lib/alloc_page.c - kvm-unit-tests - Git at Google

 /*
  * This work is licensed under the terms of the GNU LGPL, version 2.
  *
  * This is a simple allocator that provides contiguous physical addresses
  * with page granularity.
  */
 #include "libcflat.h"
 #include "alloc.h"
 #include "alloc_phys.h"
 #include "alloc_page.h"
 #include "bitops.h"
 #include "list.h"
 #include <asm/page.h>
 #include <asm/io.h>
 #include <asm/spinlock.h>
 #include <asm/memory_areas.h>

 #define IS_ALIGNED_ORDER(x,order) IS_ALIGNED((x),BIT_ULL(order))
 #define NLISTS ((BITS_PER_LONG) - (PAGE_SHIFT))
 #define PFN(x) ((uintptr_t)(x) >> PAGE_SHIFT)

 #define MAX_AREAS	6

 #define ORDER_MASK	0x3f
 #define ALLOC_MASK	0x40

 struct mem_area {
 	/* Physical frame number of the first usable frame in the area */
 	uintptr_t base;
 	/* Physical frame number of the first frame outside the area */
 	uintptr_t top;
 	/* Combination ALLOC_MASK and order */
 	u8 *page_states;
 	/* One freelist for each possible block size, up to NLISTS */
 	struct linked_list freelists[NLISTS];
 };

 static struct mem_area areas[MAX_AREAS];
 static unsigned int areas_mask;
 static struct spinlock lock;

 bool page_alloc_initialized(void)
 {
 	return areas_mask != 0;
 }

 static inline bool area_or_metadata_contains(struct mem_area *a, uintptr_t pfn)
 {
 	return (pfn >= PFN(a->page_states)) && (pfn < a->top);
 }

 static inline bool area_contains(struct mem_area *a, uintptr_t pfn)
 {
 	return (pfn >= a->base) && (pfn < a->top);
 }

 /*
  * Splits the free block starting at addr into 2 blocks of half the size.
  *
  * The function depends on the following assumptions:
  * - The allocator must have been initialized
  * - the block must be within the memory area
  * - all pages in the block must be free and not special
  * - the pointer must point to the start of the block
  * - all pages in the block must have the same block size.
  * - the block size must be greater than 0
  * - the block size must be smaller than the maximum allowed
  * - the block must be in a free list
  * - the function is called with the lock held
  */
 static void split(struct mem_area *a, void *addr)
 {
 	uintptr_t pfn = PFN(addr);
 	struct linked_list *p;
 	uintptr_t i, idx;
 	u8 order;

 	assert(a && area_contains(a, pfn));
 	idx = pfn - a->base;
 	order = a->page_states[idx];
 	assert(!(order & ~ORDER_MASK) && order && (order < NLISTS));
 	assert(IS_ALIGNED_ORDER(pfn, order));
 	assert(area_contains(a, pfn + BIT(order) - 1));

 	/* Remove the block from its free list */
 	p = list_remove(addr);
 	assert(p);

 	/* update the block size for each page in the block */
 	for (i = 0; i < BIT(order); i++) {
 		assert(a->page_states[idx + i] == order);
 		a->page_states[idx + i] = order - 1;
 	}
 	order--;
 	/* add the first half block to the appropriate free list */
 	list_add(a->freelists + order, p);
 	/* add the second half block to the appropriate free list */
 	list_add(a->freelists + order, (void *)((pfn + BIT(order)) * PAGE_SIZE));
 }

 /*
  * Returns a block whose alignment and size are at least the parameter values.
  * If there is not enough free memory, NULL is returned.
  *
  * Both parameters must be not larger than the largest allowed order
  */
 static void *page_memalign_order(struct mem_area *a, u8 al, u8 sz)
 {
 	struct linked_list *p, *res = NULL;
 	u8 order;

 	assert((al < NLISTS) && (sz < NLISTS));
 	/* we need the bigger of the two as starting point */
 	order = sz > al ? sz : al;

