/*
   * 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
   */

  /*

   * Following is how we use various fields and flags of underlying
   * struct page(s) to form a zspage.
   *
   * Usage of struct page fields:

   *	page->private: 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: refers to the component page after the first page

   *		If the page is first_page for huge object, it stores handle.
   *		Look at size_class->huge.

   *	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

   *	page->inuse: the number of objects that are used in this zspage

   *
   * Usage of struct page flags:
   *	PG_private: identifies the first component page
   *	PG_private2: identifies the last component page
   *
   */

  #include <linux/module.h>
  #include <linux/kernel.h>

  #include <linux/sched.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/preempt.h>

  #include <linux/spinlock.h>
  #include <linux/types.h>

  #include <linux/debugfs.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)

  #define ZS_HANDLE_SIZE (sizeof(unsigned long))

  /*
   * 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)

  
  /*
   * Memory for allocating for handle keeps object position by
   * encoding <page, obj_idx> and the encoded value has a room
   * in least bit(ie, look at obj_to_location).
   * We use the bit to synchronize between object access by
   * user and migration.
   */
  #define HANDLE_PIN_BIT	0
  
  /*
   * Head in allocated object should have OBJ_ALLOCATED_TAG
   * to identify the object was allocated or not.
   * It's okay to add the status bit in the least bit because
   * header keeps handle which is 4byte-aligned address so we
   * have room for two bit at least.
   */
  #define OBJ_ALLOCATED_TAG 1
  #define OBJ_TAG_BITS 1
  #define OBJ_INDEX_BITS	(BITS_PER_LONG - _PFN_BITS - OBJ_TAG_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))

  /* each chunk includes extra space to keep handle */

  #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)

  
  /*
   * 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
  };

  enum zs_stat_type {
  	OBJ_ALLOCATED,
  	OBJ_USED,

  	CLASS_ALMOST_FULL,
  	CLASS_ALMOST_EMPTY,

  };

  #ifdef CONFIG_ZSMALLOC_STAT
  #define NR_ZS_STAT_TYPE	(CLASS_ALMOST_EMPTY + 1)
  #else
  #define NR_ZS_STAT_TYPE	(OBJ_USED + 1)
  #endif

  struct zs_size_stat {
  	unsigned long objs[NR_ZS_STAT_TYPE];
  };

  #ifdef CONFIG_ZSMALLOC_STAT
  static struct dentry *zs_stat_root;

  #endif

  /*

   * number of size_classes
   */
  static int zs_size_classes;
  
  /*

   * 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 {

  	spinlock_t lock;
  	struct page *fullness_list[_ZS_NR_FULLNESS_GROUPS];

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

  	struct zs_size_stat stats;

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

  	/* huge object: pages_per_zspage == 1 && maxobj_per_zspage == 1 */
  	bool huge;

  };
  
  /*
   * 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 {

  	union {
  		/*
  		 * Position of next free chunk (encodes <PFN, obj_idx>)
  		 * It's valid for non-allocated object
  		 */
  		void *next;
  		/*
  		 * Handle of allocated object.
  		 */
  		unsigned long handle;
  	};

  };
  
  struct zs_pool {

  	const char *name;

  	struct size_class **size_class;

  	struct kmem_cache *handle_cachep;

  
  	gfp_t flags;	/* allocation flags used when growing pool */

  	atomic_long_t pages_allocated;

  	struct zs_pool_stats stats;

  
  	/* Compact classes */
  	struct shrinker shrinker;
  	/*
  	 * To signify that register_shrinker() was successful
  	 * and unregister_shrinker() will not Oops.
  	 */
  	bool shrinker_enabled;

  #ifdef CONFIG_ZSMALLOC_STAT
  	struct dentry *stat_dentry;
  #endif

  };

  
  /*
   * 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 */
  };

  static int create_handle_cache(struct zs_pool *pool)
  {
  	pool->handle_cachep = kmem_cache_create("zs_handle", ZS_HANDLE_SIZE,
  					0, 0, NULL);
  	return pool->handle_cachep ? 0 : 1;
  }
  
  static void destroy_handle_cache(struct zs_pool *pool)
  {

  	kmem_cache_destroy(pool->handle_cachep);

