// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2008 Oracle.  All rights reserved.
 */

#include <linux/kernel.h>
#include <linux/bio.h>
#include <linux/file.h>
#include <linux/fs.h>
#include <linux/pagemap.h>
#include <linux/pagevec.h>
#include <linux/highmem.h>
#include <linux/kthread.h>
#include <linux/time.h>
#include <linux/init.h>
#include <linux/string.h>
#include <linux/backing-dev.h>
#include <linux/writeback.h>
#include <linux/psi.h>
#include <linux/slab.h>
#include <linux/sched/mm.h>
#include <linux/log2.h>
#include <linux/shrinker.h>
#include "misc.h"
#include "ctree.h"
#include "fs.h"
#include "btrfs_inode.h"
#include "bio.h"
#include "ordered-data.h"
#include "compression.h"
#include "extent_io.h"
#include "extent_map.h"
#include "subpage.h"
#include "messages.h"
#include "super.h"

static struct bio_set btrfs_compressed_bioset;

static const char* const btrfs_compress_types[] = { "", "zlib", "lzo", "zstd" };

const char* btrfs_compress_type2str(enum btrfs_compression_type type)
{
        switch (type) {
        case BTRFS_COMPRESS_ZLIB:
        case BTRFS_COMPRESS_LZO:
        case BTRFS_COMPRESS_ZSTD:
        case BTRFS_COMPRESS_NONE:
                return btrfs_compress_types[type];
        default:
                break;
        }

        return NULL;
}

static inline struct compressed_bio *to_compressed_bio(struct btrfs_bio *bbio)
{
        return container_of(bbio, struct compressed_bio, bbio);
}

static struct compressed_bio *alloc_compressed_bio(struct btrfs_inode *inode,
                                                   u64 start, blk_opf_t op,
                                                   btrfs_bio_end_io_t end_io)
{
        struct btrfs_bio *bbio;

        bbio = btrfs_bio(bio_alloc_bioset(NULL, BTRFS_MAX_COMPRESSED_PAGES, op,
                                          GFP_NOFS, &btrfs_compressed_bioset));
        btrfs_bio_init(bbio, inode, start, end_io, NULL);
        return to_compressed_bio(bbio);
}

bool btrfs_compress_is_valid_type(const char *str, size_t len)
{
        int i;

        for (i = 1; i < ARRAY_SIZE(btrfs_compress_types); i++) {
                size_t comp_len = strlen(btrfs_compress_types[i]);

                if (len < comp_len)
                        continue;

                if (!strncmp(btrfs_compress_types[i], str, comp_len))
                        return true;
        }
        return false;
}

static int compression_decompress_bio(struct list_head *ws,
                                      struct compressed_bio *cb)
{
        switch (cb->compress_type) {
        case BTRFS_COMPRESS_ZLIB: return zlib_decompress_bio(ws, cb);
        case BTRFS_COMPRESS_LZO:  return lzo_decompress_bio(ws, cb);
        case BTRFS_COMPRESS_ZSTD: return zstd_decompress_bio(ws, cb);
        case BTRFS_COMPRESS_NONE:
        default:
                /*
                 * This can't happen, the type is validated several times
                 * before we get here.
                 */
                BUG();
        }
}

static int compression_decompress(int type, struct list_head *ws,
                const u8 *data_in, struct folio *dest_folio,
                unsigned long dest_pgoff, size_t srclen, size_t destlen)
{
        switch (type) {
        case BTRFS_COMPRESS_ZLIB: return zlib_decompress(ws, data_in, dest_folio,
                                                dest_pgoff, srclen, destlen);
        case BTRFS_COMPRESS_LZO:  return lzo_decompress(ws, data_in, dest_folio,
                                                dest_pgoff, srclen, destlen);
        case BTRFS_COMPRESS_ZSTD: return zstd_decompress(ws, data_in, dest_folio,
                                                dest_pgoff, srclen, destlen);
        case BTRFS_COMPRESS_NONE:
        default:
                /*
                 * This can't happen, the type is validated several times
                 * before we get here.
                 */
                BUG();
        }
}

static int btrfs_decompress_bio(struct compressed_bio *cb);

/*
 * Global cache of last unused pages for compression/decompression.
 */
static struct btrfs_compr_pool {
        struct shrinker *shrinker;
        spinlock_t lock;
        struct list_head list;
        int count;
        int thresh;
} compr_pool;

static unsigned long btrfs_compr_pool_count(struct shrinker *sh, struct shrink_control *sc)
{
        int ret;

        /*
         * We must not read the values more than once if 'ret' gets expanded in
         * the return statement so we don't accidentally return a negative
         * number, even if the first condition finds it positive.
         */
        ret = READ_ONCE(compr_pool.count) - READ_ONCE(compr_pool.thresh);

        return ret > 0 ? ret : 0;
}

static unsigned long btrfs_compr_pool_scan(struct shrinker *sh, struct shrink_control *sc)
{
        LIST_HEAD(remove);
        struct list_head *tmp, *next;
        int freed;

        if (compr_pool.count == 0)
                return SHRINK_STOP;

        /* For now, just simply drain the whole list. */
        spin_lock(&compr_pool.lock);
        list_splice_init(&compr_pool.list, &remove);
        freed = compr_pool.count;
        compr_pool.count = 0;
        spin_unlock(&compr_pool.lock);

        list_for_each_safe(tmp, next, &remove) {
                struct page *page = list_entry(tmp, struct page, lru);

                ASSERT(page_ref_count(page) == 1);
                put_page(page);
        }

        return freed;
}

/*
 * Common wrappers for page allocation from compression wrappers
 */
struct folio *btrfs_alloc_compr_folio(struct btrfs_fs_info *fs_info)
{
        struct folio *folio = NULL;

        /* For bs > ps cases, no cached folio pool for now. */
        if (fs_info->block_min_order)
                goto alloc;

        spin_lock(&compr_pool.lock);
        if (compr_pool.count > 0) {
                folio = list_first_entry(&compr_pool.list, struct folio, lru);
                list_del_init(&folio->lru);
                compr_pool.count--;
        }
        spin_unlock(&compr_pool.lock);

        if (folio)
                return folio;

alloc:
        return folio_alloc(GFP_NOFS, fs_info->block_min_order);
}

void btrfs_free_compr_folio(struct folio *folio)
{
        bool do_free = false;

        /* The folio is from bs > ps fs, no cached pool for now. */
        if (folio_order(folio))
                goto free;

        spin_lock(&compr_pool.lock);
        if (compr_pool.count > compr_pool.thresh) {
                do_free = true;
        } else {
                list_add(&folio->lru, &compr_pool.list);
                compr_pool.count++;
        }
        spin_unlock(&compr_pool.lock);

        if (!do_free)
                return;

free:
        ASSERT(folio_ref_count(folio) == 1);
        folio_put(folio);
}

static void end_bbio_compressed_read(struct btrfs_bio *bbio)
{
        struct compressed_bio *cb = to_compressed_bio(bbio);
        blk_status_t status = bbio->bio.bi_status;
        struct folio_iter fi;

        if (!status)
                status = errno_to_blk_status(btrfs_decompress_bio(cb));

        btrfs_bio_end_io(cb->orig_bbio, status);
        bio_for_each_folio_all(fi, &bbio->bio)
                btrfs_free_compr_folio(fi.folio);
        bio_put(&bbio->bio);
}

