Re: [PATCH] dm: remake of the verity target

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Hi

> Hi Mikulus,
> 
> This is some nice work. I like that you've been able to abstract a lot
> of the hash buffer management with dm-bufio. You got rid of the I/O queue.
> I've been meaning to do that for a while. The prefetch is also nice.
> We planned to do this but I decided to not do it now in order to get the
> base functionality in:
> 
> http://crosbug.com/25441
> 
> However, there are some things that I don't like. I don't like comments
> either but you have none. You also removed our documentation. You are

I added some comments. As for documentation, it's OK to use documentation 
from your patch because the on-disk format and the target arguments are 
the same (with an enhancement that my code supports different data and 
metadata bock size and it has variable-length salt).

> allocated a complete shash_desc per I/O. We only allocate one per CPU.

The hash of 4k block takes 174000 cycles. So trying to optimize 
memory latency that is about 250 cycles doesn't make much sense.

I actually observed better performance using verity on ramdisk with 
workqueue unbound to specific CPUs. The reason is that the ramdisk bio 
completion routine is always run on the same CPU (that one that submitted 
the request), so with bound workqueue, everything was executing on one 
CPU. With unbound workqueue, I got parallelism.

> We short-circuit the hash at any level. Your implementation can only
> shirt circuit at the lowest level.

It short-circuits hash at all levels. If the function 
"verity_verify_level" finds out that "aux->hash_verified" is non-zero, it 
doesn't do any hashing, it just copies the hash for the lower level. My 
implementation walks the tree from the top to the bottom, but it doesn't 
do hash verification if the same block has been verified before.

All this tree-walking from the root to the bottom is 50-times faster than 
the actual hashing of the data block (I measured that), so there's not 
much point in trying to optimize it. I did a simple optimization (don't 
walk the tree if the lowest block is already verified) and I don't need to 
do anything complicated given the fact that it can't improve more than by 
2%.

> I'd like to propose that we get the version we sent upstream and then work
> together on adding some of your enhancements incrementally.

If you add dm-bufio support, you end up deleting majority of the original 
code anyway. That's why I wrote it from scratch and that's why I didn't 
attempt to morph your code.

It's simpler to write the code from scratch and it is also less bug-prone. 

> Other than
> the changes we've made to cleanup for upstreaming, the version I
> submitted is the code we are using in production.
> 
> I'm happy to add prefetch now if that is required for merging.
> 
> What do you think?
> 
> Regards,
> Mandeep

This is the version with comments added:

Mikulas

----

Remake of the google dm-verity patch.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

---
 drivers/md/Kconfig     |   17 
 drivers/md/Makefile    |    1 
 drivers/md/dm-verity.c |  851 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 869 insertions(+)

Index: linux-3.3-rc6-fast/drivers/md/Kconfig
===================================================================
--- linux-3.3-rc6-fast.orig/drivers/md/Kconfig	2012-03-13 21:46:03.000000000 +0100
+++ linux-3.3-rc6-fast/drivers/md/Kconfig	2012-03-13 21:46:05.000000000 +0100
@@ -404,4 +404,21 @@ config DM_VERITY2
 
           If unsure, say N.
 