  }
  
  static unsigned long alloc_handle(struct zs_pool *pool)
  {
  	return (unsigned long)kmem_cache_alloc(pool->handle_cachep,
  		pool->flags & ~__GFP_HIGHMEM);
  }
  
  static void free_handle(struct zs_pool *pool, unsigned long handle)
  {
  	kmem_cache_free(pool->handle_cachep, (void *)handle);
  }
  
  static void record_obj(unsigned long handle, unsigned long obj)
  {

  	/*
  	 * lsb of @obj represents handle lock while other bits
  	 * represent object value the handle is pointing so
  	 * updating shouldn't do store tearing.
  	 */
  	WRITE_ONCE(*(unsigned long *)handle, obj);

  }

  /* zpool driver */
  
  #ifdef CONFIG_ZPOOL

  static void *zs_zpool_create(const char *name, gfp_t gfp,

  			     const struct zpool_ops *zpool_ops,

  			     struct zpool *zpool)

  {

  	return zs_create_pool(name, 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 */

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

  /* 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 min(zs_size_classes - 1, idx);

  }

  static inline void zs_stat_inc(struct size_class *class,
  				enum zs_stat_type type, unsigned long cnt)
  {

  	if (type < NR_ZS_STAT_TYPE)
  		class->stats.objs[type] += cnt;

  }
  
  static inline void zs_stat_dec(struct size_class *class,
  				enum zs_stat_type type, unsigned long cnt)
  {

  	if (type < NR_ZS_STAT_TYPE)
  		class->stats.objs[type] -= cnt;

  }
  
  static inline unsigned long zs_stat_get(struct size_class *class,
  				enum zs_stat_type type)
  {

  	if (type < NR_ZS_STAT_TYPE)
  		return class->stats.objs[type];
  	return 0;

  }

  #ifdef CONFIG_ZSMALLOC_STAT

  static int __init zs_stat_init(void)
  {
  	if (!debugfs_initialized())
  		return -ENODEV;
  
  	zs_stat_root = debugfs_create_dir("zsmalloc", NULL);
  	if (!zs_stat_root)
  		return -ENOMEM;
  
  	return 0;
  }
  
  static void __exit zs_stat_exit(void)
  {
  	debugfs_remove_recursive(zs_stat_root);
  }

  static unsigned long zs_can_compact(struct size_class *class);

  static int zs_stats_size_show(struct seq_file *s, void *v)
  {
  	int i;
  	struct zs_pool *pool = s->private;
  	struct size_class *class;
  	int objs_per_zspage;
  	unsigned long class_almost_full, class_almost_empty;

  	unsigned long obj_allocated, obj_used, pages_used, freeable;

  	unsigned long total_class_almost_full = 0, total_class_almost_empty = 0;
  	unsigned long total_objs = 0, total_used_objs = 0, total_pages = 0;

  	unsigned long total_freeable = 0;

  	seq_printf(s, " %5s %5s %11s %12s %13s %10s %10s %16s %8s
  ",

  			"class", "size", "almost_full", "almost_empty",
  			"obj_allocated", "obj_used", "pages_used",

  			"pages_per_zspage", "freeable");

  
  	for (i = 0; i < zs_size_classes; i++) {
  		class = pool->size_class[i];
  
  		if (class->index != i)
  			continue;
  
  		spin_lock(&class->lock);
  		class_almost_full = zs_stat_get(class, CLASS_ALMOST_FULL);
  		class_almost_empty = zs_stat_get(class, CLASS_ALMOST_EMPTY);
  		obj_allocated = zs_stat_get(class, OBJ_ALLOCATED);
  		obj_used = zs_stat_get(class, OBJ_USED);

  		freeable = zs_can_compact(class);

  		spin_unlock(&class->lock);
  
  		objs_per_zspage = get_maxobj_per_zspage(class->size,
  				class->pages_per_zspage);
  		pages_used = obj_allocated / objs_per_zspage *
  				class->pages_per_zspage;

  		seq_printf(s, " %5u %5u %11lu %12lu %13lu"
  				" %10lu %10lu %16d %8lu
  ",

  			i, class->size, class_almost_full, class_almost_empty,
  			obj_allocated, obj_used, pages_used,

  			class->pages_per_zspage, freeable);

  
  		total_class_almost_full += class_almost_full;
  		total_class_almost_empty += class_almost_empty;
  		total_objs += obj_allocated;
  		total_used_objs += obj_used;
  		total_pages += pages_used;

  		total_freeable += freeable;

