// 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/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/slab.h>
#include <linux/sched/mm.h>
#include <linux/log2.h>
#include <crypto/hash.h>
#include "misc.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"
#include "subpage.h"
#include "zoned.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];
        default:
                break;
        }

        return NULL;
}

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_compress_pages(int type, struct list_head *ws,
               struct address_space *mapping, u64 start, struct page **pages,
               unsigned long *out_pages, unsigned long *total_in,
               unsigned long *total_out)
{
        switch (type) {
        case BTRFS_COMPRESS_ZLIB:
                return zlib_compress_pages(ws, mapping, start, pages,
                                out_pages, total_in, total_out);
        case BTRFS_COMPRESS_LZO:
                return lzo_compress_pages(ws, mapping, start, pages,
                                out_pages, total_in, total_out);
        case BTRFS_COMPRESS_ZSTD:
                return zstd_compress_pages(ws, mapping, start, pages,
                                out_pages, total_in, total_out);
        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.
                 */
                *out_pages = 0;
                return -E2BIG;
        }
}

static int compression_decompress_bio(int type, struct list_head *ws,
                struct compressed_bio *cb)
{
        switch (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,
               unsigned char *data_in, struct page *dest_page,
               unsigned long start_byte, size_t srclen, size_t destlen)
{
        switch (type) {
        case BTRFS_COMPRESS_ZLIB: return zlib_decompress(ws, data_in, dest_page,
                                                start_byte, srclen, destlen);
        case BTRFS_COMPRESS_LZO:  return lzo_decompress(ws, data_in, dest_page,
                                                start_byte, srclen, destlen);
        case BTRFS_COMPRESS_ZSTD: return zstd_decompress(ws, data_in, dest_page,
                                                start_byte, 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);

static inline int compressed_bio_size(struct btrfs_fs_info *fs_info,
                                      unsigned long disk_size)
{
        return sizeof(struct compressed_bio) +
                (DIV_ROUND_UP(disk_size, fs_info->sectorsize)) * fs_info->csum_size;
}

static int check_compressed_csum(struct btrfs_inode *inode, struct bio *bio,
                                 u64 disk_start)
{
        struct btrfs_fs_info *fs_info = inode->root->fs_info;
        SHASH_DESC_ON_STACK(shash, fs_info->csum_shash);
        const u32 csum_size = fs_info->csum_size;
        const u32 sectorsize = fs_info->sectorsize;
        struct page *page;
        unsigned int i;
        char *kaddr;
        u8 csum[BTRFS_CSUM_SIZE];
        struct compressed_bio *cb = bio->bi_private;
        u8 *cb_sum = cb->sums;

        if (!fs_info->csum_root || (inode->flags & BTRFS_INODE_NODATASUM))
                return 0;

        shash->tfm = fs_info->csum_shash;

        for (i = 0; i < cb->nr_pages; i++) {
                u32 pg_offset;
                u32 bytes_left = PAGE_SIZE;
                page = cb->compressed_pages[i];

                /* Determine the remaining bytes inside the page first */
                if (i == cb->nr_pages - 1)
                        bytes_left = cb->compressed_len - i * PAGE_SIZE;

                /* Hash through the page sector by sector */
                for (pg_offset = 0; pg_offset < bytes_left;
                     pg_offset += sectorsize) {
                        kaddr = kmap_atomic(page);
                        crypto_shash_digest(shash, kaddr + pg_offset,
                                            sectorsize, csum);
                        kunmap_atomic(kaddr);

                        if (memcmp(&csum, cb_sum, csum_size) != 0) {
                                btrfs_print_data_csum_error(inode, disk_start,
                                                csum, cb_sum, cb->mirror_num);
                                if (btrfs_bio(bio)->device)
                                        btrfs_dev_stat_inc_and_print(
                                                btrfs_bio(bio)->device,
                                                BTRFS_DEV_STAT_CORRUPTION_ERRS);
                                return -EIO;
                        }
                        cb_sum += csum_size;
                        disk_start += sectorsize;
                }
        }
        return 0;
}

