CI: Add FreeBSD 14.2 RELEASE+STABLE builds

[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 https://opensource.org/licenses/CDDL-1.0.
 * 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, 2019 by Delphix. All rights reserved.
 * Copyright (c) 2024, Klara, Inc.
 */

/*
 * 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.
 *
 */

#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 <sys/zvol_impl.h>

unsigned int zvol_inhibit_dev = 0;
unsigned int zvol_volmode = ZFS_VOLMODE_GEOM;

struct hlist_head *zvol_htable;
static list_t zvol_state_list;
krwlock_t zvol_state_lock;

typedef enum {
        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 name1[MAXNAMELEN];
        char name2[MAXNAMELEN];
        uint64_t value;
} zvol_task_t;

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

/*
 * 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.
 */
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 && strcmp(zv->zv_name, name) == 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 &&
                                    strcmp(zv->zv_name, name) == 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));
}

/*
 * 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;
        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;
        }
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 (error == 0 && zv != NULL)
                zvol_os_update_volsize(zv, volsize);

        return (SET_ERROR(error));
}

/*
 * Update volthreading.
 */
int
zvol_set_volthreading(const char *name, boolean_t value)
{
        zvol_state_t *zv = zvol_find_by_name(name, RW_NONE);
        if (zv == NULL)
                return (ENOENT);
        zv->zv_threading = value;
        mutex_exit(&zv->zv_state_lock);
        return (0);
}

/*
 * Update zvol ro property.
 */
int
zvol_set_ro(const char *name, boolean_t value)
{
        zvol_state_t *zv = zvol_find_by_name(name, RW_NONE);
        if (zv == NULL)
                return (-1);
        if (value) {
                zvol_os_set_disk_ro(zv, 1);
                zv->zv_flags |= ZVOL_RDONLY;
        } else {
                zvol_os_set_disk_ro(zv, 0);
                zv->zv_flags &= ~ZVOL_RDONLY;
        }
        mutex_exit(&zv->zv_state_lock);
        return (0);
}

/*
 * 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);
}

/*
 * 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;

        ASSERT3U(lr->lr_common.lrc_reclen, >=, sizeof (*lr));

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

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

        dmu_tx_t *tx = dmu_tx_create(zv->zv_objset);
        dmu_tx_mark_netfree(tx);
        int error = dmu_tx_assign(tx, TXG_WAIT);
        if (error != 0) {
                dmu_tx_abort(tx);
        } else {
                (void) zil_replaying(zv->zv_zilog, tx);
                dmu_tx_commit(tx);
                error = dmu_free_long_range(zv->zv_objset, ZVOL_OBJ, offset,
                    length);
        }

        return (error);
}

/*
 * 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;

        ASSERT3U(lr->lr_common.lrc_reclen, >=, sizeof (*lr));

        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);
                (void) zil_replaying(zv->zv_zilog, tx);
                dmu_tx_commit(tx);
        }

        return (error);
}

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

/*
 * Callback vectors for replaying records.
 * Only TX_WRITE and TX_TRUNCATE are needed for zvol.
 */
zil_replay_func_t *const 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_replay_err,        /* TX_SETSAXATTR */
        zvol_replay_err,        /* TX_RENAME_EXCHANGE */
        zvol_replay_err,        /* TX_RENAME_WHITEOUT */
        zvol_replay_err,        /* TX_CLONE_RANGE */
};

/*
 * 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().
 */
static const ssize_t zvol_immediate_write_sz = 32768;

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

        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 (commit)
                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(zilog))
                        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;

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

                offset += len;
                size -= len;
        }

        if (write_state == WR_COPIED || write_state == WR_NEED_COPY) {
                dsl_pool_wrlog_count(zilog->zl_dmu_pool, sz, tx->tx_txg);
        }
}

/*
 * Log a DKIOCFREE/free-long-range to the ZIL with TX_TRUNCATE.
 */
void
zvol_log_truncate(zvol_state_t *zv, dmu_tx_t *tx, uint64_t off, uint64_t len)
{
        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;

        zil_itx_assign(zilog, itx, tx);
}


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

        zfs_rangelock_exit(zgd->zgd_lr);

        kmem_free(zgd, sizeof (zgd_t));
}

/*
 * Get data to generate a TX_WRITE intent log record.
 */
int
zvol_get_data(void *arg, uint64_t arg2, 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);
        ASSERT3U(size, !=, 0);

        zgd = 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 = zfs_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 */
                ASSERT3P(zio, !=, NULL);
                /*
                 * 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 = zfs_rangelock_enter(&zv->zv_rangelock, offset,
                    size, RL_READER);
                error = dmu_buf_hold_noread_by_dnode(zv->zv_dn, offset, zgd,
                    &db);
                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.
 */

void
zvol_insert(zvol_state_t *zv)
{
        ASSERT(RW_WRITE_HELD(&zvol_state_lock));
        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));

        zv->zv_zilog = NULL;
        zv->zv_flags &= ~ZVOL_WRITTEN_TO;

