dm-thin.c 68 KB
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/*
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 * Copyright (C) 2011-2012 Red Hat UK.
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 *
 * This file is released under the GPL.
 */

#include "dm-thin-metadata.h"
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#include "dm-bio-prison.h"
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#include "dm.h"
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#include <linux/device-mapper.h>
#include <linux/dm-io.h>
#include <linux/dm-kcopyd.h>
#include <linux/list.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/slab.h>

#define	DM_MSG_PREFIX	"thin"

/*
 * Tunable constants
 */
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#define ENDIO_HOOK_POOL_SIZE 1024
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#define MAPPING_POOL_SIZE 1024
#define PRISON_CELLS 1024
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#define COMMIT_PERIOD HZ
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/*
 * The block size of the device holding pool data must be
 * between 64KB and 1GB.
 */
#define DATA_DEV_BLOCK_SIZE_MIN_SECTORS (64 * 1024 >> SECTOR_SHIFT)
#define DATA_DEV_BLOCK_SIZE_MAX_SECTORS (1024 * 1024 * 1024 >> SECTOR_SHIFT)

/*
 * Device id is restricted to 24 bits.
 */
#define MAX_DEV_ID ((1 << 24) - 1)

/*
 * How do we handle breaking sharing of data blocks?
 * =================================================
 *
 * We use a standard copy-on-write btree to store the mappings for the
 * devices (note I'm talking about copy-on-write of the metadata here, not
 * the data).  When you take an internal snapshot you clone the root node
 * of the origin btree.  After this there is no concept of an origin or a
 * snapshot.  They are just two device trees that happen to point to the
 * same data blocks.
 *
 * When we get a write in we decide if it's to a shared data block using
 * some timestamp magic.  If it is, we have to break sharing.
 *
 * Let's say we write to a shared block in what was the origin.  The
 * steps are:
 *
 * i) plug io further to this physical block. (see bio_prison code).
 *
 * ii) quiesce any read io to that shared data block.  Obviously
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 * including all devices that share this block.  (see dm_deferred_set code)
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 *
 * iii) copy the data block to a newly allocate block.  This step can be
 * missed out if the io covers the block. (schedule_copy).
 *
 * iv) insert the new mapping into the origin's btree
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 * (process_prepared_mapping).  This act of inserting breaks some
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 * sharing of btree nodes between the two devices.  Breaking sharing only
 * effects the btree of that specific device.  Btrees for the other
 * devices that share the block never change.  The btree for the origin
 * device as it was after the last commit is untouched, ie. we're using
 * persistent data structures in the functional programming sense.
 *
 * v) unplug io to this physical block, including the io that triggered
 * the breaking of sharing.
 *
 * Steps (ii) and (iii) occur in parallel.
 *
 * The metadata _doesn't_ need to be committed before the io continues.  We
 * get away with this because the io is always written to a _new_ block.
 * If there's a crash, then:
 *
 * - The origin mapping will point to the old origin block (the shared
 * one).  This will contain the data as it was before the io that triggered
 * the breaking of sharing came in.
 *
 * - The snap mapping still points to the old block.  As it would after
 * the commit.
 *
 * The downside of this scheme is the timestamp magic isn't perfect, and
 * will continue to think that data block in the snapshot device is shared
 * even after the write to the origin has broken sharing.  I suspect data
 * blocks will typically be shared by many different devices, so we're
 * breaking sharing n + 1 times, rather than n, where n is the number of
 * devices that reference this data block.  At the moment I think the
 * benefits far, far outweigh the disadvantages.
 */

/*----------------------------------------------------------------*/

/*
 * Key building.
 */
static void build_data_key(struct dm_thin_device *td,
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			   dm_block_t b, struct dm_cell_key *key)
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{
	key->virtual = 0;
	key->dev = dm_thin_dev_id(td);
	key->block = b;
}

static void build_virtual_key(struct dm_thin_device *td, dm_block_t b,
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			      struct dm_cell_key *key)
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{
	key->virtual = 1;
	key->dev = dm_thin_dev_id(td);
	key->block = b;
}

/*----------------------------------------------------------------*/

/*
 * A pool device ties together a metadata device and a data device.  It
 * also provides the interface for creating and destroying internal
 * devices.
 */
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struct dm_thin_new_mapping;
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/*
 * The pool runs in 3 modes.  Ordered in degraded order for comparisons.
 */
enum pool_mode {
	PM_WRITE,		/* metadata may be changed */
	PM_READ_ONLY,		/* metadata may not be changed */
	PM_FAIL,		/* all I/O fails */
};

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struct pool_features {
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	enum pool_mode mode;

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	bool zero_new_blocks:1;
	bool discard_enabled:1;
	bool discard_passdown:1;
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};

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struct thin_c;
typedef void (*process_bio_fn)(struct thin_c *tc, struct bio *bio);
typedef void (*process_mapping_fn)(struct dm_thin_new_mapping *m);

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struct pool {
	struct list_head list;
	struct dm_target *ti;	/* Only set if a pool target is bound */

	struct mapped_device *pool_md;
	struct block_device *md_dev;
	struct dm_pool_metadata *pmd;

	dm_block_t low_water_blocks;
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	uint32_t sectors_per_block;
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	int sectors_per_block_shift;
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	struct pool_features pf;
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	unsigned low_water_triggered:1;	/* A dm event has been sent */
	unsigned no_free_space:1;	/* A -ENOSPC warning has been issued */

