Merge tag 'cocci-for-6.10' of git://git.kernel.org/pub/scm/linux/kernel/git/jlawall...

[thirdparty/kernel/linux.git] / mm / z3fold.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * 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/cpumask.h>
#include <linux/list.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/page-flags.h>
#include <linux/migrate.h>
#include <linux/node.h>
#include <linux/compaction.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>
#include <linux/kmemleak.h>

/*
 * 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         (TOTAL_CHUNKS - ZHDR_CHUNKS)

#define BUDDY_MASK      (0x3)
#define BUDDY_SHIFT     2
#define SLOTS_ALIGN     (0x40)

/*****************
 * Structures
*****************/
struct z3fold_pool;

enum buddy {
        HEADLESS = 0,
        FIRST,
        MIDDLE,
        LAST,
        BUDDIES_MAX = LAST
};

struct z3fold_buddy_slots {
        /*
         * we are using BUDDY_MASK in handle_to_buddy etc. so there should
         * be enough slots to hold all possible variants
         */
        unsigned long slot[BUDDY_MASK + 1];
        unsigned long pool; /* back link */
        rwlock_t lock;
};
#define HANDLE_FLAG_MASK        (0x03)

/*
 * 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
 * @slots:              pointer to the structure holding buddy slots
 * @pool:               pointer to the containing pool
 * @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)
 * @mapped_count:       the number of objects currently mapped
 */
struct z3fold_header {
        struct list_head buddy;
        spinlock_t page_lock;
        struct kref refcount;
        struct work_struct work;
        struct z3fold_buddy_slots *slots;
        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;
        unsigned short mapped_count:2;
        unsigned short foreign_handles:2;
};

/**
 * struct z3fold_pool - stores metadata for each z3fold pool
 * @name:       pool name
 * @lock:       protects pool unbuddied 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.
 * @stale:      list of pages marked for freeing
 * @pages_nr:   number of z3fold pages in the pool.
 * @c_handle:   cache for z3fold_buddy_slots allocation
 * @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 stale;
        atomic64_t pages_nr;
        struct kmem_cache *c_handle;
        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 */
        PAGE_MIGRATED, /* page is migrated and soon to be released */
};

/*
 * handle flags, go under HANDLE_FLAG_MASK
 */
enum z3fold_handle_flags {
        HANDLES_NOFREE = 0,
};

/*
 * Forward declarations
 */
static struct z3fold_header *__z3fold_alloc(struct z3fold_pool *, size_t, bool);
static void compact_page_work(struct work_struct *w);

/*****************
 * 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 inline struct z3fold_buddy_slots *alloc_slots(struct z3fold_pool *pool,
                                                        gfp_t gfp)
{
        struct z3fold_buddy_slots *slots = kmem_cache_zalloc(pool->c_handle,
                                                             gfp);

        if (slots) {
                /* It will be freed separately in free_handle(). */
                kmemleak_not_leak(slots);
                slots->pool = (unsigned long)pool;
                rwlock_init(&slots->lock);
        }

        return slots;
}

static inline struct z3fold_pool *slots_to_pool(struct z3fold_buddy_slots *s)
{
        return (struct z3fold_pool *)(s->pool & ~HANDLE_FLAG_MASK);
}

static inline struct z3fold_buddy_slots *handle_to_slots(unsigned long handle)
{
        return (struct z3fold_buddy_slots *)(handle & ~(SLOTS_ALIGN - 1));
}

/* 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);
}

/* return locked z3fold page if it's not headless */
static inline struct z3fold_header *get_z3fold_header(unsigned long handle)
{
        struct z3fold_buddy_slots *slots;
        struct z3fold_header *zhdr;
        int locked = 0;

        if (!(handle & (1 << PAGE_HEADLESS))) {
                slots = handle_to_slots(handle);
                do {
                        unsigned long addr;

                        read_lock(&slots->lock);
                        addr = *(unsigned long *)handle;
                        zhdr = (struct z3fold_header *)(addr & PAGE_MASK);
                        locked = z3fold_page_trylock(zhdr);
                        read_unlock(&slots->lock);
                        if (locked) {
                                struct page *page = virt_to_page(zhdr);