        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, zv, &zv->zv_dn);
        if (error)
                return (SET_ERROR(error));

        zvol_os_set_capacity(zv, volsize >> 9);
        zv->zv_volsize = volsize;

        if (ro || dmu_objset_is_snapshot(os) ||
            !spa_writeable(dmu_objset_spa(os))) {
                zvol_os_set_disk_ro(zv, 1);
                zv->zv_flags |= ZVOL_RDONLY;
        } else {
                zvol_os_set_disk_ro(zv, 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));

        if (zv->zv_flags & ZVOL_WRITTEN_TO) {
                ASSERT(zv->zv_zilog != NULL);
                zil_close(zv->zv_zilog);
        }

        zv->zv_zilog = NULL;

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

        /*
         * Evict cached data. We must write out any dirty data before
         * disowning the dataset.
         */
        if (zv->zv_flags & ZVOL_WRITTEN_TO)
                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);

        if (zv->zv_flags & ZVOL_REMOVING)
                cv_broadcast(&zv->zv_removing_cv);

        return (SET_ERROR(error));
}

int
zvol_first_open(zvol_state_t *zv, boolean_t readonly)
{
        objset_t *os;
        int error;

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

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

        zv->zv_objset = os;

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

        return (error);
}

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

        if (zv->zv_flags & ZVOL_REMOVING)
                cv_broadcast(&zv->zv_removing_cv);

        zvol_shutdown_zv(zv);

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

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_dnode(os, ZVOL_OBJ, 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 snapshot name\n", dsname);
        } else {
                minors_job_t *job;
                char *n = kmem_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);
}

/*
 * If spa_keystore_load_wkey() is called for an encrypted zvol,
 * we need to look for any clones also using the key. This function
 * is "best effort" - so we just skip over it if there are failures.
 */
static void
zvol_add_clones(const char *dsname, list_t *minors_list)
{
        /* Also check if it has clones */
        dsl_dir_t *dd = NULL;
        dsl_pool_t *dp = NULL;

        if (dsl_pool_hold(dsname, FTAG, &dp) != 0)
                return;

        if (!spa_feature_is_enabled(dp->dp_spa,
            SPA_FEATURE_ENCRYPTION))
                goto out;

        if (dsl_dir_hold(dp, dsname, FTAG, &dd, NULL) != 0)
                goto out;

        if (dsl_dir_phys(dd)->dd_clones == 0)
                goto out;

        zap_cursor_t *zc = kmem_alloc(sizeof (zap_cursor_t), KM_SLEEP);
        zap_attribute_t *za = zap_attribute_alloc();
        objset_t *mos = dd->dd_pool->dp_meta_objset;

        for (zap_cursor_init(zc, mos, dsl_dir_phys(dd)->dd_clones);
            zap_cursor_retrieve(zc, za) == 0;
            zap_cursor_advance(zc)) {
                dsl_dataset_t *clone;
                minors_job_t *job;

                if (dsl_dataset_hold_obj(dd->dd_pool,
                    za->za_first_integer, FTAG, &clone) == 0) {

                        char name[ZFS_MAX_DATASET_NAME_LEN];
                        dsl_dataset_name(clone, name);

                        char *n = kmem_strdup(name);
                        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);

                        dsl_dataset_rele(clone, FTAG);
                }
        }
        zap_cursor_fini(zc);
        zap_attribute_free(za);
        kmem_free(zc, sizeof (zap_cursor_t));

out:
        if (dd != NULL)
                dsl_dir_rele(dd, FTAG);
        dsl_pool_rele(dp, FTAG);
}

/*
 * 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 = kmem_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);

                zvol_add_clones(dsname, minors_list);

                if (snapdev == ZFS_SNAPDEV_VISIBLE) {
                        /*
                         * traverse snapshots only, do not traverse children,
                         * and skip the 'dsname'
                         */
                        (void) dmu_objset_find(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.
 */
void
zvol_create_minors_recursive(const char *name)
{
        list_t minors_list;
        minors_job_t *job;

        if (zvol_inhibit_dev)
                return;