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	struct dm_bio_prison *prison;
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	struct dm_kcopyd_client *copier;

	struct workqueue_struct *wq;
	struct work_struct worker;
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	struct delayed_work waker;
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	unsigned long last_commit_jiffies;
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	unsigned ref_count;
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	spinlock_t lock;
	struct bio_list deferred_bios;
	struct bio_list deferred_flush_bios;
	struct list_head prepared_mappings;
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	struct list_head prepared_discards;
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	struct bio_list retry_on_resume_list;

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	struct dm_deferred_set *shared_read_ds;
	struct dm_deferred_set *all_io_ds;
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	struct dm_thin_new_mapping *next_mapping;
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	mempool_t *mapping_pool;
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	process_bio_fn process_bio;
	process_bio_fn process_discard;

	process_mapping_fn process_prepared_mapping;
	process_mapping_fn process_prepared_discard;
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};

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static enum pool_mode get_pool_mode(struct pool *pool);
static void set_pool_mode(struct pool *pool, enum pool_mode mode);

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/*
 * Target context for a pool.
 */
struct pool_c {
	struct dm_target *ti;
	struct pool *pool;
	struct dm_dev *data_dev;
	struct dm_dev *metadata_dev;
	struct dm_target_callbacks callbacks;

	dm_block_t low_water_blocks;
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	struct pool_features requested_pf; /* Features requested during table load */
	struct pool_features adjusted_pf;  /* Features used after adjusting for constituent devices */
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};

/*
 * Target context for a thin.
 */
struct thin_c {
	struct dm_dev *pool_dev;
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	struct dm_dev *origin_dev;
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	dm_thin_id dev_id;

	struct pool *pool;
	struct dm_thin_device *td;
};

/*----------------------------------------------------------------*/

/*
 * A global list of pools that uses a struct mapped_device as a key.
 */
static struct dm_thin_pool_table {
	struct mutex mutex;
	struct list_head pools;
} dm_thin_pool_table;

static void pool_table_init(void)
{
	mutex_init(&dm_thin_pool_table.mutex);
	INIT_LIST_HEAD(&dm_thin_pool_table.pools);
}

static void __pool_table_insert(struct pool *pool)
{
	BUG_ON(!mutex_is_locked(&dm_thin_pool_table.mutex));
	list_add(&pool->list, &dm_thin_pool_table.pools);
}

static void __pool_table_remove(struct pool *pool)
{
	BUG_ON(!mutex_is_locked(&dm_thin_pool_table.mutex));
	list_del(&pool->list);
}

static struct pool *__pool_table_lookup(struct mapped_device *md)
{
	struct pool *pool = NULL, *tmp;

	BUG_ON(!mutex_is_locked(&dm_thin_pool_table.mutex));

	list_for_each_entry(tmp, &dm_thin_pool_table.pools, list) {
		if (tmp->pool_md == md) {
			pool = tmp;
			break;
		}
	}

	return pool;
}

static struct pool *__pool_table_lookup_metadata_dev(struct block_device *md_dev)
{
	struct pool *pool = NULL, *tmp;

	BUG_ON(!mutex_is_locked(&dm_thin_pool_table.mutex));

	list_for_each_entry(tmp, &dm_thin_pool_table.pools, list) {
		if (tmp->md_dev == md_dev) {
			pool = tmp;
			break;
		}
	}

	return pool;
}

/*----------------------------------------------------------------*/

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struct dm_thin_endio_hook {
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	struct thin_c *tc;
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	struct dm_deferred_entry *shared_read_entry;
	struct dm_deferred_entry *all_io_entry;
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	struct dm_thin_new_mapping *overwrite_mapping;
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};

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static void __requeue_bio_list(struct thin_c *tc, struct bio_list *master)
{
	struct bio *bio;
	struct bio_list bios;

	bio_list_init(&bios);
	bio_list_merge(&bios, master);
	bio_list_init(master);

	while ((bio = bio_list_pop(&bios))) {
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		struct dm_thin_endio_hook *h = dm_per_bio_data(bio, sizeof(struct dm_thin_endio_hook));
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		if (h->tc == tc)
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			bio_endio(bio, DM_ENDIO_REQUEUE);
		else
			bio_list_add(master, bio);
	}
}

static void requeue_io(struct thin_c *tc)
{
	struct pool *pool = tc->pool;
	unsigned long flags;

	spin_lock_irqsave(&pool->lock, flags);
	__requeue_bio_list(tc, &pool->deferred_bios);
	__requeue_bio_list(tc, &pool->retry_on_resume_list);
	spin_unlock_irqrestore(&pool->lock, flags);
}

/*
 * This section of code contains the logic for processing a thin device's IO.
 * Much of the code depends on pool object resources (lists, workqueues, etc)
 * but most is exclusively called from the thin target rather than the thin-pool
 * target.
 */

static dm_block_t get_bio_block(struct thin_c *tc, struct bio *bio)
{
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	sector_t block_nr = bio->bi_sector;

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	if (tc->pool->sectors_per_block_shift < 0)
		(void) sector_div(block_nr, tc->pool->sectors_per_block);
	else
		block_nr >>= tc->pool->sectors_per_block_shift;
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	return block_nr;
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}

static void remap(struct thin_c *tc, struct bio *bio, dm_block_t block)
{
	struct pool *pool = tc->pool;
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	sector_t bi_sector = bio->bi_sector;
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	bio->bi_bdev = tc->pool_dev->bdev;
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	if (tc->pool->sectors_per_block_shift < 0)
		bio->bi_sector = (block * pool->sectors_per_block) +
				 sector_div(bi_sector, pool->sectors_per_block);
	else
		bio->bi_sector = (block << pool->sectors_per_block_shift) |
				(bi_sector & (pool->sectors_per_block - 1));
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}

