_big_bins.head;
			}
			klmalloc_big_bins.level = level;
		}

		/*
		 * Make the new node.
		 */
		node = value;

		/*
		 * Run through and point the preceeding nodes
		 * for each level to the new node.
		 */
		for (i = 0; i <= level; ++i) {
			node->forward[i] = update[i]->forward[i];
			if (node->forward[i])
				assert((node->forward[i]->size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
			update[i]->forward[i] = node;
		}
	}
}

/*
 * Delete a header from the skip list.
 * Be sure you didn't change the size, or we won't be able to find it.
 */
static void klmalloc_skip_list_delete(klmalloc_big_bin_header * value) {
	/*
	 * Debug assertions
	 */
	assert(value != NULL);
	assert(value->head);

	/*
	 * Starting from the bin header, again...
	 */
	klmalloc_big_bin_header * node = &klmalloc_big_bins.head;
	klmalloc_big_bin_header * update[SKIP_MAX_LEVEL + 1];

	/*
	 * Find the node.
	 */
	int i;
	for (i = klmalloc_big_bins.level; i >= 0; --i) {
		while (node->forward[i] && node->forward[i]->size < value->size) {
			node = node->forward[i];
			if (node)
				assert((node->size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
		}
		update[i] = node;
	}
	node = node->forward[0];
	while (node != value) {
		node = node->forward[0];
	}

	if (node != value) {
		node = klmalloc_big_bins.head.forward[0];
		while (node->forward[0] && node->forward[0] != value) {
			node = node->forward[0];
		}
		node = node->forward[0];
	}
	/*
	 * If we found the node, delete it;
	 * otherwise, we do nothing.
	 */
	if (node == value) {
		for (i = 0; i <= klmalloc_big_bins.level; ++i) {
			if (update[i]->forward[i] != node) {
				break;
			}
			update[i]->forward[i] = node->forward[i];
			if (update[i]->forward[i]) {
				assert((uintptr_t)(update[i]->forward[i]) % PAGE_SIZE == 0);
				assert((update[i]->forward[i]->size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
			}
		}

		while (klmalloc_big_bins.level > 0 && klmalloc_big_bins.head.forward[klmalloc_big_bins.level] == NULL) {
			--klmalloc_big_bins.level;
		}
	}
}

/* }}} */
/* Stack {{{ */
/*
 * Pop an item from a block.
 * Free space is stored as a stack,
 * so we get a free space for a bin
 * by popping a free node from the
 * top of the stack.
 */
static void * klmalloc_stack_pop(klmalloc_bin_header *header) {
	assert(header);
	assert(header->head != NULL);

	/*
	 * Remove the current head and point
	 * the head to where the old head pointed.
	 */
	void *item = header->head;
	size_t **head = header->head;
	size_t *next = *head;
	header->head = next;
	return item;
}

/*
 * Push an item into a block.
 * When we free memory, we need
 * to add the freed cell back
 * into the stack of free spaces
 * for the block.
 */
static void klmalloc_stack_push(klmalloc_bin_header *header, void *ptr) {
	assert(ptr != NULL);
	size_t **item = (size_t **)ptr;
	*item = (size_t *)header->head;
	header->head = item;
}

/*
 * Is this cell stack empty?
 * If the head of the stack points
 * to NULL, we have exhausted the
 * stack, so there is no more free
 * space available in the block.
 */
static int __attribute__ ((always_inline)) klmalloc_stack_empty(klmalloc_bin_header *header) {
	return header->head == NULL;
}

/* }}} Stack */

/* malloc() {{{ */
static void * __attribute__ ((malloc)) klmalloc(size_t size) {
	/*
	 * C standard implementation:
	 * If size is zero, we can choose do a number of things.
	 * This implementation will return a NULL pointer.
	 */
	if (__builtin_expect(size == 0, 0))
		return NULL;

