diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
new file mode 100644
index 0000000..9bfa665
--- /dev/null
+++ b/fs/btrfs/compression.c
@@ -0,0 +1,1606 @@
+// 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/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 "ctree.h"
+#include "disk-io.h"
+#include "transaction.h"
+#include "btrfs_inode.h"
+#include "volumes.h"
+#include "ordered-data.h"
+#include "compression.h"
+#include "extent_io.h"
+#include "extent_map.h"
+
+static const char* const btrfs_compress_types[] = { "", "zlib", "lzo", "zstd" };
+
+const char* btrfs_compress_type2str(enum btrfs_compression_type type)
+{
+	switch (type) {
+	case BTRFS_COMPRESS_ZLIB:
+	case BTRFS_COMPRESS_LZO:
+	case BTRFS_COMPRESS_ZSTD:
+	case BTRFS_COMPRESS_NONE:
+		return btrfs_compress_types[type];
+	}
+
+	return NULL;
+}
+
+static int btrfs_decompress_bio(struct compressed_bio *cb);
+
+static inline int compressed_bio_size(struct btrfs_fs_info *fs_info,
+				      unsigned long disk_size)
+{
+	u16 csum_size = btrfs_super_csum_size(fs_info->super_copy);
+
+	return sizeof(struct compressed_bio) +
+		(DIV_ROUND_UP(disk_size, fs_info->sectorsize)) * csum_size;
+}
+
+static int check_compressed_csum(struct btrfs_inode *inode,
+				 struct compressed_bio *cb,
+				 u64 disk_start)
+{
+	int ret;
+	struct page *page;
+	unsigned long i;
+	char *kaddr;
+	u32 csum;
+	u32 *cb_sum = &cb->sums;
+
+	if (inode->flags & BTRFS_INODE_NODATASUM)
+		return 0;
+
+	for (i = 0; i < cb->nr_pages; i++) {
+		page = cb->compressed_pages[i];
+		csum = ~(u32)0;
+
+		kaddr = kmap_atomic(page);
+		csum = btrfs_csum_data(kaddr, csum, PAGE_SIZE);
+		btrfs_csum_final(csum, (u8 *)&csum);
+		kunmap_atomic(kaddr);
+
+		if (csum != *cb_sum) {
+			btrfs_print_data_csum_error(inode, disk_start, csum,
+					*cb_sum, cb->mirror_num);
+			ret = -EIO;
+			goto fail;
+		}
+		cb_sum++;
+
+	}
+	ret = 0;
+fail:
+	return ret;
+}
+
+/* when we finish reading compressed pages from the disk, we
+ * decompress them and then run the bio end_io routines on the
+ * decompressed pages (in the inode address space).
+ *
+ * This allows the checksumming and other IO error handling routines
+ * to work normally
+ *
+ * The compressed pages are freed here, and it must be run
+ * in process context
+ */
+static void end_compressed_bio_read(struct bio *bio)
+{
+	struct compressed_bio *cb = bio->bi_private;
+	struct inode *inode;
+	struct page *page;
+	unsigned long index;
+	unsigned int mirror = btrfs_io_bio(bio)->mirror_num;
+	int ret = 0;
+
+	if (bio->bi_status)
+		cb->errors = 1;
+
+	/* if there are more bios still pending for this compressed
+	 * extent, just exit
+	 */
+	if (!refcount_dec_and_test(&cb->pending_bios))
+		goto out;
+
+	/*
+	 * Record the correct mirror_num in cb->orig_bio so that
+	 * read-repair can work properly.
+	 */
+	ASSERT(btrfs_io_bio(cb->orig_bio));
+	btrfs_io_bio(cb->orig_bio)->mirror_num = mirror;
+	cb->mirror_num = mirror;
+
+	/*
+	 * Some IO in this cb have failed, just skip checksum as there
+	 * is no way it could be correct.
+	 */
+	if (cb->errors == 1)
+		goto csum_failed;
+
+	inode = cb->inode;
+	ret = check_compressed_csum(BTRFS_I(inode), cb,
+				    (u64)bio->bi_iter.bi_sector << 9);
+	if (ret)
+		goto csum_failed;
+
+	/* ok, we're the last bio for this extent, lets start
+	 * the decompression.
+	 */
+	ret = btrfs_decompress_bio(cb);
+
+csum_failed:
+	if (ret)
+		cb->errors = 1;
+
+	/* release the compressed pages */
+	index = 0;
+	for (index = 0; index < cb->nr_pages; index++) {
+		page = cb->compressed_pages[index];
+		page->mapping = NULL;
+		put_page(page);
+	}
+
+	/* do io completion on the original bio */
+	if (cb->errors) {
+		bio_io_error(cb->orig_bio);
+	} else {
+		int i;
+		struct bio_vec *bvec;
+
+		/*
+		 * we have verified the checksum already, set page
+		 * checked so the end_io handlers know about it
+		 */
+		ASSERT(!bio_flagged(bio, BIO_CLONED));
+		bio_for_each_segment_all(bvec, cb->orig_bio, i)
+			SetPageChecked(bvec->bv_page);
+
+		bio_endio(cb->orig_bio);
+	}
+
+	/* finally free the cb struct */
+	kfree(cb->compressed_pages);
+	kfree(cb);
+out:
+	bio_put(bio);
+}
+
+/*
+ * Clear the writeback bits on all of the file
+ * pages for a compressed write
+ */
+static noinline void end_compressed_writeback(struct inode *inode,
+					      const struct compressed_bio *cb)
+{
+	unsigned long index = cb->start >> PAGE_SHIFT;
+	unsigned long end_index = (cb->start + cb->len - 1) >> PAGE_SHIFT;
+	struct page *pages[16];
+	unsigned long nr_pages = end_index - index + 1;
+	int i;
+	int ret;
+
+	if (cb->errors)
+		mapping_set_error(inode->i_mapping, -EIO);
+
+	while (nr_pages > 0) {
+		ret = find_get_pages_contig(inode->i_mapping, index,
+				     min_t(unsigned long,
+				     nr_pages, ARRAY_SIZE(pages)), pages);
+		if (ret == 0) {
+			nr_pages -= 1;
+			index += 1;
+			continue;
+		}
+		for (i = 0; i < ret; i++) {
+			if (cb->errors)
+				SetPageError(pages[i]);
+			end_page_writeback(pages[i]);
+			put_page(pages[i]);
+		}
+		nr_pages -= ret;
+		index += ret;
+	}
+	/* the inode may be gone now */
+}
+
+/*
+ * do the cleanup once all the compressed pages hit the disk.
+ * This will clear writeback on the file pages and free the compressed
+ * pages.
+ *
+ * This also calls the writeback end hooks for the file pages so that
+ * metadata and checksums can be updated in the file.