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static void remap_to_origin(struct thin_c *tc, struct bio *bio)
{
	bio->bi_bdev = tc->origin_dev->bdev;
}

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static int bio_triggers_commit(struct thin_c *tc, struct bio *bio)
{
	return (bio->bi_rw & (REQ_FLUSH | REQ_FUA)) &&
		dm_thin_changed_this_transaction(tc->td);
}

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static void inc_all_io_entry(struct pool *pool, struct bio *bio)
{
	struct dm_thin_endio_hook *h;

	if (bio->bi_rw & REQ_DISCARD)
		return;

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	h = dm_per_bio_data(bio, sizeof(struct dm_thin_endio_hook));
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	h->all_io_entry = dm_deferred_entry_inc(pool->all_io_ds);
}

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static void issue(struct thin_c *tc, struct bio *bio)
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{
	struct pool *pool = tc->pool;
	unsigned long flags;

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	if (!bio_triggers_commit(tc, bio)) {
		generic_make_request(bio);
		return;
	}

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	/*
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	 * Complete bio with an error if earlier I/O caused changes to
	 * the metadata that can't be committed e.g, due to I/O errors
	 * on the metadata device.
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	 */
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	if (dm_thin_aborted_changes(tc->td)) {
		bio_io_error(bio);
		return;
	}

	/*
	 * Batch together any bios that trigger commits and then issue a
	 * single commit for them in process_deferred_bios().
	 */
	spin_lock_irqsave(&pool->lock, flags);
	bio_list_add(&pool->deferred_flush_bios, bio);
	spin_unlock_irqrestore(&pool->lock, flags);
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}

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static void remap_to_origin_and_issue(struct thin_c *tc, struct bio *bio)
{
	remap_to_origin(tc, bio);
	issue(tc, bio);
}

static void remap_and_issue(struct thin_c *tc, struct bio *bio,
			    dm_block_t block)
{
	remap(tc, bio, block);
	issue(tc, bio);
}

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/*
 * wake_worker() is used when new work is queued and when pool_resume is
 * ready to continue deferred IO processing.
 */
static void wake_worker(struct pool *pool)
{
	queue_work(pool->wq, &pool->worker);
}

/*----------------------------------------------------------------*/

/*
 * Bio endio functions.
 */
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struct dm_thin_new_mapping {
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	struct list_head list;

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	unsigned quiesced:1;
	unsigned prepared:1;
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	unsigned pass_discard:1;
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	struct thin_c *tc;
	dm_block_t virt_block;
	dm_block_t data_block;
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	struct dm_bio_prison_cell *cell, *cell2;
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	int err;

	/*
	 * If the bio covers the whole area of a block then we can avoid
	 * zeroing or copying.  Instead this bio is hooked.  The bio will
	 * still be in the cell, so care has to be taken to avoid issuing
	 * the bio twice.
	 */
	struct bio *bio;
	bio_end_io_t *saved_bi_end_io;
};

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static void __maybe_add_mapping(struct dm_thin_new_mapping *m)
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{
	struct pool *pool = m->tc->pool;

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	if (m->quiesced && m->prepared) {
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		list_add(&m->list, &pool->prepared_mappings);
		wake_worker(pool);
	}
}

static void copy_complete(int read_err, unsigned long write_err, void *context)
{
	unsigned long flags;
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	struct dm_thin_new_mapping *m = context;
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	struct pool *pool = m->tc->pool;

	m->err = read_err || write_err ? -EIO : 0;

	spin_lock_irqsave(&pool->lock, flags);
	m->prepared = 1;
	__maybe_add_mapping(m);
	spin_unlock_irqrestore(&pool->lock, flags);
}

static void overwrite_endio(struct bio *bio, int err)
{
	unsigned long flags;
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	struct dm_thin_endio_hook *h = dm_per_bio_data(bio, sizeof(struct dm_thin_endio_hook));
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	struct dm_thin_new_mapping *m = h->overwrite_mapping;
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	struct pool *pool = m->tc->pool;

	m->err = err;

	spin_lock_irqsave(&pool->lock, flags);
	m->prepared = 1;
	__maybe_add_mapping(m);
	spin_unlock_irqrestore(&pool->lock, flags);
}

/*----------------------------------------------------------------*/

/*
 * Workqueue.
 */

/*
 * Prepared mapping jobs.
 */

/*
 * This sends the bios in the cell back to the deferred_bios list.
 */
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static void cell_defer(struct thin_c *tc, struct dm_bio_prison_cell *cell)
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{
	struct pool *pool = tc->pool;
	unsigned long flags;

	spin_lock_irqsave(&pool->lock, flags);
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	dm_cell_release(cell, &pool->deferred_bios);
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	spin_unlock_irqrestore(&tc->pool->lock, flags);

	wake_worker(pool);
}

/*
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 * Same as cell_defer except it omits the original holder of the cell.
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 */
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static void cell_defer_no_holder(struct thin_c *tc, struct dm_bio_prison_cell *cell)
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{
	struct pool *pool = tc->pool;
	unsigned long flags;

	spin_lock_irqsave(&pool->lock, flags);
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	dm_cell_release_no_holder(cell, &pool->deferred_bios);
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	spin_unlock_irqrestore(&pool->lock, flags);

	wake_worker(pool);
}

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static void process_prepared_mapping_fail(struct dm_thin_new_mapping *m)
{
	if (m->bio)
		m->bio->bi_end_io = m->saved_bi_end_io;
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	dm_cell_error(m->cell);
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	list_del(&m->list);
	mempool_free(m, m->tc->pool->mapping_pool);
}
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static void process_prepared_mapping(struct dm_thin_new_mapping *m)
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{
	struct thin_c *tc = m->tc;
	struct bio *bio;
	int r;

	bio = m->bio;
	if (bio)
		bio->bi_end_io = m->saved_bi_end_io;

	if (m->err) {
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		dm_cell_error(m->cell);
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		goto out;
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	}