/*
 * Reduce bio and io accounting for a compressed_bio with its corresponding bio.
 *
 * Return true if there is no pending bio nor io.
 * Return false otherwise.
 */
static bool dec_and_test_compressed_bio(struct compressed_bio *cb, struct bio *bio)
{
        struct btrfs_fs_info *fs_info = btrfs_sb(cb->inode->i_sb);
        unsigned int bi_size = 0;
        bool last_io = false;
        struct bio_vec *bvec;
        struct bvec_iter_all iter_all;

        /*
         * At endio time, bi_iter.bi_size doesn't represent the real bio size.
         * Thus here we have to iterate through all segments to grab correct
         * bio size.
         */
        bio_for_each_segment_all(bvec, bio, iter_all)
                bi_size += bvec->bv_len;

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

        ASSERT(bi_size && bi_size <= cb->compressed_len);
        last_io = refcount_sub_and_test(bi_size >> fs_info->sectorsize_bits,
                                        &cb->pending_sectors);
        /*
         * Here we must wake up the possible error handler after all other
         * operations on @cb finished, or we can race with
         * finish_compressed_bio_*() which may free @cb.
         */
        wake_up_var(cb);

        return last_io;
}

static void finish_compressed_bio_read(struct compressed_bio *cb, struct bio *bio)
{
        unsigned int index;
        struct page *page;

        /* Release the compressed pages */
        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 {
                struct bio_vec *bvec;
                struct bvec_iter_all iter_all;

                ASSERT(bio);
                ASSERT(!bio->bi_status);
                /*
                 * 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, iter_all) {
                        u64 bvec_start = page_offset(bvec->bv_page) +
                                         bvec->bv_offset;

                        btrfs_page_set_checked(btrfs_sb(cb->inode->i_sb),
                                        bvec->bv_page, bvec_start,
                                        bvec->bv_len);
                }

                bio_endio(cb->orig_bio);
        }

        /* Finally free the cb struct */
        kfree(cb->compressed_pages);
        kfree(cb);
}

/* 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;
        unsigned int mirror = btrfs_bio(bio)->mirror_num;
        int ret = 0;

        if (!dec_and_test_compressed_bio(cb, bio))
                goto out;

        /*
         * Record the correct mirror_num in cb->orig_bio so that
         * read-repair can work properly.
         */
        btrfs_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), bio,
                                    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;
        finish_compressed_bio_read(cb, bio);
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)
{
        struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
        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]);
                        btrfs_page_clamp_clear_writeback(fs_info, pages[i],
                                                         cb->start, cb->len);
                        put_page(pages[i]);
                }
                nr_pages -= ret;
                index += ret;
        }
        /* the inode may be gone now */
}

static void finish_compressed_bio_write(struct compressed_bio *cb)
{
        struct inode *inode = cb->inode;
        unsigned int index;

        /*
         * 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.
         */
        btrfs_writepage_endio_finish_ordered(BTRFS_I(inode), NULL,
                        cb->start, cb->start + cb->len - 1,
                        !cb->errors);

        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
         */
        for (index = 0; index < cb->nr_pages; index++) {
                struct page *page = cb->compressed_pages[index];

                page->mapping = NULL;
                put_page(page);
        }

        /* Finally free the cb struct */
        kfree(cb->compressed_pages);
        kfree(cb);
}

/*
 * 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 compressed_bio *cb = bio->bi_private;

        if (!dec_and_test_compressed_bio(cb, bio))
                goto out;

        btrfs_record_physical_zoned(cb->inode, cb->start, bio);

        finish_compressed_bio_write(cb);
out:
        bio_put(bio);
}

static blk_status_t submit_compressed_bio(struct btrfs_fs_info *fs_info,
                                          struct compressed_bio *cb,
                                          struct bio *bio, int mirror_num)
{
        blk_status_t ret;

