zsmalloc.c

/*
 * zsmalloc memory allocator
 *
 * Copyright (C) 2011  Nitin Gupta
 * Copyright (C) 2012, 2013 Minchan Kim
 *
 * This code is released using a dual license strategy: BSD/GPL
 * You can choose the license that better fits your requirements.
 *
 * Released under the terms of 3-clause BSD License
 * Released under the terms of GNU General Public License Version 2.0
 */

/*
 * This allocator is designed for use with zram. Thus, the allocator is
 * supposed to work well under low memory conditions. In particular, it
 * never attempts higher order page allocation which is very likely to
 * fail under memory pressure. On the other hand, if we just use single
 * (0-order) pages, it would suffer from very high fragmentation --
 * any object of size PAGE_SIZE/2 or larger would occupy an entire page.
 * This was one of the major issues with its predecessor (xvmalloc).
 *
 * To overcome these issues, zsmalloc allocates a bunch of 0-order pages
 * and links them together using various 'struct page' fields. These linked
 * pages act as a single higher-order page i.e. an object can span 0-order
 * page boundaries. The code refers to these linked pages as a single entity
 * called zspage.
 *
 * For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE
 * since this satisfies the requirements of all its current users (in the
 * worst case, page is incompressible and is thus stored "as-is" i.e. in
 * uncompressed form). For allocation requests larger than this size, failure
 * is returned (see zs_malloc).
 *
 * Additionally, zs_malloc() does not return a dereferenceable pointer.
 * Instead, it returns an opaque handle (unsigned long) which encodes actual
 * location of the allocated object. The reason for this indirection is that
 * zsmalloc does not keep zspages permanently mapped since that would cause
 * issues on 32-bit systems where the VA region for kernel space mappings
 * is very small. So, before using the allocating memory, the object has to
 * be mapped using zs_map_object() to get a usable pointer and subsequently
 * unmapped using zs_unmap_object().
 *
 * Following is how we use various fields and flags of underlying
 * struct page(s) to form a zspage.
 *
 * Usage of struct page fields:
 *	page->first_page: points to the first component (0-order) page
 *	page->index (union with page->freelist): offset of the first object
 *		starting in this page. For the first page, this is
 *		always 0, so we use this field (aka freelist) to point
 *		to the first free object in zspage.
 *	page->lru: links together all component pages (except the first page)
 *		of a zspage
 *
 *	For _first_ page only:
 *
 *	page->private (union with page->first_page): refers to the
 *		component page after the first page
 *	page->freelist: points to the first free object in zspage.
 *		Free objects are linked together using in-place
 *		metadata.
 *	page->objects: maximum number of objects we can store in this
 *		zspage (class->zspage_order * PAGE_SIZE / class->size)
 *	page->lru: links together first pages of various zspages.
 *		Basically forming list of zspages in a fullness group.
 *	page->mapping: class index and fullness group of the zspage
 *
 * Usage of struct page flags:
 *	PG_private: identifies the first component page
 *	PG_private2: identifies the last component page
 *
 */

#ifdef CONFIG_ZSMALLOC_DEBUG
#define DEBUG
#endif

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/bitops.h>
#include <linux/errno.h>
#include <linux/highmem.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <asm/tlbflush.h>
#include <asm/pgtable.h>
#include <linux/cpumask.h>
#include <linux/cpu.h>
#include <linux/vmalloc.h>
#include <linux/hardirq.h>
#include <linux/spinlock.h>
#include <linux/types.h>
#include <linux/zsmalloc.h>
#include <linux/zpool.h>

/*
 * This must be power of 2 and greater than of equal to sizeof(link_free).
 * These two conditions ensure that any 'struct link_free' itself doesn't
 * span more than 1 page which avoids complex case of mapping 2 pages simply
 * to restore link_free pointer values.
 */
#define ZS_ALIGN		8

/*
 * A single 'zspage' is composed of up to 2^N discontiguous 0-order (single)
 * pages. ZS_MAX_ZSPAGE_ORDER defines upper limit on N.
 */
#define ZS_MAX_ZSPAGE_ORDER 2
#define ZS_MAX_PAGES_PER_ZSPAGE (_AC(1, UL) << ZS_MAX_ZSPAGE_ORDER)

/*
 * Object location (<PFN>, <obj_idx>) is encoded as
 * as single (unsigned long) handle value.
 *
 * Note that object index <obj_idx> is relative to system
 * page <PFN> it is stored in, so for each sub-page belonging
 * to a zspage, obj_idx starts with 0.
 *
 * This is made more complicated by various memory models and PAE.
 */

