z3fold: fix reclaim lock-ups

[mirror_ubuntu-bionic-kernel.git] / mm / z3fold.c

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

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/atomic.h>
#include <linux/sched.h>
#include <linux/list.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/percpu.h>
#include <linux/preempt.h>
#include <linux/workqueue.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/zpool.h>

/*****************
 * Structures
*****************/
struct z3fold_pool;
struct z3fold_ops {
        int (*evict)(struct z3fold_pool *pool, unsigned long handle);
};

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

/*
 * struct z3fold_header - z3fold page metadata occupying first chunks of each
 *                      z3fold page, except for HEADLESS pages
 * @buddy:              links the z3fold page into the relevant list in the
 *                      pool
 * @page_lock:          per-page lock
 * @refcount:           reference count for the z3fold page
 * @work:               work_struct for page layout optimization
 * @pool:               pointer to the pool which this page belongs to
 * @cpu:                CPU which this page "belongs" to
 * @first_chunks:       the size of the first buddy in chunks, 0 if free
 * @middle_chunks:      the size of the middle buddy in chunks, 0 if free
 * @last_chunks:        the size of the last buddy in chunks, 0 if free
 * @first_num:          the starting number (for the first handle)
 */
struct z3fold_header {
        struct list_head buddy;
        spinlock_t page_lock;
        struct kref refcount;
        struct work_struct work;
        struct z3fold_pool *pool;
        short cpu;
        unsigned short first_chunks;
        unsigned short middle_chunks;
        unsigned short last_chunks;
        unsigned short start_middle;
        unsigned short first_num:2;
};

/*
 * NCHUNKS_ORDER determines the internal allocation granularity, effectively
 * adjusting internal fragmentation.  It also determines the number of
 * freelists maintained in each pool. NCHUNKS_ORDER of 6 means that the
 * allocation granularity will be in chunks of size PAGE_SIZE/64. Some chunks
 * in the beginning of an allocated page are occupied by z3fold header, so
 * NCHUNKS will be calculated to 63 (or 62 in case CONFIG_DEBUG_SPINLOCK=y),
 * which shows the max number of free chunks in z3fold page, also there will
 * be 63, or 62, respectively, freelists per pool.
 */
#define NCHUNKS_ORDER   6

#define CHUNK_SHIFT     (PAGE_SHIFT - NCHUNKS_ORDER)
#define CHUNK_SIZE      (1 << CHUNK_SHIFT)
#define ZHDR_SIZE_ALIGNED round_up(sizeof(struct z3fold_header), CHUNK_SIZE)
#define ZHDR_CHUNKS     (ZHDR_SIZE_ALIGNED >> CHUNK_SHIFT)
#define TOTAL_CHUNKS    (PAGE_SIZE >> CHUNK_SHIFT)
#define NCHUNKS         ((PAGE_SIZE - ZHDR_SIZE_ALIGNED) >> CHUNK_SHIFT)

#define BUDDY_MASK      (0x3)

/**
 * struct z3fold_pool - stores metadata for each z3fold pool
 * @name:       pool name
 * @lock:       protects pool unbuddied/lru lists
 * @stale_lock: protects pool stale page list
 * @unbuddied:  per-cpu array of lists tracking z3fold pages that contain 2-
 *              buddies; the list each z3fold page is added to depends on
 *              the size of its free region.
 * @lru:        list tracking the z3fold pages in LRU order by most recently
 *              added buddy.
 * @stale:      list of pages marked for freeing
 * @pages_nr:   number of z3fold pages in the pool.
 * @ops:        pointer to a structure of user defined operations specified at
 *              pool creation time.
 * @compact_wq: workqueue for page layout background optimization
 * @release_wq: workqueue for safe page release
 * @work:       work_struct for safe page release
 *
 * This structure is allocated at pool creation time and maintains metadata
 * pertaining to a particular z3fold pool.
 */
struct z3fold_pool {
        const char *name;
        spinlock_t lock;
        spinlock_t stale_lock;
        struct list_head *unbuddied;
        struct list_head lru;
        struct list_head stale;
        atomic64_t pages_nr;
        const struct z3fold_ops *ops;
        struct zpool *zpool;
        const struct zpool_ops *zpool_ops;
        struct workqueue_struct *compact_wq;
        struct workqueue_struct *release_wq;
        struct work_struct work;
};