	/*
	 * Find the appropriate bin for the requested
	 * allocation and start looking through that list.
	 */
	unsigned int bucket_id = klmalloc_bin_size(size);

	if (bucket_id < BIG_BIN) {
		/*
		 * Small bins.
		 */
		klmalloc_bin_header * bin_header = klmalloc_list_head(&klmalloc_bin_head[bucket_id]);
		if (!bin_header) {
			/*
			 * Grow the heap for the new bin.
			 */
			bin_header = (klmalloc_bin_header*)sbrk(PAGE_SIZE);
			assert((uintptr_t)bin_header % PAGE_SIZE == 0);

			/*
			 * Set the head of the stack.
			 */
			bin_header->head = (void*)((uintptr_t)bin_header + sizeof(klmalloc_bin_header));
			/*
			 * Insert the new bin at the front of
			 * the list of bins for this size.
			 */
			klmalloc_list_insert(&klmalloc_bin_head[bucket_id], bin_header);
			/*
			 * Initialize the stack inside the bin.
			 * The stack is initially full, with each
			 * entry pointing to the next until the end
			 * which points to NULL.
			 */
			size_t adj = SMALLEST_BIN_LOG + bucket_id;
			size_t i, available = ((PAGE_SIZE - sizeof(klmalloc_bin_header)) >> adj) - 1;

			size_t **base = bin_header->head;
			for (i = 0; i < available; ++i) {
				/*
				 * Our available memory is made into a stack, with each
				 * piece of memory turned into a pointer to the next
				 * available piece. When we want to get a new piece
				 * of memory from this block, we just pop off a free
				 * spot and give its address.
				 */
				base[i << bucket_id] = (size_t *)&base[(i + 1) << bucket_id];
			}
			base[available << bucket_id] = NULL;
			bin_header->size = bucket_id;
		}
		size_t ** item = klmalloc_stack_pop(bin_header);
		if (klmalloc_stack_empty(bin_header)) {
			klmalloc_list_decouple(&(klmalloc_bin_head[bucket_id]),bin_header);
		}
		return item;
	} else {
		/*
		 * Big bins.
		 */
		klmalloc_big_bin_header * bin_header = klmalloc_skip_list_findbest(size);
		if (bin_header) {
			/*
			 * If we found one, delete it from the skip list
			 */
			klmalloc_skip_list_delete(bin_header);
			/*
			 * Retreive the head of the block.
			 */
			size_t ** item = klmalloc_stack_pop((klmalloc_bin_header *)bin_header);
#if 0
			/*
			 * Resize block, if necessary
			 */
			assert(bin_header->head == NULL);
			size_t old_size = bin_header->size;
			//size_t rsize = size;
			/*
			 * Round the requeste size to our full required size.
			 */
			size = ((size + sizeof(klmalloc_big_bin_header)) / PAGE_SIZE + 1) * PAGE_SIZE - sizeof(klmalloc_big_bin_header);
			assert((size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
			if (bin_header->size > size * 2) {
				assert(old_size != size);
				/*
				 * If we have extra space, start splitting.
				 */
				bin_header->size = size;
				assert(sbrk(0) >= bin_header->size + (uintptr_t)bin_header);
				/*
				 * Make a new block at the end of the needed space.
				 */
				klmalloc_big_bin_header * header_new = (klmalloc_big_bin_header *)((uintptr_t)bin_header + sizeof(klmalloc_big_bin_header) + size);
				assert((uintptr_t)header_new % PAGE_SIZE == 0);
				memset(header_new, 0, sizeof(klmalloc_big_bin_header) + sizeof(void *));
				header_new->prev = bin_header;
				if (bin_header->next) {
					bin_header->next->prev = header_new;
				}
				header_new->next = bin_header->next;
				bin_header->next = header_new;
				if (klmalloc_newest_big == bin_header) {
					klmalloc_newest_big = header_new;
				}
				header_new->size = old_size - (size + sizeof(klmalloc_big_bin_header));
				assert(((uintptr_t)header_new->size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
				fprintf(stderr, "Splitting %p [now %zx] at %p [%zx] from [%zx,%zx].\n", (void*)bin_header, bin_header->size, (void*)header_new, header_new->size, old_size, size);
				/*
				 * Free the new block.
				 */
				klfree((void *)((uintptr_t)header_new + sizeof(klmalloc_big_bin_header)));
			}
#endif
			return item;
		} else {
			/*
			 * Round requested size to a set of pages, plus the header size.
			 */
			size_t pages = (size + sizeof(klmalloc_big_bin_header)) / PAGE_SIZE + 1;
			bin_header = (klmalloc_big_bin_header*)sbrk(PAGE_SIZE * pages);
			assert((uintptr_t)bin_header % PAGE_SIZE == 0);
			/*
			 * Give the header the remaining space.
			 */
			bin_header->size = pages * PAGE_SIZE - sizeof(klmalloc_big_bin_header);
			assert((bin_header->size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
			/*
			 * Link the block in physical memory.
			 */
			bin_header->prev = klmalloc_newest_big;
			if (bin_header->prev) {
				bin_header->prev->next = bin_header;
			}
			klmalloc_newest_big = bin_header;
			bin_header->next = NULL;
			/*
			 * Return the head of the block.
			 */
			bin_header->head = NULL;
			return (void*)((uintptr_t)bin_header + sizeof(klmalloc_big_bin_header));
		}
	}
}
/* }}} */
/* free() {{{ */
static void klfree(void *ptr) {
	/*
	 * C standard implementation: Do nothing when NULL is passed to free.
	 */
	if (__builtin_expect(ptr == NULL, 0)) {
		return;
	}

