zsmalloc: factor page chain functionality out

[mirror_ubuntu-zesty-kernel.git] / mm / zsmalloc.c

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

/*
 * 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->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
 *
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#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;
        int objs_per_zspage;
        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;

        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 FULLNESS_BITS   4
#define CLASS_BITS      28

#define FULLNESS_SHIFT  0
#define CLASS_SHIFT     (FULLNESS_SHIFT + FULLNESS_BITS)

#define FULLNESS_MASK   ((1UL << FULLNESS_BITS) - 1)
#define CLASS_MASK      ((1UL << CLASS_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, gfp_t gfp)
{
        return (unsigned long)kmem_cache_alloc(pool->handle_cachep,
                        gfp & ~__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)
{
        /*
         * Ignore global gfp flags: zs_malloc() may be invoked from
         * different contexts and its caller must provide a valid
         * gfp mask.
         */
        return zs_create_pool(name);
}

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, gfp);
        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 inline int get_zspage_inuse(struct page *first_page)
{
        return first_page->inuse;
}

static inline void set_zspage_inuse(struct page *first_page, int val)
{
        first_page->inuse = val;
}

static inline void mod_zspage_inuse(struct page *first_page, int val)
{
        first_page->inuse += val;
}

static inline int get_first_obj_offset(struct page *page)
{
        return page->index;
}

static inline void set_first_obj_offset(struct page *page, int offset)
{
        page->index = offset;
}

static inline unsigned long get_freeobj(struct page *first_page)
{
        return (unsigned long)first_page->freelist;
}

static inline void set_freeobj(struct page *first_page, unsigned long obj)
{
        first_page->freelist = (void *)obj;
}

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

        m = (unsigned long)first_page->mapping;
        *fullness = (m >> FULLNESS_SHIFT) & FULLNESS_MASK;
        *class_idx = (m >> CLASS_SHIFT) & CLASS_MASK;
}

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

        m = (class_idx << CLASS_SHIFT) | (fullness << FULLNESS_SHIFT);
        first_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 void __init zs_stat_init(void)
{
        if (!debugfs_initialized()) {
                pr_warn("debugfs not available, stat dir not created\n");
                return;
        }

        zs_stat_root = debugfs_create_dir("zsmalloc", NULL);
        if (!zs_stat_root)
                pr_warn("debugfs 'zsmalloc' stat dir creation failed\n");
}

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\n",
                        "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\n",
                        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, "\n");
        seq_printf(s, " %5s %5s %11lu %12lu %13lu %10lu %10lu %16s %8lu\n",
                        "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 void zs_pool_stat_create(struct zs_pool *pool, const char *name)
{
        struct dentry *entry;

        if (!zs_stat_root) {
                pr_warn("no root stat dir, not creating <%s> stat dir\n", name);
                return;
        }

        entry = debugfs_create_dir(name, zs_stat_root);
        if (!entry) {
                pr_warn("debugfs dir <%s> creation failed\n", name);
                return;
        }
        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\n",
                                name, "classes");
                debugfs_remove_recursive(pool->stat_dentry);
                pool->stat_dentry = NULL;
        }
}

static void zs_pool_stat_destroy(struct zs_pool *pool)
{
        debugfs_remove_recursive(pool->stat_dentry);
}

#else /* CONFIG_ZSMALLOC_STAT */
static void __init zs_stat_init(void)
{
}

static void __exit zs_stat_exit(void)
{
}

static inline void zs_pool_stat_create(struct zs_pool *pool, const char *name)
{
}

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 size_class *class,
                                                struct page *first_page)
{
        int inuse, objs_per_zspage;
        enum fullness_group fg;

        VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);

        inuse = get_zspage_inuse(first_page);
        objs_per_zspage = class->objs_per_zspage;

        if (inuse == 0)
                fg = ZS_EMPTY;
        else if (inuse == objs_per_zspage)
                fg = ZS_FULL;
        else if (inuse <= 3 * objs_per_zspage / 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 size_class *class,
                                enum fullness_group fullness,
                                struct page *first_page)
{
        struct page **head;

        VM_BUG_ON_PAGE(!is_first_page(first_page), first_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 = first_page;
                return;
        }

        /*
         * We want to see more ZS_FULL pages and less almost
         * empty/full. Put pages with higher ->inuse first.
         */
        list_add_tail(&first_page->lru, &(*head)->lru);
        if (get_zspage_inuse(first_page) >= get_zspage_inuse(*head))
                *head = first_page;
}

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

        VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);

        if (fullness >= _ZS_NR_FULLNESS_GROUPS)
                return;

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

        list_del_init(&first_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 *first_page)
{
        int class_idx;
        enum fullness_group currfg, newfg;

