drivers/md/dm-mpath.c

/*
 * Copyright (C) 2003 Sistina Software Limited.
 * Copyright (C) 2004-2005 Red Hat, Inc. All rights reserved.
 *
 * This file is released under the GPL.
 */

#include "dm.h"
#include "dm-path-selector.h"
#include "dm-hw-handler.h"
#include "dm-bio-list.h"
#include "dm-bio-record.h"

#include <linux/ctype.h>
#include <linux/init.h>
#include <linux/mempool.h>
#include <linux/module.h>
#include <linux/pagemap.h>
#include <linux/slab.h>
#include <linux/time.h>
#include <linux/workqueue.h>
#include <asm/atomic.h>

#define MESG_STR(x) x, sizeof(x)

/* Path properties */
struct pgpath {
	struct list_head list;

	struct priority_group *pg;	/* Owning PG */
	unsigned fail_count;		/* Cumulative failure count */

	struct path path;
};

#define path_to_pgpath(__pgp) container_of((__pgp), struct pgpath, path)

/*
 * Paths are grouped into Priority Groups and numbered from 1 upwards.
 * Each has a path selector which controls which path gets used.
 */
struct priority_group {
	struct list_head list;

	struct multipath *m;		/* Owning multipath instance */
	struct path_selector ps;

	unsigned pg_num;		/* Reference number */
	unsigned bypassed;		/* Temporarily bypass this PG? */

	unsigned nr_pgpaths;		/* Number of paths in PG */
	struct list_head pgpaths;
};

/* Multipath context */
struct multipath {
	struct list_head list;
	struct dm_target *ti;

	spinlock_t lock;

	struct hw_handler hw_handler;
	unsigned nr_priority_groups;
	struct list_head priority_groups;
	unsigned pg_init_required;	/* pg_init needs calling? */

	unsigned nr_valid_paths;	/* Total number of usable paths */
	struct pgpath *current_pgpath;
	struct priority_group *current_pg;
	struct priority_group *next_pg;	/* Switch to this PG if set */
	unsigned repeat_count;		/* I/Os left before calling PS again */

	unsigned queue_io;		/* Must we queue all I/O? */
	unsigned queue_if_no_path;	/* Queue I/O if last path fails? */
	unsigned suspended;		/* Has dm core suspended our I/O? */

	struct work_struct process_queued_ios;
	struct bio_list queued_ios;
	unsigned queue_size;

	struct work_struct trigger_event;

	/*
	 * We must use a mempool of mpath_io structs so that we
	 * can resubmit bios on error.
	 */
	mempool_t *mpio_pool;
};

/*
 * Context information attached to each bio we process.
 */
struct mpath_io {
	struct pgpath *pgpath;
	struct dm_bio_details details;
};

typedef int (*action_fn) (struct pgpath *pgpath);

#define MIN_IOS 256	/* Mempool size */

static kmem_cache_t *_mpio_cache;

struct workqueue_struct *kmultipathd;
static void process_queued_ios(void *data);
static void trigger_event(void *data);


/*-----------------------------------------------
 * Allocation routines
 *-----------------------------------------------*/

static struct pgpath *alloc_pgpath(void)
{
	struct pgpath *pgpath = kmalloc(sizeof(*pgpath), GFP_KERNEL);

	if (pgpath) {
		memset(pgpath, 0, sizeof(*pgpath));
		pgpath->path.is_active = 1;
	}

	return pgpath;
}

static inline void free_pgpath(struct pgpath *pgpath)
{
	kfree(pgpath);
}

static struct priority_group *alloc_priority_group(void)
{
	struct priority_group *pg;

	pg = kmalloc(sizeof(*pg), GFP_KERNEL);
	if (!pg)
		return NULL;

	memset(pg, 0, sizeof(*pg));
	INIT_LIST_HEAD(&pg->pgpaths);

	return pg;
}

static void free_pgpaths(struct list_head *pgpaths, struct dm_target *ti)
{
	struct pgpath *pgpath, *tmp;

	list_for_each_entry_safe(pgpath, tmp, pgpaths, list) {
		list_del(&pgpath->list);
		dm_put_device(ti, pgpath->path.dev);
		free_pgpath(pgpath);
	}
}

static void free_priority_group(struct priority_group *pg,
				struct dm_target *ti)
{
	struct path_selector *ps = &pg->ps;

	if (ps->type) {
		ps->type->destroy(ps);
		dm_put_path_selector(ps->type);
	}

	free_pgpaths(&pg->pgpaths, ti);
	kfree(pg);
}

static struct multipath *alloc_multipath(void)
{
	struct multipath *m;

