Don't enter zvol's rangelock for read bio with size 0

[mirror_zfs.git] / module / zfs / zvol.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright (C) 2008-2010 Lawrence Livermore National Security, LLC.
 * Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
 * Rewritten for Linux by Brian Behlendorf <behlendorf1@llnl.gov>.
 * LLNL-CODE-403049.
 *
 * ZFS volume emulation driver.
 *
 * Makes a DMU object look like a volume of arbitrary size, up to 2^64 bytes.
 * Volumes are accessed through the symbolic links named:
 *
 * /dev/<pool_name>/<dataset_name>
 *
 * Volumes are persistent through reboot and module load.  No user command
 * needs to be run before opening and using a device.
 *
 * Copyright 2014 Nexenta Systems, Inc.  All rights reserved.
 * Copyright (c) 2016 Actifio, Inc. All rights reserved.
 * Copyright (c) 2012, 2018 by Delphix. All rights reserved.
 */

/*
 * Note on locking of zvol state structures.
 *
 * These structures are used to maintain internal state used to emulate block
 * devices on top of zvols. In particular, management of device minor number
 * operations - create, remove, rename, and set_snapdev - involves access to
 * these structures. The zvol_state_lock is primarily used to protect the
 * zvol_state_list. The zv->zv_state_lock is used to protect the contents
 * of the zvol_state_t structures, as well as to make sure that when the
 * time comes to remove the structure from the list, it is not in use, and
 * therefore, it can be taken off zvol_state_list and freed.
 *
 * The zv_suspend_lock was introduced to allow for suspending I/O to a zvol,
 * e.g. for the duration of receive and rollback operations. This lock can be
 * held for significant periods of time. Given that it is undesirable to hold
 * mutexes for long periods of time, the following lock ordering applies:
 * - take zvol_state_lock if necessary, to protect zvol_state_list
 * - take zv_suspend_lock if necessary, by the code path in question
 * - take zv_state_lock to protect zvol_state_t
 *
 * The minor operations are issued to spa->spa_zvol_taskq queues, that are
 * single-threaded (to preserve order of minor operations), and are executed
 * through the zvol_task_cb that dispatches the specific operations. Therefore,
 * these operations are serialized per pool. Consequently, we can be certain
 * that for a given zvol, there is only one operation at a time in progress.
 * That is why one can be sure that first, zvol_state_t for a given zvol is
 * allocated and placed on zvol_state_list, and then other minor operations
 * for this zvol are going to proceed in the order of issue.
 *
 * It is also worth keeping in mind that once add_disk() is called, the zvol is
 * announced to the world, and zvol_open()/zvol_release() can be called at any
 * time. Incidentally, add_disk() itself calls zvol_open()->zvol_first_open()
 * and zvol_release()->zvol_last_close() directly as well.
 */

#include <sys/dataset_kstats.h>
#include <sys/dbuf.h>
#include <sys/dmu_traverse.h>
#include <sys/dsl_dataset.h>
#include <sys/dsl_prop.h>
#include <sys/dsl_dir.h>
#include <sys/zap.h>
#include <sys/zfeature.h>
#include <sys/zil_impl.h>
#include <sys/dmu_tx.h>
#include <sys/zio.h>
#include <sys/zfs_rlock.h>
#include <sys/spa_impl.h>
#include <sys/zvol.h>

#include <linux/blkdev_compat.h>
#include <linux/task_io_accounting_ops.h>

unsigned int zvol_inhibit_dev = 0;
unsigned int zvol_major = ZVOL_MAJOR;
unsigned int zvol_threads = 32;
unsigned int zvol_request_sync = 0;
unsigned int zvol_prefetch_bytes = (128 * 1024);
unsigned long zvol_max_discard_blocks = 16384;
unsigned int zvol_volmode = ZFS_VOLMODE_GEOM;

static taskq_t *zvol_taskq;
static krwlock_t zvol_state_lock;
static list_t zvol_state_list;

#define ZVOL_HT_SIZE    1024
static struct hlist_head *zvol_htable;
#define ZVOL_HT_HEAD(hash)      (&zvol_htable[(hash) & (ZVOL_HT_SIZE-1)])

static struct ida zvol_ida;

/*
 * The in-core state of each volume.
 */
struct zvol_state {
        char                    zv_name[MAXNAMELEN];    /* name */
        uint64_t                zv_volsize;             /* advertised space */
        uint64_t                zv_volblocksize;        /* volume block size */
        objset_t                *zv_objset;     /* objset handle */
        uint32_t                zv_flags;       /* ZVOL_* flags */
        uint32_t                zv_open_count;  /* open counts */
        uint32_t                zv_changed;     /* disk changed */
        zilog_t                 *zv_zilog;      /* ZIL handle */
        rangelock_t             zv_rangelock;   /* for range locking */
        dnode_t                 *zv_dn;         /* dnode hold */
        dev_t                   zv_dev;         /* device id */
        struct gendisk          *zv_disk;       /* generic disk */
        struct request_queue    *zv_queue;      /* request queue */
        dataset_kstats_t        zv_kstat;       /* zvol kstats */
        list_node_t             zv_next;        /* next zvol_state_t linkage */
        uint64_t                zv_hash;        /* name hash */
        struct hlist_node       zv_hlink;       /* hash link */
        kmutex_t                zv_state_lock;  /* protects zvol_state_t */
        atomic_t                zv_suspend_ref; /* refcount for suspend */
        krwlock_t               zv_suspend_lock;        /* suspend lock */
};

typedef enum {
        ZVOL_ASYNC_CREATE_MINORS,
        ZVOL_ASYNC_REMOVE_MINORS,
        ZVOL_ASYNC_RENAME_MINORS,
        ZVOL_ASYNC_SET_SNAPDEV,
        ZVOL_ASYNC_SET_VOLMODE,
        ZVOL_ASYNC_MAX
} zvol_async_op_t;

typedef struct {
        zvol_async_op_t op;
        char pool[MAXNAMELEN];
        char name1[MAXNAMELEN];
        char name2[MAXNAMELEN];
        zprop_source_t source;
        uint64_t value;
} zvol_task_t;

#define ZVOL_RDONLY     0x1

static uint64_t
zvol_name_hash(const char *name)
{
        int i;
        uint64_t crc = -1ULL;
        uint8_t *p = (uint8_t *)name;
        ASSERT(zfs_crc64_table[128] == ZFS_CRC64_POLY);
        for (i = 0; i < MAXNAMELEN - 1 && *p; i++, p++) {
                crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (*p)) & 0xFF];
        }
        return (crc);
}

/*
 * Find a zvol_state_t given the full major+minor dev_t. If found,
 * return with zv_state_lock taken, otherwise, return (NULL) without
 * taking zv_state_lock.
 */
static zvol_state_t *
zvol_find_by_dev(dev_t dev)
{
        zvol_state_t *zv;

        rw_enter(&zvol_state_lock, RW_READER);
        for (zv = list_head(&zvol_state_list); zv != NULL;
            zv = list_next(&zvol_state_list, zv)) {
                mutex_enter(&zv->zv_state_lock);
                if (zv->zv_dev == dev) {
                        rw_exit(&zvol_state_lock);
                        return (zv);
                }
                mutex_exit(&zv->zv_state_lock);
        }
        rw_exit(&zvol_state_lock);

        return (NULL);
}

/*
 * Find a zvol_state_t given the name and hash generated by zvol_name_hash.
 * If found, return with zv_suspend_lock and zv_state_lock taken, otherwise,
 * return (NULL) without the taking locks. The zv_suspend_lock is always taken
 * before zv_state_lock. The mode argument indicates the mode (including none)
 * for zv_suspend_lock to be taken.
 */
static zvol_state_t *
zvol_find_by_name_hash(const char *name, uint64_t hash, int mode)
{
        zvol_state_t *zv;
        struct hlist_node *p = NULL;

        rw_enter(&zvol_state_lock, RW_READER);
        hlist_for_each(p, ZVOL_HT_HEAD(hash)) {
                zv = hlist_entry(p, zvol_state_t, zv_hlink);
                mutex_enter(&zv->zv_state_lock);
                if (zv->zv_hash == hash &&
                    strncmp(zv->zv_name, name, MAXNAMELEN) == 0) {
                        /*
                         * this is the right zvol, take the locks in the
                         * right order
                         */
                        if (mode != RW_NONE &&
                            !rw_tryenter(&zv->zv_suspend_lock, mode)) {
                                mutex_exit(&zv->zv_state_lock);
                                rw_enter(&zv->zv_suspend_lock, mode);
                                mutex_enter(&zv->zv_state_lock);
                                /*
                                 * zvol cannot be renamed as we continue
                                 * to hold zvol_state_lock
                                 */
                                ASSERT(zv->zv_hash == hash &&
                                    strncmp(zv->zv_name, name, MAXNAMELEN)
                                    == 0);
                        }
                        rw_exit(&zvol_state_lock);
                        return (zv);
                }
                mutex_exit(&zv->zv_state_lock);
        }
        rw_exit(&zvol_state_lock);

        return (NULL);
}

/*
 * Find a zvol_state_t given the name.
 * If found, return with zv_suspend_lock and zv_state_lock taken, otherwise,
 * return (NULL) without the taking locks. The zv_suspend_lock is always taken
 * before zv_state_lock. The mode argument indicates the mode (including none)
 * for zv_suspend_lock to be taken.
 */
static zvol_state_t *
zvol_find_by_name(const char *name, int mode)
{
        return (zvol_find_by_name_hash(name, zvol_name_hash(name), mode));
}


/*
 * Given a path, return TRUE if path is a ZVOL.
 */
boolean_t
zvol_is_zvol(const char *device)
{
        struct block_device *bdev;
        unsigned int major;

        bdev = vdev_lookup_bdev(device);
        if (IS_ERR(bdev))
                return (B_FALSE);

        major = MAJOR(bdev->bd_dev);
        bdput(bdev);

        if (major == zvol_major)
                return (B_TRUE);

        return (B_FALSE);
}

/*
 * ZFS_IOC_CREATE callback handles dmu zvol and zap object creation.
 */
void
zvol_create_cb(objset_t *os, void *arg, cred_t *cr, dmu_tx_t *tx)
{
        zfs_creat_t *zct = arg;
        nvlist_t *nvprops = zct->zct_props;
        int error;
        uint64_t volblocksize, volsize;

        VERIFY(nvlist_lookup_uint64(nvprops,
            zfs_prop_to_name(ZFS_PROP_VOLSIZE), &volsize) == 0);
        if (nvlist_lookup_uint64(nvprops,
            zfs_prop_to_name(ZFS_PROP_VOLBLOCKSIZE), &volblocksize) != 0)
                volblocksize = zfs_prop_default_numeric(ZFS_PROP_VOLBLOCKSIZE);

        /*
         * These properties must be removed from the list so the generic
         * property setting step won't apply to them.
         */
        VERIFY(nvlist_remove_all(nvprops,
            zfs_prop_to_name(ZFS_PROP_VOLSIZE)) == 0);
        (void) nvlist_remove_all(nvprops,
            zfs_prop_to_name(ZFS_PROP_VOLBLOCKSIZE));

        error = dmu_object_claim(os, ZVOL_OBJ, DMU_OT_ZVOL, volblocksize,
            DMU_OT_NONE, 0, tx);
        ASSERT(error == 0);

        error = zap_create_claim(os, ZVOL_ZAP_OBJ, DMU_OT_ZVOL_PROP,
            DMU_OT_NONE, 0, tx);
        ASSERT(error == 0);

        error = zap_update(os, ZVOL_ZAP_OBJ, "size", 8, 1, &volsize, tx);
        ASSERT(error == 0);
}