                                if (!test_bit(PAGE_MIGRATED, &page->private))
                                        break;
                                z3fold_page_unlock(zhdr);
                        }
                        cpu_relax();
                } while (true);
        } else {
                zhdr = (struct z3fold_header *)(handle & PAGE_MASK);
        }

        return zhdr;
}

static inline void put_z3fold_header(struct z3fold_header *zhdr)
{
        struct page *page = virt_to_page(zhdr);

        if (!test_bit(PAGE_HEADLESS, &page->private))
                z3fold_page_unlock(zhdr);
}

static inline void free_handle(unsigned long handle, struct z3fold_header *zhdr)
{
        struct z3fold_buddy_slots *slots;
        int i;
        bool is_free;

        if (WARN_ON(*(unsigned long *)handle == 0))
                return;

        slots = handle_to_slots(handle);
        write_lock(&slots->lock);
        *(unsigned long *)handle = 0;

        if (test_bit(HANDLES_NOFREE, &slots->pool)) {
                write_unlock(&slots->lock);
                return; /* simple case, nothing else to do */
        }

        if (zhdr->slots != slots)
                zhdr->foreign_handles--;

        is_free = true;
        for (i = 0; i <= BUDDY_MASK; i++) {
                if (slots->slot[i]) {
                        is_free = false;
                        break;
                }
        }
        write_unlock(&slots->lock);

        if (is_free) {
                struct z3fold_pool *pool = slots_to_pool(slots);

                if (zhdr->slots == slots)
                        zhdr->slots = NULL;
                kmem_cache_free(pool->c_handle, slots);
        }
}

/* Initializes the z3fold header of a newly allocated z3fold page */
static struct z3fold_header *init_z3fold_page(struct page *page, bool headless,
                                        struct z3fold_pool *pool, gfp_t gfp)
{
        struct z3fold_header *zhdr = page_address(page);
        struct z3fold_buddy_slots *slots;

        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);
        clear_bit(PAGE_MIGRATED, &page->private);
        if (headless)
                return zhdr;

        slots = alloc_slots(pool, gfp);
        if (!slots)
                return NULL;

        memset(zhdr, 0, sizeof(*zhdr));
        spin_lock_init(&zhdr->page_lock);
        kref_init(&zhdr->refcount);
        zhdr->cpu = -1;
        zhdr->slots = slots;
        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, bool headless)
{
        if (!headless) {
                lock_page(page);
                __ClearPageMovable(page);
                unlock_page(page);
        }
        __free_page(page);
}

/* Helper function to build the index */
static inline int __idx(struct z3fold_header *zhdr, enum buddy bud)
{
        return (bud + zhdr->first_num) & BUDDY_MASK;
}

/*
 * Encodes the handle of a particular buddy within a z3fold page.
 * Zhdr->page_lock should be held as this function accesses first_num
 * if bud != HEADLESS.
 */
static unsigned long __encode_handle(struct z3fold_header *zhdr,
                                struct z3fold_buddy_slots *slots,
                                enum buddy bud)
{
        unsigned long h = (unsigned long)zhdr;
        int idx = 0;

        /*
         * For a headless page, its handle is its pointer with the extra
         * PAGE_HEADLESS bit set
         */
        if (bud == HEADLESS)
                return h | (1 << PAGE_HEADLESS);

        /* otherwise, return pointer to encoded handle */
        idx = __idx(zhdr, bud);
        h += idx;
        if (bud == LAST)
                h |= (zhdr->last_chunks << BUDDY_SHIFT);

        write_lock(&slots->lock);
        slots->slot[idx] = h;
        write_unlock(&slots->lock);
        return (unsigned long)&slots->slot[idx];
}

static unsigned long encode_handle(struct z3fold_header *zhdr, enum buddy bud)
{
        return __encode_handle(zhdr, zhdr->slots, bud);
}