  	}
  
  	seq_puts(s, "
  ");

  	seq_printf(s, " %5s %5s %11lu %12lu %13lu %10lu %10lu %16s %8lu
  ",

  			"Total", "", total_class_almost_full,
  			total_class_almost_empty, total_objs,

  			total_used_objs, total_pages, "", total_freeable);

  
  	return 0;
  }
  
  static int zs_stats_size_open(struct inode *inode, struct file *file)
  {
  	return single_open(file, zs_stats_size_show, inode->i_private);
  }
  
  static const struct file_operations zs_stat_size_ops = {
  	.open           = zs_stats_size_open,
  	.read           = seq_read,
  	.llseek         = seq_lseek,
  	.release        = single_release,
  };

  static int zs_pool_stat_create(const char *name, struct zs_pool *pool)

  {
  	struct dentry *entry;
  
  	if (!zs_stat_root)
  		return -ENODEV;
  
  	entry = debugfs_create_dir(name, zs_stat_root);
  	if (!entry) {
  		pr_warn("debugfs dir <%s> creation failed
  ", name);
  		return -ENOMEM;
  	}
  	pool->stat_dentry = entry;
  
  	entry = debugfs_create_file("classes", S_IFREG | S_IRUGO,
  			pool->stat_dentry, pool, &zs_stat_size_ops);
  	if (!entry) {
  		pr_warn("%s: debugfs file entry <%s> creation failed
  ",
  				name, "classes");
  		return -ENOMEM;
  	}
  
  	return 0;
  }
  
  static void zs_pool_stat_destroy(struct zs_pool *pool)
  {
  	debugfs_remove_recursive(pool->stat_dentry);
  }
  
  #else /* CONFIG_ZSMALLOC_STAT */

  static int __init zs_stat_init(void)
  {
  	return 0;
  }
  
  static void __exit zs_stat_exit(void)
  {
  }

  static inline int zs_pool_stat_create(const char *name, struct zs_pool *pool)

  {
  	return 0;
  }
  
  static inline void zs_pool_stat_destroy(struct zs_pool *pool)
  {
  }

  #endif

  /*
   * 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 <= 3 * 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;

  	zs_stat_inc(class, fullness == ZS_ALMOST_EMPTY ?
  			CLASS_ALMOST_EMPTY : CLASS_ALMOST_FULL, 1);

  
  	head = &class->fullness_list[fullness];
  	if (!*head) {
  		*head = page;
  		return;
  	}
  
  	/*
  	 * We want to see more ZS_FULL pages and less almost
  	 * empty/full. Put pages with higher ->inuse first.
  	 */
  	list_add_tail(&page->lru, &(*head)->lru);
  	if (page->inuse >= (*head)->inuse)
  		*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);

  	zs_stat_dec(class, fullness == ZS_ALMOST_EMPTY ?
  			CLASS_ALMOST_EMPTY : CLASS_ALMOST_FULL, 1);

  }

  /*
   * 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 size_class *class,

  						struct page *page)
  {
  	int class_idx;

  	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;

  	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 % class_size
   *     usage = Zp - wastage

   * 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 (struct page *)page_private(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.

   * We use the least bit of handle for tagging.

   */

  static void *location_to_obj(struct page *page, unsigned long obj_idx)

  {

  	unsigned long obj;

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

  	obj = page_to_pfn(page) << OBJ_INDEX_BITS;
  	obj |= ((obj_idx) & OBJ_INDEX_MASK);
  	obj <<= OBJ_TAG_BITS;

  	return (void *)obj;

  }

  /*
   * 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

   * location_to_obj().