/*
 * ZFS_IOC_OBJSET_STATS entry point.
 */
int
zvol_get_stats(objset_t *os, nvlist_t *nv)
{
        int error;
        dmu_object_info_t *doi;
        uint64_t val;

        error = zap_lookup(os, ZVOL_ZAP_OBJ, "size", 8, 1, &val);
        if (error)
                return (SET_ERROR(error));

        dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_VOLSIZE, val);
        doi = kmem_alloc(sizeof (dmu_object_info_t), KM_SLEEP);
        error = dmu_object_info(os, ZVOL_OBJ, doi);

        if (error == 0) {
                dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_VOLBLOCKSIZE,
                    doi->doi_data_block_size);
        }

        kmem_free(doi, sizeof (dmu_object_info_t));

        return (SET_ERROR(error));
}

/*
 * Sanity check volume size.
 */
int
zvol_check_volsize(uint64_t volsize, uint64_t blocksize)
{
        if (volsize == 0)
                return (SET_ERROR(EINVAL));

        if (volsize % blocksize != 0)
                return (SET_ERROR(EINVAL));

#ifdef _ILP32
        if (volsize - 1 > SPEC_MAXOFFSET_T)
                return (SET_ERROR(EOVERFLOW));
#endif
        return (0);
}

/*
 * Ensure the zap is flushed then inform the VFS of the capacity change.
 */
static int
zvol_update_volsize(uint64_t volsize, objset_t *os)
{
        dmu_tx_t *tx;
        int error;
        uint64_t txg;

        tx = dmu_tx_create(os);
        dmu_tx_hold_zap(tx, ZVOL_ZAP_OBJ, TRUE, NULL);
        dmu_tx_mark_netfree(tx);
        error = dmu_tx_assign(tx, TXG_WAIT);
        if (error) {
                dmu_tx_abort(tx);
                return (SET_ERROR(error));
        }
        txg = dmu_tx_get_txg(tx);

        error = zap_update(os, ZVOL_ZAP_OBJ, "size", 8, 1,
            &volsize, tx);
        dmu_tx_commit(tx);

        txg_wait_synced(dmu_objset_pool(os), txg);

        if (error == 0)
                error = dmu_free_long_range(os,
                    ZVOL_OBJ, volsize, DMU_OBJECT_END);

        return (error);
}

/*
 * Set ZFS_PROP_VOLSIZE set entry point.  Note that modifying the volume
 * size will result in a udev "change" event being generated.
 */
int
zvol_set_volsize(const char *name, uint64_t volsize)
{
        objset_t *os = NULL;
        struct gendisk *disk = NULL;
        uint64_t readonly;
        int error;
        boolean_t owned = B_FALSE;

        error = dsl_prop_get_integer(name,
            zfs_prop_to_name(ZFS_PROP_READONLY), &readonly, NULL);
        if (error != 0)
                return (SET_ERROR(error));
        if (readonly)
                return (SET_ERROR(EROFS));

        zvol_state_t *zv = zvol_find_by_name(name, RW_READER);

        ASSERT(zv == NULL || (MUTEX_HELD(&zv->zv_state_lock) &&
            RW_READ_HELD(&zv->zv_suspend_lock)));

        if (zv == NULL || zv->zv_objset == NULL) {
                if (zv != NULL)
                        rw_exit(&zv->zv_suspend_lock);
                if ((error = dmu_objset_own(name, DMU_OST_ZVOL, B_FALSE, B_TRUE,
                    FTAG, &os)) != 0) {
                        if (zv != NULL)
                                mutex_exit(&zv->zv_state_lock);
                        return (SET_ERROR(error));
                }
                owned = B_TRUE;
                if (zv != NULL)
                        zv->zv_objset = os;
        } else {
                os = zv->zv_objset;
        }

        dmu_object_info_t *doi = kmem_alloc(sizeof (*doi), KM_SLEEP);

        if ((error = dmu_object_info(os, ZVOL_OBJ, doi)) ||
            (error = zvol_check_volsize(volsize, doi->doi_data_block_size)))
                goto out;

        error = zvol_update_volsize(volsize, os);
        if (error == 0 && zv != NULL) {
                zv->zv_volsize = volsize;
                zv->zv_changed = 1;
                disk = zv->zv_disk;
        }
out:
        kmem_free(doi, sizeof (dmu_object_info_t));

        if (owned) {
                dmu_objset_disown(os, B_TRUE, FTAG);
                if (zv != NULL)
                        zv->zv_objset = NULL;
        } else {
                rw_exit(&zv->zv_suspend_lock);
        }

        if (zv != NULL)
                mutex_exit(&zv->zv_state_lock);

        if (disk != NULL)
                revalidate_disk(disk);

        return (SET_ERROR(error));
}

/*
 * Sanity check volume block size.
 */
int
zvol_check_volblocksize(const char *name, uint64_t volblocksize)
{
        /* Record sizes above 128k need the feature to be enabled */
        if (volblocksize > SPA_OLD_MAXBLOCKSIZE) {
                spa_t *spa;
                int error;

                if ((error = spa_open(name, &spa, FTAG)) != 0)
                        return (error);

                if (!spa_feature_is_enabled(spa, SPA_FEATURE_LARGE_BLOCKS)) {
                        spa_close(spa, FTAG);
                        return (SET_ERROR(ENOTSUP));
                }

                /*
                 * We don't allow setting the property above 1MB,
                 * unless the tunable has been changed.
                 */
                if (volblocksize > zfs_max_recordsize)
                        return (SET_ERROR(EDOM));

                spa_close(spa, FTAG);
        }

        if (volblocksize < SPA_MINBLOCKSIZE ||
            volblocksize > SPA_MAXBLOCKSIZE ||
            !ISP2(volblocksize))
                return (SET_ERROR(EDOM));

        return (0);
}

/*
 * Set ZFS_PROP_VOLBLOCKSIZE set entry point.
 */
int
zvol_set_volblocksize(const char *name, uint64_t volblocksize)
{
        zvol_state_t *zv;
        dmu_tx_t *tx;
        int error;

        zv = zvol_find_by_name(name, RW_READER);

        if (zv == NULL)
                return (SET_ERROR(ENXIO));

        ASSERT(MUTEX_HELD(&zv->zv_state_lock));
        ASSERT(RW_READ_HELD(&zv->zv_suspend_lock));

        if (zv->zv_flags & ZVOL_RDONLY) {
                mutex_exit(&zv->zv_state_lock);
                rw_exit(&zv->zv_suspend_lock);
                return (SET_ERROR(EROFS));
        }

        tx = dmu_tx_create(zv->zv_objset);
        dmu_tx_hold_bonus(tx, ZVOL_OBJ);
        error = dmu_tx_assign(tx, TXG_WAIT);
        if (error) {
                dmu_tx_abort(tx);
        } else {
                error = dmu_object_set_blocksize(zv->zv_objset, ZVOL_OBJ,
                    volblocksize, 0, tx);
                if (error == ENOTSUP)
                        error = SET_ERROR(EBUSY);
                dmu_tx_commit(tx);
                if (error == 0)
                        zv->zv_volblocksize = volblocksize;
        }

        mutex_exit(&zv->zv_state_lock);
        rw_exit(&zv->zv_suspend_lock);

        return (SET_ERROR(error));
}

/*
 * Replay a TX_TRUNCATE ZIL transaction if asked.  TX_TRUNCATE is how we
 * implement DKIOCFREE/free-long-range.
 */
static int
zvol_replay_truncate(void *arg1, void *arg2, boolean_t byteswap)
{
        zvol_state_t *zv = arg1;
        lr_truncate_t *lr = arg2;
        uint64_t offset, length;

        if (byteswap)
                byteswap_uint64_array(lr, sizeof (*lr));

        offset = lr->lr_offset;
        length = lr->lr_length;

        return (dmu_free_long_range(zv->zv_objset, ZVOL_OBJ, offset, length));
}

/*
 * Replay a TX_WRITE ZIL transaction that didn't get committed
 * after a system failure
 */
static int
zvol_replay_write(void *arg1, void *arg2, boolean_t byteswap)
{
        zvol_state_t *zv = arg1;
        lr_write_t *lr = arg2;
        objset_t *os = zv->zv_objset;
        char *data = (char *)(lr + 1);  /* data follows lr_write_t */
        uint64_t offset, length;
        dmu_tx_t *tx;
        int error;

        if (byteswap)
                byteswap_uint64_array(lr, sizeof (*lr));

        offset = lr->lr_offset;
        length = lr->lr_length;

        /* If it's a dmu_sync() block, write the whole block */
        if (lr->lr_common.lrc_reclen == sizeof (lr_write_t)) {
                uint64_t blocksize = BP_GET_LSIZE(&lr->lr_blkptr);
                if (length < blocksize) {
                        offset -= offset % blocksize;
                        length = blocksize;
                }
        }

        tx = dmu_tx_create(os);
        dmu_tx_hold_write(tx, ZVOL_OBJ, offset, length);
        error = dmu_tx_assign(tx, TXG_WAIT);
        if (error) {
                dmu_tx_abort(tx);
        } else {
                dmu_write(os, ZVOL_OBJ, offset, length, data, tx);
                dmu_tx_commit(tx);
        }

        return (error);
}

static int
zvol_replay_err(void *arg1, void *arg2, boolean_t byteswap)
{
        return (SET_ERROR(ENOTSUP));
}

/*
 * Callback vectors for replaying records.
 * Only TX_WRITE and TX_TRUNCATE are needed for zvol.
 */
zil_replay_func_t *zvol_replay_vector[TX_MAX_TYPE] = {
        zvol_replay_err,        /* no such transaction type */
        zvol_replay_err,        /* TX_CREATE */
        zvol_replay_err,        /* TX_MKDIR */
        zvol_replay_err,        /* TX_MKXATTR */
        zvol_replay_err,        /* TX_SYMLINK */
        zvol_replay_err,        /* TX_REMOVE */
        zvol_replay_err,        /* TX_RMDIR */
        zvol_replay_err,        /* TX_LINK */
        zvol_replay_err,        /* TX_RENAME */
        zvol_replay_write,      /* TX_WRITE */
        zvol_replay_truncate,   /* TX_TRUNCATE */
        zvol_replay_err,        /* TX_SETATTR */
        zvol_replay_err,        /* TX_ACL */
        zvol_replay_err,        /* TX_CREATE_ATTR */
        zvol_replay_err,        /* TX_CREATE_ACL_ATTR */
        zvol_replay_err,        /* TX_MKDIR_ACL */
        zvol_replay_err,        /* TX_MKDIR_ATTR */
        zvol_replay_err,        /* TX_MKDIR_ACL_ATTR */
        zvol_replay_err,        /* TX_WRITE2 */
};

/*
 * zvol_log_write() handles synchronous writes using TX_WRITE ZIL transactions.
 *
 * We store data in the log buffers if it's small enough.
 * Otherwise we will later flush the data out via dmu_sync().
 */
ssize_t zvol_immediate_write_sz = 32768;

static void
zvol_log_write(zvol_state_t *zv, dmu_tx_t *tx, uint64_t offset,
    uint64_t size, int sync)
{
        uint32_t blocksize = zv->zv_volblocksize;
        zilog_t *zilog = zv->zv_zilog;
        itx_wr_state_t write_state;

        if (zil_replaying(zilog, tx))
                return;

        if (zilog->zl_logbias == ZFS_LOGBIAS_THROUGHPUT)
                write_state = WR_INDIRECT;
        else if (!spa_has_slogs(zilog->zl_spa) &&
            size >= blocksize && blocksize > zvol_immediate_write_sz)
                write_state = WR_INDIRECT;
        else if (sync)
                write_state = WR_COPIED;
        else
                write_state = WR_NEED_COPY;

        while (size) {
                itx_t *itx;
                lr_write_t *lr;
                itx_wr_state_t wr_state = write_state;
                ssize_t len = size;

                if (wr_state == WR_COPIED && size > ZIL_MAX_COPIED_DATA)
                        wr_state = WR_NEED_COPY;
                else if (wr_state == WR_INDIRECT)
                        len = MIN(blocksize - P2PHASE(offset, blocksize), size);

                itx = zil_itx_create(TX_WRITE, sizeof (*lr) +
                    (wr_state == WR_COPIED ? len : 0));
                lr = (lr_write_t *)&itx->itx_lr;
                if (wr_state == WR_COPIED && dmu_read_by_dnode(zv->zv_dn,
                    offset, len, lr+1, DMU_READ_NO_PREFETCH) != 0) {
                        zil_itx_destroy(itx);
                        itx = zil_itx_create(TX_WRITE, sizeof (*lr));
                        lr = (lr_write_t *)&itx->itx_lr;
                        wr_state = WR_NEED_COPY;
                }

                itx->itx_wr_state = wr_state;
                lr->lr_foid = ZVOL_OBJ;
                lr->lr_offset = offset;
                lr->lr_length = len;
                lr->lr_blkoff = 0;
                BP_ZERO(&lr->lr_blkptr);

                itx->itx_private = zv;
                itx->itx_sync = sync;

