Date:	Mon, 07 Oct 2013 04:41:20 +0100
From:	Phillip Lougher <phillip@...gher.demon.co.uk>
To:	Minchan Kim <minchan@...nel.org>
CC:	linux-kernel@...r.kernel.org, ch0.han@....com, gunho.lee@....com
Subject: re: [RFC,4/5] squashfs: support multiple decompress stream buffer

Hi,

This a partial review, based on the stuff I've managed to review
so far!

1. This is a substantial performance improvement, which is great
    stuff!

    But like the "squashfs: remove cache for normal data page" patch
    it needs to be optional, with the previous behaviour retained as
    default.  Again, without wanting to sound like a broken (vinyl)
    record, this is because as maintainer I get to worry about breaking
    things for existing users of Squashfs when they upgrade their kernel.

    I know from consulting experience, many users of Squashfs are "on the
    edge" of memory and CPU performance, and are using Squashfs to squeeze
    a bit more performance out of a maxed out system.

    In these cases, changing Squashfs so it uses more memory and more
    CPU than previously (and in this patch a lot more memory and CPU as
    it will try and kick off multiple decompressors per core) is a bit
    like robbing Peter to pay Paul, Squashfs may take CPU and memory
    that are needed elsewhere, and used to be available.

    So, basically, users need to be able to explicitly select this.

2. The patch breaks the decompressor interface.  Compressor option
    parsing is implemented in the decompressor init() function, which
    means everytime a new decompressor is dynamically instantiated, we
    need to read and parse the compression options again and again.  This
    is an unnecessary performance degradation.

    Compressor option parsing and reading should be split out of init()
    and into a separate function.

    Compression option parsing and reading is quite obscure, it is a
    late addition to the filesystem format, and had to be squeezed into
    the existing format.  This means it can be difficult to get it right
    as the specification exists only in my head.

    I'll help you here.

Specific comments follow in the patch.