	m = kmalloc(sizeof(*m), GFP_KERNEL);
	if (m) {
		memset(m, 0, sizeof(*m));
		INIT_LIST_HEAD(&m->priority_groups);
		spin_lock_init(&m->lock);
		m->queue_io = 1;
		INIT_WORK(&m->process_queued_ios, process_queued_ios, m);
		INIT_WORK(&m->trigger_event, trigger_event, m);
		m->mpio_pool = mempool_create(MIN_IOS, mempool_alloc_slab,
					      mempool_free_slab, _mpio_cache);
		if (!m->mpio_pool) {
			kfree(m);
			return NULL;
		}
	}

	return m;
}

static void free_multipath(struct multipath *m)
{
	struct priority_group *pg, *tmp;
	struct hw_handler *hwh = &m->hw_handler;

	list_for_each_entry_safe(pg, tmp, &m->priority_groups, list) {
		list_del(&pg->list);
		free_priority_group(pg, m->ti);
	}

	if (hwh->type) {
		hwh->type->destroy(hwh);
		dm_put_hw_handler(hwh->type);
	}

	mempool_destroy(m->mpio_pool);
	kfree(m);
}


/*-----------------------------------------------
 * Path selection
 *-----------------------------------------------*/

static void __switch_pg(struct multipath *m, struct pgpath *pgpath)
{
	struct hw_handler *hwh = &m->hw_handler;

	m->current_pg = pgpath->pg;

	/* Must we initialise the PG first, and queue I/O till it's ready? */
	if (hwh->type && hwh->type->pg_init) {
		m->pg_init_required = 1;
		m->queue_io = 1;
	} else {
		m->pg_init_required = 0;
		m->queue_io = 0;
	}
}

static int __choose_path_in_pg(struct multipath *m, struct priority_group *pg)
{
	struct path *path;

	path = pg->ps.type->select_path(&pg->ps, &m->repeat_count);
	if (!path)
		return -ENXIO;

	m->current_pgpath = path_to_pgpath(path);

	if (m->current_pg != pg)
		__switch_pg(m, m->current_pgpath);

	return 0;
}

static void __choose_pgpath(struct multipath *m)
{
	struct priority_group *pg;
	unsigned bypassed = 1;

	if (!m->nr_valid_paths)
		goto failed;

	/* Were we instructed to switch PG? */
	if (m->next_pg) {
		pg = m->next_pg;
		m->next_pg = NULL;
		if (!__choose_path_in_pg(m, pg))
			return;
	}

	/* Don't change PG until it has no remaining paths */
	if (m->current_pg && !__choose_path_in_pg(m, m->current_pg))
		return;

	/*
	 * Loop through priority groups until we find a valid path.
	 * First time we skip PGs marked 'bypassed'.
	 * Second time we only try the ones we skipped.
	 */
	do {
		list_for_each_entry(pg, &m->priority_groups, list) {
			if (pg->bypassed == bypassed)
				continue;
			if (!__choose_path_in_pg(m, pg))
				return;
		}
	} while (bypassed--);

failed:
	m->current_pgpath = NULL;
	m->current_pg = NULL;
}

static int map_io(struct multipath *m, struct bio *bio, struct mpath_io *mpio,
		  unsigned was_queued)
{
	int r = 1;
	unsigned long flags;
	struct pgpath *pgpath;

	spin_lock_irqsave(&m->lock, flags);

	/* Do we need to select a new pgpath? */
	if (!m->current_pgpath ||
	    (!m->queue_io && (m->repeat_count && --m->repeat_count == 0)))
		__choose_pgpath(m);

	pgpath = m->current_pgpath;

	if (was_queued)
		m->queue_size--;

	if ((pgpath && m->queue_io) ||
	    (!pgpath && m->queue_if_no_path && !m->suspended)) {
		/* Queue for the daemon to resubmit */
		bio_list_add(&m->queued_ios, bio);
		m->queue_size++;
		if (m->pg_init_required || !m->queue_io)
			queue_work(kmultipathd, &m->process_queued_ios);
		pgpath = NULL;
		r = 0;
	} else if (!pgpath)
		r = -EIO;		/* Failed */
	else
		bio->bi_bdev = pgpath->path.dev->bdev;

	mpio->pgpath = pgpath;

	spin_unlock_irqrestore(&m->lock, flags);

	return r;
}

/*
 * If we run out of usable paths, should we queue I/O or error it?
 */
static int queue_if_no_path(struct multipath *m, unsigned queue_if_no_path)
{
	unsigned long flags;