/*
 * Internal z3fold page flags
 */
enum z3fold_page_flags {
        PAGE_HEADLESS = 0,
        MIDDLE_CHUNK_MAPPED,
        NEEDS_COMPACTING,
        PAGE_STALE,
        UNDER_RECLAIM
};

/*****************
 * Helpers
*****************/

/* Converts an allocation size in bytes to size in z3fold chunks */
static int size_to_chunks(size_t size)
{
        return (size + CHUNK_SIZE - 1) >> CHUNK_SHIFT;
}

#define for_each_unbuddied_list(_iter, _begin) \
        for ((_iter) = (_begin); (_iter) < NCHUNKS; (_iter)++)

static void compact_page_work(struct work_struct *w);

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

        INIT_LIST_HEAD(&page->lru);
        clear_bit(PAGE_HEADLESS, &page->private);
        clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
        clear_bit(NEEDS_COMPACTING, &page->private);
        clear_bit(PAGE_STALE, &page->private);
        clear_bit(UNDER_RECLAIM, &page->private);

        spin_lock_init(&zhdr->page_lock);
        kref_init(&zhdr->refcount);
        zhdr->first_chunks = 0;
        zhdr->middle_chunks = 0;
        zhdr->last_chunks = 0;
        zhdr->first_num = 0;
        zhdr->start_middle = 0;
        zhdr->cpu = -1;
        zhdr->pool = pool;
        INIT_LIST_HEAD(&zhdr->buddy);
        INIT_WORK(&zhdr->work, compact_page_work);
        return zhdr;
}

/* Resets the struct page fields and frees the page */
static void free_z3fold_page(struct page *page)
{
        __free_page(page);
}

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

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

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

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

        handle = (unsigned long)zhdr;
        if (bud != HEADLESS)
                handle += (bud + zhdr->first_num) & BUDDY_MASK;
        return handle;
}

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

/*
 * (handle & BUDDY_MASK) < zhdr->first_num is possible in encode_handle
 *  but that doesn't matter. because the masking will result in the
 *  correct buddy number.
 */
static enum buddy handle_to_buddy(unsigned long handle)
{
        struct z3fold_header *zhdr = handle_to_z3fold_header(handle);
        return (handle - zhdr->first_num) & BUDDY_MASK;
}

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

        WARN_ON(!list_empty(&zhdr->buddy));
        set_bit(PAGE_STALE, &page->private);
        clear_bit(NEEDS_COMPACTING, &page->private);
        spin_lock(&pool->lock);
        if (!list_empty(&page->lru))
                list_del(&page->lru);
        spin_unlock(&pool->lock);
        if (locked)
                z3fold_page_unlock(zhdr);
        spin_lock(&pool->stale_lock);
        list_add(&zhdr->buddy, &pool->stale);
        queue_work(pool->release_wq, &pool->work);
        spin_unlock(&pool->stale_lock);
}

static void __attribute__((__unused__))
                        release_z3fold_page(struct kref *ref)
{
        struct z3fold_header *zhdr = container_of(ref, struct z3fold_header,
                                                refcount);
        __release_z3fold_page(zhdr, false);
}

static void release_z3fold_page_locked(struct kref *ref)
{
        struct z3fold_header *zhdr = container_of(ref, struct z3fold_header,
                                                refcount);
        WARN_ON(z3fold_page_trylock(zhdr));
        __release_z3fold_page(zhdr, true);
}

static void release_z3fold_page_locked_list(struct kref *ref)
{
        struct z3fold_header *zhdr = container_of(ref, struct z3fold_header,
                                               refcount);
        spin_lock(&zhdr->pool->lock);
        list_del_init(&zhdr->buddy);
        spin_unlock(&zhdr->pool->lock);