        ASSERT(bio->bi_iter.bi_size);
        ret = btrfs_bio_wq_end_io(fs_info, bio, BTRFS_WQ_ENDIO_DATA);
        if (ret)
                return ret;
        ret = btrfs_map_bio(fs_info, bio, mirror_num);
        return ret;
}

/*
 * Allocate a compressed_bio, which will be used to read/write on-disk
 * (aka, compressed) * data.
 *
 * @cb:                 The compressed_bio structure, which records all the needed
 *                      information to bind the compressed data to the uncompressed
 *                      page cache.
 * @disk_byten:         The logical bytenr where the compressed data will be read
 *                      from or written to.
 * @endio_func:         The endio function to call after the IO for compressed data
 *                      is finished.
 * @next_stripe_start:  Return value of logical bytenr of where next stripe starts.
 *                      Let the caller know to only fill the bio up to the stripe
 *                      boundary.
 */


static struct bio *alloc_compressed_bio(struct compressed_bio *cb, u64 disk_bytenr,
                                        unsigned int opf, bio_end_io_t endio_func,
                                        u64 *next_stripe_start)
{
        struct btrfs_fs_info *fs_info = btrfs_sb(cb->inode->i_sb);
        struct btrfs_io_geometry geom;
        struct extent_map *em;
        struct bio *bio;
        int ret;

        bio = btrfs_bio_alloc(BIO_MAX_VECS);

        bio->bi_iter.bi_sector = disk_bytenr >> SECTOR_SHIFT;
        bio->bi_opf = opf;
        bio->bi_private = cb;
        bio->bi_end_io = endio_func;

        em = btrfs_get_chunk_map(fs_info, disk_bytenr, fs_info->sectorsize);
        if (IS_ERR(em)) {
                bio_put(bio);
                return ERR_CAST(em);
        }

        if (bio_op(bio) == REQ_OP_ZONE_APPEND)
                bio_set_dev(bio, em->map_lookup->stripes[0].dev->bdev);

        ret = btrfs_get_io_geometry(fs_info, em, btrfs_op(bio), disk_bytenr, &geom);
        free_extent_map(em);
        if (ret < 0) {
                bio_put(bio);
                return ERR_PTR(ret);
        }
        *next_stripe_start = disk_bytenr + geom.len;

        return 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 btrfs_inode *inode, u64 start,
                                 unsigned int len, u64 disk_start,
                                 unsigned int compressed_len,
                                 struct page **compressed_pages,
                                 unsigned int nr_pages,
                                 unsigned int write_flags,
                                 struct cgroup_subsys_state *blkcg_css)
{
        struct btrfs_fs_info *fs_info = inode->root->fs_info;
        struct bio *bio = NULL;
        struct compressed_bio *cb;
        u64 cur_disk_bytenr = disk_start;
        u64 next_stripe_start;
        blk_status_t ret;
        int skip_sum = inode->flags & BTRFS_INODE_NODATASUM;
        const bool use_append = btrfs_use_zone_append(inode, disk_start);
        const unsigned int bio_op = use_append ? REQ_OP_ZONE_APPEND : REQ_OP_WRITE;

        ASSERT(IS_ALIGNED(start, fs_info->sectorsize) &&
               IS_ALIGNED(len, fs_info->sectorsize));
        cb = kmalloc(compressed_bio_size(fs_info, compressed_len), GFP_NOFS);
        if (!cb)
                return BLK_STS_RESOURCE;
        refcount_set(&cb->pending_sectors, compressed_len >> fs_info->sectorsize_bits);
        cb->errors = 0;
        cb->inode = &inode->vfs_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;

        while (cur_disk_bytenr < disk_start + compressed_len) {
                u64 offset = cur_disk_bytenr - disk_start;
                unsigned int index = offset >> PAGE_SHIFT;
                unsigned int real_size;
                unsigned int added;
                struct page *page = compressed_pages[index];
                bool submit = false;