	spin_lock_irqsave(&m->lock, flags);

	m->queue_if_no_path = queue_if_no_path;
	if (!m->queue_if_no_path)
		queue_work(kmultipathd, &m->process_queued_ios);

	spin_unlock_irqrestore(&m->lock, flags);

	return 0;
}

/*-----------------------------------------------------------------
 * The multipath daemon is responsible for resubmitting queued ios.
 *---------------------------------------------------------------*/

static void dispatch_queued_ios(struct multipath *m)
{
	int r;
	unsigned long flags;
	struct bio *bio = NULL, *next;
	struct mpath_io *mpio;
	union map_info *info;

	spin_lock_irqsave(&m->lock, flags);
	bio = bio_list_get(&m->queued_ios);
	spin_unlock_irqrestore(&m->lock, flags);

	while (bio) {
		next = bio->bi_next;
		bio->bi_next = NULL;

		info = dm_get_mapinfo(bio);
		mpio = info->ptr;

		r = map_io(m, bio, mpio, 1);
		if (r < 0)
			bio_endio(bio, bio->bi_size, r);
		else if (r == 1)
			generic_make_request(bio);

		bio = next;
	}
}

static void process_queued_ios(void *data)
{
	struct multipath *m = (struct multipath *) data;
	struct hw_handler *hwh = &m->hw_handler;
	struct pgpath *pgpath;
	unsigned init_required, must_queue = 0;
	unsigned long flags;

	spin_lock_irqsave(&m->lock, flags);

	if (!m->current_pgpath)
		__choose_pgpath(m);

	pgpath = m->current_pgpath;

	if ((pgpath && m->queue_io) ||
	    (!pgpath && m->queue_if_no_path && !m->suspended))
		must_queue = 1;

	init_required = m->pg_init_required;
	if (init_required)
		m->pg_init_required = 0;

	spin_unlock_irqrestore(&m->lock, flags);

	if (init_required)
		hwh->type->pg_init(hwh, pgpath->pg->bypassed, &pgpath->path);

	if (!must_queue)
		dispatch_queued_ios(m);
}

/*
 * An event is triggered whenever a path is taken out of use.
 * Includes path failure and PG bypass.
 */
static void trigger_event(void *data)
{
	struct multipath *m = (struct multipath *) data;

	dm_table_event(m->ti->table);
}

/*-----------------------------------------------------------------
 * Constructor/argument parsing:
 * <#multipath feature args> [<arg>]*
 * <#hw_handler args> [hw_handler [<arg>]*]
 * <#priority groups>
 * <initial priority group>
 *     [<selector> <#selector args> [<arg>]*
 *      <#paths> <#per-path selector args>
 *         [<path> [<arg>]* ]+ ]+
 *---------------------------------------------------------------*/
struct param {
	unsigned min;
	unsigned max;
	char *error;
};

#define ESTR(s) ("dm-multipath: " s)

static int read_param(struct param *param, char *str, unsigned *v, char **error)
{
	if (!str ||
	    (sscanf(str, "%u", v) != 1) ||
	    (*v < param->min) ||
	    (*v > param->max)) {
		*error = param->error;
		return -EINVAL;
	}

	return 0;
}

struct arg_set {
	unsigned argc;
	char **argv;
};

static char *shift(struct arg_set *as)
{
	char *r;

	if (as->argc) {
		as->argc--;
		r = *as->argv;
		as->argv++;
		return r;
	}

	return NULL;
}

static void consume(struct arg_set *as, unsigned n)
{
	BUG_ON (as->argc < n);
	as->argc -= n;
	as->argv += n;
}

static int parse_path_selector(struct arg_set *as, struct priority_group *pg,
			       struct dm_target *ti)
{
	int r;
	struct path_selector_type *pst;
	unsigned ps_argc;

	static struct param _params[] = {
		{0, 1024, ESTR("invalid number of path selector args")},
	};

	pst = dm_get_path_selector(shift(as));
	if (!pst) {
		ti->error = ESTR("unknown path selector type");
		return -EINVAL;
	}

	r = read_param(_params, shift(as), &ps_argc, &ti->error);
	if (r)
		return -EINVAL;

	r = pst->create(&pg->ps, ps_argc, as->argv);
	if (r) {
		dm_put_path_selector(pst);
		ti->error = ESTR("path selector constructor failed");
		return r;
	}

	pg->ps.type = pst;
	consume(as, ps_argc);

	return 0;
}

static struct pgpath *parse_path(struct arg_set *as, struct path_selector *ps,
			       struct dm_target *ti)
{
	int r;
	struct pgpath *p;

