/*
 * 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-rq.h"
#include "dm-bio-record.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>
#include <linux/blk-mq.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 fail_count;            /* Cumulative failure count */

        struct dm_path path;
        struct delayed_work activate_path;

        bool is_active:1;               /* Path status */
};

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

        bool bypassed:1;                /* Temporarily bypass this PG? */
};

/* 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 */

        struct pgpath *current_pgpath;
        struct priority_group *current_pg;
        struct priority_group *next_pg; /* Switch to this PG if set */

        unsigned long flags;            /* Multipath state flags */

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

        atomic_t nr_valid_paths;        /* Total number of usable paths */
        atomic_t pg_init_in_progress;   /* Only one pg_init allowed at once */
        atomic_t pg_init_count;         /* Number of times pg_init called */

        enum dm_queue_mode queue_mode;

        struct mutex work_mutex;
        struct work_struct trigger_event;

        struct work_struct process_queued_bios;
        struct bio_list queued_bios;
};

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

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

static struct workqueue_struct *kmultipathd, *kmpath_handlerd;
static void trigger_event(struct work_struct *work);
static void activate_or_offline_path(struct pgpath *pgpath);
static void activate_path_work(struct work_struct *work);
static void process_queued_bios(struct work_struct *work);

/*-----------------------------------------------
 * Multipath state flags.
 *-----------------------------------------------*/

#define MPATHF_QUEUE_IO 0                       /* Must we queue all I/O? */
#define MPATHF_QUEUE_IF_NO_PATH 1               /* Queue I/O if last path fails? */
#define MPATHF_SAVED_QUEUE_IF_NO_PATH 2         /* Saved state during suspension */
#define MPATHF_RETAIN_ATTACHED_HW_HANDLER 3     /* If there's already a hw_handler present, don't change it. */
#define MPATHF_PG_INIT_DISABLED 4               /* pg_init is not currently allowed */
#define MPATHF_PG_INIT_REQUIRED 5               /* pg_init needs calling? */
#define MPATHF_PG_INIT_DELAY_RETRY 6            /* Delay pg_init retry? */

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

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

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

        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;

        m = kzalloc(sizeof(*m), GFP_KERNEL);
        if (m) {
                INIT_LIST_HEAD(&m->priority_groups);
                spin_lock_init(&m->lock);
                set_bit(MPATHF_QUEUE_IO, &m->flags);
                atomic_set(&m->nr_valid_paths, 0);
                atomic_set(&m->pg_init_in_progress, 0);
                atomic_set(&m->pg_init_count, 0);
                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->queue_mode = DM_TYPE_NONE;

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

        return m;
}

static int alloc_multipath_stage2(struct dm_target *ti, struct multipath *m)
{
        if (m->queue_mode == DM_TYPE_NONE) {
                /*
                 * Default to request-based.
                 */
                if (dm_use_blk_mq(dm_table_get_md(ti->table)))
                        m->queue_mode = DM_TYPE_MQ_REQUEST_BASED;
                else
                        m->queue_mode = DM_TYPE_REQUEST_BASED;
        } else if (m->queue_mode == DM_TYPE_BIO_BASED) {
                INIT_WORK(&m->process_queued_bios, process_queued_bios);
                /*
                 * bio-based doesn't support any direct scsi_dh management;
                 * it just discovers if a scsi_dh is attached.
                 */
                set_bit(MPATHF_RETAIN_ATTACHED_HW_HANDLER, &m->flags);
        }

        dm_table_set_type(ti->table, m->queue_mode);

        return 0;
}

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);
        kfree(m);
}

static struct dm_mpath_io *get_mpio(union map_info *info)
{
        return info->ptr;
}

static size_t multipath_per_bio_data_size(void)
{
        return sizeof(struct dm_mpath_io) + sizeof(struct dm_bio_details);
}

static struct dm_mpath_io *get_mpio_from_bio(struct bio *bio)
{
        return dm_per_bio_data(bio, multipath_per_bio_data_size());
}

static struct dm_bio_details *get_bio_details_from_bio(struct bio *bio)
{
        /* dm_bio_details is immediately after the dm_mpath_io in bio's per-bio-data */
        struct dm_mpath_io *mpio = get_mpio_from_bio(bio);
        void *bio_details = mpio + 1;

        return bio_details;
}

static void multipath_init_per_bio_data(struct bio *bio, struct dm_mpath_io **mpio_p,
                                        struct dm_bio_details **bio_details_p)
{
        struct dm_mpath_io *mpio = get_mpio_from_bio(bio);
        struct dm_bio_details *bio_details = get_bio_details_from_bio(bio);

        memset(mpio, 0, sizeof(*mpio));
        memset(bio_details, 0, sizeof(*bio_details));
        dm_bio_record(bio_details, bio);

        if (mpio_p)
                *mpio_p = mpio;
        if (bio_details_p)
                *bio_details_p = bio_details;
}