 	/* search all free lists for some memory */
 	for ( ; order < NLISTS; order++) {
 		p = a->freelists[order].next;
 		if (!is_list_empty(p))
 			break;
 	}
 	/* out of memory */
 	if (order >= NLISTS)
 		return NULL;

 	/*
 	 * the block is bigger than what we need because either there were
 	 * no smaller blocks, or the smaller blocks were not aligned to our
 	 * needs; therefore we split the block until we reach the needed size
 	 */
 	for (; order > sz; order--)
 		split(a, p);

 	res = list_remove(p);
 	memset(a->page_states + (PFN(res) - a->base), ALLOC_MASK | order, BIT(order));
 	return res;
 }

 /*
  * Try to merge two blocks into a bigger one.
  * Returns true in case of a successful merge.
  * Merging will succeed only if both blocks have the same block size and are
  * both free.
  *
  * The function depends on the following assumptions:
  * - the first parameter is strictly smaller than the second
  * - the parameters must point each to the start of their block
  * - the two parameters point to adjacent blocks
  * - the two blocks are both in a free list
  * - all of the pages of the two blocks must be free
  * - all of the pages of the two blocks must have the same block size
  * - the function is called with the lock held
  */
 static bool coalesce(struct mem_area *a, u8 order, uintptr_t pfn, uintptr_t pfn2)
 {
 	uintptr_t first, second, i;
 	struct linked_list *li;

 	assert(IS_ALIGNED_ORDER(pfn, order) && IS_ALIGNED_ORDER(pfn2, order));
 	assert(pfn2 == pfn + BIT(order));
 	assert(a);

 	/* attempting to coalesce two blocks that belong to different areas */
 	if (!area_contains(a, pfn) || !area_contains(a, pfn2 + BIT(order) - 1))
 		return false;
 	first = pfn - a->base;
 	second = pfn2 - a->base;
 	/* the two blocks have different sizes, cannot coalesce */
 	if ((a->page_states[first] != order) || (a->page_states[second] != order))
 		return false;

 	/* we can coalesce, remove both blocks from their freelists */
 	li = list_remove((void *)(pfn2 << PAGE_SHIFT));
 	assert(li);
 	li = list_remove((void *)(pfn << PAGE_SHIFT));
 	assert(li);
 	/* check the metadata entries and update with the new size */
 	for (i = 0; i < (2ull << order); i++) {
 		assert(a->page_states[first + i] == order);
 		a->page_states[first + i] = order + 1;
 	}
 	/* finally add the newly coalesced block to the appropriate freelist */
 	list_add(a->freelists + order + 1, li);
 	return true;
 }

 /*
  * Free a block of memory.
  * The parameter can be NULL, in which case nothing happens.
  *
  * The function depends on the following assumptions:
  * - the parameter is page aligned
  * - the parameter belongs to an existing memory area
  * - the parameter points to the beginning of the block
  * - the size of the block is less than the maximum allowed
  * - the block is completely contained in its memory area
  * - all pages in the block have the same block size
  * - no pages in the memory block were already free
  * - no pages in the memory block are special
  */
 static void _free_pages(void *mem)
 {
 	uintptr_t pfn2, pfn = PFN(mem);
 	struct mem_area *a = NULL;
 	uintptr_t i, p;
 	u8 order;

 	if (!mem)
 		return;
 	assert(IS_ALIGNED((uintptr_t)mem, PAGE_SIZE));

 	/* find which area this pointer belongs to*/
 	for (i = 0; !a && (i < MAX_AREAS); i++) {
 		if ((areas_mask & BIT(i)) && area_contains(areas + i, pfn))
 			a = areas + i;
 	}
 	assert_msg(a, "memory does not belong to any area: %p", mem);

 	p = pfn - a->base;
 	order = a->page_states[p] & ORDER_MASK;

