dm: allow immutable request-based targets to use blk-mq pdu

[deliverable/linux.git] / 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 <linux/device-mapper.h>

#include "dm.h"
#include "dm-path-selector.h"
#include "dm-uevent.h"

#include <linux/blkdev.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 <linux/delay.h>
#include <scsi/scsi_dh.h>
#include <linux/atomic.h>

#define DM_MSG_PREFIX "multipath"
#define DM_PG_INIT_DELAY_MSECS 2000
#define DM_PG_INIT_DELAY_DEFAULT ((unsigned) -1)

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

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

        struct dm_path path;
        struct delayed_work activate_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;

        const char *hw_handler_name;
        char *hw_handler_params;

        spinlock_t lock;

        unsigned nr_priority_groups;
        struct list_head priority_groups;

        wait_queue_head_t pg_init_wait; /* Wait for pg_init completion */

        unsigned pg_init_required;      /* pg_init needs calling? */
        unsigned pg_init_in_progress;   /* Only one pg_init allowed at once */
        unsigned pg_init_delay_retry;   /* Delay pg_init retry? */

        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:1;            /* Must we queue all I/O? */
        unsigned queue_if_no_path:1;    /* Queue I/O if last path fails? */
        unsigned saved_queue_if_no_path:1; /* Saved state during suspension */
        unsigned retain_attached_hw_handler:1; /* If there's already a hw_handler present, don't change it. */
        unsigned pg_init_disabled:1;    /* pg_init is not currently allowed */

        unsigned pg_init_retries;       /* Number of times to retry pg_init */
        unsigned pg_init_count;         /* Number of times pg_init called */
        unsigned pg_init_delay_msecs;   /* Number of msecs before pg_init retry */

        struct work_struct trigger_event;

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

        struct mutex work_mutex;
};

/*
 * Context information attached to each bio we process.
 */
struct dm_mpath_io {
        struct pgpath *pgpath;
        size_t nr_bytes;
};

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

static struct kmem_cache *_mpio_cache;

static struct workqueue_struct *kmultipathd, *kmpath_handlerd;
static void trigger_event(struct work_struct *work);
static void activate_path(struct work_struct *work);
static int __pgpath_busy(struct pgpath *pgpath);


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

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

        if (pgpath) {
                pgpath->is_active = 1;
                INIT_DELAYED_WORK(&pgpath->activate_path, activate_path);
        }

        return pgpath;
}

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

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

        pg = kzalloc(sizeof(*pg), GFP_KERNEL);

        if (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(struct dm_target *ti)
{
        struct multipath *m;
        unsigned min_ios = dm_get_reserved_rq_based_ios();

        m = kzalloc(sizeof(*m), GFP_KERNEL);
        if (m) {
                INIT_LIST_HEAD(&m->priority_groups);
                spin_lock_init(&m->lock);
                m->queue_io = 1;
                m->pg_init_delay_msecs = DM_PG_INIT_DELAY_DEFAULT;
                INIT_WORK(&m->trigger_event, trigger_event);
                init_waitqueue_head(&m->pg_init_wait);
                mutex_init(&m->work_mutex);
                m->mpio_pool = mempool_create_slab_pool(min_ios, _mpio_cache);
                if (!m->mpio_pool) {
                        kfree(m);
                        return NULL;
                }
                m->ti = ti;
                ti->private = m;
        }

        return m;
}

static void free_multipath(struct multipath *m)
{
        struct priority_group *pg, *tmp;

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

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

static int set_mapinfo(struct multipath *m, union map_info *info)
{
        struct dm_mpath_io *mpio;

        mpio = mempool_alloc(m->mpio_pool, GFP_ATOMIC);
        if (!mpio)
                return -ENOMEM;

        memset(mpio, 0, sizeof(*mpio));
        info->ptr = mpio;

        return 0;
}

static void clear_mapinfo(struct multipath *m, union map_info *info)
{
        struct dm_mpath_io *mpio = info->ptr;

        info->ptr = NULL;
        mempool_free(mpio, m->mpio_pool);
}

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

static int __pg_init_all_paths(struct multipath *m)
{
        struct pgpath *pgpath;
        unsigned long pg_init_delay = 0;

        if (m->pg_init_in_progress || m->pg_init_disabled)
                return 0;

        m->pg_init_count++;
        m->pg_init_required = 0;