#ifndef MAX_PHYSMEM_BITS
#ifdef CONFIG_HIGHMEM64G
#define MAX_PHYSMEM_BITS 36
#else /* !CONFIG_HIGHMEM64G */
/*
 * If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
 * be PAGE_SHIFT
 */
#define MAX_PHYSMEM_BITS BITS_PER_LONG
#endif
#endif
#define _PFN_BITS		(MAX_PHYSMEM_BITS - PAGE_SHIFT)
#define OBJ_INDEX_BITS	(BITS_PER_LONG - _PFN_BITS)
#define OBJ_INDEX_MASK	((_AC(1, UL) << OBJ_INDEX_BITS) - 1)

#define MAX(a, b) ((a) >= (b) ? (a) : (b))
/* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
#define ZS_MIN_ALLOC_SIZE \
	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OBJ_INDEX_BITS))
#define ZS_MAX_ALLOC_SIZE	PAGE_SIZE

/*
 * On systems with 4K page size, this gives 255 size classes! There is a
 * trader-off here:
 *  - Large number of size classes is potentially wasteful as free page are
 *    spread across these classes
 *  - Small number of size classes causes large internal fragmentation
 *  - Probably its better to use specific size classes (empirically
 *    determined). NOTE: all those class sizes must be set as multiple of
 *    ZS_ALIGN to make sure link_free itself never has to span 2 pages.
 *
 *  ZS_MIN_ALLOC_SIZE and ZS_SIZE_CLASS_DELTA must be multiple of ZS_ALIGN
 *  (reason above)
 */
#define ZS_SIZE_CLASS_DELTA	(PAGE_SIZE >> 8)
#define ZS_SIZE_CLASSES		((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / \
					ZS_SIZE_CLASS_DELTA + 1)

/*
 * We do not maintain any list for completely empty or full pages
 */
enum fullness_group {
	ZS_ALMOST_FULL,
	ZS_ALMOST_EMPTY,
	_ZS_NR_FULLNESS_GROUPS,

	ZS_EMPTY,
	ZS_FULL
};

/*
 * We assign a page to ZS_ALMOST_EMPTY fullness group when:
 *	n <= N / f, where
 * n = number of allocated objects
 * N = total number of objects zspage can store
 * f = fullness_threshold_frac
 *
 * Similarly, we assign zspage to:
 *	ZS_ALMOST_FULL	when n > N / f
 *	ZS_EMPTY	when n == 0
 *	ZS_FULL		when n == N
 *
 * (see: fix_fullness_group())
 */
static const int fullness_threshold_frac = 4;

struct size_class {
	/*
	 * Size of objects stored in this class. Must be multiple
	 * of ZS_ALIGN.
	 */
	int size;
	unsigned int index;

	/* Number of PAGE_SIZE sized pages to combine to form a 'zspage' */
	int pages_per_zspage;

	spinlock_t lock;

	struct page *fullness_list[_ZS_NR_FULLNESS_GROUPS];
};

/*
 * Placed within free objects to form a singly linked list.
 * For every zspage, first_page->freelist gives head of this list.
 *
 * This must be power of 2 and less than or equal to ZS_ALIGN
 */
struct link_free {
	/* Handle of next free chunk (encodes <PFN, obj_idx>) */
	void *next;
};

struct zs_pool {
	struct size_class *size_class[ZS_SIZE_CLASSES];

	gfp_t flags;	/* allocation flags used when growing pool */
	atomic_long_t pages_allocated;
};

/*
 * A zspage's class index and fullness group
 * are encoded in its (first)page->mapping
 */
#define CLASS_IDX_BITS	28
#define FULLNESS_BITS	4
#define CLASS_IDX_MASK	((1 << CLASS_IDX_BITS) - 1)
#define FULLNESS_MASK	((1 << FULLNESS_BITS) - 1)

struct mapping_area {
#ifdef CONFIG_PGTABLE_MAPPING
	struct vm_struct *vm; /* vm area for mapping object that span pages */
#else
	char *vm_buf; /* copy buffer for objects that span pages */
#endif
	char *vm_addr; /* address of kmap_atomic()'ed pages */
	enum zs_mapmode vm_mm; /* mapping mode */
};

/* zpool driver */

#ifdef CONFIG_ZPOOL

static void *zs_zpool_create(gfp_t gfp, struct zpool_ops *zpool_ops)
{
	return zs_create_pool(gfp);
}

static void zs_zpool_destroy(void *pool)
{
	zs_destroy_pool(pool);
}

static int zs_zpool_malloc(void *pool, size_t size, gfp_t gfp,
			unsigned long *handle)
{
	*handle = zs_malloc(pool, size);
	return *handle ? 0 : -1;
}
static void zs_zpool_free(void *pool, unsigned long handle)
{
	zs_free(pool, handle);
}

static int zs_zpool_shrink(void *pool, unsigned int pages,
			unsigned int *reclaimed)
{
	return -EINVAL;
}

