/*
   * z3fold.c
   *
   * Author: Vitaly Wool <vitaly.wool@konsulko.com>
   * Copyright (C) 2016, Sony Mobile Communications Inc.
   *
   * This implementation is based on zbud written by Seth Jennings.
   *
   * z3fold is an special purpose allocator for storing compressed pages. It
   * can store up to three compressed pages per page which improves the
   * compression ratio of zbud while retaining its main concepts (e. g. always
   * storing an integral number of objects per page) and simplicity.
   * It still has simple and deterministic reclaim properties that make it
   * preferable to a higher density approach (with no requirement on integral
   * number of object per page) when reclaim is used.
   *
   * As in zbud, pages are divided into "chunks".  The size of the chunks is
   * fixed at compile time and is determined by NCHUNKS_ORDER below.
   *
   * z3fold doesn't export any API and is meant to be used via zpool API.
   */
  
  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  
  #include <linux/atomic.h>

  #include <linux/sched.h>

  #include <linux/list.h>
  #include <linux/mm.h>
  #include <linux/module.h>

  #include <linux/percpu.h>

  #include <linux/preempt.h>

  #include <linux/workqueue.h>

  #include <linux/slab.h>
  #include <linux/spinlock.h>
  #include <linux/zpool.h>
  
  /*****************
   * Structures
  *****************/

  struct z3fold_pool;
  struct z3fold_ops {
  	int (*evict)(struct z3fold_pool *pool, unsigned long handle);
  };
  
  enum buddy {
  	HEADLESS = 0,
  	FIRST,
  	MIDDLE,
  	LAST,
  	BUDDIES_MAX
  };
  
  /*

   * struct z3fold_header - z3fold page metadata occupying first chunks of each

   *			z3fold page, except for HEADLESS pages

   * @buddy:		links the z3fold page into the relevant list in the
   *			pool

   * @page_lock:		per-page lock

   * @refcount:		reference count for the z3fold page
   * @work:		work_struct for page layout optimization
   * @pool:		pointer to the pool which this page belongs to
   * @cpu:		CPU which this page "belongs" to

   * @first_chunks:	the size of the first buddy in chunks, 0 if free
   * @middle_chunks:	the size of the middle buddy in chunks, 0 if free
   * @last_chunks:	the size of the last buddy in chunks, 0 if free
   * @first_num:		the starting number (for the first handle)
   */
  struct z3fold_header {
  	struct list_head buddy;

  	spinlock_t page_lock;

  	struct kref refcount;

  	struct work_struct work;
  	struct z3fold_pool *pool;
  	short cpu;

  	unsigned short first_chunks;
  	unsigned short middle_chunks;
  	unsigned short last_chunks;
  	unsigned short start_middle;
  	unsigned short first_num:2;
  };

  /*
   * NCHUNKS_ORDER determines the internal allocation granularity, effectively
   * adjusting internal fragmentation.  It also determines the number of
   * freelists maintained in each pool. NCHUNKS_ORDER of 6 means that the

   * allocation granularity will be in chunks of size PAGE_SIZE/64. Some chunks
   * in the beginning of an allocated page are occupied by z3fold header, so
   * NCHUNKS will be calculated to 63 (or 62 in case CONFIG_DEBUG_SPINLOCK=y),
   * which shows the max number of free chunks in z3fold page, also there will
   * be 63, or 62, respectively, freelists per pool.

   */
  #define NCHUNKS_ORDER	6
  
  #define CHUNK_SHIFT	(PAGE_SHIFT - NCHUNKS_ORDER)
  #define CHUNK_SIZE	(1 << CHUNK_SHIFT)

  #define ZHDR_SIZE_ALIGNED round_up(sizeof(struct z3fold_header), CHUNK_SIZE)
  #define ZHDR_CHUNKS	(ZHDR_SIZE_ALIGNED >> CHUNK_SHIFT)
  #define TOTAL_CHUNKS	(PAGE_SIZE >> CHUNK_SHIFT)

  #define NCHUNKS		((PAGE_SIZE - ZHDR_SIZE_ALIGNED) >> CHUNK_SHIFT)

  #define BUDDY_MASK	(0x3)

  #define BUDDY_SHIFT	2

  /**
   * struct z3fold_pool - stores metadata for each z3fold pool

   * @name:	pool name
   * @lock:	protects pool unbuddied/lru lists
   * @stale_lock:	protects pool stale page list
   * @unbuddied:	per-cpu array of lists tracking z3fold pages that contain 2-
   *		buddies; the list each z3fold page is added to depends on
   *		the size of its free region.

   * @lru:	list tracking the z3fold pages in LRU order by most recently
   *		added buddy.

   * @stale:	list of pages marked for freeing

   * @pages_nr:	number of z3fold pages in the pool.
   * @ops:	pointer to a structure of user defined operations specified at
   *		pool creation time.