	/*
	 * Commit the prepared block into the mapping btree.
	 * Any I/O for this block arriving after this point will get
	 * remapped to it directly.
	 */
	r = dm_thin_insert_block(tc->td, m->virt_block, m->data_block);
	if (r) {
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		DMERR_LIMIT("dm_thin_insert_block() failed");
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		dm_cell_error(m->cell);
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		goto out;
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	}

	/*
	 * Release any bios held while the block was being provisioned.
	 * If we are processing a write bio that completely covers the block,
	 * we already processed it so can ignore it now when processing
	 * the bios in the cell.
	 */
	if (bio) {
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		cell_defer_no_holder(tc, m->cell);
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		bio_endio(bio, 0);
	} else
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		cell_defer(tc, m->cell);
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out:
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	list_del(&m->list);
	mempool_free(m, tc->pool->mapping_pool);
}

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static void process_prepared_discard_fail(struct dm_thin_new_mapping *m)
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{
	struct thin_c *tc = m->tc;

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	bio_io_error(m->bio);
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	cell_defer_no_holder(tc, m->cell);
	cell_defer_no_holder(tc, m->cell2);
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	mempool_free(m, tc->pool->mapping_pool);
}

static void process_prepared_discard_passdown(struct dm_thin_new_mapping *m)
{
	struct thin_c *tc = m->tc;
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	inc_all_io_entry(tc->pool, m->bio);
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	cell_defer_no_holder(tc, m->cell);
	cell_defer_no_holder(tc, m->cell2);
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	if (m->pass_discard)
		remap_and_issue(tc, m->bio, m->data_block);
	else
		bio_endio(m->bio, 0);

	mempool_free(m, tc->pool->mapping_pool);
}

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static void process_prepared_discard(struct dm_thin_new_mapping *m)
{
	int r;
	struct thin_c *tc = m->tc;

	r = dm_thin_remove_block(tc->td, m->virt_block);
	if (r)
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		DMERR_LIMIT("dm_thin_remove_block() failed");
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	process_prepared_discard_passdown(m);
}

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static void process_prepared(struct pool *pool, struct list_head *head,
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			     process_mapping_fn *fn)
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{
	unsigned long flags;
	struct list_head maps;
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	struct dm_thin_new_mapping *m, *tmp;
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	INIT_LIST_HEAD(&maps);
	spin_lock_irqsave(&pool->lock, flags);
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	list_splice_init(head, &maps);
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	spin_unlock_irqrestore(&pool->lock, flags);

	list_for_each_entry_safe(m, tmp, &maps, list)
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		(*fn)(m);
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}

/*
 * Deferred bio jobs.
 */
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static int io_overlaps_block(struct pool *pool, struct bio *bio)
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{
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	return bio->bi_size == (pool->sectors_per_block << SECTOR_SHIFT);
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}

static int io_overwrites_block(struct pool *pool, struct bio *bio)
{
	return (bio_data_dir(bio) == WRITE) &&
		io_overlaps_block(pool, bio);
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}

static void save_and_set_endio(struct bio *bio, bio_end_io_t **save,
			       bio_end_io_t *fn)
{
	*save = bio->bi_end_io;
	bio->bi_end_io = fn;
}

static int ensure_next_mapping(struct pool *pool)
{
	if (pool->next_mapping)
		return 0;

	pool->next_mapping = mempool_alloc(pool->mapping_pool, GFP_ATOMIC);

	return pool->next_mapping ? 0 : -ENOMEM;
}

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static struct dm_thin_new_mapping *get_next_mapping(struct pool *pool)
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{
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	struct dm_thin_new_mapping *r = pool->next_mapping;
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	BUG_ON(!pool->next_mapping);

	pool->next_mapping = NULL;

	return r;
}

static void schedule_copy(struct thin_c *tc, dm_block_t virt_block,
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			  struct dm_dev *origin, dm_block_t data_origin,
			  dm_block_t data_dest,
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			  struct dm_bio_prison_cell *cell, struct bio *bio)
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{
	int r;
	struct pool *pool = tc->pool;
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	struct dm_thin_new_mapping *m = get_next_mapping(pool);
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	INIT_LIST_HEAD(&m->list);
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	m->quiesced = 0;
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	m->prepared = 0;
	m->tc = tc;
	m->virt_block = virt_block;
	m->data_block = data_dest;
	m->cell = cell;
	m->err = 0;
	m->bio = NULL;

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	if (!dm_deferred_set_add_work(pool->shared_read_ds, &m->list))
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		m->quiesced = 1;
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	/*
	 * IO to pool_dev remaps to the pool target's data_dev.
	 *
	 * If the whole block of data is being overwritten, we can issue the
	 * bio immediately. Otherwise we use kcopyd to clone the data first.
	 */
	if (io_overwrites_block(pool, bio)) {
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		struct dm_thin_endio_hook *h = dm_per_bio_data(bio, sizeof(struct dm_thin_endio_hook));
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		h->overwrite_mapping = m;
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		m->bio = bio;
		save_and_set_endio(bio, &m->saved_bi_end_io, overwrite_endio);
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		inc_all_io_entry(pool, bio);
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		remap_and_issue(tc, bio, data_dest);
	} else {
		struct dm_io_region from, to;