+ */
+static void end_compressed_bio_write(struct bio *bio)
+{
+	struct extent_io_tree *tree;
+	struct compressed_bio *cb = bio->bi_private;
+	struct inode *inode;
+	struct page *page;
+	unsigned long index;
+
+	if (bio->bi_status)
+		cb->errors = 1;
+
+	/* if there are more bios still pending for this compressed
+	 * extent, just exit
+	 */
+	if (!refcount_dec_and_test(&cb->pending_bios))
+		goto out;
+
+	/* ok, we're the last bio for this extent, step one is to
+	 * call back into the FS and do all the end_io operations
+	 */
+	inode = cb->inode;
+	tree = &BTRFS_I(inode)->io_tree;
+	cb->compressed_pages[0]->mapping = cb->inode->i_mapping;
+	tree->ops->writepage_end_io_hook(cb->compressed_pages[0],
+					 cb->start,
+					 cb->start + cb->len - 1,
+					 NULL,
+					 bio->bi_status ?
+					 BLK_STS_OK : BLK_STS_NOTSUPP);
+	cb->compressed_pages[0]->mapping = NULL;
+
+	end_compressed_writeback(inode, cb);
+	/* note, our inode could be gone now */
+
+	/*
+	 * release the compressed pages, these came from alloc_page and
+	 * are not attached to the inode at all
+	 */
+	index = 0;
+	for (index = 0; index < cb->nr_pages; index++) {
+		page = cb->compressed_pages[index];
+		page->mapping = NULL;
+		put_page(page);
+	}
+
+	/* finally free the cb struct */
+	kfree(cb->compressed_pages);
+	kfree(cb);
+out:
+	bio_put(bio);
+}
+
+/*
+ * worker function to build and submit bios for previously compressed pages.
+ * The corresponding pages in the inode should be marked for writeback
+ * and the compressed pages should have a reference on them for dropping
+ * when the IO is complete.
+ *
+ * This also checksums the file bytes and gets things ready for
+ * the end io hooks.
+ */
+blk_status_t btrfs_submit_compressed_write(struct inode *inode, u64 start,
+				 unsigned long len, u64 disk_start,
+				 unsigned long compressed_len,
+				 struct page **compressed_pages,
+				 unsigned long nr_pages,
+				 unsigned int write_flags)
+{
+	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
+	struct bio *bio = NULL;
+	struct compressed_bio *cb;
+	unsigned long bytes_left;
+	int pg_index = 0;
+	struct page *page;
+	u64 first_byte = disk_start;
+	struct block_device *bdev;
+	blk_status_t ret;
+	int skip_sum = BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM;
+
+	WARN_ON(start & ((u64)PAGE_SIZE - 1));
+	cb = kmalloc(compressed_bio_size(fs_info, compressed_len), GFP_NOFS);
+	if (!cb)
+		return BLK_STS_RESOURCE;
+	refcount_set(&cb->pending_bios, 0);
+	cb->errors = 0;
+	cb->inode = inode;
+	cb->start = start;
+	cb->len = len;
+	cb->mirror_num = 0;
+	cb->compressed_pages = compressed_pages;
+	cb->compressed_len = compressed_len;
+	cb->orig_bio = NULL;
+	cb->nr_pages = nr_pages;
+
+	bdev = fs_info->fs_devices->latest_bdev;
+
+	bio = btrfs_bio_alloc(bdev, first_byte);
+	bio->bi_opf = REQ_OP_WRITE | write_flags;
+	bio->bi_private = cb;
+	bio->bi_end_io = end_compressed_bio_write;
+	refcount_set(&cb->pending_bios, 1);
+
+	/* create and submit bios for the compressed pages */
+	bytes_left = compressed_len;
+	for (pg_index = 0; pg_index < cb->nr_pages; pg_index++) {
+		int submit = 0;
+
+		page = compressed_pages[pg_index];
+		page->mapping = inode->i_mapping;
+		if (bio->bi_iter.bi_size)
+			submit = btrfs_merge_bio_hook(page, 0, PAGE_SIZE, bio, 0);
+
+		page->mapping = NULL;
+		if (submit || bio_add_page(bio, page, PAGE_SIZE, 0) <
+		    PAGE_SIZE) {
+			/*
+			 * inc the count before we submit the bio so
+			 * we know the end IO handler won't happen before
+			 * we inc the count.  Otherwise, the cb might get
+			 * freed before we're done setting it up
+			 */
+			refcount_inc(&cb->pending_bios);
+			ret = btrfs_bio_wq_end_io(fs_info, bio,
+						  BTRFS_WQ_ENDIO_DATA);
+			BUG_ON(ret); /* -ENOMEM */
+
+			if (!skip_sum) {
+				ret = btrfs_csum_one_bio(inode, bio, start, 1);
+				BUG_ON(ret); /* -ENOMEM */
+			}
+
+			ret = btrfs_map_bio(fs_info, bio, 0, 1);
+			if (ret) {
+				bio->bi_status = ret;
+				bio_endio(bio);
+			}
+
+			bio = btrfs_bio_alloc(bdev, first_byte);
+			bio->bi_opf = REQ_OP_WRITE | write_flags;
+			bio->bi_private = cb;
+			bio->bi_end_io = end_compressed_bio_write;
+			bio_add_page(bio, page, PAGE_SIZE, 0);
+		}
+		if (bytes_left < PAGE_SIZE) {
+			btrfs_info(fs_info,
+					"bytes left %lu compress len %lu nr %lu",
+			       bytes_left, cb->compressed_len, cb->nr_pages);
+		}
+		bytes_left -= PAGE_SIZE;
+		first_byte += PAGE_SIZE;
+		cond_resched();
+	}
+
+	ret = btrfs_bio_wq_end_io(fs_info, bio, BTRFS_WQ_ENDIO_DATA);
+	BUG_ON(ret); /* -ENOMEM */
+
+	if (!skip_sum) {
+		ret = btrfs_csum_one_bio(inode, bio, start, 1);
+		BUG_ON(ret); /* -ENOMEM */
+	}
+
+	ret = btrfs_map_bio(fs_info, bio, 0, 1);
+	if (ret) {
+		bio->bi_status = ret;
+		bio_endio(bio);
+	}
+
+	return 0;
+}
+
+static u64 bio_end_offset(struct bio *bio)
+{
+	struct bio_vec *last = bio_last_bvec_all(bio);
+
+	return page_offset(last->bv_page) + last->bv_len + last->bv_offset;
+}
+
+static noinline int add_ra_bio_pages(struct inode *inode,
+				     u64 compressed_end,
+				     struct compressed_bio *cb)
+{
+	unsigned long end_index;
+	unsigned long pg_index;
+	u64 last_offset;
+	u64 isize = i_size_read(inode);
+	int ret;
+	struct page *page;
+	unsigned long nr_pages = 0;
+	struct extent_map *em;
+	struct address_space *mapping = inode->i_mapping;
+	struct extent_map_tree *em_tree;
+	struct extent_io_tree *tree;
+	u64 end;
+	int misses = 0;
+
+	last_offset = bio_end_offset(cb->orig_bio);
+	em_tree = &BTRFS_I(inode)->extent_tree;
+	tree = &BTRFS_I(inode)->io_tree;
+
+	if (isize == 0)
+		return 0;
+
+	end_index = (i_size_read(inode) - 1) >> PAGE_SHIFT;
+
+	while (last_offset < compressed_end) {
+		pg_index = last_offset >> PAGE_SHIFT;
+
+		if (pg_index > end_index)
+			break;
+
+		rcu_read_lock();
+		page = radix_tree_lookup(&mapping->i_pages, pg_index);
+		rcu_read_unlock();
+		if (page && !radix_tree_exceptional_entry(page)) {
+			misses++;
+			if (misses > 4)
+				break;
+			goto next;
+		}
+
+		page = __page_cache_alloc(mapping_gfp_constraint(mapping,
+								 ~__GFP_FS));
+		if (!page)
+			break;
+
+		if (add_to_page_cache_lru(page, mapping, pg_index, GFP_NOFS)) {
+			put_page(page);
+			goto next;
+		}
+
+		end = last_offset + PAGE_SIZE - 1;
+		/*
+		 * at this point, we have a locked page in the page cache
+		 * for these bytes in the file.  But, we have to make
+		 * sure they map to this compressed extent on disk.