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

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

        lockdep_assert_held(&m->lock);

        if (atomic_read(&m->pg_init_in_progress) || test_bit(MPATHF_PG_INIT_DISABLED, &m->flags))
                return 0;

        atomic_inc(&m->pg_init_count);
        clear_bit(MPATHF_PG_INIT_REQUIRED, &m->flags);

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

        if (test_bit(MPATHF_PG_INIT_DELAY_RETRY, &m->flags))
                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))
                        atomic_inc(&m->pg_init_in_progress);
        }
        return atomic_read(&m->pg_init_in_progress);
}

static int pg_init_all_paths(struct multipath *m)
{
        int ret;
        unsigned long flags;

        spin_lock_irqsave(&m->lock, flags);
        ret = __pg_init_all_paths(m);
        spin_unlock_irqrestore(&m->lock, flags);

        return ret;
}

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

        /* Must we initialise the PG first, and queue I/O till it's ready? */
        if (m->hw_handler_name) {
                set_bit(MPATHF_PG_INIT_REQUIRED, &m->flags);
                set_bit(MPATHF_QUEUE_IO, &m->flags);
        } else {
                clear_bit(MPATHF_PG_INIT_REQUIRED, &m->flags);
                clear_bit(MPATHF_QUEUE_IO, &m->flags);
        }

        atomic_set(&m->pg_init_count, 0);
}

static struct pgpath *choose_path_in_pg(struct multipath *m,
                                        struct priority_group *pg,
                                        size_t nr_bytes)
{
        unsigned long flags;
        struct dm_path *path;
        struct pgpath *pgpath;

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

        pgpath = path_to_pgpath(path);

        if (unlikely(lockless_dereference(m->current_pg) != pg)) {
                /* Only update current_pgpath if pg changed */
                spin_lock_irqsave(&m->lock, flags);
                m->current_pgpath = pgpath;
                __switch_pg(m, pg);
                spin_unlock_irqrestore(&m->lock, flags);
        }

        return pgpath;
}

static struct pgpath *choose_pgpath(struct multipath *m, size_t nr_bytes)
{
        unsigned long flags;
        struct priority_group *pg;
        struct pgpath *pgpath;
        unsigned bypassed = 1;

        if (!atomic_read(&m->nr_valid_paths)) {
                clear_bit(MPATHF_QUEUE_IO, &m->flags);
                goto failed;
        }

        /* Were we instructed to switch PG? */
        if (lockless_dereference(m->next_pg)) {
                spin_lock_irqsave(&m->lock, flags);
                pg = m->next_pg;
                if (!pg) {
                        spin_unlock_irqrestore(&m->lock, flags);
                        goto check_current_pg;
                }
                m->next_pg = NULL;
                spin_unlock_irqrestore(&m->lock, flags);
                pgpath = choose_path_in_pg(m, pg, nr_bytes);
                if (!IS_ERR_OR_NULL(pgpath))
                        return pgpath;
        }

        /* Don't change PG until it has no remaining paths */
check_current_pg:
        pg = lockless_dereference(m->current_pg);
        if (pg) {
                pgpath = choose_path_in_pg(m, pg, nr_bytes);
                if (!IS_ERR_OR_NULL(pgpath))
                        return pgpath;
        }