	/*
	 * Get our pointer to the head of this block by
	 * page aligning it.
	 */
	klmalloc_bin_header * header = (klmalloc_bin_header *)((uintptr_t)ptr & (size_t)~PAGE_MASK);
	assert((uintptr_t)header % PAGE_SIZE == 0);

	/*
	 * For small bins, the bin number is stored in the size
	 * field of the header. For large bins, the actual size
	 * available in the bin is stored in this field. It's
	 * easy to tell which is which, though.
	 */
	size_t bucket_id = header->size;
	if (bucket_id > (size_t)NUM_BINS) {
		bucket_id = BIG_BIN;
		klmalloc_big_bin_header *bheader = (klmalloc_big_bin_header*)header;
		
		assert(bheader);
		assert(bheader->head == NULL);
		assert((bheader->size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
		/*
		 * Coalesce forward blocks into us.
		 */
#if 0
		if (bheader != klmalloc_newest_big) {
			/*
			 * If we are not the newest big bin, there is most definitely
			 * something in front of us that we can read.
			 */
			assert((bheader->size + sizeof(klmalloc_big_bin_header)) % PAGE_SIZE == 0);
			klmalloc_big_bin_header * next = (void *)((uintptr_t)bheader + sizeof(klmalloc_big_bin_header) + bheader->size);
			assert((uintptr_t)next % PAGE_SIZE == 0);
			if (next == bheader->next && next->head) { //next->size > NUM_BINS && next->head) {
				/*
				 * If that something is an available big bin, we can
				 * coalesce it into us to form one larger bin.
				 */

				// XXX
				size_t old_size = bheader->size;
				// XXX

				klmalloc_skip_list_delete(next);
				bheader->size = (size_t)bheader->size + (size_t)sizeof(klmalloc_big_bin_header) + next->size;
				assert((bheader->size + sizeof(klmalloc_big_bin_header))  % PAGE_SIZE == 0);

				if (next == klmalloc_newest_big) {
					/*
					 * If the guy in front of us was the newest,
					 * we are now the newest (as we are him).
					 */
					klmalloc_newest_big = bheader;
				} else {
					if (next->next) {
						next->next->prev = bheader;
					}
				}
				fprintf(stderr,"Coelesced (forwards)  %p [%zx] <- %p [%zx] = %zx\n", (void*)bheader, old_size, (void*)next, next->size, bheader->size);
			}
		}
#endif
		/*
		 * Coalesce backwards
		 */
#if 0
		if (bheader->prev && bheader->prev->head) {
			/*
			 * If there is something behind us, it is available, and there is nothing between
			 * it and us, we can coalesce ourselves into it to form a big block.
			 */
			if ((uintptr_t)bheader->prev + (bheader->prev->size + sizeof(klmalloc_big_bin_header)) == (uintptr_t)bheader) {

				size_t old_size = bheader->prev->size;

