btrfs: Add zstd support

Add zstd compression and decompression support to BtrFS. zstd at its
fastest level compresses almost as well as zlib, while offering much
faster compression and decompression, approaching lzo speeds.

I benchmarked btrfs with zstd compression against no compression, lzo
compression, and zlib compression. I benchmarked two scenarios. Copying
a set of files to btrfs, and then reading the files. Copying a tarball
to btrfs, extracting it to btrfs, and then reading the extracted files.
After every operation, I call `sync` and include the sync time.
Between every pair of operations I unmount and remount the filesystem
to avoid caching. The benchmark files can be found in the upstream
zstd source repository under
`contrib/linux-kernel/{btrfs-benchmark.sh,btrfs-extract-benchmark.sh}`
[1] [2].

I ran the benchmarks on a Ubuntu 14.04 VM with 2 cores and 4 GiB of RAM.
The VM is running on a MacBook Pro with a 3.1 GHz Intel Core i7 processor,
16 GB of RAM, and a SSD.

The first compression benchmark is copying 10 copies of the unzipped
Silesia corpus [3] into a BtrFS filesystem mounted with
`-o compress-force=Method`. The decompression benchmark times how long
it takes to `tar` all 10 copies into `/dev/null`. The compression ratio is
measured by comparing the output of `df` and `du`. See the benchmark file
[1] for details. I benchmarked multiple zstd compression levels, although
the patch uses zstd level 1.

| Method | Ratio | Compression MB/s | Decompression speed |
|---------|-------|------------------|---------------------|
| None | 0.99 | 504 | 686 |
| lzo | 1.66 | 398 | 442 |
| zlib | 2.58 | 65 | 241 |
| zstd 1 | 2.57 | 260 | 383 |
| zstd 3 | 2.71 | 174 | 408 |
| zstd 6 | 2.87 | 70 | 398 |
| zstd 9 | 2.92 | 43 | 406 |
| zstd 12 | 2.93 | 21 | 408 |
| zstd 15 | 3.01 | 11 | 354 |

The next benchmark first copies `linux-4.11.6.tar` [4] to btrfs. Then it
measures the compression ratio, extracts the tar, and deletes the tar.
Then it measures the compression ratio again, and `tar`s the extracted
files into `/dev/null`. See the benchmark file [2] for details.

| Method | Tar Ratio | Extract Ratio | Copy (s) | Extract (s)| Read (s) |
|--------|-----------|---------------|----------|------------|----------|
| None | 0.97 | 0.78 | 0.981 | 5.501 | 8.807 |
| lzo | 2.06 | 1.38 | 1.631 | 8.458 | 8.585 |
| zlib | 3.40 | 1.86 | 7.750 | 21.544 | 11.744 |
| zstd 1 | 3.57 | 1.85 | 2.579 | 11.479 | 9.389 |

[1] https://github.com/facebook/zstd/blob/dev/contrib/linux-kernel/btrfs-benchmark.sh
[2] https://github.com/facebook/zstd/blob/dev/contrib/linux-kernel/btrfs-extract-benchmark.sh
[3] http://sun.aei.polsl.pl/~sdeor/index.php?page=silesia
[4] https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.11.6.tar.xz

zstd source repository: https://github.com/facebook/zstd

Signed-off-by: Nick Terrell <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/Kconfig

···
 	select ZLIB_DEFLATE
 	select LZO_COMPRESS
 	select LZO_DECOMPRESS
+	select ZSTD_COMPRESS
+	select ZSTD_DECOMPRESS
 	select RAID6_PQ
 	select XOR_BLOCKS
 	select SRCU

fs/btrfs/Makefile

···
 	   transaction.o inode.o file.o tree-defrag.o \
 	   extent_map.o sysfs.o struct-funcs.o xattr.o ordered-data.o \
 	   extent_io.o volumes.o async-thread.o ioctl.o locking.o orphan.o \
-	   export.o tree-log.o free-space-cache.o zlib.o lzo.o \
+	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
 	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
 	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
 	   uuid-tree.o props.o hash.o free-space-tree.o

fs/btrfs/compression.c

···
 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)

fs/btrfs/compression.h

···
 	BTRFS_COMPRESS_NONE  = 0,
 	BTRFS_COMPRESS_ZLIB  = 1,
 	BTRFS_COMPRESS_LZO   = 2,
-	BTRFS_COMPRESS_TYPES = 2,
-	BTRFS_COMPRESS_LAST  = 3,
+	BTRFS_COMPRESS_ZSTD  = 3,
+	BTRFS_COMPRESS_TYPES = 3,
+	BTRFS_COMPRESS_LAST  = 4,
 };
 
 struct btrfs_compress_op {
···
 
 extern const struct btrfs_compress_op btrfs_zlib_compress;
 extern const struct btrfs_compress_op btrfs_lzo_compress;
+extern const struct btrfs_compress_op btrfs_zstd_compress;
 