+		 */
+		set_page_extent_mapped(page);
+		lock_extent(tree, last_offset, end);
+		read_lock(&em_tree->lock);
+		em = lookup_extent_mapping(em_tree, last_offset,
+					   PAGE_SIZE);
+		read_unlock(&em_tree->lock);
+
+		if (!em || last_offset < em->start ||
+		    (last_offset + PAGE_SIZE > extent_map_end(em)) ||
+		    (em->block_start >> 9) != cb->orig_bio->bi_iter.bi_sector) {
+			free_extent_map(em);
+			unlock_extent(tree, last_offset, end);
+			unlock_page(page);
+			put_page(page);
+			break;
+		}
+		free_extent_map(em);
+
+		if (page->index == end_index) {
+			char *userpage;
+			size_t zero_offset = isize & (PAGE_SIZE - 1);
+
+			if (zero_offset) {
+				int zeros;
+				zeros = PAGE_SIZE - zero_offset;
+				userpage = kmap_atomic(page);
+				memset(userpage + zero_offset, 0, zeros);
+				flush_dcache_page(page);
+				kunmap_atomic(userpage);
+			}
+		}
+
+		ret = bio_add_page(cb->orig_bio, page,
+				   PAGE_SIZE, 0);
+
+		if (ret == PAGE_SIZE) {
+			nr_pages++;
+			put_page(page);
+		} else {
+			unlock_extent(tree, last_offset, end);
+			unlock_page(page);
+			put_page(page);
+			break;
+		}
+next:
+		last_offset += PAGE_SIZE;
+	}
+	return 0;
+}
+
+/*
+ * for a compressed read, the bio we get passed has all the inode pages
+ * in it.  We don't actually do IO on those pages but allocate new ones
+ * to hold the compressed pages on disk.
+ *
+ * bio->bi_iter.bi_sector points to the compressed extent on disk
+ * bio->bi_io_vec points to all of the inode pages
+ *
+ * After the compressed pages are read, we copy the bytes into the
+ * bio we were passed and then call the bio end_io calls
+ */
+blk_status_t btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
+				 int mirror_num, unsigned long bio_flags)
+{
+	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
+	struct extent_io_tree *tree;
+	struct extent_map_tree *em_tree;
+	struct compressed_bio *cb;
+	unsigned long compressed_len;
+	unsigned long nr_pages;
+	unsigned long pg_index;
+	struct page *page;
+	struct block_device *bdev;
+	struct bio *comp_bio;
+	u64 cur_disk_byte = (u64)bio->bi_iter.bi_sector << 9;
+	u64 em_len;
+	u64 em_start;
+	struct extent_map *em;
+	blk_status_t ret = BLK_STS_RESOURCE;
+	int faili = 0;
+	u32 *sums;
+
+	tree = &BTRFS_I(inode)->io_tree;
+	em_tree = &BTRFS_I(inode)->extent_tree;
+
+	/* we need the actual starting offset of this extent in the file */
+	read_lock(&em_tree->lock);
+	em = lookup_extent_mapping(em_tree,
+				   page_offset(bio_first_page_all(bio)),
+				   PAGE_SIZE);
+	read_unlock(&em_tree->lock);
+	if (!em)
+		return BLK_STS_IOERR;
+
+	compressed_len = em->block_len;
+	cb = kmalloc(compressed_bio_size(fs_info, compressed_len), GFP_NOFS);
+	if (!cb)
+		goto out;
+
+	refcount_set(&cb->pending_bios, 0);
+	cb->errors = 0;
+	cb->inode = inode;
+	cb->mirror_num = mirror_num;
+	sums = &cb->sums;
+
+	cb->start = em->orig_start;
+	em_len = em->len;
+	em_start = em->start;
+
+	free_extent_map(em);
+	em = NULL;
+
+	cb->len = bio->bi_iter.bi_size;
+	cb->compressed_len = compressed_len;
+	cb->compress_type = extent_compress_type(bio_flags);
+	cb->orig_bio = bio;
+
+	nr_pages = DIV_ROUND_UP(compressed_len, PAGE_SIZE);
+	cb->compressed_pages = kcalloc(nr_pages, sizeof(struct page *),
+				       GFP_NOFS);
+	if (!cb->compressed_pages)
+		goto fail1;
+
+	bdev = fs_info->fs_devices->latest_bdev;
+
+	for (pg_index = 0; pg_index < nr_pages; pg_index++) {
+		cb->compressed_pages[pg_index] = alloc_page(GFP_NOFS |
+							      __GFP_HIGHMEM);
+		if (!cb->compressed_pages[pg_index]) {
+			faili = pg_index - 1;
+			ret = BLK_STS_RESOURCE;
+			goto fail2;
+		}
+	}
+	faili = nr_pages - 1;
+	cb->nr_pages = nr_pages;
+
+	add_ra_bio_pages(inode, em_start + em_len, cb);
+
+	/* include any pages we added in add_ra-bio_pages */
+	cb->len = bio->bi_iter.bi_size;
+
+	comp_bio = btrfs_bio_alloc(bdev, cur_disk_byte);
+	comp_bio->bi_opf = REQ_OP_READ;
+	comp_bio->bi_private = cb;
+	comp_bio->bi_end_io = end_compressed_bio_read;
+	refcount_set(&cb->pending_bios, 1);
+
+	for (pg_index = 0; pg_index < nr_pages; pg_index++) {
+		int submit = 0;
+
+		page = cb->compressed_pages[pg_index];
+		page->mapping = inode->i_mapping;
+		page->index = em_start >> PAGE_SHIFT;
+
+		if (comp_bio->bi_iter.bi_size)
+			submit = btrfs_merge_bio_hook(page, 0, PAGE_SIZE,
+					comp_bio, 0);
+
+		page->mapping = NULL;
+		if (submit || bio_add_page(comp_bio, page, PAGE_SIZE, 0) <
+		    PAGE_SIZE) {
+			ret = btrfs_bio_wq_end_io(fs_info, comp_bio,
+						  BTRFS_WQ_ENDIO_DATA);
+			BUG_ON(ret); /* -ENOMEM */
+
+			/*
+			 * inc the count before we submit the bio so
+			 * we know the end IO handler won't happen before
+			 * we inc the count.  Otherwise, the cb might get
+			 * freed before we're done setting it up
+			 */
+			refcount_inc(&cb->pending_bios);
+
+			if (!(BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)) {
+				ret = btrfs_lookup_bio_sums(inode, comp_bio,
+							    sums);
+				BUG_ON(ret); /* -ENOMEM */
+			}
+			sums += DIV_ROUND_UP(comp_bio->bi_iter.bi_size,
+					     fs_info->sectorsize);
+
+			ret = btrfs_map_bio(fs_info, comp_bio, mirror_num, 0);
+			if (ret) {