                (void) zil_itx_assign(zilog, itx, tx);

                offset += len;
                size -= len;
        }
}

typedef struct zv_request {
        zvol_state_t    *zv;
        struct bio      *bio;
        locked_range_t  *lr;
} zv_request_t;

static void
uio_from_bio(uio_t *uio, struct bio *bio)
{
        uio->uio_bvec = &bio->bi_io_vec[BIO_BI_IDX(bio)];
        uio->uio_skip = BIO_BI_SKIP(bio);
        uio->uio_resid = BIO_BI_SIZE(bio);
        uio->uio_iovcnt = bio->bi_vcnt - BIO_BI_IDX(bio);
        uio->uio_loffset = BIO_BI_SECTOR(bio) << 9;
        uio->uio_limit = MAXOFFSET_T;
        uio->uio_segflg = UIO_BVEC;
}

static void
zvol_write(void *arg)
{
        int error = 0;

        zv_request_t *zvr = arg;
        struct bio *bio = zvr->bio;
        uio_t uio;
        uio_from_bio(&uio, bio);

        zvol_state_t *zv = zvr->zv;
        ASSERT(zv && zv->zv_open_count > 0);

        ssize_t start_resid = uio.uio_resid;
        unsigned long start_jif = jiffies;
        blk_generic_start_io_acct(zv->zv_queue, WRITE, bio_sectors(bio),
            &zv->zv_disk->part0);

        boolean_t sync =
            bio_is_fua(bio) || zv->zv_objset->os_sync == ZFS_SYNC_ALWAYS;

        uint64_t volsize = zv->zv_volsize;
        while (uio.uio_resid > 0 && uio.uio_loffset < volsize) {
                uint64_t bytes = MIN(uio.uio_resid, DMU_MAX_ACCESS >> 1);
                uint64_t off = uio.uio_loffset;
                dmu_tx_t *tx = dmu_tx_create(zv->zv_objset);

                if (bytes > volsize - off)      /* don't write past the end */
                        bytes = volsize - off;

                dmu_tx_hold_write(tx, ZVOL_OBJ, off, bytes);

                /* This will only fail for ENOSPC */
                error = dmu_tx_assign(tx, TXG_WAIT);
                if (error) {
                        dmu_tx_abort(tx);
                        break;
                }
                error = dmu_write_uio_dnode(zv->zv_dn, &uio, bytes, tx);
                if (error == 0) {
                        zvol_log_write(zv, tx, off, bytes, sync);
                }
                dmu_tx_commit(tx);

                if (error)
                        break;
        }
        rangelock_exit(zvr->lr);

        int64_t nwritten = start_resid - uio.uio_resid;
        dataset_kstats_update_write_kstats(&zv->zv_kstat, nwritten);
        task_io_account_write(nwritten);

        if (sync)
                zil_commit(zv->zv_zilog, ZVOL_OBJ);

        rw_exit(&zv->zv_suspend_lock);
        blk_generic_end_io_acct(zv->zv_queue, WRITE, &zv->zv_disk->part0,
            start_jif);
        BIO_END_IO(bio, -error);
        kmem_free(zvr, sizeof (zv_request_t));
}

/*
 * Log a DKIOCFREE/free-long-range to the ZIL with TX_TRUNCATE.
 */
static void
zvol_log_truncate(zvol_state_t *zv, dmu_tx_t *tx, uint64_t off, uint64_t len,
    boolean_t sync)
{
        itx_t *itx;
        lr_truncate_t *lr;
        zilog_t *zilog = zv->zv_zilog;

        if (zil_replaying(zilog, tx))
                return;

        itx = zil_itx_create(TX_TRUNCATE, sizeof (*lr));
        lr = (lr_truncate_t *)&itx->itx_lr;
        lr->lr_foid = ZVOL_OBJ;
        lr->lr_offset = off;
        lr->lr_length = len;

        itx->itx_sync = sync;
        zil_itx_assign(zilog, itx, tx);
}

static void
zvol_discard(void *arg)
{
        zv_request_t *zvr = arg;
        struct bio *bio = zvr->bio;
        zvol_state_t *zv = zvr->zv;
        uint64_t start = BIO_BI_SECTOR(bio) << 9;
        uint64_t size = BIO_BI_SIZE(bio);
        uint64_t end = start + size;
        boolean_t sync;
        int error = 0;
        dmu_tx_t *tx;
        unsigned long start_jif;

        ASSERT(zv && zv->zv_open_count > 0);

        start_jif = jiffies;
        blk_generic_start_io_acct(zv->zv_queue, WRITE, bio_sectors(bio),
            &zv->zv_disk->part0);

        sync = bio_is_fua(bio) || zv->zv_objset->os_sync == ZFS_SYNC_ALWAYS;

        if (end > zv->zv_volsize) {
                error = SET_ERROR(EIO);
                goto unlock;
        }

        /*
         * Align the request to volume block boundaries when a secure erase is
         * not required.  This will prevent dnode_free_range() from zeroing out
         * the unaligned parts which is slow (read-modify-write) and useless
         * since we are not freeing any space by doing so.
         */
        if (!bio_is_secure_erase(bio)) {
                start = P2ROUNDUP(start, zv->zv_volblocksize);
                end = P2ALIGN(end, zv->zv_volblocksize);
                size = end - start;
        }

        if (start >= end)
                goto unlock;

        tx = dmu_tx_create(zv->zv_objset);
        dmu_tx_mark_netfree(tx);
        error = dmu_tx_assign(tx, TXG_WAIT);
        if (error != 0) {
                dmu_tx_abort(tx);
        } else {
                zvol_log_truncate(zv, tx, start, size, B_TRUE);
                dmu_tx_commit(tx);
                error = dmu_free_long_range(zv->zv_objset,
                    ZVOL_OBJ, start, size);
        }
unlock:
        rangelock_exit(zvr->lr);

        if (error == 0 && sync)
                zil_commit(zv->zv_zilog, ZVOL_OBJ);

        rw_exit(&zv->zv_suspend_lock);
        blk_generic_end_io_acct(zv->zv_queue, WRITE, &zv->zv_disk->part0,
            start_jif);
        BIO_END_IO(bio, -error);
        kmem_free(zvr, sizeof (zv_request_t));
}

static void
zvol_read(void *arg)
{
        int error = 0;

        zv_request_t *zvr = arg;
        struct bio *bio = zvr->bio;
        uio_t uio;
        uio_from_bio(&uio, bio);

        zvol_state_t *zv = zvr->zv;
        ASSERT(zv && zv->zv_open_count > 0);

        ssize_t start_resid = uio.uio_resid;
        unsigned long start_jif = jiffies;
        blk_generic_start_io_acct(zv->zv_queue, READ, bio_sectors(bio),
            &zv->zv_disk->part0);

        uint64_t volsize = zv->zv_volsize;
        while (uio.uio_resid > 0 && uio.uio_loffset < volsize) {
                uint64_t bytes = MIN(uio.uio_resid, DMU_MAX_ACCESS >> 1);

                /* don't read past the end */
                if (bytes > volsize - uio.uio_loffset)
                        bytes = volsize - uio.uio_loffset;

                error = dmu_read_uio_dnode(zv->zv_dn, &uio, bytes);
                if (error) {
                        /* convert checksum errors into IO errors */
                        if (error == ECKSUM)
                                error = SET_ERROR(EIO);
                        break;
                }
        }
        rangelock_exit(zvr->lr);

        int64_t nread = start_resid - uio.uio_resid;
        dataset_kstats_update_read_kstats(&zv->zv_kstat, nread);
        task_io_account_read(nread);

        rw_exit(&zv->zv_suspend_lock);
        blk_generic_end_io_acct(zv->zv_queue, READ, &zv->zv_disk->part0,
            start_jif);
        BIO_END_IO(bio, -error);
        kmem_free(zvr, sizeof (zv_request_t));
}

static MAKE_REQUEST_FN_RET
zvol_request(struct request_queue *q, struct bio *bio)
{
        zvol_state_t *zv = q->queuedata;
        fstrans_cookie_t cookie = spl_fstrans_mark();
        uint64_t offset = BIO_BI_SECTOR(bio) << 9;
        uint64_t size = BIO_BI_SIZE(bio);
        int rw = bio_data_dir(bio);
        zv_request_t *zvr;

        if (bio_has_data(bio) && offset + size > zv->zv_volsize) {
                printk(KERN_INFO
                    "%s: bad access: offset=%llu, size=%lu\n",
                    zv->zv_disk->disk_name,
                    (long long unsigned)offset,
                    (long unsigned)size);

                BIO_END_IO(bio, -SET_ERROR(EIO));
                goto out;
        }

        if (rw == WRITE) {
                boolean_t need_sync = B_FALSE;

                if (unlikely(zv->zv_flags & ZVOL_RDONLY)) {
                        BIO_END_IO(bio, -SET_ERROR(EROFS));
                        goto out;
                }

                /*
                 * To be released in the I/O function. See the comment on
                 * rangelock_enter() below.
                 */
                rw_enter(&zv->zv_suspend_lock, RW_READER);

                /* bio marked as FLUSH need to flush before write */
                if (bio_is_flush(bio))
                        zil_commit(zv->zv_zilog, ZVOL_OBJ);

                /* Some requests are just for flush and nothing else. */
                if (size == 0) {
                        rw_exit(&zv->zv_suspend_lock);
                        BIO_END_IO(bio, 0);
                        goto out;
                }

                zvr = kmem_alloc(sizeof (zv_request_t), KM_SLEEP);
                zvr->zv = zv;
                zvr->bio = bio;

