/*
   * 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>
  
  /*
   * 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 254 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 = 1/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;
  
  	/* stats */
  	u64 pages_allocated;
  
  	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 */
  };
  
  /*
   * 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 */
  };
  
  
  /* 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, objs_on_page;
  
  		/*
  		 * 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;
  
  		link = (struct link_free *)kmap_atomic(page) +
  						off / sizeof(*link);
  		objs_on_page = (PAGE_SIZE - off) / class->size;
  
  		for (i = 1; i <= objs_on_page; i++) {
  			off += class->size;
  			if (off < 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(link);
  		page = next_page;
  		off = (off + class->size) % 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_exit(void)
  {
  	int cpu;
  
  	for_each_online_cpu(cpu)
  		zs_cpu_notifier(NULL, CPU_DEAD, (void *)(long)cpu);
  	unregister_cpu_notifier(&zs_cpu_nb);
  }
  
  static int zs_init(void)
  {
  	int cpu, ret;
  
  	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))
  			goto fail;
  	}
  	return 0;
  fail:
  	zs_exit();
  	return notifier_to_errno(ret);
  }
  
  /**
   * 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;
  
  	for (i = 0; i < ZS_SIZE_CLASSES; i++) {
  		int size;
  		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;
  
  		class = &pool->size_class[i];
  		class->size = size;
  		class->index = i;
  		spin_lock_init(&class->lock);
  		class->pages_per_zspage = get_pages_per_zspage(size);
  
  	}
  
  	pool->flags = flags;
  
  	return pool;
  }
  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];
  
  		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(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;
  	int class_idx;
  	struct size_class *class;
  
  	struct page *first_page, *m_page;
  	unsigned long m_objidx, m_offset;
  
  	if (unlikely(!size || size > ZS_MAX_ALLOC_SIZE))
  		return 0;
  
  	class_idx = get_size_class_index(size);
  	class = &pool->size_class[class_idx];
  	BUG_ON(class_idx != class->index);
  
  	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);
  		spin_lock(&class->lock);
  		class->pages_allocated += class->pages_per_zspage;
  	}
  
  	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);
  
  	link = (struct link_free *)kmap_atomic(m_page) +
  					m_offset / sizeof(*link);
  	first_page->freelist = link->next;
  	memset(link, POISON_INUSE, sizeof(*link));
  	kunmap_atomic(link);
  
  	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;
  
  	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 */
  	link = (struct link_free *)((unsigned char *)kmap_atomic(f_page)
  							+ f_offset);
  	link->next = first_page->freelist;
  	kunmap_atomic(link);
  	first_page->freelist = (void *)obj;
  
  	first_page->inuse--;
  	fullness = fix_fullness_group(pool, first_page);
  
  	if (fullness == ZS_EMPTY)
  		class->pages_allocated -= class->pages_per_zspage;
  
  	spin_unlock(&class->lock);
  
  	if (fullness == ZS_EMPTY)
  		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 = &__get_cpu_var(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);
  
  u64 zs_get_total_size_bytes(struct zs_pool *pool)
  {
  	int i;
  	u64 npages = 0;
  
  	for (i = 0; i < ZS_SIZE_CLASSES; i++)
  		npages += pool->size_class[i].pages_allocated;
  
  	return npages << PAGE_SHIFT;
  }
  EXPORT_SYMBOL_GPL(zs_get_total_size_bytes);
  
  module_init(zs_init);
  module_exit(zs_exit);
  
  MODULE_LICENSE("Dual BSD/GPL");
  MODULE_AUTHOR("Nitin Gupta <ngupta@vflare.org>");