 #endif

fs/btrfs/ctree.h

···
 	 BTRFS_FEATURE_INCOMPAT_MIXED_GROUPS |		\
 	 BTRFS_FEATURE_INCOMPAT_BIG_METADATA |		\
 	 BTRFS_FEATURE_INCOMPAT_COMPRESS_LZO |		\
+	 BTRFS_FEATURE_INCOMPAT_COMPRESS_ZSTD |		\
 	 BTRFS_FEATURE_INCOMPAT_RAID56 |		\
 	 BTRFS_FEATURE_INCOMPAT_EXTENDED_IREF |		\
 	 BTRFS_FEATURE_INCOMPAT_SKINNY_METADATA |	\

fs/btrfs/disk-io.c

···
 	features |= BTRFS_FEATURE_INCOMPAT_MIXED_BACKREF;
 	if (fs_info->compress_type == BTRFS_COMPRESS_LZO)
 		features |= BTRFS_FEATURE_INCOMPAT_COMPRESS_LZO;
+	else if (fs_info->compress_type == BTRFS_COMPRESS_ZSTD)
+		features |= BTRFS_FEATURE_INCOMPAT_COMPRESS_ZSTD;
 
 	if (features & BTRFS_FEATURE_INCOMPAT_SKINNY_METADATA)
 		btrfs_info(fs_info, "has skinny extents");

fs/btrfs/ioctl.c

···
 
 		if (fs_info->compress_type == BTRFS_COMPRESS_LZO)
 			comp = "lzo";
-		else
+		else if (fs_info->compress_type == BTRFS_COMPRESS_ZLIB)
 			comp = "zlib";
+		else
+			comp = "zstd";
 		ret = btrfs_set_prop(inode, "btrfs.compression",
 				     comp, strlen(comp), 0);
 		if (ret)
···
 
 	if (range->compress_type == BTRFS_COMPRESS_LZO) {
 		btrfs_set_fs_incompat(fs_info, COMPRESS_LZO);
+	} else if (range->compress_type == BTRFS_COMPRESS_ZSTD) {
+		btrfs_set_fs_incompat(fs_info, COMPRESS_ZSTD);
 	}
 
 	ret = defrag_count;

fs/btrfs/props.c

···
 		return 0;
 	else if (!strncmp("zlib", value, len))
 		return 0;
+	else if (!strncmp("zstd", value, len))
+		return 0;
 
 	return -EINVAL;
 }
···
 		type = BTRFS_COMPRESS_LZO;
 	else if (!strncmp("zlib", value, len))
 		type = BTRFS_COMPRESS_ZLIB;
+	else if (!strncmp("zstd", value, len))
+		type = BTRFS_COMPRESS_ZSTD;
 	else
 		return -EINVAL;
 
···
 		return "zlib";
 	case BTRFS_COMPRESS_LZO:
 		return "lzo";
+	case BTRFS_COMPRESS_ZSTD:
+		return "zstd";
 	}
 
 	return NULL;

fs/btrfs/super.c

···
 				btrfs_clear_opt(info->mount_opt, NODATASUM);
 				btrfs_set_fs_incompat(info, COMPRESS_LZO);
 				no_compress = 0;
+			} else if (strcmp(args[0].from, "zstd") == 0) {
+				compress_type = "zstd";
+				info->compress_type = BTRFS_COMPRESS_ZSTD;
+				btrfs_set_opt(info->mount_opt, COMPRESS);
+				btrfs_clear_opt(info->mount_opt, NODATACOW);
+				btrfs_clear_opt(info->mount_opt, NODATASUM);
+				btrfs_set_fs_incompat(info, COMPRESS_ZSTD);
+				no_compress = 0;
 			} else if (strncmp(args[0].from, "no", 2) == 0) {
 				compress_type = "no";
 				btrfs_clear_opt(info->mount_opt, COMPRESS);
···
 	if (btrfs_test_opt(info, COMPRESS)) {
 		if (info->compress_type == BTRFS_COMPRESS_ZLIB)
 			compress_type = "zlib";
-		else
+		else if (info->compress_type == BTRFS_COMPRESS_LZO)
 			compress_type = "lzo";
+		else
+			compress_type = "zstd";
 		if (btrfs_test_opt(info, FORCE_COMPRESS))
 			seq_printf(seq, ",compress-force=%s", compress_type);
 		else

