Linux 4.18.10

[linux/fpc-iii.git] / fs / btrfs / compression.c

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

#include <linux/kernel.h>
#include <linux/bio.h>
#include <linux/buffer_head.h>
#include <linux/file.h>
#include <linux/fs.h>
#include <linux/pagemap.h>
#include <linux/highmem.h>
#include <linux/time.h>
#include <linux/init.h>
#include <linux/string.h>
#include <linux/backing-dev.h>
#include <linux/mpage.h>
#include <linux/swap.h>
#include <linux/writeback.h>
#include <linux/bit_spinlock.h>
#include <linux/slab.h>
#include <linux/sched/mm.h>
#include <linux/log2.h>
#include "ctree.h"
#include "disk-io.h"
#include "transaction.h"
#include "btrfs_inode.h"
#include "volumes.h"
#include "ordered-data.h"
#include "compression.h"
#include "extent_io.h"
#include "extent_map.h"

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];
        }

        return NULL;
}

static int btrfs_decompress_bio(struct compressed_bio *cb);

static inline int compressed_bio_size(struct btrfs_fs_info *fs_info,
                                      unsigned long disk_size)
{
        u16 csum_size = btrfs_super_csum_size(fs_info->super_copy);

        return sizeof(struct compressed_bio) +
                (DIV_ROUND_UP(disk_size, fs_info->sectorsize)) * csum_size;
}

static int check_compressed_csum(struct btrfs_inode *inode,
                                 struct compressed_bio *cb,
                                 u64 disk_start)
{
        int ret;
        struct page *page;
        unsigned long i;
        char *kaddr;
        u32 csum;
        u32 *cb_sum = &cb->sums;

        if (inode->flags & BTRFS_INODE_NODATASUM)
                return 0;

        for (i = 0; i < cb->nr_pages; i++) {
                page = cb->compressed_pages[i];
                csum = ~(u32)0;

                kaddr = kmap_atomic(page);
                csum = btrfs_csum_data(kaddr, csum, PAGE_SIZE);
                btrfs_csum_final(csum, (u8 *)&csum);
                kunmap_atomic(kaddr);

                if (csum != *cb_sum) {
                        btrfs_print_data_csum_error(inode, disk_start, csum,
                                        *cb_sum, cb->mirror_num);
                        ret = -EIO;
                        goto fail;
                }
                cb_sum++;

        }
        ret = 0;
fail:
        return ret;
}

/* when we finish reading compressed pages from the disk, we
 * decompress them and then run the bio end_io routines on the
 * decompressed pages (in the inode address space).
 *
 * This allows the checksumming and other IO error handling routines
 * to work normally
 *
 * The compressed pages are freed here, and it must be run
 * in process context
 */
static void end_compressed_bio_read(struct bio *bio)
{
        struct compressed_bio *cb = bio->bi_private;
        struct inode *inode;
        struct page *page;
        unsigned long index;
        unsigned int mirror = btrfs_io_bio(bio)->mirror_num;
        int ret = 0;

        if (bio->bi_status)
                cb->errors = 1;

        /* if there are more bios still pending for this compressed
         * extent, just exit
         */
        if (!refcount_dec_and_test(&cb->pending_bios))
                goto out;

        /*
         * Record the correct mirror_num in cb->orig_bio so that
         * read-repair can work properly.
         */
        ASSERT(btrfs_io_bio(cb->orig_bio));
        btrfs_io_bio(cb->orig_bio)->mirror_num = mirror;
        cb->mirror_num = mirror;

        /*
         * Some IO in this cb have failed, just skip checksum as there
         * is no way it could be correct.
         */
        if (cb->errors == 1)
                goto csum_failed;

        inode = cb->inode;
        ret = check_compressed_csum(BTRFS_I(inode), cb,
                                    (u64)bio->bi_iter.bi_sector << 9);
        if (ret)
                goto csum_failed;

        /* ok, we're the last bio for this extent, lets start
         * the decompression.
         */
        ret = btrfs_decompress_bio(cb);

csum_failed:
        if (ret)
                cb->errors = 1;

        /* release the compressed pages */
        index = 0;
        for (index = 0; index < cb->nr_pages; index++) {
                page = cb->compressed_pages[index];
                page->mapping = NULL;
                put_page(page);
        }