        /* Check here to reset pg_init_required */
        if (!m->current_pg)
                return 0;

        if (m->pg_init_delay_retry)
                pg_init_delay = msecs_to_jiffies(m->pg_init_delay_msecs != DM_PG_INIT_DELAY_DEFAULT ?
                                                 m->pg_init_delay_msecs : DM_PG_INIT_DELAY_MSECS);
        list_for_each_entry(pgpath, &m->current_pg->pgpaths, list) {
                /* Skip failed paths */
                if (!pgpath->is_active)
                        continue;
                if (queue_delayed_work(kmpath_handlerd, &pgpath->activate_path,
                                       pg_init_delay))
                        m->pg_init_in_progress++;
        }
        return m->pg_init_in_progress;
}

static void __switch_pg(struct multipath *m, struct pgpath *pgpath)
{
        m->current_pg = pgpath->pg;

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

        m->pg_init_count = 0;
}

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

        path = pg->ps.type->select_path(&pg->ps, &m->repeat_count, nr_bytes);
        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, size_t nr_bytes)
{
        struct priority_group *pg;
        unsigned bypassed = 1;

        if (!m->nr_valid_paths) {
                m->queue_io = 0;
                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, nr_bytes))
                        return;
        }

        /* Don't change PG until it has no remaining paths */
        if (m->current_pg && !__choose_path_in_pg(m, m->current_pg, nr_bytes))
                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, but set
         * pg_init_delay_retry so we do not hammer controllers.
         */
        do {
                list_for_each_entry(pg, &m->priority_groups, list) {
                        if (pg->bypassed == bypassed)
                                continue;
                        if (!__choose_path_in_pg(m, pg, nr_bytes)) {
                                if (!bypassed)
                                        m->pg_init_delay_retry = 1;
                                return;
                        }
                }
        } while (bypassed--);

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

/*
 * Check whether bios must be queued in the device-mapper core rather
 * than here in the target.
 *
 * m->lock must be held on entry.
 *
 * If m->queue_if_no_path and m->saved_queue_if_no_path hold the
 * same value then we are not between multipath_presuspend()
 * and multipath_resume() calls and we have no need to check
 * for the DMF_NOFLUSH_SUSPENDING flag.
 */
static int __must_push_back(struct multipath *m)
{
        return (m->queue_if_no_path ||
                (m->queue_if_no_path != m->saved_queue_if_no_path &&
                 dm_noflush_suspending(m->ti)));
}

/*
 * Map cloned requests
 */
static int __multipath_map(struct dm_target *ti, struct request *clone,
                           union map_info *map_context,
                           struct request *rq, struct request **__clone)
{
        struct multipath *m = (struct multipath *) ti->private;
        int r = DM_MAPIO_REQUEUE;
        size_t nr_bytes = clone ? blk_rq_bytes(clone) : blk_rq_bytes(rq);
        struct pgpath *pgpath;
        struct block_device *bdev;
        struct dm_mpath_io *mpio;

        spin_lock_irq(&m->lock);

        /* 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, nr_bytes);

        pgpath = m->current_pgpath;

        if (!pgpath) {
                if (!__must_push_back(m))
                        r = -EIO;       /* Failed */
                goto out_unlock;
        } else if (m->queue_io || m->pg_init_required) {
                __pg_init_all_paths(m);
                goto out_unlock;
        }

        if (set_mapinfo(m, map_context) < 0)
                /* ENOMEM, requeue */
                goto out_unlock;

        mpio = map_context->ptr;
        mpio->pgpath = pgpath;
        mpio->nr_bytes = nr_bytes;

        bdev = pgpath->path.dev->bdev;

        spin_unlock_irq(&m->lock);

        if (clone) {
                /*
                 * Old request-based interface: allocated clone is passed in.
                 * Used by: .request_fn stacked on .request_fn path(s).
                 */
                clone->q = bdev_get_queue(bdev);
                clone->rq_disk = bdev->bd_disk;
                clone->cmd_flags |= REQ_FAILFAST_TRANSPORT;
        } else {
                /*
                 * blk-mq request-based interface; used by both:
                 * .request_fn stacked on blk-mq path(s) and
                 * blk-mq stacked on blk-mq path(s).
                 */
                *__clone = blk_get_request(bdev_get_queue(bdev),
                                           rq_data_dir(rq), GFP_ATOMIC);
                if (IS_ERR(*__clone)) {
                        /* ENOMEM, requeue */
                        clear_mapinfo(m, map_context);
                        return r;
                }
                (*__clone)->bio = (*__clone)->biotail = NULL;
                (*__clone)->rq_disk = bdev->bd_disk;
                (*__clone)->cmd_flags |= REQ_FAILFAST_TRANSPORT;
        }

        if (pgpath->pg->ps.type->start_io)
                pgpath->pg->ps.type->start_io(&pgpath->pg->ps,
                                              &pgpath->path,
                                              nr_bytes);
        return DM_MAPIO_REMAPPED;

out_unlock:
        spin_unlock_irq(&m->lock);

        return r;
}

static int multipath_map(struct dm_target *ti, struct request *clone,
                         union map_info *map_context)
{
        return __multipath_map(ti, clone, map_context, NULL, NULL);
}

static int multipath_clone_and_map(struct dm_target *ti, struct request *rq,
                                   union map_info *map_context,
                                   struct request **clone)
{
        return __multipath_map(ti, NULL, map_context, rq, clone);
}

static void multipath_release_clone(struct request *clone)
{
        blk_put_request(clone);
}