   * @compact_wq:	workqueue for page layout background optimization
   * @release_wq:	workqueue for safe page release
   * @work:	work_struct for safe page release

   *
   * This structure is allocated at pool creation time and maintains metadata
   * pertaining to a particular z3fold pool.
   */
  struct z3fold_pool {

  	const char *name;

  	spinlock_t lock;

  	spinlock_t stale_lock;
  	struct list_head *unbuddied;

  	struct list_head lru;

  	struct list_head stale;

  	atomic64_t pages_nr;

  	const struct z3fold_ops *ops;
  	struct zpool *zpool;
  	const struct zpool_ops *zpool_ops;

  	struct workqueue_struct *compact_wq;
  	struct workqueue_struct *release_wq;
  	struct work_struct work;

  };

  /*
   * Internal z3fold page flags
   */
  enum z3fold_page_flags {

  	PAGE_HEADLESS = 0,

  	MIDDLE_CHUNK_MAPPED,

  	NEEDS_COMPACTING,

  	PAGE_STALE,

  	PAGE_CLAIMED, /* by either reclaim or free */

  };
  
  /*****************
   * Helpers
  *****************/
  
  /* Converts an allocation size in bytes to size in z3fold chunks */
  static int size_to_chunks(size_t size)
  {
  	return (size + CHUNK_SIZE - 1) >> CHUNK_SHIFT;
  }
  
  #define for_each_unbuddied_list(_iter, _begin) \
  	for ((_iter) = (_begin); (_iter) < NCHUNKS; (_iter)++)

  static void compact_page_work(struct work_struct *w);

  /* Initializes the z3fold header of a newly allocated z3fold page */

  static struct z3fold_header *init_z3fold_page(struct page *page,
  					struct z3fold_pool *pool)

  {
  	struct z3fold_header *zhdr = page_address(page);
  
  	INIT_LIST_HEAD(&page->lru);

  	clear_bit(PAGE_HEADLESS, &page->private);
  	clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);

  	clear_bit(NEEDS_COMPACTING, &page->private);
  	clear_bit(PAGE_STALE, &page->private);

  	clear_bit(PAGE_CLAIMED, &page->private);

  	spin_lock_init(&zhdr->page_lock);

  	kref_init(&zhdr->refcount);

  	zhdr->first_chunks = 0;
  	zhdr->middle_chunks = 0;
  	zhdr->last_chunks = 0;
  	zhdr->first_num = 0;
  	zhdr->start_middle = 0;

  	zhdr->cpu = -1;
  	zhdr->pool = pool;

  	INIT_LIST_HEAD(&zhdr->buddy);

  	INIT_WORK(&zhdr->work, compact_page_work);

  	return zhdr;
  }
  
  /* Resets the struct page fields and frees the page */

  static void free_z3fold_page(struct page *page)

  {

  	__free_page(page);
  }

  /* Lock a z3fold page */
  static inline void z3fold_page_lock(struct z3fold_header *zhdr)
  {
  	spin_lock(&zhdr->page_lock);
  }

  /* Try to lock a z3fold page */
  static inline int z3fold_page_trylock(struct z3fold_header *zhdr)
  {
  	return spin_trylock(&zhdr->page_lock);
  }

  /* Unlock a z3fold page */
  static inline void z3fold_page_unlock(struct z3fold_header *zhdr)
  {
  	spin_unlock(&zhdr->page_lock);
  }

  /*
   * Encodes the handle of a particular buddy within a z3fold page
   * Pool lock should be held as this function accesses first_num
   */
  static unsigned long encode_handle(struct z3fold_header *zhdr, enum buddy bud)
  {
  	unsigned long handle;
  
  	handle = (unsigned long)zhdr;

  	if (bud != HEADLESS) {
  		handle |= (bud + zhdr->first_num) & BUDDY_MASK;
  		if (bud == LAST)
  			handle |= (zhdr->last_chunks << BUDDY_SHIFT);
  	}

  	return handle;
  }
  
  /* Returns the z3fold page where a given handle is stored */
  static struct z3fold_header *handle_to_z3fold_header(unsigned long handle)
  {
  	return (struct z3fold_header *)(handle & PAGE_MASK);
  }

  /* only for LAST bud, returns zero otherwise */
  static unsigned short handle_to_chunks(unsigned long handle)
  {
  	return (handle & ~PAGE_MASK) >> BUDDY_SHIFT;
  }

  /*
   * (handle & BUDDY_MASK) < zhdr->first_num is possible in encode_handle
   *  but that doesn't matter. because the masking will result in the
   *  correct buddy number.
   */

  static enum buddy handle_to_buddy(unsigned long handle)
  {
  	struct z3fold_header *zhdr = handle_to_z3fold_header(handle);
  	return (handle - zhdr->first_num) & BUDDY_MASK;
  }

  static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
  {
  	struct page *page = virt_to_page(zhdr);
  	struct z3fold_pool *pool = zhdr->pool;
  