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		from.bdev = origin->bdev;
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		from.sector = data_origin * pool->sectors_per_block;
		from.count = pool->sectors_per_block;

		to.bdev = tc->pool_dev->bdev;
		to.sector = data_dest * pool->sectors_per_block;
		to.count = pool->sectors_per_block;

		r = dm_kcopyd_copy(pool->copier, &from, 1, &to,
				   0, copy_complete, m);
		if (r < 0) {
			mempool_free(m, pool->mapping_pool);
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			DMERR_LIMIT("dm_kcopyd_copy() failed");
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			dm_cell_error(cell);
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		}
	}
}

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static void schedule_internal_copy(struct thin_c *tc, dm_block_t virt_block,
				   dm_block_t data_origin, dm_block_t data_dest,
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				   struct dm_bio_prison_cell *cell, struct bio *bio)
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{
	schedule_copy(tc, virt_block, tc->pool_dev,
		      data_origin, data_dest, cell, bio);
}

static void schedule_external_copy(struct thin_c *tc, dm_block_t virt_block,
				   dm_block_t data_dest,
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				   struct dm_bio_prison_cell *cell, struct bio *bio)
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{
	schedule_copy(tc, virt_block, tc->origin_dev,
		      virt_block, data_dest, cell, bio);
}

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static void schedule_zero(struct thin_c *tc, dm_block_t virt_block,
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			  dm_block_t data_block, struct dm_bio_prison_cell *cell,
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			  struct bio *bio)
{
	struct pool *pool = tc->pool;
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	struct dm_thin_new_mapping *m = get_next_mapping(pool);
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	INIT_LIST_HEAD(&m->list);
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	m->quiesced = 1;
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	m->prepared = 0;
	m->tc = tc;
	m->virt_block = virt_block;
	m->data_block = data_block;
	m->cell = cell;
	m->err = 0;
	m->bio = NULL;

	/*
	 * If the whole block of data is being overwritten or we are not
	 * zeroing pre-existing data, we can issue the bio immediately.
	 * Otherwise we use kcopyd to zero the data first.
	 */
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	if (!pool->pf.zero_new_blocks)
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		process_prepared_mapping(m);

	else if (io_overwrites_block(pool, bio)) {
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		struct dm_thin_endio_hook *h = dm_per_bio_data(bio, sizeof(struct dm_thin_endio_hook));
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		h->overwrite_mapping = m;
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		m->bio = bio;
		save_and_set_endio(bio, &m->saved_bi_end_io, overwrite_endio);
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		inc_all_io_entry(pool, bio);
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		remap_and_issue(tc, bio, data_block);
	} else {
		int r;
		struct dm_io_region to;

		to.bdev = tc->pool_dev->bdev;
		to.sector = data_block * pool->sectors_per_block;
		to.count = pool->sectors_per_block;

		r = dm_kcopyd_zero(pool->copier, 1, &to, 0, copy_complete, m);
		if (r < 0) {
			mempool_free(m, pool->mapping_pool);
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			DMERR_LIMIT("dm_kcopyd_zero() failed");
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			dm_cell_error(cell);
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		}
	}
}

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static int commit(struct pool *pool)
{
	int r;

	r = dm_pool_commit_metadata(pool->pmd);
	if (r)
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		DMERR_LIMIT("commit failed: error = %d", r);
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	return r;
}

/*
 * A non-zero return indicates read_only or fail_io mode.
 * Many callers don't care about the return value.
 */
static int commit_or_fallback(struct pool *pool)
{
	int r;

	if (get_pool_mode(pool) != PM_WRITE)
		return -EINVAL;

	r = commit(pool);
	if (r)
		set_pool_mode(pool, PM_READ_ONLY);

	return r;
}

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static int alloc_data_block(struct thin_c *tc, dm_block_t *result)
{
	int r;
	dm_block_t free_blocks;
	unsigned long flags;
	struct pool *pool = tc->pool;

	r = dm_pool_get_free_block_count(pool->pmd, &free_blocks);
	if (r)
		return r;

	if (free_blocks <= pool->low_water_blocks && !pool->low_water_triggered) {
		DMWARN("%s: reached low water mark, sending event.",
		       dm_device_name(pool->pool_md));
		spin_lock_irqsave(&pool->lock, flags);
		pool->low_water_triggered = 1;
		spin_unlock_irqrestore(&pool->lock, flags);
		dm_table_event(pool->ti->table);
	}

	if (!free_blocks) {
		if (pool->no_free_space)
			return -ENOSPC;
		else {
			/*
			 * Try to commit to see if that will free up some
			 * more space.
			 */
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			(void) commit_or_fallback(pool);
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			r = dm_pool_get_free_block_count(pool->pmd, &free_blocks);
			if (r)
				return r;

			/*
			 * If we still have no space we set a flag to avoid
			 * doing all this checking and return -ENOSPC.
			 */
			if (!free_blocks) {
				DMWARN("%s: no free space available.",
				       dm_device_name(pool->pool_md));
				spin_lock_irqsave(&pool->lock, flags);
				pool->no_free_space = 1;
				spin_unlock_irqrestore(&pool->lock, flags);
				return -ENOSPC;
			}
		}
	}

	r = dm_pool_alloc_data_block(pool->pmd, result);
	if (r)
		return r;

	return 0;
}

/*
 * If we have run out of space, queue bios until the device is
 * resumed, presumably after having been reloaded with more space.
 */
static void retry_on_resume(struct bio *bio)
{
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	struct dm_thin_endio_hook *h = dm_per_bio_data(bio, sizeof(struct dm_thin_endio_hook));
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	struct thin_c *tc = h->tc;
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	struct pool *pool = tc->pool;
	unsigned long flags;

	spin_lock_irqsave(&pool->lock, flags);
	bio_list_add(&pool->retry_on_resume_list, bio);
	spin_unlock_irqrestore(&pool->lock, flags);
}