   */

  static void obj_to_location(unsigned long obj, struct page **page,

  				unsigned long *obj_idx)
  {

  	obj >>= OBJ_TAG_BITS;
  	*page = pfn_to_page(obj >> OBJ_INDEX_BITS);
  	*obj_idx = (obj & OBJ_INDEX_MASK);

  }

  static unsigned long handle_to_obj(unsigned long handle)
  {
  	return *(unsigned long *)handle;
  }

  static unsigned long obj_to_head(struct size_class *class, struct page *page,
  			void *obj)

  {

  	if (class->huge) {
  		VM_BUG_ON(!is_first_page(page));

  		return page_private(page);

  	} else
  		return *(unsigned long *)obj;

  }

  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 inline int trypin_tag(unsigned long handle)
  {
  	unsigned long *ptr = (unsigned long *)handle;
  
  	return !test_and_set_bit_lock(HANDLE_PIN_BIT, ptr);
  }
  
  static void pin_tag(unsigned long handle)
  {
  	while (!trypin_tag(handle));
  }
  
  static void unpin_tag(unsigned long handle)
  {
  	unsigned long *ptr = (unsigned long *)handle;
  
  	clear_bit_unlock(HANDLE_PIN_BIT, ptr);
  }

  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 = location_to_obj(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 = location_to_obj(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->private

  	 *
  	 * 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)

  			set_page_private(page, (unsigned long)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 = location_to_obj(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 = kmalloc(ZS_MAX_ALLOC_SIZE, GFP_KERNEL);

  	if (!area->vm_buf)
  		return -ENOMEM;
  	return 0;
  }
  
  static inline void __zs_cpu_down(struct mapping_area *area)
  {

  	kfree(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;

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

  	buf = area->vm_buf;

  	buf = buf + ZS_HANDLE_SIZE;
  	size -= ZS_HANDLE_SIZE;
  	off += ZS_HANDLE_SIZE;

  	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 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 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 void init_zs_size_classes(void)

  {

  	int nr;

  	nr = (ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / ZS_SIZE_CLASS_DELTA + 1;
  	if ((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) % ZS_SIZE_CLASS_DELTA)
  		nr += 1;

  	zs_size_classes = nr;

  }

  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;
  }

  static bool zspage_full(struct page *page)
  {
  	BUG_ON(!is_first_page(page));
  
  	return page->inuse == page->objects;
  }

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

  /**

   * 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, obj_idx, off;

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

  	void *ret;

  	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());

  	/* From now on, migration cannot move the object */
  	pin_tag(handle);

  	obj = handle_to_obj(handle);
  	obj_to_location(obj, &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);

  		ret = area->vm_addr + off;
  		goto out;

  	}

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

  	ret = __zs_map_object(area, pages, off, class->size);
  out:

  	if (!class->huge)
  		ret += ZS_HANDLE_SIZE;
  
  	return ret;

  }

  EXPORT_SYMBOL_GPL(zs_map_object);

  void zs_unmap_object(struct zs_pool *pool, unsigned long handle)

  {

  	struct page *page;

  	unsigned long obj, 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_obj(handle);
  	obj_to_location(obj, &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);

  	unpin_tag(handle);

  }

  EXPORT_SYMBOL_GPL(zs_unmap_object);

  static unsigned long obj_malloc(struct page *first_page,
  		struct size_class *class, unsigned long handle)
  {
  	unsigned long obj;
  	struct link_free *link;
  
  	struct page *m_page;
  	unsigned long m_objidx, m_offset;
  	void *vaddr;

  	handle |= OBJ_ALLOCATED_TAG;

  	obj = (unsigned long)first_page->freelist;
  	obj_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;

  	if (!class->huge)
  		/* record handle in the header of allocated chunk */
  		link->handle = handle;
  	else
  		/* record handle in first_page->private */
  		set_page_private(first_page, handle);

  	kunmap_atomic(vaddr);
  	first_page->inuse++;
  	zs_stat_inc(class, OBJ_USED, 1);
  
  	return obj;
  }

  /**
   * 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 handle, obj;

  	struct size_class *class;

  	struct page *first_page;

  	if (unlikely(!size || size > ZS_MAX_ALLOC_SIZE))

  		return 0;
  
  	handle = alloc_handle(pool);
  	if (!handle)

  		return 0;