/*
 * 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 save_old_value)
{
        unsigned long flags;

        spin_lock_irqsave(&m->lock, flags);

        if (save_old_value)
                m->saved_queue_if_no_path = m->queue_if_no_path;
        else
                m->saved_queue_if_no_path = queue_if_no_path;
        m->queue_if_no_path = queue_if_no_path;
        spin_unlock_irqrestore(&m->lock, flags);

        if (!queue_if_no_path)
                dm_table_run_md_queue_async(m->ti->table);

        return 0;
}

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

        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>]* ]+ ]+
 *---------------------------------------------------------------*/
static int parse_path_selector(struct dm_arg_set *as, struct priority_group *pg,
                               struct dm_target *ti)
{
        int r;
        struct path_selector_type *pst;
        unsigned ps_argc;

        static struct dm_arg _args[] = {
                {0, 1024, "invalid number of path selector args"},
        };

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

        r = dm_read_arg_group(_args, as, &ps_argc, &ti->error);
        if (r) {
                dm_put_path_selector(pst);
                return -EINVAL;
        }

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

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

        return 0;
}

static struct pgpath *parse_path(struct dm_arg_set *as, struct path_selector *ps,
                               struct dm_target *ti)
{
        int r;
        struct pgpath *p;
        struct multipath *m = ti->private;
        struct request_queue *q = NULL;
        const char *attached_handler_name;

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

        p = alloc_pgpath();
        if (!p)
                return ERR_PTR(-ENOMEM);

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

        if (m->retain_attached_hw_handler || m->hw_handler_name)
                q = bdev_get_queue(p->path.dev->bdev);

        if (m->retain_attached_hw_handler) {
retain:
                attached_handler_name = scsi_dh_attached_handler_name(q, GFP_KERNEL);
                if (attached_handler_name) {
                        /*
                         * Reset hw_handler_name to match the attached handler
                         * and clear any hw_handler_params associated with the
                         * ignored handler.
                         *
                         * NB. This modifies the table line to show the actual
                         * handler instead of the original table passed in.
                         */
                        kfree(m->hw_handler_name);
                        m->hw_handler_name = attached_handler_name;

                        kfree(m->hw_handler_params);
                        m->hw_handler_params = NULL;
                }
        }

        if (m->hw_handler_name) {
                r = scsi_dh_attach(q, m->hw_handler_name);
                if (r == -EBUSY) {
                        char b[BDEVNAME_SIZE];

                        printk(KERN_INFO "dm-mpath: retaining handler on device %s\n",
                                bdevname(p->path.dev->bdev, b));
                        goto retain;
                }
                if (r < 0) {
                        ti->error = "error attaching hardware handler";
                        dm_put_device(ti, p->path.dev);
                        goto bad;
                }

                if (m->hw_handler_params) {
                        r = scsi_dh_set_params(q, m->hw_handler_params);
                        if (r < 0) {
                                ti->error = "unable to set hardware "
                                                        "handler parameters";
                                dm_put_device(ti, p->path.dev);
                                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 ERR_PTR(r);
}

static struct priority_group *parse_priority_group(struct dm_arg_set *as,
                                                   struct multipath *m)
{
        static struct dm_arg _args[] = {
                {1, 1024, "invalid number of paths"},
                {0, 1024, "invalid number of selector args"}
        };

        int r;
        unsigned i, nr_selector_args, nr_args;
        struct priority_group *pg;
        struct dm_target *ti = m->ti;

        if (as->argc < 2) {
                as->argc = 0;
                ti->error = "not enough priority group arguments";
                return ERR_PTR(-EINVAL);
        }

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

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

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

        r = dm_read_arg(_args + 1, as, &nr_selector_args, &ti->error);
        if (r)
                goto bad;

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

                if (as->argc < nr_args) {
                        ti->error = "not enough path parameters";
                        r = -EINVAL;
                        goto bad;
                }