  	WARN_ON(!list_empty(&zhdr->buddy));
  	set_bit(PAGE_STALE, &page->private);

  	clear_bit(NEEDS_COMPACTING, &page->private);

  	spin_lock(&pool->lock);
  	if (!list_empty(&page->lru))
  		list_del(&page->lru);
  	spin_unlock(&pool->lock);
  	if (locked)
  		z3fold_page_unlock(zhdr);
  	spin_lock(&pool->stale_lock);
  	list_add(&zhdr->buddy, &pool->stale);
  	queue_work(pool->release_wq, &pool->work);
  	spin_unlock(&pool->stale_lock);
  }
  
  static void __attribute__((__unused__))
  			release_z3fold_page(struct kref *ref)
  {
  	struct z3fold_header *zhdr = container_of(ref, struct z3fold_header,
  						refcount);
  	__release_z3fold_page(zhdr, false);
  }
  
  static void release_z3fold_page_locked(struct kref *ref)
  {
  	struct z3fold_header *zhdr = container_of(ref, struct z3fold_header,
  						refcount);
  	WARN_ON(z3fold_page_trylock(zhdr));
  	__release_z3fold_page(zhdr, true);
  }
  
  static void release_z3fold_page_locked_list(struct kref *ref)
  {
  	struct z3fold_header *zhdr = container_of(ref, struct z3fold_header,
  					       refcount);
  	spin_lock(&zhdr->pool->lock);
  	list_del_init(&zhdr->buddy);
  	spin_unlock(&zhdr->pool->lock);
  
  	WARN_ON(z3fold_page_trylock(zhdr));
  	__release_z3fold_page(zhdr, true);
  }
  
  static void free_pages_work(struct work_struct *w)
  {
  	struct z3fold_pool *pool = container_of(w, struct z3fold_pool, work);
  
  	spin_lock(&pool->stale_lock);
  	while (!list_empty(&pool->stale)) {
  		struct z3fold_header *zhdr = list_first_entry(&pool->stale,
  						struct z3fold_header, buddy);
  		struct page *page = virt_to_page(zhdr);
  
  		list_del(&zhdr->buddy);
  		if (WARN_ON(!test_bit(PAGE_STALE, &page->private)))
  			continue;

  		spin_unlock(&pool->stale_lock);
  		cancel_work_sync(&zhdr->work);
  		free_z3fold_page(page);
  		cond_resched();
  		spin_lock(&pool->stale_lock);
  	}
  	spin_unlock(&pool->stale_lock);
  }

  /*
   * Returns the number of free chunks in a z3fold page.
   * NB: can't be used with HEADLESS pages.
   */
  static int num_free_chunks(struct z3fold_header *zhdr)
  {
  	int nfree;
  	/*
  	 * If there is a middle object, pick up the bigger free space
  	 * either before or after it. Otherwise just subtract the number
  	 * of chunks occupied by the first and the last objects.
  	 */
  	if (zhdr->middle_chunks != 0) {
  		int nfree_before = zhdr->first_chunks ?

  			0 : zhdr->start_middle - ZHDR_CHUNKS;

  		int nfree_after = zhdr->last_chunks ?

  			0 : TOTAL_CHUNKS -
  				(zhdr->start_middle + zhdr->middle_chunks);

  		nfree = max(nfree_before, nfree_after);
  	} else
  		nfree = NCHUNKS - zhdr->first_chunks - zhdr->last_chunks;
  	return nfree;
  }

  static inline void *mchunk_memmove(struct z3fold_header *zhdr,
  				unsigned short dst_chunk)
  {
  	void *beg = zhdr;
  	return memmove(beg + (dst_chunk << CHUNK_SHIFT),
  		       beg + (zhdr->start_middle << CHUNK_SHIFT),
  		       zhdr->middle_chunks << CHUNK_SHIFT);
  }

  #define BIG_CHUNK_GAP	3

  /* Has to be called with lock held */
  static int z3fold_compact_page(struct z3fold_header *zhdr)
  {
  	struct page *page = virt_to_page(zhdr);

  	if (test_bit(MIDDLE_CHUNK_MAPPED, &page->private))
  		return 0; /* can't move middle chunk, it's used */

  	if (zhdr->middle_chunks == 0)
  		return 0; /* nothing to compact */
  
  	if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
  		/* move to the beginning */
  		mchunk_memmove(zhdr, ZHDR_CHUNKS);

  		zhdr->first_chunks = zhdr->middle_chunks;
  		zhdr->middle_chunks = 0;
  		zhdr->start_middle = 0;
  		zhdr->first_num++;

  		return 1;

  	}

  
  	/*
  	 * moving data is expensive, so let's only do that if
  	 * there's substantial gain (at least BIG_CHUNK_GAP chunks)
  	 */
  	if (zhdr->first_chunks != 0 && zhdr->last_chunks == 0 &&
  	    zhdr->start_middle - (zhdr->first_chunks + ZHDR_CHUNKS) >=
  			BIG_CHUNK_GAP) {
  		mchunk_memmove(zhdr, zhdr->first_chunks + ZHDR_CHUNKS);
  		zhdr->start_middle = zhdr->first_chunks + ZHDR_CHUNKS;
  		return 1;
  	} else if (zhdr->last_chunks != 0 && zhdr->first_chunks == 0 &&
  		   TOTAL_CHUNKS - (zhdr->last_chunks + zhdr->start_middle
  					+ zhdr->middle_chunks) >=
  			BIG_CHUNK_GAP) {
  		unsigned short new_start = TOTAL_CHUNKS - zhdr->last_chunks -
  			zhdr->middle_chunks;
  		mchunk_memmove(zhdr, new_start);
  		zhdr->start_middle = new_start;
  		return 1;
  	}
  
  	return 0;