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

        remove_zspage(class, currfg, first_page);
        insert_zspage(class, newfg, first_page);
        set_zspage_mapping(first_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) {
                VM_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_PAGE(!is_first_page(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 = get_first_obj_offset(page);

        return off + obj_idx * class_size;
}

static inline int trypin_tag(unsigned long handle)
{
        return bit_spin_trylock(HANDLE_PIN_BIT, (unsigned long *)handle);
}

static void pin_tag(unsigned long handle)
{
        bit_spin_lock(HANDLE_PIN_BIT, (unsigned long *)handle);
}

static void unpin_tag(unsigned long handle)
{
        bit_spin_unlock(HANDLE_PIN_BIT, (unsigned long *)handle);
}

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;

        VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);
        VM_BUG_ON_PAGE(get_zspage_inuse(first_page), first_page);

        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 size_class *class, struct page *first_page)
{
        unsigned long off = 0;
        struct page *page = first_page;

        first_page->freelist = NULL;
        set_zspage_inuse(first_page, 0);

        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)
                        set_first_obj_offset(page, 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;
        }

        set_freeobj(first_page, (unsigned long)location_to_obj(first_page, 0));
}

static void create_page_chain(struct page *pages[], int nr_pages)
{
        int i;
        struct page *page;
        struct page *prev_page = NULL;
        struct page *first_page = NULL;

        /*
         * 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.
         */
        for (i = 0; i < nr_pages; i++) {
                page = pages[i];

                INIT_LIST_HEAD(&page->lru);
                if (i == 0) {
                        SetPagePrivate(page);
                        set_page_private(page, 0);
                        first_page = page;
                }
                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 == nr_pages - 1)
                        SetPagePrivate2(page);
                prev_page = page;
        }
}

/*
 * Allocate a zspage for the given size class
 */
static struct page *alloc_zspage(struct size_class *class, gfp_t flags)
{
        int i;
        struct page *first_page = NULL;
        struct page *pages[ZS_MAX_PAGES_PER_ZSPAGE];

        for (i = 0; i < class->pages_per_zspage; i++) {
                struct page *page;

                page = alloc_page(flags);
                if (!page) {
                        while (--i >= 0)
                                __free_page(pages[i]);
                        return NULL;
                }
                pages[i] = page;
        }

        create_page_chain(pages, class->pages_per_zspage);
        first_page = pages[0];
        init_zspage(class, first_page);

        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 size_class *class, struct page *first_page)
{
        VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);

        return get_zspage_inuse(first_page) == class->objs_per_zspage;
}

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;

        /*
         * 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.
         */
        WARN_ON_ONCE(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;

        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 size_class *class,
                                struct page *first_page, 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 = get_freeobj(first_page);
        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);
        set_freeobj(first_page, (unsigned long)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);
        mod_zspage_inuse(first_page, 1);
        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, gfp_t gfp)
{
        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, gfp);
        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, gfp);
                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(class, first_page, 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 size_class *class, unsigned long obj)
{
        struct link_free *link;
        struct page *first_page, *f_page;
        unsigned long f_objidx, f_offset;
        void *vaddr;

        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 = (void *)get_freeobj(first_page);
        if (class->huge)
                set_page_private(first_page, 0);
        kunmap_atomic(vaddr);
        set_freeobj(first_page, obj);
        mod_zspage_inuse(first_page, -1);
        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(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(struct size_class *class, unsigned long dst,
                                unsigned long src)
{
        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);
                        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);
                        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 size_class *class,
                                        struct page *page, int index)
{
        unsigned long head;
        int offset = 0;
        unsigned long handle = 0;
        void *addr = kmap_atomic(page);

        if (!is_first_page(page))
                offset = get_first_obj_offset(page);
        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(class, s_page, index);
                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(class, d_page)) {
                        unpin_tag(handle);
                        ret = -ENOMEM;
                        break;
                }

                used_obj = handle_to_obj(handle);
                free_obj = obj_malloc(class, d_page, handle);
                zs_object_copy(class, free_obj, used_obj);
                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(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(class, i, page);
                        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;

        fullness = get_fullness_group(class, first_page);
        insert_zspage(class, fullness, first_page);
        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(class, i, page);
                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;
        unsigned long obj_allocated = zs_stat_get(class, OBJ_ALLOCATED);
        unsigned long obj_used = zs_stat_get(class, OBJ_USED);

        if (obj_allocated <= obj_used)
                return 0;

        obj_wasted = obj_allocated - 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))) {

                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)
{
        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;
                class->objs_per_zspage = class->pages_per_zspage *
                                                PAGE_SIZE / class->size;
                if (pages_per_zspage == 1 && class->objs_per_zspage == 1)
                        class->huge = true;
                spin_lock_init(&class->lock);
                pool->size_class[i] = class;

                prev_class = class;
        }

        /* debug only, don't abort if it fails */
        zs_pool_stat_create(pool, name);

        /*
         * 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\n",
                                        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

        zs_stat_init();

        return 0;

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>");