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static void no_space(struct dm_bio_prison_cell *cell)
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{
	struct bio *bio;
	struct bio_list bios;

	bio_list_init(&bios);
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	dm_cell_release(cell, &bios);
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	while ((bio = bio_list_pop(&bios)))
		retry_on_resume(bio);
}

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static void process_discard(struct thin_c *tc, struct bio *bio)
{
	int r;
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	unsigned long flags;
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	struct pool *pool = tc->pool;
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	struct dm_bio_prison_cell *cell, *cell2;
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	struct dm_cell_key key, key2;
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	dm_block_t block = get_bio_block(tc, bio);
	struct dm_thin_lookup_result lookup_result;
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	struct dm_thin_new_mapping *m;
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	build_virtual_key(tc->td, block, &key);
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	if (dm_bio_detain(tc->pool->prison, &key, bio, &cell))
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		return;

	r = dm_thin_find_block(tc->td, block, 1, &lookup_result);
	switch (r) {
	case 0:
		/*
		 * Check nobody is fiddling with this pool block.  This can
		 * happen if someone's in the process of breaking sharing
		 * on this block.
		 */
		build_data_key(tc->td, lookup_result.block, &key2);
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		if (dm_bio_detain(tc->pool->prison, &key2, bio, &cell2)) {
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			cell_defer_no_holder(tc, cell);
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			break;
		}

		if (io_overlaps_block(pool, bio)) {
			/*
			 * IO may still be going to the destination block.  We must
			 * quiesce before we can do the removal.
			 */
			m = get_next_mapping(pool);
			m->tc = tc;
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			m->pass_discard = (!lookup_result.shared) && pool->pf.discard_passdown;
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			m->virt_block = block;
			m->data_block = lookup_result.block;
			m->cell = cell;
			m->cell2 = cell2;
			m->err = 0;
			m->bio = bio;

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			if (!dm_deferred_set_add_work(pool->all_io_ds, &m->list)) {
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				spin_lock_irqsave(&pool->lock, flags);
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				list_add(&m->list, &pool->prepared_discards);
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				spin_unlock_irqrestore(&pool->lock, flags);
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				wake_worker(pool);
			}
		} else {
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			inc_all_io_entry(pool, bio);
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			cell_defer_no_holder(tc, cell);
			cell_defer_no_holder(tc, cell2);
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			/*
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			 * The DM core makes sure that the discard doesn't span
			 * a block boundary.  So we submit the discard of a
			 * partial block appropriately.
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			 */
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			if ((!lookup_result.shared) && pool->pf.discard_passdown)
				remap_and_issue(tc, bio, lookup_result.block);
			else
				bio_endio(bio, 0);
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		}
		break;

	case -ENODATA:
		/*
		 * It isn't provisioned, just forget it.
		 */
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		cell_defer_no_holder(tc, cell);
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		bio_endio(bio, 0);
		break;

	default:
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		DMERR_LIMIT("%s: dm_thin_find_block() failed: error = %d",
			    __func__, r);
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		cell_defer_no_holder(tc, cell);
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		bio_io_error(bio);
		break;
	}
}

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static void break_sharing(struct thin_c *tc, struct bio *bio, dm_block_t block,
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			  struct dm_cell_key *key,
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			  struct dm_thin_lookup_result *lookup_result,
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			  struct dm_bio_prison_cell *cell)
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{
	int r;
	dm_block_t data_block;

	r = alloc_data_block(tc, &data_block);
	switch (r) {
	case 0:
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		schedule_internal_copy(tc, block, lookup_result->block,
				       data_block, cell, bio);
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		break;

	case -ENOSPC:
		no_space(cell);
		break;

	default:
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		DMERR_LIMIT("%s: alloc_data_block() failed: error = %d",
			    __func__, r);
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		dm_cell_error(cell);
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		break;
	}
}

static void process_shared_bio(struct thin_c *tc, struct bio *bio,
			       dm_block_t block,
			       struct dm_thin_lookup_result *lookup_result)
{
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	struct dm_bio_prison_cell *cell;
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	struct pool *pool = tc->pool;
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	struct dm_cell_key key;
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	/*
	 * If cell is already occupied, then sharing is already in the process
	 * of being broken so we have nothing further to do here.
	 */
	build_data_key(tc->td, lookup_result->block, &key);
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	if (dm_bio_detain(pool->prison, &key, bio, &cell))
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		return;

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	if (bio_data_dir(bio) == WRITE && bio->bi_size)
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		break_sharing(tc, bio, block, &key, lookup_result, cell);
	else {
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		struct dm_thin_endio_hook *h = dm_per_bio_data(bio, sizeof(struct dm_thin_endio_hook));
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		h->shared_read_entry = dm_deferred_entry_inc(pool->shared_read_ds);
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		inc_all_io_entry(pool, bio);
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		cell_defer_no_holder(tc, cell);
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		remap_and_issue(tc, bio, lookup_result->block);
	}
}

static void provision_block(struct thin_c *tc, struct bio *bio, dm_block_t block,
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			    struct dm_bio_prison_cell *cell)
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{
	int r;
	dm_block_t data_block;

	/*
	 * Remap empty bios (flushes) immediately, without provisioning.
	 */
	if (!bio->bi_size) {
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		inc_all_io_entry(tc->pool, bio);
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		cell_defer_no_holder(tc, cell);
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		remap_and_issue(tc, bio, 0);
		return;
	}

