// SPDX-License-Identifier: GPL-2.0-only
/*
 * zbud.c
 *
 * Copyright (C) 2013, Seth Jennings, IBM
 *
 * Concepts based on zcache internal zbud allocator by Dan Magenheimer.
 *
 * zbud is an special purpose allocator for storing compressed pages.  Contrary
 * to what its name may suggest, zbud is not a buddy allocator, but rather an
 * allocator that "buddies" two compressed pages together in a single memory
 * page.
 *
 * While this design limits storage density, it has simple and deterministic
 * reclaim properties that make it preferable to a higher density approach when
 * reclaim will be used.
 *
 * zbud works by storing compressed pages, or "zpages", together in pairs in a
 * single memory page called a "zbud page".  The first buddy is "left
 * justified" at the beginning of the zbud page, and the last buddy is "right
 * justified" at the end of the zbud page.  The benefit is that if either
 * buddy is freed, the freed buddy space, coalesced with whatever slack space
 * that existed between the buddies, results in the largest possible free region
 * within the zbud page.
 *
 * zbud also provides an attractive lower bound on density. The ratio of zpages
 * to zbud pages can not be less than 1.  This ensures that zbud can never "do
 * harm" by using more pages to store zpages than the uncompressed zpages would
 * have used on their own.
 *
 * zbud pages are divided into "chunks".  The size of the chunks is fixed at
 * compile time and determined by NCHUNKS_ORDER below.  Dividing zbud pages
 * into chunks allows organizing unbuddied zbud pages into a manageable number
 * of unbuddied lists according to the number of free chunks available in the
 * zbud page.
 *
 * The zbud API differs from that of conventional allocators in that the
 * allocation function, zbud_alloc(), returns an opaque handle to the user,
 * not a dereferenceable pointer.  The user must map the handle using
 * zbud_map() in order to get a usable pointer by which to access the
 * allocation data and unmap the handle with zbud_unmap() when operations
 * on the allocation data are complete.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

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

/*****************
 * Structures
*****************/
/*
 * 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. As one chunk
 * in allocated page is occupied by zbud header, NCHUNKS will be calculated to
 * 63 which shows the max number of free chunks in zbud page, also there will be
 * 63 freelists per pool.
 */
#define NCHUNKS_ORDER   6

#define CHUNK_SHIFT     (PAGE_SHIFT - NCHUNKS_ORDER)
#define CHUNK_SIZE      (1 << CHUNK_SHIFT)
#define ZHDR_SIZE_ALIGNED CHUNK_SIZE
#define NCHUNKS         ((PAGE_SIZE - ZHDR_SIZE_ALIGNED) >> CHUNK_SHIFT)

struct zbud_pool;

struct zbud_ops {
        int (*evict)(struct zbud_pool *pool, unsigned long handle);
};

/**
 * struct zbud_pool - stores metadata for each zbud pool
 * @lock:       protects all pool fields and first|last_chunk fields of any
 *              zbud page in the pool
 * @unbuddied:  array of lists tracking zbud pages that only contain one buddy;
 *              the lists each zbud page is added to depends on the size of
 *              its free region.
 * @buddied:    list tracking the zbud pages that contain two buddies;
 *              these zbud pages are full
 * @lru:        list tracking the zbud pages in LRU order by most recently
 *              added buddy.
 * @pages_nr:   number of zbud pages in the pool.
 * @ops:        pointer to a structure of user defined operations specified at
 *              pool creation time.
 * @zpool:      zpool driver
 * @zpool_ops:  zpool operations structure with an evict callback
 *
 * This structure is allocated at pool creation time and maintains metadata
 * pertaining to a particular zbud pool.
 */
struct zbud_pool {
        spinlock_t lock;
        union {
                /*
                 * Reuse unbuddied[0] as buddied on the ground that
                 * unbuddied[0] is unused.
                 */
                struct list_head buddied;
                struct list_head unbuddied[NCHUNKS];
        };
        struct list_head lru;
        u64 pages_nr;
        const struct zbud_ops *ops;
        struct zpool *zpool;
        const struct zpool_ops *zpool_ops;
};