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

static void free_pages_work(struct work_struct *w)
{
        struct z3fold_pool *pool = container_of(w, struct z3fold_pool, work);

        spin_lock(&pool->stale_lock);
        while (!list_empty(&pool->stale)) {
                struct z3fold_header *zhdr = list_first_entry(&pool->stale,
                                                struct z3fold_header, buddy);
                struct page *page = virt_to_page(zhdr);

                list_del(&zhdr->buddy);
                if (WARN_ON(!test_bit(PAGE_STALE, &page->private)))
                        continue;
                spin_unlock(&pool->stale_lock);
                cancel_work_sync(&zhdr->work);
                free_z3fold_page(page);
                cond_resched();
                spin_lock(&pool->stale_lock);
        }
        spin_unlock(&pool->stale_lock);
}

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

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

#define BIG_CHUNK_GAP   3
/* Has to be called with lock held */
static int z3fold_compact_page(struct z3fold_header *zhdr)
{
        struct page *page = virt_to_page(zhdr);

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

        if (zhdr->middle_chunks == 0)
                return 0; /* nothing to compact */

        if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
                /* move to the beginning */
                mchunk_memmove(zhdr, ZHDR_CHUNKS);
                zhdr->first_chunks = zhdr->middle_chunks;
                zhdr->middle_chunks = 0;
                zhdr->start_middle = 0;
                zhdr->first_num++;
                return 1;
        }

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

        return 0;
}

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

        page = virt_to_page(zhdr);
        if (locked)
                WARN_ON(z3fold_page_trylock(zhdr));
        else
                z3fold_page_lock(zhdr);
        if (WARN_ON(!test_and_clear_bit(NEEDS_COMPACTING, &page->private))) {
                z3fold_page_unlock(zhdr);
                return;
        }
        spin_lock(&pool->lock);
        list_del_init(&zhdr->buddy);
        spin_unlock(&pool->lock);

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

        z3fold_compact_page(zhdr);
        unbuddied = get_cpu_ptr(pool->unbuddied);
        fchunks = num_free_chunks(zhdr);
        if (fchunks < NCHUNKS &&
            (!zhdr->first_chunks || !zhdr->middle_chunks ||
                        !zhdr->last_chunks)) {
                /* the page's not completely free and it's unbuddied */
                spin_lock(&pool->lock);
                list_add(&zhdr->buddy, &unbuddied[fchunks]);
                spin_unlock(&pool->lock);
                zhdr->cpu = smp_processor_id();
        }
        put_cpu_ptr(pool->unbuddied);
        z3fold_page_unlock(zhdr);
}

static void compact_page_work(struct work_struct *w)
{
        struct z3fold_header *zhdr = container_of(w, struct z3fold_header,
                                                work);

        do_compact_page(zhdr, false);
}


/*
 * API Functions
 */

/**
 * z3fold_create_pool() - create a new z3fold pool
 * @name:       pool name
 * @gfp:        gfp flags when allocating the z3fold pool structure
 * @ops:        user-defined operations for the z3fold pool
 *
 * Return: pointer to the new z3fold pool or NULL if the metadata allocation
 * failed.
 */
static struct z3fold_pool *z3fold_create_pool(const char *name, gfp_t gfp,
                const struct z3fold_ops *ops)
{
        struct z3fold_pool *pool = NULL;
        int i, cpu;

        pool = kzalloc(sizeof(struct z3fold_pool), gfp);
        if (!pool)
                goto out;
        spin_lock_init(&pool->lock);
        spin_lock_init(&pool->stale_lock);
        pool->unbuddied = __alloc_percpu(sizeof(struct list_head)*NCHUNKS, 2);
        for_each_possible_cpu(cpu) {
                struct list_head *unbuddied =
                                per_cpu_ptr(pool->unbuddied, cpu);
                for_each_unbuddied_list(i, 0)
                        INIT_LIST_HEAD(&unbuddied[i]);
        }
        INIT_LIST_HEAD(&pool->lru);
        INIT_LIST_HEAD(&pool->stale);
        atomic64_set(&pool->pages_nr, 0);
        pool->name = name;
        pool->compact_wq = create_singlethread_workqueue(pool->name);
        if (!pool->compact_wq)
                goto out;
        pool->release_wq = create_singlethread_workqueue(pool->name);
        if (!pool->release_wq)
                goto out_wq;
        INIT_WORK(&pool->work, free_pages_work);
        pool->ops = ops;
        return pool;

out_wq:
        destroy_workqueue(pool->compact_wq);
out:
        kfree(pool);
        return NULL;
}