	/* we need at least a path arg */
	if (as->argc < 1) {
		ti->error = ESTR("no device given");
		return NULL;
	}

	p = alloc_pgpath();
	if (!p)
		return NULL;

	r = dm_get_device(ti, shift(as), ti->begin, ti->len,
			  dm_table_get_mode(ti->table), &p->path.dev);
	if (r) {
		ti->error = ESTR("error getting device");
		goto bad;
	}

	r = ps->type->add_path(ps, &p->path, as->argc, as->argv, &ti->error);
	if (r) {
		dm_put_device(ti, p->path.dev);
		goto bad;
	}

	return p;

 bad:
	free_pgpath(p);
	return NULL;
}

static struct priority_group *parse_priority_group(struct arg_set *as,
						   struct multipath *m,
						   struct dm_target *ti)
{
	static struct param _params[] = {
		{1, 1024, ESTR("invalid number of paths")},
		{0, 1024, ESTR("invalid number of selector args")}
	};

	int r;
	unsigned i, nr_selector_args, nr_params;
	struct priority_group *pg;

	if (as->argc < 2) {
		as->argc = 0;
		ti->error = ESTR("not enough priority group aruments");
		return NULL;
	}

	pg = alloc_priority_group();
	if (!pg) {
		ti->error = ESTR("couldn't allocate priority group");
		return NULL;
	}
	pg->m = m;

	r = parse_path_selector(as, pg, ti);
	if (r)
		goto bad;

	/*
	 * read the paths
	 */
	r = read_param(_params, shift(as), &pg->nr_pgpaths, &ti->error);
	if (r)
		goto bad;

	r = read_param(_params + 1, shift(as), &nr_selector_args, &ti->error);
	if (r)
		goto bad;

	nr_params = 1 + nr_selector_args;
	for (i = 0; i < pg->nr_pgpaths; i++) {
		struct pgpath *pgpath;
		struct arg_set path_args;

		if (as->argc < nr_params)
			goto bad;

		path_args.argc = nr_params;
		path_args.argv = as->argv;

		pgpath = parse_path(&path_args, &pg->ps, ti);
		if (!pgpath)
			goto bad;

		pgpath->pg = pg;
		list_add_tail(&pgpath->list, &pg->pgpaths);
		consume(as, nr_params);
	}

	return pg;

 bad:
	free_priority_group(pg, ti);
	return NULL;
}

static int parse_hw_handler(struct arg_set *as, struct multipath *m,
			    struct dm_target *ti)
{
	int r;
	struct hw_handler_type *hwht;
	unsigned hw_argc;

	static struct param _params[] = {
		{0, 1024, ESTR("invalid number of hardware handler args")},
	};

	r = read_param(_params, shift(as), &hw_argc, &ti->error);
	if (r)
		return -EINVAL;

	if (!hw_argc)
		return 0;

	hwht = dm_get_hw_handler(shift(as));
	if (!hwht) {
		ti->error = ESTR("unknown hardware handler type");
		return -EINVAL;
	}

	r = hwht->create(&m->hw_handler, hw_argc - 1, as->argv);
	if (r) {
		dm_put_hw_handler(hwht);
		ti->error = ESTR("hardware handler constructor failed");
		return r;
	}

	m->hw_handler.type = hwht;
	consume(as, hw_argc - 1);

	return 0;
}

static int parse_features(struct arg_set *as, struct multipath *m,
			  struct dm_target *ti)
{
	int r;
	unsigned argc;

	static struct param _params[] = {
		{0, 1, ESTR("invalid number of feature args")},
	};

	r = read_param(_params, shift(as), &argc, &ti->error);
	if (r)
		return -EINVAL;

	if (!argc)
		return 0;

	if (!strnicmp(shift(as), MESG_STR("queue_if_no_path")))
		return queue_if_no_path(m, 1);
	else {
		ti->error = "Unrecognised multipath feature request";
		return -EINVAL;
	}
}

static int multipath_ctr(struct dm_target *ti, unsigned int argc,
			 char **argv)
{
	/* target parameters */
	static struct param _params[] = {
		{1, 1024, ESTR("invalid number of priority groups")},
		{1, 1024, ESTR("invalid initial priority group number")},
	};

	int r;
	struct multipath *m;
	struct arg_set as;
	unsigned pg_count = 0;
	unsigned next_pg_num;

	as.argc = argc;
	as.argv = argv;

	m = alloc_multipath();
	if (!m) {
		ti->error = ESTR("can't allocate multipath");
		return -EINVAL;
	}

	r = parse_features(&as, m, ti);
	if (r)
		goto bad;

	r = parse_hw_handler(&as, m, ti);
	if (r)
		goto bad;

	r = read_param(_params, shift(&as), &m->nr_priority_groups, &ti->error);
	if (r)
		goto bad;

	r = read_param(_params + 1, shift(&as), &next_pg_num, &ti->error);
	if (r)
		goto bad;