+				comp_bio->bi_status = ret;
+				bio_endio(comp_bio);
+			}
+
+			comp_bio = btrfs_bio_alloc(bdev, cur_disk_byte);
+			comp_bio->bi_opf = REQ_OP_READ;
+			comp_bio->bi_private = cb;
+			comp_bio->bi_end_io = end_compressed_bio_read;
+
+			bio_add_page(comp_bio, page, PAGE_SIZE, 0);
+		}
+		cur_disk_byte += PAGE_SIZE;
+	}
+
+	ret = btrfs_bio_wq_end_io(fs_info, comp_bio, BTRFS_WQ_ENDIO_DATA);
+	BUG_ON(ret); /* -ENOMEM */
+
+	if (!(BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)) {
+		ret = btrfs_lookup_bio_sums(inode, comp_bio, sums);
+		BUG_ON(ret); /* -ENOMEM */
+	}
+
+	ret = btrfs_map_bio(fs_info, comp_bio, mirror_num, 0);
+	if (ret) {
+		comp_bio->bi_status = ret;
+		bio_endio(comp_bio);
+	}
+
+	return 0;
+
+fail2:
+	while (faili >= 0) {
+		__free_page(cb->compressed_pages[faili]);
+		faili--;
+	}
+
+	kfree(cb->compressed_pages);
+fail1:
+	kfree(cb);
+out:
+	free_extent_map(em);
+	return ret;
+}
+
+/*
+ * Heuristic uses systematic sampling to collect data from the input data
+ * range, the logic can be tuned by the following constants:
+ *
+ * @SAMPLING_READ_SIZE - how many bytes will be copied from for each sample
+ * @SAMPLING_INTERVAL  - range from which the sampled data can be collected
+ */
+#define SAMPLING_READ_SIZE	(16)
+#define SAMPLING_INTERVAL	(256)
+
+/*
+ * For statistical analysis of the input data we consider bytes that form a
+ * Galois Field of 256 objects. Each object has an attribute count, ie. how
+ * many times the object appeared in the sample.
+ */
+#define BUCKET_SIZE		(256)
+
+/*
+ * The size of the sample is based on a statistical sampling rule of thumb.
+ * The common way is to perform sampling tests as long as the number of
+ * elements in each cell is at least 5.
+ *
+ * Instead of 5, we choose 32 to obtain more accurate results.
+ * If the data contain the maximum number of symbols, which is 256, we obtain a
+ * sample size bound by 8192.
+ *
+ * For a sample of at most 8KB of data per data range: 16 consecutive bytes
+ * from up to 512 locations.
+ */
+#define MAX_SAMPLE_SIZE		(BTRFS_MAX_UNCOMPRESSED *		\
+				 SAMPLING_READ_SIZE / SAMPLING_INTERVAL)
+
+struct bucket_item {
+	u32 count;
+};
+
+struct heuristic_ws {
+	/* Partial copy of input data */
+	u8 *sample;
+	u32 sample_size;
+	/* Buckets store counters for each byte value */
+	struct bucket_item *bucket;
+	/* Sorting buffer */
+	struct bucket_item *bucket_b;
+	struct list_head list;
+};
+
+static void free_heuristic_ws(struct list_head *ws)
+{
+	struct heuristic_ws *workspace;
+
+	workspace = list_entry(ws, struct heuristic_ws, list);
+
+	kvfree(workspace->sample);
+	kfree(workspace->bucket);
+	kfree(workspace->bucket_b);
+	kfree(workspace);
+}
+
+static struct list_head *alloc_heuristic_ws(void)
+{
+	struct heuristic_ws *ws;
+
+	ws = kzalloc(sizeof(*ws), GFP_KERNEL);
+	if (!ws)
+		return ERR_PTR(-ENOMEM);
+
+	ws->sample = kvmalloc(MAX_SAMPLE_SIZE, GFP_KERNEL);
+	if (!ws->sample)
+		goto fail;
+
+	ws->bucket = kcalloc(BUCKET_SIZE, sizeof(*ws->bucket), GFP_KERNEL);
+	if (!ws->bucket)
+		goto fail;
+
+	ws->bucket_b = kcalloc(BUCKET_SIZE, sizeof(*ws->bucket_b), GFP_KERNEL);
+	if (!ws->bucket_b)
+		goto fail;
+
+	INIT_LIST_HEAD(&ws->list);
+	return &ws->list;
+fail:
+	free_heuristic_ws(&ws->list);
+	return ERR_PTR(-ENOMEM);
+}
+
+struct workspaces_list {
+	struct list_head idle_ws;
+	spinlock_t ws_lock;
+	/* Number of free workspaces */
+	int free_ws;
+	/* Total number of allocated workspaces */
+	atomic_t total_ws;
+	/* Waiters for a free workspace */
+	wait_queue_head_t ws_wait;
+};
+
+static struct workspaces_list btrfs_comp_ws[BTRFS_COMPRESS_TYPES];
+
+static struct workspaces_list btrfs_heuristic_ws;
+
+static const struct btrfs_compress_op * const btrfs_compress_op[] = {
+	&btrfs_zlib_compress,
+	&btrfs_lzo_compress,
+	&btrfs_zstd_compress,
+};
+
+void __init btrfs_init_compress(void)
+{
+	struct list_head *workspace;
+	int i;
+
+	INIT_LIST_HEAD(&btrfs_heuristic_ws.idle_ws);
+	spin_lock_init(&btrfs_heuristic_ws.ws_lock);
+	atomic_set(&btrfs_heuristic_ws.total_ws, 0);
+	init_waitqueue_head(&btrfs_heuristic_ws.ws_wait);
+
+	workspace = alloc_heuristic_ws();
+	if (IS_ERR(workspace)) {
+		pr_warn(
+	"BTRFS: cannot preallocate heuristic workspace, will try later\n");
+	} else {
+		atomic_set(&btrfs_heuristic_ws.total_ws, 1);
+		btrfs_heuristic_ws.free_ws = 1;
+		list_add(workspace, &btrfs_heuristic_ws.idle_ws);
+	}
+
+	for (i = 0; i < BTRFS_COMPRESS_TYPES; i++) {
+		INIT_LIST_HEAD(&btrfs_comp_ws[i].idle_ws);
+		spin_lock_init(&btrfs_comp_ws[i].ws_lock);
+		atomic_set(&btrfs_comp_ws[i].total_ws, 0);
+		init_waitqueue_head(&btrfs_comp_ws[i].ws_wait);
+
+		/*
+		 * Preallocate one workspace for each compression type so
+		 * we can guarantee forward progress in the worst case
+		 */
+		workspace = btrfs_compress_op[i]->alloc_workspace();
+		if (IS_ERR(workspace)) {
+			pr_warn("BTRFS: cannot preallocate compression workspace, will try later\n");
+		} else {
+			atomic_set(&btrfs_comp_ws[i].total_ws, 1);
+			btrfs_comp_ws[i].free_ws = 1;
+			list_add(workspace, &btrfs_comp_ws[i].idle_ws);
+		}
+	}
+}
+
+/*
+ * This finds an available workspace or allocates a new one.
