/*
 * Copyright (C) 2008 Oracle.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public
 * License along with this program; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 021110-1307, USA.
 */

#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 "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"

struct compressed_bio {
        /* number of bios pending for this compressed extent */
        refcount_t pending_bios;

        /* the pages with the compressed data on them */
        struct page **compressed_pages;

        /* inode that owns this data */
        struct inode *inode;

        /* starting offset in the inode for our pages */
        u64 start;

        /* number of bytes in the inode we're working on */
        unsigned long len;

        /* number of bytes on disk */
        unsigned long compressed_len;

        /* the compression algorithm for this bio */
        int compress_type;

        /* number of compressed pages in the array */
        unsigned long nr_pages;

        /* IO errors */
        int errors;
        int mirror_num;

        /* for reads, this is the bio we are copying the data into */
        struct bio *orig_bio;

        /*
         * the start of a variable length array of checksums only
         * used by reads
         */
        u32 sums;
};

static int btrfs_decompress_bio(int type, struct page **pages_in,
                                   u64 disk_start, struct bio *orig_bio,
                                   size_t srclen);

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 struct bio *compressed_bio_alloc(struct block_device *bdev,
                                        u64 first_byte, gfp_t gfp_flags)
{
        return btrfs_bio_alloc(bdev, first_byte >> 9, BIO_MAX_PAGES, gfp_flags);
}

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;
        int ret;

        if (bio->bi_error)
                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;

        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->compress_type,
                                      cb->compressed_pages,
                                      cb->start,
                                      cb->orig_bio,
                                      cb->compressed_len);
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
                 */
                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_error)
                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_error ? 0 : 1);
        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.
 */
int 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)
{
        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;
        int 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 -ENOMEM;
        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 = compressed_bio_alloc(bdev, first_byte, GFP_NOFS);
        if (!bio) {
                kfree(cb);
                return -ENOMEM;
        }
        bio_set_op_attrs(bio, REQ_OP_WRITE, 0);
        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++) {
                page = compressed_pages[pg_index];
                page->mapping = inode->i_mapping;
                if (bio->bi_iter.bi_size)
                        ret = io_tree->ops->merge_bio_hook(page, 0,
                                                           PAGE_SIZE,
                                                           bio, 0);
                else
                        ret = 0;

                page->mapping = NULL;
                if (ret || bio_add_page(bio, page, PAGE_SIZE, 0) <
                    PAGE_SIZE) {
                        bio_get(bio);

                        /*
                         * 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_error = ret;
                                bio_endio(bio);
                        }

                        bio_put(bio);

                        bio = compressed_bio_alloc(bdev, first_byte, GFP_NOFS);
                        BUG_ON(!bio);
                        bio_set_op_attrs(bio, REQ_OP_WRITE, 0);
                        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();
        }
        bio_get(bio);

        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_error = ret;
                bio_endio(bio);
        }

        bio_put(bio);
        return 0;
}

static u64 bio_end_offset(struct bio *bio)
{
        struct bio_vec *last = &bio->bi_io_vec[bio->bi_vcnt - 1];

        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->page_tree, 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
 */
int 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;
        int ret = -ENOMEM;
        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->bi_io_vec->bv_page),
                                   PAGE_SIZE);
        read_unlock(&em_tree->lock);
        if (!em)
                return -EIO;

        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 = -ENOMEM;
                        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 = compressed_bio_alloc(bdev, cur_disk_byte, GFP_NOFS);
        if (!comp_bio)
                goto fail2;
        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++) {
                page = cb->compressed_pages[pg_index];
                page->mapping = inode->i_mapping;
                page->index = em_start >> PAGE_SHIFT;

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

                page->mapping = NULL;
                if (ret || bio_add_page(comp_bio, page, PAGE_SIZE, 0) <
                    PAGE_SIZE) {
                        bio_get(comp_bio);

                        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_error = ret;
                                bio_endio(comp_bio);
                        }

                        bio_put(comp_bio);

                        comp_bio = compressed_bio_alloc(bdev, cur_disk_byte,
                                                        GFP_NOFS);
                        BUG_ON(!comp_bio);
                        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;
        }
        bio_get(comp_bio);

        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_error = ret;
                bio_endio(comp_bio);
        }

        bio_put(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;
}

static struct {
        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;
} btrfs_comp_ws[BTRFS_COMPRESS_TYPES];

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

void __init btrfs_init_compress(void)
{
        int i;

        for (i = 0; i < BTRFS_COMPRESS_TYPES; i++) {
                struct list_head *workspace;

                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)
{
        struct list_head *workspace;
        int cpus = num_online_cpus();
        int idx = type - 1;

        struct list_head *idle_ws       = &btrfs_comp_ws[idx].idle_ws;
        spinlock_t *ws_lock             = &btrfs_comp_ws[idx].ws_lock;
        atomic_t *total_ws              = &btrfs_comp_ws[idx].total_ws;
        wait_queue_head_t *ws_wait      = &btrfs_comp_ws[idx].ws_wait;
        int *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);

        workspace = btrfs_compress_op[idx]->alloc_workspace();
        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;
}

/*
 * 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)
{
        int idx = type - 1;
        struct list_head *idle_ws       = &btrfs_comp_ws[idx].idle_ws;
        spinlock_t *ws_lock             = &btrfs_comp_ws[idx].ws_lock;
        atomic_t *total_ws              = &btrfs_comp_ws[idx].total_ws;
        wait_queue_head_t *ws_wait      = &btrfs_comp_ws[idx].ws_wait;
        int *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);

        btrfs_compress_op[idx]->free_workspace(workspace);
        atomic_dec(total_ws);
wake:
        /*
         * Make sure counter is updated before we wake up waiters.
         */
        smp_mb();
        if (waitqueue_active(ws_wait))
                wake_up(ws_wait);
}

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

        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.
 *
 * @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(int type, 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;

        workspace = find_workspace(type);

        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(int type, struct page **pages_in,
                                   u64 disk_start, struct bio *orig_bio,
                                   size_t srclen)
{
        struct list_head *workspace;
        int ret;

        workspace = find_workspace(type);

        ret = btrfs_compress_op[type-1]->decompress_bio(workspace, pages_in,
                                                         disk_start, orig_bio,
                                                         srclen);
        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 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;
}