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

                pgpath = parse_path(&path_args, &pg->ps, ti);
                if (IS_ERR(pgpath)) {
                        r = PTR_ERR(pgpath);
                        goto bad;
                }

                pgpath->pg = pg;
                list_add_tail(&pgpath->list, &pg->pgpaths);
                dm_consume_args(as, nr_args);
        }

        return pg;

 bad:
        free_priority_group(pg, ti);
        return ERR_PTR(r);
}

static int parse_hw_handler(struct dm_arg_set *as, struct multipath *m)
{
        unsigned hw_argc;
        int ret;
        struct dm_target *ti = m->ti;

        static struct dm_arg _args[] = {
                {0, 1024, "invalid number of hardware handler args"},
        };

        if (dm_read_arg_group(_args, as, &hw_argc, &ti->error))
                return -EINVAL;

        if (!hw_argc)
                return 0;

        m->hw_handler_name = kstrdup(dm_shift_arg(as), GFP_KERNEL);

        if (hw_argc > 1) {
                char *p;
                int i, j, len = 4;

                for (i = 0; i <= hw_argc - 2; i++)
                        len += strlen(as->argv[i]) + 1;
                p = m->hw_handler_params = kzalloc(len, GFP_KERNEL);
                if (!p) {
                        ti->error = "memory allocation failed";
                        ret = -ENOMEM;
                        goto fail;
                }
                j = sprintf(p, "%d", hw_argc - 1);
                for (i = 0, p+=j+1; i <= hw_argc - 2; i++, p+=j+1)
                        j = sprintf(p, "%s", as->argv[i]);
        }
        dm_consume_args(as, hw_argc - 1);

        return 0;
fail:
        kfree(m->hw_handler_name);
        m->hw_handler_name = NULL;
        return ret;
}

static int parse_features(struct dm_arg_set *as, struct multipath *m)
{
        int r;
        unsigned argc;
        struct dm_target *ti = m->ti;
        const char *arg_name;

        static struct dm_arg _args[] = {
                {0, 6, "invalid number of feature args"},
                {1, 50, "pg_init_retries must be between 1 and 50"},
                {0, 60000, "pg_init_delay_msecs must be between 0 and 60000"},
        };

        r = dm_read_arg_group(_args, as, &argc, &ti->error);
        if (r)
                return -EINVAL;

        if (!argc)
                return 0;

        do {
                arg_name = dm_shift_arg(as);
                argc--;

                if (!strcasecmp(arg_name, "queue_if_no_path")) {
                        r = queue_if_no_path(m, 1, 0);
                        continue;
                }

                if (!strcasecmp(arg_name, "retain_attached_hw_handler")) {
                        m->retain_attached_hw_handler = 1;
                        continue;
                }

                if (!strcasecmp(arg_name, "pg_init_retries") &&
                    (argc >= 1)) {
                        r = dm_read_arg(_args + 1, as, &m->pg_init_retries, &ti->error);
                        argc--;
                        continue;
                }

                if (!strcasecmp(arg_name, "pg_init_delay_msecs") &&
                    (argc >= 1)) {
                        r = dm_read_arg(_args + 2, as, &m->pg_init_delay_msecs, &ti->error);
                        argc--;
                        continue;
                }

                ti->error = "Unrecognised multipath feature request";
                r = -EINVAL;
        } while (argc && !r);

        return r;
}

static int multipath_ctr(struct dm_target *ti, unsigned int argc,
                         char **argv)
{
        /* target arguments */
        static struct dm_arg _args[] = {
                {0, 1024, "invalid number of priority groups"},
                {0, 1024, "invalid initial priority group number"},
        };

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

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

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

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

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

        r = dm_read_arg(_args, &as, &m->nr_priority_groups, &ti->error);
        if (r)
                goto bad;

        r = dm_read_arg(_args + 1, &as, &next_pg_num, &ti->error);
        if (r)
                goto bad;

        if ((!m->nr_priority_groups && next_pg_num) ||
            (m->nr_priority_groups && !next_pg_num)) {
                ti->error = "invalid initial priority group";
                r = -EINVAL;
                goto bad;
        }