        /* do io completion on the original bio */
        if (cb->errors) {
                bio_io_error(cb->orig_bio);
        } else {
                int i;
                struct bio_vec *bvec;

                /*
                 * we have verified the checksum already, set page
                 * checked so the end_io handlers know about it
                 */
                ASSERT(!bio_flagged(bio, BIO_CLONED));
                bio_for_each_segment_all(bvec, cb->orig_bio, i)
                        SetPageChecked(bvec->bv_page);

                bio_endio(cb->orig_bio);
        }

        /* finally free the cb struct */
        kfree(cb->compressed_pages);
        kfree(cb);
out:
        bio_put(bio);
}

/*
 * Clear the writeback bits on all of the file
 * pages for a compressed write
 */
static noinline void end_compressed_writeback(struct inode *inode,
                                              const struct compressed_bio *cb)
{
        unsigned long index = cb->start >> PAGE_SHIFT;
        unsigned long end_index = (cb->start + cb->len - 1) >> PAGE_SHIFT;
        struct page *pages[16];
        unsigned long nr_pages = end_index - index + 1;
        int i;
        int ret;

        if (cb->errors)
                mapping_set_error(inode->i_mapping, -EIO);

        while (nr_pages > 0) {
                ret = find_get_pages_contig(inode->i_mapping, index,
                                     min_t(unsigned long,
                                     nr_pages, ARRAY_SIZE(pages)), pages);
                if (ret == 0) {
                        nr_pages -= 1;
                        index += 1;
                        continue;
                }
                for (i = 0; i < ret; i++) {
                        if (cb->errors)
                                SetPageError(pages[i]);
                        end_page_writeback(pages[i]);
                        put_page(pages[i]);
                }
                nr_pages -= ret;
                index += ret;
        }
        /* 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_compressed_bio_write(struct bio *bio)
{
        struct extent_io_tree *tree;
        struct compressed_bio *cb = bio->bi_private;
        struct inode *inode;
        struct page *page;
        unsigned long index;

        if (bio->bi_status)
                cb->errors = 1;

        /* if there are more bios still pending for this compressed
         * extent, just exit
         */
        if (!refcount_dec_and_test(&cb->pending_bios))
                goto out;

        /* ok, we're the last bio for this extent, step one is to
         * call back into the FS and do all the end_io operations
         */
        inode = cb->inode;
        tree = &BTRFS_I(inode)->io_tree;
        cb->compressed_pages[0]->mapping = cb->inode->i_mapping;
        tree->ops->writepage_end_io_hook(cb->compressed_pages[0],
                                         cb->start,
                                         cb->start + cb->len - 1,
                                         NULL,
                                         bio->bi_status ?
                                         BLK_STS_OK : BLK_STS_NOTSUPP);
        cb->compressed_pages[0]->mapping = NULL;

        end_compressed_writeback(inode, cb);
        /* note, our inode could be gone now */

        /*
         * release the compressed pages, these came from alloc_page and
         * are not attached to the inode at all
         */
        index = 0;
        for (index = 0; index < cb->nr_pages; index++) {
                page = cb->compressed_pages[index];
                page->mapping = NULL;
                put_page(page);
        }

        /* finally free the cb struct */
        kfree(cb->compressed_pages);
        kfree(cb);
out:
        bio_put(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.
 */
blk_status_t btrfs_submit_compressed_write(struct inode *inode, u64 start,
                                 unsigned long len, u64 disk_start,
                                 unsigned long compressed_len,
                                 struct page **compressed_pages,
                                 unsigned long nr_pages,
                                 unsigned int write_flags)
{
        struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
        struct bio *bio = NULL;
        struct compressed_bio *cb;
        unsigned long bytes_left;
        struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
        int pg_index = 0;
        struct page *page;
        u64 first_byte = disk_start;
        struct block_device *bdev;
        blk_status_t ret;
        int skip_sum = BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM;