/*
 * Clear the writeback bits on all of the file
 * pages for a compressed write
 */
static noinline void end_compressed_writeback(const struct compressed_bio *cb)
{
        struct inode *inode = &cb->bbio.inode->vfs_inode;
        struct btrfs_fs_info *fs_info = inode_to_fs_info(inode);
        pgoff_t index = cb->start >> PAGE_SHIFT;
        const pgoff_t end_index = (cb->start + cb->len - 1) >> PAGE_SHIFT;
        struct folio_batch fbatch;
        int i;
        int ret;

        ret = blk_status_to_errno(cb->bbio.bio.bi_status);
        if (ret)
                mapping_set_error(inode->i_mapping, ret);

        folio_batch_init(&fbatch);
        while (index <= end_index) {
                ret = filemap_get_folios(inode->i_mapping, &index, end_index,
                                &fbatch);

                if (ret == 0)
                        return;

                for (i = 0; i < ret; i++) {
                        struct folio *folio = fbatch.folios[i];

                        btrfs_folio_clamp_clear_writeback(fs_info, folio,
                                                          cb->start, cb->len);
                }
                folio_batch_release(&fbatch);
        }
        /* the inode may be gone now */
}

/*
 * Do the cleanup once all the compressed pages hit the disk.  This will clear
 * writeback on the file pages and free the compressed pages.
 *
 * This also calls the writeback end hooks for the file pages so that metadata
 * and checksums can be updated in the file.
 */
static void end_bbio_compressed_write(struct btrfs_bio *bbio)
{
        struct compressed_bio *cb = to_compressed_bio(bbio);
        struct folio_iter fi;

        btrfs_finish_ordered_extent(cb->bbio.ordered, NULL, cb->start, cb->len,
                                    cb->bbio.bio.bi_status == BLK_STS_OK);

        if (cb->writeback)
                end_compressed_writeback(cb);
        /* Note, our inode could be gone now. */
        bio_for_each_folio_all(fi, &bbio->bio)
                btrfs_free_compr_folio(fi.folio);
        bio_put(&cb->bbio.bio);
}

/*
 * worker function to build and submit bios for previously compressed pages.
 * The corresponding pages in the inode should be marked for writeback
 * and the compressed pages should have a reference on them for dropping
 * when the IO is complete.
 *
 * This also checksums the file bytes and gets things ready for
 * the end io hooks.
 */
void btrfs_submit_compressed_write(struct btrfs_ordered_extent *ordered,
                                   struct compressed_bio *cb)
{
        struct btrfs_inode *inode = ordered->inode;
        struct btrfs_fs_info *fs_info = inode->root->fs_info;

        ASSERT(IS_ALIGNED(ordered->file_offset, fs_info->sectorsize));
        ASSERT(IS_ALIGNED(ordered->num_bytes, fs_info->sectorsize));
        /*
         * This flag determines if we should clear the writeback flag from the
         * page cache. But this function is only utilized by encoded writes, it
         * never goes through the page cache.
         */
        ASSERT(!cb->writeback);

        cb->start = ordered->file_offset;
        cb->len = ordered->num_bytes;
        ASSERT(cb->bbio.bio.bi_iter.bi_size == ordered->disk_num_bytes);
        cb->compressed_len = ordered->disk_num_bytes;
        cb->bbio.bio.bi_iter.bi_sector = ordered->disk_bytenr >> SECTOR_SHIFT;
        cb->bbio.ordered = ordered;

        btrfs_submit_bbio(&cb->bbio, 0);
}

/*
 * Allocate a compressed write bio for @inode file offset @start length @len.
 *
 * The caller still needs to properly queue all folios and populate involved
 * members.
 */
struct compressed_bio *btrfs_alloc_compressed_write(struct btrfs_inode *inode,
                                                    u64 start, u64 len)
{
        struct compressed_bio *cb;

        cb = alloc_compressed_bio(inode, start, REQ_OP_WRITE, end_bbio_compressed_write);
        cb->start = start;
        cb->len = len;
        cb->writeback = false;
        return cb;
}

/*
 * Add extra pages in the same compressed file extent so that we don't need to
 * re-read the same extent again and again.
 *
 * NOTE: this won't work well for subpage, as for subpage read, we lock the
 * full page then submit bio for each compressed/regular extents.
 *
 * This means, if we have several sectors in the same page points to the same
 * on-disk compressed data, we will re-read the same extent many times and
 * this function can only help for the next page.
 */
static noinline int add_ra_bio_pages(struct inode *inode,
                                     u64 compressed_end,
                                     struct compressed_bio *cb,
                                     int *memstall, unsigned long *pflags)
{
        struct btrfs_fs_info *fs_info = inode_to_fs_info(inode);
        pgoff_t end_index;
        struct bio *orig_bio = &cb->orig_bbio->bio;
        u64 cur = cb->orig_bbio->file_offset + orig_bio->bi_iter.bi_size;
        u64 isize = i_size_read(inode);
        int ret;
        struct folio *folio;
        struct extent_map *em;
        struct address_space *mapping = inode->i_mapping;
        struct extent_map_tree *em_tree;
        struct extent_io_tree *tree;
        int sectors_missed = 0;

        em_tree = &BTRFS_I(inode)->extent_tree;
        tree = &BTRFS_I(inode)->io_tree;

        if (isize == 0)
                return 0;

        /*
         * For current subpage support, we only support 64K page size,
         * which means maximum compressed extent size (128K) is just 2x page
         * size.
         * This makes readahead less effective, so here disable readahead for
         * subpage for now, until full compressed write is supported.
         */
        if (fs_info->sectorsize < PAGE_SIZE)
                return 0;

        /* For bs > ps cases, we don't support readahead for compressed folios for now. */
        if (fs_info->block_min_order)
                return 0;

        end_index = (i_size_read(inode) - 1) >> PAGE_SHIFT;

        while (cur < compressed_end) {
                pgoff_t page_end;
                pgoff_t pg_index = cur >> PAGE_SHIFT;
                u32 add_size;

                if (pg_index > end_index)
                        break;

                folio = filemap_get_folio(mapping, pg_index);
                if (!IS_ERR(folio)) {
                        u64 folio_sz = folio_size(folio);
                        u64 offset = offset_in_folio(folio, cur);

                        folio_put(folio);
                        sectors_missed += (folio_sz - offset) >>
                                          fs_info->sectorsize_bits;

                        /* Beyond threshold, no need to continue */
                        if (sectors_missed > 4)
                                break;