 	/* ensure that the first page is allocated and not special */
 	assert(a->page_states[p] == (order | ALLOC_MASK));
 	/* ensure that the order has a sane value */
 	assert(order < NLISTS);
 	/* ensure that the block is aligned properly for its size */
 	assert(IS_ALIGNED_ORDER(pfn, order));
 	/* ensure that the area can contain the whole block */
 	assert(area_contains(a, pfn + BIT(order) - 1));

 	for (i = 0; i < BIT(order); i++) {
 		/* check that all pages of the block have consistent metadata */
 		assert(a->page_states[p + i] == (ALLOC_MASK | order));
 		/* set the page as free */
 		a->page_states[p + i] &= ~ALLOC_MASK;
 	}
 	/* provisionally add the block to the appropriate free list */
 	list_add(a->freelists + order, mem);
 	/* try to coalesce the block with neighbouring blocks if possible */
 	do {
 		/*
 		 * get the order again since it might have changed after
 		 * coalescing in a previous iteration
 		 */
 		order = a->page_states[p] & ORDER_MASK;
 		/*
 		 * let's consider this block and the next one if this block
 		 * is aligned to the next size, otherwise let's consider the
 		 * previous block and this one
 		 */
 		if (!IS_ALIGNED_ORDER(pfn, order + 1))
 			pfn = pfn - BIT(order);
 		pfn2 = pfn + BIT(order);
 		/* repeat as long as we manage to coalesce something */
 	} while (coalesce(a, order, pfn, pfn2));
 }

 void free_pages(void *mem)
 {
 	spin_lock(&lock);
 	_free_pages(mem);
 	spin_unlock(&lock);
 }

 static void *page_memalign_order_area(unsigned area, u8 ord, u8 al)
 {
 	void *res = NULL;
 	int i;

 	spin_lock(&lock);
 	area &= areas_mask;
 	for (i = 0; !res && (i < MAX_AREAS); i++)
 		if (area & BIT(i))
 			res = page_memalign_order(areas + i, ord, al);
 	spin_unlock(&lock);
 	return res;
 }

 /*
  * Allocates (1 << order) physically contiguous and naturally aligned pages.
  * Returns NULL if the allocation was not possible.
  */
 void *alloc_pages_area(unsigned int area, unsigned int order)
 {
 	return page_memalign_order_area(area, order, order);
 }

 void *alloc_pages(unsigned int order)
 {
 	return alloc_pages_area(AREA_ANY, order);
 }

 /*
  * Allocates (1 << order) physically contiguous aligned pages.
  * Returns NULL if the allocation was not possible.
  */
 void *memalign_pages_area(unsigned int area, size_t alignment, size_t size)
 {
 	assert(is_power_of_2(alignment));
 	alignment = get_order(PAGE_ALIGN(alignment) >> PAGE_SHIFT);
 	size = get_order(PAGE_ALIGN(size) >> PAGE_SHIFT);
 	assert(alignment < NLISTS);
 	assert(size < NLISTS);
 	return page_memalign_order_area(area, size, alignment);
 }

 void *memalign_pages(size_t alignment, size_t size)
 {
 	return memalign_pages_area(AREA_ANY, alignment, size);
 }

 /*
  * Allocates one page
  */
 void *alloc_page()
 {
 	return alloc_pages(0);
 }

 static struct alloc_ops page_alloc_ops = {
 	.memalign = memalign_pages,
 	.free = free_pages,
 	.align_min = PAGE_SIZE,
 };

 /*
  * Enables the page allocator.
  *
  * Prerequisites:
  * - at least one memory area has been initialized
  */
 void page_alloc_ops_enable(void)
 {
 	spin_lock(&lock);
 	assert(page_alloc_initialized());
 	alloc_ops = &page_alloc_ops;
 	spin_unlock(&lock);
 }

 /*
  * Adds a new memory area to the pool of available memory.
  *
  * Prerequisites:
  * - the lock is held
  * - start and top are page frame numbers
  * - start is smaller than top
  * - top does not fall outside of addressable memory
  * - there is at least one more slot free for memory areas
  * - if a specific memory area number has been indicated, it needs to be free
  * - the memory area to add does not overlap with existing areas
  * - the memory area to add has at least 5 pages available
  */
 static void _page_alloc_init_area(u8 n, uintptr_t start_pfn, uintptr_t top_pfn)
 {
 	size_t table_size, npages, i;
 	struct mem_area *a;
 	u8 order = 0;