+ * If it's not possible to allocate a new one, waits until there's one.
+ * Preallocation makes a forward progress guarantees and we do not return
+ * errors.
+ */
+static struct list_head *__find_workspace(int type, bool heuristic)
+{
+	struct list_head *workspace;
+	int cpus = num_online_cpus();
+	int idx = type - 1;
+	unsigned nofs_flag;
+	struct list_head *idle_ws;
+	spinlock_t *ws_lock;
+	atomic_t *total_ws;
+	wait_queue_head_t *ws_wait;
+	int *free_ws;
+
+	if (heuristic) {
+		idle_ws	 = &btrfs_heuristic_ws.idle_ws;
+		ws_lock	 = &btrfs_heuristic_ws.ws_lock;
+		total_ws = &btrfs_heuristic_ws.total_ws;
+		ws_wait	 = &btrfs_heuristic_ws.ws_wait;
+		free_ws	 = &btrfs_heuristic_ws.free_ws;
+	} else {
+		idle_ws	 = &btrfs_comp_ws[idx].idle_ws;
+		ws_lock	 = &btrfs_comp_ws[idx].ws_lock;
+		total_ws = &btrfs_comp_ws[idx].total_ws;
+		ws_wait	 = &btrfs_comp_ws[idx].ws_wait;
+		free_ws	 = &btrfs_comp_ws[idx].free_ws;
+	}
+
+again:
+	spin_lock(ws_lock);
+	if (!list_empty(idle_ws)) {
+		workspace = idle_ws->next;
+		list_del(workspace);
+		(*free_ws)--;
+		spin_unlock(ws_lock);
+		return workspace;
+
+	}
+	if (atomic_read(total_ws) > cpus) {
+		DEFINE_WAIT(wait);
+
+		spin_unlock(ws_lock);
+		prepare_to_wait(ws_wait, &wait, TASK_UNINTERRUPTIBLE);
+		if (atomic_read(total_ws) > cpus && !*free_ws)
+			schedule();
+		finish_wait(ws_wait, &wait);
+		goto again;
+	}
+	atomic_inc(total_ws);
+	spin_unlock(ws_lock);
+
+	/*
+	 * Allocation helpers call vmalloc that can't use GFP_NOFS, so we have
+	 * to turn it off here because we might get called from the restricted
+	 * context of btrfs_compress_bio/btrfs_compress_pages
+	 */
+	nofs_flag = memalloc_nofs_save();
+	if (heuristic)
+		workspace = alloc_heuristic_ws();
+	else
+		workspace = btrfs_compress_op[idx]->alloc_workspace();
+	memalloc_nofs_restore(nofs_flag);
+
+	if (IS_ERR(workspace)) {
+		atomic_dec(total_ws);
+		wake_up(ws_wait);
+
+		/*
+		 * Do not return the error but go back to waiting. There's a
+		 * workspace preallocated for each type and the compression
+		 * time is bounded so we get to a workspace eventually. This
+		 * makes our caller's life easier.
+		 *
+		 * To prevent silent and low-probability deadlocks (when the
+		 * initial preallocation fails), check if there are any
+		 * workspaces at all.
+		 */
+		if (atomic_read(total_ws) == 0) {
+			static DEFINE_RATELIMIT_STATE(_rs,
+					/* once per minute */ 60 * HZ,
+					/* no burst */ 1);
+
+			if (__ratelimit(&_rs)) {
+				pr_warn("BTRFS: no compression workspaces, low memory, retrying\n");
+			}
+		}
+		goto again;
+	}
+	return workspace;
+}
+
+static struct list_head *find_workspace(int type)
+{
+	return __find_workspace(type, false);
+}
+
+/*
+ * put a workspace struct back on the list or free it if we have enough
+ * idle ones sitting around
+ */
+static void __free_workspace(int type, struct list_head *workspace,
+			     bool heuristic)
+{
+	int idx = type - 1;
+	struct list_head *idle_ws;
+	spinlock_t *ws_lock;
+	atomic_t *total_ws;
+	wait_queue_head_t *ws_wait;
+	int *free_ws;
+
+	if (heuristic) {
+		idle_ws	 = &btrfs_heuristic_ws.idle_ws;
+		ws_lock	 = &btrfs_heuristic_ws.ws_lock;
+		total_ws = &btrfs_heuristic_ws.total_ws;
+		ws_wait	 = &btrfs_heuristic_ws.ws_wait;
+		free_ws	 = &btrfs_heuristic_ws.free_ws;
+	} else {
+		idle_ws	 = &btrfs_comp_ws[idx].idle_ws;
+		ws_lock	 = &btrfs_comp_ws[idx].ws_lock;
+		total_ws = &btrfs_comp_ws[idx].total_ws;
+		ws_wait	 = &btrfs_comp_ws[idx].ws_wait;
+		free_ws	 = &btrfs_comp_ws[idx].free_ws;
+	}
+
+	spin_lock(ws_lock);
+	if (*free_ws <= num_online_cpus()) {
+		list_add(workspace, idle_ws);
+		(*free_ws)++;
+		spin_unlock(ws_lock);
+		goto wake;
+	}
+	spin_unlock(ws_lock);
+
+	if (heuristic)
+		free_heuristic_ws(workspace);
+	else
+		btrfs_compress_op[idx]->free_workspace(workspace);
+	atomic_dec(total_ws);
+wake:
+	cond_wake_up(ws_wait);
+}
+
+static void free_workspace(int type, struct list_head *ws)
+{
+	return __free_workspace(type, ws, false);
+}
+
+/*
+ * cleanup function for module exit
+ */
+static void free_workspaces(void)
+{
+	struct list_head *workspace;
+	int i;
+
+	while (!list_empty(&btrfs_heuristic_ws.idle_ws)) {
+		workspace = btrfs_heuristic_ws.idle_ws.next;
+		list_del(workspace);
+		free_heuristic_ws(workspace);
+		atomic_dec(&btrfs_heuristic_ws.total_ws);
+	}
+
+	for (i = 0; i < BTRFS_COMPRESS_TYPES; i++) {
+		while (!list_empty(&btrfs_comp_ws[i].idle_ws)) {
+			workspace = btrfs_comp_ws[i].idle_ws.next;
+			list_del(workspace);
+			btrfs_compress_op[i]->free_workspace(workspace);
+			atomic_dec(&btrfs_comp_ws[i].total_ws);
+		}
+	}
+}
+
+/*
+ * Given an address space and start and length, compress the bytes into @pages
+ * that are allocated on demand.