        /*
         * 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;
                        pgpath = choose_path_in_pg(m, pg, nr_bytes);
                        if (!IS_ERR_OR_NULL(pgpath)) {
                                if (!bypassed)
                                        set_bit(MPATHF_PG_INIT_DELAY_RETRY, &m->flags);
                                return pgpath;
                        }
                }
        } while (bypassed--);

failed:
        spin_lock_irqsave(&m->lock, flags);
        m->current_pgpath = NULL;
        m->current_pg = NULL;
        spin_unlock_irqrestore(&m->lock, flags);

        return NULL;
}

/*
 * dm_report_EIO() is a macro instead of a function to make pr_debug()
 * report the function name and line number of the function from which
 * it has been invoked.
 */
#define dm_report_EIO(m)                                                \
do {                                                                    \
        struct mapped_device *md = dm_table_get_md((m)->ti->table);     \
                                                                        \
        pr_debug("%s: returning EIO; QIFNP = %d; SQIFNP = %d; DNFS = %d\n", \
                 dm_device_name(md),                                    \
                 test_bit(MPATHF_QUEUE_IF_NO_PATH, &(m)->flags),        \
                 test_bit(MPATHF_SAVED_QUEUE_IF_NO_PATH, &(m)->flags),  \
                 dm_noflush_suspending((m)->ti));                       \
} while (0)

/*
 * Map cloned requests (request-based multipath)
 */
static int multipath_clone_and_map(struct dm_target *ti, struct request *rq,
                                   union map_info *map_context,
                                   struct request **__clone)
{
        struct multipath *m = ti->private;
        size_t nr_bytes = blk_rq_bytes(rq);
        struct pgpath *pgpath;
        struct block_device *bdev;
        struct dm_mpath_io *mpio = get_mpio(map_context);
        struct request_queue *q;
        struct request *clone;

        /* Do we need to select a new pgpath? */
        pgpath = lockless_dereference(m->current_pgpath);
        if (!pgpath || !test_bit(MPATHF_QUEUE_IO, &m->flags))
                pgpath = choose_pgpath(m, nr_bytes);

        if (!pgpath) {
                if (test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags))
                        return DM_MAPIO_DELAY_REQUEUE;
                dm_report_EIO(m);       /* Failed */
                return DM_MAPIO_KILL;
        } else if (test_bit(MPATHF_QUEUE_IO, &m->flags) ||
                   test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags)) {
                if (pg_init_all_paths(m))
                        return DM_MAPIO_DELAY_REQUEUE;
                return DM_MAPIO_REQUEUE;
        }

        memset(mpio, 0, sizeof(*mpio));
        mpio->pgpath = pgpath;
        mpio->nr_bytes = nr_bytes;

        bdev = pgpath->path.dev->bdev;
        q = bdev_get_queue(bdev);
        clone = blk_get_request(q, rq->cmd_flags | REQ_NOMERGE, GFP_ATOMIC);
        if (IS_ERR(clone)) {
                /* EBUSY, ENODEV or EWOULDBLOCK: requeue */
                bool queue_dying = blk_queue_dying(q);
                DMERR_LIMIT("blk_get_request() returned %ld%s - requeuing",
                            PTR_ERR(clone), queue_dying ? " (path offline)" : "");
                if (queue_dying) {
                        atomic_inc(&m->pg_init_in_progress);
                        activate_or_offline_path(pgpath);
                }
                return DM_MAPIO_DELAY_REQUEUE;
        }
        clone->bio = clone->biotail = NULL;
        clone->rq_disk = bdev->bd_disk;
        clone->cmd_flags |= REQ_FAILFAST_TRANSPORT;
        *__clone = clone;

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

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

/*
 * Map cloned bios (bio-based multipath)
 */
static int __multipath_map_bio(struct multipath *m, struct bio *bio, struct dm_mpath_io *mpio)
{
        size_t nr_bytes = bio->bi_iter.bi_size;
        struct pgpath *pgpath;
        unsigned long flags;
        bool queue_io;

        /* Do we need to select a new pgpath? */
        pgpath = lockless_dereference(m->current_pgpath);
        queue_io = test_bit(MPATHF_QUEUE_IO, &m->flags);
        if (!pgpath || !queue_io)
                pgpath = choose_pgpath(m, nr_bytes);

        if ((pgpath && queue_io) ||
            (!pgpath && test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags))) {
                /* Queue for the daemon to resubmit */
                spin_lock_irqsave(&m->lock, flags);
                bio_list_add(&m->queued_bios, bio);
                spin_unlock_irqrestore(&m->lock, flags);
                /* PG_INIT_REQUIRED cannot be set without QUEUE_IO */
                if (queue_io || test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags))
                        pg_init_all_paths(m);
                else if (!queue_io)
                        queue_work(kmultipathd, &m->process_queued_bios);
                return DM_MAPIO_SUBMITTED;
        }