	/*
	 * Fill read bios with zeroes and complete them immediately.
	 */
	if (bio_data_dir(bio) == READ) {
		zero_fill_bio(bio);
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		cell_defer_no_holder(tc, cell);
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		bio_endio(bio, 0);
		return;
	}

	r = alloc_data_block(tc, &data_block);
	switch (r) {
	case 0:
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		if (tc->origin_dev)
			schedule_external_copy(tc, block, data_block, cell, bio);
		else
			schedule_zero(tc, block, data_block, cell, bio);
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		break;

	case -ENOSPC:
		no_space(cell);
		break;

	default:
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		DMERR_LIMIT("%s: alloc_data_block() failed: error = %d",
			    __func__, r);
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		set_pool_mode(tc->pool, PM_READ_ONLY);
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		dm_cell_error(cell);
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		break;
	}
}

static void process_bio(struct thin_c *tc, struct bio *bio)
{
	int r;
	dm_block_t block = get_bio_block(tc, bio);
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	struct dm_bio_prison_cell *cell;
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	struct dm_cell_key key;
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	struct dm_thin_lookup_result lookup_result;

	/*
	 * If cell is already occupied, then the block is already
	 * being provisioned so we have nothing further to do here.
	 */
	build_virtual_key(tc->td, block, &key);
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	if (dm_bio_detain(tc->pool->prison, &key, bio, &cell))
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		return;

	r = dm_thin_find_block(tc->td, block, 1, &lookup_result);
	switch (r) {
	case 0:
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		if (lookup_result.shared) {
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			process_shared_bio(tc, bio, block, &lookup_result);
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			cell_defer_no_holder(tc, cell);
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		} else {
			inc_all_io_entry(tc->pool, bio);
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			cell_defer_no_holder(tc, cell);
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			remap_and_issue(tc, bio, lookup_result.block);
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		}
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		break;

	case -ENODATA:
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		if (bio_data_dir(bio) == READ && tc->origin_dev) {
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			inc_all_io_entry(tc->pool, bio);
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			cell_defer_no_holder(tc, cell);
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			remap_to_origin_and_issue(tc, bio);
		} else
			provision_block(tc, bio, block, cell);
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		break;

	default:
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		DMERR_LIMIT("%s: dm_thin_find_block() failed: error = %d",
			    __func__, r);
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		cell_defer_no_holder(tc, cell);
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		bio_io_error(bio);
		break;
	}
}

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static void process_bio_read_only(struct thin_c *tc, struct bio *bio)
{
	int r;
	int rw = bio_data_dir(bio);
	dm_block_t block = get_bio_block(tc, bio);
	struct dm_thin_lookup_result lookup_result;

	r = dm_thin_find_block(tc->td, block, 1, &lookup_result);
	switch (r) {
	case 0:
		if (lookup_result.shared && (rw == WRITE) && bio->bi_size)
			bio_io_error(bio);
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		else {
			inc_all_io_entry(tc->pool, bio);
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			remap_and_issue(tc, bio, lookup_result.block);
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		}
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		break;

	case -ENODATA:
		if (rw != READ) {
			bio_io_error(bio);
			break;
		}

		if (tc->origin_dev) {
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			inc_all_io_entry(tc->pool, bio);
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			remap_to_origin_and_issue(tc, bio);
			break;
		}

		zero_fill_bio(bio);
		bio_endio(bio, 0);
		break;

	default:
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		DMERR_LIMIT("%s: dm_thin_find_block() failed: error = %d",
			    __func__, r);
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		bio_io_error(bio);
		break;
	}
}

static void process_bio_fail(struct thin_c *tc, struct bio *bio)
{
	bio_io_error(bio);
}

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static int need_commit_due_to_time(struct pool *pool)
{
	return jiffies < pool->last_commit_jiffies ||
	       jiffies > pool->last_commit_jiffies + COMMIT_PERIOD;
}

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static void process_deferred_bios(struct pool *pool)
{
	unsigned long flags;
	struct bio *bio;
	struct bio_list bios;

	bio_list_init(&bios);

	spin_lock_irqsave(&pool->lock, flags);
	bio_list_merge(&bios, &pool->deferred_bios);
	bio_list_init(&pool->deferred_bios);
	spin_unlock_irqrestore(&pool->lock, flags);

	while ((bio = bio_list_pop(&bios))) {
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		struct dm_thin_endio_hook *h = dm_per_bio_data(bio, sizeof(struct dm_thin_endio_hook));
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		struct thin_c *tc = h->tc;

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		/*
		 * If we've got no free new_mapping structs, and processing
		 * this bio might require one, we pause until there are some
		 * prepared mappings to process.
		 */
		if (ensure_next_mapping(pool)) {
			spin_lock_irqsave(&pool->lock, flags);
			bio_list_merge(&pool->deferred_bios, &bios);
			spin_unlock_irqrestore(&pool->lock, flags);

			break;
		}
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		if (bio->bi_rw & REQ_DISCARD)
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			pool->process_discard(tc, bio);
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		else
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			pool->process_bio(tc, bio);
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	}

	/*
	 * If there are any deferred flush bios, we must commit
	 * the metadata before issuing them.
	 */
	bio_list_init(&bios);
	spin_lock_irqsave(&pool->lock, flags);
	bio_list_merge(&bios, &pool->deferred_flush_bios);
	bio_list_init(&pool->deferred_flush_bios);
	spin_unlock_irqrestore(&pool->lock, flags);

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	if (bio_list_empty(&bios) && !need_commit_due_to_time(pool))
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		return;