/* only for LAST bud, returns zero otherwise */
static unsigned short handle_to_chunks(unsigned long handle)
{
        struct z3fold_buddy_slots *slots = handle_to_slots(handle);
        unsigned long addr;

        read_lock(&slots->lock);
        addr = *(unsigned long *)handle;
        read_unlock(&slots->lock);
        return (addr & ~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;
        struct z3fold_buddy_slots *slots = handle_to_slots(handle);
        unsigned long addr;

        read_lock(&slots->lock);
        WARN_ON(handle & (1 << PAGE_HEADLESS));
        addr = *(unsigned long *)handle;
        read_unlock(&slots->lock);
        zhdr = (struct z3fold_header *)(addr & PAGE_MASK);
        return (addr - zhdr->first_num) & BUDDY_MASK;
}

static inline struct z3fold_pool *zhdr_to_pool(struct z3fold_header *zhdr)
{
        return zhdr->pool;
}

static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
{
        struct page *page = virt_to_page(zhdr);
        struct z3fold_pool *pool = zhdr_to_pool(zhdr);

        WARN_ON(!list_empty(&zhdr->buddy));
        set_bit(PAGE_STALE, &page->private);
        clear_bit(NEEDS_COMPACTING, &page->private);
        spin_lock(&pool->lock);
        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);

        atomic64_dec(&pool->pages_nr);
}

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);
        struct z3fold_pool *pool = zhdr_to_pool(zhdr);

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

        WARN_ON(z3fold_page_trylock(zhdr));
        __release_z3fold_page(zhdr, true);
}

static inline int put_z3fold_locked(struct z3fold_header *zhdr)
{
        return kref_put(&zhdr->refcount, release_z3fold_page_locked);
}

static inline int put_z3fold_locked_list(struct z3fold_header *zhdr)
{
        return kref_put(&zhdr->refcount, release_z3fold_page_locked_list);
}

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, false);
                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;
}

/* Add to the appropriate unbuddied list */
static inline void add_to_unbuddied(struct z3fold_pool *pool,
                                struct z3fold_header *zhdr)
{
        if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0 ||
                        zhdr->middle_chunks == 0) {
                struct list_head *unbuddied;
                int freechunks = num_free_chunks(zhdr);

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

static inline enum buddy get_free_buddy(struct z3fold_header *zhdr, int chunks)
{
        enum buddy bud = HEADLESS;

        if (zhdr->middle_chunks) {
                if (!zhdr->first_chunks &&
                    chunks <= zhdr->start_middle - ZHDR_CHUNKS)
                        bud = FIRST;
                else if (!zhdr->last_chunks)
                        bud = LAST;
        } else {
                if (!zhdr->first_chunks)
                        bud = FIRST;
                else if (!zhdr->last_chunks)
                        bud = LAST;
                else
                        bud = MIDDLE;
        }

        return bud;
}

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);
}

static inline bool buddy_single(struct z3fold_header *zhdr)
{
        return !((zhdr->first_chunks && zhdr->middle_chunks) ||
                        (zhdr->first_chunks && zhdr->last_chunks) ||
                        (zhdr->middle_chunks && zhdr->last_chunks));
}

static struct z3fold_header *compact_single_buddy(struct z3fold_header *zhdr)
{
        struct z3fold_pool *pool = zhdr_to_pool(zhdr);
        void *p = zhdr;
        unsigned long old_handle = 0;
        size_t sz = 0;
        struct z3fold_header *new_zhdr = NULL;
        int first_idx = __idx(zhdr, FIRST);
        int middle_idx = __idx(zhdr, MIDDLE);
        int last_idx = __idx(zhdr, LAST);
        unsigned short *moved_chunks = NULL;