  }

  static void do_compact_page(struct z3fold_header *zhdr, bool locked)
  {
  	struct z3fold_pool *pool = zhdr->pool;
  	struct page *page;
  	struct list_head *unbuddied;
  	int fchunks;
  
  	page = virt_to_page(zhdr);
  	if (locked)
  		WARN_ON(z3fold_page_trylock(zhdr));
  	else
  		z3fold_page_lock(zhdr);

  	if (WARN_ON(!test_and_clear_bit(NEEDS_COMPACTING, &page->private))) {

  		z3fold_page_unlock(zhdr);
  		return;
  	}
  	spin_lock(&pool->lock);
  	list_del_init(&zhdr->buddy);
  	spin_unlock(&pool->lock);

  	if (kref_put(&zhdr->refcount, release_z3fold_page_locked)) {
  		atomic64_dec(&pool->pages_nr);
  		return;
  	}

  	z3fold_compact_page(zhdr);
  	unbuddied = get_cpu_ptr(pool->unbuddied);
  	fchunks = num_free_chunks(zhdr);
  	if (fchunks < NCHUNKS &&
  	    (!zhdr->first_chunks || !zhdr->middle_chunks ||
  			!zhdr->last_chunks)) {
  		/* the page's not completely free and it's unbuddied */
  		spin_lock(&pool->lock);
  		list_add(&zhdr->buddy, &unbuddied[fchunks]);
  		spin_unlock(&pool->lock);
  		zhdr->cpu = smp_processor_id();
  	}
  	put_cpu_ptr(pool->unbuddied);
  	z3fold_page_unlock(zhdr);
  }
  
  static void compact_page_work(struct work_struct *w)
  {
  	struct z3fold_header *zhdr = container_of(w, struct z3fold_header,
  						work);
  
  	do_compact_page(zhdr, false);
  }
  
  
  /*
   * API Functions
   */
  
  /**
   * z3fold_create_pool() - create a new z3fold pool
   * @name:	pool name
   * @gfp:	gfp flags when allocating the z3fold pool structure
   * @ops:	user-defined operations for the z3fold pool
   *
   * Return: pointer to the new z3fold pool or NULL if the metadata allocation
   * failed.
   */
  static struct z3fold_pool *z3fold_create_pool(const char *name, gfp_t gfp,
  		const struct z3fold_ops *ops)
  {
  	struct z3fold_pool *pool = NULL;
  	int i, cpu;
  
  	pool = kzalloc(sizeof(struct z3fold_pool), gfp);
  	if (!pool)
  		goto out;
  	spin_lock_init(&pool->lock);
  	spin_lock_init(&pool->stale_lock);
  	pool->unbuddied = __alloc_percpu(sizeof(struct list_head)*NCHUNKS, 2);

  	if (!pool->unbuddied)
  		goto out_pool;

  	for_each_possible_cpu(cpu) {
  		struct list_head *unbuddied =
  				per_cpu_ptr(pool->unbuddied, cpu);
  		for_each_unbuddied_list(i, 0)
  			INIT_LIST_HEAD(&unbuddied[i]);
  	}
  	INIT_LIST_HEAD(&pool->lru);
  	INIT_LIST_HEAD(&pool->stale);
  	atomic64_set(&pool->pages_nr, 0);
  	pool->name = name;
  	pool->compact_wq = create_singlethread_workqueue(pool->name);
  	if (!pool->compact_wq)

  		goto out_unbuddied;

  	pool->release_wq = create_singlethread_workqueue(pool->name);
  	if (!pool->release_wq)
  		goto out_wq;
  	INIT_WORK(&pool->work, free_pages_work);
  	pool->ops = ops;
  	return pool;
  
  out_wq:
  	destroy_workqueue(pool->compact_wq);

  out_unbuddied:
  	free_percpu(pool->unbuddied);
  out_pool:

  	kfree(pool);

  out:

  	return NULL;
  }
  
  /**
   * z3fold_destroy_pool() - destroys an existing z3fold pool
   * @pool:	the z3fold pool to be destroyed
   *
   * The pool should be emptied before this function is called.
   */
  static void z3fold_destroy_pool(struct z3fold_pool *pool)
  {
  	destroy_workqueue(pool->release_wq);
  	destroy_workqueue(pool->compact_wq);
  	kfree(pool);
  }