Phillip



>Now squashfs have used for only one stream buffer for decompression
>so it hurts concurrent read performance due to locking lock of getting
>stream buffer.
>
>When file system mount, the number of stream buffer is started from
>num_online_cpus() and grows up to num_online_cpus() * 2M / block_size * 2.
>The rationale is MM does readahead chunk into 2M unit to prevent too much
>memory pin and while one request is waitting, we should request another
>chunk. That's why I multiply by 2.
>
>If it reveals too much memory problem, we can add shrinker routine.
>
>I did test following as
>
>Two 1G file dd read
>
>dd if=test/test1.dat of=/dev/null &
>dd if=test/test2.dat of=/dev/null &
>
>old : 60sec -> new : 30 sec
>
>Signed-off-by: Minchan Kim <minchan@...nel.org>
>
>---
>fs/squashfs/block.c          |    9 ++--
> fs/squashfs/decompressor.c   |  105 ++++++++++++++++++++++++++++++++++++++----
> fs/squashfs/decompressor.h   |   27 +++++------
> fs/squashfs/lzo_wrapper.c    |   12 ++---
> fs/squashfs/squashfs.h       |    3 +-
> fs/squashfs/squashfs_fs_sb.h |    7 ++-
> fs/squashfs/super.c          |   40 ++++++++++++----
> fs/squashfs/xz_wrapper.c     |   20 ++++----
> fs/squashfs/zlib_wrapper.c   |   12 ++---
> 9 files changed, 168 insertions(+), 67 deletions(-)
>
>diff --git a/fs/squashfs/block.c b/fs/squashfs/block.c
>index f33c6ef..d41bac8 100644
>--- a/fs/squashfs/block.c
>+++ b/fs/squashfs/block.c
>@@ -78,14 +78,14 @@ static struct buffer_head *get_block_length(struct super_block *sb,
>
>
>
>-int squashfs_decompress_block(struct squashfs_sb_info *msblk, int compressed,
>+int squashfs_decompress_block(struct super_block *sb, int compressed,
> 		void **buffer, struct buffer_head **bh, int nr_bh,
> 		int offset, int length, int srclength, int pages)
> {
> 	int k = 0;
>
> 	if (compressed) {
>-		length = squashfs_decompress(msblk, buffer, bh, nr_bh,
>+		length = squashfs_decompress(sb, buffer, bh, nr_bh,
> 				offset, length, srclength, pages);
> 		if (length < 0)
> 			goto out;
>@@ -93,6 +93,7 @@ int squashfs_decompress_block(struct squashfs_sb_info *msblk, int compressed,
> 		/*
> 		 * Block is uncompressed.
> 		 */
>+		struct squashfs_sb_info *msblk = sb->s_fs_info;
> 		int bytes, in, avail, pg_offset = 0, page = 0;
>
> 		for (bytes = length; k < nr_bh; k++) {
>@@ -262,8 +263,8 @@ int squashfs_read_metablock(struct super_block *sb, void **buffer, u64 index,
> 	}
> 	ll_rw_block(READ, b - 1, bh + 1);
>
>-	length = squashfs_decompress_block(msblk, compressed, buffer, bh, b,
>-				offset, length, srclength, pages);
>+	length = squashfs_decompress_block(sb, compressed, buffer, bh,
>+				b, offset, length, srclength, pages);
> 	if (length < 0)
> 		goto read_failure;
>
>diff --git a/fs/squashfs/decompressor.c b/fs/squashfs/decompressor.c
>index e47453e..ed35b32 100644
>--- a/fs/squashfs/decompressor.c
>+++ b/fs/squashfs/decompressor.c
>@@ -25,6 +25,8 @@
> #include <linux/mutex.h>
> #include <linux/slab.h>
> #include <linux/buffer_head.h>
>+#include <linux/sched.h>
>+#include <linux/wait.h>
>
> #include "squashfs_fs.h"
> #include "squashfs_fs_sb.h"
>@@ -70,6 +72,80 @@ static const struct squashfs_decompressor *decompressor[] = {
> 	&squashfs_unknown_comp_ops
> };
>
>+void squashfs_decompressor_free(struct squashfs_sb_info *msblk,
>+	struct squashfs_decomp_strm *stream)
>+{
>+	if (msblk->decompressor)
>+		msblk->decompressor->free(stream->strm);
>+	kfree(stream);
>+}
>+
>+static void *squashfs_get_decomp_strm(struct squashfs_sb_info *msblk)
>+{
>+	struct squashfs_decomp_strm *strm = NULL;
>+	mutex_lock(&msblk->comp_strm_mutex);
>+	if (!list_empty(&msblk->strm_list)) {
>+		strm = list_entry(msblk->strm_list.next,
>+				struct squashfs_decomp_strm, list);
>+		list_del(&strm->list);
>+		msblk->nr_avail_decomp--;
>+		WARN_ON(msblk->nr_avail_decomp < 0);
>+	}
>+	mutex_unlock(&msblk->comp_strm_mutex);
>+	return strm;
>+}
>+
>+static bool full_decomp_strm(struct squashfs_sb_info *msblk)
>+{
>+	/* MM do readahread 2M unit */
>+	int blocks = 2 * 1024 * 1024 / msblk->block_size;
>+	return msblk->nr_avail_decomp > (num_online_cpus() * blocks * 2);
>+}
>+
>+static void squashfs_put_decomp_strm(struct squashfs_sb_info *msblk,
>+					struct squashfs_decomp_strm *strm)
>+{
>+	mutex_lock(&msblk->comp_strm_mutex);
>+	if (full_decomp_strm(msblk)) {
>+		mutex_unlock(&msblk->comp_strm_mutex);
>+		squashfs_decompressor_free(msblk, strm);
>+		return;
>+	}
>+
>+	list_add(&strm->list, &msblk->strm_list);
>+	msblk->nr_avail_decomp++;
>+	mutex_unlock(&msblk->comp_strm_mutex);
>+	wake_up(&msblk->decomp_wait_queue);
>+}
>+
>+int squashfs_decompress(struct super_block *sb, void **buffer,
>+			struct buffer_head **bh, int b, int offset, int length,
>+			int srclength, int pages)
>+{
>+	int ret;
>+	struct squashfs_decomp_strm *strm;
>+	struct squashfs_sb_info *msblk = sb->s_fs_info;
>+	while (1) {
>+		strm = squashfs_get_decomp_strm(msblk);
>+		if (strm)
>+			break;
>+
>+		if (!full_decomp_strm(msblk)) {
>+			strm = squashfs_decompressor_init(sb);

here you call squashfs_decompressor_dynamically to instantiate a new
decompressor,  this needs to read and parse the compression options again.