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

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

        if (!size || (gfp & __GFP_HIGHMEM))
                return -EINVAL;

        if (size > PAGE_SIZE)
                return -ENOSPC;

        if (size > PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE)
                bud = HEADLESS;
        else {
                struct list_head *unbuddied;
                chunks = size_to_chunks(size);

lookup:
                /* First, try to find an unbuddied z3fold page. */
                unbuddied = get_cpu_ptr(pool->unbuddied);
                for_each_unbuddied_list(i, chunks) {
                        struct list_head *l = &unbuddied[i];

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

                        if (!zhdr)
                                continue;

                        /* Re-check under lock. */
                        spin_lock(&pool->lock);
                        l = &unbuddied[i];
                        if (unlikely(zhdr != list_first_entry(READ_ONCE(l),
                                        struct z3fold_header, buddy)) ||
                            !z3fold_page_trylock(zhdr)) {
                                spin_unlock(&pool->lock);
                                put_cpu_ptr(pool->unbuddied);
                                goto lookup;
                        }
                        list_del_init(&zhdr->buddy);
                        zhdr->cpu = -1;
                        spin_unlock(&pool->lock);

                        page = virt_to_page(zhdr);
                        if (test_bit(NEEDS_COMPACTING, &page->private)) {
                                z3fold_page_unlock(zhdr);
                                zhdr = NULL;
                                put_cpu_ptr(pool->unbuddied);
                                if (can_sleep)
                                        cond_resched();
                                goto lookup;
                        }

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

                if (zhdr) {
                        if (zhdr->first_chunks == 0) {
                                if (zhdr->middle_chunks != 0 &&
                                    chunks >= zhdr->start_middle)
                                        bud = LAST;
                                else
                                        bud = FIRST;
                        } else if (zhdr->last_chunks == 0)
                                bud = LAST;
                        else if (zhdr->middle_chunks == 0)
                                bud = MIDDLE;
                        else {
                                if (kref_put(&zhdr->refcount,
                                             release_z3fold_page_locked))
                                        atomic64_dec(&pool->pages_nr);
                                else
                                        z3fold_page_unlock(zhdr);
                                pr_err("No free chunks in unbuddied\n");
                                WARN_ON(1);
                                goto lookup;
                        }
                        goto found;
                }
                bud = FIRST;
        }

        spin_lock(&pool->stale_lock);
        zhdr = list_first_entry_or_null(&pool->stale,
                                        struct z3fold_header, buddy);
        /*
         * Before allocating a page, let's see if we can take one from the
         * stale pages list. cancel_work_sync() can sleep so we must make
         * sure it won't be called in case we're in atomic context.
         */
        if (zhdr && (can_sleep || !work_pending(&zhdr->work))) {
                list_del(&zhdr->buddy);
                spin_unlock(&pool->stale_lock);
                if (can_sleep)
                        cancel_work_sync(&zhdr->work);
                page = virt_to_page(zhdr);
        } else {
                spin_unlock(&pool->stale_lock);
                page = alloc_page(gfp);
        }

        if (!page)
                return -ENOMEM;

        atomic64_inc(&pool->pages_nr);
        zhdr = init_z3fold_page(page, pool);

        if (bud == HEADLESS) {
                set_bit(PAGE_HEADLESS, &page->private);
                goto headless;
        }
        z3fold_page_lock(zhdr);

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

        if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0 ||
                        zhdr->middle_chunks == 0) {
                struct list_head *unbuddied = get_cpu_ptr(pool->unbuddied);