        if (!pgpath) {
                if (test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags))
                        return DM_MAPIO_REQUEUE;
                dm_report_EIO(m);
                return DM_MAPIO_KILL;
        }

        mpio->pgpath = pgpath;
        mpio->nr_bytes = nr_bytes;

        bio->bi_status = 0;
        bio_set_dev(bio, pgpath->path.dev->bdev);
        bio->bi_opf |= 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;
}

static int multipath_map_bio(struct dm_target *ti, struct bio *bio)
{
        struct multipath *m = ti->private;
        struct dm_mpath_io *mpio = NULL;

        multipath_init_per_bio_data(bio, &mpio, NULL);

        return __multipath_map_bio(m, bio, mpio);
}

static void process_queued_io_list(struct multipath *m)
{
        if (m->queue_mode == DM_TYPE_MQ_REQUEST_BASED)
                dm_mq_kick_requeue_list(dm_table_get_md(m->ti->table));
        else if (m->queue_mode == DM_TYPE_BIO_BASED)
                queue_work(kmultipathd, &m->process_queued_bios);
}

static void process_queued_bios(struct work_struct *work)
{
        int r;
        unsigned long flags;
        struct bio *bio;
        struct bio_list bios;
        struct blk_plug plug;
        struct multipath *m =
                container_of(work, struct multipath, process_queued_bios);

        bio_list_init(&bios);

        spin_lock_irqsave(&m->lock, flags);

        if (bio_list_empty(&m->queued_bios)) {
                spin_unlock_irqrestore(&m->lock, flags);
                return;
        }

        bio_list_merge(&bios, &m->queued_bios);
        bio_list_init(&m->queued_bios);

        spin_unlock_irqrestore(&m->lock, flags);

        blk_start_plug(&plug);
        while ((bio = bio_list_pop(&bios))) {
                r = __multipath_map_bio(m, bio, get_mpio_from_bio(bio));
                switch (r) {
                case DM_MAPIO_KILL:
                        bio->bi_status = BLK_STS_IOERR;
                        bio_endio(bio);
                        break;
                case DM_MAPIO_REQUEUE:
                        bio->bi_status = BLK_STS_DM_REQUEUE;
                        bio_endio(bio);
                        break;
                case DM_MAPIO_REMAPPED:
                        generic_make_request(bio);
                        break;
                case 0:
                        break;
                default:
                        WARN_ONCE(true, "__multipath_map_bio() returned %d\n", r);
                }
        }
        blk_finish_plug(&plug);
}

static void assign_bit(bool value, long nr, unsigned long *addr)
{
        if (value)
                set_bit(nr, addr);
        else
                clear_bit(nr, addr);
}

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

        spin_lock_irqsave(&m->lock, flags);
        assign_bit((save_old_value && test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags)) ||
                   (!save_old_value && queue_if_no_path),
                   MPATHF_SAVED_QUEUE_IF_NO_PATH, &m->flags);
        assign_bit(queue_if_no_path || dm_noflush_suspending(m->ti),
                   MPATHF_QUEUE_IF_NO_PATH, &m->flags);
        spin_unlock_irqrestore(&m->lock, flags);

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

        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 const 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 (test_bit(MPATHF_RETAIN_ATTACHED_HW_HANDLER, &m->flags) || m->hw_handler_name)
                q = bdev_get_queue(p->path.dev->bdev);

        if (test_bit(MPATHF_RETAIN_ATTACHED_HW_HANDLER, &m->flags)) {
retain:
                attached_handler_name = scsi_dh_attached_handler_name(q, GFP_KERNEL);
                if (attached_handler_name) {
                        /*
                         * Clear any hw_handler_params associated with a
                         * handler that isn't already attached.
                         */
                        if (m->hw_handler_name && strcmp(attached_handler_name, m->hw_handler_name)) {
                                kfree(m->hw_handler_params);
                                m->hw_handler_params = NULL;
                        }

                        /*
                         * Reset hw_handler_name to match the attached 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;
                }
        }

        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 const 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 const 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;

        if (m->queue_mode == DM_TYPE_BIO_BASED) {
                dm_consume_args(as, hw_argc);
                DMERR("bio-based multipath doesn't allow hardware handler args");
                return 0;
        }

        m->hw_handler_name = kstrdup(dm_shift_arg(as), GFP_KERNEL);
        if (!m->hw_handler_name)
                return -EINVAL;