                        /*
                         * Jump to next page start as we already have page for
                         * current offset.
                         */
                        cur += (folio_sz - offset);
                        continue;
                }

                folio = filemap_alloc_folio(mapping_gfp_constraint(mapping, ~__GFP_FS),
                                            0, NULL);
                if (!folio)
                        break;

                if (filemap_add_folio(mapping, folio, pg_index, GFP_NOFS)) {
                        /* There is already a page, skip to page end */
                        cur += folio_size(folio);
                        folio_put(folio);
                        continue;
                }

                if (!*memstall && folio_test_workingset(folio)) {
                        psi_memstall_enter(pflags);
                        *memstall = 1;
                }

                ret = set_folio_extent_mapped(folio);
                if (ret < 0) {
                        folio_unlock(folio);
                        folio_put(folio);
                        break;
                }

                page_end = (pg_index << PAGE_SHIFT) + folio_size(folio) - 1;
                btrfs_lock_extent(tree, cur, page_end, NULL);
                read_lock(&em_tree->lock);
                em = btrfs_lookup_extent_mapping(em_tree, cur, page_end + 1 - cur);
                read_unlock(&em_tree->lock);

                /*
                 * At this point, we have a locked page in the page cache for
                 * these bytes in the file.  But, we have to make sure they map
                 * to this compressed extent on disk.
                 */
                if (!em || cur < em->start ||
                    (cur + fs_info->sectorsize > btrfs_extent_map_end(em)) ||
                    (btrfs_extent_map_block_start(em) >> SECTOR_SHIFT) !=
                    orig_bio->bi_iter.bi_sector) {
                        btrfs_free_extent_map(em);
                        btrfs_unlock_extent(tree, cur, page_end, NULL);
                        folio_unlock(folio);
                        folio_put(folio);
                        break;
                }
                add_size = min(btrfs_extent_map_end(em), page_end + 1) - cur;
                btrfs_free_extent_map(em);
                btrfs_unlock_extent(tree, cur, page_end, NULL);

                if (folio_contains(folio, end_index)) {
                        size_t zero_offset = offset_in_folio(folio, isize);

                        if (zero_offset) {
                                int zeros;
                                zeros = folio_size(folio) - zero_offset;
                                folio_zero_range(folio, zero_offset, zeros);
                        }
                }

                if (!bio_add_folio(orig_bio, folio, add_size,
                                   offset_in_folio(folio, cur))) {
                        folio_unlock(folio);
                        folio_put(folio);
                        break;
                }
                /*
                 * If it's subpage, we also need to increase its
                 * subpage::readers number, as at endio we will decrease
                 * subpage::readers and to unlock the page.
                 */
                if (fs_info->sectorsize < PAGE_SIZE)
                        btrfs_folio_set_lock(fs_info, folio, cur, add_size);
                folio_put(folio);
                cur += add_size;
        }
        return 0;
}

/*
 * for a compressed read, the bio we get passed has all the inode pages
 * in it.  We don't actually do IO on those pages but allocate new ones
 * to hold the compressed pages on disk.
 *
 * bio->bi_iter.bi_sector points to the compressed extent on disk
 * bio->bi_io_vec points to all of the inode pages
 *
 * After the compressed pages are read, we copy the bytes into the
 * bio we were passed and then call the bio end_io calls
 */
void btrfs_submit_compressed_read(struct btrfs_bio *bbio)
{
        struct btrfs_inode *inode = bbio->inode;
        struct btrfs_fs_info *fs_info = inode->root->fs_info;
        struct extent_map_tree *em_tree = &inode->extent_tree;
        struct compressed_bio *cb;
        unsigned int compressed_len;
        const u32 min_folio_size = btrfs_min_folio_size(fs_info);
        u64 file_offset = bbio->file_offset;
        u64 em_len;
        u64 em_start;
        struct extent_map *em;
        unsigned long pflags;
        int memstall = 0;
        int ret;

        /* we need the actual starting offset of this extent in the file */
        read_lock(&em_tree->lock);
        em = btrfs_lookup_extent_mapping(em_tree, file_offset, fs_info->sectorsize);
        read_unlock(&em_tree->lock);
        if (!em) {
                ret = -EIO;
                goto out;
        }

        ASSERT(btrfs_extent_map_is_compressed(em));
        compressed_len = em->disk_num_bytes;

        cb = alloc_compressed_bio(inode, file_offset, REQ_OP_READ,
                                  end_bbio_compressed_read);

        cb->start = em->start - em->offset;
        em_len = em->len;
        em_start = em->start;

        cb->len = bbio->bio.bi_iter.bi_size;
        cb->compressed_len = compressed_len;
        cb->compress_type = btrfs_extent_map_compression(em);
        cb->orig_bbio = bbio;
        cb->bbio.csum_search_commit_root = bbio->csum_search_commit_root;

        btrfs_free_extent_map(em);

        for (int i = 0; i * min_folio_size < compressed_len; i++) {
                struct folio *folio;
                u32 cur_len = min(compressed_len - i * min_folio_size, min_folio_size);

                folio = btrfs_alloc_compr_folio(fs_info);
                if (!folio) {
                        ret = -ENOMEM;
                        goto out_free_bio;
                }

                ret = bio_add_folio(&cb->bbio.bio, folio, cur_len, 0);
                if (unlikely(!ret)) {
                        folio_put(folio);
                        ret = -EINVAL;
                        goto out_free_bio;
                }
        }
        ASSERT(cb->bbio.bio.bi_iter.bi_size == compressed_len);

        add_ra_bio_pages(&inode->vfs_inode, em_start + em_len, cb, &memstall,
                         &pflags);

        cb->len = bbio->bio.bi_iter.bi_size;
        cb->bbio.bio.bi_iter.bi_sector = bbio->bio.bi_iter.bi_sector;

        if (memstall)
                psi_memstall_leave(&pflags);

        btrfs_submit_bbio(&cb->bbio, 0);
        return;

out_free_bio:
        cleanup_compressed_bio(cb);
out:
        btrfs_bio_end_io(bbio, errno_to_blk_status(ret));
}

/*
 * Heuristic uses systematic sampling to collect data from the input data
 * range, the logic can be tuned by the following constants:
 *
 * @SAMPLING_READ_SIZE - how many bytes will be copied from for each sample
 * @SAMPLING_INTERVAL  - range from which the sampled data can be collected
 */
#define SAMPLING_READ_SIZE      (16)
#define SAMPLING_INTERVAL       (256)

/*
 * For statistical analysis of the input data we consider bytes that form a
 * Galois Field of 256 objects. Each object has an attribute count, ie. how
 * many times the object appeared in the sample.
 */
#define BUCKET_SIZE             (256)