+config DM_VERITY
+	tristate "Verity target support"
+	depends on BLK_DEV_DM
+	select CRYPTO
+	select CRYPTO_HASH
+	select DM_BUFIO
+	---help---
+	  This device-mapper target allows you to create a device that
+	  transparently integrity checks the data on it. You'll need to
+	  activate the digests you're going to use in the cryptoapi
+	  configuration.
+
+	  To compile this code as a module, choose M here: the module will
+	  be called dm-verity.
+
+	  If unsure, say N.
+
 endif # MD
Index: linux-3.3-rc6-fast/drivers/md/Makefile
===================================================================
--- linux-3.3-rc6-fast.orig/drivers/md/Makefile	2012-03-13 21:46:03.000000000 +0100
+++ linux-3.3-rc6-fast/drivers/md/Makefile	2012-03-13 21:46:05.000000000 +0100
@@ -29,6 +29,7 @@ obj-$(CONFIG_MD_FAULTY)		+= faulty.o
 obj-$(CONFIG_BLK_DEV_MD)	+= md-mod.o
 obj-$(CONFIG_BLK_DEV_DM)	+= dm-mod.o
 obj-$(CONFIG_DM_BUFIO)		+= dm-bufio.o
+obj-$(CONFIG_DM_VERITY)		+= dm-verity.o
 obj-$(CONFIG_DM_CRYPT)		+= dm-crypt.o
 obj-$(CONFIG_DM_DELAY)		+= dm-delay.o
 obj-$(CONFIG_DM_FLAKEY)		+= dm-flakey.o
Index: linux-3.3-rc6-fast/drivers/md/dm-verity.c
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ linux-3.3-rc6-fast/drivers/md/dm-verity.c	2012-03-13 22:02:05.000000000 +0100
@@ -0,0 +1,851 @@
+/*
+ * Copyright (C) 2012 Red Hat, Inc.
+ *
+ * Author: Mikulas Patocka <mpatocka@xxxxxxxxxx>
+ *
+ * Based on Chromium dm-verity driver (C) 2011 The Chromium OS Authors
+ *
+ * This file is released under the GPLv2.
+ *
+ * Device mapper target parameters:
+ *	<version>	0
+ *	<data device>
+ *	<hash device>
+ *	<hash start>	(typically 0)
+ *	<block size>	(typically 4096)
+ *	<algorithm>
+ *	<digest>
+ *	optional parameters:
+ *		<salt> (should have 32 bytes for compatibility with Google code)
+ *		<hash block size> (by default it is the same as data block size)
+ *
+ * In the file "/sys/module/dm_verity/parameters/prefetch_cluster" you can set
+ * default prefetch value. Data are read in "prefetch_cluster" chunks from the
+ * hash device. Prefetch cluster greatly improves performance when data and hash
+ * are on the same disk on different partitions.
+ */
+
+#include <linux/module.h>
+#include <linux/device-mapper.h>
+#include <crypto/hash.h>
+#include "dm-bufio.h"
+
+#define DM_MSG_PREFIX			"verity"
+
+#define DM_VERITY_IO_VEC_INLINE		16
+#define DM_VERITY_MEMPOOL_SIZE		4
+#define DM_VERITY_DEFAULT_PREFETCH_SIZE	262144
+
+#define DM_VERITY_MAX_LEVELS		63
+
+static unsigned prefetch_cluster = DM_VERITY_DEFAULT_PREFETCH_SIZE;
+
+module_param_named(prefetch_cluster, prefetch_cluster, uint, S_IRUGO | S_IWUSR);
+
+struct dm_verity {
+	struct dm_dev *data_dev;
+	struct dm_dev *hash_dev;
+	struct dm_target *ti;
+	struct dm_bufio_client *bufio;
+	char *alg_name;
+	struct crypto_shash *tfm;
+	u8 *root_digest;	/* digest of the root block */
+	u8 *salt;		/* salt, its size is salt_size */
+	unsigned salt_size;
+	sector_t data_start;	/* data offset in 512-byte sectors */
+	sector_t hash_start;	/* hash start in blocks */
+	sector_t data_blocks;	/* the number of data blocks */
+	sector_t hash_blocks;	/* the number of hash blocks */
+	unsigned char data_dev_block_bits;	/* log2(data blocksize) */
+	unsigned char hash_dev_block_bits;	/* log2(hash blocksize) */
+	unsigned char hash_per_block_bits;	/* log2(hashes in hash block) */
+	unsigned char levels;	/* the number of tree levels */
+	unsigned digest_size;	/* digest size for the current hash algorithm */
+	unsigned shash_descsize;/* the size of temporary space for crypto */
+
+	mempool_t *io_mempool;	/* mempool of struct dm_verity_io */
+	mempool_t *vec_mempool;	/* mempool of bio vector */
+
+	struct workqueue_struct *verify_wq;
+
+	/* starting blocks for each tree level. 0 is the lowest level. */
+	sector_t hash_level_block[DM_VERITY_MAX_LEVELS];
+};
+
+struct dm_verity_io {
+	struct dm_verity *v;
+	struct bio *bio;
+
+	/* original values of bio->bi_end_io and bio->bi_private */
+	bio_end_io_t *orig_bi_end_io;
+	void *orig_bi_private;
+
+	sector_t block;
+	unsigned n_blocks;
+
+	/* saved bio vector */
+	struct bio_vec *io_vec;
+	unsigned io_vec_size;
+
+	struct work_struct work;
+
+	/* a space for short vectors; longer vectors are allocated separately */
+	struct bio_vec io_vec_inline[DM_VERITY_IO_VEC_INLINE];
+
+	/* variable-size fields, accessible with functions
+		io_hash_desc, io_real_digest, io_want_digest */
+	/* u8 hash_desc[crypto_shash_descsize(v->tfm)]; */
+	/* u8 real_digest[v->digest_size]; */
+	/* u8 want_digest[v->digest_size]; */
+};
+
+static struct shash_desc *io_hash_desc(struct dm_verity *v, struct dm_verity_io *io)
+{
+	return (struct shash_desc *)(io + 1);
+}
+
+static u8 *io_real_digest(struct dm_verity *v, struct dm_verity_io *io)
+{
+	return (u8 *)(io + 1) + v->shash_descsize;
+}
+
+static u8 *io_want_digest(struct dm_verity *v, struct dm_verity_io *io)
+{
+	return (u8 *)(io + 1) + v->shash_descsize + v->digest_size;
+}
+
+/*