                /*
                 * To be released in the I/O function. Since the I/O functions
                 * are asynchronous, we take it here synchronously to make
                 * sure overlapped I/Os are properly ordered.
                 */
                zvr->lr = rangelock_enter(&zv->zv_rangelock, offset, size,
                    RL_WRITER);
                /*
                 * Sync writes and discards execute zil_commit() which may need
                 * to take a RL_READER lock on the whole block being modified
                 * via its zillog->zl_get_data(): to avoid circular dependency
                 * issues with taskq threads execute these requests
                 * synchronously here in zvol_request().
                 */
                need_sync = bio_is_fua(bio) ||
                    zv->zv_objset->os_sync == ZFS_SYNC_ALWAYS;
                if (bio_is_discard(bio) || bio_is_secure_erase(bio)) {
                        if (zvol_request_sync || need_sync ||
                            taskq_dispatch(zvol_taskq, zvol_discard, zvr,
                            TQ_SLEEP) == TASKQID_INVALID)
                                zvol_discard(zvr);
                } else {
                        if (zvol_request_sync || need_sync ||
                            taskq_dispatch(zvol_taskq, zvol_write, zvr,
                            TQ_SLEEP) == TASKQID_INVALID)
                                zvol_write(zvr);
                }
        } else {
                /*
                 * The SCST driver, and possibly others, may issue READ I/Os
                 * with a length of zero bytes.  These empty I/Os contain no
                 * data and require no additional handling.
                 */
                if (size == 0) {
                        BIO_END_IO(bio, 0);
                        goto out;
                }

                zvr = kmem_alloc(sizeof (zv_request_t), KM_SLEEP);
                zvr->zv = zv;
                zvr->bio = bio;

                rw_enter(&zv->zv_suspend_lock, RW_READER);

                zvr->lr = rangelock_enter(&zv->zv_rangelock, offset, size,
                    RL_READER);
                if (zvol_request_sync || taskq_dispatch(zvol_taskq,
                    zvol_read, zvr, TQ_SLEEP) == TASKQID_INVALID)
                        zvol_read(zvr);
        }

out:
        spl_fstrans_unmark(cookie);
#ifdef HAVE_MAKE_REQUEST_FN_RET_INT
        return (0);
#elif defined(HAVE_MAKE_REQUEST_FN_RET_QC)
        return (BLK_QC_T_NONE);
#endif
}

/* ARGSUSED */
static void
zvol_get_done(zgd_t *zgd, int error)
{
        if (zgd->zgd_db)
                dmu_buf_rele(zgd->zgd_db, zgd);

        rangelock_exit(zgd->zgd_lr);

        kmem_free(zgd, sizeof (zgd_t));
}

/*
 * Get data to generate a TX_WRITE intent log record.
 */
static int
zvol_get_data(void *arg, lr_write_t *lr, char *buf, struct lwb *lwb, zio_t *zio)
{
        zvol_state_t *zv = arg;
        uint64_t offset = lr->lr_offset;
        uint64_t size = lr->lr_length;
        dmu_buf_t *db;
        zgd_t *zgd;
        int error;

        ASSERT3P(lwb, !=, NULL);
        ASSERT3P(zio, !=, NULL);
        ASSERT3U(size, !=, 0);

        zgd = (zgd_t *)kmem_zalloc(sizeof (zgd_t), KM_SLEEP);
        zgd->zgd_lwb = lwb;

        /*
         * Write records come in two flavors: immediate and indirect.
         * For small writes it's cheaper to store the data with the
         * log record (immediate); for large writes it's cheaper to
         * sync the data and get a pointer to it (indirect) so that
         * we don't have to write the data twice.
         */
        if (buf != NULL) { /* immediate write */
                zgd->zgd_lr = rangelock_enter(&zv->zv_rangelock, offset, size,
                    RL_READER);
                error = dmu_read_by_dnode(zv->zv_dn, offset, size, buf,
                    DMU_READ_NO_PREFETCH);
        } else { /* indirect write */
                /*
                 * Have to lock the whole block to ensure when it's written out
                 * and its checksum is being calculated that no one can change
                 * the data. Contrarily to zfs_get_data we need not re-check
                 * blocksize after we get the lock because it cannot be changed.
                 */
                size = zv->zv_volblocksize;
                offset = P2ALIGN_TYPED(offset, size, uint64_t);
                zgd->zgd_lr = rangelock_enter(&zv->zv_rangelock, offset, size,
                    RL_READER);
                error = dmu_buf_hold_by_dnode(zv->zv_dn, offset, zgd, &db,
                    DMU_READ_NO_PREFETCH);
                if (error == 0) {
                        blkptr_t *bp = &lr->lr_blkptr;

                        zgd->zgd_db = db;
                        zgd->zgd_bp = bp;

                        ASSERT(db != NULL);
                        ASSERT(db->db_offset == offset);
                        ASSERT(db->db_size == size);

                        error = dmu_sync(zio, lr->lr_common.lrc_txg,
                            zvol_get_done, zgd);

                        if (error == 0)
                                return (0);
                }
        }

        zvol_get_done(zgd, error);

        return (SET_ERROR(error));
}

/*
 * The zvol_state_t's are inserted into zvol_state_list and zvol_htable.
 */
static void
zvol_insert(zvol_state_t *zv)
{
        ASSERT(RW_WRITE_HELD(&zvol_state_lock));
        ASSERT3U(MINOR(zv->zv_dev) & ZVOL_MINOR_MASK, ==, 0);
        list_insert_head(&zvol_state_list, zv);
        hlist_add_head(&zv->zv_hlink, ZVOL_HT_HEAD(zv->zv_hash));
}

/*
 * Simply remove the zvol from to list of zvols.
 */
static void
zvol_remove(zvol_state_t *zv)
{
        ASSERT(RW_WRITE_HELD(&zvol_state_lock));
        list_remove(&zvol_state_list, zv);
        hlist_del(&zv->zv_hlink);
}

/*
 * Setup zv after we just own the zv->objset
 */
static int
zvol_setup_zv(zvol_state_t *zv)
{
        uint64_t volsize;
        int error;
        uint64_t ro;
        objset_t *os = zv->zv_objset;

        ASSERT(MUTEX_HELD(&zv->zv_state_lock));
        ASSERT(RW_LOCK_HELD(&zv->zv_suspend_lock));

        error = dsl_prop_get_integer(zv->zv_name, "readonly", &ro, NULL);
        if (error)
                return (SET_ERROR(error));

        error = zap_lookup(os, ZVOL_ZAP_OBJ, "size", 8, 1, &volsize);
        if (error)
                return (SET_ERROR(error));

        error = dnode_hold(os, ZVOL_OBJ, FTAG, &zv->zv_dn);
        if (error)
                return (SET_ERROR(error));

        set_capacity(zv->zv_disk, volsize >> 9);
        zv->zv_volsize = volsize;
        zv->zv_zilog = zil_open(os, zvol_get_data);

        if (ro || dmu_objset_is_snapshot(os) ||
            !spa_writeable(dmu_objset_spa(os))) {
                set_disk_ro(zv->zv_disk, 1);
                zv->zv_flags |= ZVOL_RDONLY;
        } else {
                set_disk_ro(zv->zv_disk, 0);
                zv->zv_flags &= ~ZVOL_RDONLY;
        }
        return (0);
}

/*
 * Shutdown every zv_objset related stuff except zv_objset itself.
 * The is the reverse of zvol_setup_zv.
 */
static void
zvol_shutdown_zv(zvol_state_t *zv)
{
        ASSERT(MUTEX_HELD(&zv->zv_state_lock) &&
            RW_LOCK_HELD(&zv->zv_suspend_lock));

        zil_close(zv->zv_zilog);
        zv->zv_zilog = NULL;

        dnode_rele(zv->zv_dn, FTAG);
        zv->zv_dn = NULL;

        /*
         * Evict cached data. We must write out any dirty data before
         * disowning the dataset.
         */
        if (!(zv->zv_flags & ZVOL_RDONLY))
                txg_wait_synced(dmu_objset_pool(zv->zv_objset), 0);
        (void) dmu_objset_evict_dbufs(zv->zv_objset);
}

/*
 * return the proper tag for rollback and recv
 */
void *
zvol_tag(zvol_state_t *zv)
{
        ASSERT(RW_WRITE_HELD(&zv->zv_suspend_lock));
        return (zv->zv_open_count > 0 ? zv : NULL);
}

/*
 * Suspend the zvol for recv and rollback.
 */
zvol_state_t *
zvol_suspend(const char *name)
{
        zvol_state_t *zv;

        zv = zvol_find_by_name(name, RW_WRITER);

        if (zv == NULL)
                return (NULL);

        /* block all I/O, release in zvol_resume. */
        ASSERT(MUTEX_HELD(&zv->zv_state_lock));
        ASSERT(RW_WRITE_HELD(&zv->zv_suspend_lock));

        atomic_inc(&zv->zv_suspend_ref);

        if (zv->zv_open_count > 0)
                zvol_shutdown_zv(zv);

        /*
         * do not hold zv_state_lock across suspend/resume to
         * avoid locking up zvol lookups
         */
        mutex_exit(&zv->zv_state_lock);

        /* zv_suspend_lock is released in zvol_resume() */
        return (zv);
}

int
zvol_resume(zvol_state_t *zv)
{
        int error = 0;

        ASSERT(RW_WRITE_HELD(&zv->zv_suspend_lock));

        mutex_enter(&zv->zv_state_lock);

        if (zv->zv_open_count > 0) {
                VERIFY0(dmu_objset_hold(zv->zv_name, zv, &zv->zv_objset));
                VERIFY3P(zv->zv_objset->os_dsl_dataset->ds_owner, ==, zv);
                VERIFY(dsl_dataset_long_held(zv->zv_objset->os_dsl_dataset));
                dmu_objset_rele(zv->zv_objset, zv);

                error = zvol_setup_zv(zv);
        }

        mutex_exit(&zv->zv_state_lock);

        rw_exit(&zv->zv_suspend_lock);
        /*
         * We need this because we don't hold zvol_state_lock while releasing
         * zv_suspend_lock. zvol_remove_minors_impl thus cannot check
         * zv_suspend_lock to determine it is safe to free because rwlock is
         * not inherent atomic.
         */
        atomic_dec(&zv->zv_suspend_ref);

        return (SET_ERROR(error));
}

static int
zvol_first_open(zvol_state_t *zv, boolean_t readonly)
{
        objset_t *os;
        int error, locked = 0;
        boolean_t ro;

        ASSERT(RW_READ_HELD(&zv->zv_suspend_lock));
        ASSERT(MUTEX_HELD(&zv->zv_state_lock));

        /*
         * In all other cases the spa_namespace_lock is taken before the
         * bdev->bd_mutex lock.  But in this case the Linux __blkdev_get()
         * function calls fops->open() with the bdev->bd_mutex lock held.
         * This deadlock can be easily observed with zvols used as vdevs.
         *
         * To avoid a potential lock inversion deadlock we preemptively
         * try to take the spa_namespace_lock().  Normally it will not
         * be contended and this is safe because spa_open_common() handles
         * the case where the caller already holds the spa_namespace_lock.
         *
         * When it is contended we risk a lock inversion if we were to
         * block waiting for the lock.  Luckily, the __blkdev_get()
         * function allows us to return -ERESTARTSYS which will result in
         * bdev->bd_mutex being dropped, reacquired, and fops->open() being
         * called again.  This process can be repeated safely until both
         * locks are acquired.
         */
        if (!mutex_owned(&spa_namespace_lock)) {
                locked = mutex_tryenter(&spa_namespace_lock);
                if (!locked)
                        return (-SET_ERROR(ERESTARTSYS));
        }

        ro = (readonly || (strchr(zv->zv_name, '@') != NULL));
        error = dmu_objset_own(zv->zv_name, DMU_OST_ZVOL, ro, B_TRUE, zv, &os);
        if (error)
                goto out_mutex;

        zv->zv_objset = os;

        error = zvol_setup_zv(zv);

        if (error) {
                dmu_objset_disown(os, 1, zv);
                zv->zv_objset = NULL;
        }

out_mutex:
        if (locked)
                mutex_exit(&spa_namespace_lock);
        return (SET_ERROR(-error));
}

static void
zvol_last_close(zvol_state_t *zv)
{
        ASSERT(RW_READ_HELD(&zv->zv_suspend_lock));
        ASSERT(MUTEX_HELD(&zv->zv_state_lock));

        zvol_shutdown_zv(zv);

        dmu_objset_disown(zv->zv_objset, 1, zv);
        zv->zv_objset = NULL;
}

static int
zvol_open(struct block_device *bdev, fmode_t flag)
{
        zvol_state_t *zv;
        int error = 0;
        boolean_t drop_suspend = B_TRUE;

        rw_enter(&zvol_state_lock, RW_READER);
        /*
         * Obtain a copy of private_data under the zvol_state_lock to make
         * sure that either the result of zvol free code path setting
         * bdev->bd_disk->private_data to NULL is observed, or zvol_free()
         * is not called on this zv because of the positive zv_open_count.
         */
        zv = bdev->bd_disk->private_data;
        if (zv == NULL) {
                rw_exit(&zvol_state_lock);
                return (SET_ERROR(-ENXIO));
        }

        mutex_enter(&zv->zv_state_lock);
        /*
         * make sure zvol is not suspended during first open
         * (hold zv_suspend_lock) and respect proper lock acquisition
         * ordering - zv_suspend_lock before zv_state_lock
         */
        if (zv->zv_open_count == 0) {
                if (!rw_tryenter(&zv->zv_suspend_lock, RW_READER)) {
                        mutex_exit(&zv->zv_state_lock);
                        rw_enter(&zv->zv_suspend_lock, RW_READER);
                        mutex_enter(&zv->zv_state_lock);
                        /* check to see if zv_suspend_lock is needed */
                        if (zv->zv_open_count != 0) {
                                rw_exit(&zv->zv_suspend_lock);
                                drop_suspend = B_FALSE;
                        }
                }
        } else {
                drop_suspend = B_FALSE;
        }
        rw_exit(&zvol_state_lock);