        WARN_ON(start & ((u64)PAGE_SIZE - 1));
        cb = kmalloc(compressed_bio_size(fs_info, compressed_len), GFP_NOFS);
        if (!cb)
                return BLK_STS_RESOURCE;
        refcount_set(&cb->pending_bios, 0);
        cb->errors = 0;
        cb->inode = inode;
        cb->start = start;
        cb->len = len;
        cb->mirror_num = 0;
        cb->compressed_pages = compressed_pages;
        cb->compressed_len = compressed_len;
        cb->orig_bio = NULL;
        cb->nr_pages = nr_pages;

        bdev = fs_info->fs_devices->latest_bdev;

        bio = btrfs_bio_alloc(bdev, first_byte);
        bio->bi_opf = REQ_OP_WRITE | write_flags;
        bio->bi_private = cb;
        bio->bi_end_io = end_compressed_bio_write;
        refcount_set(&cb->pending_bios, 1);

        /* create and submit bios for the compressed pages */
        bytes_left = compressed_len;
        for (pg_index = 0; pg_index < cb->nr_pages; pg_index++) {
                int submit = 0;

                page = compressed_pages[pg_index];
                page->mapping = inode->i_mapping;
                if (bio->bi_iter.bi_size)
                        submit = io_tree->ops->merge_bio_hook(page, 0,
                                                           PAGE_SIZE,
                                                           bio, 0);

                page->mapping = NULL;
                if (submit || bio_add_page(bio, page, PAGE_SIZE, 0) <
                    PAGE_SIZE) {
                        /*
                         * inc the count before we submit the bio so
                         * we know the end IO handler won't happen before
                         * we inc the count.  Otherwise, the cb might get
                         * freed before we're done setting it up
                         */
                        refcount_inc(&cb->pending_bios);
                        ret = btrfs_bio_wq_end_io(fs_info, bio,
                                                  BTRFS_WQ_ENDIO_DATA);
                        BUG_ON(ret); /* -ENOMEM */

                        if (!skip_sum) {
                                ret = btrfs_csum_one_bio(inode, bio, start, 1);
                                BUG_ON(ret); /* -ENOMEM */
                        }

                        ret = btrfs_map_bio(fs_info, bio, 0, 1);
                        if (ret) {
                                bio->bi_status = ret;
                                bio_endio(bio);
                        }

                        bio = btrfs_bio_alloc(bdev, first_byte);
                        bio->bi_opf = REQ_OP_WRITE | write_flags;
                        bio->bi_private = cb;
                        bio->bi_end_io = end_compressed_bio_write;
                        bio_add_page(bio, page, PAGE_SIZE, 0);
                }
                if (bytes_left < PAGE_SIZE) {
                        btrfs_info(fs_info,
                                        "bytes left %lu compress len %lu nr %lu",
                               bytes_left, cb->compressed_len, cb->nr_pages);
                }
                bytes_left -= PAGE_SIZE;
                first_byte += PAGE_SIZE;
                cond_resched();
        }

        ret = btrfs_bio_wq_end_io(fs_info, bio, BTRFS_WQ_ENDIO_DATA);
        BUG_ON(ret); /* -ENOMEM */

        if (!skip_sum) {
                ret = btrfs_csum_one_bio(inode, bio, start, 1);
                BUG_ON(ret); /* -ENOMEM */
        }

        ret = btrfs_map_bio(fs_info, bio, 0, 1);
        if (ret) {
                bio->bi_status = ret;
                bio_endio(bio);
        }

        return 0;
}

static u64 bio_end_offset(struct bio *bio)
{
        struct bio_vec *last = bio_last_bvec_all(bio);

        return page_offset(last->bv_page) + last->bv_len + last->bv_offset;
}

static noinline int add_ra_bio_pages(struct inode *inode,
                                     u64 compressed_end,
                                     struct compressed_bio *cb)
{
        unsigned long end_index;
        unsigned long pg_index;
        u64 last_offset;
        u64 isize = i_size_read(inode);
        int ret;
        struct page *page;
        unsigned long nr_pages = 0;
        struct extent_map *em;
        struct address_space *mapping = inode->i_mapping;
        struct extent_map_tree *em_tree;
        struct extent_io_tree *tree;
        u64 end;
        int misses = 0;

        last_offset = bio_end_offset(cb->orig_bio);
        em_tree = &BTRFS_I(inode)->extent_tree;
        tree = &BTRFS_I(inode)->io_tree;

        if (isize == 0)
                return 0;