                /* Allocate new bio if submitted or not yet allocated */
                if (!bio) {
                        bio = alloc_compressed_bio(cb, cur_disk_bytenr,
                                bio_op | write_flags, end_compressed_bio_write,
                                &next_stripe_start);
                        if (IS_ERR(bio)) {
                                ret = errno_to_blk_status(PTR_ERR(bio));
                                bio = NULL;
                                goto finish_cb;
                        }
                }
                /*
                 * We should never reach next_stripe_start start as we will
                 * submit comp_bio when reach the boundary immediately.
                 */
                ASSERT(cur_disk_bytenr != next_stripe_start);

                /*
                 * We have various limits on the real read size:
                 * - stripe boundary
                 * - page boundary
                 * - compressed length boundary
                 */
                real_size = min_t(u64, U32_MAX, next_stripe_start - cur_disk_bytenr);
                real_size = min_t(u64, real_size, PAGE_SIZE - offset_in_page(offset));
                real_size = min_t(u64, real_size, compressed_len - offset);
                ASSERT(IS_ALIGNED(real_size, fs_info->sectorsize));

                if (use_append)
                        added = bio_add_zone_append_page(bio, page, real_size,
                                        offset_in_page(offset));
                else
                        added = bio_add_page(bio, page, real_size,
                                        offset_in_page(offset));
                /* Reached zoned boundary */
                if (added == 0)
                        submit = true;

                cur_disk_bytenr += added;
                /* Reached stripe boundary */
                if (cur_disk_bytenr == next_stripe_start)
                        submit = true;

                /* Finished the range */
                if (cur_disk_bytenr == disk_start + compressed_len)
                        submit = true;

                if (submit) {
                        if (!skip_sum) {
                                ret = btrfs_csum_one_bio(inode, bio, start, 1);
                                if (ret)
                                        goto finish_cb;
                        }

                        ret = submit_compressed_bio(fs_info, cb, bio, 0);
                        if (ret)
                                goto finish_cb;
                        bio = NULL;
                }
                cond_resched();
        }
        if (blkcg_css)
                kthread_associate_blkcg(NULL);

        return 0;

finish_cb:
        if (bio) {
                bio->bi_status = ret;
                bio_endio(bio);
        }
        /* Last byte of @cb is submitted, endio will free @cb */
        if (cur_disk_bytenr == disk_start + compressed_len)
                return ret;

        wait_var_event(cb, refcount_read(&cb->pending_sectors) ==
                           (disk_start + compressed_len - cur_disk_bytenr) >>
                           fs_info->sectorsize_bits);
        /*
         * Even with previous bio ended, we should still have io not yet
         * submitted, thus need to finish manually.
         */
        ASSERT(refcount_read(&cb->pending_sectors));
        /* Now we are the only one referring @cb, can finish it safely. */
        finish_compressed_bio_write(cb);
        return ret;
}

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;
}

/*
 * 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)
{
        struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
        unsigned long end_index;
        u64 cur = bio_end_offset(cb->orig_bio);
        u64 isize = i_size_read(inode);
        int ret;
        struct page *page;
        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 (btrfs_sb(inode->i_sb)->sectorsize < PAGE_SIZE)
                return 0;

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

        while (cur < compressed_end) {
                u64 page_end;
                u64 pg_index = cur >> PAGE_SHIFT;
                u32 add_size;

                if (pg_index > end_index)
                        break;

                page = xa_load(&mapping->i_pages, pg_index);
                if (page && !xa_is_value(page)) {
                        sectors_missed += (PAGE_SIZE - offset_in_page(cur)) >>
                                          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 = (pg_index << PAGE_SHIFT) + PAGE_SIZE;
                        continue;
                }