        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 const struct dm_arg _args[] = {
                {0, 8, "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, true, false);
                        continue;
                }

                if (!strcasecmp(arg_name, "retain_attached_hw_handler")) {
                        set_bit(MPATHF_RETAIN_ATTACHED_HW_HANDLER, &m->flags);
                        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;
                }

                if (!strcasecmp(arg_name, "queue_mode") &&
                    (argc >= 1)) {
                        const char *queue_mode_name = dm_shift_arg(as);

                        if (!strcasecmp(queue_mode_name, "bio"))
                                m->queue_mode = DM_TYPE_BIO_BASED;
                        else if (!strcasecmp(queue_mode_name, "rq"))
                                m->queue_mode = DM_TYPE_REQUEST_BASED;
                        else if (!strcasecmp(queue_mode_name, "mq"))
                                m->queue_mode = DM_TYPE_MQ_REQUEST_BASED;
                        else {
                                ti->error = "Unknown 'queue_mode' requested";
                                r = -EINVAL;
                        }
                        argc--;
                        continue;
                }

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

        return r;
}

static int multipath_ctr(struct dm_target *ti, unsigned argc, char **argv)
{
        /* target arguments */
        static const 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 = alloc_multipath_stage2(ti, 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;
                unsigned nr_valid_paths = atomic_read(&m->nr_valid_paths);

                pg = parse_priority_group(&as, m);
                if (IS_ERR(pg)) {
                        r = PTR_ERR(pg);
                        goto bad;
                }

                nr_valid_paths += pg->nr_pgpaths;
                atomic_set(&m->nr_valid_paths, nr_valid_paths);

                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;
        ti->num_write_zeroes_bios = 1;
        if (m->queue_mode == DM_TYPE_BIO_BASED)
                ti->per_io_data_size = multipath_per_bio_data_size();
        else
                ti->per_io_data_size = sizeof(struct dm_mpath_io);

        return 0;

 bad:
        free_multipath(m);
        return r;
}

static void multipath_wait_for_pg_init_completion(struct multipath *m)
{
        DEFINE_WAIT(wait);

        while (1) {
                prepare_to_wait(&m->pg_init_wait, &wait, TASK_UNINTERRUPTIBLE);

                if (!atomic_read(&m->pg_init_in_progress))
                        break;

                io_schedule();
        }
        finish_wait(&m->pg_init_wait, &wait);
}

static void flush_multipath_work(struct multipath *m)
{
        set_bit(MPATHF_PG_INIT_DISABLED, &m->flags);
        smp_mb__after_atomic();

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

        clear_bit(MPATHF_PG_INIT_DISABLED, &m->flags);
        smp_mb__after_atomic();
}

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 = false;
        pgpath->fail_count++;

        atomic_dec(&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, atomic_read(&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;
        unsigned nr_valid_paths;

        spin_lock_irqsave(&m->lock, flags);

        if (pgpath->is_active)
                goto out;

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

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

        pgpath->is_active = true;

        nr_valid_paths = atomic_inc_return(&m->nr_valid_paths);
        if (nr_valid_paths == 1) {
                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))
                        atomic_inc(&m->pg_init_in_progress);
        }

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

        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,
                      bool 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 ||
            !m->nr_priority_groups || (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 = false;
                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, bool bypassed)
{
        struct priority_group *pg;
        unsigned pgnum;
        char dummy;

        if (!pgstr || (sscanf(pgstr, "%u%c", &pgnum, &dummy) != 1) || !pgnum ||
            !m->nr_priority_groups || (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 bool pg_init_limit_reached(struct multipath *m, struct pgpath *pgpath)
{
        unsigned long flags;
        bool limit_reached = false;

        spin_lock_irqsave(&m->lock, flags);

        if (atomic_read(&m->pg_init_count) <= m->pg_init_retries &&
            !test_bit(MPATHF_PG_INIT_DISABLED, &m->flags))
                set_bit(MPATHF_PG_INIT_REQUIRED, &m->flags);
        else
                limit_reached = true;

        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;
        bool delay_retry = false;