  	/* extra space in chunk to keep the handle */
  	size += ZS_HANDLE_SIZE;

  	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)) {
  			free_handle(pool, handle);

  			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);

  		zs_stat_inc(class, OBJ_ALLOCATED, get_maxobj_per_zspage(
  				class->size, class->pages_per_zspage));

  	}

  	obj = obj_malloc(first_page, class, handle);

  	/* Now move the zspage to another fullness group, if required */

  	fix_fullness_group(class, first_page);

  	record_obj(handle, obj);

  	spin_unlock(&class->lock);

  	return handle;

  }
  EXPORT_SYMBOL_GPL(zs_malloc);

  static void obj_free(struct zs_pool *pool, struct size_class *class,
  			unsigned long obj)

  {
  	struct link_free *link;
  	struct page *first_page, *f_page;

  	unsigned long f_objidx, f_offset;

  	void *vaddr;

  	BUG_ON(!obj);

  	obj &= ~OBJ_ALLOCATED_TAG;

  	obj_to_location(obj, &f_page, &f_objidx);

  	first_page = get_first_page(f_page);

  	f_offset = obj_idx_to_offset(f_page, f_objidx, class->size);

  	vaddr = kmap_atomic(f_page);

  
  	/* Insert this object in containing zspage's freelist */

  	link = (struct link_free *)(vaddr + f_offset);

  	link->next = first_page->freelist;

  	if (class->huge)
  		set_page_private(first_page, 0);

  	kunmap_atomic(vaddr);

  	first_page->freelist = (void *)obj;

  	first_page->inuse--;

  	zs_stat_dec(class, OBJ_USED, 1);

  }
  
  void zs_free(struct zs_pool *pool, unsigned long handle)
  {
  	struct page *first_page, *f_page;
  	unsigned long obj, f_objidx;
  	int class_idx;
  	struct size_class *class;
  	enum fullness_group fullness;
  
  	if (unlikely(!handle))
  		return;

  	pin_tag(handle);

  	obj = handle_to_obj(handle);

  	obj_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];
  
  	spin_lock(&class->lock);
  	obj_free(pool, class, obj);
  	fullness = fix_fullness_group(class, first_page);

  	if (fullness == ZS_EMPTY) {

  		zs_stat_dec(class, OBJ_ALLOCATED, get_maxobj_per_zspage(
  				class->size, class->pages_per_zspage));

  		atomic_long_sub(class->pages_per_zspage,
  				&pool->pages_allocated);
  		free_zspage(first_page);
  	}

  	spin_unlock(&class->lock);

  	unpin_tag(handle);

  	free_handle(pool, handle);
  }
  EXPORT_SYMBOL_GPL(zs_free);

  static void zs_object_copy(unsigned long dst, unsigned long src,

  				struct size_class *class)
  {
  	struct page *s_page, *d_page;
  	unsigned long s_objidx, d_objidx;
  	unsigned long s_off, d_off;
  	void *s_addr, *d_addr;
  	int s_size, d_size, size;
  	int written = 0;
  
  	s_size = d_size = class->size;
  
  	obj_to_location(src, &s_page, &s_objidx);
  	obj_to_location(dst, &d_page, &d_objidx);
  
  	s_off = obj_idx_to_offset(s_page, s_objidx, class->size);
  	d_off = obj_idx_to_offset(d_page, d_objidx, class->size);
  
  	if (s_off + class->size > PAGE_SIZE)
  		s_size = PAGE_SIZE - s_off;
  
  	if (d_off + class->size > PAGE_SIZE)
  		d_size = PAGE_SIZE - d_off;
  
  	s_addr = kmap_atomic(s_page);
  	d_addr = kmap_atomic(d_page);
  
  	while (1) {
  		size = min(s_size, d_size);
  		memcpy(d_addr + d_off, s_addr + s_off, size);
  		written += size;
  
  		if (written == class->size)
  			break;

  		s_off += size;
  		s_size -= size;
  		d_off += size;
  		d_size -= size;
  
  		if (s_off >= PAGE_SIZE) {

  			kunmap_atomic(d_addr);
  			kunmap_atomic(s_addr);
  			s_page = get_next_page(s_page);
  			BUG_ON(!s_page);
  			s_addr = kmap_atomic(s_page);
  			d_addr = kmap_atomic(d_page);
  			s_size = class->size - written;
  			s_off = 0;

