xref: /illumos-gate/usr/src/uts/common/fs/zfs/dsl_dataset.c (revision ed22c710)

/*
 * 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) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2011 by Delphix. All rights reserved.
 */

#include <sys/dmu_objset.h>
#include <sys/dsl_dataset.h>
#include <sys/dsl_dir.h>
#include <sys/dsl_prop.h>
#include <sys/dsl_synctask.h>
#include <sys/dmu_traverse.h>
#include <sys/dmu_tx.h>
#include <sys/arc.h>
#include <sys/zio.h>
#include <sys/zap.h>
#include <sys/unique.h>
#include <sys/zfs_context.h>
#include <sys/zfs_ioctl.h>
#include <sys/spa.h>
#include <sys/zfs_znode.h>
#include <sys/zfs_onexit.h>
#include <sys/zvol.h>
#include <sys/dsl_scan.h>
#include <sys/dsl_deadlist.h>

static char *dsl_reaper = "the grim reaper";

static dsl_checkfunc_t dsl_dataset_destroy_begin_check;
static dsl_syncfunc_t dsl_dataset_destroy_begin_sync;
static dsl_syncfunc_t dsl_dataset_set_reservation_sync;

#define	SWITCH64(x, y) \
	{ \
		uint64_t __tmp = (x); \
		(x) = (y); \
		(y) = __tmp; \
	}

#define	DS_REF_MAX	(1ULL << 62)

#define	DSL_DEADLIST_BLOCKSIZE	SPA_MAXBLOCKSIZE

#define	DSL_DATASET_IS_DESTROYED(ds)	((ds)->ds_owner == dsl_reaper)


/*
 * Figure out how much of this delta should be propogated to the dsl_dir
 * layer.  If there's a refreservation, that space has already been
 * partially accounted for in our ancestors.
 */
static int64_t
parent_delta(dsl_dataset_t *ds, int64_t delta)
{
	uint64_t old_bytes, new_bytes;

	if (ds->ds_reserved == 0)
		return (delta);

	old_bytes = MAX(ds->ds_phys->ds_unique_bytes, ds->ds_reserved);
	new_bytes = MAX(ds->ds_phys->ds_unique_bytes + delta, ds->ds_reserved);

	ASSERT3U(ABS((int64_t)(new_bytes - old_bytes)), <=, ABS(delta));
	return (new_bytes - old_bytes);
}

void
dsl_dataset_block_born(dsl_dataset_t *ds, const blkptr_t *bp, dmu_tx_t *tx)
{
	int used = bp_get_dsize_sync(tx->tx_pool->dp_spa, bp);
	int compressed = BP_GET_PSIZE(bp);
	int uncompressed = BP_GET_UCSIZE(bp);
	int64_t delta;

	dprintf_bp(bp, "ds=%p", ds);

	ASSERT(dmu_tx_is_syncing(tx));
	/* It could have been compressed away to nothing */
	if (BP_IS_HOLE(bp))
		return;
	ASSERT(BP_GET_TYPE(bp) != DMU_OT_NONE);
	ASSERT3U(BP_GET_TYPE(bp), <, DMU_OT_NUMTYPES);
	if (ds == NULL) {
		/*
		 * Account for the meta-objset space in its placeholder
		 * dsl_dir.
		 */
		ASSERT3U(compressed, ==, uncompressed); /* it's all metadata */
		dsl_dir_diduse_space(tx->tx_pool->dp_mos_dir, DD_USED_HEAD,
		    used, compressed, uncompressed, tx);
		dsl_dir_dirty(tx->tx_pool->dp_mos_dir, tx);
		return;
	}
	dmu_buf_will_dirty(ds->ds_dbuf, tx);

	mutex_enter(&ds->ds_dir->dd_lock);
	mutex_enter(&ds->ds_lock);
	delta = parent_delta(ds, used);
	ds->ds_phys->ds_used_bytes += used;
	ds->ds_phys->ds_compressed_bytes += compressed;
	ds->ds_phys->ds_uncompressed_bytes += uncompressed;
	ds->ds_phys->ds_unique_bytes += used;
	mutex_exit(&ds->ds_lock);
	dsl_dir_diduse_space(ds->ds_dir, DD_USED_HEAD, delta,
	    compressed, uncompressed, tx);
	dsl_dir_transfer_space(ds->ds_dir, used - delta,
	    DD_USED_REFRSRV, DD_USED_HEAD, tx);
	mutex_exit(&ds->ds_dir->dd_lock);
}

int
dsl_dataset_block_kill(dsl_dataset_t *ds, const blkptr_t *bp, dmu_tx_t *tx,
    boolean_t async)
{
	if (BP_IS_HOLE(bp))
		return (0);