 	/* the number must be within the allowed range */
 	assert(n < MAX_AREAS);
 	/* the new area number must be unused */
 	assert(!(areas_mask & BIT(n)));

 	/* other basic sanity checks */
 	assert(top_pfn > start_pfn);
 	assert(top_pfn - start_pfn > 4);
 	assert(top_pfn < BIT_ULL(sizeof(void *) * 8 - PAGE_SHIFT));

 	/* calculate the size of the metadata table in pages */
 	table_size = (top_pfn - start_pfn + PAGE_SIZE) / (PAGE_SIZE + 1);

 	/* fill in the values of the new area */
 	a = areas + n;
 	a->page_states = (void *)(start_pfn << PAGE_SHIFT);
 	a->base = start_pfn + table_size;
 	a->top = top_pfn;
 	npages = top_pfn - a->base;
 	assert((a->base - start_pfn) * PAGE_SIZE >= npages);

 	/* check that the new area does not overlap with any existing areas */
 	for (i = 0; i < MAX_AREAS; i++) {
 		if (!(areas_mask & BIT(i)))
 			continue;
 		assert(!area_or_metadata_contains(areas + i, start_pfn));
 		assert(!area_or_metadata_contains(areas + i, top_pfn - 1));
 		assert(!area_or_metadata_contains(a, PFN(areas[i].page_states)));
 		assert(!area_or_metadata_contains(a, areas[i].top - 1));
 	}
 	/* initialize all freelists for the new area */
 	for (i = 0; i < NLISTS; i++)
 		a->freelists[i].next = a->freelists[i].prev = a->freelists + i;

 	/* initialize the metadata for the available memory */
 	for (i = a->base; i < a->top; i += 1ull << order) {
 		/* search which order to start from */
 		while (i + BIT(order) > a->top) {
 			assert(order);
 			order--;
 		}
 		/*
 		 * we need both loops, one for the start and the other for
 		 * the end of the block, in case it spans a power of two
 		 * boundary
 		 */
 		while (IS_ALIGNED_ORDER(i, order + 1) && (i + BIT(order + 1) <= a->top))
 			order++;
 		assert(order < NLISTS);
 		/* initialize the metadata and add to the freelist */
 		memset(a->page_states + (i - a->base), order, BIT(order));
 		list_add(a->freelists + order, (void *)(i << PAGE_SHIFT));
 	}
 	/* finally mark the area as present */
 	areas_mask |= BIT(n);
 }

 static void __page_alloc_init_area(u8 n, uintptr_t cutoff, uintptr_t base_pfn, uintptr_t *top_pfn)
 {
 	if (*top_pfn > cutoff) {
 		spin_lock(&lock);
 		if (base_pfn >= cutoff) {
 			_page_alloc_init_area(n, base_pfn, *top_pfn);
 			*top_pfn = 0;
 		} else {
 			_page_alloc_init_area(n, cutoff, *top_pfn);
 			*top_pfn = cutoff;
 		}
 		spin_unlock(&lock);
 	}
 }

 /*
  * Adds a new memory area to the pool of available memory.
  *
  * Prerequisites:
  * see _page_alloc_init_area
  */
 void page_alloc_init_area(u8 n, uintptr_t base_pfn, uintptr_t top_pfn)
 {
 	if (n != AREA_ANY_NUMBER) {
 		__page_alloc_init_area(n, 0, base_pfn, &top_pfn);
 		return;
 	}
 #ifdef AREA_HIGH_PFN
 	__page_alloc_init_area(AREA_HIGH_NUMBER, AREA_HIGH_PFN), base_pfn, &top_pfn);
 #endif
 	__page_alloc_init_area(AREA_NORMAL_NUMBER, AREA_NORMAL_PFN, base_pfn, &top_pfn);
 #ifdef AREA_LOW_PFN
 	__page_alloc_init_area(AREA_LOW_NUMBER, AREA_LOW_PFN, base_pfn, &top_pfn);
 #endif
 #ifdef AREA_LOWEST_PFN
 	__page_alloc_init_area(AREA_LOWEST_NUMBER, AREA_LOWEST_PFN, base_pfn, &top_pfn);
 #endif
 }
	/*
	* This work is licensed under the terms of the GNU LGPL, version 2.
	*
	* This is a simple allocator that provides contiguous physical addresses
	* with page granularity.
	*/
	#include "libcflat.h"
	#include "alloc.h"
	#include "alloc_phys.h"
	#include "alloc_page.h"
	#include "bitops.h"
	#include "list.h"
	#include <asm/page.h>
	#include <asm/io.h>
	#include <asm/spinlock.h>
	#include <asm/memory_areas.h>