>+			if (strm)
>+				break;
>+		}
>+
>+		wait_event(msblk->decomp_wait_queue, msblk->nr_avail_decomp);
>+		continue;
>+	}
>+
>+	ret = msblk->decompressor->decompress(msblk, strm->strm, buffer, bh,
>+		b, offset, length, srclength, pages);
>+
>+	squashfs_put_decomp_strm(msblk, strm);
>+	return ret;
>+}
>
> const struct squashfs_decompressor *squashfs_lookup_decompressor(int id)
> {
>@@ -82,35 +158,48 @@ const struct squashfs_decompressor *squashfs_lookup_decompressor(int id)
> 	return decompressor[i];
> }
>
>-
>-void *squashfs_decompressor_init(struct super_block *sb, unsigned short flags)
>+struct squashfs_decomp_strm *squashfs_decompressor_init(struct super_block *sb)
> {
> 	struct squashfs_sb_info *msblk = sb->s_fs_info;
>+	struct squashfs_decomp_strm *decomp_strm = NULL;
> 	void *strm, *buffer = NULL;
> 	int length = 0;
>
>+	decomp_strm = kmalloc(sizeof(struct squashfs_decomp_strm), GFP_KERNEL);
>+	if (!decomp_strm)
>+		return ERR_PTR(-ENOMEM);
> 	/*
> 	 * Read decompressor specific options from file system if present
> 	 */
>-	if (SQUASHFS_COMP_OPTS(flags)) {
>+	if (SQUASHFS_COMP_OPTS(msblk->flags)) {
> 		buffer = kmalloc(PAGE_CACHE_SIZE, GFP_KERNEL);
>-		if (buffer == NULL)
>-			return ERR_PTR(-ENOMEM);
>+		if (buffer == NULL) {
>+			decomp_strm = ERR_PTR(-ENOMEM);
>+			goto finished;
>+		}
>
> 		length = squashfs_read_metablock(sb, &buffer,
> 			sizeof(struct squashfs_super_block), 0, NULL,
> 			PAGE_CACHE_SIZE, 1);
>

This reads the compressor options each decompressor init().  This should
only be done once at mount time.

> 		if (length < 0) {
>-			strm = ERR_PTR(length);
>+			decomp_strm = ERR_PTR(length);
> 			goto finished;
> 		}
> 	}
>