fs/btrfs/sysfs.c

···
 BTRFS_FEAT_ATTR_INCOMPAT(default_subvol, DEFAULT_SUBVOL);
 BTRFS_FEAT_ATTR_INCOMPAT(mixed_groups, MIXED_GROUPS);
 BTRFS_FEAT_ATTR_INCOMPAT(compress_lzo, COMPRESS_LZO);
+BTRFS_FEAT_ATTR_INCOMPAT(compress_zstd, COMPRESS_ZSTD);
 BTRFS_FEAT_ATTR_INCOMPAT(big_metadata, BIG_METADATA);
 BTRFS_FEAT_ATTR_INCOMPAT(extended_iref, EXTENDED_IREF);
 BTRFS_FEAT_ATTR_INCOMPAT(raid56, RAID56);
···
 	BTRFS_FEAT_ATTR_PTR(default_subvol),
 	BTRFS_FEAT_ATTR_PTR(mixed_groups),
 	BTRFS_FEAT_ATTR_PTR(compress_lzo),
+	BTRFS_FEAT_ATTR_PTR(compress_zstd),
 	BTRFS_FEAT_ATTR_PTR(big_metadata),
 	BTRFS_FEAT_ATTR_PTR(extended_iref),
 	BTRFS_FEAT_ATTR_PTR(raid56),