  		}

  		if (d_off >= PAGE_SIZE) {

  			kunmap_atomic(d_addr);
  			d_page = get_next_page(d_page);
  			BUG_ON(!d_page);
  			d_addr = kmap_atomic(d_page);
  			d_size = class->size - written;
  			d_off = 0;

  		}
  	}
  
  	kunmap_atomic(d_addr);
  	kunmap_atomic(s_addr);
  }
  
  /*
   * Find alloced object in zspage from index object and
   * return handle.
   */
  static unsigned long find_alloced_obj(struct page *page, int index,
  					struct size_class *class)
  {
  	unsigned long head;
  	int offset = 0;
  	unsigned long handle = 0;
  	void *addr = kmap_atomic(page);
  
  	if (!is_first_page(page))
  		offset = page->index;
  	offset += class->size * index;
  
  	while (offset < PAGE_SIZE) {

  		head = obj_to_head(class, page, addr + offset);

  		if (head & OBJ_ALLOCATED_TAG) {
  			handle = head & ~OBJ_ALLOCATED_TAG;
  			if (trypin_tag(handle))
  				break;
  			handle = 0;
  		}
  
  		offset += class->size;
  		index++;
  	}
  
  	kunmap_atomic(addr);
  	return handle;
  }
  
  struct zs_compact_control {
  	/* Source page for migration which could be a subpage of zspage. */
  	struct page *s_page;
  	/* Destination page for migration which should be a first page
  	 * of zspage. */
  	struct page *d_page;
  	 /* Starting object index within @s_page which used for live object
  	  * in the subpage. */
  	int index;

  };
  
  static int migrate_zspage(struct zs_pool *pool, struct size_class *class,
  				struct zs_compact_control *cc)
  {
  	unsigned long used_obj, free_obj;
  	unsigned long handle;
  	struct page *s_page = cc->s_page;
  	struct page *d_page = cc->d_page;
  	unsigned long index = cc->index;

  	int ret = 0;
  
  	while (1) {
  		handle = find_alloced_obj(s_page, index, class);
  		if (!handle) {
  			s_page = get_next_page(s_page);
  			if (!s_page)
  				break;
  			index = 0;
  			continue;
  		}
  
  		/* Stop if there is no more space */
  		if (zspage_full(d_page)) {
  			unpin_tag(handle);
  			ret = -ENOMEM;
  			break;
  		}
  
  		used_obj = handle_to_obj(handle);
  		free_obj = obj_malloc(d_page, class, handle);

  		zs_object_copy(free_obj, used_obj, class);

  		index++;

  		/*
  		 * record_obj updates handle's value to free_obj and it will
  		 * invalidate lock bit(ie, HANDLE_PIN_BIT) of handle, which
  		 * breaks synchronization using pin_tag(e,g, zs_free) so
  		 * let's keep the lock bit.
  		 */
  		free_obj |= BIT(HANDLE_PIN_BIT);

  		record_obj(handle, free_obj);
  		unpin_tag(handle);
  		obj_free(pool, class, used_obj);

  	}
  
  	/* Remember last position in this iteration */
  	cc->s_page = s_page;
  	cc->index = index;

  
  	return ret;
  }

  static struct page *isolate_target_page(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) {
  			remove_zspage(page, class, i);
  			break;
  		}
  	}
  
  	return page;
  }

  /*
   * putback_zspage - add @first_page into right class's fullness list
   * @pool: target pool
   * @class: destination class
   * @first_page: target page
   *
   * Return @fist_page's fullness_group
   */
  static enum fullness_group putback_zspage(struct zs_pool *pool,
  			struct size_class *class,
  			struct page *first_page)