+ * Auxiliary structure appended to each dm-bufio buffer. If the value
+ * hash_verified is nonzero, hash of the block has been verified.
+ *
+ * There is no lock around this value, a race condition can at worst cause
+ * that multiple processes verify the hash of the same buffer simultaneously.
+ * This condition is harmless, so we don't need locking.
+ */
+struct buffer_aux {
+	int hash_verified;
+};
+
+/*
+ * Initialize struct buffer_aux for a freshly created buffer.
+ */
+static void dm_bufio_alloc_callback(struct dm_buffer *buf)
+{
+	struct buffer_aux *aux = dm_bufio_get_aux_data(buf);
+	aux->hash_verified = 0;
+}
+
+/*
+ * Translate input sector number to the sector number on the target device.
+ */
+static sector_t verity_map_sector(struct dm_verity *v, sector_t bi_sector)
+{
+	return v->data_start + dm_target_offset(v->ti, bi_sector);
+}
+
+/*
+ * Return hash position of a specified block at a specified tree level
+ * (0 is the lowest level).
+ * The lowest "hash_per_block_bits"-bits of the result denote hash position
+ * inside a hash block. The remaining bits denode location of the hash block.
+ */
+static sector_t verity_position_at_level(struct dm_verity *v, sector_t block,
+					 int level)
+{
+	return block >> (level * v->hash_per_block_bits);
+}
+
+static void verity_hash_at_level(struct dm_verity *v, sector_t block, int level,
+				 sector_t *hash_block, unsigned *offset)
+{
+	sector_t position = verity_position_at_level(v, block, level);
+
+	*hash_block = v->hash_level_block[level] + (position >> v->hash_per_block_bits);
+	if (offset)
+		*offset = v->digest_size * (position & ((1 << v->hash_per_block_bits) - 1));
+}
+
+/*
+ * Verify hash of a metadata block pertaining to the specified data block
+ * ("block" argument) at a specified level ("level" argument).
+ *
+ * On successful return, io_want_digest(v, io) contains the hash value for
+ * a lower tree level or for the data block (if we're at the lowest leve).
+ *
+ * If "skip_unverified" is true, unverified buffer is skipped an 1 is returned.
+ * If "skip_unverified" is false, unverified buffer is hashed and verified
+ * against current value of io_want_digest(v, io).
+ */
+static int verity_verify_level(struct dm_verity_io *io, sector_t block,
+			       int level, bool skip_unverified)
+{
+	struct dm_verity *v = io->v;
+	struct dm_buffer *buf;
+	struct buffer_aux *aux;
+	u8 *data;
+	int r;
+	sector_t hash_block;
+	unsigned offset;
+
+	verity_hash_at_level(v, block, level, &hash_block, &offset);
+
+	data = dm_bufio_read(v->bufio, hash_block, &buf);
+	if (unlikely(IS_ERR(data)))
+		return PTR_ERR(data);
+
+	aux = dm_bufio_get_aux_data(buf);
+
+	if (!aux->hash_verified) {
+		struct shash_desc *desc;
+		u8 *result;
+
+		if (skip_unverified) {
+			r = 1;
+			goto release_ret_r;
+		}
+
+		desc = io_hash_desc(v, io);
+		desc->tfm = v->tfm;
+		desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
+		r = crypto_shash_init(desc);
+		if (r < 0) {
+			DMERR("crypto_shash_init failed: %d", r);
+			goto release_ret_r;
+		}
+
+		r = crypto_shash_update(desc, data, 1 << v->hash_dev_block_bits);
+		if (r < 0) {
+			DMERR("crypto_shash_update failed: %d", r);
+			goto release_ret_r;
+		}
+
+		r = crypto_shash_update(desc, v->salt, v->salt_size);
+		if (r < 0) {
+			DMERR("crypto_shash_update failed: %d", r);
+			goto release_ret_r;
+		}
+
+		result = io_real_digest(v, io);
+		r = crypto_shash_final(desc, result);
+		if (r < 0) {
+			DMERR("crypto_shash_final failed: %d", r);
+			goto release_ret_r;
+		}
+		if (unlikely(memcmp(result, io_want_digest(v, io), v->digest_size))) {
+			DMERR_LIMIT("metadata block %llu is corrupted",
+				(unsigned long long)hash_block);
+			r = -EIO;
+			goto release_ret_r;
+		} else
+			aux->hash_verified = 1;
+	}
+
+	data += offset;
+
+	memcpy(io_want_digest(v, io), data, v->digest_size);
+
+	dm_bufio_release(buf);
+	return 0;
+
+release_ret_r:
+	dm_bufio_release(buf);
+	return r;
+}
+
+/*
+ * Verify one "dm_verity_io" structure.
+ */
+static int verity_verify_io(struct dm_verity_io *io)
+{
+	struct dm_verity *v = io->v;
+	unsigned b;
+	int i;
+	unsigned vector = 0, offset = 0;
+	for (b = 0; b < io->n_blocks; b++) {
+		struct shash_desc *desc;
+		u8 *result;
+		int r;
+		unsigned todo;
+
+		if (likely(v->levels)) {
+			/*
+			 * First, we try to get the requested hash for
+			 * the current block. If the hash block itself is
+			 * verified, zero is returned. If it isn't, this
+			 * function returns 0 and we fall back to whole
+			 * chain verification.
+			 */
+			int r = verity_verify_level(io, io->block + b, 0, true);
+			if (likely(!r))
+				goto test_block_hash;
+			if (r < 0)
+				return r;
+		}
+
+		memcpy(io_want_digest(v, io), v->root_digest, v->digest_size);
+
+		for (i = v->levels - 1; i >= 0; i--) {
+			int r = verity_verify_level(io, io->block + b, i, false);
+			if (unlikely(r))
+				return r;
+		}
+
+test_block_hash:
+		desc = io_hash_desc(v, io);
+		desc->tfm = v->tfm;
+		desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