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

                pg = parse_priority_group(&as, m);
                if (IS_ERR(pg)) {
                        r = PTR_ERR(pg);
                        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 = "priority group count mismatch";
                r = -EINVAL;
                goto bad;
        }

        ti->num_flush_bios = 1;
        ti->num_discard_bios = 1;
        ti->num_write_same_bios = 1;

        return 0;

 bad:
        free_multipath(m);
        return r;
}

static void multipath_wait_for_pg_init_completion(struct multipath *m)
{
        DECLARE_WAITQUEUE(wait, current);
        unsigned long flags;

        add_wait_queue(&m->pg_init_wait, &wait);

        while (1) {
                set_current_state(TASK_UNINTERRUPTIBLE);

                spin_lock_irqsave(&m->lock, flags);
                if (!m->pg_init_in_progress) {
                        spin_unlock_irqrestore(&m->lock, flags);
                        break;
                }
                spin_unlock_irqrestore(&m->lock, flags);

                io_schedule();
        }
        set_current_state(TASK_RUNNING);

        remove_wait_queue(&m->pg_init_wait, &wait);
}

static void flush_multipath_work(struct multipath *m)
{
        unsigned long flags;

        spin_lock_irqsave(&m->lock, flags);
        m->pg_init_disabled = 1;
        spin_unlock_irqrestore(&m->lock, flags);

        flush_workqueue(kmpath_handlerd);
        multipath_wait_for_pg_init_completion(m);
        flush_workqueue(kmultipathd);
        flush_work(&m->trigger_event);

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

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

        flush_multipath_work(m);
        free_multipath(m);
}

/*
 * 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->is_active)
                goto out;

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

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

        m->nr_valid_paths--;

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

        dm_path_uevent(DM_UEVENT_PATH_FAILED, m->ti,
                      pgpath->path.dev->name, m->nr_valid_paths);

        schedule_work(&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, run_queue = 0;
        unsigned long flags;
        struct multipath *m = pgpath->pg->m;

        spin_lock_irqsave(&m->lock, flags);

        if (pgpath->is_active)
                goto out;

        if (!pgpath->pg->ps.type->reinstate_path) {
                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->is_active = 1;

        if (!m->nr_valid_paths++) {
                m->current_pgpath = NULL;
                run_queue = 1;
        } else if (m->hw_handler_name && (m->current_pg == pgpath->pg)) {
                if (queue_work(kmpath_handlerd, &pgpath->activate_path.work))
                        m->pg_init_in_progress++;
        }

        dm_path_uevent(DM_UEVENT_PATH_REINSTATED, m->ti,
                      pgpath->path.dev->name, m->nr_valid_paths);

        schedule_work(&m->trigger_event);

out:
        spin_unlock_irqrestore(&m->lock, flags);
        if (run_queue)
                dm_table_run_md_queue_async(m->ti->table);

        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 = -EINVAL;
        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);

        schedule_work(&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;
        char dummy;

        if (!pgstr || (sscanf(pgstr, "%u%c", &pgnum, &dummy) != 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);

        schedule_work(&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;
        char dummy;

        if (!pgstr || (sscanf(pgstr, "%u%c", &pgnum, &dummy) != 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;
}

/*
 * Should we retry pg_init immediately?
 */
static int pg_init_limit_reached(struct multipath *m, struct pgpath *pgpath)
{
        unsigned long flags;
        int limit_reached = 0;

        spin_lock_irqsave(&m->lock, flags);

        if (m->pg_init_count <= m->pg_init_retries && !m->pg_init_disabled)
                m->pg_init_required = 1;
        else
                limit_reached = 1;

        spin_unlock_irqrestore(&m->lock, flags);

        return limit_reached;
}

static void pg_init_done(void *data, int errors)
{
        struct pgpath *pgpath = data;
        struct priority_group *pg = pgpath->pg;
        struct multipath *m = pg->m;
        unsigned long flags;
        unsigned delay_retry = 0;

        /* device or driver problems */
        switch (errors) {
        case SCSI_DH_OK:
                break;
        case SCSI_DH_NOSYS:
                if (!m->hw_handler_name) {
                        errors = 0;
                        break;
                }
                DMERR("Could not failover the device: Handler scsi_dh_%s "
                      "Error %d.", m->hw_handler_name, errors);
                /*
                 * Fail path for now, so we do not ping pong
                 */
                fail_path(pgpath);
                break;
        case SCSI_DH_DEV_TEMP_BUSY:
                /*
                 * Probably doing something like FW upgrade on the
                 * controller so try the other pg.
                 */
                bypass_pg(m, pg, 1);
                break;
        case SCSI_DH_RETRY:
                /* Wait before retrying. */
                delay_retry = 1;
        case SCSI_DH_IMM_RETRY:
        case SCSI_DH_RES_TEMP_UNAVAIL:
                if (pg_init_limit_reached(m, pgpath))
                        fail_path(pgpath);
                errors = 0;
                break;
        default:
                /*
                 * We probably do not want to fail the path for a device
                 * error, but this is what the old dm did. In future
                 * patches we can do more advanced handling.
                 */
                fail_path(pgpath);
        }

        spin_lock_irqsave(&m->lock, flags);
        if (errors) {
                if (pgpath == m->current_pgpath) {
                        DMERR("Could not failover device. Error %d.", errors);
                        m->current_pgpath = NULL;
                        m->current_pg = NULL;
                }
        } else if (!m->pg_init_required)
                pg->bypassed = 0;

        if (--m->pg_init_in_progress)
                /* Activations of other paths are still on going */
                goto out;

        if (m->pg_init_required) {
                m->pg_init_delay_retry = delay_retry;
                if (__pg_init_all_paths(m))
                        goto out;
        }
        m->queue_io = 0;