  {

  	enum fullness_group fullness;
  
  	BUG_ON(!is_first_page(first_page));

  	fullness = get_fullness_group(first_page);

  	insert_zspage(first_page, class, fullness);

  	set_zspage_mapping(first_page, class->index, fullness);

  	if (fullness == ZS_EMPTY) {

  		zs_stat_dec(class, OBJ_ALLOCATED, get_maxobj_per_zspage(
  			class->size, class->pages_per_zspage));

  		atomic_long_sub(class->pages_per_zspage,
  				&pool->pages_allocated);

  		free_zspage(first_page);

  	}

  
  	return fullness;

  }

  
  static struct page *isolate_source_page(struct size_class *class)
  {

  	int i;
  	struct page *page = NULL;
  
  	for (i = ZS_ALMOST_EMPTY; i >= ZS_ALMOST_FULL; i--) {
  		page = class->fullness_list[i];
  		if (!page)
  			continue;

  		remove_zspage(page, class, i);
  		break;
  	}

  
  	return page;
  }

  /*
   *
   * Based on the number of unused allocated objects calculate
   * and return the number of pages that we can free.

   */
  static unsigned long zs_can_compact(struct size_class *class)
  {
  	unsigned long obj_wasted;

  	obj_wasted = zs_stat_get(class, OBJ_ALLOCATED) -
  		zs_stat_get(class, OBJ_USED);
  
  	obj_wasted /= get_maxobj_per_zspage(class->size,
  			class->pages_per_zspage);

  	return obj_wasted * class->pages_per_zspage;

  }

  static void __zs_compact(struct zs_pool *pool, struct size_class *class)

  {

  	struct zs_compact_control cc;
  	struct page *src_page;
  	struct page *dst_page = NULL;

  	spin_lock(&class->lock);
  	while ((src_page = isolate_source_page(class))) {
  
  		BUG_ON(!is_first_page(src_page));

  		if (!zs_can_compact(class))
  			break;

  		cc.index = 0;
  		cc.s_page = src_page;

  		while ((dst_page = isolate_target_page(class))) {

  			cc.d_page = dst_page;
  			/*

  			 * If there is no more space in dst_page, resched
  			 * and see if anyone had allocated another zspage.

  			 */
  			if (!migrate_zspage(pool, class, &cc))
  				break;
  
  			putback_zspage(pool, class, dst_page);

  		}
  
  		/* Stop if we couldn't find slot */
  		if (dst_page == NULL)
  			break;
  
  		putback_zspage(pool, class, dst_page);

  		if (putback_zspage(pool, class, src_page) == ZS_EMPTY)

  			pool->stats.pages_compacted += class->pages_per_zspage;

  		spin_unlock(&class->lock);

  		cond_resched();
  		spin_lock(&class->lock);
  	}
  
  	if (src_page)
  		putback_zspage(pool, class, src_page);

  	spin_unlock(&class->lock);

  }
  
  unsigned long zs_compact(struct zs_pool *pool)
  {
  	int i;

  	struct size_class *class;
  
  	for (i = zs_size_classes - 1; i >= 0; i--) {
  		class = pool->size_class[i];
  		if (!class)
  			continue;
  		if (class->index != i)
  			continue;

  		__zs_compact(pool, class);

  	}

  	return pool->stats.pages_compacted;

  }
  EXPORT_SYMBOL_GPL(zs_compact);

  void zs_pool_stats(struct zs_pool *pool, struct zs_pool_stats *stats)
  {
  	memcpy(stats, &pool->stats, sizeof(struct zs_pool_stats));
  }
  EXPORT_SYMBOL_GPL(zs_pool_stats);

  static unsigned long zs_shrinker_scan(struct shrinker *shrinker,
  		struct shrink_control *sc)
  {
  	unsigned long pages_freed;
  	struct zs_pool *pool = container_of(shrinker, struct zs_pool,
  			shrinker);
  