                /* Add to unbuddied list */
                freechunks = num_free_chunks(zhdr);
                spin_lock(&pool->lock);
                list_add(&zhdr->buddy, &unbuddied[freechunks]);
                spin_unlock(&pool->lock);
                zhdr->cpu = smp_processor_id();
                put_cpu_ptr(pool->unbuddied);
        }

headless:
        spin_lock(&pool->lock);
        /* Add/move z3fold page to beginning of LRU */
        if (!list_empty(&page->lru))
                list_del(&page->lru);

        list_add(&page->lru, &pool->lru);

        *handle = encode_handle(zhdr, bud);
        spin_unlock(&pool->lock);
        if (bud != HEADLESS)
                z3fold_page_unlock(zhdr);

        return 0;
}

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

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

        if (test_bit(PAGE_HEADLESS, &page->private)) {
                /* HEADLESS page stored */
                bud = HEADLESS;
        } else {
                z3fold_page_lock(zhdr);
                bud = handle_to_buddy(handle);

                switch (bud) {
                case FIRST:
                        zhdr->first_chunks = 0;
                        break;
                case MIDDLE:
                        zhdr->middle_chunks = 0;
                        zhdr->start_middle = 0;
                        break;
                case LAST:
                        zhdr->last_chunks = 0;
                        break;
                default:
                        pr_err("%s: unknown bud %d\n", __func__, bud);
                        WARN_ON(1);
                        z3fold_page_unlock(zhdr);
                        return;
                }
        }

        if (bud == HEADLESS) {
                spin_lock(&pool->lock);
                list_del(&page->lru);
                spin_unlock(&pool->lock);
                free_z3fold_page(page);
                atomic64_dec(&pool->pages_nr);
                return;
        }

        if (kref_put(&zhdr->refcount, release_z3fold_page_locked_list)) {
                atomic64_dec(&pool->pages_nr);
                return;
        }
        if (test_bit(UNDER_RECLAIM, &page->private)) {
                z3fold_page_unlock(zhdr);
                return;
        }
        if (test_and_set_bit(NEEDS_COMPACTING, &page->private)) {
                z3fold_page_unlock(zhdr);
                return;
        }
        if (zhdr->cpu < 0 || !cpu_online(zhdr->cpu)) {
                spin_lock(&pool->lock);
                list_del_init(&zhdr->buddy);
                spin_unlock(&pool->lock);
                zhdr->cpu = -1;
                kref_get(&zhdr->refcount);
                do_compact_page(zhdr, true);
                return;
        }
        kref_get(&zhdr->refcount);
        queue_work_on(zhdr->cpu, pool->compact_wq, &zhdr->work);
        z3fold_page_unlock(zhdr);
}

/**
 * z3fold_reclaim_page() - evicts allocations from a pool page and frees it
 * @pool:       pool from which a page will attempt to be evicted
 * @retires:    number of pages on the LRU list for which eviction will
 *              be attempted before failing
 *
 * z3fold reclaim is different from normal system reclaim in that it is done
 * from the bottom, up. This is because only the bottom layer, z3fold, has
 * information on how the allocations are organized within each z3fold page.
 * This has the potential to create interesting locking situations between
 * z3fold and the user, however.
 *
 * To avoid these, this is how z3fold_reclaim_page() should be called:

 * The user detects a page should be reclaimed and calls z3fold_reclaim_page().
 * z3fold_reclaim_page() will remove a z3fold page from the pool LRU list and
 * call the user-defined eviction handler with the pool and handle as
 * arguments.
 *
 * If the handle can not be evicted, the eviction handler should return
 * non-zero. z3fold_reclaim_page() will add the z3fold page back to the
 * appropriate list and try the next z3fold page on the LRU up to
 * a user defined number of retries.
 *
 * If the handle is successfully evicted, the eviction handler should
 * return 0 _and_ should have called z3fold_free() on the handle. z3fold_free()
 * contains logic to delay freeing the page if the page is under reclaim,
 * as indicated by the setting of the PG_reclaim flag on the underlying page.
 *
 * If all buddies in the z3fold page are successfully evicted, then the
 * z3fold page can be freed.
 *
 * Returns: 0 if page is successfully freed, otherwise -EINVAL if there are
 * no pages to evict or an eviction handler is not registered, -EAGAIN if
 * the retry limit was hit.
 */
static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
{
        int i, ret = 0;
        struct z3fold_header *zhdr = NULL;
        struct page *page = NULL;
        struct list_head *pos;
        unsigned long first_handle = 0, middle_handle = 0, last_handle = 0;