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

        while (last_offset < compressed_end) {
                pg_index = last_offset >> PAGE_SHIFT;

                if (pg_index > end_index)
                        break;

                rcu_read_lock();
                page = radix_tree_lookup(&mapping->i_pages, pg_index);
                rcu_read_unlock();
                if (page && !radix_tree_exceptional_entry(page)) {
                        misses++;
                        if (misses > 4)
                                break;
                        goto next;
                }

                page = __page_cache_alloc(mapping_gfp_constraint(mapping,
                                                                 ~__GFP_FS));
                if (!page)
                        break;

                if (add_to_page_cache_lru(page, mapping, pg_index, GFP_NOFS)) {
                        put_page(page);
                        goto next;
                }

                end = last_offset + PAGE_SIZE - 1;
                /*
                 * 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.
                 */
                set_page_extent_mapped(page);
                lock_extent(tree, last_offset, end);
                read_lock(&em_tree->lock);
                em = lookup_extent_mapping(em_tree, last_offset,
                                           PAGE_SIZE);
                read_unlock(&em_tree->lock);

                if (!em || last_offset < em->start ||
                    (last_offset + PAGE_SIZE > extent_map_end(em)) ||
                    (em->block_start >> 9) != cb->orig_bio->bi_iter.bi_sector) {
                        free_extent_map(em);
                        unlock_extent(tree, last_offset, end);
                        unlock_page(page);
                        put_page(page);
                        break;
                }
                free_extent_map(em);

                if (page->index == end_index) {
                        char *userpage;
                        size_t zero_offset = isize & (PAGE_SIZE - 1);

                        if (zero_offset) {
                                int zeros;
                                zeros = PAGE_SIZE - zero_offset;
                                userpage = kmap_atomic(page);
                                memset(userpage + zero_offset, 0, zeros);
                                flush_dcache_page(page);
                                kunmap_atomic(userpage);
                        }
                }

                ret = bio_add_page(cb->orig_bio, page,
                                   PAGE_SIZE, 0);

                if (ret == PAGE_SIZE) {
                        nr_pages++;
                        put_page(page);
                } else {
                        unlock_extent(tree, last_offset, end);
                        unlock_page(page);
                        put_page(page);
                        break;
                }
next:
                last_offset += PAGE_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
 */
blk_status_t btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
                                 int mirror_num, unsigned long bio_flags)
{
        struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
        struct extent_io_tree *tree;
        struct extent_map_tree *em_tree;
        struct compressed_bio *cb;
        unsigned long compressed_len;
        unsigned long nr_pages;
        unsigned long pg_index;
        struct page *page;
        struct block_device *bdev;
        struct bio *comp_bio;
        u64 cur_disk_byte = (u64)bio->bi_iter.bi_sector << 9;
        u64 em_len;
        u64 em_start;
        struct extent_map *em;
        blk_status_t ret = BLK_STS_RESOURCE;
        int faili = 0;
        u32 *sums;

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

        /* we need the actual starting offset of this extent in the file */
        read_lock(&em_tree->lock);
        em = lookup_extent_mapping(em_tree,
                                   page_offset(bio_first_page_all(bio)),
                                   PAGE_SIZE);
        read_unlock(&em_tree->lock);
        if (!em)
                return BLK_STS_IOERR;

        compressed_len = em->block_len;
        cb = kmalloc(compressed_bio_size(fs_info, compressed_len), GFP_NOFS);
        if (!cb)
                goto out;

        refcount_set(&cb->pending_bios, 0);
        cb->errors = 0;
        cb->inode = inode;
        cb->mirror_num = mirror_num;
        sums = &cb->sums;

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

        free_extent_map(em);
        em = NULL;

        cb->len = bio->bi_iter.bi_size;
        cb->compressed_len = compressed_len;
        cb->compress_type = extent_compress_type(bio_flags);
        cb->orig_bio = bio;

        nr_pages = DIV_ROUND_UP(compressed_len, PAGE_SIZE);
        cb->compressed_pages = kcalloc(nr_pages, sizeof(struct page *),
                                       GFP_NOFS);
        if (!cb->compressed_pages)
                goto fail1;

        bdev = fs_info->fs_devices->latest_bdev;

        for (pg_index = 0; pg_index < nr_pages; pg_index++) {
                cb->compressed_pages[pg_index] = alloc_page(GFP_NOFS |
                                                              __GFP_HIGHMEM);
                if (!cb->compressed_pages[pg_index]) {
                        faili = pg_index - 1;
                        ret = BLK_STS_RESOURCE;
                        goto fail2;
                }
        }
        faili = nr_pages - 1;
        cb->nr_pages = nr_pages;

        add_ra_bio_pages(inode, em_start + em_len, cb);