static void *zs_zpool_map(void *pool, unsigned long handle,
			enum zpool_mapmode mm)
{
	enum zs_mapmode zs_mm;

	switch (mm) {
	case ZPOOL_MM_RO:
		zs_mm = ZS_MM_RO;
		break;
	case ZPOOL_MM_WO:
		zs_mm = ZS_MM_WO;
		break;
	case ZPOOL_MM_RW: /* fallthru */
	default:
		zs_mm = ZS_MM_RW;
		break;
	}

	return zs_map_object(pool, handle, zs_mm);
}
static void zs_zpool_unmap(void *pool, unsigned long handle)
{
	zs_unmap_object(pool, handle);
}

static u64 zs_zpool_total_size(void *pool)
{
	return zs_get_total_pages(pool) << PAGE_SHIFT;
}

static struct zpool_driver zs_zpool_driver = {
	.type =		"zsmalloc",
	.owner =	THIS_MODULE,
	.create =	zs_zpool_create,
	.destroy =	zs_zpool_destroy,
	.malloc =	zs_zpool_malloc,
	.free =		zs_zpool_free,
	.shrink =	zs_zpool_shrink,
	.map =		zs_zpool_map,
	.unmap =	zs_zpool_unmap,
	.total_size =	zs_zpool_total_size,
};

MODULE_ALIAS("zpool-zsmalloc");
#endif /* CONFIG_ZPOOL */

/* per-cpu VM mapping areas for zspage accesses that cross page boundaries */
static DEFINE_PER_CPU(struct mapping_area, zs_map_area);

static int is_first_page(struct page *page)
{
	return PagePrivate(page);
}

static int is_last_page(struct page *page)
{
	return PagePrivate2(page);
}

static void get_zspage_mapping(struct page *page, unsigned int *class_idx,
				enum fullness_group *fullness)
{
	unsigned long m;
	BUG_ON(!is_first_page(page));

	m = (unsigned long)page->mapping;
	*fullness = m & FULLNESS_MASK;
	*class_idx = (m >> FULLNESS_BITS) & CLASS_IDX_MASK;
}

static void set_zspage_mapping(struct page *page, unsigned int class_idx,
				enum fullness_group fullness)
{
	unsigned long m;
	BUG_ON(!is_first_page(page));

	m = ((class_idx & CLASS_IDX_MASK) << FULLNESS_BITS) |
			(fullness & FULLNESS_MASK);
	page->mapping = (struct address_space *)m;
}

/*
 * zsmalloc divides the pool into various size classes where each
 * class maintains a list of zspages where each zspage is divided
 * into equal sized chunks. Each allocation falls into one of these
 * classes depending on its size. This function returns index of the
 * size class which has chunk size big enough to hold the give size.
 */
static int get_size_class_index(int size)
{
	int idx = 0;

	if (likely(size > ZS_MIN_ALLOC_SIZE))
		idx = DIV_ROUND_UP(size - ZS_MIN_ALLOC_SIZE,
				ZS_SIZE_CLASS_DELTA);

	return idx;
}

/*
 * For each size class, zspages are divided into different groups
 * depending on how "full" they are. This was done so that we could
 * easily find empty or nearly empty zspages when we try to shrink
 * the pool (not yet implemented). This function returns fullness
 * status of the given page.
 */
static enum fullness_group get_fullness_group(struct page *page)
{
	int inuse, max_objects;
	enum fullness_group fg;
	BUG_ON(!is_first_page(page));

	inuse = page->inuse;
	max_objects = page->objects;

	if (inuse == 0)
		fg = ZS_EMPTY;
	else if (inuse == max_objects)
		fg = ZS_FULL;
	else if (inuse <= max_objects / fullness_threshold_frac)
		fg = ZS_ALMOST_EMPTY;
	else
		fg = ZS_ALMOST_FULL;

	return fg;
}

/*
 * Each size class maintains various freelists and zspages are assigned
 * to one of these freelists based on the number of live objects they
 * have. This functions inserts the given zspage into the freelist
 * identified by <class, fullness_group>.
 */
static void insert_zspage(struct page *page, struct size_class *class,
				enum fullness_group fullness)
{
	struct page **head;

	BUG_ON(!is_first_page(page));

	if (fullness >= _ZS_NR_FULLNESS_GROUPS)
		return;

	head = &class->fullness_list[fullness];
	if (*head)
		list_add_tail(&page->lru, &(*head)->lru);

	*head = page;
}

/*
 * This function removes the given zspage from the freelist identified
 * by <class, fullness_group>.
 */
static void remove_zspage(struct page *page, struct size_class *class,
				enum fullness_group fullness)
{
	struct page **head;