+		r = crypto_shash_init(desc);
+		if (r < 0) {
+			DMERR("crypto_shash_init failed: %d", r);
+			return r;
+		}
+
+		todo = 1 << v->data_dev_block_bits;
+		do {
+			struct bio_vec *bv;
+			u8 *page;
+			unsigned len;
+
+			BUG_ON(vector >= io->io_vec_size);
+			bv = &io->io_vec[vector];
+			page = kmap_atomic(bv->bv_page, KM_USER0);
+			len = bv->bv_len - offset;
+			if (likely(len >= todo))
+				len = todo;
+			r = crypto_shash_update(desc,
+					page + bv->bv_offset + offset, len);
+			kunmap_atomic(page, KM_USER0);
+			if (r < 0) {
+				DMERR("crypto_shash_update failed: %d", r);
+				return r;
+			}
+			offset += len;
+			if (likely(offset == bv->bv_len)) {
+				offset = 0;
+				vector++;
+			}
+			todo -= len;
+		} while (todo);
+
+		r = crypto_shash_update(desc, v->salt, v->salt_size);
+		if (r < 0) {
+			DMERR("crypto_shash_update failed: %d", r);
+			return r;
+		}
+
+		result = io_real_digest(v, io);
+		r = crypto_shash_final(desc, result);
+		if (r < 0) {
+			DMERR("crypto_shash_final failed: %d", r);
+			return r;
+		}
+		if (unlikely(memcmp(result, io_want_digest(v, io), v->digest_size))) {
+			DMERR_LIMIT("data block %llu is corrupted",
+				(unsigned long long)(io->block + b));
+			return -EIO;
+		}
+	}
+	BUG_ON(vector != io->io_vec_size);
+	BUG_ON(offset);
+	return 0;
+}
+
+/*
+ * End one "io" structure with a given error.
+ */
+static void verity_finish_io(struct dm_verity_io *io, int error)
+{
+	struct bio *bio = io->bio;
+	struct dm_verity *v = io->v;
+
+	bio->bi_end_io = io->orig_bi_end_io;
+	bio->bi_private = io->orig_bi_private;
+
+	if (io->io_vec != io->io_vec_inline)
+		mempool_free(io->io_vec, v->vec_mempool);
+	mempool_free(io, v->io_mempool);
+
+	bio_endio(bio, error);
+}
+
+static void verity_work(struct work_struct *w)
+{
+	struct dm_verity_io *io = container_of(w, struct dm_verity_io, work);
+
+	verity_finish_io(io, verity_verify_io(io));
+}
+
+static void verity_end_io(struct bio *bio, int error)
+{
+	struct dm_verity_io *io = bio->bi_private;
+	if (error) {
+		verity_finish_io(io, error);
+		return;
+	}
+
+	INIT_WORK(&io->work, verity_work);
+	queue_work(io->v->verify_wq, &io->work);
+}
+
+/*
+ * Prefetch buffers for the specified io.
+ * The root buffer is not prefetched, it is assumed that it will be cached
+ * all the time.
+ */
+static void verity_prefetch_io(struct dm_verity *v, struct dm_verity_io *io)
+{
+	int i;
+	for (i = v->levels - 2; i >= 0; i--) {
+		sector_t hash_block_start;
+		sector_t hash_block_end;
+		verity_hash_at_level(v, io->block, i, &hash_block_start, NULL);
+		verity_hash_at_level(v, io->block + io->n_blocks - 1, i, &hash_block_end, NULL);
+		if (!i) {
+			unsigned cluster = prefetch_cluster;
+	 /* barrier to stop GCC from re-reading prefetch_cluster again */
+			barrier();
+			cluster >>= v->data_dev_block_bits;
+			if (unlikely(!cluster))
+				goto no_prefetch_cluster;
+			if (unlikely(cluster & (cluster - 1)))
+				cluster = 1 << (fls(cluster) - 1);
+
+			hash_block_start &= ~(sector_t)(cluster - 1);
+			hash_block_end |= cluster - 1;
+			if (unlikely(hash_block_end >= v->hash_blocks))
+				hash_block_end = v->hash_blocks - 1;
+		}
+no_prefetch_cluster:
+		dm_bufio_prefetch(v->bufio, hash_block_start,
+					hash_block_end - hash_block_start + 1);
+	}
+}
+
+/*
+ * Bio map function. It allocates dm_verity_io structure and bio vector and
+ * fills them. Then it issues prefetches and the I/O.
+ */
+static int verity_map(struct dm_target *ti, struct bio *bio,
+		      union map_info *map_context)
+{
+	struct dm_verity *v = ti->private;
+	struct dm_verity_io *io;
+
+	if (((unsigned)bio->bi_sector | bio_sectors(bio)) &
+	    ((1 << (v->data_dev_block_bits - SECTOR_SHIFT)) - 1)) {
+		DMERR_LIMIT("unaligned io");
+		return -EIO;
+	}
+
+	if ((bio->bi_sector + bio_sectors(bio)) >>
+	    (v->data_dev_block_bits - SECTOR_SHIFT) > v->data_blocks) {
+		DMERR_LIMIT("io out of range");
+		return -EIO;
+	}
+
+	if (bio_data_dir(bio) == WRITE)
+		return -EIO;
+
+	io = mempool_alloc(v->io_mempool, GFP_NOIO);
+	io->v = v;
+	io->bio = bio;
+	io->orig_bi_end_io = bio->bi_end_io;
+	io->orig_bi_private = bio->bi_private;
+	io->block = bio->bi_sector >> (v->data_dev_block_bits - SECTOR_SHIFT);
+	io->n_blocks = bio->bi_size >> v->data_dev_block_bits;
+
+	bio->bi_end_io = verity_end_io;
+	bio->bi_private = io;
+	bio->bi_bdev = v->data_dev->bdev;
+	bio->bi_sector = verity_map_sector(v, bio->bi_sector);
+
+	io->io_vec_size = bio->bi_vcnt - bio->bi_idx;
+	if (io->io_vec_size < DM_VERITY_IO_VEC_INLINE)
+		io->io_vec = io->io_vec_inline;
+	else
+		io->io_vec = mempool_alloc(v->vec_mempool, GFP_NOIO);
+	memcpy(io->io_vec, bio_iovec(bio),
+	       io->io_vec_size * sizeof(struct bio_vec));
+
+	verity_prefetch_io(v, io);
+
+	generic_make_request(bio);
+
+	return DM_MAPIO_SUBMITTED;
+}
+
+static int verity_status(struct dm_target *ti, status_type_t type,
+			 char *result, unsigned maxlen)
+{
+	struct dm_verity *v = ti->private;
+	unsigned sz = 0;