	#define IS_ALIGNED_ORDER(x,order) IS_ALIGNED((x),BIT_ULL(order))
	#define NLISTS ((BITS_PER_LONG) - (PAGE_SHIFT))
	#define PFN(x) ((uintptr_t)(x) >> PAGE_SHIFT)

	#define MAX_AREAS 6

	#define ORDER_MASK 0x3f
	#define ALLOC_MASK 0x40

	struct mem_area {
	/* Physical frame number of the first usable frame in the area */
	uintptr_t base;
	/* Physical frame number of the first frame outside the area */
	uintptr_t top;
	/* Combination ALLOC_MASK and order */
	u8 *page_states;
	/* One freelist for each possible block size, up to NLISTS */
	struct linked_list freelists[NLISTS];
	};

	static struct mem_area areas[MAX_AREAS];
	static unsigned int areas_mask;
	static struct spinlock lock;

	bool page_alloc_initialized(void)
	{
	return areas_mask != 0;
	}

	static inline bool area_or_metadata_contains(struct mem_area *a, uintptr_t pfn)
	{
	return (pfn >= PFN(a->page_states)) && (pfn < a->top);
	}

	static inline bool area_contains(struct mem_area *a, uintptr_t pfn)
	{
	return (pfn >= a->base) && (pfn < a->top);
	}

	/*
	* Splits the free block starting at addr into 2 blocks of half the size.
	*
	* The function depends on the following assumptions:
	* - The allocator must have been initialized
	* - the block must be within the memory area
	* - all pages in the block must be free and not special
	* - the pointer must point to the start of the block
	* - all pages in the block must have the same block size.
	* - the block size must be greater than 0
	* - the block size must be smaller than the maximum allowed
	* - the block must be in a free list
	* - the function is called with the lock held
	*/
	static void split(struct mem_area a, void addr)
	{
	uintptr_t pfn = PFN(addr);
	struct linked_list *p;
	uintptr_t i, idx;
	u8 order;

	assert(a && area_contains(a, pfn));
	idx = pfn - a->base;
	order = a->page_states[idx];
	assert(!(order & ~ORDER_MASK) && order && (order < NLISTS));
	assert(IS_ALIGNED_ORDER(pfn, order));
	assert(area_contains(a, pfn + BIT(order) - 1));

	/* Remove the block from its free list */
	p = list_remove(addr);
	assert(p);

	/* update the block size for each page in the block */
	for (i = 0; i < BIT(order); i++) {
	assert(a->page_states[idx + i] == order);
	a->page_states[idx + i] = order - 1;
	}
	order--;
	/* add the first half block to the appropriate free list */
	list_add(a->freelists + order, p);
	/* add the second half block to the appropriate free list */
	list_add(a->freelists + order, (void )((pfn + BIT(order)) PAGE_SIZE));
	}

	/*
	* Returns a block whose alignment and size are at least the parameter values.
	* If there is not enough free memory, NULL is returned.
	*
	* Both parameters must be not larger than the largest allowed order
	*/
	static void page_memalign_order(struct mem_area a, u8 al, u8 sz)
	{
	struct linked_list p, res = NULL;
	u8 order;

	assert((al < NLISTS) && (sz < NLISTS));
	/* we need the bigger of the two as starting point */
	order = sz > al ? sz : al;