	BUG_ON(!is_first_page(page));

	if (fullness >= _ZS_NR_FULLNESS_GROUPS)
		return;

	head = &class->fullness_list[fullness];
	BUG_ON(!*head);
	if (list_empty(&(*head)->lru))
		*head = NULL;
	else if (*head == page)
		*head = (struct page *)list_entry((*head)->lru.next,
					struct page, lru);

	list_del_init(&page->lru);
}

/*
 * Each size class maintains zspages in different fullness groups depending
 * on the number of live objects they contain. When allocating or freeing
 * objects, the fullness status of the page can change, say, from ALMOST_FULL
 * to ALMOST_EMPTY when freeing an object. This function checks if such
 * a status change has occurred for the given page and accordingly moves the
 * page from the freelist of the old fullness group to that of the new
 * fullness group.
 */
static enum fullness_group fix_fullness_group(struct zs_pool *pool,
						struct page *page)
{
	int class_idx;
	struct size_class *class;
	enum fullness_group currfg, newfg;

	BUG_ON(!is_first_page(page));

	get_zspage_mapping(page, &class_idx, &currfg);
	newfg = get_fullness_group(page);
	if (newfg == currfg)
		goto out;

	class = pool->size_class[class_idx];
	remove_zspage(page, class, currfg);
	insert_zspage(page, class, newfg);
	set_zspage_mapping(page, class_idx, newfg);

out:
	return newfg;
}

/*
 * We have to decide on how many pages to link together
 * to form a zspage for each size class. This is important
 * to reduce wastage due to unusable space left at end of
 * each zspage which is given as:
 *	wastage = Zp - Zp % size_class
 * where Zp = zspage size = k * PAGE_SIZE where k = 1, 2, ...
 *
 * For example, for size class of 3/8 * PAGE_SIZE, we should
 * link together 3 PAGE_SIZE sized pages to form a zspage
 * since then we can perfectly fit in 8 such objects.
 */
static int get_pages_per_zspage(int class_size)
{
	int i, max_usedpc = 0;
	/* zspage order which gives maximum used size per KB */
	int max_usedpc_order = 1;

	for (i = 1; i <= ZS_MAX_PAGES_PER_ZSPAGE; i++) {
		int zspage_size;
		int waste, usedpc;

		zspage_size = i * PAGE_SIZE;
		waste = zspage_size % class_size;
		usedpc = (zspage_size - waste) * 100 / zspage_size;

		if (usedpc > max_usedpc) {
			max_usedpc = usedpc;
			max_usedpc_order = i;
		}
	}

	return max_usedpc_order;
}

/*
 * A single 'zspage' is composed of many system pages which are
 * linked together using fields in struct page. This function finds
 * the first/head page, given any component page of a zspage.
 */
static struct page *get_first_page(struct page *page)
{
	if (is_first_page(page))
		return page;
	else
		return page->first_page;
}

static struct page *get_next_page(struct page *page)
{
	struct page *next;

	if (is_last_page(page))
		next = NULL;
	else if (is_first_page(page))
		next = (struct page *)page_private(page);
	else
		next = list_entry(page->lru.next, struct page, lru);

	return next;
}

/*
 * Encode <page, obj_idx> as a single handle value.
 * On hardware platforms with physical memory starting at 0x0 the pfn
 * could be 0 so we ensure that the handle will never be 0 by adjusting the
 * encoded obj_idx value before encoding.
 */
static void *obj_location_to_handle(struct page *page, unsigned long obj_idx)
{
	unsigned long handle;

	if (!page) {
		BUG_ON(obj_idx);
		return NULL;
	}

	handle = page_to_pfn(page) << OBJ_INDEX_BITS;
	handle |= ((obj_idx + 1) & OBJ_INDEX_MASK);

	return (void *)handle;
}

/*
 * Decode <page, obj_idx> pair from the given object handle. We adjust the
 * decoded obj_idx back to its original value since it was adjusted in
 * obj_location_to_handle().
 */
static void obj_handle_to_location(unsigned long handle, struct page **page,
				unsigned long *obj_idx)
{
	*page = pfn_to_page(handle >> OBJ_INDEX_BITS);
	*obj_idx = (handle & OBJ_INDEX_MASK) - 1;
}

static unsigned long obj_idx_to_offset(struct page *page,
				unsigned long obj_idx, int class_size)
{
	unsigned long off = 0;

	if (!is_first_page(page))
		off = page->index;

	return off + obj_idx * class_size;
}

static void reset_page(struct page *page)
{
	clear_bit(PG_private, &page->flags);
	clear_bit(PG_private_2, &page->flags);
	set_page_private(page, 0);
	page->mapping = NULL;
	page->freelist = NULL;
	page_mapcount_reset(page);
}

static void free_zspage(struct page *first_page)
{
	struct page *nextp, *tmp, *head_extra;