+	unsigned x;
+
+	switch (type) {
+	case STATUSTYPE_INFO:
+		result[0] = 0;
+		break;
+	case STATUSTYPE_TABLE:
+		DMEMIT("%u %s %s %llu %u %s ",
+			0,
+			v->data_dev->name,
+			v->hash_dev->name,
+			(unsigned long long)v->hash_start << (v->hash_dev_block_bits - SECTOR_SHIFT),
+			1 << v->data_dev_block_bits,
+			v->alg_name
+			);
+		for (x = 0; x < v->digest_size; x++)
+			DMEMIT("%02x", v->root_digest[x]);
+		DMEMIT(" ");
+		if (!v->salt_size)
+			DMEMIT("-");
+		else
+			for (x = 0; x < v->salt_size; x++)
+				DMEMIT("%02x", v->salt[x]);
+		if (v->data_dev_block_bits != v->hash_dev_block_bits)
+			DMEMIT(" %u", 1 << v->hash_dev_block_bits);
+		break;
+	}
+	return 0;
+}
+
+static int verity_ioctl(struct dm_target *ti, unsigned cmd,
+			unsigned long arg)
+{
+	struct dm_verity *v = ti->private;
+	int r = 0;
+
+	if (v->data_start ||
+	    ti->len != i_size_read(v->data_dev->bdev->bd_inode) >> SECTOR_SHIFT)
+		r = scsi_verify_blk_ioctl(NULL, cmd);
+
+	return r ? : __blkdev_driver_ioctl(v->data_dev->bdev, v->data_dev->mode,
+				     cmd, arg);
+}
+
+static int verity_merge(struct dm_target *ti, struct bvec_merge_data *bvm,
+			struct bio_vec *biovec, int max_size)
+{
+	struct dm_verity *v = ti->private;
+	struct request_queue *q = bdev_get_queue(v->data_dev->bdev);
+
+	if (!q->merge_bvec_fn)
+		return max_size;
+
+	bvm->bi_bdev = v->data_dev->bdev;
+	bvm->bi_sector = verity_map_sector(v, bvm->bi_sector);
+
+	return min(max_size, q->merge_bvec_fn(q, bvm, biovec));
+}
+
+static int verity_iterate_devices(struct dm_target *ti,
+				  iterate_devices_callout_fn fn, void *data)
+{
+	struct dm_verity *v = ti->private;
+	return fn(ti, v->data_dev, v->data_start, ti->len, data);
+}
+
+static void verity_io_hints(struct dm_target *ti, struct queue_limits *limits)
+{
+	struct dm_verity *v = ti->private;
+
+	if (limits->logical_block_size < 1 << v->data_dev_block_bits)
+		limits->logical_block_size = 1 << v->data_dev_block_bits;
+	if (limits->physical_block_size < 1 << v->data_dev_block_bits)
+		limits->physical_block_size = 1 << v->data_dev_block_bits;
+	blk_limits_io_min(limits, limits->logical_block_size);
+}
+
+static void verity_dtr(struct dm_target *ti);
+
+static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
+{
+	struct dm_verity *v;
+	unsigned num;
+	unsigned long long hs;
+	int r;
+	int i;
+	sector_t hash_position;
+	char dummy;
+
+	v = kzalloc(sizeof(struct dm_verity), GFP_KERNEL);
+	if (!v) {
+		ti->error = "Cannot allocate verity structure";
+		return -ENOMEM;
+	}
+	ti->private = v;
+	v->ti = ti;
+
+	if ((dm_table_get_mode(ti->table) & ~FMODE_READ) != 0) {
+		ti->error = "Device must be readonly";
+		r = -EINVAL;
+		goto bad;
+	}
+
+	if (argc < 7) {
+		ti->error = "Not enough arguments";
+		r = -EINVAL;
+		goto bad;
+	}
+
+	if (sscanf(argv[0], "%d%c", &num, &dummy) != 1 ||
+	    num != 0) {
+		ti->error = "Invalid version";
+		r = -EINVAL;
+		goto bad;
+	}
+
+	r = dm_get_device(ti, argv[1], FMODE_READ, &v->data_dev);
+	if (r) {
+		ti->error = "Data device lookup failed";
+		goto bad;
+	}
+
+	r = dm_get_device(ti, argv[2], FMODE_READ, &v->hash_dev);
+	if (r) {
+		ti->error = "Data device lookup failed";
+		goto bad;
+	}
+
+	if (sscanf(argv[3], "%llu%c", &hs, &dummy) != 1 ||
+	    hs != (sector_t)hs) {
+		ti->error = "Invalid hash start";
+		r = -EINVAL;
+		goto bad;
+	}
+
+	if (sscanf(argv[4], "%u%c", &num, &dummy) != 1 ||
+	    !num || (num & (num - 1)) ||
+	    num < bdev_logical_block_size(v->data_dev->bdev) ||
+	    num > PAGE_SIZE) {
+		ti->error = "Invalid data device block size";
+		r = -EINVAL;
+		goto bad;
+	}
+	v->data_dev_block_bits = ffs(num) - 1;
+	v->hash_dev_block_bits = ffs(num) - 1;
+
+	v->alg_name = kstrdup(argv[5], GFP_KERNEL);
+	if (!v->alg_name) {
+		ti->error = "Cannot allocate algorithm name";
+		r = -ENOMEM;
+		goto bad;
+	}
+
+	v->tfm = crypto_alloc_shash(v->alg_name, 0, 0);
+	if (IS_ERR(v->tfm)) {
+		ti->error = "Cannot initialize hash function";
+		r = PTR_ERR(v->tfm);
+		v->tfm = NULL;
+		goto bad;
+	}
+	v->digest_size = crypto_shash_digestsize(v->tfm);
+	if ((1 << v->hash_dev_block_bits) < v->digest_size * 2) {
+		ti->error = "Digest size too big";
+		r = -EINVAL;
+		goto bad;
+	}
+	v->shash_descsize =
+		sizeof(struct shash_desc) + crypto_shash_descsize(v->tfm);
+
+	v->root_digest = kmalloc(v->digest_size, GFP_KERNEL);
+	if (!v->root_digest) {
+		ti->error = "Cannot allocate root digest";
+		r = -ENOMEM;
+		goto bad;
+	}
+	if (strlen(argv[6]) != v->digest_size * 2 ||
+	    hex2bin(v->root_digest, argv[6], v->digest_size)) {
+		ti->error = "Invalid root digest";
+		r = -EINVAL;
+		goto bad;
+	}
+
+	if (argc > 7 && strcmp(argv[7], "-")) {
+		v->salt_size = strlen(argv[7]) / 2;
+		v->salt = kmalloc(v->salt_size, GFP_KERNEL);
+		if (!v->salt) {
+			ti->error = "Cannot allocate salt";
+			r = -ENOMEM;
+			goto bad;
+		}
+		if (strlen(argv[7]) != v->salt_size * 2 ||
+		    hex2bin(v->salt, argv[7], v->salt_size)) {
+			ti->error = "Invalid salt";