	/* parse the priority groups */
	while (as.argc) {
		struct priority_group *pg;

		pg = parse_priority_group(&as, m, ti);
		if (!pg) {
			r = -EINVAL;
			goto bad;
		}

		m->nr_valid_paths += pg->nr_pgpaths;
		list_add_tail(&pg->list, &m->priority_groups);
		pg_count++;
		pg->pg_num = pg_count;
		if (!--next_pg_num)
			m->next_pg = pg;
	}

	if (pg_count != m->nr_priority_groups) {
		ti->error = ESTR("priority group count mismatch");
		r = -EINVAL;
		goto bad;
	}

	ti->private = m;
	m->ti = ti;

	return 0;

 bad:
	free_multipath(m);
	return r;
}

static void multipath_dtr(struct dm_target *ti)
{
	struct multipath *m = (struct multipath *) ti->private;
	free_multipath(m);
}

/*
 * Map bios, recording original fields for later in case we have to resubmit
 */
static int multipath_map(struct dm_target *ti, struct bio *bio,
			 union map_info *map_context)
{
	int r;
	struct mpath_io *mpio;
	struct multipath *m = (struct multipath *) ti->private;

	if (bio_barrier(bio))
		return -EOPNOTSUPP;

	mpio = mempool_alloc(m->mpio_pool, GFP_NOIO);
	dm_bio_record(&mpio->details, bio);

	map_context->ptr = mpio;
	bio->bi_rw |= (1 << BIO_RW_FAILFAST);
	r = map_io(m, bio, mpio, 0);
	if (r < 0)
		mempool_free(mpio, m->mpio_pool);

	return r;
}

/*
 * Take a path out of use.
 */
static int fail_path(struct pgpath *pgpath)
{
	unsigned long flags;
	struct multipath *m = pgpath->pg->m;

	spin_lock_irqsave(&m->lock, flags);

	if (!pgpath->path.is_active)
		goto out;

	DMWARN("dm-multipath: Failing path %s.", pgpath->path.dev->name);

	pgpath->pg->ps.type->fail_path(&pgpath->pg->ps, &pgpath->path);
	pgpath->path.is_active = 0;
	pgpath->fail_count++;

	m->nr_valid_paths--;

	if (pgpath == m->current_pgpath)
		m->current_pgpath = NULL;

	queue_work(kmultipathd, &m->trigger_event);

out:
	spin_unlock_irqrestore(&m->lock, flags);

	return 0;
}

/*
 * Reinstate a previously-failed path
 */
static int reinstate_path(struct pgpath *pgpath)
{
	int r = 0;
	unsigned long flags;
	struct multipath *m = pgpath->pg->m;

	spin_lock_irqsave(&m->lock, flags);

	if (pgpath->path.is_active)
		goto out;

	if (!pgpath->pg->ps.type) {
		DMWARN("Reinstate path not supported by path selector %s",
		       pgpath->pg->ps.type->name);
		r = -EINVAL;
		goto out;
	}

	r = pgpath->pg->ps.type->reinstate_path(&pgpath->pg->ps, &pgpath->path);
	if (r)
		goto out;

	pgpath->path.is_active = 1;

	m->current_pgpath = NULL;
	if (!m->nr_valid_paths++)
		queue_work(kmultipathd, &m->process_queued_ios);

	queue_work(kmultipathd, &m->trigger_event);

out:
	spin_unlock_irqrestore(&m->lock, flags);

	return r;
}

/*
 * Fail or reinstate all paths that match the provided struct dm_dev.
 */
static int action_dev(struct multipath *m, struct dm_dev *dev,
		      action_fn action)
{
	int r = 0;
	struct pgpath *pgpath;
	struct priority_group *pg;

	list_for_each_entry(pg, &m->priority_groups, list) {
		list_for_each_entry(pgpath, &pg->pgpaths, list) {
			if (pgpath->path.dev == dev)
				r = action(pgpath);
		}
	}

	return r;
}

/*
 * Temporarily try to avoid having to use the specified PG
 */
static void bypass_pg(struct multipath *m, struct priority_group *pg,
		      int bypassed)
{
	unsigned long flags;

