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 // 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");

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