/*
 * The size of the sample is based on a statistical sampling rule of thumb.
 * The common way is to perform sampling tests as long as the number of
 * elements in each cell is at least 5.
 *
 * Instead of 5, we choose 32 to obtain more accurate results.
 * If the data contain the maximum number of symbols, which is 256, we obtain a
 * sample size bound by 8192.
 *
 * For a sample of at most 8KB of data per data range: 16 consecutive bytes
 * from up to 512 locations.
 */
#define MAX_SAMPLE_SIZE         (BTRFS_MAX_UNCOMPRESSED *               \
                                 SAMPLING_READ_SIZE / SAMPLING_INTERVAL)

struct bucket_item {
        u32 count;
};

struct heuristic_ws {
        /* Partial copy of input data */
        u8 *sample;
        u32 sample_size;
        /* Buckets store counters for each byte value */
        struct bucket_item *bucket;
        /* Sorting buffer */
        struct bucket_item *bucket_b;
        struct list_head list;
};

static void free_heuristic_ws(struct list_head *ws)
{
        struct heuristic_ws *workspace;

        workspace = list_entry(ws, struct heuristic_ws, list);

        kvfree(workspace->sample);
        kfree(workspace->bucket);
        kfree(workspace->bucket_b);
        kfree(workspace);
}

static struct list_head *alloc_heuristic_ws(struct btrfs_fs_info *fs_info)
{
        struct heuristic_ws *ws;

        ws = kzalloc_obj(*ws);
        if (!ws)
                return ERR_PTR(-ENOMEM);

        ws->sample = kvmalloc(MAX_SAMPLE_SIZE, GFP_KERNEL);
        if (!ws->sample)
                goto fail;

        ws->bucket = kzalloc_objs(*ws->bucket, BUCKET_SIZE);
        if (!ws->bucket)
                goto fail;

        ws->bucket_b = kzalloc_objs(*ws->bucket_b, BUCKET_SIZE);
        if (!ws->bucket_b)
                goto fail;

        INIT_LIST_HEAD(&ws->list);
        return &ws->list;
fail:
        free_heuristic_ws(&ws->list);
        return ERR_PTR(-ENOMEM);
}

const struct btrfs_compress_levels btrfs_heuristic_compress = { 0 };

static const struct btrfs_compress_levels * const btrfs_compress_levels[] = {
        /* The heuristic is represented as compression type 0 */
        &btrfs_heuristic_compress,
        &btrfs_zlib_compress,
        &btrfs_lzo_compress,
        &btrfs_zstd_compress,
};

static struct list_head *alloc_workspace(struct btrfs_fs_info *fs_info, int type, int level)
{
        switch (type) {
        case BTRFS_COMPRESS_NONE: return alloc_heuristic_ws(fs_info);
        case BTRFS_COMPRESS_ZLIB: return zlib_alloc_workspace(fs_info, level);
        case BTRFS_COMPRESS_LZO:  return lzo_alloc_workspace(fs_info);
        case BTRFS_COMPRESS_ZSTD: return zstd_alloc_workspace(fs_info, level);
        default:
                /*
                 * This can't happen, the type is validated several times
                 * before we get here.
                 */
                BUG();
        }
}

static void free_workspace(int type, struct list_head *ws)
{
        switch (type) {
        case BTRFS_COMPRESS_NONE: return free_heuristic_ws(ws);
        case BTRFS_COMPRESS_ZLIB: return zlib_free_workspace(ws);
        case BTRFS_COMPRESS_LZO:  return lzo_free_workspace(ws);
        case BTRFS_COMPRESS_ZSTD: return zstd_free_workspace(ws);
        default:
                /*
                 * This can't happen, the type is validated several times
                 * before we get here.
                 */
                BUG();
        }
}

static int alloc_workspace_manager(struct btrfs_fs_info *fs_info,
                                   enum btrfs_compression_type type)
{
        struct workspace_manager *gwsm;
        struct list_head *workspace;

        ASSERT(fs_info->compr_wsm[type] == NULL);
        gwsm = kzalloc_obj(*gwsm);
        if (!gwsm)
                return -ENOMEM;

        INIT_LIST_HEAD(&gwsm->idle_ws);
        spin_lock_init(&gwsm->ws_lock);
        atomic_set(&gwsm->total_ws, 0);
        init_waitqueue_head(&gwsm->ws_wait);
        fs_info->compr_wsm[type] = gwsm;

        /*
         * Preallocate one workspace for each compression type so we can
         * guarantee forward progress in the worst case
         */
        workspace = alloc_workspace(fs_info, type, 0);
        if (IS_ERR(workspace)) {
                btrfs_warn(fs_info,
        "cannot preallocate compression workspace for %s, will try later",
                           btrfs_compress_type2str(type));
        } else {
                atomic_set(&gwsm->total_ws, 1);
                gwsm->free_ws = 1;
                list_add(workspace, &gwsm->idle_ws);
        }
        return 0;
}

static void free_workspace_manager(struct btrfs_fs_info *fs_info,
                                   enum btrfs_compression_type type)
{
        struct list_head *ws;
        struct workspace_manager *gwsm = fs_info->compr_wsm[type];

        /* ZSTD uses its own workspace manager, should enter here. */
        ASSERT(type != BTRFS_COMPRESS_ZSTD && type < BTRFS_NR_COMPRESS_TYPES);
        if (!gwsm)
                return;
        fs_info->compr_wsm[type] = NULL;
        while (!list_empty(&gwsm->idle_ws)) {
                ws = gwsm->idle_ws.next;
                list_del(ws);
                free_workspace(type, ws);
                atomic_dec(&gwsm->total_ws);
        }
        kfree(gwsm);
}

/*
 * This finds an available workspace or allocates a new one.
 * If it's not possible to allocate a new one, waits until there's one.
 * Preallocation makes a forward progress guarantees and we do not return
 * errors.
 */
struct list_head *btrfs_get_workspace(struct btrfs_fs_info *fs_info, int type, int level)
{
        struct workspace_manager *wsm = fs_info->compr_wsm[type];
        struct list_head *workspace;
        int cpus = num_online_cpus();
        unsigned nofs_flag;
        struct list_head *idle_ws;
        spinlock_t *ws_lock;
        atomic_t *total_ws;
        wait_queue_head_t *ws_wait;
        int *free_ws;

        ASSERT(wsm);
        idle_ws  = &wsm->idle_ws;
        ws_lock  = &wsm->ws_lock;
        total_ws = &wsm->total_ws;
        ws_wait  = &wsm->ws_wait;
        free_ws  = &wsm->free_ws;

again:
        spin_lock(ws_lock);
        if (!list_empty(idle_ws)) {
                workspace = idle_ws->next;
                list_del(workspace);
                (*free_ws)--;
                spin_unlock(ws_lock);
                return workspace;

        }
        if (atomic_read(total_ws) > cpus) {
                DEFINE_WAIT(wait);

                spin_unlock(ws_lock);
                prepare_to_wait(ws_wait, &wait, TASK_UNINTERRUPTIBLE);
                if (atomic_read(total_ws) > cpus && !*free_ws)
                        schedule();
                finish_wait(ws_wait, &wait);
                goto again;
        }
        atomic_inc(total_ws);
        spin_unlock(ws_lock);