        /*
         * Wake up any thread waiting to suspend.
         */
        wake_up(&m->pg_init_wait);

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

static void activate_path(struct work_struct *work)
{
        struct pgpath *pgpath =
                container_of(work, struct pgpath, activate_path.work);

        if (pgpath->is_active)
                scsi_dh_activate(bdev_get_queue(pgpath->path.dev->bdev),
                                 pg_init_done, pgpath);
        else
                pg_init_done(pgpath, SCSI_DH_DEV_OFFLINED);
}

static int noretry_error(int error)
{
        switch (error) {
        case -EOPNOTSUPP:
        case -EREMOTEIO:
        case -EILSEQ:
        case -ENODATA:
        case -ENOSPC:
                return 1;
        }

        /* Anything else could be a path failure, so should be retried */
        return 0;
}

/*
 * end_io handling
 */
static int do_end_io(struct multipath *m, struct request *clone,
                     int error, struct dm_mpath_io *mpio)
{
        /*
         * We don't queue any clone request inside the multipath target
         * during end I/O handling, since those clone requests don't have
         * bio clones.  If we queue them inside the multipath target,
         * we need to make bio clones, that requires memory allocation.
         * (See drivers/md/dm.c:end_clone_bio() about why the clone requests
         *  don't have bio clones.)
         * Instead of queueing the clone request here, we queue the original
         * request into dm core, which will remake a clone request and
         * clone bios for it and resubmit it later.
         */
        int r = DM_ENDIO_REQUEUE;
        unsigned long flags;

        if (!error && !clone->errors)
                return 0;       /* I/O complete */

        if (noretry_error(error))
                return error;

        if (mpio->pgpath)
                fail_path(mpio->pgpath);

        spin_lock_irqsave(&m->lock, flags);
        if (!m->nr_valid_paths) {
                if (!m->queue_if_no_path) {
                        if (!__must_push_back(m))
                                r = -EIO;
                } else {
                        if (error == -EBADE)
                                r = error;
                }
        }
        spin_unlock_irqrestore(&m->lock, flags);

        return r;
}

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

        BUG_ON(!mpio);

        r  = do_end_io(m, clone, error, mpio);
        pgpath = mpio->pgpath;
        if (pgpath) {
                ps = &pgpath->pg->ps;
                if (ps->type->end_io)
                        ps->type->end_io(ps, &pgpath->path, mpio->nr_bytes);
        }
        clear_mapinfo(m, map_context);

        return r;
}

/*
 * Suspend can't complete until all the I/O is processed so if
 * the last path fails we must error any remaining I/O.
 * Note that if the freeze_bdev fails while suspending, the
 * queue_if_no_path state is lost - userspace should reset it.
 */
static void multipath_presuspend(struct dm_target *ti)
{
        struct multipath *m = (struct multipath *) ti->private;

        queue_if_no_path(m, 0, 1);
}

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

        mutex_lock(&m->work_mutex);
        flush_multipath_work(m);
        mutex_unlock(&m->work_mutex);
}

/*
 * Restore the queue_if_no_path setting.
 */
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->queue_if_no_path = m->saved_queue_if_no_path;
        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 void multipath_status(struct dm_target *ti, status_type_t type,
                             unsigned status_flags, char *result, unsigned maxlen)
{
        int sz = 0;
        unsigned long flags;
        struct multipath *m = (struct multipath *) ti->private;
        struct priority_group *pg;
        struct pgpath *p;
        unsigned pg_num;
        char state;

        spin_lock_irqsave(&m->lock, flags);