				klmalloc_skip_list_delete(bheader->prev);
				bheader->prev->size = (size_t)bheader->prev->size + (size_t)bheader->size + sizeof(klmalloc_big_bin_header);
				assert((bheader->prev->size + sizeof(klmalloc_big_bin_header))  % PAGE_SIZE == 0);
				klmalloc_skip_list_insert(bheader->prev);
				if (klmalloc_newest_big == bheader) {
					klmalloc_newest_big = bheader->prev;
				} else {
					if (bheader->next) {
						bheader->next->prev = bheader->prev;
					}
				}
				fprintf(stderr,"Coelesced (backwards) %p [%zx] <- %p [%zx] = %zx\n", (void*)bheader->prev, old_size, (void*)bheader, bheader->size, bheader->size);
				/*
				 * If we coalesced backwards, we are done.
				 */
				return;
			}
		}
#endif
		/*
		 * Push new space back into the stack.
		 */
		klmalloc_stack_push((klmalloc_bin_header *)bheader, (void *)((uintptr_t)bheader + sizeof(klmalloc_big_bin_header)));
		assert(bheader->head != NULL);
		/*
		 * Insert the block into list of available slabs.
		 */
		klmalloc_skip_list_insert(bheader);
	} else {
		/*
		 * If the stack is empty, we are freeing
		 * a block from a previously full bin.
		 * Return it to the busy bins list.
		 */
		if (klmalloc_stack_empty(header)) {
			klmalloc_list_insert(&klmalloc_bin_head[bucket_id], header);
		}
		/*
		 * Push new space back into the stack.
		 */
		klmalloc_stack_push(header, ptr);
	}

}
/* }}} */
/* realloc() {{{ */
static void * __attribute__ ((malloc)) klrealloc(void *ptr, size_t size) {
	/*
	 * C standard implementation: When NULL is passed to realloc,
	 * simply malloc the requested size and return a pointer to that.
	 */
	if (__builtin_expect(ptr == NULL, 0))
		return malloc(size);

	/*
	 * C standard implementation: For a size of zero, free the
	 * pointer and return NULL, allocating no new memory.
	 */
	if (__builtin_expect(size == 0, 0))
	{
		free(ptr);
		return NULL;
	}

	/*
	 * Find the bin for the given pointer
	 * by aligning it to a page.
	 */
	klmalloc_bin_header * header_old = (void *)((uintptr_t)ptr & (size_t)~PAGE_MASK);


	/*
	 * (This will only happen for a big bin, mathematically speaking)
	 * If we still have room in our bin for the additonal space,
	 * we don't need to do anything.
	 */
	if (header_old->size >= size) {

		/*
		 * TODO: Break apart blocks here, which is far more important
		 *       than breaking them up on allocations.
		 */
		return ptr;
	}

	/*
	 * Reallocate more memory.
	 */
	void * newptr = klmalloc(size);
	if (__builtin_expect(newptr != NULL, 1)) {
		size_t old_size = header_old->size;
		if (old_size < (size_t)BIG_BIN) {
			/*
			 * If we are copying from a small bin,
			 * we need to get the size of the bin
			 * from its id.
			 */
			old_size = (1UL << (SMALLEST_BIN_LOG + old_size));
		}

		/*
		 * Copy the old value into the new value.
		 * Be sure to only copy as much as was in
		 * the old block.
		 */
		memcpy(newptr, ptr, old_size);
		klfree(ptr);
		return newptr;
	}

	/*
	 * We failed to allocate more memory,
	 * which means we're probably out.
	 *
	 * Bail and return NULL.
	 */
	return NULL;
}
/* }}} */
/* calloc() {{{ */
static void * __attribute__ ((malloc)) klcalloc(size_t nmemb, size_t size) {
	/*
	 * Allocate memory and zero it before returning
	 * a pointer to the newly allocated memory.
	 * 
	 * Implemented by way of a simple malloc followed
	 * by a memset to 0x00 across the length of the
	 * requested memory chunk.
	 */

	void *ptr = klmalloc(nmemb * size);
	if (__builtin_expect(ptr != NULL, 1))
		memset(ptr,0x00,nmemb * size);
	return ptr;
}
/* }}} */