fs/btrfs/zstd.c

···
+/*
+ * Copyright (c) 2016-present, Facebook, Inc.
+ * 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.
+ */
+#include <linux/bio.h>
+#include <linux/err.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/pagemap.h>
+#include <linux/refcount.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/zstd.h>
+#include "compression.h"
+
+#define ZSTD_BTRFS_MAX_WINDOWLOG 17
+#define ZSTD_BTRFS_MAX_INPUT (1 << ZSTD_BTRFS_MAX_WINDOWLOG)
+#define ZSTD_BTRFS_DEFAULT_LEVEL 3
+
+static ZSTD_parameters zstd_get_btrfs_parameters(size_t src_len)
+{
+	ZSTD_parameters params = ZSTD_getParams(ZSTD_BTRFS_DEFAULT_LEVEL,
+						src_len, 0);
+
+	if (params.cParams.windowLog > ZSTD_BTRFS_MAX_WINDOWLOG)
+		params.cParams.windowLog = ZSTD_BTRFS_MAX_WINDOWLOG;
+	WARN_ON(src_len > ZSTD_BTRFS_MAX_INPUT);
+	return params;
+}
+
+struct workspace {
+	void *mem;
+	size_t size;
+	char *buf;
+	struct list_head list;
+};
+
+static void zstd_free_workspace(struct list_head *ws)
+{
+	struct workspace *workspace = list_entry(ws, struct workspace, list);
+
+	kvfree(workspace->mem);
+	kfree(workspace->buf);
+	kfree(workspace);
+}
+
+static struct list_head *zstd_alloc_workspace(void)
+{
+	ZSTD_parameters params =
+			zstd_get_btrfs_parameters(ZSTD_BTRFS_MAX_INPUT);
+	struct workspace *workspace;
+
+	workspace = kzalloc(sizeof(*workspace), GFP_KERNEL);
+	if (!workspace)
+		return ERR_PTR(-ENOMEM);
+
+	workspace->size = max_t(size_t,
+			ZSTD_CStreamWorkspaceBound(params.cParams),
+			ZSTD_DStreamWorkspaceBound(ZSTD_BTRFS_MAX_INPUT));
+	workspace->mem = kvmalloc(workspace->size, GFP_KERNEL);
+	workspace->buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
+	if (!workspace->mem || !workspace->buf)
+		goto fail;
+
+	INIT_LIST_HEAD(&workspace->list);
+
+	return &workspace->list;
+fail:
+	zstd_free_workspace(&workspace->list);
+	return ERR_PTR(-ENOMEM);
+}
+
+static int zstd_compress_pages(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)
+{
+	struct workspace *workspace = list_entry(ws, struct workspace, list);
+	ZSTD_CStream *stream;
+	int ret = 0;
+	int nr_pages = 0;
+	struct page *in_page = NULL;  /* The current page to read */
+	struct page *out_page = NULL; /* The current page to write to */
+	ZSTD_inBuffer in_buf = { NULL, 0, 0 };
+	ZSTD_outBuffer out_buf = { NULL, 0, 0 };
+	unsigned long tot_in = 0;
+	unsigned long tot_out = 0;
+	unsigned long len = *total_out;
+	const unsigned long nr_dest_pages = *out_pages;
+	unsigned long max_out = nr_dest_pages * PAGE_SIZE;
+	ZSTD_parameters params = zstd_get_btrfs_parameters(len);
+
+	*out_pages = 0;
+	*total_out = 0;
+	*total_in = 0;
+
+	/* Initialize the stream */
+	stream = ZSTD_initCStream(params, len, workspace->mem,
+			workspace->size);
+	if (!stream) {
+		pr_warn("BTRFS: ZSTD_initCStream failed\n");
+		ret = -EIO;
+		goto out;
+	}
+
+	/* map in the first page of input data */
+	in_page = find_get_page(mapping, start >> PAGE_SHIFT);
+	in_buf.src = kmap(in_page);
+	in_buf.pos = 0;
+	in_buf.size = min_t(size_t, len, PAGE_SIZE);
+
+
+	/* Allocate and map in the output buffer */
+	out_page = alloc_page(GFP_NOFS | __GFP_HIGHMEM);
+	if (out_page == NULL) {
+		ret = -ENOMEM;
+		goto out;
+	}
+	pages[nr_pages++] = out_page;
+	out_buf.dst = kmap(out_page);
+	out_buf.pos = 0;
+	out_buf.size = min_t(size_t, max_out, PAGE_SIZE);
+
+	while (1) {
+		size_t ret2;
+
+		ret2 = ZSTD_compressStream(stream, &out_buf, &in_buf);
+		if (ZSTD_isError(ret2)) {
+			pr_debug("BTRFS: ZSTD_compressStream returned %d\n",
+					ZSTD_getErrorCode(ret2));
+			ret = -EIO;
+			goto out;
+		}
+
+		/* Check to see if we are making it bigger */
+		if (tot_in + in_buf.pos > 8192 &&