        /*
         * No need to protect slots here -- all the slots are "local" and
         * the page lock is already taken
         */
        if (zhdr->first_chunks && zhdr->slots->slot[first_idx]) {
                p += ZHDR_SIZE_ALIGNED;
                sz = zhdr->first_chunks << CHUNK_SHIFT;
                old_handle = (unsigned long)&zhdr->slots->slot[first_idx];
                moved_chunks = &zhdr->first_chunks;
        } else if (zhdr->middle_chunks && zhdr->slots->slot[middle_idx]) {
                p += zhdr->start_middle << CHUNK_SHIFT;
                sz = zhdr->middle_chunks << CHUNK_SHIFT;
                old_handle = (unsigned long)&zhdr->slots->slot[middle_idx];
                moved_chunks = &zhdr->middle_chunks;
        } else if (zhdr->last_chunks && zhdr->slots->slot[last_idx]) {
                p += PAGE_SIZE - (zhdr->last_chunks << CHUNK_SHIFT);
                sz = zhdr->last_chunks << CHUNK_SHIFT;
                old_handle = (unsigned long)&zhdr->slots->slot[last_idx];
                moved_chunks = &zhdr->last_chunks;
        }

        if (sz > 0) {
                enum buddy new_bud = HEADLESS;
                short chunks = size_to_chunks(sz);
                void *q;

                new_zhdr = __z3fold_alloc(pool, sz, false);
                if (!new_zhdr)
                        return NULL;

                if (WARN_ON(new_zhdr == zhdr))
                        goto out_fail;

                new_bud = get_free_buddy(new_zhdr, chunks);
                q = new_zhdr;
                switch (new_bud) {
                case FIRST:
                        new_zhdr->first_chunks = chunks;
                        q += ZHDR_SIZE_ALIGNED;
                        break;
                case MIDDLE:
                        new_zhdr->middle_chunks = chunks;
                        new_zhdr->start_middle =
                                new_zhdr->first_chunks + ZHDR_CHUNKS;
                        q += new_zhdr->start_middle << CHUNK_SHIFT;
                        break;
                case LAST:
                        new_zhdr->last_chunks = chunks;
                        q += PAGE_SIZE - (new_zhdr->last_chunks << CHUNK_SHIFT);
                        break;
                default:
                        goto out_fail;
                }
                new_zhdr->foreign_handles++;
                memcpy(q, p, sz);
                write_lock(&zhdr->slots->lock);
                *(unsigned long *)old_handle = (unsigned long)new_zhdr +
                        __idx(new_zhdr, new_bud);
                if (new_bud == LAST)
                        *(unsigned long *)old_handle |=
                                        (new_zhdr->last_chunks << BUDDY_SHIFT);
                write_unlock(&zhdr->slots->lock);
                add_to_unbuddied(pool, new_zhdr);
                z3fold_page_unlock(new_zhdr);

                *moved_chunks = 0;
        }

        return new_zhdr;

out_fail:
        if (new_zhdr && !put_z3fold_locked(new_zhdr)) {
                add_to_unbuddied(pool, new_zhdr);
                z3fold_page_unlock(new_zhdr);
        }
        return NULL;

}

#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 (unlikely(PageIsolated(page)))
                return 0;

        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_to_pool(zhdr);
        struct page *page;

        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 (put_z3fold_locked(zhdr))
                return;

        if (test_bit(PAGE_STALE, &page->private) ||
            test_and_set_bit(PAGE_CLAIMED, &page->private)) {
                z3fold_page_unlock(zhdr);
                return;
        }

        if (!zhdr->foreign_handles && buddy_single(zhdr) &&
            zhdr->mapped_count == 0 && compact_single_buddy(zhdr)) {
                if (!put_z3fold_locked(zhdr)) {
                        clear_bit(PAGE_CLAIMED, &page->private);
                        z3fold_page_unlock(zhdr);
                }
                return;
        }

        z3fold_compact_page(zhdr);
        add_to_unbuddied(pool, zhdr);
        clear_bit(PAGE_CLAIMED, &page->private);
        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);
}

/* returns _locked_ z3fold page header or NULL */
static inline struct z3fold_header *__z3fold_alloc(struct z3fold_pool *pool,
                                                size_t size, bool can_sleep)
{
        struct z3fold_header *zhdr = NULL;
        struct page *page;
        struct list_head *unbuddied;
        int chunks = size_to_chunks(size), i;

lookup:
        migrate_disable();
        /* First, try to find an unbuddied z3fold page. */
        unbuddied = this_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);
                if (unlikely(zhdr != list_first_entry(READ_ONCE(l),
                                                struct z3fold_header, buddy)) ||
                    !z3fold_page_trylock(zhdr)) {
                        spin_unlock(&pool->lock);
                        zhdr = NULL;
                        migrate_enable();
                        if (can_sleep)
                                cond_resched();
                        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) ||
                    test_bit(PAGE_CLAIMED, &page->private)) {
                        z3fold_page_unlock(zhdr);
                        zhdr = NULL;
                        migrate_enable();
                        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;
        }
        migrate_enable();

        if (!zhdr) {
                int cpu;