> 	strm = msblk->decompressor->init(msblk, buffer, length);
>+	if (IS_ERR(strm)) {
>+		decomp_strm = strm;
>+		goto finished;
>+	}
>
>-finished:
>+	decomp_strm->strm = strm;
> 	kfree(buffer);
>+	return decomp_strm;
>
>-	return strm;
>+finished:
>+	kfree(decomp_strm);
>+	kfree(buffer);
>+	return decomp_strm;
> }
>diff --git a/fs/squashfs/decompressor.h b/fs/squashfs/decompressor.h
>index 330073e..207c810 100644
>--- a/fs/squashfs/decompressor.h
>+++ b/fs/squashfs/decompressor.h
>@@ -26,27 +26,24 @@
> struct squashfs_decompressor {
> 	void	*(*init)(struct squashfs_sb_info *, void *, int);
> 	void	(*free)(void *);
>-	int	(*decompress)(struct squashfs_sb_info *, void **,
>+	int	(*decompress)(struct squashfs_sb_info *, void*, void **,
> 		struct buffer_head **, int, int, int, int, int);
> 	int	id;
> 	char	*name;
> 	int	supported;
> };
>
>-static inline void squashfs_decompressor_free(struct squashfs_sb_info *msblk,
>-	void *s)
>-{
>-	if (msblk->decompressor)
>-		msblk->decompressor->free(s);
>-}
>-
>-static inline int squashfs_decompress(struct squashfs_sb_info *msblk,
>-	void **buffer, struct buffer_head **bh, int b, int offset, int length,
>-	int srclength, int pages)
>-{
>-	return msblk->decompressor->decompress(msblk, buffer, bh, b, offset,
>-		length, srclength, pages);
>-}
>+struct squashfs_decomp_strm {
>+	void *strm;
>+	struct list_head list;
>+};
>+
>+int squashfs_decompress(struct super_block *sb, void **buffer,
>+			struct buffer_head **bh, int b, int offset, int length,
>+			int srclength, int pages);
>+
>+void squashfs_decompressor_free(struct squashfs_sb_info *msblk,
>+	struct squashfs_decomp_strm *stream);
>
> #ifdef CONFIG_SQUASHFS_XZ
> extern const struct squashfs_decompressor squashfs_xz_comp_ops;
>diff --git a/fs/squashfs/lzo_wrapper.c b/fs/squashfs/lzo_wrapper.c
>index 00f4dfc..e1bf135 100644
>--- a/fs/squashfs/lzo_wrapper.c
>+++ b/fs/squashfs/lzo_wrapper.c
>@@ -74,17 +74,15 @@ static void lzo_free(void *strm)
> }
>
>
>-static int lzo_uncompress(struct squashfs_sb_info *msblk, void **buffer,
>-	struct buffer_head **bh, int b, int offset, int length, int srclength,
>-	int pages)
>+static int lzo_uncompress(struct squashfs_sb_info *msblk, void *strm,
>+		void **buffer, struct buffer_head **bh, int b, int offset,
>+		int length, int srclength, int pages)
> {
>-	struct squashfs_lzo *stream = msblk->stream;
>+	struct squashfs_lzo *stream = strm;
> 	void *buff = stream->input;
> 	int avail, i, bytes = length, res;
> 	size_t out_len = srclength;
>
>-	mutex_lock(&msblk->read_data_mutex);
>-
> 	for (i = 0; i < b; i++) {
> 		wait_on_buffer(bh[i]);
> 		if (!buffer_uptodate(bh[i]))
>@@ -111,7 +109,6 @@ static int lzo_uncompress(struct squashfs_sb_info *msblk, void **buffer,
> 		bytes -= avail;
> 	}
>
>-	mutex_unlock(&msblk->read_data_mutex);
> 	return res;
>
> block_release:
>@@ -119,7 +116,6 @@ block_release:
> 		put_bh(bh[i]);
>
> failed:
>-	mutex_unlock(&msblk->read_data_mutex);
>
> 	ERROR("lzo decompression failed, data probably corrupt\n");
> 	return -EIO;
>diff --git a/fs/squashfs/squashfs.h b/fs/squashfs/squashfs.h
>index 405baed..4bb1f90 100644
>--- a/fs/squashfs/squashfs.h
>+++ b/fs/squashfs/squashfs.h
>@@ -52,7 +52,8 @@ extern void *squashfs_read_table(struct super_block *, u64, int);
>
> /* decompressor.c */
> extern const struct squashfs_decompressor *squashfs_lookup_decompressor(int);
>-extern void *squashfs_decompressor_init(struct super_block *, unsigned short);
>+extern struct squashfs_decomp_strm *squashfs_decompressor_init(
>+				struct super_block *);
>
> /* export.c */
> extern __le64 *squashfs_read_inode_lookup_table(struct super_block *, u64, u64,
>diff --git a/fs/squashfs/squashfs_fs_sb.h b/fs/squashfs/squashfs_fs_sb.h
>index 52934a2..0a209e6 100644
>--- a/fs/squashfs/squashfs_fs_sb.h
>+++ b/fs/squashfs/squashfs_fs_sb.h
>@@ -63,10 +63,10 @@ struct squashfs_sb_info {
> 	__le64					*id_table;
> 	__le64					*fragment_index;
> 	__le64					*xattr_id_table;
>-	struct mutex				read_data_mutex;
>+	struct mutex                            comp_strm_mutex;
> 	struct mutex				meta_index_mutex;
> 	struct meta_index			*meta_index;
>-	void					*stream;
>+	struct list_head			strm_list;
> 	__le64					*inode_lookup_table;
> 	u64					inode_table;
> 	u64					directory_table;
>@@ -76,5 +76,8 @@ struct squashfs_sb_info {
> 	long long				bytes_used;
> 	unsigned int				inodes;
> 	int					xattr_ids;
>+	wait_queue_head_t			decomp_wait_queue;
>+	int					nr_avail_decomp;
>+	unsigned short				flags;
> };
> #endif
>diff --git a/fs/squashfs/super.c b/fs/squashfs/super.c
>index 60553a9..70aa9c4 100644
>--- a/fs/squashfs/super.c
>+++ b/fs/squashfs/super.c
>@@ -84,7 +84,7 @@ static int squashfs_fill_super(struct super_block *sb, void *data, int silent)
> 	unsigned short flags;
> 	unsigned int fragments;
> 	u64 lookup_table_start, xattr_id_table_start, next_table;
>-	int err;
>+	int err, i;
>
> 	TRACE("Entered squashfs_fill_superblock\n");
>
>@@ -98,7 +98,9 @@ static int squashfs_fill_super(struct super_block *sb, void *data, int silent)
> 	msblk->devblksize = sb_min_blocksize(sb, SQUASHFS_DEVBLK_SIZE);
> 	msblk->devblksize_log2 = ffz(~msblk->devblksize);
>
>-	mutex_init(&msblk->read_data_mutex);
>+	INIT_LIST_HEAD(&msblk->strm_list);
>+	mutex_init(&msblk->comp_strm_mutex);
>+	init_waitqueue_head(&msblk->decomp_wait_queue);