	BUG_ON(!is_first_page(first_page));
	BUG_ON(first_page->inuse);

	head_extra = (struct page *)page_private(first_page);

	reset_page(first_page);
	__free_page(first_page);

	/* zspage with only 1 system page */
	if (!head_extra)
		return;

	list_for_each_entry_safe(nextp, tmp, &head_extra->lru, lru) {
		list_del(&nextp->lru);
		reset_page(nextp);
		__free_page(nextp);
	}
	reset_page(head_extra);
	__free_page(head_extra);
}

/* Initialize a newly allocated zspage */
static void init_zspage(struct page *first_page, struct size_class *class)
{
	unsigned long off = 0;
	struct page *page = first_page;

	BUG_ON(!is_first_page(first_page));
	while (page) {
		struct page *next_page;
		struct link_free *link;
		unsigned int i = 1;
		void *vaddr;

		/*
		 * page->index stores offset of first object starting
		 * in the page. For the first page, this is always 0,
		 * so we use first_page->index (aka ->freelist) to store
		 * head of corresponding zspage's freelist.
		 */
		if (page != first_page)
			page->index = off;

		vaddr = kmap_atomic(page);
		link = (struct link_free *)vaddr + off / sizeof(*link);

		while ((off += class->size) < PAGE_SIZE) {
			link->next = obj_location_to_handle(page, i++);
			link += class->size / sizeof(*link);
		}

		/*
		 * We now come to the last (full or partial) object on this
		 * page, which must point to the first object on the next
		 * page (if present)
		 */
		next_page = get_next_page(page);
		link->next = obj_location_to_handle(next_page, 0);
		kunmap_atomic(vaddr);
		page = next_page;
		off %= PAGE_SIZE;
	}
}

/*
 * Allocate a zspage for the given size class
 */
static struct page *alloc_zspage(struct size_class *class, gfp_t flags)
{
	int i, error;
	struct page *first_page = NULL, *uninitialized_var(prev_page);

	/*
	 * Allocate individual pages and link them together as:
	 * 1. first page->private = first sub-page
	 * 2. all sub-pages are linked together using page->lru
	 * 3. each sub-page is linked to the first page using page->first_page
	 *
	 * For each size class, First/Head pages are linked together using
	 * page->lru. Also, we set PG_private to identify the first page
	 * (i.e. no other sub-page has this flag set) and PG_private_2 to
	 * identify the last page.
	 */
	error = -ENOMEM;
	for (i = 0; i < class->pages_per_zspage; i++) {
		struct page *page;

		page = alloc_page(flags);
		if (!page)
			goto cleanup;

		INIT_LIST_HEAD(&page->lru);
		if (i == 0) {	/* first page */
			SetPagePrivate(page);
			set_page_private(page, 0);
			first_page = page;
			first_page->inuse = 0;
		}
		if (i == 1)
			set_page_private(first_page, (unsigned long)page);
		if (i >= 1)
			page->first_page = first_page;
		if (i >= 2)
			list_add(&page->lru, &prev_page->lru);
		if (i == class->pages_per_zspage - 1)	/* last page */
			SetPagePrivate2(page);
		prev_page = page;
	}

	init_zspage(first_page, class);

	first_page->freelist = obj_location_to_handle(first_page, 0);
	/* Maximum number of objects we can store in this zspage */
	first_page->objects = class->pages_per_zspage * PAGE_SIZE / class->size;

	error = 0; /* Success */

cleanup:
	if (unlikely(error) && first_page) {
		free_zspage(first_page);
		first_page = NULL;
	}

	return first_page;
}

static struct page *find_get_zspage(struct size_class *class)
{
	int i;
	struct page *page;

	for (i = 0; i < _ZS_NR_FULLNESS_GROUPS; i++) {
		page = class->fullness_list[i];
		if (page)
			break;
	}

	return page;
}

#ifdef CONFIG_PGTABLE_MAPPING
static inline int __zs_cpu_up(struct mapping_area *area)
{
	/*
	 * Make sure we don't leak memory if a cpu UP notification
	 * and zs_init() race and both call zs_cpu_up() on the same cpu
	 */
	if (area->vm)
		return 0;
	area->vm = alloc_vm_area(PAGE_SIZE * 2, NULL);
	if (!area->vm)
		return -ENOMEM;
	return 0;
}

static inline void __zs_cpu_down(struct mapping_area *area)
{
	if (area->vm)
		free_vm_area(area->vm);
	area->vm = NULL;
}

static inline void *__zs_map_object(struct mapping_area *area,
				struct page *pages[2], int off, int size)
{
	BUG_ON(map_vm_area(area->vm, PAGE_KERNEL, pages));
	area->vm_addr = area->vm->addr;
	return area->vm_addr + off;
}