/*
 * struct zbud_header - zbud page metadata occupying the first chunk of each
 *                      zbud page.
 * @buddy:      links the zbud page into the unbuddied/buddied lists in the pool
 * @lru:        links the zbud page into the lru list in the pool
 * @first_chunks:       the size of the first buddy in chunks, 0 if free
 * @last_chunks:        the size of the last buddy in chunks, 0 if free
 */
struct zbud_header {
        struct list_head buddy;
        struct list_head lru;
        unsigned int first_chunks;
        unsigned int last_chunks;
        bool under_reclaim;
};

/*****************
 * Helpers
*****************/
/* Just to make the code easier to read */
enum buddy {
        FIRST,
        LAST
};

/* Converts an allocation size in bytes to size in zbud 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)++)

/* Initializes the zbud header of a newly allocated zbud page */
static struct zbud_header *init_zbud_page(struct page *page)
{
        struct zbud_header *zhdr = page_address(page);
        zhdr->first_chunks = 0;
        zhdr->last_chunks = 0;
        INIT_LIST_HEAD(&zhdr->buddy);
        INIT_LIST_HEAD(&zhdr->lru);
        zhdr->under_reclaim = false;
        return zhdr;
}

/* Resets the struct page fields and frees the page */
static void free_zbud_page(struct zbud_header *zhdr)
{
        __free_page(virt_to_page(zhdr));
}

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

        /*
         * For now, the encoded handle is actually just the pointer to the data
         * but this might not always be the case.  A little information hiding.
         * Add CHUNK_SIZE to the handle if it is the first allocation to jump
         * over the zbud header in the first chunk.
         */
        handle = (unsigned long)zhdr;
        if (bud == FIRST)
                /* skip over zbud header */
                handle += ZHDR_SIZE_ALIGNED;
        else /* bud == LAST */
                handle += PAGE_SIZE - (zhdr->last_chunks  << CHUNK_SHIFT);
        return handle;
}

/* Returns the zbud page where a given handle is stored */
static struct zbud_header *handle_to_zbud_header(unsigned long handle)
{
        return (struct zbud_header *)(handle & PAGE_MASK);
}

/* Returns the number of free chunks in a zbud page */
static int num_free_chunks(struct zbud_header *zhdr)
{
        /*
         * Rather than branch for different situations, just use the fact that
         * free buddies have a length of zero to simplify everything.
         */
        return NCHUNKS - zhdr->first_chunks - zhdr->last_chunks;
}

/*****************
 * API Functions
*****************/
/**
 * zbud_create_pool() - create a new zbud pool
 * @gfp:        gfp flags when allocating the zbud pool structure
 * @ops:        user-defined operations for the zbud pool
 *
 * Return: pointer to the new zbud pool or NULL if the metadata allocation
 * failed.
 */
static struct zbud_pool *zbud_create_pool(gfp_t gfp, const struct zbud_ops *ops)
{
        struct zbud_pool *pool;
        int i;

        pool = kzalloc(sizeof(struct zbud_pool), gfp);
        if (!pool)
                return NULL;
        spin_lock_init(&pool->lock);
        for_each_unbuddied_list(i, 0)
                INIT_LIST_HEAD(&pool->unbuddied[i]);
        INIT_LIST_HEAD(&pool->buddied);
        INIT_LIST_HEAD(&pool->lru);
        pool->pages_nr = 0;
        pool->ops = ops;
        return pool;
}

/**
 * zbud_destroy_pool() - destroys an existing zbud pool
 * @pool:       the zbud pool to be destroyed
 *
 * The pool should be emptied before this function is called.
 */
static void zbud_destroy_pool(struct zbud_pool *pool)
{
        kfree(pool);
}

/**
 * zbud_alloc() - allocates a region of a given size
 * @pool:       zbud 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 zbud 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 zbud_alloc(struct zbud_pool *pool, size_t size, gfp_t gfp,
                        unsigned long *handle)
{
        int chunks, i, freechunks;
        struct zbud_header *zhdr = NULL;
        enum buddy bud;
        struct page *page;

        if (!size || (gfp & __GFP_HIGHMEM))
                return -EINVAL;
        if (size > PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE)
                return -ENOSPC;
        chunks = size_to_chunks(size);
        spin_lock(&pool->lock);