        ASSERT(MUTEX_HELD(&zv->zv_state_lock));
        ASSERT(zv->zv_open_count != 0 || RW_READ_HELD(&zv->zv_suspend_lock));

        if (zv->zv_open_count == 0) {
                error = zvol_first_open(zv, !(flag & FMODE_WRITE));
                if (error)
                        goto out_mutex;
        }

        if ((flag & FMODE_WRITE) && (zv->zv_flags & ZVOL_RDONLY)) {
                error = -EROFS;
                goto out_open_count;
        }

        zv->zv_open_count++;

        mutex_exit(&zv->zv_state_lock);
        if (drop_suspend)
                rw_exit(&zv->zv_suspend_lock);

        check_disk_change(bdev);

        return (0);

out_open_count:
        if (zv->zv_open_count == 0)
                zvol_last_close(zv);

out_mutex:
        mutex_exit(&zv->zv_state_lock);
        if (drop_suspend)
                rw_exit(&zv->zv_suspend_lock);
        if (error == -ERESTARTSYS)
                schedule();

        return (SET_ERROR(error));
}

#ifdef HAVE_BLOCK_DEVICE_OPERATIONS_RELEASE_VOID
static void
#else
static int
#endif
zvol_release(struct gendisk *disk, fmode_t mode)
{
        zvol_state_t *zv;
        boolean_t drop_suspend = B_TRUE;

        rw_enter(&zvol_state_lock, RW_READER);
        zv = disk->private_data;

        mutex_enter(&zv->zv_state_lock);
        ASSERT(zv->zv_open_count > 0);
        /*
         * make sure zvol is not suspended during last close
         * (hold zv_suspend_lock) and respect proper lock acquisition
         * ordering - zv_suspend_lock before zv_state_lock
         */
        if (zv->zv_open_count == 1) {
                if (!rw_tryenter(&zv->zv_suspend_lock, RW_READER)) {
                        mutex_exit(&zv->zv_state_lock);
                        rw_enter(&zv->zv_suspend_lock, RW_READER);
                        mutex_enter(&zv->zv_state_lock);
                        /* check to see if zv_suspend_lock is needed */
                        if (zv->zv_open_count != 1) {
                                rw_exit(&zv->zv_suspend_lock);
                                drop_suspend = B_FALSE;
                        }
                }
        } else {
                drop_suspend = B_FALSE;
        }
        rw_exit(&zvol_state_lock);

        ASSERT(MUTEX_HELD(&zv->zv_state_lock));
        ASSERT(zv->zv_open_count != 1 || RW_READ_HELD(&zv->zv_suspend_lock));

        zv->zv_open_count--;
        if (zv->zv_open_count == 0)
                zvol_last_close(zv);

        mutex_exit(&zv->zv_state_lock);

        if (drop_suspend)
                rw_exit(&zv->zv_suspend_lock);

#ifndef HAVE_BLOCK_DEVICE_OPERATIONS_RELEASE_VOID
        return (0);
#endif
}

static int
zvol_ioctl(struct block_device *bdev, fmode_t mode,
    unsigned int cmd, unsigned long arg)
{
        zvol_state_t *zv = bdev->bd_disk->private_data;
        int error = 0;

        ASSERT3U(zv->zv_open_count, >, 0);

        switch (cmd) {
        case BLKFLSBUF:
                fsync_bdev(bdev);
                invalidate_bdev(bdev);
                rw_enter(&zv->zv_suspend_lock, RW_READER);

                if (!(zv->zv_flags & ZVOL_RDONLY))
                        txg_wait_synced(dmu_objset_pool(zv->zv_objset), 0);

                rw_exit(&zv->zv_suspend_lock);
                break;

        case BLKZNAME:
                mutex_enter(&zv->zv_state_lock);
                error = copy_to_user((void *)arg, zv->zv_name, MAXNAMELEN);
                mutex_exit(&zv->zv_state_lock);
                break;

        default:
                error = -ENOTTY;
                break;
        }

        return (SET_ERROR(error));
}

#ifdef CONFIG_COMPAT
static int
zvol_compat_ioctl(struct block_device *bdev, fmode_t mode,
    unsigned cmd, unsigned long arg)
{
        return (zvol_ioctl(bdev, mode, cmd, arg));
}
#else
#define zvol_compat_ioctl       NULL
#endif

/*
 * Linux 2.6.38 preferred interface.
 */
#ifdef HAVE_BLOCK_DEVICE_OPERATIONS_CHECK_EVENTS
static unsigned int
zvol_check_events(struct gendisk *disk, unsigned int clearing)
{
        unsigned int mask = 0;

        rw_enter(&zvol_state_lock, RW_READER);

        zvol_state_t *zv = disk->private_data;
        if (zv != NULL) {
                mutex_enter(&zv->zv_state_lock);
                mask = zv->zv_changed ? DISK_EVENT_MEDIA_CHANGE : 0;
                zv->zv_changed = 0;
                mutex_exit(&zv->zv_state_lock);
        }

        rw_exit(&zvol_state_lock);

        return (mask);
}
#else
static int zvol_media_changed(struct gendisk *disk)
{
        int changed = 0;

        rw_enter(&zvol_state_lock, RW_READER);

        zvol_state_t *zv = disk->private_data;
        if (zv != NULL) {
                mutex_enter(&zv->zv_state_lock);
                changed = zv->zv_changed;
                zv->zv_changed = 0;
                mutex_exit(&zv->zv_state_lock);
        }

        rw_exit(&zvol_state_lock);

        return (changed);
}
#endif

static int zvol_revalidate_disk(struct gendisk *disk)
{
        rw_enter(&zvol_state_lock, RW_READER);

        zvol_state_t *zv = disk->private_data;
        if (zv != NULL) {
                mutex_enter(&zv->zv_state_lock);
                set_capacity(zv->zv_disk, zv->zv_volsize >> SECTOR_BITS);
                mutex_exit(&zv->zv_state_lock);
        }

        rw_exit(&zvol_state_lock);

        return (0);
}

/*
 * Provide a simple virtual geometry for legacy compatibility.  For devices
 * smaller than 1 MiB a small head and sector count is used to allow very
 * tiny devices.  For devices over 1 Mib a standard head and sector count
 * is used to keep the cylinders count reasonable.
 */
static int
zvol_getgeo(struct block_device *bdev, struct hd_geometry *geo)
{
        zvol_state_t *zv = bdev->bd_disk->private_data;
        sector_t sectors;

        ASSERT3U(zv->zv_open_count, >, 0);

        sectors = get_capacity(zv->zv_disk);

        if (sectors > 2048) {
                geo->heads = 16;
                geo->sectors = 63;
        } else {
                geo->heads = 2;
                geo->sectors = 4;
        }

        geo->start = 0;
        geo->cylinders = sectors / (geo->heads * geo->sectors);

        return (0);
}

static struct kobject *
zvol_probe(dev_t dev, int *part, void *arg)
{
        zvol_state_t *zv;
        struct kobject *kobj;

        zv = zvol_find_by_dev(dev);
        kobj = zv ? get_disk_and_module(zv->zv_disk) : NULL;
        ASSERT(zv == NULL || MUTEX_HELD(&zv->zv_state_lock));
        if (zv)
                mutex_exit(&zv->zv_state_lock);

        return (kobj);
}

static struct block_device_operations zvol_ops = {
        .open                   = zvol_open,
        .release                = zvol_release,
        .ioctl                  = zvol_ioctl,
        .compat_ioctl           = zvol_compat_ioctl,
#ifdef HAVE_BLOCK_DEVICE_OPERATIONS_CHECK_EVENTS
        .check_events           = zvol_check_events,
#else
        .media_changed          = zvol_media_changed,
#endif
        .revalidate_disk        = zvol_revalidate_disk,
        .getgeo                 = zvol_getgeo,
        .owner                  = THIS_MODULE,
};

/*
 * Allocate memory for a new zvol_state_t and setup the required
 * request queue and generic disk structures for the block device.
 */
static zvol_state_t *
zvol_alloc(dev_t dev, const char *name)
{
        zvol_state_t *zv;
        uint64_t volmode;

        if (dsl_prop_get_integer(name, "volmode", &volmode, NULL) != 0)
                return (NULL);

        if (volmode == ZFS_VOLMODE_DEFAULT)
                volmode = zvol_volmode;

        if (volmode == ZFS_VOLMODE_NONE)
                return (NULL);

        zv = kmem_zalloc(sizeof (zvol_state_t), KM_SLEEP);

        list_link_init(&zv->zv_next);

        mutex_init(&zv->zv_state_lock, NULL, MUTEX_DEFAULT, NULL);

        zv->zv_queue = blk_alloc_queue(GFP_ATOMIC);
        if (zv->zv_queue == NULL)
                goto out_kmem;

        blk_queue_make_request(zv->zv_queue, zvol_request);
        blk_queue_set_write_cache(zv->zv_queue, B_TRUE, B_TRUE);

        /* Limit read-ahead to a single page to prevent over-prefetching. */
        blk_queue_set_read_ahead(zv->zv_queue, 1);

        /* Disable write merging in favor of the ZIO pipeline. */
        blk_queue_flag_set(QUEUE_FLAG_NOMERGES, zv->zv_queue);

        zv->zv_disk = alloc_disk(ZVOL_MINORS);
        if (zv->zv_disk == NULL)
                goto out_queue;

        zv->zv_queue->queuedata = zv;
        zv->zv_dev = dev;
        zv->zv_open_count = 0;
        strlcpy(zv->zv_name, name, MAXNAMELEN);

        rangelock_init(&zv->zv_rangelock, NULL, NULL);
        rw_init(&zv->zv_suspend_lock, NULL, RW_DEFAULT, NULL);

        zv->zv_disk->major = zvol_major;
#ifdef HAVE_BLOCK_DEVICE_OPERATIONS_CHECK_EVENTS
        zv->zv_disk->events = DISK_EVENT_MEDIA_CHANGE;
#endif

        if (volmode == ZFS_VOLMODE_DEV) {
                /*
                 * ZFS_VOLMODE_DEV disable partitioning on ZVOL devices: set
                 * gendisk->minors = 1 as noted in include/linux/genhd.h.
                 * Also disable extended partition numbers (GENHD_FL_EXT_DEVT)
                 * and suppresses partition scanning (GENHD_FL_NO_PART_SCAN)
                 * setting gendisk->flags accordingly.
                 */
                zv->zv_disk->minors = 1;
#if defined(GENHD_FL_EXT_DEVT)
                zv->zv_disk->flags &= ~GENHD_FL_EXT_DEVT;
#endif
#if defined(GENHD_FL_NO_PART_SCAN)
                zv->zv_disk->flags |= GENHD_FL_NO_PART_SCAN;
#endif
        }
        zv->zv_disk->first_minor = (dev & MINORMASK);
        zv->zv_disk->fops = &zvol_ops;
        zv->zv_disk->private_data = zv;
        zv->zv_disk->queue = zv->zv_queue;
        snprintf(zv->zv_disk->disk_name, DISK_NAME_LEN, "%s%d",
            ZVOL_DEV_NAME, (dev & MINORMASK));

        return (zv);

out_queue:
        blk_cleanup_queue(zv->zv_queue);
out_kmem:
        kmem_free(zv, sizeof (zvol_state_t));

        return (NULL);
}

/*
 * Cleanup then free a zvol_state_t which was created by zvol_alloc().
 * At this time, the structure is not opened by anyone, is taken off
 * the zvol_state_list, and has its private data set to NULL.
 * The zvol_state_lock is dropped.
 */
static void
zvol_free(void *arg)
{
        zvol_state_t *zv = arg;