static inline void __zs_unmap_object(struct mapping_area *area,
				struct page *pages[2], int off, int size)
{
	unsigned long addr = (unsigned long)area->vm_addr;

	unmap_kernel_range(addr, PAGE_SIZE * 2);
}

#else /* CONFIG_PGTABLE_MAPPING */

static inline int __zs_cpu_up(struct mapping_area *area)
{
	/*
	 * Make sure we don't leak memory if a cpu UP notification
	 * and zs_init() race and both call zs_cpu_up() on the same cpu
	 */
	if (area->vm_buf)
		return 0;
	area->vm_buf = (char *)__get_free_page(GFP_KERNEL);
	if (!area->vm_buf)
		return -ENOMEM;
	return 0;
}

static inline void __zs_cpu_down(struct mapping_area *area)
{
	if (area->vm_buf)
		free_page((unsigned long)area->vm_buf);
	area->vm_buf = NULL;
}

static void *__zs_map_object(struct mapping_area *area,
			struct page *pages[2], int off, int size)
{
	int sizes[2];
	void *addr;
	char *buf = area->vm_buf;

	/* disable page faults to match kmap_atomic() return conditions */
	pagefault_disable();

	/* no read fastpath */
	if (area->vm_mm == ZS_MM_WO)
		goto out;

	sizes[0] = PAGE_SIZE - off;
	sizes[1] = size - sizes[0];

	/* copy object to per-cpu buffer */
	addr = kmap_atomic(pages[0]);
	memcpy(buf, addr + off, sizes[0]);
	kunmap_atomic(addr);
	addr = kmap_atomic(pages[1]);
	memcpy(buf + sizes[0], addr, sizes[1]);
	kunmap_atomic(addr);
out:
	return area->vm_buf;
}

static void __zs_unmap_object(struct mapping_area *area,
			struct page *pages[2], int off, int size)
{
	int sizes[2];
	void *addr;
	char *buf = area->vm_buf;

	/* no write fastpath */
	if (area->vm_mm == ZS_MM_RO)
		goto out;

	sizes[0] = PAGE_SIZE - off;
	sizes[1] = size - sizes[0];

	/* copy per-cpu buffer to object */
	addr = kmap_atomic(pages[0]);
	memcpy(addr + off, buf, sizes[0]);
	kunmap_atomic(addr);
	addr = kmap_atomic(pages[1]);
	memcpy(addr, buf + sizes[0], sizes[1]);
	kunmap_atomic(addr);

out:
	/* enable page faults to match kunmap_atomic() return conditions */
	pagefault_enable();
}

#endif /* CONFIG_PGTABLE_MAPPING */

static int zs_cpu_notifier(struct notifier_block *nb, unsigned long action,
				void *pcpu)
{
	int ret, cpu = (long)pcpu;
	struct mapping_area *area;

	switch (action) {
	case CPU_UP_PREPARE:
		area = &per_cpu(zs_map_area, cpu);
		ret = __zs_cpu_up(area);
		if (ret)
			return notifier_from_errno(ret);
		break;
	case CPU_DEAD:
	case CPU_UP_CANCELED:
		area = &per_cpu(zs_map_area, cpu);
		__zs_cpu_down(area);
		break;
	}

	return NOTIFY_OK;
}

static struct notifier_block zs_cpu_nb = {
	.notifier_call = zs_cpu_notifier
};

static void zs_unregister_cpu_notifier(void)
{
	int cpu;

	cpu_notifier_register_begin();

	for_each_online_cpu(cpu)
		zs_cpu_notifier(NULL, CPU_DEAD, (void *)(long)cpu);
	__unregister_cpu_notifier(&zs_cpu_nb);

	cpu_notifier_register_done();
}

static int zs_register_cpu_notifier(void)
{
	int cpu, uninitialized_var(ret);

	cpu_notifier_register_begin();

	__register_cpu_notifier(&zs_cpu_nb);
	for_each_online_cpu(cpu) {
		ret = zs_cpu_notifier(NULL, CPU_UP_PREPARE, (void *)(long)cpu);
		if (notifier_to_errno(ret))
			break;
	}

	cpu_notifier_register_done();
	return notifier_to_errno(ret);
}

static void __exit zs_exit(void)
{
#ifdef CONFIG_ZPOOL
	zpool_unregister_driver(&zs_zpool_driver);
#endif
	zs_unregister_cpu_notifier();
}

static int __init zs_init(void)
{
	int ret = zs_register_cpu_notifier();

	if (ret) {
		zs_unregister_cpu_notifier();
		return ret;
	}

#ifdef CONFIG_ZPOOL
	zpool_register_driver(&zs_zpool_driver);
#endif
	return 0;
}