        /* Features */
        if (type == STATUSTYPE_INFO)
                DMEMIT("2 %u %u ", m->queue_io, m->pg_init_count);
        else {
                DMEMIT("%u ", m->queue_if_no_path +
                              (m->pg_init_retries > 0) * 2 +
                              (m->pg_init_delay_msecs != DM_PG_INIT_DELAY_DEFAULT) * 2 +
                              m->retain_attached_hw_handler);
                if (m->queue_if_no_path)
                        DMEMIT("queue_if_no_path ");
                if (m->pg_init_retries)
                        DMEMIT("pg_init_retries %u ", m->pg_init_retries);
                if (m->pg_init_delay_msecs != DM_PG_INIT_DELAY_DEFAULT)
                        DMEMIT("pg_init_delay_msecs %u ", m->pg_init_delay_msecs);
                if (m->retain_attached_hw_handler)
                        DMEMIT("retain_attached_hw_handler ");
        }

        if (!m->hw_handler_name || type == STATUSTYPE_INFO)
                DMEMIT("0 ");
        else
                DMEMIT("1 %s ", m->hw_handler_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 = (m->nr_priority_groups ? 1 : 0);

        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->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);
}

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

        mutex_lock(&m->work_mutex);

        if (dm_suspended(ti)) {
                r = -EBUSY;
                goto out;
        }

        if (argc == 1) {
                if (!strcasecmp(argv[0], "queue_if_no_path")) {
                        r = queue_if_no_path(m, 1, 0);
                        goto out;
                } else if (!strcasecmp(argv[0], "fail_if_no_path")) {
                        r = queue_if_no_path(m, 0, 0);
                        goto out;
                }
        }

        if (argc != 2) {
                DMWARN("Invalid multipath message arguments. Expected 2 arguments, got %d.", argc);
                goto out;
        }

        if (!strcasecmp(argv[0], "disable_group")) {
                r = bypass_pg_num(m, argv[1], 1);
                goto out;
        } else if (!strcasecmp(argv[0], "enable_group")) {
                r = bypass_pg_num(m, argv[1], 0);
                goto out;
        } else if (!strcasecmp(argv[0], "switch_group")) {
                r = switch_pg_num(m, argv[1]);
                goto out;
        } else if (!strcasecmp(argv[0], "reinstate_path"))
                action = reinstate_path;
        else if (!strcasecmp(argv[0], "fail_path"))
                action = fail_path;
        else {
                DMWARN("Unrecognised multipath message received: %s", argv[0]);
                goto out;
        }

        r = dm_get_device(ti, argv[1], dm_table_get_mode(ti->table), &dev);
        if (r) {
                DMWARN("message: error getting device %s",
                       argv[1]);
                goto out;
        }

        r = action_dev(m, dev, action);

        dm_put_device(ti, dev);

out:
        mutex_unlock(&m->work_mutex);
        return r;
}

static int multipath_prepare_ioctl(struct dm_target *ti,
                struct block_device **bdev, fmode_t *mode)
{
        struct multipath *m = ti->private;
        unsigned long flags;
        int r;

        spin_lock_irqsave(&m->lock, flags);

        if (!m->current_pgpath)
                __choose_pgpath(m, 0);

        if (m->current_pgpath) {
                if (!m->queue_io) {
                        *bdev = m->current_pgpath->path.dev->bdev;
                        *mode = m->current_pgpath->path.dev->mode;
                        r = 0;
                } else {
                        /* pg_init has not started or completed */
                        r = -ENOTCONN;
                }
        } else {
                /* No path is available */
                if (m->queue_if_no_path)
                        r = -ENOTCONN;
                else
                        r = -EIO;
        }

        spin_unlock_irqrestore(&m->lock, flags);

        if (r == -ENOTCONN) {
                spin_lock_irqsave(&m->lock, flags);
                if (!m->current_pg) {
                        /* Path status changed, redo selection */
                        __choose_pgpath(m, 0);
                }
                if (m->pg_init_required)
                        __pg_init_all_paths(m);
                spin_unlock_irqrestore(&m->lock, flags);
                dm_table_run_md_queue_async(m->ti->table);
        }

        /*
         * Only pass ioctls through if the device sizes match exactly.
         */
        if (!r && ti->len != i_size_read((*bdev)->bd_inode) >> SECTOR_SHIFT)
                return 1;
        return r;
}

static int multipath_iterate_devices(struct dm_target *ti,
                                     iterate_devices_callout_fn fn, void *data)
{
        struct multipath *m = ti->private;
        struct priority_group *pg;
        struct pgpath *p;
        int ret = 0;

        list_for_each_entry(pg, &m->priority_groups, list) {
                list_for_each_entry(p, &pg->pgpaths, list) {
                        ret = fn(ti, p->path.dev, ti->begin, ti->len, data);
                        if (ret)
                                goto out;
                }
        }

out:
        return ret;
}

static int __pgpath_busy(struct pgpath *pgpath)
{
        struct request_queue *q = bdev_get_queue(pgpath->path.dev->bdev);

        return blk_lld_busy(q);
}

/*
 * We return "busy", only when we can map I/Os but underlying devices
 * are busy (so even if we map I/Os now, the I/Os will wait on
 * the underlying queue).
 * In other words, if we want to kill I/Os or queue them inside us
 * due to map unavailability, we don't return "busy".  Otherwise,
 * dm core won't give us the I/Os and we can't do what we want.
 */
static int multipath_busy(struct dm_target *ti)
{
        int busy = 0, has_active = 0;
        struct multipath *m = ti->private;
        struct priority_group *pg;
        struct pgpath *pgpath;
        unsigned long flags;

        spin_lock_irqsave(&m->lock, flags);