        /* First, try to find an unbuddied zbud page. */
        for_each_unbuddied_list(i, chunks) {
                if (!list_empty(&pool->unbuddied[i])) {
                        zhdr = list_first_entry(&pool->unbuddied[i],
                                        struct zbud_header, buddy);
                        list_del(&zhdr->buddy);
                        if (zhdr->first_chunks == 0)
                                bud = FIRST;
                        else
                                bud = LAST;
                        goto found;
                }
        }

        /* Couldn't find unbuddied zbud page, create new one */
        spin_unlock(&pool->lock);
        page = alloc_page(gfp);
        if (!page)
                return -ENOMEM;
        spin_lock(&pool->lock);
        pool->pages_nr++;
        zhdr = init_zbud_page(page);
        bud = FIRST;

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

        if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0) {
                /* Add to unbuddied list */
                freechunks = num_free_chunks(zhdr);
                list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
        } else {
                /* Add to buddied list */
                list_add(&zhdr->buddy, &pool->buddied);
        }

        /* Add/move zbud page to beginning of LRU */
        if (!list_empty(&zhdr->lru))
                list_del(&zhdr->lru);
        list_add(&zhdr->lru, &pool->lru);

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

        return 0;
}

/**
 * zbud_free() - frees the allocation associated with the given handle
 * @pool:       pool in which the allocation resided
 * @handle:     handle associated with the allocation returned by zbud_alloc()
 *
 * In the case that the zbud 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 zbud_reclaim_page() below).
 */
static void zbud_free(struct zbud_pool *pool, unsigned long handle)
{
        struct zbud_header *zhdr;
        int freechunks;

        spin_lock(&pool->lock);
        zhdr = handle_to_zbud_header(handle);

        /* If first buddy, handle will be page aligned */
        if ((handle - ZHDR_SIZE_ALIGNED) & ~PAGE_MASK)
                zhdr->last_chunks = 0;
        else
                zhdr->first_chunks = 0;

        if (zhdr->under_reclaim) {
                /* zbud page is under reclaim, reclaim will free */
                spin_unlock(&pool->lock);
                return;
        }

        /* Remove from existing buddy list */
        list_del(&zhdr->buddy);

        if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
                /* zbud page is empty, free */
                list_del(&zhdr->lru);
                free_zbud_page(zhdr);
                pool->pages_nr--;
        } else {
                /* Add to unbuddied list */
                freechunks = num_free_chunks(zhdr);
                list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
        }

        spin_unlock(&pool->lock);
}

/**
 * zbud_reclaim_page() - evicts allocations from a pool page and frees it
 * @pool:       pool from which a page will attempt to be evicted
 * @retries:    number of pages on the LRU list for which eviction will
 *              be attempted before failing
 *
 * zbud reclaim is different from normal system reclaim in that the reclaim is
 * done from the bottom, up.  This is because only the bottom layer, zbud, has
 * information on how the allocations are organized within each zbud page. This
 * has the potential to create interesting locking situations between zbud and
 * the user, however.
 *
 * To avoid these, this is how zbud_reclaim_page() should be called:
 *
 * The user detects a page should be reclaimed and calls zbud_reclaim_page().
 * zbud_reclaim_page() will remove a zbud 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. zbud_reclaim_page() will add the zbud page back to the
 * appropriate list and try the next zbud 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 zbud_free() on the handle. zbud_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 zbud page are successfully evicted, then the
 * zbud 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 zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
{
        int i, ret, freechunks;
        struct zbud_header *zhdr;
        unsigned long first_handle = 0, last_handle = 0;

        spin_lock(&pool->lock);
        if (!pool->ops || !pool->ops->evict || list_empty(&pool->lru) ||
                        retries == 0) {
                spin_unlock(&pool->lock);
                return -EINVAL;
        }
        for (i = 0; i < retries; i++) {
                zhdr = list_last_entry(&pool->lru, struct zbud_header, lru);
                list_del(&zhdr->lru);
                list_del(&zhdr->buddy);
                /* Protect zbud page against free */
                zhdr->under_reclaim = true;
                /*
                 * 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;
                if (zhdr->first_chunks)
                        first_handle = encode_handle(zhdr, FIRST);
                if (zhdr->last_chunks)
                        last_handle = encode_handle(zhdr, LAST);
                spin_unlock(&pool->lock);