+			r = -EINVAL;
+			goto bad;
+		}
+	}
+
+	if (argc > 8) {
+		if (sscanf(argv[8], "%u%c", &num, &dummy) != 1 ||
+		    !num || (num & (num - 1)) ||
+		    num < bdev_logical_block_size(v->hash_dev->bdev) ||
+		    num > INT_MAX) {
+			ti->error = "Invalid hash device block size";
+			r = -EINVAL;
+			goto bad;
+		}
+		v->hash_dev_block_bits = ffs(num) - 1;
+	}
+
+	if (hs & ((1 << (v->hash_dev_block_bits - SECTOR_SHIFT)) - 1)) {
+		ti->error = "Hash start not aligned on block boundary";
+		r = -EINVAL;
+		goto bad;
+	}
+	v->hash_start = hs >> (v->hash_dev_block_bits - SECTOR_SHIFT);
+
+	if (ti->len > i_size_read(v->data_dev->bdev->bd_inode) >> SECTOR_SHIFT) {
+		ti->error = "Data device si too small";
+		r = -EINVAL;
+		goto bad;
+	}
+
+	if (ti->len & ((1 << (v->data_dev_block_bits - SECTOR_SHIFT)) - 1)) {
+		ti->error = "Data device length is not aligned to block size";
+		r = -EINVAL;
+		goto bad;
+	}
+
+	v->data_blocks = ti->len >> (v->data_dev_block_bits - SECTOR_SHIFT);
+
+	v->hash_per_block_bits =
+		fls((1 << v->hash_dev_block_bits) / v->digest_size) - 1;
+
+	v->levels = 0;
+	if (v->data_blocks)
+		while (v->hash_per_block_bits * v->levels < 64 &&
+		       (unsigned long long)(v->data_blocks - 1) >>
+		       (v->hash_per_block_bits * v->levels))
+			v->levels++;
+
+	if (v->levels > DM_VERITY_MAX_LEVELS) {
+		ti->error = "Too many tree levels";
+		r = -E2BIG;
+		goto bad;
+	}
+
+	hash_position = v->hash_start;
+	for (i = v->levels - 1; i >= 0; i--) {
+		sector_t s;
+		v->hash_level_block[i] = hash_position;
+		s = verity_position_at_level(v, v->data_blocks, i);
+		s = (s >> v->hash_per_block_bits) +
+		    !!(s & ((1 << v->hash_per_block_bits) - 1));
+		if (hash_position + s < hash_position) {
+			ti->error = "Hash device offset overflow";
+			r = -E2BIG;
+			goto bad;
+		}
+		hash_position += s;
+	}
+	v->hash_blocks = hash_position;
+
+	v->bufio = dm_bufio_client_create(v->hash_dev->bdev,
+		1 << v->hash_dev_block_bits, 1, sizeof(struct buffer_aux),
+		dm_bufio_alloc_callback, NULL);
+	if (IS_ERR(v->bufio)) {
+		ti->error = "Cannot initialize dm-bufio";
+		r = PTR_ERR(v->bufio);
+		v->bufio = NULL;
+		goto bad;
+	}
+
+	if (dm_bufio_get_device_size(v->bufio) < v->hash_blocks) {
+		ti->error = "Hash device is too small";
+		r = -E2BIG;
+		goto bad;
+	}
+
+	v->io_mempool = mempool_create_kmalloc_pool(DM_VERITY_MEMPOOL_SIZE,
+	  sizeof(struct dm_verity_io) + v->shash_descsize + v->digest_size * 2);
+	if (!v->io_mempool) {
+		ti->error = "Cannot allocate io mempool";
+		r = -ENOMEM;
+		goto bad;
+	}
+
+	v->vec_mempool = mempool_create_kmalloc_pool(DM_VERITY_MEMPOOL_SIZE,
+					BIO_MAX_PAGES * sizeof(struct bio_vec));
+	if (!v->vec_mempool) {
+		ti->error = "Cannot allocate vector mempool";
+		r = -ENOMEM;
+		goto bad;
+	}
+
+	/*v->verify_wq = alloc_workqueue("verityd", WQ_CPU_INTENSIVE | WQ_MEM_RECLAIM, 1);*/
+	/* WQ_UNBOUND greatly improves performance when running on ramdisk */
+	v->verify_wq = alloc_workqueue("verityd", WQ_CPU_INTENSIVE | WQ_MEM_RECLAIM | WQ_UNBOUND, num_online_cpus());
+	if (!v->verify_wq) {
+		ti->error = "Cannot allocate workqueue";
+		r = -ENOMEM;
+		goto bad;
+	}
+
+	return 0;
+
+bad:
+	verity_dtr(ti);
+	return r;
+}
+
+static void verity_dtr(struct dm_target *ti)
+{
+	struct dm_verity *v = ti->private;
+
+	if (v->verify_wq)
+		destroy_workqueue(v->verify_wq);
+	if (v->vec_mempool)
+		mempool_destroy(v->vec_mempool);
+	if (v->io_mempool)
+		mempool_destroy(v->io_mempool);
+	if (v->bufio)
+		dm_bufio_client_destroy(v->bufio);
+	kfree(v->salt);
+	kfree(v->root_digest);
+	if (v->tfm)
+		crypto_free_shash(v->tfm);
+	kfree(v->alg_name);
+	if (v->hash_dev)
+		dm_put_device(ti, v->hash_dev);
+	if (v->data_dev)
+		dm_put_device(ti, v->data_dev);
+	kfree(v);
+}
+
+static struct target_type verity_target = {
+	.name		= "verity",
+	.version	= {1, 0, 0},
+	.module		= THIS_MODULE,
+	.ctr		= verity_ctr,
+	.dtr		= verity_dtr,
+	.map		= verity_map,
+	.status		= verity_status,
+	.ioctl		= verity_ioctl,
+	.merge		= verity_merge,
+	.iterate_devices = verity_iterate_devices,
+	.io_hints	= verity_io_hints,
+};
+
+static int __init dm_verity_init(void)
+{
+	int r;
+	r = dm_register_target(&verity_target);
+	if (r < 0)
+		DMERR("register failed %d", r);
+	return r;
+}
+
+static void __exit dm_verity_exit(void)
+{
+	dm_unregister_target(&verity_target);
+}
+
+module_init(dm_verity_init);
+module_exit(dm_verity_exit);
+
+MODULE_AUTHOR("Mikulas Patocka <mpatocka@xxxxxxxxxx>");
+MODULE_DESCRIPTION(DM_NAME " target for transparent disk integrity checking");
+MODULE_LICENSE("GPL");
+
Index: linux-3.3-rc6-fast/drivers/md/dm-bufio.c
===================================================================
--- linux-3.3-rc6-fast.orig/drivers/md/dm-bufio.c	2012-03-12 22:43:23.000000000 +0100
+++ linux-3.3-rc6-fast/drivers/md/dm-bufio.c	2012-03-13 15:41:02.000000000 +0100
@@ -579,7 +579,7 @@ static void write_endio(struct bio *bio,
 	struct dm_buffer *b = container_of(bio, struct dm_buffer, bio);
 