                /* look for _exact_ match on other cpus' lists */
                for_each_online_cpu(cpu) {
                        struct list_head *l;

                        unbuddied = per_cpu_ptr(pool->unbuddied, cpu);
                        spin_lock(&pool->lock);
                        l = &unbuddied[chunks];

                        zhdr = list_first_entry_or_null(READ_ONCE(l),
                                                struct z3fold_header, buddy);

                        if (!zhdr || !z3fold_page_trylock(zhdr)) {
                                spin_unlock(&pool->lock);
                                zhdr = NULL;
                                continue;
                        }
                        list_del_init(&zhdr->buddy);
                        zhdr->cpu = -1;
                        spin_unlock(&pool->lock);

                        page = virt_to_page(zhdr);
                        if (test_bit(NEEDS_COMPACTING, &page->private) ||
                            test_bit(PAGE_CLAIMED, &page->private)) {
                                z3fold_page_unlock(zhdr);
                                zhdr = NULL;
                                if (can_sleep)
                                        cond_resched();
                                continue;
                        }
                        kref_get(&zhdr->refcount);
                        break;
                }
        }

        if (zhdr && !zhdr->slots) {
                zhdr->slots = alloc_slots(pool, GFP_ATOMIC);
                if (!zhdr->slots)
                        goto out_fail;
        }
        return zhdr;

out_fail:
        if (!put_z3fold_locked(zhdr)) {
                add_to_unbuddied(pool, zhdr);
                z3fold_page_unlock(zhdr);
        }
        return NULL;
}

/*
 * API Functions
 */

/**
 * z3fold_create_pool() - create a new z3fold pool
 * @name:       pool name
 * @gfp:        gfp flags when allocating the z3fold pool structure
 *
 * 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)
{
        struct z3fold_pool *pool = NULL;
        int i, cpu;

        pool = kzalloc(sizeof(struct z3fold_pool), gfp);
        if (!pool)
                goto out;
        pool->c_handle = kmem_cache_create("z3fold_handle",
                                sizeof(struct z3fold_buddy_slots),
                                SLOTS_ALIGN, 0, NULL);
        if (!pool->c_handle)
                goto out_c;
        spin_lock_init(&pool->lock);
        spin_lock_init(&pool->stale_lock);
        pool->unbuddied = __alloc_percpu(sizeof(struct list_head) * NCHUNKS,
                                         __alignof__(struct list_head));
        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->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);
        return pool;

out_wq:
        destroy_workqueue(pool->compact_wq);
out_unbuddied:
        free_percpu(pool->unbuddied);
out_pool:
        kmem_cache_destroy(pool->c_handle);
out_c:
        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)
{
        kmem_cache_destroy(pool->c_handle);

        /*
         * We need to destroy pool->compact_wq before pool->release_wq,
         * as any pending work on pool->compact_wq will call
         * queue_work(pool->release_wq, &pool->work).
         *
         * There are still outstanding pages until both workqueues are drained,
         * so we cannot unregister migration until then.
         */

        destroy_workqueue(pool->compact_wq);
        destroy_workqueue(pool->release_wq);
        free_percpu(pool->unbuddied);
        kfree(pool);
}

static const struct movable_operations z3fold_mops;