This should be done via a helper in decompressor.c, i.e. the implementation
shouldn't be visible here.

When I added the decompressor framework, I should have done this.

> 	mutex_init(&msblk->meta_index_mutex);
>
> 	/*
>@@ -176,6 +178,7 @@ static int squashfs_fill_super(struct super_block *sb, void *data, int silent)
> 	msblk->directory_table = le64_to_cpu(sblk->directory_table_start);
> 	msblk->inodes = le32_to_cpu(sblk->inodes);
> 	flags = le16_to_cpu(sblk->flags);
>+	msblk->flags = flags;
>

ha, the superblock flags should only be needed at mount time.  After
mount time there shouldn't be anything in flags we need to look at.

You need to do this because flags is needed for the decompressor init
function (COMP_OPTS(flags)) which is now called after mount time.

Once the compressor options parsing is moved back to being only
at mount time, you won't need to store the flags anymore.

> 	TRACE("Found valid superblock on %s\n", bdevname(sb->s_bdev, b));
> 	TRACE("Inodes are %scompressed\n", SQUASHFS_UNCOMPRESSED_INODES(flags)
>@@ -212,11 +215,16 @@ static int squashfs_fill_super(struct super_block *sb, void *data, int silent)
> 		goto failed_mount;
> 	}
>
>-	msblk->stream = squashfs_decompressor_init(sb, flags);
>-	if (IS_ERR(msblk->stream)) {
>-		err = PTR_ERR(msblk->stream);
>-		msblk->stream = NULL;
>-		goto failed_mount;
>+	/* Allocate mutliple decompressor */
>+	for (i = 0; i < num_online_cpus(); i++) {
>+		struct squashfs_decomp_strm *decomp_strm;
>+		decomp_strm = squashfs_decompressor_init(sb);
>+		if (IS_ERR(decomp_strm)) {
>+			err = PTR_ERR(decomp_strm);
>+			goto failed_mount;
>+		}
>+		list_add(&decomp_strm->list, &msblk->strm_list);
>+		msblk->nr_avail_decomp++;

Again, this is decompressor implementation specific stuff, and should be
done via a helper in decompressor.c.

> 	}
>
> 	/* Handle xattrs */
>@@ -336,7 +344,14 @@ failed_mount:
> 	squashfs_cache_delete(msblk->block_cache);
> 	squashfs_cache_delete(msblk->fragment_cache);
> 	squashfs_cache_delete(msblk->read_page);
>-	squashfs_decompressor_free(msblk, msblk->stream);
>+	while (!list_empty(&msblk->strm_list)) {
>+		struct squashfs_decomp_strm *decomp_strm =
>+			list_entry(msblk->strm_list.prev,
>+					struct squashfs_decomp_strm, list);
>+		list_del(&decomp_strm->list);
>+		squashfs_decompressor_free(msblk, decomp_strm);
>+	}
>+	msblk->nr_avail_decomp = 0;

helper in decompressor.c

> 	kfree(msblk->inode_lookup_table);
> 	kfree(msblk->fragment_index);
> 	kfree(msblk->id_table);
>@@ -383,7 +398,14 @@ static void squashfs_put_super(struct super_block *sb)
> 		squashfs_cache_delete(sbi->block_cache);
> 		squashfs_cache_delete(sbi->fragment_cache);
> 		squashfs_cache_delete(sbi->read_page);
>-		squashfs_decompressor_free(sbi, sbi->stream);
>+		while (!list_empty(&sbi->strm_list)) {
>+			struct squashfs_decomp_strm *decomp_strm =
>+				list_entry(sbi->strm_list.prev,
>+					struct squashfs_decomp_strm, list);
>+			list_del(&decomp_strm->list);
>+			squashfs_decompressor_free(sbi, decomp_strm);
>+		}
>+		sbi->nr_avail_decomp = 0;