	spin_lock_irqsave(&m->lock, flags);

	pg->bypassed = bypassed;
	m->current_pgpath = NULL;
	m->current_pg = NULL;

	spin_unlock_irqrestore(&m->lock, flags);

	queue_work(kmultipathd, &m->trigger_event);
}

/*
 * Switch to using the specified PG from the next I/O that gets mapped
 */
static int switch_pg_num(struct multipath *m, const char *pgstr)
{
	struct priority_group *pg;
	unsigned pgnum;
	unsigned long flags;

	if (!pgstr || (sscanf(pgstr, "%u", &pgnum) != 1) || !pgnum ||
	    (pgnum > m->nr_priority_groups)) {
		DMWARN("invalid PG number supplied to switch_pg_num");
		return -EINVAL;
	}

	spin_lock_irqsave(&m->lock, flags);
	list_for_each_entry(pg, &m->priority_groups, list) {
		pg->bypassed = 0;
		if (--pgnum)
			continue;

		m->current_pgpath = NULL;
		m->current_pg = NULL;
		m->next_pg = pg;
	}
	spin_unlock_irqrestore(&m->lock, flags);

	queue_work(kmultipathd, &m->trigger_event);
	return 0;
}

/*
 * Set/clear bypassed status of a PG.
 * PGs are numbered upwards from 1 in the order they were declared.
 */
static int bypass_pg_num(struct multipath *m, const char *pgstr, int bypassed)
{
	struct priority_group *pg;
	unsigned pgnum;

	if (!pgstr || (sscanf(pgstr, "%u", &pgnum) != 1) || !pgnum ||
	    (pgnum > m->nr_priority_groups)) {
		DMWARN("invalid PG number supplied to bypass_pg");
		return -EINVAL;
	}

	list_for_each_entry(pg, &m->priority_groups, list) {
		if (!--pgnum)
			break;
	}

	bypass_pg(m, pg, bypassed);
	return 0;
}

/*
 * pg_init must call this when it has completed its initialisation
 */
void dm_pg_init_complete(struct path *path, unsigned err_flags)
{
	struct pgpath *pgpath = path_to_pgpath(path);
	struct priority_group *pg = pgpath->pg;
	struct multipath *m = pg->m;
	unsigned long flags;

	/* We insist on failing the path if the PG is already bypassed. */
	if (err_flags && pg->bypassed)
		err_flags |= MP_FAIL_PATH;

	if (err_flags & MP_FAIL_PATH)
		fail_path(pgpath);

	if (err_flags & MP_BYPASS_PG)
		bypass_pg(m, pg, 1);

	spin_lock_irqsave(&m->lock, flags);
	if (!err_flags)
		m->queue_io = 0;
	else {
		m->current_pgpath = NULL;
		m->current_pg = NULL;
	}
	queue_work(kmultipathd, &m->process_queued_ios);
	spin_unlock_irqrestore(&m->lock, flags);
}

/*
 * end_io handling
 */
static int do_end_io(struct multipath *m, struct bio *bio,
		     int error, struct mpath_io *mpio)
{
	struct hw_handler *hwh = &m->hw_handler;
	unsigned err_flags = MP_FAIL_PATH;	/* Default behavior */

	if (!error)
		return 0;	/* I/O complete */

	if ((error == -EWOULDBLOCK) && bio_rw_ahead(bio))
		return error;

	if (error == -EOPNOTSUPP)
		return error;

	spin_lock(&m->lock);
	if (!m->nr_valid_paths) {
		if (!m->queue_if_no_path || m->suspended) {
			spin_unlock(&m->lock);
			return -EIO;
		} else {
			spin_unlock(&m->lock);
			goto requeue;
		}
	}
	spin_unlock(&m->lock);

	if (hwh->type && hwh->type->error)
		err_flags = hwh->type->error(hwh, bio);

	if (mpio->pgpath) {
		if (err_flags & MP_FAIL_PATH)
			fail_path(mpio->pgpath);

		if (err_flags & MP_BYPASS_PG)
			bypass_pg(m, mpio->pgpath->pg, 1);
	}

	if (err_flags & MP_ERROR_IO)
		return -EIO;

      requeue:
	dm_bio_restore(&mpio->details, bio);