/**
 * 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.
 *
 * 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 = size_to_chunks(size);
        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 {
retry:
                zhdr = __z3fold_alloc(pool, size, can_sleep);
                if (zhdr) {
                        bud = get_free_buddy(zhdr, chunks);
                        if (bud == HEADLESS) {
                                if (!put_z3fold_locked(zhdr))
                                        z3fold_page_unlock(zhdr);
                                pr_err("No free chunks in unbuddied\n");
                                WARN_ON(1);
                                goto retry;
                        }
                        page = virt_to_page(zhdr);
                        goto found;
                }
                bud = FIRST;
        }

        page = alloc_page(gfp);
        if (!page)
                return -ENOMEM;

        zhdr = init_z3fold_page(page, bud == HEADLESS, pool, gfp);
        if (!zhdr) {
                __free_page(page);
                return -ENOMEM;
        }
        atomic64_inc(&pool->pages_nr);

        if (bud == HEADLESS) {
                set_bit(PAGE_HEADLESS, &page->private);
                goto headless;
        }
        if (can_sleep) {
                lock_page(page);
                __SetPageMovable(page, &z3fold_mops);
                unlock_page(page);
        } else {
                WARN_ON(!trylock_page(page));
                __SetPageMovable(page, &z3fold_mops);
                unlock_page(page);
        }
        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;
        }
        add_to_unbuddied(pool, zhdr);

headless:
        spin_lock(&pool->lock);
        *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 PAGE_CLAIMED flag being set, this function
 * only sets the first|middle|last_chunks to 0.  The page is actually freed
 * once all 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;
        bool page_claimed;

        zhdr = get_z3fold_header(handle);
        page = virt_to_page(zhdr);
        page_claimed = test_and_set_bit(PAGE_CLAIMED, &page->private);

        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 (!page_claimed) {
                        put_z3fold_header(zhdr);
                        free_z3fold_page(page, true);
                        atomic64_dec(&pool->pages_nr);
                }
                return;
        }

        /* Non-headless case */
        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\n", __func__, bud);
                WARN_ON(1);
                put_z3fold_header(zhdr);
                return;
        }

        if (!page_claimed)
                free_handle(handle, zhdr);
        if (put_z3fold_locked_list(zhdr))
                return;
        if (page_claimed) {
                /* the page has not been claimed by us */
                put_z3fold_header(zhdr);
                return;
        }
        if (test_and_set_bit(NEEDS_COMPACTING, &page->private)) {
                clear_bit(PAGE_CLAIMED, &page->private);
                put_z3fold_header(zhdr);
                return;
        }
        if (zhdr->cpu < 0 || !cpu_online(zhdr->cpu)) {
                zhdr->cpu = -1;
                kref_get(&zhdr->refcount);
                clear_bit(PAGE_CLAIMED, &page->private);
                do_compact_page(zhdr, true);
                return;
        }
        kref_get(&zhdr->refcount);
        clear_bit(PAGE_CLAIMED, &page->private);
        queue_work_on(zhdr->cpu, pool->compact_wq, &zhdr->work);
        put_z3fold_header(zhdr);
}

/**
 * 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 = get_z3fold_header(handle);
        addr = zhdr;
        page = virt_to_page(zhdr);

        if (test_bit(PAGE_HEADLESS, &page->private))
                goto out;

        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\n", buddy);
                WARN_ON(1);
                addr = NULL;
                break;
        }

        if (addr)
                zhdr->mapped_count++;
out:
        put_z3fold_header(zhdr);
        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 = get_z3fold_header(handle);
        page = virt_to_page(zhdr);

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

        buddy = handle_to_buddy(handle);
        if (buddy == MIDDLE)
                clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
        zhdr->mapped_count--;
        put_z3fold_header(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);
}

static bool z3fold_page_isolate(struct page *page, isolate_mode_t mode)
{
        struct z3fold_header *zhdr;
        struct z3fold_pool *pool;

        VM_BUG_ON_PAGE(PageIsolated(page), page);

        if (test_bit(PAGE_HEADLESS, &page->private))
                return false;

        zhdr = page_address(page);
        z3fold_page_lock(zhdr);
        if (test_bit(NEEDS_COMPACTING, &page->private) ||
            test_bit(PAGE_STALE, &page->private))
                goto out;

        if (zhdr->mapped_count != 0 || zhdr->foreign_handles != 0)
                goto out;

        if (test_and_set_bit(PAGE_CLAIMED, &page->private))
                goto out;
        pool = zhdr_to_pool(zhdr);
        spin_lock(&pool->lock);
        if (!list_empty(&zhdr->buddy))
                list_del_init(&zhdr->buddy);
        spin_unlock(&pool->lock);

        kref_get(&zhdr->refcount);
        z3fold_page_unlock(zhdr);
        return true;

out:
        z3fold_page_unlock(zhdr);
        return false;
}

static int z3fold_page_migrate(struct page *newpage, struct page *page,
                enum migrate_mode mode)
{
        struct z3fold_header *zhdr, *new_zhdr;
        struct z3fold_pool *pool;