static unsigned int get_maxobj_per_zspage(int size, int pages_per_zspage)
{
	return pages_per_zspage * PAGE_SIZE / size;
}

static bool can_merge(struct size_class *prev, int size, int pages_per_zspage)
{
	if (prev->pages_per_zspage != pages_per_zspage)
		return false;

	if (get_maxobj_per_zspage(prev->size, prev->pages_per_zspage)
		!= get_maxobj_per_zspage(size, pages_per_zspage))
		return false;

	return true;
}

/**
 * zs_create_pool - Creates an allocation pool to work from.
 * @flags: allocation flags used to allocate pool metadata
 *
 * This function must be called before anything when using
 * the zsmalloc allocator.
 *
 * On success, a pointer to the newly created pool is returned,
 * otherwise NULL.
 */
struct zs_pool *zs_create_pool(gfp_t flags)
{
	int i, ovhd_size;
	struct zs_pool *pool;

	ovhd_size = roundup(sizeof(*pool), PAGE_SIZE);
	pool = kzalloc(ovhd_size, GFP_KERNEL);
	if (!pool)
		return NULL;

	/*
	 * Iterate reversly, because, size of size_class that we want to use
	 * for merging should be larger or equal to current size.
	 */
	for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
		int size;
		int pages_per_zspage;
		struct size_class *class;
		struct size_class *prev_class;

		size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
		if (size > ZS_MAX_ALLOC_SIZE)
			size = ZS_MAX_ALLOC_SIZE;
		pages_per_zspage = get_pages_per_zspage(size);

		/*
		 * size_class is used for normal zsmalloc operation such
		 * as alloc/free for that size. Although it is natural that we
		 * have one size_class for each size, there is a chance that we
		 * can get more memory utilization if we use one size_class for
		 * many different sizes whose size_class have same
		 * characteristics. So, we makes size_class point to
		 * previous size_class if possible.
		 */
		if (i < ZS_SIZE_CLASSES - 1) {
			prev_class = pool->size_class[i + 1];
			if (can_merge(prev_class, size, pages_per_zspage)) {
				pool->size_class[i] = prev_class;
				continue;
			}
		}

		class = kzalloc(sizeof(struct size_class), GFP_KERNEL);
		if (!class)
			goto err;

		class->size = size;
		class->index = i;
		class->pages_per_zspage = pages_per_zspage;
		spin_lock_init(&class->lock);
		pool->size_class[i] = class;
	}

	pool->flags = flags;

	return pool;

err:
	zs_destroy_pool(pool);
	return NULL;
}
EXPORT_SYMBOL_GPL(zs_create_pool);

void zs_destroy_pool(struct zs_pool *pool)
{
	int i;

	for (i = 0; i < ZS_SIZE_CLASSES; i++) {
		int fg;
		struct size_class *class = pool->size_class[i];

		if (!class)
			continue;

		if (class->index != i)
			continue;

		for (fg = 0; fg < _ZS_NR_FULLNESS_GROUPS; fg++) {
			if (class->fullness_list[fg]) {
				pr_info("Freeing non-empty class with size %db, fullness group %d\n",
					class->size, fg);
			}
		}
		kfree(class);
	}
	kfree(pool);
}
EXPORT_SYMBOL_GPL(zs_destroy_pool);

/**
 * zs_malloc - Allocate block of given size from pool.
 * @pool: pool to allocate from
 * @size: size of block to allocate
 *
 * On success, handle to the allocated object is returned,
 * otherwise 0.
 * Allocation requests with size > ZS_MAX_ALLOC_SIZE will fail.
 */
unsigned long zs_malloc(struct zs_pool *pool, size_t size)
{
	unsigned long obj;
	struct link_free *link;
	struct size_class *class;
	void *vaddr;

	struct page *first_page, *m_page;
	unsigned long m_objidx, m_offset;

	if (unlikely(!size || size > ZS_MAX_ALLOC_SIZE))
		return 0;

	class = pool->size_class[get_size_class_index(size)];

	spin_lock(&class->lock);
	first_page = find_get_zspage(class);

	if (!first_page) {
		spin_unlock(&class->lock);
		first_page = alloc_zspage(class, pool->flags);
		if (unlikely(!first_page))
			return 0;

		set_zspage_mapping(first_page, class->index, ZS_EMPTY);
		atomic_long_add(class->pages_per_zspage,
					&pool->pages_allocated);
		spin_lock(&class->lock);
	}

	obj = (unsigned long)first_page->freelist;
	obj_handle_to_location(obj, &m_page, &m_objidx);
	m_offset = obj_idx_to_offset(m_page, m_objidx, class->size);