+ *
+ * @type_level is encoded algorithm and level, where level 0 means whatever
+ * default the algorithm chooses and is opaque here;
+ * - compression algo are 0-3
+ * - the level are bits 4-7
+ *
+ * @out_pages is an in/out parameter, holds maximum number of pages to allocate
+ * and returns number of actually allocated pages
+ *
+ * @total_in is used to return the number of bytes actually read.  It
+ * may be smaller than the input length if we had to exit early because we
+ * ran out of room in the pages array or because we cross the
+ * max_out threshold.
+ *
+ * @total_out is an in/out parameter, must be set to the input length and will
+ * be also used to return the total number of compressed bytes
+ *
+ * @max_out tells us the max number of bytes that we're allowed to
+ * stuff into pages
+ */
+int btrfs_compress_pages(unsigned int type_level, struct address_space *mapping,
+			 u64 start, struct page **pages,
+			 unsigned long *out_pages,
+			 unsigned long *total_in,
+			 unsigned long *total_out)
+{
+	struct list_head *workspace;
+	int ret;
+	int type = type_level & 0xF;
+
+	workspace = find_workspace(type);
+
+	btrfs_compress_op[type - 1]->set_level(workspace, type_level);
+	ret = btrfs_compress_op[type-1]->compress_pages(workspace, mapping,
+						      start, pages,
+						      out_pages,
+						      total_in, total_out);
+	free_workspace(type, workspace);
+	return ret;
+}
+
+/*
+ * pages_in is an array of pages with compressed data.
+ *
+ * disk_start is the starting logical offset of this array in the file
+ *
+ * orig_bio contains the pages from the file that we want to decompress into
+ *
+ * srclen is the number of bytes in pages_in
+ *
+ * The basic idea is that we have a bio that was created by readpages.
+ * The pages in the bio are for the uncompressed data, and they may not
+ * be contiguous.  They all correspond to the range of bytes covered by
+ * the compressed extent.
+ */
+static int btrfs_decompress_bio(struct compressed_bio *cb)
+{
+	struct list_head *workspace;
+	int ret;
+	int type = cb->compress_type;
+
+	workspace = find_workspace(type);
+	ret = btrfs_compress_op[type - 1]->decompress_bio(workspace, cb);
+	free_workspace(type, workspace);
+
+	return ret;
+}
+
+/*
+ * a less complex decompression routine.  Our compressed data fits in a
+ * single page, and we want to read a single page out of it.
+ * start_byte tells us the offset into the compressed data we're interested in
+ */
+int btrfs_decompress(int type, unsigned char *data_in, struct page *dest_page,
+		     unsigned long start_byte, size_t srclen, size_t destlen)
+{
+	struct list_head *workspace;
+	int ret;
+
+	workspace = find_workspace(type);
+
+	ret = btrfs_compress_op[type-1]->decompress(workspace, data_in,
+						  dest_page, start_byte,
+						  srclen, destlen);
+
+	free_workspace(type, workspace);
+	return ret;
+}
+
+void __cold btrfs_exit_compress(void)
+{
+	free_workspaces();
+}
+
+/*
+ * Copy uncompressed data from working buffer to pages.
+ *
+ * buf_start is the byte offset we're of the start of our workspace buffer.
+ *
+ * total_out is the last byte of the buffer
+ */
+int btrfs_decompress_buf2page(const char *buf, unsigned long buf_start,
+			      unsigned long total_out, u64 disk_start,
+			      struct bio *bio)
+{
+	unsigned long buf_offset;
+	unsigned long current_buf_start;
+	unsigned long start_byte;
+	unsigned long prev_start_byte;
+	unsigned long working_bytes = total_out - buf_start;
+	unsigned long bytes;
+	char *kaddr;
+	struct bio_vec bvec = bio_iter_iovec(bio, bio->bi_iter);
+
+	/*
+	 * start byte is the first byte of the page we're currently
+	 * copying into relative to the start of the compressed data.
+	 */
+	start_byte = page_offset(bvec.bv_page) - disk_start;
+
+	/* we haven't yet hit data corresponding to this page */
+	if (total_out <= start_byte)
+		return 1;
+
+	/*
+	 * the start of the data we care about is offset into
+	 * the middle of our working buffer
+	 */
+	if (total_out > start_byte && buf_start < start_byte) {
+		buf_offset = start_byte - buf_start;
+		working_bytes -= buf_offset;
+	} else {
+		buf_offset = 0;
+	}
+	current_buf_start = buf_start;
+
+	/* copy bytes from the working buffer into the pages */
+	while (working_bytes > 0) {
+		bytes = min_t(unsigned long, bvec.bv_len,
+				PAGE_SIZE - buf_offset);
+		bytes = min(bytes, working_bytes);
+
+		kaddr = kmap_atomic(bvec.bv_page);
+		memcpy(kaddr + bvec.bv_offset, buf + buf_offset, bytes);
+		kunmap_atomic(kaddr);
+		flush_dcache_page(bvec.bv_page);
+
+		buf_offset += bytes;
+		working_bytes -= bytes;
+		current_buf_start += bytes;
+
+		/* check if we need to pick another page */
+		bio_advance(bio, bytes);
+		if (!bio->bi_iter.bi_size)
+			return 0;
+		bvec = bio_iter_iovec(bio, bio->bi_iter);
+		prev_start_byte = start_byte;
+		start_byte = page_offset(bvec.bv_page) - disk_start;
+
+		/*
+		 * We need to make sure we're only adjusting
+		 * our offset into compression working buffer when
+		 * we're switching pages.  Otherwise we can incorrectly
+		 * keep copying when we were actually done.