  /**
   * z3fold_alloc() - allocates a region of a given size
   * @pool:	z3fold pool from which to allocate
   * @size:	size in bytes of the desired allocation
   * @gfp:	gfp flags used if the pool needs to grow
   * @handle:	handle of the new allocation
   *
   * This function will attempt to find a free region in the pool large enough to
   * satisfy the allocation request.  A search of the unbuddied lists is
   * performed first. If no suitable free region is found, then a new page is
   * allocated and added to the pool to satisfy the request.
   *
   * gfp should not set __GFP_HIGHMEM as highmem pages cannot be used
   * as z3fold pool pages.
   *
   * Return: 0 if success and handle is set, otherwise -EINVAL if the size or
   * gfp arguments are invalid or -ENOMEM if the pool was unable to allocate
   * a new page.
   */
  static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
  			unsigned long *handle)
  {
  	int chunks = 0, i, freechunks;
  	struct z3fold_header *zhdr = NULL;

  	struct page *page = NULL;

  	enum buddy bud;

  	bool can_sleep = gfpflags_allow_blocking(gfp);

  
  	if (!size || (gfp & __GFP_HIGHMEM))
  		return -EINVAL;
  
  	if (size > PAGE_SIZE)
  		return -ENOSPC;
  
  	if (size > PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE)
  		bud = HEADLESS;
  	else {

  		struct list_head *unbuddied;

  		chunks = size_to_chunks(size);

  lookup:

  		/* First, try to find an unbuddied z3fold page. */

  		unbuddied = get_cpu_ptr(pool->unbuddied);

  		for_each_unbuddied_list(i, chunks) {

  			struct list_head *l = &unbuddied[i];
  
  			zhdr = list_first_entry_or_null(READ_ONCE(l),

  						struct z3fold_header, buddy);

  
  			if (!zhdr)

  				continue;

  
  			/* Re-check under lock. */
  			spin_lock(&pool->lock);
  			l = &unbuddied[i];
  			if (unlikely(zhdr != list_first_entry(READ_ONCE(l),
  					struct z3fold_header, buddy)) ||
  			    !z3fold_page_trylock(zhdr)) {
  				spin_unlock(&pool->lock);
  				put_cpu_ptr(pool->unbuddied);
  				goto lookup;

  			}
  			list_del_init(&zhdr->buddy);

  			zhdr->cpu = -1;

  			spin_unlock(&pool->lock);
  
  			page = virt_to_page(zhdr);

  			if (test_bit(NEEDS_COMPACTING, &page->private)) {
  				z3fold_page_unlock(zhdr);
  				zhdr = NULL;
  				put_cpu_ptr(pool->unbuddied);
  				if (can_sleep)
  					cond_resched();
  				goto lookup;
  			}
  
  			/*
  			 * this page could not be removed from its unbuddied
  			 * list while pool lock was held, and then we've taken
  			 * page lock so kref_put could not be called before
  			 * we got here, so it's safe to just call kref_get()
  			 */
  			kref_get(&zhdr->refcount);
  			break;
  		}
  		put_cpu_ptr(pool->unbuddied);
  
  		if (zhdr) {

  			if (zhdr->first_chunks == 0) {
  				if (zhdr->middle_chunks != 0 &&
  				    chunks >= zhdr->start_middle)

  					bud = LAST;

  				else
  					bud = FIRST;
  			} else if (zhdr->last_chunks == 0)
  				bud = LAST;
  			else if (zhdr->middle_chunks == 0)
  				bud = MIDDLE;
  			else {

  				if (kref_put(&zhdr->refcount,

  					     release_z3fold_page_locked))

  					atomic64_dec(&pool->pages_nr);

  				else
  					z3fold_page_unlock(zhdr);

  				pr_err("No free chunks in unbuddied
  ");
  				WARN_ON(1);

  				goto lookup;

  			}

  			goto found;

  		}
  		bud = FIRST;

  	}

  	page = NULL;
  	if (can_sleep) {
  		spin_lock(&pool->stale_lock);
  		zhdr = list_first_entry_or_null(&pool->stale,
  						struct z3fold_header, buddy);
  		/*
  		 * Before allocating a page, let's see if we can take one from
  		 * the stale pages list. cancel_work_sync() can sleep so we
  		 * limit this case to the contexts where we can sleep
  		 */
  		if (zhdr) {
  			list_del(&zhdr->buddy);
  			spin_unlock(&pool->stale_lock);

  			cancel_work_sync(&zhdr->work);

  			page = virt_to_page(zhdr);
  		} else {
  			spin_unlock(&pool->stale_lock);
  		}

  	}

  	if (!page)
  		page = alloc_page(gfp);

  	if (!page)
  		return -ENOMEM;

  	atomic64_inc(&pool->pages_nr);

  	zhdr = init_z3fold_page(page, pool);

  
  	if (bud == HEADLESS) {
  		set_bit(PAGE_HEADLESS, &page->private);
  		goto headless;
  	}

  	z3fold_page_lock(zhdr);

  
  found:
  	if (bud == FIRST)
  		zhdr->first_chunks = chunks;
  	else if (bud == LAST)
  		zhdr->last_chunks = chunks;
  	else {
  		zhdr->middle_chunks = chunks;

  		zhdr->start_middle = zhdr->first_chunks + ZHDR_CHUNKS;

  	}
  
  	if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0 ||
  			zhdr->middle_chunks == 0) {

  		struct list_head *unbuddied = get_cpu_ptr(pool->unbuddied);

  		/* Add to unbuddied list */
  		freechunks = num_free_chunks(zhdr);

  		spin_lock(&pool->lock);
  		list_add(&zhdr->buddy, &unbuddied[freechunks]);
  		spin_unlock(&pool->lock);
  		zhdr->cpu = smp_processor_id();
  		put_cpu_ptr(pool->unbuddied);