        /* include any pages we added in add_ra-bio_pages */
        cb->len = bio->bi_iter.bi_size;

        comp_bio = btrfs_bio_alloc(bdev, cur_disk_byte);
        bio_set_op_attrs (comp_bio, REQ_OP_READ, 0);
        comp_bio->bi_private = cb;
        comp_bio->bi_end_io = end_compressed_bio_read;
        refcount_set(&cb->pending_bios, 1);

        for (pg_index = 0; pg_index < nr_pages; pg_index++) {
                int submit = 0;

                page = cb->compressed_pages[pg_index];
                page->mapping = inode->i_mapping;
                page->index = em_start >> PAGE_SHIFT;

                if (comp_bio->bi_iter.bi_size)
                        submit = tree->ops->merge_bio_hook(page, 0,
                                                        PAGE_SIZE,
                                                        comp_bio, 0);

                page->mapping = NULL;
                if (submit || bio_add_page(comp_bio, page, PAGE_SIZE, 0) <
                    PAGE_SIZE) {
                        ret = btrfs_bio_wq_end_io(fs_info, comp_bio,
                                                  BTRFS_WQ_ENDIO_DATA);
                        BUG_ON(ret); /* -ENOMEM */

                        /*
                         * inc the count before we submit the bio so
                         * we know the end IO handler won't happen before
                         * we inc the count.  Otherwise, the cb might get
                         * freed before we're done setting it up
                         */
                        refcount_inc(&cb->pending_bios);

                        if (!(BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)) {
                                ret = btrfs_lookup_bio_sums(inode, comp_bio,
                                                            sums);
                                BUG_ON(ret); /* -ENOMEM */
                        }
                        sums += DIV_ROUND_UP(comp_bio->bi_iter.bi_size,
                                             fs_info->sectorsize);

                        ret = btrfs_map_bio(fs_info, comp_bio, mirror_num, 0);
                        if (ret) {
                                comp_bio->bi_status = ret;
                                bio_endio(comp_bio);
                        }

                        comp_bio = btrfs_bio_alloc(bdev, cur_disk_byte);
                        bio_set_op_attrs(comp_bio, REQ_OP_READ, 0);
                        comp_bio->bi_private = cb;
                        comp_bio->bi_end_io = end_compressed_bio_read;

                        bio_add_page(comp_bio, page, PAGE_SIZE, 0);
                }
                cur_disk_byte += PAGE_SIZE;
        }

        ret = btrfs_bio_wq_end_io(fs_info, comp_bio, BTRFS_WQ_ENDIO_DATA);
        BUG_ON(ret); /* -ENOMEM */

        if (!(BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)) {
                ret = btrfs_lookup_bio_sums(inode, comp_bio, sums);
                BUG_ON(ret); /* -ENOMEM */
        }

        ret = btrfs_map_bio(fs_info, comp_bio, mirror_num, 0);
        if (ret) {
                comp_bio->bi_status = ret;
                bio_endio(comp_bio);
        }

        return 0;

fail2:
        while (faili >= 0) {
                __free_page(cb->compressed_pages[faili]);
                faili--;
        }

        kfree(cb->compressed_pages);
fail1:
        kfree(cb);
out:
        free_extent_map(em);
        return 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(void)
{
        struct heuristic_ws *ws;

        ws = kzalloc(sizeof(*ws), GFP_KERNEL);
        if (!ws)
                return ERR_PTR(-ENOMEM);

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

        ws->bucket = kcalloc(BUCKET_SIZE, sizeof(*ws->bucket), GFP_KERNEL);
        if (!ws->bucket)
                goto fail;

        ws->bucket_b = kcalloc(BUCKET_SIZE, sizeof(*ws->bucket_b), GFP_KERNEL);
        if (!ws->bucket_b)
                goto fail;