        /*
         * Allocation helpers call vmalloc that can't use GFP_NOFS, so we have
         * to turn it off here because we might get called from the restricted
         * context of btrfs_compress_bio/btrfs_compress_pages
         */
        nofs_flag = memalloc_nofs_save();
        workspace = alloc_workspace(fs_info, type, level);
        memalloc_nofs_restore(nofs_flag);

        if (IS_ERR(workspace)) {
                atomic_dec(total_ws);
                wake_up(ws_wait);

                /*
                 * Do not return the error but go back to waiting. There's a
                 * workspace preallocated for each type and the compression
                 * time is bounded so we get to a workspace eventually. This
                 * makes our caller's life easier.
                 *
                 * To prevent silent and low-probability deadlocks (when the
                 * initial preallocation fails), check if there are any
                 * workspaces at all.
                 */
                if (atomic_read(total_ws) == 0) {
                        static DEFINE_RATELIMIT_STATE(_rs,
                                        /* once per minute */ 60 * HZ,
                                        /* no burst */ 1);

                        if (__ratelimit(&_rs))
                                btrfs_warn(fs_info,
                                "no compression workspaces, low memory, retrying");
                }
                goto again;
        }
        return workspace;
}

static struct list_head *get_workspace(struct btrfs_fs_info *fs_info, int type, int level)
{
        switch (type) {
        case BTRFS_COMPRESS_NONE: return btrfs_get_workspace(fs_info, type, level);
        case BTRFS_COMPRESS_ZLIB: return zlib_get_workspace(fs_info, level);
        case BTRFS_COMPRESS_LZO:  return btrfs_get_workspace(fs_info, type, level);
        case BTRFS_COMPRESS_ZSTD: return zstd_get_workspace(fs_info, level);
        default:
                /*
                 * This can't happen, the type is validated several times
                 * before we get here.
                 */
                BUG();
        }
}

/*
 * put a workspace struct back on the list or free it if we have enough
 * idle ones sitting around
 */
void btrfs_put_workspace(struct btrfs_fs_info *fs_info, int type, struct list_head *ws)
{
        struct workspace_manager *gwsm = fs_info->compr_wsm[type];
        struct list_head *idle_ws;
        spinlock_t *ws_lock;
        atomic_t *total_ws;
        wait_queue_head_t *ws_wait;
        int *free_ws;

        ASSERT(gwsm);
        idle_ws  = &gwsm->idle_ws;
        ws_lock  = &gwsm->ws_lock;
        total_ws = &gwsm->total_ws;
        ws_wait  = &gwsm->ws_wait;
        free_ws  = &gwsm->free_ws;

        spin_lock(ws_lock);
        if (*free_ws <= num_online_cpus()) {
                list_add(ws, idle_ws);
                (*free_ws)++;
                spin_unlock(ws_lock);
                goto wake;
        }
        spin_unlock(ws_lock);

        free_workspace(type, ws);
        atomic_dec(total_ws);
wake:
        cond_wake_up(ws_wait);
}

static void put_workspace(struct btrfs_fs_info *fs_info, int type, struct list_head *ws)
{
        switch (type) {
        case BTRFS_COMPRESS_NONE: return btrfs_put_workspace(fs_info, type, ws);
        case BTRFS_COMPRESS_ZLIB: return btrfs_put_workspace(fs_info, type, ws);
        case BTRFS_COMPRESS_LZO:  return btrfs_put_workspace(fs_info, type, ws);
        case BTRFS_COMPRESS_ZSTD: return zstd_put_workspace(fs_info, ws);
        default:
                /*
                 * This can't happen, the type is validated several times
                 * before we get here.
                 */
                BUG();
        }
}

/*
 * Adjust @level according to the limits of the compression algorithm or
 * fallback to default
 */
static int btrfs_compress_set_level(unsigned int type, int level)
{
        const struct btrfs_compress_levels *levels = btrfs_compress_levels[type];

        if (level == 0)
                level = levels->default_level;
        else
                level = clamp(level, levels->min_level, levels->max_level);

        return level;
}

/*
 * Check whether the @level is within the valid range for the given type.
 */
bool btrfs_compress_level_valid(unsigned int type, int level)
{
        const struct btrfs_compress_levels *levels = btrfs_compress_levels[type];

        return levels->min_level <= level && level <= levels->max_level;
}

/* Wrapper around find_get_page(), with extra error message. */
int btrfs_compress_filemap_get_folio(struct address_space *mapping, u64 start,
                                     struct folio **in_folio_ret)
{
        struct folio *in_folio;

        /*
         * The compressed write path should have the folio locked already, thus
         * we only need to grab one reference.
         */
        in_folio = filemap_get_folio(mapping, start >> PAGE_SHIFT);
        if (IS_ERR(in_folio)) {
                struct btrfs_inode *inode = BTRFS_I(mapping->host);

                btrfs_crit(inode->root->fs_info,
                "failed to get page cache, root %lld ino %llu file offset %llu",
                           btrfs_root_id(inode->root), btrfs_ino(inode), start);
                return -ENOENT;
        }
        *in_folio_ret = in_folio;
        return 0;
}

/*
 * Given an address space and start and length, compress the page cache
 * contents into @cb.
 *
 * @type_level:      is encoded algorithm and level, where level 0 means whatever
 *                   default the algorithm chooses and is opaque here;
 *                   - compression algo are 0-3
 *                   - the level are bits 4-7
 *
 * @cb->bbio.bio.bi_iter.bi_size will indicate the compressed data size.
 * The bi_size may not be sectorsize aligned, thus the caller still need
 * to do the round up before submission.
 *
 * This function will allocate compressed folios with btrfs_alloc_compr_folio(),
 * thus callers must make sure the endio function and error handling are using
 * btrfs_free_compr_folio() to release those folios.
 * This is already done in end_bbio_compressed_write() and cleanup_compressed_bio().
 */
struct compressed_bio *btrfs_compress_bio(struct btrfs_inode *inode,
                                          u64 start, u32 len, unsigned int type,
                                          int level, blk_opf_t write_flags)
{
        struct btrfs_fs_info *fs_info = inode->root->fs_info;
        struct list_head *workspace;
        struct compressed_bio *cb;
        int ret;

        cb = alloc_compressed_bio(inode, start, REQ_OP_WRITE | write_flags,
                                  end_bbio_compressed_write);
        cb->start = start;
        cb->len = len;
        cb->writeback = true;
        cb->compress_type = type;