 	b->write_error = error;
-	if (error) {
+	if (unlikely(error)) {
 		struct dm_bufio_client *c = b->c;
 		(void)cmpxchg(&c->async_write_error, 0, error);
 	}
@@ -698,13 +698,20 @@ static void __wait_for_free_buffer(struc
 	dm_bufio_lock(c);
 }
 
+enum new_flag {
+	NF_FRESH = 0,
+	NF_READ = 1,
+	NF_GET = 2,
+	NF_PREFETCH = 3
+};
+
 /*
  * Allocate a new buffer. If the allocation is not possible, wait until
  * some other thread frees a buffer.
  *
  * May drop the lock and regain it.
  */
-static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client *c)
+static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client *c, enum new_flag nf)
 {
 	struct dm_buffer *b;
 
@@ -727,6 +734,9 @@ static struct dm_buffer *__alloc_buffer_
 				return b;
 		}
 
+		if (nf == NF_PREFETCH)
+			return NULL;
+
 		if (!list_empty(&c->reserved_buffers)) {
 			b = list_entry(c->reserved_buffers.next,
 				       struct dm_buffer, lru_list);
@@ -744,9 +754,12 @@ static struct dm_buffer *__alloc_buffer_
 	}
 }
 
-static struct dm_buffer *__alloc_buffer_wait(struct dm_bufio_client *c)
+static struct dm_buffer *__alloc_buffer_wait(struct dm_bufio_client *c, enum new_flag nf)
 {
-	struct dm_buffer *b = __alloc_buffer_wait_no_callback(c);
+	struct dm_buffer *b = __alloc_buffer_wait_no_callback(c, nf);
+
+	if (!b)
+		return NULL;
 
 	if (c->alloc_callback)
 		c->alloc_callback(b);
@@ -866,15 +879,8 @@ static struct dm_buffer *__find(struct d
  * Getting a buffer
  *--------------------------------------------------------------*/
 