        spin_lock(&pool->lock);
        if (!pool->ops || !pool->ops->evict || retries == 0) {
                spin_unlock(&pool->lock);
                return -EINVAL;
        }
        for (i = 0; i < retries; i++) {
                if (list_empty(&pool->lru)) {
                        spin_unlock(&pool->lock);
                        return -EINVAL;
                }
                list_for_each_prev(pos, &pool->lru) {
                        page = list_entry(pos, struct page, lru);
                        if (test_bit(PAGE_HEADLESS, &page->private))
                                /* candidate found */
                                break;

                        zhdr = page_address(page);
                        if (!z3fold_page_trylock(zhdr))
                                continue; /* can't evict at this point */
                        kref_get(&zhdr->refcount);
                        list_del_init(&zhdr->buddy);
                        zhdr->cpu = -1;
                        set_bit(UNDER_RECLAIM, &page->private);
                        break;
                }

                list_del_init(&page->lru);
                spin_unlock(&pool->lock);

                if (!test_bit(PAGE_HEADLESS, &page->private)) {
                        /*
                         * We need encode the handles before unlocking, since
                         * we can race with free that will set
                         * (first|last)_chunks to 0
                         */
                        first_handle = 0;
                        last_handle = 0;
                        middle_handle = 0;
                        if (zhdr->first_chunks)
                                first_handle = encode_handle(zhdr, FIRST);
                        if (zhdr->middle_chunks)
                                middle_handle = encode_handle(zhdr, MIDDLE);
                        if (zhdr->last_chunks)
                                last_handle = encode_handle(zhdr, LAST);
                        /*
                         * it's safe to unlock here because we hold a
                         * reference to this page
                         */
                        z3fold_page_unlock(zhdr);
                } else {
                        first_handle = encode_handle(zhdr, HEADLESS);
                        last_handle = middle_handle = 0;
                }

                /* Issue the eviction callback(s) */
                if (middle_handle) {
                        ret = pool->ops->evict(pool, middle_handle);
                        if (ret)
                                goto next;
                }
                if (first_handle) {
                        ret = pool->ops->evict(pool, first_handle);
                        if (ret)
                                goto next;
                }
                if (last_handle) {
                        ret = pool->ops->evict(pool, last_handle);
                        if (ret)
                                goto next;
                }
next:
                if (test_bit(PAGE_HEADLESS, &page->private)) {
                        if (ret == 0) {
                                free_z3fold_page(page);
                                return 0;
                        }
                        spin_lock(&pool->lock);
                        list_add(&page->lru, &pool->lru);
                        spin_unlock(&pool->lock);
                } else {
                        z3fold_page_lock(zhdr);
                        clear_bit(UNDER_RECLAIM, &page->private);
                        if (kref_put(&zhdr->refcount,
                                        release_z3fold_page_locked)) {
                                atomic64_dec(&pool->pages_nr);
                                return 0;
                        }
                        /*
                         * if we are here, the page is still not completely
                         * free. Take the global pool lock then to be able
                         * to add it back to the lru list
                         */
                        spin_lock(&pool->lock);
                        list_add(&page->lru, &pool->lru);
                        spin_unlock(&pool->lock);
                        z3fold_page_unlock(zhdr);
                }