                /* Issue the eviction callback(s) */
                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:
                spin_lock(&pool->lock);
                zhdr->under_reclaim = false;
                if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
                        /*
                         * Both buddies are now free, free the zbud page and
                         * return success.
                         */
                        free_zbud_page(zhdr);
                        pool->pages_nr--;
                        spin_unlock(&pool->lock);
                        return 0;
                } else if (zhdr->first_chunks == 0 ||
                                zhdr->last_chunks == 0) {
                        /* add to unbuddied list */
                        freechunks = num_free_chunks(zhdr);
                        list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
                } else {
                        /* add to buddied list */
                        list_add(&zhdr->buddy, &pool->buddied);
                }

                /* add to beginning of LRU */
                list_add(&zhdr->lru, &pool->lru);
        }
        spin_unlock(&pool->lock);
        return -EAGAIN;
}

/**
 * zbud_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
 *
 * While trivial for zbud, the mapping functions for others allocators
 * implementing this allocation API could have more complex information encoded
 * in the handle and could create temporary mappings to make the data
 * accessible to the user.
 *
 * Returns: a pointer to the mapped allocation
 */
static void *zbud_map(struct zbud_pool *pool, unsigned long handle)
{
        return (void *)(handle);
}

/**
 * zbud_unmap() - maps 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 zbud_unmap(struct zbud_pool *pool, unsigned long handle)
{
}

/**
 * zbud_get_pool_size() - gets the zbud pool size in pages
 * @pool:       pool whose size is being queried
 *
 * Returns: size in pages of the given pool.  The pool lock need not be
 * taken to access pages_nr.
 */
static u64 zbud_get_pool_size(struct zbud_pool *pool)
{
        return pool->pages_nr;
}

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

static int zbud_zpool_evict(struct zbud_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 zbud_ops zbud_zpool_ops = {
        .evict =        zbud_zpool_evict
};

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

        pool = zbud_create_pool(gfp, zpool_ops ? &zbud_zpool_ops : NULL);
        if (pool) {
                pool->zpool = zpool;
                pool->zpool_ops = zpool_ops;
        }
        return pool;
}

static void zbud_zpool_destroy(void *pool)
{
        zbud_destroy_pool(pool);
}

static int zbud_zpool_malloc(void *pool, size_t size, gfp_t gfp,
                        unsigned long *handle)
{
        return zbud_alloc(pool, size, gfp, handle);
}
static void zbud_zpool_free(void *pool, unsigned long handle)
{
        zbud_free(pool, handle);
}

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

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

        if (reclaimed)
                *reclaimed = total;

        return ret;
}

static void *zbud_zpool_map(void *pool, unsigned long handle,
                        enum zpool_mapmode mm)
{
        return zbud_map(pool, handle);
}
static void zbud_zpool_unmap(void *pool, unsigned long handle)
{
        zbud_unmap(pool, handle);
}

static u64 zbud_zpool_total_size(void *pool)
{
        return zbud_get_pool_size(pool) * PAGE_SIZE;
}

static struct zpool_driver zbud_zpool_driver = {
        .type =         "zbud",
        .sleep_mapped = true,
        .owner =        THIS_MODULE,
        .create =       zbud_zpool_create,
        .destroy =      zbud_zpool_destroy,
        .malloc =       zbud_zpool_malloc,
        .free =         zbud_zpool_free,
        .shrink =       zbud_zpool_shrink,
        .map =          zbud_zpool_map,
        .unmap =        zbud_zpool_unmap,
        .total_size =   zbud_zpool_total_size,
};

MODULE_ALIAS("zpool-zbud");

static int __init init_zbud(void)
{
        /* Make sure the zbud header will fit in one chunk */
        BUILD_BUG_ON(sizeof(struct zbud_header) > ZHDR_SIZE_ALIGNED);
        pr_info("loaded\n");

        zpool_register_driver(&zbud_zpool_driver);

        return 0;
}

static void __exit exit_zbud(void)
{
        zpool_unregister_driver(&zbud_zpool_driver);
        pr_info("unloaded\n");
}

module_init(init_zbud);
module_exit(exit_zbud);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Seth Jennings <sjennings@variantweb.net>");
MODULE_DESCRIPTION("Buddy Allocator for Compressed Pages");