	/* search all free lists for some memory */
	for ( ; order < NLISTS; order++) {
	p = a->freelists[order].next;
	if (!is_list_empty(p))
	break;
	}
	/* out of memory */
	if (order >= NLISTS)
	return NULL;

	/*
	* the block is bigger than what we need because either there were
	* no smaller blocks, or the smaller blocks were not aligned to our
	* needs; therefore we split the block until we reach the needed size
	*/
	for (; order > sz; order--)
	split(a, p);

	res = list_remove(p);
	memset(a->page_states + (PFN(res) - a->base), ALLOC_MASK \| order, BIT(order));
	return res;
	}

	/*
	* Try to merge two blocks into a bigger one.
	* Returns true in case of a successful merge.
	* Merging will succeed only if both blocks have the same block size and are
	* both free.
	*
	* The function depends on the following assumptions:
	* - the first parameter is strictly smaller than the second
	* - the parameters must point each to the start of their block
	* - the two parameters point to adjacent blocks
	* - the two blocks are both in a free list
	* - all of the pages of the two blocks must be free
	* - all of the pages of the two blocks must have the same block size
	* - the function is called with the lock held
	*/
	static bool coalesce(struct mem_area *a, u8 order, uintptr_t pfn, uintptr_t pfn2)
	{
	uintptr_t first, second, i;
	struct linked_list *li;

	assert(IS_ALIGNED_ORDER(pfn, order) && IS_ALIGNED_ORDER(pfn2, order));
	assert(pfn2 == pfn + BIT(order));
	assert(a);

	/* attempting to coalesce two blocks that belong to different areas */
	if (!area_contains(a, pfn) \|\| !area_contains(a, pfn2 + BIT(order) - 1))
	return false;
	first = pfn - a->base;
	second = pfn2 - a->base;
	/* the two blocks have different sizes, cannot coalesce */
	if ((a->page_states[first] != order) \|\| (a->page_states[second] != order))
	return false;

	/* we can coalesce, remove both blocks from their freelists */
	li = list_remove((void *)(pfn2 << PAGE_SHIFT));
	assert(li);
	li = list_remove((void *)(pfn << PAGE_SHIFT));
	assert(li);
	/* check the metadata entries and update with the new size */
	for (i = 0; i < (2ull << order); i++) {
	assert(a->page_states[first + i] == order);
	a->page_states[first + i] = order + 1;
	}
	/* finally add the newly coalesced block to the appropriate freelist */
	list_add(a->freelists + order + 1, li);
	return true;
	}

	/*
	* Free a block of memory.
	* The parameter can be NULL, in which case nothing happens.
	*
	* The function depends on the following assumptions:
	* - the parameter is page aligned
	* - the parameter belongs to an existing memory area
	* - the parameter points to the beginning of the block
	* - the size of the block is less than the maximum allowed
	* - the block is completely contained in its memory area
	* - all pages in the block have the same block size
	* - no pages in the memory block were already free
	* - no pages in the memory block are special
	*/
	static void _free_pages(void *mem)
	{
	uintptr_t pfn2, pfn = PFN(mem);
	struct mem_area *a = NULL;
	uintptr_t i, p;
	u8 order;

	if (!mem)
	return;
	assert(IS_ALIGNED((uintptr_t)mem, PAGE_SIZE));

	/* find which area this pointer belongs to*/
	for (i = 0; !a && (i < MAX_AREAS); i++) {
	if ((areas_mask & BIT(i)) && area_contains(areas + i, pfn))
	a = areas + i;
	}
	assert_msg(a, "memory does not belong to any area: %p", mem);

	p = pfn - a->base;
	order = a->page_states[p] & ORDER_MASK;