+				tot_in + in_buf.pos <
+				tot_out + out_buf.pos) {
+			ret = -E2BIG;
+			goto out;
+		}
+
+		/* We've reached the end of our output range */
+		if (out_buf.pos >= max_out) {
+			tot_out += out_buf.pos;
+			ret = -E2BIG;
+			goto out;
+		}
+
+		/* Check if we need more output space */
+		if (out_buf.pos == out_buf.size) {
+			tot_out += PAGE_SIZE;
+			max_out -= PAGE_SIZE;
+			kunmap(out_page);
+			if (nr_pages == nr_dest_pages) {
+				out_page = NULL;
+				ret = -E2BIG;
+				goto out;
+			}
+			out_page = alloc_page(GFP_NOFS | __GFP_HIGHMEM);
+			if (out_page == NULL) {
+				ret = -ENOMEM;
+				goto out;
+			}
+			pages[nr_pages++] = out_page;
+			out_buf.dst = kmap(out_page);
+			out_buf.pos = 0;
+			out_buf.size = min_t(size_t, max_out, PAGE_SIZE);
+		}
+
+		/* We've reached the end of the input */
+		if (in_buf.pos >= len) {
+			tot_in += in_buf.pos;
+			break;
+		}
+
+		/* Check if we need more input */
+		if (in_buf.pos == in_buf.size) {
+			tot_in += PAGE_SIZE;
+			kunmap(in_page);
+			put_page(in_page);
+
+			start += PAGE_SIZE;
+			len -= PAGE_SIZE;
+			in_page = find_get_page(mapping, start >> PAGE_SHIFT);
+			in_buf.src = kmap(in_page);
+			in_buf.pos = 0;
+			in_buf.size = min_t(size_t, len, PAGE_SIZE);
+		}
+	}
+	while (1) {
+		size_t ret2;
+
+		ret2 = ZSTD_endStream(stream, &out_buf);
+		if (ZSTD_isError(ret2)) {
+			pr_debug("BTRFS: ZSTD_endStream returned %d\n",
+					ZSTD_getErrorCode(ret2));
+			ret = -EIO;
+			goto out;
+		}
+		if (ret2 == 0) {
+			tot_out += out_buf.pos;
+			break;
+		}
+		if (out_buf.pos >= max_out) {
+			tot_out += out_buf.pos;
+			ret = -E2BIG;
+			goto out;
+		}
+
+		tot_out += PAGE_SIZE;
+		max_out -= PAGE_SIZE;
+		kunmap(out_page);
+		if (nr_pages == nr_dest_pages) {
+			out_page = NULL;
+			ret = -E2BIG;
+			goto out;
+		}
+		out_page = alloc_page(GFP_NOFS | __GFP_HIGHMEM);
+		if (out_page == NULL) {
+			ret = -ENOMEM;
+			goto out;
+		}
+		pages[nr_pages++] = out_page;
+		out_buf.dst = kmap(out_page);
+		out_buf.pos = 0;
+		out_buf.size = min_t(size_t, max_out, PAGE_SIZE);
+	}
+
+	if (tot_out >= tot_in) {
+		ret = -E2BIG;
+		goto out;
+	}
+
+	ret = 0;
+	*total_in = tot_in;
+	*total_out = tot_out;
+out:
+	*out_pages = nr_pages;
+	/* Cleanup */
+	if (in_page) {
+		kunmap(in_page);
+		put_page(in_page);
+	}
+	if (out_page)
+		kunmap(out_page);
+	return ret;
+}
+
+static int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
+{
+	struct workspace *workspace = list_entry(ws, struct workspace, list);
+	struct page **pages_in = cb->compressed_pages;
+	u64 disk_start = cb->start;
+	struct bio *orig_bio = cb->orig_bio;
+	size_t srclen = cb->compressed_len;
+	ZSTD_DStream *stream;
+	int ret = 0;
+	unsigned long page_in_index = 0;
+	unsigned long total_pages_in = DIV_ROUND_UP(srclen, PAGE_SIZE);
+	unsigned long buf_start;
+	unsigned long total_out = 0;
+	ZSTD_inBuffer in_buf = { NULL, 0, 0 };
+	ZSTD_outBuffer out_buf = { NULL, 0, 0 };
+
+	stream = ZSTD_initDStream(
+			ZSTD_BTRFS_MAX_INPUT, workspace->mem, workspace->size);
+	if (!stream) {
+		pr_debug("BTRFS: ZSTD_initDStream failed\n");
+		ret = -EIO;
+		goto done;
+	}
+
+	in_buf.src = kmap(pages_in[page_in_index]);
+	in_buf.pos = 0;
+	in_buf.size = min_t(size_t, srclen, PAGE_SIZE);
+
+	out_buf.dst = workspace->buf;
+	out_buf.pos = 0;
+	out_buf.size = PAGE_SIZE;
+
+	while (1) {
+		size_t ret2;
+
+		ret2 = ZSTD_decompressStream(stream, &out_buf, &in_buf);
+		if (ZSTD_isError(ret2)) {
+			pr_debug("BTRFS: ZSTD_decompressStream returned %d\n",
+					ZSTD_getErrorCode(ret2));
+			ret = -EIO;
+			goto done;
+		}
+		buf_start = total_out;
+		total_out += out_buf.pos;
+		out_buf.pos = 0;