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

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

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

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

        z3fold_page_lock(zhdr);
        buddy = handle_to_buddy(handle);
        switch (buddy) {
        case FIRST:
                addr += ZHDR_SIZE_ALIGNED;
                break;
        case MIDDLE:
                addr += zhdr->start_middle << CHUNK_SHIFT;
                set_bit(MIDDLE_CHUNK_MAPPED, &page->private);
                break;
        case LAST:
                addr += PAGE_SIZE - (zhdr->last_chunks << CHUNK_SHIFT);
                break;
        default:
                pr_err("unknown buddy id %d\n", buddy);
                WARN_ON(1);
                addr = NULL;
                break;
        }

        z3fold_page_unlock(zhdr);
out:
        return addr;
}

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

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

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

        z3fold_page_lock(zhdr);
        buddy = handle_to_buddy(handle);
        if (buddy == MIDDLE)
                clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
        z3fold_page_unlock(zhdr);
}

/**
 * z3fold_get_pool_size() - gets the z3fold pool size in pages
 * @pool:       pool whose size is being queried
 *
 * Returns: size in pages of the given pool.
 */
static u64 z3fold_get_pool_size(struct z3fold_pool *pool)
{
        return atomic64_read(&pool->pages_nr);
}

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

static int z3fold_zpool_evict(struct z3fold_pool *pool, unsigned long handle)
{
        if (pool->zpool && pool->zpool_ops && pool->zpool_ops->evict)
                return pool->zpool_ops->evict(pool->zpool, handle);
        else
                return -ENOENT;
}

static const struct z3fold_ops z3fold_zpool_ops = {
        .evict =        z3fold_zpool_evict
};

static void *z3fold_zpool_create(const char *name, gfp_t gfp,
                               const struct zpool_ops *zpool_ops,
                               struct zpool *zpool)
{
        struct z3fold_pool *pool;

        pool = z3fold_create_pool(name, gfp,
                                zpool_ops ? &z3fold_zpool_ops : NULL);
        if (pool) {
                pool->zpool = zpool;
                pool->zpool_ops = zpool_ops;
        }
        return pool;
}

static void z3fold_zpool_destroy(void *pool)
{
        z3fold_destroy_pool(pool);
}

static int z3fold_zpool_malloc(void *pool, size_t size, gfp_t gfp,
                        unsigned long *handle)
{
        return z3fold_alloc(pool, size, gfp, handle);
}
static void z3fold_zpool_free(void *pool, unsigned long handle)
{
        z3fold_free(pool, handle);
}

static int z3fold_zpool_shrink(void *pool, unsigned int pages,
                        unsigned int *reclaimed)
{
        unsigned int total = 0;
        int ret = -EINVAL;

        while (total < pages) {
                ret = z3fold_reclaim_page(pool, 8);
                if (ret < 0)
                        break;
                total++;
        }

        if (reclaimed)
                *reclaimed = total;

        return ret;
}

static void *z3fold_zpool_map(void *pool, unsigned long handle,
                        enum zpool_mapmode mm)
{
        return z3fold_map(pool, handle);
}
static void z3fold_zpool_unmap(void *pool, unsigned long handle)
{
        z3fold_unmap(pool, handle);
}

static u64 z3fold_zpool_total_size(void *pool)
{
        return z3fold_get_pool_size(pool) * PAGE_SIZE;
}

static struct zpool_driver z3fold_zpool_driver = {
        .type =         "z3fold",
        .owner =        THIS_MODULE,
        .create =       z3fold_zpool_create,
        .destroy =      z3fold_zpool_destroy,
        .malloc =       z3fold_zpool_malloc,
        .free =         z3fold_zpool_free,
        .shrink =       z3fold_zpool_shrink,
        .map =          z3fold_zpool_map,
        .unmap =        z3fold_zpool_unmap,
        .total_size =   z3fold_zpool_total_size,
};

MODULE_ALIAS("zpool-z3fold");

static int __init init_z3fold(void)
{
        /* Make sure the z3fold header is not larger than the page size */
        BUILD_BUG_ON(ZHDR_SIZE_ALIGNED > PAGE_SIZE);
        zpool_register_driver(&z3fold_zpool_driver);

        return 0;
}

static void __exit exit_z3fold(void)
{
        zpool_unregister_driver(&z3fold_zpool_driver);
}

module_init(init_z3fold);
module_exit(exit_z3fold);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Vitaly Wool <vitalywool@gmail.com>");
MODULE_DESCRIPTION("3-Fold Allocator for Compressed Pages");