	ASSERT(dmu_tx_is_syncing(tx));
	ASSERT(bp->blk_birth <= tx->tx_txg);

	int used = bp_get_dsize_sync(tx->tx_pool->dp_spa, bp);
	int compressed = BP_GET_PSIZE(bp);
	int uncompressed = BP_GET_UCSIZE(bp);

	ASSERT(used > 0);
	if (ds == NULL) {
		/*
		 * Account for the meta-objset space in its placeholder
		 * dataset.
		 */
		dsl_free(tx->tx_pool, tx->tx_txg, bp);

		dsl_dir_diduse_space(tx->tx_pool->dp_mos_dir, DD_USED_HEAD,
		    -used, -compressed, -uncompressed, tx);
		dsl_dir_dirty(tx->tx_pool->dp_mos_dir, tx);
		return (used);
	}
	ASSERT3P(tx->tx_pool, ==, ds->ds_dir->dd_pool);

	ASSERT(!dsl_dataset_is_snapshot(ds));
	dmu_buf_will_dirty(ds->ds_dbuf, tx);

	if (bp->blk_birth > ds->ds_phys->ds_prev_snap_txg) {
		int64_t delta;

		dprintf_bp(bp, "freeing ds=%llu", ds->ds_object);
		dsl_free(tx->tx_pool, tx->tx_txg, bp);

		mutex_enter(&ds->ds_dir->dd_lock);
		mutex_enter(&ds->ds_lock);
		ASSERT(ds->ds_phys->ds_unique_bytes >= used ||
		    !DS_UNIQUE_IS_ACCURATE(ds));
		delta = parent_delta(ds, -used);
		ds->ds_phys->ds_unique_bytes -= used;
		mutex_exit(&ds->ds_lock);
		dsl_dir_diduse_space(ds->ds_dir, DD_USED_HEAD,
		    delta, -compressed, -uncompressed, tx);
		dsl_dir_transfer_space(ds->ds_dir, -used - delta,
		    DD_USED_REFRSRV, DD_USED_HEAD, tx);
		mutex_exit(&ds->ds_dir->dd_lock);
	} else {
		dprintf_bp(bp, "putting on dead list: %s", "");
		if (async) {
			/*
			 * We are here as part of zio's write done callback,
			 * which means we're a zio interrupt thread.  We can't
			 * call dsl_deadlist_insert() now because it may block
			 * waiting for I/O.  Instead, put bp on the deferred
			 * queue and let dsl_pool_sync() finish the job.
			 */
			bplist_append(&ds->ds_pending_deadlist, bp);
		} else {
			dsl_deadlist_insert(&ds->ds_deadlist, bp, tx);
		}
		ASSERT3U(ds->ds_prev->ds_object, ==,
		    ds->ds_phys->ds_prev_snap_obj);
		ASSERT(ds->ds_prev->ds_phys->ds_num_children > 0);
		/* if (bp->blk_birth > prev prev snap txg) prev unique += bs */
		if (ds->ds_prev->ds_phys->ds_next_snap_obj ==
		    ds->ds_object && bp->blk_birth >
		    ds->ds_prev->ds_phys->ds_prev_snap_txg) {
			dmu_buf_will_dirty(ds->ds_prev->ds_dbuf, tx);
			mutex_enter(&ds->ds_prev->ds_lock);
			ds->ds_prev->ds_phys->ds_unique_bytes += used;
			mutex_exit(&ds->ds_prev->ds_lock);
		}
		if (bp->blk_birth > ds->ds_dir->dd_origin_txg) {
			dsl_dir_transfer_space(ds->ds_dir, used,
			    DD_USED_HEAD, DD_USED_SNAP, tx);
		}
	}
	mutex_enter(&ds->ds_lock);
	ASSERT3U(ds->ds_phys->ds_used_bytes, >=, used);
	ds->ds_phys->ds_used_bytes -= used;
	ASSERT3U(ds->ds_phys->ds_compressed_bytes, >=, compressed);
	ds->ds_phys->ds_compressed_bytes -= compressed;
	ASSERT3U(ds->ds_phys->ds_uncompressed_bytes, >=, uncompressed);
	ds->ds_phys->ds_uncompressed_bytes -= uncompressed;
	mutex_exit(&ds->ds_lock);