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

struct workspaces_list {
        struct list_head idle_ws;
        spinlock_t ws_lock;
        /* Number of free workspaces */
        int free_ws;
        /* Total number of allocated workspaces */
        atomic_t total_ws;
        /* Waiters for a free workspace */
        wait_queue_head_t ws_wait;
};

static struct workspaces_list btrfs_comp_ws[BTRFS_COMPRESS_TYPES];

static struct workspaces_list btrfs_heuristic_ws;

static const struct btrfs_compress_op * const btrfs_compress_op[] = {
        &btrfs_zlib_compress,
        &btrfs_lzo_compress,
        &btrfs_zstd_compress,
};

void __init btrfs_init_compress(void)
{
        struct list_head *workspace;
        int i;

        INIT_LIST_HEAD(&btrfs_heuristic_ws.idle_ws);
        spin_lock_init(&btrfs_heuristic_ws.ws_lock);
        atomic_set(&btrfs_heuristic_ws.total_ws, 0);
        init_waitqueue_head(&btrfs_heuristic_ws.ws_wait);

        workspace = alloc_heuristic_ws();
        if (IS_ERR(workspace)) {
                pr_warn(
        "BTRFS: cannot preallocate heuristic workspace, will try later\n");
        } else {
                atomic_set(&btrfs_heuristic_ws.total_ws, 1);
                btrfs_heuristic_ws.free_ws = 1;
                list_add(workspace, &btrfs_heuristic_ws.idle_ws);
        }

        for (i = 0; i < BTRFS_COMPRESS_TYPES; i++) {
                INIT_LIST_HEAD(&btrfs_comp_ws[i].idle_ws);
                spin_lock_init(&btrfs_comp_ws[i].ws_lock);
                atomic_set(&btrfs_comp_ws[i].total_ws, 0);
                init_waitqueue_head(&btrfs_comp_ws[i].ws_wait);

                /*
                 * Preallocate one workspace for each compression type so
                 * we can guarantee forward progress in the worst case
                 */
                workspace = btrfs_compress_op[i]->alloc_workspace();
                if (IS_ERR(workspace)) {
                        pr_warn("BTRFS: cannot preallocate compression workspace, will try later\n");
                } else {
                        atomic_set(&btrfs_comp_ws[i].total_ws, 1);
                        btrfs_comp_ws[i].free_ws = 1;
                        list_add(workspace, &btrfs_comp_ws[i].idle_ws);
                }
        }
}

/*
 * 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.
 */
static struct list_head *__find_workspace(int type, bool heuristic)
{
        struct list_head *workspace;
        int cpus = num_online_cpus();
        int idx = type - 1;
        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;

        if (heuristic) {
                idle_ws  = &btrfs_heuristic_ws.idle_ws;
                ws_lock  = &btrfs_heuristic_ws.ws_lock;
                total_ws = &btrfs_heuristic_ws.total_ws;
                ws_wait  = &btrfs_heuristic_ws.ws_wait;
                free_ws  = &btrfs_heuristic_ws.free_ws;
        } else {
                idle_ws  = &btrfs_comp_ws[idx].idle_ws;
                ws_lock  = &btrfs_comp_ws[idx].ws_lock;
                total_ws = &btrfs_comp_ws[idx].total_ws;
                ws_wait  = &btrfs_comp_ws[idx].ws_wait;
                free_ws  = &btrfs_comp_ws[idx].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();
        if (heuristic)
                workspace = alloc_heuristic_ws();
        else
                workspace = btrfs_compress_op[idx]->alloc_workspace();
        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)) {
                                pr_warn("BTRFS: no compression workspaces, low memory, retrying\n");
                        }
                }
                goto again;
        }
        return workspace;
}

static struct list_head *find_workspace(int type)
{
        return __find_workspace(type, false);
}

/*
 * put a workspace struct back on the list or free it if we have enough
 * idle ones sitting around
 */
static void __free_workspace(int type, struct list_head *workspace,
                             bool heuristic)
{
        int idx = type - 1;
        struct list_head *idle_ws;
        spinlock_t *ws_lock;
        atomic_t *total_ws;
        wait_queue_head_t *ws_wait;
        int *free_ws;

        if (heuristic) {
                idle_ws  = &btrfs_heuristic_ws.idle_ws;
                ws_lock  = &btrfs_heuristic_ws.ws_lock;
                total_ws = &btrfs_heuristic_ws.total_ws;
                ws_wait  = &btrfs_heuristic_ws.ws_wait;
                free_ws  = &btrfs_heuristic_ws.free_ws;
        } else {
                idle_ws  = &btrfs_comp_ws[idx].idle_ws;
                ws_lock  = &btrfs_comp_ws[idx].ws_lock;
                total_ws = &btrfs_comp_ws[idx].total_ws;
                ws_wait  = &btrfs_comp_ws[idx].ws_wait;
                free_ws  = &btrfs_comp_ws[idx].free_ws;
        }