        /* pg_init in progress or no paths available */
        if (m->pg_init_in_progress ||
            (!m->nr_valid_paths && m->queue_if_no_path)) {
                busy = 1;
                goto out;
        }
        /* Guess which priority_group will be used at next mapping time */
        if (unlikely(!m->current_pgpath && m->next_pg))
                pg = m->next_pg;
        else if (likely(m->current_pg))
                pg = m->current_pg;
        else
                /*
                 * We don't know which pg will be used at next mapping time.
                 * We don't call __choose_pgpath() here to avoid to trigger
                 * pg_init just by busy checking.
                 * So we don't know whether underlying devices we will be using
                 * at next mapping time are busy or not. Just try mapping.
                 */
                goto out;

        /*
         * If there is one non-busy active path at least, the path selector
         * will be able to select it. So we consider such a pg as not busy.
         */
        busy = 1;
        list_for_each_entry(pgpath, &pg->pgpaths, list)
                if (pgpath->is_active) {
                        has_active = 1;

                        if (!__pgpath_busy(pgpath)) {
                                busy = 0;
                                break;
                        }
                }

        if (!has_active)
                /*
                 * No active path in this pg, so this pg won't be used and
                 * the current_pg will be changed at next mapping time.
                 * We need to try mapping to determine it.
                 */
                busy = 0;

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

        return busy;
}

/*-----------------------------------------------------------------
 * Module setup
 *---------------------------------------------------------------*/
static struct target_type multipath_target = {
        .name = "multipath",
        .version = {1, 11, 0},
        .features = DM_TARGET_SINGLETON | DM_TARGET_IMMUTABLE,
        .module = THIS_MODULE,
        .ctr = multipath_ctr,
        .dtr = multipath_dtr,
        .map_rq = multipath_map,
        .clone_and_map_rq = multipath_clone_and_map,
        .release_clone_rq = multipath_release_clone,
        .rq_end_io = multipath_end_io,
        .presuspend = multipath_presuspend,
        .postsuspend = multipath_postsuspend,
        .resume = multipath_resume,
        .status = multipath_status,
        .message = multipath_message,
        .prepare_ioctl = multipath_prepare_ioctl,
        .iterate_devices = multipath_iterate_devices,
        .busy = multipath_busy,
};

static int __init dm_multipath_init(void)
{
        int r;

        /* allocate a slab for the dm_ios */
        _mpio_cache = KMEM_CACHE(dm_mpath_io, 0);
        if (!_mpio_cache)
                return -ENOMEM;

        r = dm_register_target(&multipath_target);
        if (r < 0) {
                DMERR("register failed %d", r);
                r = -EINVAL;
                goto bad_register_target;
        }

        kmultipathd = alloc_workqueue("kmpathd", WQ_MEM_RECLAIM, 0);
        if (!kmultipathd) {
                DMERR("failed to create workqueue kmpathd");
                r = -ENOMEM;
                goto bad_alloc_kmultipathd;
        }

        /*
         * A separate workqueue is used to handle the device handlers
         * to avoid overloading existing workqueue. Overloading the
         * old workqueue would also create a bottleneck in the
         * path of the storage hardware device activation.
         */
        kmpath_handlerd = alloc_ordered_workqueue("kmpath_handlerd",
                                                  WQ_MEM_RECLAIM);
        if (!kmpath_handlerd) {
                DMERR("failed to create workqueue kmpath_handlerd");
                r = -ENOMEM;
                goto bad_alloc_kmpath_handlerd;
        }

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

        return 0;

bad_alloc_kmpath_handlerd:
        destroy_workqueue(kmultipathd);
bad_alloc_kmultipathd:
        dm_unregister_target(&multipath_target);
bad_register_target:
        kmem_cache_destroy(_mpio_cache);

        return r;
}

static void __exit dm_multipath_exit(void)
{
        destroy_workqueue(kmpath_handlerd);
        destroy_workqueue(kmultipathd);

        dm_unregister_target(&multipath_target);
        kmem_cache_destroy(_mpio_cache);
}

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");