  	pages_freed = pool->stats.pages_compacted;
  	/*
  	 * Compact classes and calculate compaction delta.
  	 * Can run concurrently with a manually triggered
  	 * (by user) compaction.
  	 */
  	pages_freed = zs_compact(pool) - pages_freed;
  
  	return pages_freed ? pages_freed : SHRINK_STOP;
  }
  
  static unsigned long zs_shrinker_count(struct shrinker *shrinker,
  		struct shrink_control *sc)
  {
  	int i;
  	struct size_class *class;
  	unsigned long pages_to_free = 0;
  	struct zs_pool *pool = container_of(shrinker, struct zs_pool,
  			shrinker);

  	for (i = zs_size_classes - 1; i >= 0; i--) {
  		class = pool->size_class[i];
  		if (!class)
  			continue;
  		if (class->index != i)
  			continue;

  		pages_to_free += zs_can_compact(class);

  	}
  
  	return pages_to_free;
  }
  
  static void zs_unregister_shrinker(struct zs_pool *pool)
  {
  	if (pool->shrinker_enabled) {
  		unregister_shrinker(&pool->shrinker);
  		pool->shrinker_enabled = false;
  	}
  }
  
  static int zs_register_shrinker(struct zs_pool *pool)
  {
  	pool->shrinker.scan_objects = zs_shrinker_scan;
  	pool->shrinker.count_objects = zs_shrinker_count;
  	pool->shrinker.batch = 0;
  	pool->shrinker.seeks = DEFAULT_SEEKS;
  
  	return register_shrinker(&pool->shrinker);
  }

  /**

   * 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(const char *name, gfp_t flags)

  {

  	int i;
  	struct zs_pool *pool;
  	struct size_class *prev_class = NULL;

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

  	pool->size_class = kcalloc(zs_size_classes, sizeof(struct size_class *),
  			GFP_KERNEL);
  	if (!pool->size_class) {
  		kfree(pool);
  		return NULL;
  	}

  	pool->name = kstrdup(name, GFP_KERNEL);
  	if (!pool->name)
  		goto err;
  
  	if (create_handle_cache(pool))
  		goto err;

  	/*

  	 * 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;

  		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 (prev_class) {
  			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;

  		if (pages_per_zspage == 1 &&
  			get_maxobj_per_zspage(size, pages_per_zspage) == 1)
  			class->huge = true;

  		spin_lock_init(&class->lock);
  		pool->size_class[i] = class;
  
  		prev_class = class;

  	}

  	pool->flags = flags;

  	if (zs_pool_stat_create(name, pool))
  		goto err;

  	/*
  	 * Not critical, we still can use the pool
  	 * and user can trigger compaction manually.
  	 */
  	if (zs_register_shrinker(pool) == 0)
  		pool->shrinker_enabled = true;

  	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;

  	zs_unregister_shrinker(pool);

  	zs_pool_stat_destroy(pool);

  	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
  ",
  					class->size, fg);
  			}
  		}
  		kfree(class);
  	}

  	destroy_handle_cache(pool);

  	kfree(pool->size_class);

  	kfree(pool->name);

  	kfree(pool);
  }
  EXPORT_SYMBOL_GPL(zs_destroy_pool);

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

  	if (ret)
  		goto notifier_fail;

  
  	init_zs_size_classes();
  
  #ifdef CONFIG_ZPOOL
  	zpool_register_driver(&zs_zpool_driver);
  #endif

  
  	ret = zs_stat_init();
  	if (ret) {
  		pr_err("zs stat initialization failed
  ");
  		goto stat_fail;
  	}

  	return 0;

  
  stat_fail:
  #ifdef CONFIG_ZPOOL
  	zpool_unregister_driver(&zs_zpool_driver);
  #endif
  notifier_fail:
  	zs_unregister_cpu_notifier();
  
  	return ret;

  }

  static void __exit zs_exit(void)

  {

  #ifdef CONFIG_ZPOOL
  	zpool_unregister_driver(&zs_zpool_driver);
  #endif
  	zs_unregister_cpu_notifier();

  
  	zs_stat_exit();

  }

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