                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);
                        /* There is already a page, skip to page end */
                        cur = (pg_index << PAGE_SHIFT) + PAGE_SIZE;
                        continue;
                }

                ret = set_page_extent_mapped(page);
                if (ret < 0) {
                        unlock_page(page);
                        put_page(page);
                        break;
                }

                page_end = (pg_index << PAGE_SHIFT) + PAGE_SIZE - 1;
                lock_extent(tree, cur, page_end);
                read_lock(&em_tree->lock);
                em = 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 > extent_map_end(em)) ||
                    (em->block_start >> 9) != cb->orig_bio->bi_iter.bi_sector) {
                        free_extent_map(em);
                        unlock_extent(tree, cur, page_end);
                        unlock_page(page);
                        put_page(page);
                        break;
                }
                free_extent_map(em);

                if (page->index == end_index) {
                        size_t zero_offset = offset_in_page(isize);

                        if (zero_offset) {
                                int zeros;
                                zeros = PAGE_SIZE - zero_offset;
                                memzero_page(page, zero_offset, zeros);
                                flush_dcache_page(page);
                        }
                }

                add_size = min(em->start + em->len, page_end + 1) - cur;
                ret = bio_add_page(cb->orig_bio, page, add_size, offset_in_page(cur));
                if (ret != add_size) {
                        unlock_extent(tree, cur, page_end);
                        unlock_page(page);
                        put_page(page);
                        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_subpage_start_reader(fs_info, page, cur, add_size);
                put_page(page);
                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
 */
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_map_tree *em_tree;
        struct compressed_bio *cb;
        unsigned int compressed_len;
        unsigned int nr_pages;
        unsigned int pg_index;
        struct bio *comp_bio = NULL;
        const u64 disk_bytenr = bio->bi_iter.bi_sector << SECTOR_SHIFT;
        u64 cur_disk_byte = disk_bytenr;
        u64 next_stripe_start;
        u64 file_offset;
        u64 em_len;
        u64 em_start;
        struct extent_map *em;
        blk_status_t ret = BLK_STS_RESOURCE;
        int faili = 0;
        u8 *sums;

        em_tree = &BTRFS_I(inode)->extent_tree;

        file_offset = bio_first_bvec_all(bio)->bv_offset +
                      page_offset(bio_first_page_all(bio));

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

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

        refcount_set(&cb->pending_sectors, compressed_len >> fs_info->sectorsize_bits);
        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;

        for (pg_index = 0; pg_index < nr_pages; pg_index++) {
                cb->compressed_pages[pg_index] = alloc_page(GFP_NOFS);
                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;

        while (cur_disk_byte < disk_bytenr + compressed_len) {
                u64 offset = cur_disk_byte - disk_bytenr;
                unsigned int index = offset >> PAGE_SHIFT;
                unsigned int real_size;
                unsigned int added;
                struct page *page = cb->compressed_pages[index];
                bool submit = false;

                /* Allocate new bio if submitted or not yet allocated */
                if (!comp_bio) {
                        comp_bio = alloc_compressed_bio(cb, cur_disk_byte,
                                        REQ_OP_READ, end_compressed_bio_read,
                                        &next_stripe_start);
                        if (IS_ERR(comp_bio)) {
                                ret = errno_to_blk_status(PTR_ERR(comp_bio));
                                comp_bio = NULL;
                                goto finish_cb;
                        }
                }
                /*
                 * We should never reach next_stripe_start start as we will
                 * submit comp_bio when reach the boundary immediately.
                 */
                ASSERT(cur_disk_byte != next_stripe_start);
                /*
                 * We have various limit on the real read size:
                 * - stripe boundary
                 * - page boundary
                 * - compressed length boundary
                 */
                real_size = min_t(u64, U32_MAX, next_stripe_start - cur_disk_byte);
                real_size = min_t(u64, real_size, PAGE_SIZE - offset_in_page(offset));
                real_size = min_t(u64, real_size, compressed_len - offset);
                ASSERT(IS_ALIGNED(real_size, fs_info->sectorsize));

                added = bio_add_page(comp_bio, page, real_size, offset_in_page(offset));
                /*
                 * Maximum compressed extent is smaller than bio size limit,
                 * thus bio_add_page() should always success.
                 */
                ASSERT(added == real_size);
                cur_disk_byte += added;