	/* queue for the daemon to resubmit or fail */
	spin_lock(&m->lock);
	bio_list_add(&m->queued_ios, bio);
	m->queue_size++;
	if (!m->queue_io)
		queue_work(kmultipathd, &m->process_queued_ios);
	spin_unlock(&m->lock);

	return 1;	/* io not complete */
}

static int multipath_end_io(struct dm_target *ti, struct bio *bio,
			    int error, union map_info *map_context)
{
	struct multipath *m = (struct multipath *) ti->private;
	struct mpath_io *mpio = (struct mpath_io *) map_context->ptr;
	struct pgpath *pgpath = mpio->pgpath;
	struct path_selector *ps;
	int r;

	r  = do_end_io(m, bio, error, mpio);
	if (pgpath) {
		ps = &pgpath->pg->ps;
		if (ps->type->end_io)
			ps->type->end_io(ps, &pgpath->path);
	}
	if (r <= 0)
		mempool_free(mpio, m->mpio_pool);

	return r;
}

/*
 * Suspend can't complete until all the I/O is processed so if
 * the last path failed we will now error any queued I/O.
 */
static void multipath_presuspend(struct dm_target *ti)
{
	struct multipath *m = (struct multipath *) ti->private;
	unsigned long flags;

	spin_lock_irqsave(&m->lock, flags);
	m->suspended = 1;
	if (m->queue_if_no_path)
		queue_work(kmultipathd, &m->process_queued_ios);
	spin_unlock_irqrestore(&m->lock, flags);
}

static void multipath_resume(struct dm_target *ti)
{
	struct multipath *m = (struct multipath *) ti->private;
	unsigned long flags;

	spin_lock_irqsave(&m->lock, flags);
	m->suspended = 0;
	spin_unlock_irqrestore(&m->lock, flags);
}

/*
 * Info output has the following format:
 * num_multipath_feature_args [multipath_feature_args]*
 * num_handler_status_args [handler_status_args]*
 * num_groups init_group_number
 *            [A|D|E num_ps_status_args [ps_status_args]*
 *             num_paths num_selector_args
 *             [path_dev A|F fail_count [selector_args]* ]+ ]+
 *
 * Table output has the following format (identical to the constructor string):
 * num_feature_args [features_args]*
 * num_handler_args hw_handler [hw_handler_args]*
 * num_groups init_group_number
 *     [priority selector-name num_ps_args [ps_args]*
 *      num_paths num_selector_args [path_dev [selector_args]* ]+ ]+
 */
static int multipath_status(struct dm_target *ti, status_type_t type,
			    char *result, unsigned int maxlen)
{
	int sz = 0;
	unsigned long flags;
	struct multipath *m = (struct multipath *) ti->private;
	struct hw_handler *hwh = &m->hw_handler;
	struct priority_group *pg;
	struct pgpath *p;
	unsigned pg_num;
	char state;

	spin_lock_irqsave(&m->lock, flags);

	/* Features */
	if (type == STATUSTYPE_INFO)
		DMEMIT("1 %u ", m->queue_size);
	else if (m->queue_if_no_path)
		DMEMIT("1 queue_if_no_path ");
	else
		DMEMIT("0 ");

	if (hwh->type && hwh->type->status)
		sz += hwh->type->status(hwh, type, result + sz, maxlen - sz);
	else if (!hwh->type || type == STATUSTYPE_INFO)
		DMEMIT("0 ");
	else
		DMEMIT("1 %s ", hwh->type->name);

	DMEMIT("%u ", m->nr_priority_groups);

	if (m->next_pg)
		pg_num = m->next_pg->pg_num;
	else if (m->current_pg)
		pg_num = m->current_pg->pg_num;
	else
			pg_num = 1;

	DMEMIT("%u ", pg_num);

	switch (type) {
	case STATUSTYPE_INFO:
		list_for_each_entry(pg, &m->priority_groups, list) {
			if (pg->bypassed)
				state = 'D';	/* Disabled */
			else if (pg == m->current_pg)
				state = 'A';	/* Currently Active */
			else
				state = 'E';	/* Enabled */

			DMEMIT("%c ", state);

			if (pg->ps.type->status)
				sz += pg->ps.type->status(&pg->ps, NULL, type,
							  result + sz,
							  maxlen - sz);
			else
				DMEMIT("0 ");

			DMEMIT("%u %u ", pg->nr_pgpaths,
			       pg->ps.type->info_args);

			list_for_each_entry(p, &pg->pgpaths, list) {
				DMEMIT("%s %s %u ", p->path.dev->name,
				       p->path.is_active ? "A" : "F",
				       p->fail_count);
				if (pg->ps.type->status)
					sz += pg->ps.type->status(&pg->ps,
					      &p->path, type, result + sz,
					      maxlen - sz);
			}
		}
		break;

	case STATUSTYPE_TABLE:
		list_for_each_entry(pg, &m->priority_groups, list) {
			DMEMIT("%s ", pg->ps.type->name);

			if (pg->ps.type->status)
				sz += pg->ps.type->status(&pg->ps, NULL, type,
							  result + sz,
							  maxlen - sz);
			else
				DMEMIT("0 ");