        level = btrfs_compress_set_level(type, level);
        workspace = get_workspace(fs_info, type, level);
        switch (type) {
        case BTRFS_COMPRESS_ZLIB:
                ret = zlib_compress_bio(workspace, cb);
                break;
        case BTRFS_COMPRESS_LZO:
                ret = lzo_compress_bio(workspace, cb);
                break;
        case BTRFS_COMPRESS_ZSTD:
                ret = zstd_compress_bio(workspace, cb);
                break;
        case BTRFS_COMPRESS_NONE:
        default:
                /*
                 * This can happen when compression races with remount setting
                 * it to 'no compress', while caller doesn't call
                 * inode_need_compress() to check if we really need to
                 * compress.
                 *
                 * Not a big deal, just need to inform caller that we
                 * haven't allocated any pages yet.
                 */
                ret = -E2BIG;
        }

        put_workspace(fs_info, type, workspace);
        if (ret < 0) {
                cleanup_compressed_bio(cb);
                return ERR_PTR(ret);
        }
        return cb;
}

static int btrfs_decompress_bio(struct compressed_bio *cb)
{
        struct btrfs_fs_info *fs_info = cb_to_fs_info(cb);
        struct list_head *workspace;
        int ret;
        int type = cb->compress_type;

        workspace = get_workspace(fs_info, type, 0);
        ret = compression_decompress_bio(workspace, cb);
        put_workspace(fs_info, type, workspace);

        if (!ret)
                zero_fill_bio(&cb->orig_bbio->bio);
        return ret;
}

/*
 * a less complex decompression routine.  Our compressed data fits in a
 * single page, and we want to read a single page out of it.
 * dest_pgoff tells us the offset into the destination folio where we write the
 * decompressed data.
 */
int btrfs_decompress(int type, const u8 *data_in, struct folio *dest_folio,
                     unsigned long dest_pgoff, size_t srclen, size_t destlen)
{
        struct btrfs_fs_info *fs_info = folio_to_fs_info(dest_folio);
        struct list_head *workspace;
        const u32 sectorsize = fs_info->sectorsize;
        int ret;

        /*
         * The full destination folio range should not exceed the folio size.
         * And the @destlen should not exceed sectorsize, as this is only called for
         * inline file extents, which should not exceed sectorsize.
         */
        ASSERT(dest_pgoff + destlen <= folio_size(dest_folio) && destlen <= sectorsize);

        workspace = get_workspace(fs_info, type, 0);
        ret = compression_decompress(type, workspace, data_in, dest_folio,
                                     dest_pgoff, srclen, destlen);
        put_workspace(fs_info, type, workspace);

        return ret;
}

int btrfs_alloc_compress_wsm(struct btrfs_fs_info *fs_info)
{
        int ret;

        ret = alloc_workspace_manager(fs_info, BTRFS_COMPRESS_NONE);
        if (ret < 0)
                goto error;
        ret = alloc_workspace_manager(fs_info, BTRFS_COMPRESS_ZLIB);
        if (ret < 0)
                goto error;
        ret = alloc_workspace_manager(fs_info, BTRFS_COMPRESS_LZO);
        if (ret < 0)
                goto error;
        ret = zstd_alloc_workspace_manager(fs_info);
        if (ret < 0)
                goto error;
        return 0;
error:
        btrfs_free_compress_wsm(fs_info);
        return ret;
}

void btrfs_free_compress_wsm(struct btrfs_fs_info *fs_info)
{
        free_workspace_manager(fs_info, BTRFS_COMPRESS_NONE);
        free_workspace_manager(fs_info, BTRFS_COMPRESS_ZLIB);
        free_workspace_manager(fs_info, BTRFS_COMPRESS_LZO);
        zstd_free_workspace_manager(fs_info);
}

int __init btrfs_init_compress(void)
{
        if (bioset_init(&btrfs_compressed_bioset, BIO_POOL_SIZE,
                        offsetof(struct compressed_bio, bbio.bio),
                        BIOSET_NEED_BVECS))
                return -ENOMEM;

        compr_pool.shrinker = shrinker_alloc(SHRINKER_NONSLAB, "btrfs-compr-pages");
        if (!compr_pool.shrinker)
                return -ENOMEM;

        spin_lock_init(&compr_pool.lock);
        INIT_LIST_HEAD(&compr_pool.list);
        compr_pool.count = 0;
        /* 128K / 4K = 32, for 8 threads is 256 pages. */
        compr_pool.thresh = BTRFS_MAX_COMPRESSED / PAGE_SIZE * 8;
        compr_pool.shrinker->count_objects = btrfs_compr_pool_count;
        compr_pool.shrinker->scan_objects = btrfs_compr_pool_scan;
        compr_pool.shrinker->batch = 32;
        compr_pool.shrinker->seeks = DEFAULT_SEEKS;
        shrinker_register(compr_pool.shrinker);

        return 0;
}

void __cold btrfs_exit_compress(void)
{
        /* For now scan drains all pages and does not touch the parameters. */
        btrfs_compr_pool_scan(NULL, NULL);
        shrinker_free(compr_pool.shrinker);

        bioset_exit(&btrfs_compressed_bioset);
}

/*
 * The bvec is a single page bvec from a bio that contains folios from a filemap.
 *
 * Since the folio may be a large one, and if the bv_page is not a head page of
 * a large folio, then page->index is unreliable.
 *
 * Thus we need this helper to grab the proper file offset.
 */
static u64 file_offset_from_bvec(const struct bio_vec *bvec)
{
        const struct page *page = bvec->bv_page;
        const struct folio *folio = page_folio(page);

        return (page_pgoff(folio, page) << PAGE_SHIFT) + bvec->bv_offset;
}

/*
 * Copy decompressed data from working buffer to pages.
 *
 * @buf:                The decompressed data buffer
 * @buf_len:            The decompressed data length
 * @decompressed:       Number of bytes that are already decompressed inside the
 *                      compressed extent
 * @cb:                 The compressed extent descriptor
 * @orig_bio:           The original bio that the caller wants to read for
 *
 * An easier to understand graph is like below:
 *
 *              |<- orig_bio ->|     |<- orig_bio->|
 *      |<-------      full decompressed extent      ----->|
 *      |<-----------    @cb range   ---->|
 *      |                       |<-- @buf_len -->|
 *      |<--- @decompressed --->|
 *
 * Note that, @cb can be a subpage of the full decompressed extent, but
 * @cb->start always has the same as the orig_file_offset value of the full
 * decompressed extent.
 *
 * When reading compressed extent, we have to read the full compressed extent,
 * while @orig_bio may only want part of the range.
 * Thus this function will ensure only data covered by @orig_bio will be copied
 * to.
 *
 * Return 0 if we have copied all needed contents for @orig_bio.
 * Return >0 if we need continue decompress.
 */
int btrfs_decompress_buf2page(const char *buf, u32 buf_len,
                              struct compressed_bio *cb, u32 decompressed)
{
        struct bio *orig_bio = &cb->orig_bbio->bio;
        /* Offset inside the full decompressed extent */
        u32 cur_offset;

        cur_offset = decompressed;
        /* The main loop to do the copy */
        while (cur_offset < decompressed + buf_len) {
                struct bio_vec bvec;
                size_t copy_len;
                u32 copy_start;
                /* Offset inside the full decompressed extent */
                u32 bvec_offset;
                void *kaddr;

                bvec = bio_iter_iovec(orig_bio, orig_bio->bi_iter);
                /*
                 * cb->start may underflow, but subtracting that value can still
                 * give us correct offset inside the full decompressed extent.
                 */
                bvec_offset = file_offset_from_bvec(&bvec) - cb->start;