        VM_BUG_ON_PAGE(!PageIsolated(page), page);
        VM_BUG_ON_PAGE(!test_bit(PAGE_CLAIMED, &page->private), page);
        VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);

        zhdr = page_address(page);
        pool = zhdr_to_pool(zhdr);

        if (!z3fold_page_trylock(zhdr))
                return -EAGAIN;
        if (zhdr->mapped_count != 0 || zhdr->foreign_handles != 0) {
                clear_bit(PAGE_CLAIMED, &page->private);
                z3fold_page_unlock(zhdr);
                return -EBUSY;
        }
        if (work_pending(&zhdr->work)) {
                z3fold_page_unlock(zhdr);
                return -EAGAIN;
        }
        new_zhdr = page_address(newpage);
        memcpy(new_zhdr, zhdr, PAGE_SIZE);
        newpage->private = page->private;
        set_bit(PAGE_MIGRATED, &page->private);
        z3fold_page_unlock(zhdr);
        spin_lock_init(&new_zhdr->page_lock);
        INIT_WORK(&new_zhdr->work, compact_page_work);
        /*
         * z3fold_page_isolate() ensures that new_zhdr->buddy is empty,
         * so we only have to reinitialize it.
         */
        INIT_LIST_HEAD(&new_zhdr->buddy);
        __ClearPageMovable(page);

        get_page(newpage);
        z3fold_page_lock(new_zhdr);
        if (new_zhdr->first_chunks)
                encode_handle(new_zhdr, FIRST);
        if (new_zhdr->last_chunks)
                encode_handle(new_zhdr, LAST);
        if (new_zhdr->middle_chunks)
                encode_handle(new_zhdr, MIDDLE);
        set_bit(NEEDS_COMPACTING, &newpage->private);
        new_zhdr->cpu = smp_processor_id();
        __SetPageMovable(newpage, &z3fold_mops);
        z3fold_page_unlock(new_zhdr);

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

        /* PAGE_CLAIMED and PAGE_MIGRATED are cleared now. */
        page->private = 0;
        put_page(page);
        return 0;
}

static void z3fold_page_putback(struct page *page)
{
        struct z3fold_header *zhdr;
        struct z3fold_pool *pool;

        zhdr = page_address(page);
        pool = zhdr_to_pool(zhdr);

        z3fold_page_lock(zhdr);
        if (!list_empty(&zhdr->buddy))
                list_del_init(&zhdr->buddy);
        INIT_LIST_HEAD(&page->lru);
        if (put_z3fold_locked(zhdr))
                return;
        if (list_empty(&zhdr->buddy))
                add_to_unbuddied(pool, zhdr);
        clear_bit(PAGE_CLAIMED, &page->private);
        z3fold_page_unlock(zhdr);
}

static const struct movable_operations z3fold_mops = {
        .isolate_page = z3fold_page_isolate,
        .migrate_page = z3fold_page_migrate,
        .putback_page = z3fold_page_putback,
};

/*****************
 * zpool
 ****************/

static void *z3fold_zpool_create(const char *name, gfp_t gfp)
{
        return z3fold_create_pool(name, gfp);
}

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 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",
        .sleep_mapped = true,
        .owner =        THIS_MODULE,
        .create =       z3fold_zpool_create,
        .destroy =      z3fold_zpool_destroy,
        .malloc =       z3fold_zpool_malloc,
        .free =         z3fold_zpool_free,
        .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 and
         * there has remaining spaces for its buddy.
         */
        BUILD_BUG_ON(ZHDR_SIZE_ALIGNED > PAGE_SIZE - CHUNK_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");