			DMEMIT("%u %u ", pg->nr_pgpaths,
			       pg->ps.type->table_args);

			list_for_each_entry(p, &pg->pgpaths, list) {
				DMEMIT("%s ", p->path.dev->name);
				if (pg->ps.type->status)
					sz += pg->ps.type->status(&pg->ps,
					      &p->path, type, result + sz,
					      maxlen - sz);
			}
		}
		break;
	}

	spin_unlock_irqrestore(&m->lock, flags);

	return 0;
}

static int multipath_message(struct dm_target *ti, unsigned argc, char **argv)
{
	int r;
	struct dm_dev *dev;
	struct multipath *m = (struct multipath *) ti->private;
	action_fn action;

	if (argc == 1) {
		if (!strnicmp(argv[0], MESG_STR("queue_if_no_path")))
			return queue_if_no_path(m, 1);
		else if (!strnicmp(argv[0], MESG_STR("fail_if_no_path")))
			return queue_if_no_path(m, 0);
	}

	if (argc != 2)
		goto error;

	if (!strnicmp(argv[0], MESG_STR("disable_group")))
		return bypass_pg_num(m, argv[1], 1);
	else if (!strnicmp(argv[0], MESG_STR("enable_group")))
		return bypass_pg_num(m, argv[1], 0);
	else if (!strnicmp(argv[0], MESG_STR("switch_group")))
		return switch_pg_num(m, argv[1]);
	else if (!strnicmp(argv[0], MESG_STR("reinstate_path")))
		action = reinstate_path;
	else if (!strnicmp(argv[0], MESG_STR("fail_path")))
		action = fail_path;
	else
		goto error;

	r = dm_get_device(ti, argv[1], ti->begin, ti->len,
			  dm_table_get_mode(ti->table), &dev);
	if (r) {
		DMWARN("dm-multipath message: error getting device %s",
		       argv[1]);
		return -EINVAL;
	}

	r = action_dev(m, dev, action);

	dm_put_device(ti, dev);

	return r;

error:
	DMWARN("Unrecognised multipath message received.");
	return -EINVAL;
}

/*-----------------------------------------------------------------
 * Module setup
 *---------------------------------------------------------------*/
static struct target_type multipath_target = {
	.name = "multipath",
	.version = {1, 0, 4},
	.module = THIS_MODULE,
	.ctr = multipath_ctr,
	.dtr = multipath_dtr,
	.map = multipath_map,
	.end_io = multipath_end_io,
	.presuspend = multipath_presuspend,
	.resume = multipath_resume,
	.status = multipath_status,
	.message = multipath_message,
};

static int __init dm_multipath_init(void)
{
	int r;

	/* allocate a slab for the dm_ios */
	_mpio_cache = kmem_cache_create("dm_mpath", sizeof(struct mpath_io),
					0, 0, NULL, NULL);
	if (!_mpio_cache)
		return -ENOMEM;

	r = dm_register_target(&multipath_target);
	if (r < 0) {
		DMERR("%s: register failed %d", multipath_target.name, r);
		kmem_cache_destroy(_mpio_cache);
		return -EINVAL;
	}

	kmultipathd = create_workqueue("kmpathd");
	if (!kmultipathd) {
		DMERR("%s: failed to create workqueue kmpathd",
				multipath_target.name);
		dm_unregister_target(&multipath_target);
		kmem_cache_destroy(_mpio_cache);
		return -ENOMEM;
	}

	DMINFO("dm-multipath version %u.%u.%u loaded",
	       multipath_target.version[0], multipath_target.version[1],
	       multipath_target.version[2]);

	return r;
}

static void __exit dm_multipath_exit(void)
{
	int r;

	destroy_workqueue(kmultipathd);

	r = dm_unregister_target(&multipath_target);
	if (r < 0)
		DMERR("%s: target unregister failed %d",
		      multipath_target.name, r);
	kmem_cache_destroy(_mpio_cache);
}

EXPORT_SYMBOL_GPL(dm_pg_init_complete);

module_init(dm_multipath_init);
module_exit(dm_multipath_exit);

MODULE_DESCRIPTION(DM_NAME " multipath target");
MODULE_AUTHOR("Sistina Software <dm-devel@redhat.com>");
MODULE_LICENSE("GPL");