                /* Haven't reached the bvec range, exit */
                if (decompressed + buf_len <= bvec_offset)
                        return 1;

                copy_start = max(cur_offset, bvec_offset);
                copy_len = min(bvec_offset + bvec.bv_len,
                               decompressed + buf_len) - copy_start;
                ASSERT(copy_len);

                /*
                 * Extra range check to ensure we didn't go beyond
                 * @buf + @buf_len.
                 */
                ASSERT(copy_start - decompressed < buf_len);

                kaddr = bvec_kmap_local(&bvec);
                memcpy(kaddr, buf + copy_start - decompressed, copy_len);
                kunmap_local(kaddr);

                cur_offset += copy_len;
                bio_advance(orig_bio, copy_len);
                /* Finished the bio */
                if (!orig_bio->bi_iter.bi_size)
                        return 0;
        }
        return 1;
}

/*
 * Shannon Entropy calculation
 *
 * Pure byte distribution analysis fails to determine compressibility of data.
 * Try calculating entropy to estimate the average minimum number of bits
 * needed to encode the sampled data.
 *
 * For convenience, return the percentage of needed bits, instead of amount of
 * bits directly.
 *
 * @ENTROPY_LVL_ACEPTABLE - below that threshold, sample has low byte entropy
 *                          and can be compressible with high probability
 *
 * @ENTROPY_LVL_HIGH - data are not compressible with high probability
 *
 * Use of ilog2() decreases precision, we lower the LVL to 5 to compensate.
 */
#define ENTROPY_LVL_ACEPTABLE           (65)
#define ENTROPY_LVL_HIGH                (80)

/*
 * For increased precision in shannon_entropy calculation,
 * let's do pow(n, M) to save more digits after comma:
 *
 * - maximum int bit length is 64
 * - ilog2(MAX_SAMPLE_SIZE)     -> 13
 * - 13 * 4 = 52 < 64           -> M = 4
 *
 * So use pow(n, 4).
 */
static inline u32 ilog2_w(u64 n)
{
        return ilog2(n * n * n * n);
}

static u32 shannon_entropy(struct heuristic_ws *ws)
{
        const u32 entropy_max = 8 * ilog2_w(2);
        u32 entropy_sum = 0;
        u32 p, p_base, sz_base;
        u32 i;

        sz_base = ilog2_w(ws->sample_size);
        for (i = 0; i < BUCKET_SIZE && ws->bucket[i].count > 0; i++) {
                p = ws->bucket[i].count;
                p_base = ilog2_w(p);
                entropy_sum += p * (sz_base - p_base);
        }

        entropy_sum /= ws->sample_size;
        return entropy_sum * 100 / entropy_max;
}

#define RADIX_BASE              4U
#define COUNTERS_SIZE           (1U << RADIX_BASE)

static u8 get4bits(u64 num, int shift) {
        u8 low4bits;

        num >>= shift;
        /* Reverse order */
        low4bits = (COUNTERS_SIZE - 1) - (num % COUNTERS_SIZE);
        return low4bits;
}

/*
 * Use 4 bits as radix base
 * Use 16 u32 counters for calculating new position in buf array
 *
 * @array     - array that will be sorted
 * @array_buf - buffer array to store sorting results
 *              must be equal in size to @array
 * @num       - array size
 */
static void radix_sort(struct bucket_item *array, struct bucket_item *array_buf,
                       int num)
{
        u64 max_num;
        u64 buf_num;
        u32 counters[COUNTERS_SIZE];
        u32 new_addr;
        u32 addr;
        int bitlen;
        int shift;
        int i;

        /*
         * Try avoid useless loop iterations for small numbers stored in big
         * counters.  Example: 48 33 4 ... in 64bit array
         */
        max_num = array[0].count;
        for (i = 1; i < num; i++) {
                buf_num = array[i].count;
                if (buf_num > max_num)
                        max_num = buf_num;
        }

        buf_num = ilog2(max_num);
        bitlen = ALIGN(buf_num, RADIX_BASE * 2);

        shift = 0;
        while (shift < bitlen) {
                memset(counters, 0, sizeof(counters));

                for (i = 0; i < num; i++) {
                        buf_num = array[i].count;
                        addr = get4bits(buf_num, shift);
                        counters[addr]++;
                }

                for (i = 1; i < COUNTERS_SIZE; i++)
                        counters[i] += counters[i - 1];

                for (i = num - 1; i >= 0; i--) {
                        buf_num = array[i].count;
                        addr = get4bits(buf_num, shift);
                        counters[addr]--;
                        new_addr = counters[addr];
                        array_buf[new_addr] = array[i];
                }

                shift += RADIX_BASE;

                /*
                 * Normal radix expects to move data from a temporary array, to
                 * the main one.  But that requires some CPU time. Avoid that
                 * by doing another sort iteration to original array instead of
                 * memcpy()
                 */
                memset(counters, 0, sizeof(counters));

                for (i = 0; i < num; i ++) {
                        buf_num = array_buf[i].count;
                        addr = get4bits(buf_num, shift);
                        counters[addr]++;
                }

                for (i = 1; i < COUNTERS_SIZE; i++)
                        counters[i] += counters[i - 1];

                for (i = num - 1; i >= 0; i--) {
                        buf_num = array_buf[i].count;
                        addr = get4bits(buf_num, shift);
                        counters[addr]--;
                        new_addr = counters[addr];
                        array[new_addr] = array_buf[i];
                }

                shift += RADIX_BASE;
        }
}

/*
 * Size of the core byte set - how many bytes cover 90% of the sample
 *
 * There are several types of structured binary data that use nearly all byte
 * values. The distribution can be uniform and counts in all buckets will be
 * nearly the same (eg. encrypted data). Unlikely to be compressible.
 *
 * Other possibility is normal (Gaussian) distribution, where the data could
 * be potentially compressible, but we have to take a few more steps to decide
 * how much.
 *
 * @BYTE_CORE_SET_LOW  - main part of byte values repeated frequently,
 *                       compression algo can easy fix that
 * @BYTE_CORE_SET_HIGH - data have uniform distribution and with high
 *                       probability is not compressible
 */
#define BYTE_CORE_SET_LOW               (64)
#define BYTE_CORE_SET_HIGH              (200)

static int byte_core_set_size(struct heuristic_ws *ws)
{
        u32 i;
        u32 coreset_sum = 0;
        const u32 core_set_threshold = ws->sample_size * 90 / 100;
        struct bucket_item *bucket = ws->bucket;