+
+		ret = btrfs_decompress_buf2page(out_buf.dst, buf_start,
+				total_out, disk_start, orig_bio);
+		if (ret == 0)
+			break;
+
+		if (in_buf.pos >= srclen)
+			break;
+
+		/* Check if we've hit the end of a frame */
+		if (ret2 == 0)
+			break;
+
+		if (in_buf.pos == in_buf.size) {
+			kunmap(pages_in[page_in_index++]);
+			if (page_in_index >= total_pages_in) {
+				in_buf.src = NULL;
+				ret = -EIO;
+				goto done;
+			}
+			srclen -= PAGE_SIZE;
+			in_buf.src = kmap(pages_in[page_in_index]);
+			in_buf.pos = 0;
+			in_buf.size = min_t(size_t, srclen, PAGE_SIZE);
+		}
+	}
+	ret = 0;
+	zero_fill_bio(orig_bio);
+done:
+	if (in_buf.src)
+		kunmap(pages_in[page_in_index]);
+	return ret;
+}
+
+static int zstd_decompress(struct list_head *ws, unsigned char *data_in,
+		struct page *dest_page,
+		unsigned long start_byte,
+		size_t srclen, size_t destlen)
+{
+	struct workspace *workspace = list_entry(ws, struct workspace, list);
+	ZSTD_DStream *stream;
+	int ret = 0;
+	size_t ret2;
+	ZSTD_inBuffer in_buf = { NULL, 0, 0 };
+	ZSTD_outBuffer out_buf = { NULL, 0, 0 };
+	unsigned long total_out = 0;
+	unsigned long pg_offset = 0;
+	char *kaddr;
+
+	stream = ZSTD_initDStream(
+			ZSTD_BTRFS_MAX_INPUT, workspace->mem, workspace->size);
+	if (!stream) {
+		pr_warn("BTRFS: ZSTD_initDStream failed\n");
+		ret = -EIO;
+		goto finish;
+	}
+
+	destlen = min_t(size_t, destlen, PAGE_SIZE);
+
+	in_buf.src = data_in;
+	in_buf.pos = 0;
+	in_buf.size = srclen;
+
+	out_buf.dst = workspace->buf;
+	out_buf.pos = 0;
+	out_buf.size = PAGE_SIZE;
+
+	ret2 = 1;
+	while (pg_offset < destlen && in_buf.pos < in_buf.size) {
+		unsigned long buf_start;
+		unsigned long buf_offset;
+		unsigned long bytes;
+
+		/* Check if the frame is over and we still need more input */
+		if (ret2 == 0) {
+			pr_debug("BTRFS: ZSTD_decompressStream ended early\n");
+			ret = -EIO;
+			goto finish;
+		}
+		ret2 = ZSTD_decompressStream(stream, &out_buf, &in_buf);
+		if (ZSTD_isError(ret2)) {
+			pr_debug("BTRFS: ZSTD_decompressStream returned %d\n",
+					ZSTD_getErrorCode(ret2));
+			ret = -EIO;
+			goto finish;
+		}
+
+		buf_start = total_out;
+		total_out += out_buf.pos;
+		out_buf.pos = 0;
+
+		if (total_out <= start_byte)
+			continue;
+
+		if (total_out > start_byte && buf_start < start_byte)
+			buf_offset = start_byte - buf_start;
+		else
+			buf_offset = 0;
+
+		bytes = min_t(unsigned long, destlen - pg_offset,
+				out_buf.size - buf_offset);
+
+		kaddr = kmap_atomic(dest_page);
+		memcpy(kaddr + pg_offset, out_buf.dst + buf_offset, bytes);
+		kunmap_atomic(kaddr);
+
+		pg_offset += bytes;
+	}
+	ret = 0;
+finish:
+	if (pg_offset < destlen) {
+		kaddr = kmap_atomic(dest_page);
+		memset(kaddr + pg_offset, 0, destlen - pg_offset);
+		kunmap_atomic(kaddr);
+	}
+	return ret;
+}
+
+const struct btrfs_compress_op btrfs_zstd_compress = {
+	.alloc_workspace = zstd_alloc_workspace,
+	.free_workspace = zstd_free_workspace,
+	.compress_pages = zstd_compress_pages,
+	.decompress_bio = zstd_decompress_bio,
+	.decompress = zstd_decompress,
+};

include/uapi/linux/btrfs.h

···
 #define BTRFS_FEATURE_INCOMPAT_DEFAULT_SUBVOL	(1ULL << 1)
 #define BTRFS_FEATURE_INCOMPAT_MIXED_GROUPS	(1ULL << 2)
 #define BTRFS_FEATURE_INCOMPAT_COMPRESS_LZO	(1ULL << 3)
-/*
- * some patches floated around with a second compression method
- * lets save that incompat here for when they do get in
- * Note we don't actually support it, we're just reserving the
- * number
- */
-#define BTRFS_FEATURE_INCOMPAT_COMPRESS_LZOv2	(1ULL << 4)
+#define BTRFS_FEATURE_INCOMPAT_COMPRESS_ZSTD	(1ULL << 4)
 
 /*
  * older kernels tried to do bigger metadata blocks, but the