	/* ensure that the first page is allocated and not special */
	assert(a->page_states[p] == (order \| ALLOC_MASK));
	/* ensure that the order has a sane value */
	assert(order < NLISTS);
	/* ensure that the block is aligned properly for its size */
	assert(IS_ALIGNED_ORDER(pfn, order));
	/* ensure that the area can contain the whole block */
	assert(area_contains(a, pfn + BIT(order) - 1));

	for (i = 0; i < BIT(order); i++) {
	/* check that all pages of the block have consistent metadata */
	assert(a->page_states[p + i] == (ALLOC_MASK \| order));
	/* set the page as free */
	a->page_states[p + i] &= ~ALLOC_MASK;
	}
	/* provisionally add the block to the appropriate free list */
	list_add(a->freelists + order, mem);
	/* try to coalesce the block with neighbouring blocks if possible */
	do {
	/*
	* get the order again since it might have changed after
	* coalescing in a previous iteration
	*/
	order = a->page_states[p] & ORDER_MASK;
	/*
	* let's consider this block and the next one if this block
	* is aligned to the next size, otherwise let's consider the
	* previous block and this one
	*/
	if (!IS_ALIGNED_ORDER(pfn, order + 1))
	pfn = pfn - BIT(order);
	pfn2 = pfn + BIT(order);
	/* repeat as long as we manage to coalesce something */
	} while (coalesce(a, order, pfn, pfn2));
	}

	void free_pages(void *mem)
	{
	spin_lock(&lock);
	_free_pages(mem);
	spin_unlock(&lock);
	}

	static void *page_memalign_order_area(unsigned area, u8 ord, u8 al)
	{
	void *res = NULL;
	int i;

	spin_lock(&lock);
	area &= areas_mask;
	for (i = 0; !res && (i < MAX_AREAS); i++)
	if (area & BIT(i))
	res = page_memalign_order(areas + i, ord, al);
	spin_unlock(&lock);
	return res;
	}

	/*
	* Allocates (1 << order) physically contiguous and naturally aligned pages.
	* Returns NULL if the allocation was not possible.
	*/
	void *alloc_pages_area(unsigned int area, unsigned int order)
	{
	return page_memalign_order_area(area, order, order);
	}

	void *alloc_pages(unsigned int order)
	{
	return alloc_pages_area(AREA_ANY, order);
	}

	/*
	* Allocates (1 << order) physically contiguous aligned pages.
	* Returns NULL if the allocation was not possible.
	*/
	void *memalign_pages_area(unsigned int area, size_t alignment, size_t size)
	{
	assert(is_power_of_2(alignment));
	alignment = get_order(PAGE_ALIGN(alignment) >> PAGE_SHIFT);
	size = get_order(PAGE_ALIGN(size) >> PAGE_SHIFT);
	assert(alignment < NLISTS);
	assert(size < NLISTS);
	return page_memalign_order_area(area, size, alignment);
	}

	void *memalign_pages(size_t alignment, size_t size)
	{
	return memalign_pages_area(AREA_ANY, alignment, size);
	}

	/*
	* Allocates one page
	*/
	void *alloc_page()
	{
	return alloc_pages(0);
	}

	static struct alloc_ops page_alloc_ops = {
	.memalign = memalign_pages,
	.free = free_pages,
	.align_min = PAGE_SIZE,
	};

	/*
	* Enables the page allocator.
	*
	* Prerequisites:
	* - at least one memory area has been initialized
	*/
	void page_alloc_ops_enable(void)
	{
	spin_lock(&lock);
	assert(page_alloc_initialized());
	alloc_ops = &page_alloc_ops;
	spin_unlock(&lock);
	}

	/*
	* Adds a new memory area to the pool of available memory.
	*
	* Prerequisites:
	* - the lock is held
	* - start and top are page frame numbers
	* - start is smaller than top
	* - top does not fall outside of addressable memory
	* - there is at least one more slot free for memory areas
	* - if a specific memory area number has been indicated, it needs to be free
	* - the memory area to add does not overlap with existing areas
	* - the memory area to add has at least 5 pages available
	*/
	static void _page_alloc_init_area(u8 n, uintptr_t start_pfn, uintptr_t top_pfn)
	{
	size_t table_size, npages, i;
	struct mem_area *a;
	u8 order = 0;