                /* Reached stripe boundary, need to submit */
                if (cur_disk_byte == next_stripe_start)
                        submit = true;

                /* Has finished the range, need to submit */
                if (cur_disk_byte == disk_bytenr + compressed_len)
                        submit = true;

                if (submit) {
                        unsigned int nr_sectors;

                        ret = btrfs_lookup_bio_sums(inode, comp_bio, sums);
                        if (ret)
                                goto finish_cb;

                        nr_sectors = DIV_ROUND_UP(comp_bio->bi_iter.bi_size,
                                                  fs_info->sectorsize);
                        sums += fs_info->csum_size * nr_sectors;

                        ret = submit_compressed_bio(fs_info, cb, comp_bio, mirror_num);
                        if (ret)
                                goto finish_cb;
                        comp_bio = NULL;
                }
        }
        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;
finish_cb:
        if (comp_bio) {
                comp_bio->bi_status = ret;
                bio_endio(comp_bio);
        }
        /* All bytes of @cb is submitted, endio will free @cb */
        if (cur_disk_byte == disk_bytenr + compressed_len)
                return ret;

        wait_var_event(cb, refcount_read(&cb->pending_sectors) ==
                           (disk_bytenr + compressed_len - cur_disk_byte) >>
                           fs_info->sectorsize_bits);
        /*
         * Even with previous bio ended, we should still have io not yet
         * submitted, thus need to finish @cb manually.
         */
        ASSERT(refcount_read(&cb->pending_sectors));
        /* Now we are the only one referring @cb, can finish it safely. */
        finish_compressed_bio_read(cb, NULL);
        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 struct workspace_manager heuristic_wsm;

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(unsigned int level)
{
        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);
}

const struct btrfs_compress_op btrfs_heuristic_compress = {
        .workspace_manager = &heuristic_wsm,
};

static const struct btrfs_compress_op * const btrfs_compress_op[] = {
        /* 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(int type, unsigned int level)
{
        switch (type) {
        case BTRFS_COMPRESS_NONE: return alloc_heuristic_ws(level);
        case BTRFS_COMPRESS_ZLIB: return zlib_alloc_workspace(level);
        case BTRFS_COMPRESS_LZO:  return lzo_alloc_workspace(level);
        case BTRFS_COMPRESS_ZSTD: return zstd_alloc_workspace(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 void btrfs_init_workspace_manager(int type)
{
        struct workspace_manager *wsm;
        struct list_head *workspace;

        wsm = btrfs_compress_op[type]->workspace_manager;
        INIT_LIST_HEAD(&wsm->idle_ws);
        spin_lock_init(&wsm->ws_lock);
        atomic_set(&wsm->total_ws, 0);
        init_waitqueue_head(&wsm->ws_wait);

        /*
         * Preallocate one workspace for each compression type so we can
         * guarantee forward progress in the worst case
         */
        workspace = alloc_workspace(type, 0);
        if (IS_ERR(workspace)) {
                pr_warn(
        "BTRFS: cannot preallocate compression workspace, will try later\n");
        } else {
                atomic_set(&wsm->total_ws, 1);
                wsm->free_ws = 1;
                list_add(workspace, &wsm->idle_ws);
        }
}

static void btrfs_cleanup_workspace_manager(int type)
{
        struct workspace_manager *wsman;
        struct list_head *ws;

        wsman = btrfs_compress_op[type]->workspace_manager;
        while (!list_empty(&wsman->idle_ws)) {
                ws = wsman->idle_ws.next;
                list_del(ws);
                free_workspace(type, ws);
                atomic_dec(&wsman->total_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.
 */
struct list_head *btrfs_get_workspace(int type, unsigned int level)
{
        struct workspace_manager *wsm;
        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;