  	}
  
  headless:

  	spin_lock(&pool->lock);

  	/* Add/move z3fold page to beginning of LRU */
  	if (!list_empty(&page->lru))
  		list_del(&page->lru);
  
  	list_add(&page->lru, &pool->lru);
  
  	*handle = encode_handle(zhdr, bud);
  	spin_unlock(&pool->lock);

  	if (bud != HEADLESS)
  		z3fold_page_unlock(zhdr);

  
  	return 0;
  }
  
  /**
   * z3fold_free() - frees the allocation associated with the given handle
   * @pool:	pool in which the allocation resided
   * @handle:	handle associated with the allocation returned by z3fold_alloc()
   *
   * In the case that the z3fold page in which the allocation resides is under
   * reclaim, as indicated by the PG_reclaim flag being set, this function
   * only sets the first|last_chunks to 0.  The page is actually freed
   * once both buddies are evicted (see z3fold_reclaim_page() below).
   */
  static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
  {
  	struct z3fold_header *zhdr;

  	struct page *page;
  	enum buddy bud;

  	zhdr = handle_to_z3fold_header(handle);
  	page = virt_to_page(zhdr);
  
  	if (test_bit(PAGE_HEADLESS, &page->private)) {

  		/* if a headless page is under reclaim, just leave.
  		 * NB: we use test_and_set_bit for a reason: if the bit
  		 * has not been set before, we release this page
  		 * immediately so we don't care about its value any more.
  		 */
  		if (!test_and_set_bit(PAGE_CLAIMED, &page->private)) {
  			spin_lock(&pool->lock);
  			list_del(&page->lru);
  			spin_unlock(&pool->lock);
  			free_z3fold_page(page);
  			atomic64_dec(&pool->pages_nr);

  		}

  		return;

  	}

  	/* Non-headless case */
  	z3fold_page_lock(zhdr);
  	bud = handle_to_buddy(handle);
  
  	switch (bud) {
  	case FIRST:
  		zhdr->first_chunks = 0;
  		break;
  	case MIDDLE:
  		zhdr->middle_chunks = 0;
  		break;
  	case LAST:
  		zhdr->last_chunks = 0;
  		break;
  	default:
  		pr_err("%s: unknown bud %d
  ", __func__, bud);
  		WARN_ON(1);
  		z3fold_page_unlock(zhdr);

  		return;
  	}
  
  	if (kref_put(&zhdr->refcount, release_z3fold_page_locked_list)) {
  		atomic64_dec(&pool->pages_nr);
  		return;
  	}

  	if (test_bit(PAGE_CLAIMED, &page->private)) {

  		z3fold_page_unlock(zhdr);
  		return;
  	}

  	if (test_and_set_bit(NEEDS_COMPACTING, &page->private)) {

  		z3fold_page_unlock(zhdr);

  		return;
  	}
  	if (zhdr->cpu < 0 || !cpu_online(zhdr->cpu)) {

  		spin_lock(&pool->lock);

  		list_del_init(&zhdr->buddy);

  		spin_unlock(&pool->lock);

  		zhdr->cpu = -1;

  		kref_get(&zhdr->refcount);

  		do_compact_page(zhdr, true);
  		return;

  	}

  	kref_get(&zhdr->refcount);

  	queue_work_on(zhdr->cpu, pool->compact_wq, &zhdr->work);
  	z3fold_page_unlock(zhdr);

  }
  
  /**
   * z3fold_reclaim_page() - evicts allocations from a pool page and frees it
   * @pool:	pool from which a page will attempt to be evicted

   * @retries:	number of pages on the LRU list for which eviction will

   *		be attempted before failing
   *
   * z3fold reclaim is different from normal system reclaim in that it is done
   * from the bottom, up. This is because only the bottom layer, z3fold, has
   * information on how the allocations are organized within each z3fold page.
   * This has the potential to create interesting locking situations between
   * z3fold and the user, however.
   *
   * To avoid these, this is how z3fold_reclaim_page() should be called:

   *

   * The user detects a page should be reclaimed and calls z3fold_reclaim_page().
   * z3fold_reclaim_page() will remove a z3fold page from the pool LRU list and
   * call the user-defined eviction handler with the pool and handle as
   * arguments.
   *
   * If the handle can not be evicted, the eviction handler should return
   * non-zero. z3fold_reclaim_page() will add the z3fold page back to the
   * appropriate list and try the next z3fold page on the LRU up to
   * a user defined number of retries.
   *
   * If the handle is successfully evicted, the eviction handler should
   * return 0 _and_ should have called z3fold_free() on the handle. z3fold_free()
   * contains logic to delay freeing the page if the page is under reclaim,
   * as indicated by the setting of the PG_reclaim flag on the underlying page.
   *
   * If all buddies in the z3fold page are successfully evicted, then the
   * z3fold page can be freed.
   *
   * Returns: 0 if page is successfully freed, otherwise -EINVAL if there are
   * no pages to evict or an eviction handler is not registered, -EAGAIN if
   * the retry limit was hit.
   */
  static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
  {

  	int i, ret = 0;
  	struct z3fold_header *zhdr = NULL;
  	struct page *page = NULL;
  	struct list_head *pos;

  	unsigned long first_handle = 0, middle_handle = 0, last_handle = 0;
  
  	spin_lock(&pool->lock);

  	if (!pool->ops || !pool->ops->evict || retries == 0) {

  		spin_unlock(&pool->lock);
  		return -EINVAL;
  	}
  	for (i = 0; i < retries; i++) {

  		if (list_empty(&pool->lru)) {
  			spin_unlock(&pool->lock);
  			return -EINVAL;
  		}

  		list_for_each_prev(pos, &pool->lru) {
  			page = list_entry(pos, struct page, lru);