	vaddr = kmap_atomic(m_page);
	link = (struct link_free *)vaddr + m_offset / sizeof(*link);
	first_page->freelist = link->next;
	memset(link, POISON_INUSE, sizeof(*link));
	kunmap_atomic(vaddr);

	first_page->inuse++;
	/* Now move the zspage to another fullness group, if required */
	fix_fullness_group(pool, first_page);
	spin_unlock(&class->lock);

	return obj;
}
EXPORT_SYMBOL_GPL(zs_malloc);

void zs_free(struct zs_pool *pool, unsigned long obj)
{
	struct link_free *link;
	struct page *first_page, *f_page;
	unsigned long f_objidx, f_offset;
	void *vaddr;

	int class_idx;
	struct size_class *class;
	enum fullness_group fullness;

	if (unlikely(!obj))
		return;

	obj_handle_to_location(obj, &f_page, &f_objidx);
	first_page = get_first_page(f_page);

	get_zspage_mapping(first_page, &class_idx, &fullness);
	class = pool->size_class[class_idx];
	f_offset = obj_idx_to_offset(f_page, f_objidx, class->size);

	spin_lock(&class->lock);

	/* Insert this object in containing zspage's freelist */
	vaddr = kmap_atomic(f_page);
	link = (struct link_free *)(vaddr + f_offset);
	link->next = first_page->freelist;
	kunmap_atomic(vaddr);
	first_page->freelist = (void *)obj;

	first_page->inuse--;
	fullness = fix_fullness_group(pool, first_page);
	spin_unlock(&class->lock);

	if (fullness == ZS_EMPTY) {
		atomic_long_sub(class->pages_per_zspage,
				&pool->pages_allocated);
		free_zspage(first_page);
	}
}
EXPORT_SYMBOL_GPL(zs_free);

/**
 * zs_map_object - get address of allocated object from handle.
 * @pool: pool from which the object was allocated
 * @handle: handle returned from zs_malloc
 *
 * Before using an object allocated from zs_malloc, it must be mapped using
 * this function. When done with the object, it must be unmapped using
 * zs_unmap_object.
 *
 * Only one object can be mapped per cpu at a time. There is no protection
 * against nested mappings.
 *
 * This function returns with preemption and page faults disabled.
 */
void *zs_map_object(struct zs_pool *pool, unsigned long handle,
			enum zs_mapmode mm)
{
	struct page *page;
	unsigned long obj_idx, off;

	unsigned int class_idx;
	enum fullness_group fg;
	struct size_class *class;
	struct mapping_area *area;
	struct page *pages[2];

	BUG_ON(!handle);

	/*
	 * Because we use per-cpu mapping areas shared among the
	 * pools/users, we can't allow mapping in interrupt context
	 * because it can corrupt another users mappings.
	 */
	BUG_ON(in_interrupt());

	obj_handle_to_location(handle, &page, &obj_idx);
	get_zspage_mapping(get_first_page(page), &class_idx, &fg);
	class = pool->size_class[class_idx];
	off = obj_idx_to_offset(page, obj_idx, class->size);

	area = &get_cpu_var(zs_map_area);
	area->vm_mm = mm;
	if (off + class->size <= PAGE_SIZE) {
		/* this object is contained entirely within a page */
		area->vm_addr = kmap_atomic(page);
		return area->vm_addr + off;
	}

	/* this object spans two pages */
	pages[0] = page;
	pages[1] = get_next_page(page);
	BUG_ON(!pages[1]);

	return __zs_map_object(area, pages, off, class->size);
}
EXPORT_SYMBOL_GPL(zs_map_object);

void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
{
	struct page *page;
	unsigned long obj_idx, off;

	unsigned int class_idx;
	enum fullness_group fg;
	struct size_class *class;
	struct mapping_area *area;

	BUG_ON(!handle);

	obj_handle_to_location(handle, &page, &obj_idx);
	get_zspage_mapping(get_first_page(page), &class_idx, &fg);
	class = pool->size_class[class_idx];
	off = obj_idx_to_offset(page, obj_idx, class->size);

	area = this_cpu_ptr(&zs_map_area);
	if (off + class->size <= PAGE_SIZE)
		kunmap_atomic(area->vm_addr);
	else {
		struct page *pages[2];

		pages[0] = page;
		pages[1] = get_next_page(page);
		BUG_ON(!pages[1]);

		__zs_unmap_object(area, pages, off, class->size);
	}
	put_cpu_var(zs_map_area);
}
EXPORT_SYMBOL_GPL(zs_unmap_object);

unsigned long zs_get_total_pages(struct zs_pool *pool)
{
	return atomic_long_read(&pool->pages_allocated);
}
EXPORT_SYMBOL_GPL(zs_get_total_pages);

module_init(zs_init);
module_exit(zs_exit);

MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("Nitin Gupta <ngupta@vflare.org>");