        ASSERT(!RW_LOCK_HELD(&zv->zv_suspend_lock));
        ASSERT(!MUTEX_HELD(&zv->zv_state_lock));
        ASSERT(zv->zv_open_count == 0);
        ASSERT(zv->zv_disk->private_data == NULL);

        rw_destroy(&zv->zv_suspend_lock);
        rangelock_fini(&zv->zv_rangelock);

        del_gendisk(zv->zv_disk);
        blk_cleanup_queue(zv->zv_queue);
        put_disk(zv->zv_disk);

        ida_simple_remove(&zvol_ida, MINOR(zv->zv_dev) >> ZVOL_MINOR_BITS);

        mutex_destroy(&zv->zv_state_lock);
        dataset_kstats_destroy(&zv->zv_kstat);

        kmem_free(zv, sizeof (zvol_state_t));
}

/*
 * Create a block device minor node and setup the linkage between it
 * and the specified volume.  Once this function returns the block
 * device is live and ready for use.
 */
static int
zvol_create_minor_impl(const char *name)
{
        zvol_state_t *zv;
        objset_t *os;
        dmu_object_info_t *doi;
        uint64_t volsize;
        uint64_t len;
        unsigned minor = 0;
        int error = 0;
        int idx;
        uint64_t hash = zvol_name_hash(name);

        if (zvol_inhibit_dev)
                return (0);

        idx = ida_simple_get(&zvol_ida, 0, 0, kmem_flags_convert(KM_SLEEP));
        if (idx < 0)
                return (SET_ERROR(-idx));
        minor = idx << ZVOL_MINOR_BITS;

        zv = zvol_find_by_name_hash(name, hash, RW_NONE);
        if (zv) {
                ASSERT(MUTEX_HELD(&zv->zv_state_lock));
                mutex_exit(&zv->zv_state_lock);
                ida_simple_remove(&zvol_ida, idx);
                return (SET_ERROR(EEXIST));
        }

        doi = kmem_alloc(sizeof (dmu_object_info_t), KM_SLEEP);

        error = dmu_objset_own(name, DMU_OST_ZVOL, B_TRUE, B_TRUE, FTAG, &os);
        if (error)
                goto out_doi;

        error = dmu_object_info(os, ZVOL_OBJ, doi);
        if (error)
                goto out_dmu_objset_disown;

        error = zap_lookup(os, ZVOL_ZAP_OBJ, "size", 8, 1, &volsize);
        if (error)
                goto out_dmu_objset_disown;

        zv = zvol_alloc(MKDEV(zvol_major, minor), name);
        if (zv == NULL) {
                error = SET_ERROR(EAGAIN);
                goto out_dmu_objset_disown;
        }
        zv->zv_hash = hash;

        if (dmu_objset_is_snapshot(os))
                zv->zv_flags |= ZVOL_RDONLY;

        zv->zv_volblocksize = doi->doi_data_block_size;
        zv->zv_volsize = volsize;
        zv->zv_objset = os;

        set_capacity(zv->zv_disk, zv->zv_volsize >> 9);

        blk_queue_max_hw_sectors(zv->zv_queue, (DMU_MAX_ACCESS / 4) >> 9);
        blk_queue_max_segments(zv->zv_queue, UINT16_MAX);
        blk_queue_max_segment_size(zv->zv_queue, UINT_MAX);
        blk_queue_physical_block_size(zv->zv_queue, zv->zv_volblocksize);
        blk_queue_io_opt(zv->zv_queue, zv->zv_volblocksize);
        blk_queue_max_discard_sectors(zv->zv_queue,
            (zvol_max_discard_blocks * zv->zv_volblocksize) >> 9);
        blk_queue_discard_granularity(zv->zv_queue, zv->zv_volblocksize);
        blk_queue_flag_set(QUEUE_FLAG_DISCARD, zv->zv_queue);
#ifdef QUEUE_FLAG_NONROT
        blk_queue_flag_set(QUEUE_FLAG_NONROT, zv->zv_queue);
#endif
#ifdef QUEUE_FLAG_ADD_RANDOM
        blk_queue_flag_clear(QUEUE_FLAG_ADD_RANDOM, zv->zv_queue);
#endif

        if (spa_writeable(dmu_objset_spa(os))) {
                if (zil_replay_disable)
                        zil_destroy(dmu_objset_zil(os), B_FALSE);
                else
                        zil_replay(os, zv, zvol_replay_vector);
        }
        ASSERT3P(zv->zv_kstat.dk_kstats, ==, NULL);
        dataset_kstats_create(&zv->zv_kstat, zv->zv_objset);

        /*
         * When udev detects the addition of the device it will immediately
         * invoke blkid(8) to determine the type of content on the device.
         * Prefetching the blocks commonly scanned by blkid(8) will speed
         * up this process.
         */
        len = MIN(MAX(zvol_prefetch_bytes, 0), SPA_MAXBLOCKSIZE);
        if (len > 0) {
                dmu_prefetch(os, ZVOL_OBJ, 0, 0, len, ZIO_PRIORITY_SYNC_READ);
                dmu_prefetch(os, ZVOL_OBJ, 0, volsize - len, len,
                    ZIO_PRIORITY_SYNC_READ);
        }

        zv->zv_objset = NULL;
out_dmu_objset_disown:
        dmu_objset_disown(os, B_TRUE, FTAG);
out_doi:
        kmem_free(doi, sizeof (dmu_object_info_t));

        if (error == 0) {
                rw_enter(&zvol_state_lock, RW_WRITER);
                zvol_insert(zv);
                rw_exit(&zvol_state_lock);
                add_disk(zv->zv_disk);
        } else {
                ida_simple_remove(&zvol_ida, idx);
        }

        return (SET_ERROR(error));
}

/*
 * Rename a block device minor mode for the specified volume.
 */
static void
zvol_rename_minor(zvol_state_t *zv, const char *newname)
{
        int readonly = get_disk_ro(zv->zv_disk);

        ASSERT(RW_LOCK_HELD(&zvol_state_lock));
        ASSERT(MUTEX_HELD(&zv->zv_state_lock));

        strlcpy(zv->zv_name, newname, sizeof (zv->zv_name));

        /* move to new hashtable entry  */
        zv->zv_hash = zvol_name_hash(zv->zv_name);
        hlist_del(&zv->zv_hlink);
        hlist_add_head(&zv->zv_hlink, ZVOL_HT_HEAD(zv->zv_hash));

        /*
         * The block device's read-only state is briefly changed causing
         * a KOBJ_CHANGE uevent to be issued.  This ensures udev detects
         * the name change and fixes the symlinks.  This does not change
         * ZVOL_RDONLY in zv->zv_flags so the actual read-only state never
         * changes.  This would normally be done using kobject_uevent() but
         * that is a GPL-only symbol which is why we need this workaround.
         */
        set_disk_ro(zv->zv_disk, !readonly);
        set_disk_ro(zv->zv_disk, readonly);
}

typedef struct minors_job {
        list_t *list;
        list_node_t link;
        /* input */
        char *name;
        /* output */
        int error;
} minors_job_t;

/*
 * Prefetch zvol dnodes for the minors_job
 */
static void
zvol_prefetch_minors_impl(void *arg)
{
        minors_job_t *job = arg;
        char *dsname = job->name;
        objset_t *os = NULL;

        job->error = dmu_objset_own(dsname, DMU_OST_ZVOL, B_TRUE, B_TRUE,
            FTAG, &os);
        if (job->error == 0) {
                dmu_prefetch(os, ZVOL_OBJ, 0, 0, 0, ZIO_PRIORITY_SYNC_READ);
                dmu_objset_disown(os, B_TRUE, FTAG);
        }
}

/*
 * Mask errors to continue dmu_objset_find() traversal
 */
static int
zvol_create_snap_minor_cb(const char *dsname, void *arg)
{
        minors_job_t *j = arg;
        list_t *minors_list = j->list;
        const char *name = j->name;

        ASSERT0(MUTEX_HELD(&spa_namespace_lock));

        /* skip the designated dataset */
        if (name && strcmp(dsname, name) == 0)
                return (0);

        /* at this point, the dsname should name a snapshot */
        if (strchr(dsname, '@') == 0) {
                dprintf("zvol_create_snap_minor_cb(): "
                    "%s is not a shapshot name\n", dsname);
        } else {
                minors_job_t *job;
                char *n = strdup(dsname);
                if (n == NULL)
                        return (0);

                job = kmem_alloc(sizeof (minors_job_t), KM_SLEEP);
                job->name = n;
                job->list = minors_list;
                job->error = 0;
                list_insert_tail(minors_list, job);
                /* don't care if dispatch fails, because job->error is 0 */
                taskq_dispatch(system_taskq, zvol_prefetch_minors_impl, job,
                    TQ_SLEEP);
        }

        return (0);
}

/*
 * Mask errors to continue dmu_objset_find() traversal
 */
static int
zvol_create_minors_cb(const char *dsname, void *arg)
{
        uint64_t snapdev;
        int error;
        list_t *minors_list = arg;

        ASSERT0(MUTEX_HELD(&spa_namespace_lock));

        error = dsl_prop_get_integer(dsname, "snapdev", &snapdev, NULL);
        if (error)
                return (0);

        /*
         * Given the name and the 'snapdev' property, create device minor nodes
         * with the linkages to zvols/snapshots as needed.
         * If the name represents a zvol, create a minor node for the zvol, then
         * check if its snapshots are 'visible', and if so, iterate over the
         * snapshots and create device minor nodes for those.
         */
        if (strchr(dsname, '@') == 0) {
                minors_job_t *job;
                char *n = strdup(dsname);
                if (n == NULL)
                        return (0);

                job = kmem_alloc(sizeof (minors_job_t), KM_SLEEP);
                job->name = n;
                job->list = minors_list;
                job->error = 0;
                list_insert_tail(minors_list, job);
                /* don't care if dispatch fails, because job->error is 0 */
                taskq_dispatch(system_taskq, zvol_prefetch_minors_impl, job,
                    TQ_SLEEP);

                if (snapdev == ZFS_SNAPDEV_VISIBLE) {
                        /*
                         * traverse snapshots only, do not traverse children,
                         * and skip the 'dsname'
                         */
                        error = dmu_objset_find((char *)dsname,
                            zvol_create_snap_minor_cb, (void *)job,
                            DS_FIND_SNAPSHOTS);
                }
        } else {
                dprintf("zvol_create_minors_cb(): %s is not a zvol name\n",
                    dsname);
        }

        return (0);
}

/*
 * Create minors for the specified dataset, including children and snapshots.
 * Pay attention to the 'snapdev' property and iterate over the snapshots
 * only if they are 'visible'. This approach allows one to assure that the
 * snapshot metadata is read from disk only if it is needed.
 *
 * The name can represent a dataset to be recursively scanned for zvols and
 * their snapshots, or a single zvol snapshot. If the name represents a
 * dataset, the scan is performed in two nested stages:
 * - scan the dataset for zvols, and
 * - for each zvol, create a minor node, then check if the zvol's snapshots
 *   are 'visible', and only then iterate over the snapshots if needed
 *
 * If the name represents a snapshot, a check is performed if the snapshot is
 * 'visible' (which also verifies that the parent is a zvol), and if so,
 * a minor node for that snapshot is created.
 */
static int
zvol_create_minors_impl(const char *name)
{
        int error = 0;
        fstrans_cookie_t cookie;
        char *atp, *parent;
        list_t minors_list;
        minors_job_t *job;

        if (zvol_inhibit_dev)
                return (0);