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

        if (heuristic)
                free_heuristic_ws(workspace);
        else
                btrfs_compress_op[idx]->free_workspace(workspace);
        atomic_dec(total_ws);
wake:
        cond_wake_up(ws_wait);
}

static void free_workspace(int type, struct list_head *ws)
{
        return __free_workspace(type, ws, false);
}

/*
 * cleanup function for module exit
 */
static void free_workspaces(void)
{
        struct list_head *workspace;
        int i;

        while (!list_empty(&btrfs_heuristic_ws.idle_ws)) {
                workspace = btrfs_heuristic_ws.idle_ws.next;
                list_del(workspace);
                free_heuristic_ws(workspace);
                atomic_dec(&btrfs_heuristic_ws.total_ws);
        }

        for (i = 0; i < BTRFS_COMPRESS_TYPES; i++) {
                while (!list_empty(&btrfs_comp_ws[i].idle_ws)) {
                        workspace = btrfs_comp_ws[i].idle_ws.next;
                        list_del(workspace);
                        btrfs_compress_op[i]->free_workspace(workspace);
                        atomic_dec(&btrfs_comp_ws[i].total_ws);
                }
        }
}

/*
 * Given an address space and start and length, compress the bytes into @pages
 * that are allocated on demand.
 *
 * @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
 *
 * @out_pages is an in/out parameter, holds maximum number of pages to allocate
 * and returns number of actually allocated pages
 *
 * @total_in is used to return the number of bytes actually read.  It
 * may be smaller than the input length if we had to exit early because we
 * ran out of room in the pages array or because we cross the
 * max_out threshold.
 *
 * @total_out is an in/out parameter, must be set to the input length and will
 * be also used to return the total number of compressed bytes
 *
 * @max_out tells us the max number of bytes that we're allowed to
 * stuff into pages
 */
int btrfs_compress_pages(unsigned int type_level, struct address_space *mapping,
                         u64 start, struct page **pages,
                         unsigned long *out_pages,
                         unsigned long *total_in,
                         unsigned long *total_out)
{
        struct list_head *workspace;
        int ret;
        int type = type_level & 0xF;

        workspace = find_workspace(type);

        btrfs_compress_op[type - 1]->set_level(workspace, type_level);
        ret = btrfs_compress_op[type-1]->compress_pages(workspace, mapping,
                                                      start, pages,
                                                      out_pages,
                                                      total_in, total_out);
        free_workspace(type, workspace);
        return ret;
}

/*
 * pages_in is an array of pages with compressed data.
 *
 * disk_start is the starting logical offset of this array in the file
 *
 * orig_bio contains the pages from the file that we want to decompress into
 *
 * srclen is the number of bytes in pages_in
 *
 * The basic idea is that we have a bio that was created by readpages.
 * The pages in the bio are for the uncompressed data, and they may not
 * be contiguous.  They all correspond to the range of bytes covered by
 * the compressed extent.
 */
static int btrfs_decompress_bio(struct compressed_bio *cb)
{
        struct list_head *workspace;
        int ret;
        int type = cb->compress_type;

        workspace = find_workspace(type);
        ret = btrfs_compress_op[type - 1]->decompress_bio(workspace, cb);
        free_workspace(type, workspace);

        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.
 * start_byte tells us the offset into the compressed data we're interested in
 */
int btrfs_decompress(int type, unsigned char *data_in, struct page *dest_page,
                     unsigned long start_byte, size_t srclen, size_t destlen)
{
        struct list_head *workspace;
        int ret;

        workspace = find_workspace(type);

        ret = btrfs_compress_op[type-1]->decompress(workspace, data_in,
                                                  dest_page, start_byte,
                                                  srclen, destlen);

        free_workspace(type, workspace);
        return ret;
}

void __cold btrfs_exit_compress(void)
{
        free_workspaces();
}

/*
 * Copy uncompressed data from working buffer to pages.
 *
 * buf_start is the byte offset we're of the start of our workspace buffer.
 *
 * total_out is the last byte of the buffer
 */
int btrfs_decompress_buf2page(const char *buf, unsigned long buf_start,
                              unsigned long total_out, u64 disk_start,
                              struct bio *bio)
{
        unsigned long buf_offset;
        unsigned long current_buf_start;
        unsigned long start_byte;
        unsigned long prev_start_byte;
        unsigned long working_bytes = total_out - buf_start;
        unsigned long bytes;
        char *kaddr;
        struct bio_vec bvec = bio_iter_iovec(bio, bio->bi_iter);