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	if (commit_or_fallback(pool)) {
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		while ((bio = bio_list_pop(&bios)))
			bio_io_error(bio);
		return;
	}
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	pool->last_commit_jiffies = jiffies;
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	while ((bio = bio_list_pop(&bios)))
		generic_make_request(bio);
}

static void do_worker(struct work_struct *ws)
{
	struct pool *pool = container_of(ws, struct pool, worker);

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	process_prepared(pool, &pool->prepared_mappings, &pool->process_prepared_mapping);
	process_prepared(pool, &pool->prepared_discards, &pool->process_prepared_discard);
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	process_deferred_bios(pool);
}

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/*
 * We want to commit periodically so that not too much
 * unwritten data builds up.
 */
static void do_waker(struct work_struct *ws)
{
	struct pool *pool = container_of(to_delayed_work(ws), struct pool, waker);
	wake_worker(pool);
	queue_delayed_work(pool->wq, &pool->waker, COMMIT_PERIOD);
}

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/*----------------------------------------------------------------*/

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static enum pool_mode get_pool_mode(struct pool *pool)
{
	return pool->pf.mode;
}

static void set_pool_mode(struct pool *pool, enum pool_mode mode)
{
	int r;

	pool->pf.mode = mode;

	switch (mode) {
	case PM_FAIL:
		DMERR("switching pool to failure mode");
		pool->process_bio = process_bio_fail;
		pool->process_discard = process_bio_fail;
		pool->process_prepared_mapping = process_prepared_mapping_fail;
		pool->process_prepared_discard = process_prepared_discard_fail;
		break;

	case PM_READ_ONLY:
		DMERR("switching pool to read-only mode");
		r = dm_pool_abort_metadata(pool->pmd);
		if (r) {
			DMERR("aborting transaction failed");
			set_pool_mode(pool, PM_FAIL);
		} else {
			dm_pool_metadata_read_only(pool->pmd);
			pool->process_bio = process_bio_read_only;
			pool->process_discard = process_discard;
			pool->process_prepared_mapping = process_prepared_mapping_fail;
			pool->process_prepared_discard = process_prepared_discard_passdown;
		}
		break;

	case PM_WRITE:
		pool->process_bio = process_bio;
		pool->process_discard = process_discard;
		pool->process_prepared_mapping = process_prepared_mapping;
		pool->process_prepared_discard = process_prepared_discard;
		break;
	}
}

/*----------------------------------------------------------------*/

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/*
 * Mapping functions.
 */

/*
 * Called only while mapping a thin bio to hand it over to the workqueue.
 */
static void thin_defer_bio(struct thin_c *tc, struct bio *bio)
{
	unsigned long flags;
	struct pool *pool = tc->pool;

	spin_lock_irqsave(&pool->lock, flags);
	bio_list_add(&pool->deferred_bios, bio);
	spin_unlock_irqrestore(&pool->lock, flags);

	wake_worker(pool);
}

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static void thin_hook_bio(struct thin_c *tc, struct bio *bio)
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{
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	struct dm_thin_endio_hook *h = dm_per_bio_data(bio, sizeof(struct dm_thin_endio_hook));
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	h->tc = tc;
	h->shared_read_entry = NULL;
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	h->all_io_entry = NULL;
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	h->overwrite_mapping = NULL;
}

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/*
 * Non-blocking function called from the thin target's map function.
 */
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static int thin_bio_map(struct dm_target *ti, struct bio *bio)
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{
	int r;
	struct thin_c *tc = ti->private;
	dm_block_t block = get_bio_block(tc, bio);
	struct dm_thin_device *td = tc->td;
	struct dm_thin_lookup_result result;
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	struct dm_bio_prison_cell *cell1, *cell2;
	struct dm_cell_key key;
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	thin_hook_bio(tc, bio);
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	if (get_pool_mode(tc->pool) == PM_FAIL) {
		bio_io_error(bio);
		return DM_MAPIO_SUBMITTED;
	}

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	if (bio->bi_rw & (REQ_DISCARD | REQ_FLUSH | REQ_FUA)) {
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		thin_defer_bio(tc, bio);
		return DM_MAPIO_SUBMITTED;
	}

	r = dm_thin_find_block(td, block, 0, &result);

	/*
	 * Note that we defer readahead too.
	 */
	switch (r) {
	case 0:
		if (unlikely(result.shared)) {
			/*
			 * We have a race condition here between the
			 * result.shared value returned by the lookup and
			 * snapshot creation, which may cause new
			 * sharing.
			 *
			 * To avoid this always quiesce the origin before
			 * taking the snap.  You want to do this anyway to
			 * ensure a consistent application view
			 * (i.e. lockfs).
			 *
			 * More distant ancestors are irrelevant. The
			 * shared flag will be set in their case.
			 */
			thin_defer_bio(tc, bio);
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			return DM_MAPIO_SUBMITTED;
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		}
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		build_virtual_key(tc->td, block, &key);
		if (dm_bio_detain(tc->pool->prison, &key, bio, &cell1))
			return DM_MAPIO_SUBMITTED;

		build_data_key(tc->td, result.block, &key);
		if (dm_bio_detain(tc->pool->prison, &key, bio, &cell2)) {
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			cell_defer_no_holder(tc, cell1);
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			return DM_MAPIO_SUBMITTED;
		}

		inc_all_io_entry(tc->pool, bio);
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		cell_defer_no_holder(tc, cell2);
		cell_defer_no_holder(tc, cell1);
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		remap(tc, bio, result.block);
		return DM_MAPIO_REMAPPED;
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	case -ENODATA:
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