        /* 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, true);
                break;
        case SCSI_DH_RETRY:
                /* Wait before retrying. */
                delay_retry = 1;
                /* fall through */
        case SCSI_DH_IMM_RETRY:
        case SCSI_DH_RES_TEMP_UNAVAIL:
                if (pg_init_limit_reached(m, pgpath))
                        fail_path(pgpath);
                errors = 0;
                break;
        case SCSI_DH_DEV_OFFLINED:
        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 (!test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags))
                pg->bypassed = false;

        if (atomic_dec_return(&m->pg_init_in_progress) > 0)
                /* Activations of other paths are still on going */
                goto out;

        if (test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags)) {
                if (delay_retry)
                        set_bit(MPATHF_PG_INIT_DELAY_RETRY, &m->flags);
                else
                        clear_bit(MPATHF_PG_INIT_DELAY_RETRY, &m->flags);

                if (__pg_init_all_paths(m))
                        goto out;
        }
        clear_bit(MPATHF_QUEUE_IO, &m->flags);

        process_queued_io_list(m);

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

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

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

        if (pgpath->is_active && !blk_queue_dying(q))
                scsi_dh_activate(q, pg_init_done, pgpath);
        else
                pg_init_done(pgpath, SCSI_DH_DEV_OFFLINED);
}

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

        activate_or_offline_path(pgpath);
}

static int noretry_error(blk_status_t error)
{
        switch (error) {
        case BLK_STS_NOTSUPP:
        case BLK_STS_NOSPC:
        case BLK_STS_TARGET:
        case BLK_STS_NEXUS:
        case BLK_STS_MEDIUM:
                return 1;
        }

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

static int multipath_end_io(struct dm_target *ti, struct request *clone,
                            blk_status_t error, union map_info *map_context)
{
        struct dm_mpath_io *mpio = get_mpio(map_context);
        struct pgpath *pgpath = mpio->pgpath;
        int r = DM_ENDIO_DONE;

        /*
         * 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-rq.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.
         */
        if (error && !noretry_error(error)) {
                struct multipath *m = ti->private;

                r = DM_ENDIO_REQUEUE;

                if (pgpath)
                        fail_path(pgpath);

                if (atomic_read(&m->nr_valid_paths) == 0 &&
                    !test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags)) {
                        if (error == BLK_STS_IOERR)
                                dm_report_EIO(m);
                        /* complete with the original error */
                        r = DM_ENDIO_DONE;
                }
        }

        if (pgpath) {
                struct path_selector *ps = &pgpath->pg->ps;

                if (ps->type->end_io)
                        ps->type->end_io(ps, &pgpath->path, mpio->nr_bytes);
        }

        return r;
}

static int multipath_end_io_bio(struct dm_target *ti, struct bio *clone,
                blk_status_t *error)
{
        struct multipath *m = ti->private;
        struct dm_mpath_io *mpio = get_mpio_from_bio(clone);
        struct pgpath *pgpath = mpio->pgpath;
        unsigned long flags;
        int r = DM_ENDIO_DONE;

        if (!*error || noretry_error(*error))
                goto done;

        if (pgpath)
                fail_path(pgpath);

        if (atomic_read(&m->nr_valid_paths) == 0 &&
            !test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags)) {
                dm_report_EIO(m);
                *error = BLK_STS_IOERR;
                goto done;
        }

        /* Queue for the daemon to resubmit */
        dm_bio_restore(get_bio_details_from_bio(clone), clone);

        spin_lock_irqsave(&m->lock, flags);
        bio_list_add(&m->queued_bios, clone);
        spin_unlock_irqrestore(&m->lock, flags);
        if (!test_bit(MPATHF_QUEUE_IO, &m->flags))
                queue_work(kmultipathd, &m->process_queued_bios);

        r = DM_ENDIO_INCOMPLETE;
done:
        if (pgpath) {
                struct path_selector *ps = &pgpath->pg->ps;

                if (ps->type->end_io)
                        ps->type->end_io(ps, &pgpath->path, mpio->nr_bytes);
        }