        /* Sort in reverse order */
        radix_sort(ws->bucket, ws->bucket_b, BUCKET_SIZE);

        for (i = 0; i < BYTE_CORE_SET_LOW; i++)
                coreset_sum += bucket[i].count;

        if (coreset_sum > core_set_threshold)
                return i;

        for (; i < BYTE_CORE_SET_HIGH && bucket[i].count > 0; i++) {
                coreset_sum += bucket[i].count;
                if (coreset_sum > core_set_threshold)
                        break;
        }

        return i;
}

/*
 * Count byte values in buckets.
 * This heuristic can detect textual data (configs, xml, json, html, etc).
 * Because in most text-like data byte set is restricted to limited number of
 * possible characters, and that restriction in most cases makes data easy to
 * compress.
 *
 * @BYTE_SET_THRESHOLD - consider all data within this byte set size:
 *      less - compressible
 *      more - need additional analysis
 */
#define BYTE_SET_THRESHOLD              (64)

static u32 byte_set_size(const struct heuristic_ws *ws)
{
        u32 i;
        u32 byte_set_size = 0;

        for (i = 0; i < BYTE_SET_THRESHOLD; i++) {
                if (ws->bucket[i].count > 0)
                        byte_set_size++;
        }

        /*
         * Continue collecting count of byte values in buckets.  If the byte
         * set size is bigger then the threshold, it's pointless to continue,
         * the detection technique would fail for this type of data.
         */
        for (; i < BUCKET_SIZE; i++) {
                if (ws->bucket[i].count > 0) {
                        byte_set_size++;
                        if (byte_set_size > BYTE_SET_THRESHOLD)
                                return byte_set_size;
                }
        }

        return byte_set_size;
}

static bool sample_repeated_patterns(struct heuristic_ws *ws)
{
        const u32 half_of_sample = ws->sample_size / 2;
        const u8 *data = ws->sample;

        return memcmp(&data[0], &data[half_of_sample], half_of_sample) == 0;
}

static void heuristic_collect_sample(struct inode *inode, u64 start, u64 end,
                                     struct heuristic_ws *ws)
{
        struct page *page;
        pgoff_t index, index_end;
        u32 i, curr_sample_pos;
        u8 *in_data;

        /*
         * Compression handles the input data by chunks of 128KiB
         * (defined by BTRFS_MAX_UNCOMPRESSED)
         *
         * We do the same for the heuristic and loop over the whole range.
         *
         * MAX_SAMPLE_SIZE - calculated under assumption that heuristic will
         * process no more than BTRFS_MAX_UNCOMPRESSED at a time.
         */
        if (end - start > BTRFS_MAX_UNCOMPRESSED)
                end = start + BTRFS_MAX_UNCOMPRESSED;

        index = start >> PAGE_SHIFT;
        index_end = end >> PAGE_SHIFT;

        /* Don't miss unaligned end */
        if (!PAGE_ALIGNED(end))
                index_end++;

        curr_sample_pos = 0;
        while (index < index_end) {
                page = find_get_page(inode->i_mapping, index);
                in_data = kmap_local_page(page);
                /* Handle case where the start is not aligned to PAGE_SIZE */
                i = start % PAGE_SIZE;
                while (i < PAGE_SIZE - SAMPLING_READ_SIZE) {
                        /* Don't sample any garbage from the last page */
                        if (start > end - SAMPLING_READ_SIZE)
                                break;
                        memcpy(&ws->sample[curr_sample_pos], &in_data[i],
                                        SAMPLING_READ_SIZE);
                        i += SAMPLING_INTERVAL;
                        start += SAMPLING_INTERVAL;
                        curr_sample_pos += SAMPLING_READ_SIZE;
                }
                kunmap_local(in_data);
                put_page(page);

                index++;
        }

        ws->sample_size = curr_sample_pos;
}

/*
 * Compression heuristic.
 *
 * The following types of analysis can be performed:
 * - detect mostly zero data
 * - detect data with low "byte set" size (text, etc)
 * - detect data with low/high "core byte" set
 *
 * Return non-zero if the compression should be done, 0 otherwise.
 */
int btrfs_compress_heuristic(struct btrfs_inode *inode, u64 start, u64 end)
{
        struct btrfs_fs_info *fs_info = inode->root->fs_info;
        struct list_head *ws_list = get_workspace(fs_info, 0, 0);
        struct heuristic_ws *ws;
        u32 i;
        u8 byte;
        int ret = 0;

        ws = list_entry(ws_list, struct heuristic_ws, list);

        heuristic_collect_sample(&inode->vfs_inode, start, end, ws);

        if (sample_repeated_patterns(ws)) {
                ret = 1;
                goto out;
        }

        memset(ws->bucket, 0, sizeof(*ws->bucket)*BUCKET_SIZE);

        for (i = 0; i < ws->sample_size; i++) {
                byte = ws->sample[i];
                ws->bucket[byte].count++;
        }

        i = byte_set_size(ws);
        if (i < BYTE_SET_THRESHOLD) {
                ret = 2;
                goto out;
        }

        i = byte_core_set_size(ws);
        if (i <= BYTE_CORE_SET_LOW) {
                ret = 3;
                goto out;
        }

        if (i >= BYTE_CORE_SET_HIGH) {
                ret = 0;
                goto out;
        }

        i = shannon_entropy(ws);
        if (i <= ENTROPY_LVL_ACEPTABLE) {
                ret = 4;
                goto out;
        }

        /*
         * For the levels below ENTROPY_LVL_HIGH, additional analysis would be
         * needed to give green light to compression.
         *
         * For now just assume that compression at that level is not worth the
         * resources because:
         *
         * 1. it is possible to defrag the data later
         *
         * 2. the data would turn out to be hardly compressible, eg. 150 byte
         * values, every bucket has counter at level ~54. The heuristic would
         * be confused. This can happen when data have some internal repeated
         * patterns like "abbacbbc...". This can be detected by analyzing
         * pairs of bytes, which is too costly.
         */
        if (i < ENTROPY_LVL_HIGH) {
                ret = 5;
                goto out;
        } else {
                ret = 0;
                goto out;
        }

out:
        put_workspace(fs_info, 0, ws_list);
        return ret;
}

/*
 * Convert the compression suffix (eg. after "zlib" starting with ":") to level.
 *
 * If the resulting level exceeds the algo's supported levels, it will be clamped.
 *
 * Return <0 if no valid string can be found.
 * Return 0 if everything is fine.
 */
int btrfs_compress_str2level(unsigned int type, const char *str, int *level_ret)
{
        int level = 0;
        int ret;

        if (!type) {
                *level_ret = btrfs_compress_set_level(type, level);
                return 0;
        }

        if (str[0] == ':') {
                ret = kstrtoint(str + 1, 10, &level);
                if (ret)
                        return ret;
        }

        *level_ret = btrfs_compress_set_level(type, level);
        return 0;
}