        /*
         * This is the list for prefetch jobs. Whenever we found a match
         * during dmu_objset_find, we insert a minors_job to the list and do
         * taskq_dispatch to parallel prefetch zvol dnodes. Note we don't need
         * any lock because all list operation is done on the current thread.
         *
         * We will use this list to do zvol_create_minor_impl after prefetch
         * so we don't have to traverse using dmu_objset_find again.
         */
        list_create(&minors_list, sizeof (minors_job_t),
            offsetof(minors_job_t, link));

        parent = kmem_alloc(MAXPATHLEN, KM_SLEEP);
        (void) strlcpy(parent, name, MAXPATHLEN);

        if ((atp = strrchr(parent, '@')) != NULL) {
                uint64_t snapdev;

                *atp = '\0';
                error = dsl_prop_get_integer(parent, "snapdev",
                    &snapdev, NULL);

                if (error == 0 && snapdev == ZFS_SNAPDEV_VISIBLE)
                        error = zvol_create_minor_impl(name);
        } else {
                cookie = spl_fstrans_mark();
                error = dmu_objset_find(parent, zvol_create_minors_cb,
                    &minors_list, DS_FIND_CHILDREN);
                spl_fstrans_unmark(cookie);
        }

        kmem_free(parent, MAXPATHLEN);
        taskq_wait_outstanding(system_taskq, 0);

        /*
         * Prefetch is completed, we can do zvol_create_minor_impl
         * sequentially.
         */
        while ((job = list_head(&minors_list)) != NULL) {
                list_remove(&minors_list, job);
                if (!job->error)
                        zvol_create_minor_impl(job->name);
                strfree(job->name);
                kmem_free(job, sizeof (minors_job_t));
        }

        list_destroy(&minors_list);

        return (SET_ERROR(error));
}

/*
 * Remove minors for specified dataset including children and snapshots.
 */
static void
zvol_remove_minors_impl(const char *name)
{
        zvol_state_t *zv, *zv_next;
        int namelen = ((name) ? strlen(name) : 0);
        taskqid_t t, tid = TASKQID_INVALID;
        list_t free_list;

        if (zvol_inhibit_dev)
                return;

        list_create(&free_list, sizeof (zvol_state_t),
            offsetof(zvol_state_t, zv_next));

        rw_enter(&zvol_state_lock, RW_WRITER);

        for (zv = list_head(&zvol_state_list); zv != NULL; zv = zv_next) {
                zv_next = list_next(&zvol_state_list, zv);

                mutex_enter(&zv->zv_state_lock);
                if (name == NULL || strcmp(zv->zv_name, name) == 0 ||
                    (strncmp(zv->zv_name, name, namelen) == 0 &&
                    (zv->zv_name[namelen] == '/' ||
                    zv->zv_name[namelen] == '@'))) {
                        /*
                         * By holding zv_state_lock here, we guarantee that no
                         * one is currently using this zv
                         */

                        /* If in use, leave alone */
                        if (zv->zv_open_count > 0 ||
                            atomic_read(&zv->zv_suspend_ref)) {
                                mutex_exit(&zv->zv_state_lock);
                                continue;
                        }

                        zvol_remove(zv);

                        /*
                         * Cleared while holding zvol_state_lock as a writer
                         * which will prevent zvol_open() from opening it.
                         */
                        zv->zv_disk->private_data = NULL;

                        /* Drop zv_state_lock before zvol_free() */
                        mutex_exit(&zv->zv_state_lock);

                        /* Try parallel zv_free, if failed do it in place */
                        t = taskq_dispatch(system_taskq, zvol_free, zv,
                            TQ_SLEEP);
                        if (t == TASKQID_INVALID)
                                list_insert_head(&free_list, zv);
                        else
                                tid = t;
                } else {
                        mutex_exit(&zv->zv_state_lock);
                }
        }
        rw_exit(&zvol_state_lock);

        /* Drop zvol_state_lock before calling zvol_free() */
        while ((zv = list_head(&free_list)) != NULL) {
                list_remove(&free_list, zv);
                zvol_free(zv);
        }

        if (tid != TASKQID_INVALID)
                taskq_wait_outstanding(system_taskq, tid);
}

/* Remove minor for this specific volume only */
static void
zvol_remove_minor_impl(const char *name)
{
        zvol_state_t *zv = NULL, *zv_next;

        if (zvol_inhibit_dev)
                return;

        rw_enter(&zvol_state_lock, RW_WRITER);

        for (zv = list_head(&zvol_state_list); zv != NULL; zv = zv_next) {
                zv_next = list_next(&zvol_state_list, zv);

                mutex_enter(&zv->zv_state_lock);
                if (strcmp(zv->zv_name, name) == 0) {
                        /*
                         * By holding zv_state_lock here, we guarantee that no
                         * one is currently using this zv
                         */

                        /* If in use, leave alone */
                        if (zv->zv_open_count > 0 ||
                            atomic_read(&zv->zv_suspend_ref)) {
                                mutex_exit(&zv->zv_state_lock);
                                continue;
                        }
                        zvol_remove(zv);

                        /*
                         * Cleared while holding zvol_state_lock as a writer
                         * which will prevent zvol_open() from opening it.
                         */
                        zv->zv_disk->private_data = NULL;

                        mutex_exit(&zv->zv_state_lock);
                        break;
                } else {
                        mutex_exit(&zv->zv_state_lock);
                }
        }

        /* Drop zvol_state_lock before calling zvol_free() */
        rw_exit(&zvol_state_lock);

        if (zv != NULL)
                zvol_free(zv);
}

/*
 * Rename minors for specified dataset including children and snapshots.
 */
static void
zvol_rename_minors_impl(const char *oldname, const char *newname)
{
        zvol_state_t *zv, *zv_next;
        int oldnamelen, newnamelen;

        if (zvol_inhibit_dev)
                return;

        oldnamelen = strlen(oldname);
        newnamelen = strlen(newname);

        rw_enter(&zvol_state_lock, RW_READER);

        for (zv = list_head(&zvol_state_list); zv != NULL; zv = zv_next) {
                zv_next = list_next(&zvol_state_list, zv);

                mutex_enter(&zv->zv_state_lock);

                /* If in use, leave alone */
                if (zv->zv_open_count > 0) {
                        mutex_exit(&zv->zv_state_lock);
                        continue;
                }

                if (strcmp(zv->zv_name, oldname) == 0) {
                        zvol_rename_minor(zv, newname);
                } else if (strncmp(zv->zv_name, oldname, oldnamelen) == 0 &&
                    (zv->zv_name[oldnamelen] == '/' ||
                    zv->zv_name[oldnamelen] == '@')) {
                        char *name = kmem_asprintf("%s%c%s", newname,
                            zv->zv_name[oldnamelen],
                            zv->zv_name + oldnamelen + 1);
                        zvol_rename_minor(zv, name);
                        strfree(name);
                }

                mutex_exit(&zv->zv_state_lock);
        }

        rw_exit(&zvol_state_lock);
}

typedef struct zvol_snapdev_cb_arg {
        uint64_t snapdev;
} zvol_snapdev_cb_arg_t;

static int
zvol_set_snapdev_cb(const char *dsname, void *param)
{
        zvol_snapdev_cb_arg_t *arg = param;

        if (strchr(dsname, '@') == NULL)
                return (0);

        switch (arg->snapdev) {
                case ZFS_SNAPDEV_VISIBLE:
                        (void) zvol_create_minor_impl(dsname);
                        break;
                case ZFS_SNAPDEV_HIDDEN:
                        (void) zvol_remove_minor_impl(dsname);
                        break;
        }

        return (0);
}

static void
zvol_set_snapdev_impl(char *name, uint64_t snapdev)
{
        zvol_snapdev_cb_arg_t arg = {snapdev};
        fstrans_cookie_t cookie = spl_fstrans_mark();
        /*
         * The zvol_set_snapdev_sync() sets snapdev appropriately
         * in the dataset hierarchy. Here, we only scan snapshots.
         */
        dmu_objset_find(name, zvol_set_snapdev_cb, &arg, DS_FIND_SNAPSHOTS);
        spl_fstrans_unmark(cookie);
}

typedef struct zvol_volmode_cb_arg {
        uint64_t volmode;
} zvol_volmode_cb_arg_t;

static void
zvol_set_volmode_impl(char *name, uint64_t volmode)
{
        fstrans_cookie_t cookie = spl_fstrans_mark();

        if (strchr(name, '@') != NULL)
                return;

        /*
         * It's unfortunate we need to remove minors before we create new ones:
         * this is necessary because our backing gendisk (zvol_state->zv_disk)
         * coule be different when we set, for instance, volmode from "geom"
         * to "dev" (or vice versa).
         * A possible optimization is to modify our consumers so we don't get
         * called when "volmode" does not change.
         */
        switch (volmode) {
                case ZFS_VOLMODE_NONE:
                        (void) zvol_remove_minor_impl(name);
                        break;
                case ZFS_VOLMODE_GEOM:
                case ZFS_VOLMODE_DEV:
                        (void) zvol_remove_minor_impl(name);
                        (void) zvol_create_minor_impl(name);
                        break;
                case ZFS_VOLMODE_DEFAULT:
                        (void) zvol_remove_minor_impl(name);
                        if (zvol_volmode == ZFS_VOLMODE_NONE)
                                break;
                        else /* if zvol_volmode is invalid defaults to "geom" */
                                (void) zvol_create_minor_impl(name);
                        break;
        }

        spl_fstrans_unmark(cookie);
}

static zvol_task_t *
zvol_task_alloc(zvol_async_op_t op, const char *name1, const char *name2,
    uint64_t value)
{
        zvol_task_t *task;
        char *delim;

        /* Never allow tasks on hidden names. */
        if (name1[0] == '$')
                return (NULL);

        task = kmem_zalloc(sizeof (zvol_task_t), KM_SLEEP);
        task->op = op;
        task->value = value;
        delim = strchr(name1, '/');
        strlcpy(task->pool, name1, delim ? (delim - name1 + 1) : MAXNAMELEN);

        strlcpy(task->name1, name1, MAXNAMELEN);
        if (name2 != NULL)
                strlcpy(task->name2, name2, MAXNAMELEN);

        return (task);
}

static void
zvol_task_free(zvol_task_t *task)
{
        kmem_free(task, sizeof (zvol_task_t));
}

/*
 * The worker thread function performed asynchronously.
 */
static void
zvol_task_cb(void *param)
{
        zvol_task_t *task = (zvol_task_t *)param;

        switch (task->op) {
        case ZVOL_ASYNC_CREATE_MINORS:
                (void) zvol_create_minors_impl(task->name1);
                break;
        case ZVOL_ASYNC_REMOVE_MINORS:
                zvol_remove_minors_impl(task->name1);
                break;
        case ZVOL_ASYNC_RENAME_MINORS:
                zvol_rename_minors_impl(task->name1, task->name2);
                break;
        case ZVOL_ASYNC_SET_SNAPDEV:
                zvol_set_snapdev_impl(task->name1, task->value);
                break;
        case ZVOL_ASYNC_SET_VOLMODE:
                zvol_set_volmode_impl(task->name1, task->value);
                break;
        default:
                VERIFY(0);
                break;
        }

        zvol_task_free(task);
}

typedef struct zvol_set_prop_int_arg {
        const char *zsda_name;
        uint64_t zsda_value;
        zprop_source_t zsda_source;
        dmu_tx_t *zsda_tx;
} zvol_set_prop_int_arg_t;