        /*
         * 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_os_create_minor 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));


        if (strchr(name, '@') != NULL) {
                uint64_t snapdev;

                int error = dsl_prop_get_integer(name, "snapdev",
                    &snapdev, NULL);

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

        taskq_wait_outstanding(system_taskq, 0);

        /*
         * Prefetch is completed, we can do zvol_os_create_minor
         * sequentially.
         */
        while ((job = list_remove_head(&minors_list)) != NULL) {
                if (!job->error)
                        (void) zvol_os_create_minor(job->name);
                kmem_strfree(job->name);
                kmem_free(job, sizeof (minors_job_t));
        }

        list_destroy(&minors_list);
}

void
zvol_create_minor(const char *name)
{
        /*
         * Note: the dsl_pool_config_lock must not be held.
         * Minor node creation needs to obtain the zvol_state_lock.
         * zvol_open() obtains the zvol_state_lock and then the dsl pool
         * config lock.  Therefore, we can't have the config lock now if
         * we are going to wait for the zvol_state_lock, because it
         * would be a lock order inversion which could lead to deadlock.
         */

        if (zvol_inhibit_dev)
                return;

        if (strchr(name, '@') != NULL) {
                uint64_t snapdev;

                int error = dsl_prop_get_integer(name,
                    "snapdev", &snapdev, NULL);

                if (error == 0 && snapdev == ZFS_SNAPDEV_VISIBLE)
                        (void) zvol_os_create_minor(name);
        } else {
                (void) zvol_os_create_minor(name);
        }
}

/*
 * Remove minors for specified dataset including children and snapshots.
 */

/*
 * Remove the minor for a given zvol. This will do it all:
 *  - flag the zvol for removal, so new requests are rejected
 *  - wait until outstanding requests are completed
 *  - remove it from lists
 *  - free it
 * It's also usable as a taskq task, and smells nice too.
 */
static void
zvol_remove_minor_task(void *arg)
{
        zvol_state_t *zv = (zvol_state_t *)arg;

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

        mutex_enter(&zv->zv_state_lock);
        while (zv->zv_open_count > 0 || atomic_read(&zv->zv_suspend_ref)) {
                zv->zv_flags |= ZVOL_REMOVING;
                cv_wait(&zv->zv_removing_cv, &zv->zv_state_lock);
        }
        mutex_exit(&zv->zv_state_lock);

        rw_enter(&zvol_state_lock, RW_WRITER);
        mutex_enter(&zv->zv_state_lock);

        zvol_remove(zv);
        zvol_os_clear_private(zv);

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

        zvol_os_free(zv);
}

static void
zvol_free_task(void *arg)
{
        zvol_os_free(arg);
}

void
zvol_remove_minors_impl(const char *name)
{
        zvol_state_t *zv, *zv_next;
        int namelen = ((name) ? strlen(name) : 0);
        taskqid_t t;
        list_t delay_list, free_list;

        if (zvol_inhibit_dev)
                return;

        list_create(&delay_list, sizeof (zvol_state_t),
            offsetof(zvol_state_t, zv_next));
        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, try to throw everyone off and try again
                         * later.
                         */
                        if (zv->zv_open_count > 0 ||
                            atomic_read(&zv->zv_suspend_ref)) {
                                zv->zv_flags |= ZVOL_REMOVING;
                                t = taskq_dispatch(
                                    zv->zv_objset->os_spa->spa_zvol_taskq,
                                    zvol_remove_minor_task, zv, TQ_SLEEP);
                                if (t == TASKQID_INVALID) {
                                        /*
                                         * Couldn't create the task, so we'll
                                         * do it in place once the loop is
                                         * finished.
                                         */
                                        list_insert_head(&delay_list, zv);
                                }
                                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.
                         */
                        zvol_os_clear_private(zv);

                        /* 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_task, zv,
                            TQ_SLEEP);
                        if (t == TASKQID_INVALID)
                                list_insert_head(&free_list, zv);
                } else {
                        mutex_exit(&zv->zv_state_lock);
                }
        }
        rw_exit(&zvol_state_lock);

        /* Wait for zvols that we couldn't create a remove task for */
        while ((zv = list_remove_head(&delay_list)) != NULL)
                zvol_remove_minor_task(zv);

        /* Free any that we couldn't free in parallel earlier */
        while ((zv = list_remove_head(&free_list)) != NULL)
                zvol_os_free(zv);
}

/* 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)
                        /* Found, leave the the loop with zv_lock held */
                        break;
                mutex_exit(&zv->zv_state_lock);
        }

        if (zv == NULL) {
                rw_exit(&zvol_state_lock);
                return;
        }