+		 */
+		if (start_byte != prev_start_byte) {
+			/*
+			 * make sure our new page is covered by this
+			 * working buffer
+			 */
+			if (total_out <= start_byte)
+				return 1;
+
+			/*
+			 * the next page in the biovec might not be adjacent
+			 * to the last page, but it might still be found
+			 * inside this working buffer. bump our offset pointer
+			 */
+			if (total_out > start_byte &&
+			    current_buf_start < start_byte) {
+				buf_offset = start_byte - buf_start;
+				working_bytes = total_out - start_byte;
+				current_buf_start = buf_start + buf_offset;
+			}
+		}
+	}
+
+	return 1;
+}
+
+/*
+ * Shannon Entropy calculation
+ *
+ * Pure byte distribution analysis fails to determine compressiability of data.
+ * Try calculating entropy to estimate the average minimum number of bits
+ * needed to encode the sampled data.
+ *
+ * For convenience, return the percentage of needed bits, instead of amount of
+ * bits directly.
+ *
+ * @ENTROPY_LVL_ACEPTABLE - below that threshold, sample has low byte entropy
+ *			    and can be compressible with high probability
+ *
+ * @ENTROPY_LVL_HIGH - data are not compressible with high probability
+ *
+ * Use of ilog2() decreases precision, we lower the LVL to 5 to compensate.
+ */
+#define ENTROPY_LVL_ACEPTABLE		(65)
+#define ENTROPY_LVL_HIGH		(80)
+
+/*
+ * For increasead precision in shannon_entropy calculation,
+ * let's do pow(n, M) to save more digits after comma:
+ *
+ * - maximum int bit length is 64
+ * - ilog2(MAX_SAMPLE_SIZE)	-> 13
+ * - 13 * 4 = 52 < 64		-> M = 4
+ *
+ * So use pow(n, 4).
+ */
+static inline u32 ilog2_w(u64 n)
+{
+	return ilog2(n * n * n * n);
+}
+
+static u32 shannon_entropy(struct heuristic_ws *ws)
+{
+	const u32 entropy_max = 8 * ilog2_w(2);
+	u32 entropy_sum = 0;
+	u32 p, p_base, sz_base;
+	u32 i;
+
+	sz_base = ilog2_w(ws->sample_size);
+	for (i = 0; i < BUCKET_SIZE && ws->bucket[i].count > 0; i++) {
+		p = ws->bucket[i].count;
+		p_base = ilog2_w(p);
+		entropy_sum += p * (sz_base - p_base);
+	}
+
+	entropy_sum /= ws->sample_size;
+	return entropy_sum * 100 / entropy_max;
+}
+
+#define RADIX_BASE		4U
+#define COUNTERS_SIZE		(1U << RADIX_BASE)
+
+static u8 get4bits(u64 num, int shift) {
+	u8 low4bits;
+
+	num >>= shift;
+	/* Reverse order */
+	low4bits = (COUNTERS_SIZE - 1) - (num % COUNTERS_SIZE);
+	return low4bits;
+}
+
+/*
+ * Use 4 bits as radix base
+ * Use 16 u32 counters for calculating new possition in buf array
+ *
+ * @array     - array that will be sorted
+ * @array_buf - buffer array to store sorting results
+ *              must be equal in size to @array
+ * @num       - array size
+ */
+static void radix_sort(struct bucket_item *array, struct bucket_item *array_buf,
+		       int num)
+{
+	u64 max_num;
+	u64 buf_num;
+	u32 counters[COUNTERS_SIZE];
+	u32 new_addr;
+	u32 addr;
+	int bitlen;
+	int shift;
+	int i;
+
+	/*
+	 * Try avoid useless loop iterations for small numbers stored in big
+	 * counters.  Example: 48 33 4 ... in 64bit array
+	 */
+	max_num = array[0].count;
+	for (i = 1; i < num; i++) {
+		buf_num = array[i].count;
+		if (buf_num > max_num)
+			max_num = buf_num;
+	}
+
+	buf_num = ilog2(max_num);
+	bitlen = ALIGN(buf_num, RADIX_BASE * 2);
+
+	shift = 0;
+	while (shift < bitlen) {
+		memset(counters, 0, sizeof(counters));
+
+		for (i = 0; i < num; i++) {
+			buf_num = array[i].count;
+			addr = get4bits(buf_num, shift);
+			counters[addr]++;
+		}
+
+		for (i = 1; i < COUNTERS_SIZE; i++)
+			counters[i] += counters[i - 1];
+
+		for (i = num - 1; i >= 0; i--) {
+			buf_num = array[i].count;
+			addr = get4bits(buf_num, shift);
+			counters[addr]--;
+			new_addr = counters[addr];
+			array_buf[new_addr] = array[i];
+		}
+
+		shift += RADIX_BASE;
+
+		/*
+		 * Normal radix expects to move data from a temporary array, to
+		 * the main one.  But that requires some CPU time. Avoid that
+		 * by doing another sort iteration to original array instead of
+		 * memcpy()
+		 */
+		memset(counters, 0, sizeof(counters));
+
+		for (i = 0; i < num; i ++) {
+			buf_num = array_buf[i].count;
+			addr = get4bits(buf_num, shift);
+			counters[addr]++;
+		}
+
+		for (i = 1; i < COUNTERS_SIZE; i++)
+			counters[i] += counters[i - 1];
+
+		for (i = num - 1; i >= 0; i--) {
+			buf_num = array_buf[i].count;
+			addr = get4bits(buf_num, shift);
+			counters[addr]--;
+			new_addr = counters[addr];
+			array[new_addr] = array_buf[i];
+		}
+
+		shift += RADIX_BASE;
+	}
+}
+
+/*
+ * Size of the core byte set - how many bytes cover 90% of the sample
+ *
+ * There are several types of structured binary data that use nearly all byte
+ * values. The distribution can be uniform and counts in all buckets will be
+ * nearly the same (eg. encrypted data). Unlikely to be compressible.
+ *
+ * Other possibility is normal (Gaussian) distribution, where the data could
+ * be potentially compressible, but we have to take a few more steps to decide
+ * how much.
+ *
+ * @BYTE_CORE_SET_LOW  - main part of byte values repeated frequently,
+ *                       compression algo can easy fix that
+ * @BYTE_CORE_SET_HIGH - data have uniform distribution and with high
+ *                       probability is not compressible
+ */
+#define BYTE_CORE_SET_LOW		(64)
+#define BYTE_CORE_SET_HIGH		(200)
+
+static int byte_core_set_size(struct heuristic_ws *ws)
+{
+	u32 i;
+	u32 coreset_sum = 0;
+	const u32 core_set_threshold = ws->sample_size * 90 / 100;
+	struct bucket_item *bucket = ws->bucket;
+
+	/* Sort in reverse order */
+	radix_sort(ws->bucket, ws->bucket_b, BUCKET_SIZE);
+
+	for (i = 0; i < BYTE_CORE_SET_LOW; i++)
+		coreset_sum += bucket[i].count;
+
+	if (coreset_sum > core_set_threshold)
+		return i;
+
+	for (; i < BYTE_CORE_SET_HIGH && bucket[i].count > 0; i++) {
+		coreset_sum += bucket[i].count;
+		if (coreset_sum > core_set_threshold)
+			break;
+	}
+
+	return i;
+}
+
+/*
+ * Count byte values in buckets.