  
  			/* this bit could have been set by free, in which case
  			 * we pass over to the next page in the pool.
  			 */
  			if (test_and_set_bit(PAGE_CLAIMED, &page->private))
  				continue;
  
  			zhdr = page_address(page);

  			if (test_bit(PAGE_HEADLESS, &page->private))

  				break;

  			if (!z3fold_page_trylock(zhdr)) {
  				zhdr = NULL;

  				continue; /* can't evict at this point */

  			}

  			kref_get(&zhdr->refcount);
  			list_del_init(&zhdr->buddy);
  			zhdr->cpu = -1;

  			break;

  		}

  		if (!zhdr)
  			break;

  		list_del_init(&page->lru);

  		spin_unlock(&pool->lock);

  		if (!test_bit(PAGE_HEADLESS, &page->private)) {

  			/*
  			 * We need encode the handles before unlocking, since
  			 * we can race with free that will set
  			 * (first|last)_chunks to 0
  			 */
  			first_handle = 0;
  			last_handle = 0;
  			middle_handle = 0;
  			if (zhdr->first_chunks)
  				first_handle = encode_handle(zhdr, FIRST);
  			if (zhdr->middle_chunks)
  				middle_handle = encode_handle(zhdr, MIDDLE);
  			if (zhdr->last_chunks)
  				last_handle = encode_handle(zhdr, LAST);

  			/*
  			 * it's safe to unlock here because we hold a
  			 * reference to this page
  			 */

  			z3fold_page_unlock(zhdr);

  		} else {
  			first_handle = encode_handle(zhdr, HEADLESS);
  			last_handle = middle_handle = 0;
  		}

  		/* Issue the eviction callback(s) */
  		if (middle_handle) {
  			ret = pool->ops->evict(pool, middle_handle);
  			if (ret)
  				goto next;
  		}
  		if (first_handle) {
  			ret = pool->ops->evict(pool, first_handle);
  			if (ret)
  				goto next;
  		}
  		if (last_handle) {
  			ret = pool->ops->evict(pool, last_handle);
  			if (ret)
  				goto next;
  		}
  next:

  		if (test_bit(PAGE_HEADLESS, &page->private)) {
  			if (ret == 0) {
  				free_z3fold_page(page);

  				atomic64_dec(&pool->pages_nr);

  				return 0;

  			}

  			spin_lock(&pool->lock);
  			list_add(&page->lru, &pool->lru);
  			spin_unlock(&pool->lock);
  		} else {
  			z3fold_page_lock(zhdr);

  			clear_bit(PAGE_CLAIMED, &page->private);

  			if (kref_put(&zhdr->refcount,
  					release_z3fold_page_locked)) {
  				atomic64_dec(&pool->pages_nr);
  				return 0;
  			}
  			/*
  			 * if we are here, the page is still not completely
  			 * free. Take the global pool lock then to be able
  			 * to add it back to the lru list
  			 */
  			spin_lock(&pool->lock);
  			list_add(&page->lru, &pool->lru);

  			spin_unlock(&pool->lock);

  			z3fold_page_unlock(zhdr);

  		}

  		/* We started off locked to we need to lock the pool back */
  		spin_lock(&pool->lock);

  	}
  	spin_unlock(&pool->lock);
  	return -EAGAIN;
  }
  
  /**
   * z3fold_map() - maps the allocation associated with the given handle
   * @pool:	pool in which the allocation resides
   * @handle:	handle associated with the allocation to be mapped
   *
   * Extracts the buddy number from handle and constructs the pointer to the
   * correct starting chunk within the page.
   *
   * Returns: a pointer to the mapped allocation
   */
  static void *z3fold_map(struct z3fold_pool *pool, unsigned long handle)
  {
  	struct z3fold_header *zhdr;
  	struct page *page;
  	void *addr;
  	enum buddy buddy;

  	zhdr = handle_to_z3fold_header(handle);
  	addr = zhdr;
  	page = virt_to_page(zhdr);
  
  	if (test_bit(PAGE_HEADLESS, &page->private))
  		goto out;

  	z3fold_page_lock(zhdr);

  	buddy = handle_to_buddy(handle);
  	switch (buddy) {
  	case FIRST:
  		addr += ZHDR_SIZE_ALIGNED;
  		break;
  	case MIDDLE:
  		addr += zhdr->start_middle << CHUNK_SHIFT;
  		set_bit(MIDDLE_CHUNK_MAPPED, &page->private);
  		break;
  	case LAST:

  		addr += PAGE_SIZE - (handle_to_chunks(handle) << CHUNK_SHIFT);

  		break;
  	default:
  		pr_err("unknown buddy id %d
  ", buddy);
  		WARN_ON(1);
  		addr = NULL;
  		break;
  	}

  
  	z3fold_page_unlock(zhdr);

  out:

  	return addr;
  }
  
  /**
   * z3fold_unmap() - unmaps the allocation associated with the given handle
   * @pool:	pool in which the allocation resides
   * @handle:	handle associated with the allocation to be unmapped
   */
  static void z3fold_unmap(struct z3fold_pool *pool, unsigned long handle)
  {
  	struct z3fold_header *zhdr;
  	struct page *page;
  	enum buddy buddy;

  	zhdr = handle_to_z3fold_header(handle);
  	page = virt_to_page(zhdr);

  	if (test_bit(PAGE_HEADLESS, &page->private))

  		return;

  	z3fold_page_lock(zhdr);

  	buddy = handle_to_buddy(handle);
  	if (buddy == MIDDLE)
  		clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);

  	z3fold_page_unlock(zhdr);

  }
  
  /**
   * z3fold_get_pool_size() - gets the z3fold pool size in pages
   * @pool:	pool whose size is being queried
   *

   * Returns: size in pages of the given pool.

   */
  static u64 z3fold_get_pool_size(struct z3fold_pool *pool)
  {

  	return atomic64_read(&pool->pages_nr);

  }
  
  /*****************
   * zpool
   ****************/
  
  static int z3fold_zpool_evict(struct z3fold_pool *pool, unsigned long handle)
  {
  	if (pool->zpool && pool->zpool_ops && pool->zpool_ops->evict)
  		return pool->zpool_ops->evict(pool->zpool, handle);
  	else
  		return -ENOENT;
  }
  
  static const struct z3fold_ops z3fold_zpool_ops = {
  	.evict =	z3fold_zpool_evict
  };
  
  static void *z3fold_zpool_create(const char *name, gfp_t gfp,
  			       const struct zpool_ops *zpool_ops,
  			       struct zpool *zpool)
  {
  	struct z3fold_pool *pool;

  	pool = z3fold_create_pool(name, gfp,
  				zpool_ops ? &z3fold_zpool_ops : NULL);

  	if (pool) {
  		pool->zpool = zpool;
  		pool->zpool_ops = zpool_ops;
  	}
  	return pool;
  }
  
  static void z3fold_zpool_destroy(void *pool)
  {
  	z3fold_destroy_pool(pool);
  }
  
  static int z3fold_zpool_malloc(void *pool, size_t size, gfp_t gfp,
  			unsigned long *handle)
  {
  	return z3fold_alloc(pool, size, gfp, handle);
  }
  static void z3fold_zpool_free(void *pool, unsigned long handle)
  {
  	z3fold_free(pool, handle);
  }
  
  static int z3fold_zpool_shrink(void *pool, unsigned int pages,
  			unsigned int *reclaimed)
  {
  	unsigned int total = 0;
  	int ret = -EINVAL;
  
  	while (total < pages) {
  		ret = z3fold_reclaim_page(pool, 8);
  		if (ret < 0)
  			break;
  		total++;
  	}
  
  	if (reclaimed)
  		*reclaimed = total;
  
  	return ret;
  }
  
  static void *z3fold_zpool_map(void *pool, unsigned long handle,
  			enum zpool_mapmode mm)
  {
  	return z3fold_map(pool, handle);
  }
  static void z3fold_zpool_unmap(void *pool, unsigned long handle)
  {
  	z3fold_unmap(pool, handle);
  }
  
  static u64 z3fold_zpool_total_size(void *pool)
  {
  	return z3fold_get_pool_size(pool) * PAGE_SIZE;
  }
  
  static struct zpool_driver z3fold_zpool_driver = {
  	.type =		"z3fold",
  	.owner =	THIS_MODULE,
  	.create =	z3fold_zpool_create,
  	.destroy =	z3fold_zpool_destroy,
  	.malloc =	z3fold_zpool_malloc,
  	.free =		z3fold_zpool_free,
  	.shrink =	z3fold_zpool_shrink,
  	.map =		z3fold_zpool_map,
  	.unmap =	z3fold_zpool_unmap,
  	.total_size =	z3fold_zpool_total_size,
  };
  
  MODULE_ALIAS("zpool-z3fold");
  
  static int __init init_z3fold(void)
  {

  	/* Make sure the z3fold header is not larger than the page size */
  	BUILD_BUG_ON(ZHDR_SIZE_ALIGNED > PAGE_SIZE);

  	zpool_register_driver(&z3fold_zpool_driver);
  
  	return 0;
  }
  
  static void __exit exit_z3fold(void)
  {
  	zpool_unregister_driver(&z3fold_zpool_driver);
  }
  
  module_init(init_z3fold);
  module_exit(exit_z3fold);
  
  MODULE_LICENSE("GPL");
  MODULE_AUTHOR("Vitaly Wool <vitalywool@gmail.com>");
  MODULE_DESCRIPTION("3-Fold Allocator for Compressed Pages");