        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 = ti->private;

        queue_if_no_path(m, false, true);
}

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 = ti->private;
        unsigned long flags;

        spin_lock_irqsave(&m->lock, flags);
        assign_bit(test_bit(MPATHF_SAVED_QUEUE_IF_NO_PATH, &m->flags),
                   MPATHF_QUEUE_IF_NO_PATH, &m->flags);
        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 = 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 ", test_bit(MPATHF_QUEUE_IO, &m->flags),
                       atomic_read(&m->pg_init_count));
        else {
                DMEMIT("%u ", test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags) +
                              (m->pg_init_retries > 0) * 2 +
                              (m->pg_init_delay_msecs != DM_PG_INIT_DELAY_DEFAULT) * 2 +
                              test_bit(MPATHF_RETAIN_ATTACHED_HW_HANDLER, &m->flags) +
                              (m->queue_mode != DM_TYPE_REQUEST_BASED) * 2);

                if (test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags))
                        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 (test_bit(MPATHF_RETAIN_ATTACHED_HW_HANDLER, &m->flags))
                        DMEMIT("retain_attached_hw_handler ");
                if (m->queue_mode != DM_TYPE_REQUEST_BASED) {
                        switch(m->queue_mode) {
                        case DM_TYPE_BIO_BASED:
                                DMEMIT("queue_mode bio ");
                                break;
                        case DM_TYPE_MQ_REQUEST_BASED:
                                DMEMIT("queue_mode mq ");
                                break;
                        default:
                                WARN_ON_ONCE(true);
                                break;
                        }
                }
        }

        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 = 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, true, false);
                        goto out;
                } else if (!strcasecmp(argv[0], "fail_if_no_path")) {
                        r = queue_if_no_path(m, false, false);
                        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], true);
                goto out;
        } else if (!strcasecmp(argv[0], "enable_group")) {
                r = bypass_pg_num(m, argv[1], false);
                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;
        struct pgpath *current_pgpath;
        int r;

        current_pgpath = lockless_dereference(m->current_pgpath);
        if (!current_pgpath)
                current_pgpath = choose_pgpath(m, 0);

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

        if (r == -ENOTCONN) {
                if (!lockless_dereference(m->current_pg)) {
                        /* Path status changed, redo selection */
                        (void) choose_pgpath(m, 0);
                }
                if (test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags))
                        pg_init_all_paths(m);
                dm_table_run_md_queue_async(m->ti->table);
                process_queued_io_list(m);
        }

        /*
         * 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)
{
        bool busy = false, has_active = false;
        struct multipath *m = ti->private;
        struct priority_group *pg, *next_pg;
        struct pgpath *pgpath;

        /* pg_init in progress */
        if (atomic_read(&m->pg_init_in_progress))
                return true;

        /* no paths available, for blk-mq: rely on IO mapping to delay requeue */
        if (!atomic_read(&m->nr_valid_paths) && test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags))
                return (m->queue_mode != DM_TYPE_MQ_REQUEST_BASED);

        /* Guess which priority_group will be used at next mapping time */
        pg = lockless_dereference(m->current_pg);
        next_pg = lockless_dereference(m->next_pg);
        if (unlikely(!lockless_dereference(m->current_pgpath) && next_pg))
                pg = next_pg;

        if (!pg) {
                /*
                 * 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.
                 */
                return busy;
        }

        /*
         * 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 = true;
        list_for_each_entry(pgpath, &pg->pgpaths, list) {
                if (pgpath->is_active) {
                        has_active = true;
                        if (!pgpath_busy(pgpath)) {
                                busy = false;
                                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 = false;
        }

        return busy;
}

/*-----------------------------------------------------------------
 * Module setup
 *---------------------------------------------------------------*/
static struct target_type multipath_target = {
        .name = "multipath",
        .version = {1, 12, 0},
        .features = DM_TARGET_SINGLETON | DM_TARGET_IMMUTABLE,
        .module = THIS_MODULE,
        .ctr = multipath_ctr,
        .dtr = multipath_dtr,
        .clone_and_map_rq = multipath_clone_and_map,
        .release_clone_rq = multipath_release_clone,
        .rq_end_io = multipath_end_io,
        .map = multipath_map_bio,
        .end_io = multipath_end_io_bio,
        .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;

        r = dm_register_target(&multipath_target);
        if (r < 0) {
                DMERR("request-based 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;
        }

        return 0;

bad_alloc_kmpath_handlerd:

        destroy_workqueue(kmultipathd);
bad_alloc_kmultipathd:
        dm_unregister_target(&multipath_target);
bad_register_target:
        return r;
}

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

        dm_unregister_target(&multipath_target);
}

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");