	return (used);
}

uint64_t
dsl_dataset_prev_snap_txg(dsl_dataset_t *ds)
{
	uint64_t trysnap = 0;

	if (ds == NULL)
		return (0);
	/*
	 * The snapshot creation could fail, but that would cause an
	 * incorrect FALSE return, which would only result in an
	 * overestimation of the amount of space that an operation would
	 * consume, which is OK.
	 *
	 * There's also a small window where we could miss a pending
	 * snapshot, because we could set the sync task in the quiescing
	 * phase.  So this should only be used as a guess.
	 */
	if (ds->ds_trysnap_txg >
	    spa_last_synced_txg(ds->ds_dir->dd_pool->dp_spa))
		trysnap = ds->ds_trysnap_txg;
	return (MAX(ds->ds_phys->ds_prev_snap_txg, trysnap));
}

boolean_t
dsl_dataset_block_freeable(dsl_dataset_t *ds, const blkptr_t *bp,
    uint64_t blk_birth)
{
	if (blk_birth <= dsl_dataset_prev_snap_txg(ds))
		return (B_FALSE);

	ddt_prefetch(dsl_dataset_get_spa(ds), bp);

	return (B_TRUE);
}

/* ARGSUSED */
static void
dsl_dataset_evict(dmu_buf_t *db, void *dsv)
{
	dsl_dataset_t *ds = dsv;

	ASSERT(ds->ds_owner == NULL || DSL_DATASET_IS_DESTROYED(ds));

	unique_remove(ds->ds_fsid_guid);

	if (ds->ds_objset != NULL)
		dmu_objset_evict(ds->ds_objset);

	if (ds->ds_prev) {
		dsl_dataset_drop_ref(ds->ds_prev, ds);
		ds->ds_prev = NULL;
	}

	bplist_destroy(&ds->ds_pending_deadlist);
	if (db != NULL) {
		dsl_deadlist_close(&ds->ds_deadlist);
	} else {
		ASSERT(ds->ds_deadlist.dl_dbuf == NULL);
		ASSERT(!ds->ds_deadlist.dl_oldfmt);
	}
	if (ds->ds_dir)
		dsl_dir_close(ds->ds_dir, ds);

	ASSERT(!list_link_active(&ds->ds_synced_link));

	mutex_destroy(&ds->ds_lock);
	mutex_destroy(&ds->ds_recvlock);
	mutex_destroy(&ds->ds_opening_lock);
	rw_destroy(&ds->ds_rwlock);
	cv_destroy(&ds->ds_exclusive_cv);

	kmem_free(ds, sizeof (dsl_dataset_t));
}

static int
dsl_dataset_get_snapname(dsl_dataset_t *ds)
{
	dsl_dataset_phys_t *headphys;
	int err;
	dmu_buf_t *headdbuf;
	dsl_pool_t *dp = ds->ds_dir->dd_pool;
	objset_t *mos = dp->dp_meta_objset;

	if (ds->ds_snapname[0])
		return (0);
	if (ds->ds_phys->ds_next_snap_obj == 0)
		return (0);

	err = dmu_bonus_hold(mos, ds->ds_dir->dd_phys->dd_head_dataset_obj,
	    FTAG, &headdbuf);
	if (err)
		return (err);
	headphys = headdbuf->db_data;
	err = zap_value_search(dp->dp_meta_objset,
	    headphys->ds_snapnames_zapobj, ds->ds_object, 0, ds->ds_snapname);
	dmu_buf_rele(headdbuf, FTAG);
	return (err);
}

static int
dsl_dataset_snap_lookup(dsl_dataset_t *ds, const char *name, uint64_t *value)
{
	objset_t *mos = ds->ds_dir->dd_pool->dp_meta_objset;
	uint64_t snapobj = ds->ds_phys->ds_snapnames_zapobj;
	matchtype_t mt;
	int err;

	if (ds->ds_phys->ds_flags & DS_FLAG_CI_DATASET)
		mt = MT_FIRST;
	else
		mt = MT_EXACT;