	/* the number must be within the allowed range */
	assert(n < MAX_AREAS);
	/* the new area number must be unused */
	assert(!(areas_mask & BIT(n)));

	/* other basic sanity checks */
	assert(top_pfn > start_pfn);
	assert(top_pfn - start_pfn > 4);
	assert(top_pfn < BIT_ULL(sizeof(void ) 8 - PAGE_SHIFT));

	/* calculate the size of the metadata table in pages */
	table_size = (top_pfn - start_pfn + PAGE_SIZE) / (PAGE_SIZE + 1);

	/* fill in the values of the new area */
	a = areas + n;
	a->page_states = (void *)(start_pfn << PAGE_SHIFT);
	a->base = start_pfn + table_size;
	a->top = top_pfn;
	npages = top_pfn - a->base;
	assert((a->base - start_pfn) * PAGE_SIZE >= npages);

	/* check that the new area does not overlap with any existing areas */
	for (i = 0; i < MAX_AREAS; i++) {
	if (!(areas_mask & BIT(i)))
	continue;
	assert(!area_or_metadata_contains(areas + i, start_pfn));
	assert(!area_or_metadata_contains(areas + i, top_pfn - 1));
	assert(!area_or_metadata_contains(a, PFN(areas[i].page_states)));
	assert(!area_or_metadata_contains(a, areas[i].top - 1));
	}
	/* initialize all freelists for the new area */
	for (i = 0; i < NLISTS; i++)
	a->freelists[i].next = a->freelists[i].prev = a->freelists + i;

	/* initialize the metadata for the available memory */
	for (i = a->base; i < a->top; i += 1ull << order) {
	/* search which order to start from */
	while (i + BIT(order) > a->top) {
	assert(order);
	order--;
	}
	/*
	* we need both loops, one for the start and the other for
	* the end of the block, in case it spans a power of two
	* boundary
	*/
	while (IS_ALIGNED_ORDER(i, order + 1) && (i + BIT(order + 1) <= a->top))
	order++;
	assert(order < NLISTS);
	/* initialize the metadata and add to the freelist */
	memset(a->page_states + (i - a->base), order, BIT(order));
	list_add(a->freelists + order, (void *)(i << PAGE_SHIFT));
	}
	/* finally mark the area as present */
	areas_mask \|= BIT(n);
	}

	static void __page_alloc_init_area(u8 n, uintptr_t cutoff, uintptr_t base_pfn, uintptr_t *top_pfn)
	{
	if (*top_pfn > cutoff) {
	spin_lock(&lock);
	if (base_pfn >= cutoff) {
	_page_alloc_init_area(n, base_pfn, *top_pfn);
	*top_pfn = 0;
	} else {
	_page_alloc_init_area(n, cutoff, *top_pfn);
	*top_pfn = cutoff;
	}
	spin_unlock(&lock);
	}
	}

	/*
	* Adds a new memory area to the pool of available memory.
	*
	* Prerequisites:
	* see _page_alloc_init_area
	*/
	void page_alloc_init_area(u8 n, uintptr_t base_pfn, uintptr_t top_pfn)
	{
	if (n != AREA_ANY_NUMBER) {
	__page_alloc_init_area(n, 0, base_pfn, &top_pfn);
	return;
	}
	#ifdef AREA_HIGH_PFN
	__page_alloc_init_area(AREA_HIGH_NUMBER, AREA_HIGH_PFN), base_pfn, &top_pfn);
	#endif
	__page_alloc_init_area(AREA_NORMAL_NUMBER, AREA_NORMAL_PFN, base_pfn, &top_pfn);
	#ifdef AREA_LOW_PFN
	__page_alloc_init_area(AREA_LOW_NUMBER, AREA_LOW_PFN, base_pfn, &top_pfn);
	#endif
	#ifdef AREA_LOWEST_PFN
	__page_alloc_init_area(AREA_LOWEST_NUMBER, AREA_LOWEST_PFN, base_pfn, &top_pfn);
	#endif
	}