helper in decompressor.c

> 		kfree(sbi->id_table);
> 		kfree(sbi->fragment_index);
> 		kfree(sbi->meta_index);
>diff --git a/fs/squashfs/xz_wrapper.c b/fs/squashfs/xz_wrapper.c
>index 1760b7d1..98b8bb5 100644
>--- a/fs/squashfs/xz_wrapper.c
>+++ b/fs/squashfs/xz_wrapper.c
>@@ -103,15 +103,13 @@ static void squashfs_xz_free(void *strm)
> }
>
>
>-static int squashfs_xz_uncompress(struct squashfs_sb_info *msblk, void **buffer,
>-	struct buffer_head **bh, int b, int offset, int length, int srclength,
>-	int pages)
>+static int squashfs_xz_uncompress(struct squashfs_sb_info *msblk, void *strm,
>+		void **buffer, struct buffer_head **bh, int b, int offset,
>+		int length, int srclength, int pages)
> {
> 	enum xz_ret xz_err;
> 	int avail, total = 0, k = 0, page = 0;
>-	struct squashfs_xz *stream = msblk->stream;
>-
>-	mutex_lock(&msblk->read_data_mutex);
>+	struct squashfs_xz *stream = strm;
>
> 	xz_dec_reset(stream->state);
> 	stream->buf.in_pos = 0;
>@@ -126,7 +124,7 @@ static int squashfs_xz_uncompress(struct squashfs_sb_info *msblk, void **buffer,
> 			length -= avail;
> 			wait_on_buffer(bh[k]);
> 			if (!buffer_uptodate(bh[k]))
>-				goto release_mutex;
>+				goto out;
>
> 			stream->buf.in = bh[k]->b_data + offset;
> 			stream->buf.in_size = avail;
>@@ -149,20 +147,18 @@ static int squashfs_xz_uncompress(struct squashfs_sb_info *msblk, void **buffer,
>
> 	if (xz_err != XZ_STREAM_END) {
> 		ERROR("xz_dec_run error, data probably corrupt\n");
>-		goto release_mutex;
>+		goto out;
> 	}
>
> 	if (k < b) {
> 		ERROR("xz_uncompress error, input remaining\n");
>-		goto release_mutex;
>+		goto out;
> 	}
>
> 	total += stream->buf.out_pos;
>-	mutex_unlock(&msblk->read_data_mutex);
> 	return total;
>
>-release_mutex:
>-	mutex_unlock(&msblk->read_data_mutex);
>+out:
>
> 	for (; k < b; k++)
> 		put_bh(bh[k]);
>diff --git a/fs/squashfs/zlib_wrapper.c b/fs/squashfs/zlib_wrapper.c
>index 55d918f..82c1a70 100644
>--- a/fs/squashfs/zlib_wrapper.c
>+++ b/fs/squashfs/zlib_wrapper.c
>@@ -61,15 +61,13 @@ static void zlib_free(void *strm)
> }
>
>
>-static int zlib_uncompress(struct squashfs_sb_info *msblk, void **buffer,
>-	struct buffer_head **bh, int b, int offset, int length, int srclength,
>-	int pages)
>+static int zlib_uncompress(struct squashfs_sb_info *msblk, void *strm,
>+		void **buffer, struct buffer_head **bh, int b, int offset,
>+		int length, int srclength, int pages)
> {
> 	int zlib_err, zlib_init = 0;
> 	int k = 0, page = 0;
>-	z_stream *stream = msblk->stream;
>-
>-	mutex_lock(&msblk->read_data_mutex);
>+	z_stream *stream = strm;
>
> 	stream->avail_out = 0;
> 	stream->avail_in = 0;
>@@ -126,11 +124,9 @@ static int zlib_uncompress(struct squashfs_sb_info *msblk, void **buffer,
> 	}
>
> 	length = stream->total_out;
>-	mutex_unlock(&msblk->read_data_mutex);
> 	return length;
>
> release_mutex:
>-	mutex_unlock(&msblk->read_data_mutex);
>
> 	for (; k < b; k++)
> 		put_bh(bh[k]);
--
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