-enum new_flag {
-	NF_FRESH = 0,
-	NF_READ = 1,
-	NF_GET = 2
-};
-
 static struct dm_buffer *__bufio_new(struct dm_bufio_client *c, sector_t block,
-				     enum new_flag nf, struct dm_buffer **bp,
-				     int *need_submit)
+				     enum new_flag nf, int *need_submit)
 {
 	struct dm_buffer *b, *new_b = NULL;
 
@@ -882,6 +888,19 @@ static struct dm_buffer *__bufio_new(str
 
 	b = __find(c, block);
 	if (b) {
+found_buffer:
+		if (nf == NF_PREFETCH)
+			return NULL;
+		/*
+		 * Note: it is essential that we don't wait for the buffer to be
+		 * read if dm_bufio_get function is used. Both dm_bufio_get and
+		 * dm_bufio_prefetch can be used in the driver request routine.
+		 * If the user called both dm_bufio_prefetch and dm_bufio_get on
+		 * the same buffer, it would deadlock if we waited.
+		 */
+		if (nf == NF_GET && unlikely(test_bit(B_READING, &b->state)))
+			return NULL;
+
 		b->hold_count++;
 		__relink_lru(b, test_bit(B_DIRTY, &b->state) ||
 			     test_bit(B_WRITING, &b->state));
@@ -891,7 +910,9 @@ static struct dm_buffer *__bufio_new(str
 	if (nf == NF_GET)
 		return NULL;
 
-	new_b = __alloc_buffer_wait(c);
+	new_b = __alloc_buffer_wait(c, nf);
+	if (!new_b)
+		return NULL;
 
 	/*
 	 * We've had a period where the mutex was unlocked, so need to
@@ -900,10 +921,7 @@ static struct dm_buffer *__bufio_new(str
 	b = __find(c, block);
 	if (b) {
 		__free_buffer_wake(new_b);
-		b->hold_count++;
-		__relink_lru(b, test_bit(B_DIRTY, &b->state) ||
-			     test_bit(B_WRITING, &b->state));
-		return b;
+		goto found_buffer;
 	}
 
 	__check_watermark(c);
@@ -957,7 +975,7 @@ static void *new_read(struct dm_bufio_cl
 	struct dm_buffer *b;
 
 	dm_bufio_lock(c);
-	b = __bufio_new(c, block, nf, bp, &need_submit);
+	b = __bufio_new(c, block, nf, &need_submit);
 	dm_bufio_unlock(c);
 
 	if (!b || IS_ERR(b))
@@ -1006,13 +1024,46 @@ void *dm_bufio_new(struct dm_bufio_clien
 }
 EXPORT_SYMBOL_GPL(dm_bufio_new);
 
+void dm_bufio_prefetch(struct dm_bufio_client *c,
+		       sector_t block, unsigned n_blocks)
+{
+	struct blk_plug plug;
+
+	blk_start_plug(&plug);
+	dm_bufio_lock(c);
+
+	for (; n_blocks--; block++) {
+		int need_submit;
+		struct dm_buffer *b;
+		b = __bufio_new(c, block, NF_PREFETCH, &need_submit);
+		if (unlikely(b != NULL)) {
+			dm_bufio_unlock(c);
+
+			if (need_submit)
+				submit_io(b, READ, b->block, read_endio);
+			dm_bufio_release(b);
+
+			dm_bufio_cond_resched();
+
+			if (!n_blocks)
+				goto flush_plug;
+			dm_bufio_lock(c);
+		}
+
+	}
+
+	dm_bufio_unlock(c);
+flush_plug:
+	blk_finish_plug(&plug);
+}
+EXPORT_SYMBOL_GPL(dm_bufio_prefetch);
+
 void dm_bufio_release(struct dm_buffer *b)
 {
 	struct dm_bufio_client *c = b->c;
 
 	dm_bufio_lock(c);
 
-	BUG_ON(test_bit(B_READING, &b->state));
 	BUG_ON(!b->hold_count);
 
 	b->hold_count--;
@@ -1025,6 +1076,7 @@ void dm_bufio_release(struct dm_buffer *
 		 * invalid buffer.
 		 */
 		if ((b->read_error || b->write_error) &&
+		    !test_bit(B_READING, &b->state) &&
 		    !test_bit(B_WRITING, &b->state) &&
 		    !test_bit(B_DIRTY, &b->state)) {
 			__unlink_buffer(b);
@@ -1042,6 +1094,8 @@ void dm_bufio_mark_buffer_dirty(struct d
 
 	dm_bufio_lock(c);
 
+	BUG_ON(test_bit(B_READING, &b->state));
+
 	if (!test_and_set_bit(B_DIRTY, &b->state))
 		__relink_lru(b, LIST_DIRTY);
 
Index: linux-3.3-rc6-fast/drivers/md/dm-bufio.h
===================================================================
--- linux-3.3-rc6-fast.orig/drivers/md/dm-bufio.h	2012-03-12 22:43:23.000000000 +0100
+++ linux-3.3-rc6-fast/drivers/md/dm-bufio.h	2012-03-12 22:43:25.000000000 +0100
@@ -63,6 +63,14 @@ void *dm_bufio_new(struct dm_bufio_clien
 		   struct dm_buffer **bp);
 
 /*
+ * Prefetch the specified blocks to the cache.
+ * The function starts to read the blocks and returns without waiting for
+ * I/O to finish.
+ */
+void dm_bufio_prefetch(struct dm_bufio_client *c,
+		       sector_t block, unsigned n_blocks);
+
+/*
  * Release a reference obtained with dm_bufio_{read,get,new}. The data
  * pointer and dm_buffer pointer is no longer valid after this call.
  */

--
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