/*
 * Sanity check the dataset for safe use by the sync task.  No additional
 * conditions are imposed.
 */
static int
zvol_set_snapdev_check(void *arg, dmu_tx_t *tx)
{
        zvol_set_prop_int_arg_t *zsda = arg;
        dsl_pool_t *dp = dmu_tx_pool(tx);
        dsl_dir_t *dd;
        int error;

        error = dsl_dir_hold(dp, zsda->zsda_name, FTAG, &dd, NULL);
        if (error != 0)
                return (error);

        dsl_dir_rele(dd, FTAG);

        return (error);
}

/* ARGSUSED */
static int
zvol_set_snapdev_sync_cb(dsl_pool_t *dp, dsl_dataset_t *ds, void *arg)
{
        char dsname[MAXNAMELEN];
        zvol_task_t *task;
        uint64_t snapdev;

        dsl_dataset_name(ds, dsname);
        if (dsl_prop_get_int_ds(ds, "snapdev", &snapdev) != 0)
                return (0);
        task = zvol_task_alloc(ZVOL_ASYNC_SET_SNAPDEV, dsname, NULL, snapdev);
        if (task == NULL)
                return (0);

        (void) taskq_dispatch(dp->dp_spa->spa_zvol_taskq, zvol_task_cb,
            task, TQ_SLEEP);
        return (0);
}

/*
 * Traverse all child datasets and apply snapdev appropriately.
 * We call dsl_prop_set_sync_impl() here to set the value only on the toplevel
 * dataset and read the effective "snapdev" on every child in the callback
 * function: this is because the value is not guaranteed to be the same in the
 * whole dataset hierarchy.
 */
static void
zvol_set_snapdev_sync(void *arg, dmu_tx_t *tx)
{
        zvol_set_prop_int_arg_t *zsda = arg;
        dsl_pool_t *dp = dmu_tx_pool(tx);
        dsl_dir_t *dd;
        dsl_dataset_t *ds;
        int error;

        VERIFY0(dsl_dir_hold(dp, zsda->zsda_name, FTAG, &dd, NULL));
        zsda->zsda_tx = tx;

        error = dsl_dataset_hold(dp, zsda->zsda_name, FTAG, &ds);
        if (error == 0) {
                dsl_prop_set_sync_impl(ds, zfs_prop_to_name(ZFS_PROP_SNAPDEV),
                    zsda->zsda_source, sizeof (zsda->zsda_value), 1,
                    &zsda->zsda_value, zsda->zsda_tx);
                dsl_dataset_rele(ds, FTAG);
        }
        dmu_objset_find_dp(dp, dd->dd_object, zvol_set_snapdev_sync_cb,
            zsda, DS_FIND_CHILDREN);

        dsl_dir_rele(dd, FTAG);
}

int
zvol_set_snapdev(const char *ddname, zprop_source_t source, uint64_t snapdev)
{
        zvol_set_prop_int_arg_t zsda;

        zsda.zsda_name = ddname;
        zsda.zsda_source = source;
        zsda.zsda_value = snapdev;

        return (dsl_sync_task(ddname, zvol_set_snapdev_check,
            zvol_set_snapdev_sync, &zsda, 0, ZFS_SPACE_CHECK_NONE));
}

/*
 * Sanity check the dataset for safe use by the sync task.  No additional
 * conditions are imposed.
 */
static int
zvol_set_volmode_check(void *arg, dmu_tx_t *tx)
{
        zvol_set_prop_int_arg_t *zsda = arg;
        dsl_pool_t *dp = dmu_tx_pool(tx);
        dsl_dir_t *dd;
        int error;

        error = dsl_dir_hold(dp, zsda->zsda_name, FTAG, &dd, NULL);
        if (error != 0)
                return (error);

        dsl_dir_rele(dd, FTAG);

        return (error);
}

/* ARGSUSED */
static int
zvol_set_volmode_sync_cb(dsl_pool_t *dp, dsl_dataset_t *ds, void *arg)
{
        char dsname[MAXNAMELEN];
        zvol_task_t *task;
        uint64_t volmode;

        dsl_dataset_name(ds, dsname);
        if (dsl_prop_get_int_ds(ds, "volmode", &volmode) != 0)
                return (0);
        task = zvol_task_alloc(ZVOL_ASYNC_SET_VOLMODE, dsname, NULL, volmode);
        if (task == NULL)
                return (0);

        (void) taskq_dispatch(dp->dp_spa->spa_zvol_taskq, zvol_task_cb,
            task, TQ_SLEEP);
        return (0);
}

/*
 * Traverse all child datasets and apply volmode appropriately.
 * We call dsl_prop_set_sync_impl() here to set the value only on the toplevel
 * dataset and read the effective "volmode" on every child in the callback
 * function: this is because the value is not guaranteed to be the same in the
 * whole dataset hierarchy.
 */
static void
zvol_set_volmode_sync(void *arg, dmu_tx_t *tx)
{
        zvol_set_prop_int_arg_t *zsda = arg;
        dsl_pool_t *dp = dmu_tx_pool(tx);
        dsl_dir_t *dd;
        dsl_dataset_t *ds;
        int error;

        VERIFY0(dsl_dir_hold(dp, zsda->zsda_name, FTAG, &dd, NULL));
        zsda->zsda_tx = tx;

        error = dsl_dataset_hold(dp, zsda->zsda_name, FTAG, &ds);
        if (error == 0) {
                dsl_prop_set_sync_impl(ds, zfs_prop_to_name(ZFS_PROP_VOLMODE),
                    zsda->zsda_source, sizeof (zsda->zsda_value), 1,
                    &zsda->zsda_value, zsda->zsda_tx);
                dsl_dataset_rele(ds, FTAG);
        }

        dmu_objset_find_dp(dp, dd->dd_object, zvol_set_volmode_sync_cb,
            zsda, DS_FIND_CHILDREN);

        dsl_dir_rele(dd, FTAG);
}

int
zvol_set_volmode(const char *ddname, zprop_source_t source, uint64_t volmode)
{
        zvol_set_prop_int_arg_t zsda;

        zsda.zsda_name = ddname;
        zsda.zsda_source = source;
        zsda.zsda_value = volmode;

        return (dsl_sync_task(ddname, zvol_set_volmode_check,
            zvol_set_volmode_sync, &zsda, 0, ZFS_SPACE_CHECK_NONE));
}

void
zvol_create_minors(spa_t *spa, const char *name, boolean_t async)
{
        zvol_task_t *task;
        taskqid_t id;

        task = zvol_task_alloc(ZVOL_ASYNC_CREATE_MINORS, name, NULL, ~0ULL);
        if (task == NULL)
                return;

        id = taskq_dispatch(spa->spa_zvol_taskq, zvol_task_cb, task, TQ_SLEEP);
        if ((async == B_FALSE) && (id != TASKQID_INVALID))
                taskq_wait_id(spa->spa_zvol_taskq, id);
}

void
zvol_remove_minors(spa_t *spa, const char *name, boolean_t async)
{
        zvol_task_t *task;
        taskqid_t id;

        task = zvol_task_alloc(ZVOL_ASYNC_REMOVE_MINORS, name, NULL, ~0ULL);
        if (task == NULL)
                return;

        id = taskq_dispatch(spa->spa_zvol_taskq, zvol_task_cb, task, TQ_SLEEP);
        if ((async == B_FALSE) && (id != TASKQID_INVALID))
                taskq_wait_id(spa->spa_zvol_taskq, id);
}

void
zvol_rename_minors(spa_t *spa, const char *name1, const char *name2,
    boolean_t async)
{
        zvol_task_t *task;
        taskqid_t id;

        task = zvol_task_alloc(ZVOL_ASYNC_RENAME_MINORS, name1, name2, ~0ULL);
        if (task == NULL)
                return;

        id = taskq_dispatch(spa->spa_zvol_taskq, zvol_task_cb, task, TQ_SLEEP);
        if ((async == B_FALSE) && (id != TASKQID_INVALID))
                taskq_wait_id(spa->spa_zvol_taskq, id);
}

int
zvol_init(void)
{
        int threads = MIN(MAX(zvol_threads, 1), 1024);
        int i, error;

        list_create(&zvol_state_list, sizeof (zvol_state_t),
            offsetof(zvol_state_t, zv_next));
        rw_init(&zvol_state_lock, NULL, RW_DEFAULT, NULL);
        ida_init(&zvol_ida);

        zvol_taskq = taskq_create(ZVOL_DRIVER, threads, maxclsyspri,
            threads * 2, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
        if (zvol_taskq == NULL) {
                printk(KERN_INFO "ZFS: taskq_create() failed\n");
                error = -ENOMEM;
                goto out;
        }

        zvol_htable = kmem_alloc(ZVOL_HT_SIZE * sizeof (struct hlist_head),
            KM_SLEEP);
        if (!zvol_htable) {
                error = -ENOMEM;
                goto out_taskq;
        }
        for (i = 0; i < ZVOL_HT_SIZE; i++)
                INIT_HLIST_HEAD(&zvol_htable[i]);

        error = register_blkdev(zvol_major, ZVOL_DRIVER);
        if (error) {
                printk(KERN_INFO "ZFS: register_blkdev() failed %d\n", error);
                goto out_free;
        }

        blk_register_region(MKDEV(zvol_major, 0), 1UL << MINORBITS,
            THIS_MODULE, zvol_probe, NULL, NULL);

        return (0);

out_free:
        kmem_free(zvol_htable, ZVOL_HT_SIZE * sizeof (struct hlist_head));
out_taskq:
        taskq_destroy(zvol_taskq);
out:
        ida_destroy(&zvol_ida);
        rw_destroy(&zvol_state_lock);
        list_destroy(&zvol_state_list);

        return (SET_ERROR(error));
}

void
zvol_fini(void)
{
        zvol_remove_minors_impl(NULL);

        blk_unregister_region(MKDEV(zvol_major, 0), 1UL << MINORBITS);
        unregister_blkdev(zvol_major, ZVOL_DRIVER);
        kmem_free(zvol_htable, ZVOL_HT_SIZE * sizeof (struct hlist_head));

        taskq_destroy(zvol_taskq);
        list_destroy(&zvol_state_list);
        rw_destroy(&zvol_state_lock);

        ida_destroy(&zvol_ida);
}

/* BEGIN CSTYLED */
module_param(zvol_inhibit_dev, uint, 0644);
MODULE_PARM_DESC(zvol_inhibit_dev, "Do not create zvol device nodes");

module_param(zvol_major, uint, 0444);
MODULE_PARM_DESC(zvol_major, "Major number for zvol device");

module_param(zvol_threads, uint, 0444);
MODULE_PARM_DESC(zvol_threads, "Max number of threads to handle I/O requests");

module_param(zvol_request_sync, uint, 0644);
MODULE_PARM_DESC(zvol_request_sync, "Synchronously handle bio requests");

module_param(zvol_max_discard_blocks, ulong, 0444);
MODULE_PARM_DESC(zvol_max_discard_blocks, "Max number of blocks to discard");

module_param(zvol_prefetch_bytes, uint, 0644);
MODULE_PARM_DESC(zvol_prefetch_bytes, "Prefetch N bytes at zvol start+end");

module_param(zvol_volmode, uint, 0644);
MODULE_PARM_DESC(zvol_volmode, "Default volmode property value");
/* END CSTYLED */