+ * This heuristic can detect textual data (configs, xml, json, html, etc).
+ * Because in most text-like data byte set is restricted to limited number of
+ * possible characters, and that restriction in most cases makes data easy to
+ * compress.
+ *
+ * @BYTE_SET_THRESHOLD - consider all data within this byte set size:
+ *	less - compressible
+ *	more - need additional analysis
+ */
+#define BYTE_SET_THRESHOLD		(64)
+
+static u32 byte_set_size(const struct heuristic_ws *ws)
+{
+	u32 i;
+	u32 byte_set_size = 0;
+
+	for (i = 0; i < BYTE_SET_THRESHOLD; i++) {
+		if (ws->bucket[i].count > 0)
+			byte_set_size++;
+	}
+
+	/*
+	 * Continue collecting count of byte values in buckets.  If the byte
+	 * set size is bigger then the threshold, it's pointless to continue,
+	 * the detection technique would fail for this type of data.
+	 */
+	for (; i < BUCKET_SIZE; i++) {
+		if (ws->bucket[i].count > 0) {
+			byte_set_size++;
+			if (byte_set_size > BYTE_SET_THRESHOLD)
+				return byte_set_size;
+		}
+	}
+
+	return byte_set_size;
+}
+
+static bool sample_repeated_patterns(struct heuristic_ws *ws)
+{
+	const u32 half_of_sample = ws->sample_size / 2;
+	const u8 *data = ws->sample;
+
+	return memcmp(&data[0], &data[half_of_sample], half_of_sample) == 0;
+}
+
+static void heuristic_collect_sample(struct inode *inode, u64 start, u64 end,
+				     struct heuristic_ws *ws)
+{
+	struct page *page;
+	u64 index, index_end;
+	u32 i, curr_sample_pos;
+	u8 *in_data;
+
+	/*
+	 * Compression handles the input data by chunks of 128KiB
+	 * (defined by BTRFS_MAX_UNCOMPRESSED)
+	 *
+	 * We do the same for the heuristic and loop over the whole range.
+	 *
+	 * MAX_SAMPLE_SIZE - calculated under assumption that heuristic will
+	 * process no more than BTRFS_MAX_UNCOMPRESSED at a time.
+	 */
+	if (end - start > BTRFS_MAX_UNCOMPRESSED)
+		end = start + BTRFS_MAX_UNCOMPRESSED;
+
+	index = start >> PAGE_SHIFT;
+	index_end = end >> PAGE_SHIFT;
+
+	/* Don't miss unaligned end */
+	if (!IS_ALIGNED(end, PAGE_SIZE))
+		index_end++;
+
+	curr_sample_pos = 0;
+	while (index < index_end) {
+		page = find_get_page(inode->i_mapping, index);
+		in_data = kmap(page);
+		/* Handle case where the start is not aligned to PAGE_SIZE */
+		i = start % PAGE_SIZE;
+		while (i < PAGE_SIZE - SAMPLING_READ_SIZE) {
+			/* Don't sample any garbage from the last page */
+			if (start > end - SAMPLING_READ_SIZE)
+				break;
+			memcpy(&ws->sample[curr_sample_pos], &in_data[i],
+					SAMPLING_READ_SIZE);
+			i += SAMPLING_INTERVAL;
+			start += SAMPLING_INTERVAL;
+			curr_sample_pos += SAMPLING_READ_SIZE;
+		}
+		kunmap(page);
+		put_page(page);
+
+		index++;
+	}
+
+	ws->sample_size = curr_sample_pos;
+}
+
+/*
+ * Compression heuristic.
+ *
+ * For now is's a naive and optimistic 'return true', we'll extend the logic to
+ * quickly (compared to direct compression) detect data characteristics
+ * (compressible/uncompressible) to avoid wasting CPU time on uncompressible
+ * data.
+ *
+ * The following types of analysis can be performed:
+ * - detect mostly zero data
+ * - detect data with low "byte set" size (text, etc)
+ * - detect data with low/high "core byte" set
+ *
+ * Return non-zero if the compression should be done, 0 otherwise.
+ */
+int btrfs_compress_heuristic(struct inode *inode, u64 start, u64 end)
+{
+	struct list_head *ws_list = __find_workspace(0, true);
+	struct heuristic_ws *ws;
+	u32 i;
+	u8 byte;
+	int ret = 0;
+
+	ws = list_entry(ws_list, struct heuristic_ws, list);
+
+	heuristic_collect_sample(inode, start, end, ws);
+
+	if (sample_repeated_patterns(ws)) {
+		ret = 1;
+		goto out;
+	}
+
+	memset(ws->bucket, 0, sizeof(*ws->bucket)*BUCKET_SIZE);
+
+	for (i = 0; i < ws->sample_size; i++) {
+		byte = ws->sample[i];
+		ws->bucket[byte].count++;
+	}
+
+	i = byte_set_size(ws);
+	if (i < BYTE_SET_THRESHOLD) {
+		ret = 2;
+		goto out;
+	}
+
+	i = byte_core_set_size(ws);
+	if (i <= BYTE_CORE_SET_LOW) {
+		ret = 3;
+		goto out;
+	}
+
+	if (i >= BYTE_CORE_SET_HIGH) {
+		ret = 0;
+		goto out;
+	}
+
+	i = shannon_entropy(ws);
+	if (i <= ENTROPY_LVL_ACEPTABLE) {
+		ret = 4;
+		goto out;
+	}
+
+	/*
+	 * For the levels below ENTROPY_LVL_HIGH, additional analysis would be
+	 * needed to give green light to compression.
+	 *
+	 * For now just assume that compression at that level is not worth the
+	 * resources because:
+	 *
+	 * 1. it is possible to defrag the data later
+	 *
+	 * 2. the data would turn out to be hardly compressible, eg. 150 byte
+	 * values, every bucket has counter at level ~54. The heuristic would
+	 * be confused. This can happen when data have some internal repeated
+	 * patterns like "abbacbbc...". This can be detected by analyzing
+	 * pairs of bytes, which is too costly.
+	 */
+	if (i < ENTROPY_LVL_HIGH) {
+		ret = 5;
+		goto out;
+	} else {
+		ret = 0;
+		goto out;
+	}
+
+out:
+	__free_workspace(0, ws_list, true);
+	return ret;
+}
+
+unsigned int btrfs_compress_str2level(const char *str)
+{
+	if (strncmp(str, "zlib", 4) != 0)
+		return 0;
+
+	/* Accepted form: zlib:1 up to zlib:9 and nothing left after the number */
+	if (str[4] == ':' && '1' <= str[5] && str[5] <= '9' && str[6] == 0)
+		return str[5] - '0';
+
+	return BTRFS_ZLIB_DEFAULT_LEVEL;
+}