        /*
         * start byte is the first byte of the page we're currently
         * copying into relative to the start of the compressed data.
         */
        start_byte = page_offset(bvec.bv_page) - disk_start;

        /* we haven't yet hit data corresponding to this page */
        if (total_out <= start_byte)
                return 1;

        /*
         * the start of the data we care about is offset into
         * the middle of our working buffer
         */
        if (total_out > start_byte && buf_start < start_byte) {
                buf_offset = start_byte - buf_start;
                working_bytes -= buf_offset;
        } else {
                buf_offset = 0;
        }
        current_buf_start = buf_start;

        /* copy bytes from the working buffer into the pages */
        while (working_bytes > 0) {
                bytes = min_t(unsigned long, bvec.bv_len,
                                PAGE_SIZE - buf_offset);
                bytes = min(bytes, working_bytes);

                kaddr = kmap_atomic(bvec.bv_page);
                memcpy(kaddr + bvec.bv_offset, buf + buf_offset, bytes);
                kunmap_atomic(kaddr);
                flush_dcache_page(bvec.bv_page);

                buf_offset += bytes;
                working_bytes -= bytes;
                current_buf_start += bytes;

                /* check if we need to pick another page */
                bio_advance(bio, bytes);
                if (!bio->bi_iter.bi_size)
                        return 0;
                bvec = bio_iter_iovec(bio, bio->bi_iter);
                prev_start_byte = start_byte;
                start_byte = page_offset(bvec.bv_page) - disk_start;

                /*
                 * We need to make sure we're only adjusting
                 * our offset into compression working buffer when
                 * we're switching pages.  Otherwise we can incorrectly
                 * keep copying when we were actually done.
                 */
                if (start_byte != prev_start_byte) {
                        /*
                         * make sure our new page is covered by this
                         * working buffer
                         */
                        if (total_out <= start_byte)
                                return 1;

                        /*
                         * the next page in the biovec might not be adjacent
                         * to the last page, but it might still be found
                         * inside this working buffer. bump our offset pointer
                         */
                        if (total_out > start_byte &&
                            current_buf_start < start_byte) {
                                buf_offset = start_byte - buf_start;
                                working_bytes = total_out - start_byte;
                                current_buf_start = buf_start + buf_offset;
                        }
                }
        }

        return 1;
}

/*
 * Shannon Entropy calculation
 *
 * Pure byte distribution analysis fails to determine compressiability 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 increasead 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 possition 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;
        u64 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 (!IS_ALIGNED(end, PAGE_SIZE))
                index_end++;

        curr_sample_pos = 0;
        while (index < index_end) {
                page = find_get_page(inode->i_mapping, index);
                in_data = kmap(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(page);
                put_page(page);

                index++;
        }

        ws->sample_size = curr_sample_pos;
}

/*
 * Compression heuristic.
 *
 * For now is's a naive and optimistic 'return true', we'll extend the logic to
 * quickly (compared to direct compression) detect data characteristics
 * (compressible/uncompressible) to avoid wasting CPU time on uncompressible
 * data.
 *
 * 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 inode *inode, u64 start, u64 end)
{
        struct list_head *ws_list = __find_workspace(0, true);
        struct heuristic_ws *ws;
        u32 i;
        u8 byte;
        int ret = 0;

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

        heuristic_collect_sample(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:
        __free_workspace(0, ws_list, true);
        return ret;
}

unsigned int btrfs_compress_str2level(const char *str)
{
        if (strncmp(str, "zlib", 4) != 0)
                return 0;

        /* Accepted form: zlib:1 up to zlib:9 and nothing left after the number */
        if (str[4] == ':' && '1' <= str[5] && str[5] <= '9' && str[6] == 0)
                return str[5] - '0';

        return BTRFS_ZLIB_DEFAULT_LEVEL;
}