        wsm = btrfs_compress_op[type]->workspace_manager;
        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(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)) {
                                pr_warn("BTRFS: no compression workspaces, low memory, retrying\n");
                        }
                }
                goto again;
        }
        return workspace;
}

static struct list_head *get_workspace(int type, int level)
{
        switch (type) {
        case BTRFS_COMPRESS_NONE: return btrfs_get_workspace(type, level);
        case BTRFS_COMPRESS_ZLIB: return zlib_get_workspace(level);
        case BTRFS_COMPRESS_LZO:  return btrfs_get_workspace(type, level);
        case BTRFS_COMPRESS_ZSTD: return zstd_get_workspace(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(int type, struct list_head *ws)
{
        struct workspace_manager *wsm;
        struct list_head *idle_ws;
        spinlock_t *ws_lock;
        atomic_t *total_ws;
        wait_queue_head_t *ws_wait;
        int *free_ws;

        wsm = btrfs_compress_op[type]->workspace_manager;
        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;

        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(int type, struct list_head *ws)
{
        switch (type) {
        case BTRFS_COMPRESS_NONE: return btrfs_put_workspace(type, ws);
        case BTRFS_COMPRESS_ZLIB: return btrfs_put_workspace(type, ws);
        case BTRFS_COMPRESS_LZO:  return btrfs_put_workspace(type, ws);
        case BTRFS_COMPRESS_ZSTD: return zstd_put_workspace(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 unsigned int btrfs_compress_set_level(int type, unsigned level)
{
        const struct btrfs_compress_op *ops = btrfs_compress_op[type];

        if (level == 0)
                level = ops->default_level;
        else
                level = min(level, ops->max_level);

        return level;
}

/*
 * 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
 */
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)
{
        int type = btrfs_compress_type(type_level);
        int level = btrfs_compress_level(type_level);
        struct list_head *workspace;
        int ret;

        level = btrfs_compress_set_level(type, level);
        workspace = get_workspace(type, level);
        ret = compression_compress_pages(type, workspace, mapping, start, pages,
                                         out_pages, total_in, total_out);
        put_workspace(type, workspace);
        return ret;
}

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

        workspace = get_workspace(type, 0);
        ret = compression_decompress_bio(type, workspace, cb);
        put_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 = get_workspace(type, 0);
        ret = compression_decompress(type, workspace, data_in, dest_page,
                                     start_byte, srclen, destlen);
        put_workspace(type, workspace);

        return ret;
}

void __init btrfs_init_compress(void)
{
        btrfs_init_workspace_manager(BTRFS_COMPRESS_NONE);
        btrfs_init_workspace_manager(BTRFS_COMPRESS_ZLIB);
        btrfs_init_workspace_manager(BTRFS_COMPRESS_LZO);
        zstd_init_workspace_manager();
}

void __cold btrfs_exit_compress(void)
{
        btrfs_cleanup_workspace_manager(BTRFS_COMPRESS_NONE);
        btrfs_cleanup_workspace_manager(BTRFS_COMPRESS_ZLIB);
        btrfs_cleanup_workspace_manager(BTRFS_COMPRESS_LZO);
        zstd_cleanup_workspace_manager();
}

/*
 * 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_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;

                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 = page_offset(bvec.bv_page) + bvec.bv_offset - 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);
                memcpy_to_page(bvec.bv_page, bvec.bv_offset,
                               buf + copy_start - decompressed, copy_len);
                flush_dcache_page(bvec.bv_page);
                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 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 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;
        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_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.
 *
 * 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 = get_workspace(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, 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(0, ws_list);
        return ret;
}

/*
 * Convert the compression suffix (eg. after "zlib" starting with ":") to
 * level, unrecognized string will set the default level
 */
unsigned int btrfs_compress_str2level(unsigned int type, const char *str)
{
        unsigned int level = 0;
        int ret;

        if (!type)
                return 0;

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

        level = btrfs_compress_set_level(type, level);

        return level;
}