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

        if (zv->zv_open_count > 0 || atomic_read(&zv->zv_suspend_ref)) {
                /*
                 * In use, so try to throw everyone off, then wait
                 * until finished.
                 */
                zv->zv_flags |= ZVOL_REMOVING;
                mutex_exit(&zv->zv_state_lock);
                rw_exit(&zvol_state_lock);
                zvol_remove_minor_task(zv);
                return;
        }

        zvol_remove(zv);
        zvol_os_clear_private(zv);

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

        zvol_os_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;

        if (zvol_inhibit_dev)
                return;

        oldnamelen = strlen(oldname);

        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 (strcmp(zv->zv_name, oldname) == 0) {
                        zvol_os_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_os_rename_minor(zv, name);
                        kmem_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_os_create_minor(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);
}

static void
zvol_set_volmode_impl(char *name, uint64_t volmode)
{
        fstrans_cookie_t cookie;
        uint64_t old_volmode;
        zvol_state_t *zv;

        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)
         * could be different when we set, for instance, volmode from "geom"
         * to "dev" (or vice versa).
         */
        zv = zvol_find_by_name(name, RW_NONE);
        if (zv == NULL && volmode == ZFS_VOLMODE_NONE)
                        return;
        if (zv != NULL) {
                old_volmode = zv->zv_volmode;
                mutex_exit(&zv->zv_state_lock);
                if (old_volmode == volmode)
                        return;
                zvol_wait_close(zv);
        }
        cookie = spl_fstrans_mark();
        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_os_create_minor(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_os_create_minor(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;

        /* 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;

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

        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 *arg)
{
        zvol_task_t *task = arg;

        switch (task->op) {
        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;
        zfs_prop_t zsda_prop;
} 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_common_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);
}

static int
zvol_set_common_sync_cb(dsl_pool_t *dp, dsl_dataset_t *ds, void *arg)
{
        zvol_set_prop_int_arg_t *zsda = arg;
        char dsname[ZFS_MAX_DATASET_NAME_LEN];
        zvol_task_t *task;
        uint64_t prop;

        const char *prop_name = zfs_prop_to_name(zsda->zsda_prop);
        dsl_dataset_name(ds, dsname);

        if (dsl_prop_get_int_ds(ds, prop_name, &prop) != 0)
                return (0);

        switch (zsda->zsda_prop) {
                case ZFS_PROP_VOLMODE:
                        task = zvol_task_alloc(ZVOL_ASYNC_SET_VOLMODE, dsname,
                            NULL, prop);
                        break;
                case ZFS_PROP_SNAPDEV:
                        task = zvol_task_alloc(ZVOL_ASYNC_SET_SNAPDEV, dsname,
                            NULL, prop);
                        break;
                default:
                        task = NULL;
                        break;
        }

        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 the property appropriately.
 * We call dsl_prop_set_sync_impl() here to set the value only on the toplevel
 * dataset and read the effective "property" 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_common_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));

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

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

        dsl_dir_rele(dd, FTAG);
}

int
zvol_set_common(const char *ddname, zfs_prop_t prop, zprop_source_t source,
    uint64_t val)
{
        zvol_set_prop_int_arg_t zsda;

        zsda.zsda_name = ddname;
        zsda.zsda_source = source;
        zsda.zsda_value = val;
        zsda.zsda_prop = prop;

        return (dsl_sync_task(ddname, zvol_set_common_check,
            zvol_set_common_sync, &zsda, 0, ZFS_SPACE_CHECK_NONE));
}

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);
}

boolean_t
zvol_is_zvol(const char *name)
{

        return (zvol_os_is_zvol(name));
}

int
zvol_init_impl(void)
{
        int i;

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

        zvol_htable = kmem_alloc(ZVOL_HT_SIZE * sizeof (struct hlist_head),
            KM_SLEEP);
        for (i = 0; i < ZVOL_HT_SIZE; i++)
                INIT_HLIST_HEAD(&zvol_htable[i]);

        return (0);
}

void
zvol_fini_impl(void)
{
        zvol_remove_minors_impl(NULL);

        /*
         * The call to "zvol_remove_minors_impl" may dispatch entries to
         * the system_taskq, but it doesn't wait for those entries to
         * complete before it returns. Thus, we must wait for all of the
         * removals to finish, before we can continue.
         */
        taskq_wait_outstanding(system_taskq, 0);

        kmem_free(zvol_htable, ZVOL_HT_SIZE * sizeof (struct hlist_head));
        list_destroy(&zvol_state_list);
        rw_destroy(&zvol_state_lock);
}