	err = zap_lookup_norm(mos, snapobj, name, 8, 1,
	    value, mt, NULL, 0, NULL);
	if (err == ENOTSUP && mt == MT_FIRST)
		err = zap_lookup(mos, snapobj, name, 8, 1, value);
	return (err);
}

static int
dsl_dataset_snap_remove(dsl_dataset_t *ds, char *name, dmu_tx_t *tx)
{
	objset_t *mos = ds->ds_dir->dd_pool->dp_meta_objset;
	uint64_t snapobj = ds->ds_phys->ds_snapnames_zapobj;
	matchtype_t mt;
	int err;

	dsl_dir_snap_cmtime_update(ds->ds_dir);

	if (ds->ds_phys->ds_flags & DS_FLAG_CI_DATASET)
		mt = MT_FIRST;
	else
		mt = MT_EXACT;

	err = zap_remove_norm(mos, snapobj, name, mt, tx);
	if (err == ENOTSUP && mt == MT_FIRST)
		err = zap_remove(mos, snapobj, name, tx);
	return (err);
}

static int
dsl_dataset_get_ref(dsl_pool_t *dp, uint64_t dsobj, void *tag,
    dsl_dataset_t **dsp)
{
	objset_t *mos = dp->dp_meta_objset;
	dmu_buf_t *dbuf;
	dsl_dataset_t *ds;
	int err;
	dmu_object_info_t doi;

	ASSERT(RW_LOCK_HELD(&dp->dp_config_rwlock) ||
	    dsl_pool_sync_context(dp));

	err = dmu_bonus_hold(mos, dsobj, tag, &dbuf);
	if (err)
		return (err);

	/* Make sure dsobj has the correct object type. */
	dmu_object_info_from_db(dbuf, &doi);
	if (doi.doi_type != DMU_OT_DSL_DATASET)
		return (EINVAL);

	ds = dmu_buf_get_user(dbuf);
	if (ds == NULL) {
		dsl_dataset_t *winner;

		ds = kmem_zalloc(sizeof (dsl_dataset_t), KM_SLEEP);
		ds->ds_dbuf = dbuf;
		ds->ds_object = dsobj;
		ds->ds_phys = dbuf->db_data;

		mutex_init(&ds->ds_lock, NULL, MUTEX_DEFAULT, NULL);
		mutex_init(&ds->ds_recvlock, NULL, MUTEX_DEFAULT, NULL);
		mutex_init(&ds->ds_opening_lock, NULL, MUTEX_DEFAULT, NULL);
		rw_init(&ds->ds_rwlock, 0, 0, 0);
		cv_init(&ds->ds_exclusive_cv, NULL, CV_DEFAULT, NULL);

		bplist_create(&ds->ds_pending_deadlist);
		dsl_deadlist_open(&ds->ds_deadlist,
		    mos, ds->ds_phys->ds_deadlist_obj);

		if (err == 0) {
			err = dsl_dir_open_obj(dp,
			    ds->ds_phys->ds_dir_obj, NULL, ds, &ds->ds_dir);
		}
		if (err) {
			mutex_destroy(&ds->ds_lock);
			mutex_destroy(&ds->ds_recvlock);
			mutex_destroy(&ds->ds_opening_lock);
			rw_destroy(&ds->ds_rwlock);
			cv_destroy(&ds->ds_exclusive_cv);
			bplist_destroy(&ds->ds_pending_deadlist);
			dsl_deadlist_close(&ds->ds_deadlist);
			kmem_free(ds, sizeof (dsl_dataset_t));
			dmu_buf_rele(dbuf, tag);
			return (err);
		}

		if (!dsl_dataset_is_snapshot(ds)) {
			ds->ds_snapname[0] = '\0';
			if (ds->ds_phys->ds_prev_snap_obj) {
				err = dsl_dataset_get_ref(dp,
				    ds->ds_phys->ds_prev_snap_obj,
				    ds, &ds->ds_prev);
			}
		} else {
			if (zfs_flags & ZFS_DEBUG_SNAPNAMES)
				err = dsl_dataset_get_snapname(ds);
			if (err == 0 && ds->ds_phys->ds_userrefs_obj != 0) {
				err = zap_count(
				    ds->ds_dir->dd_pool->dp_meta_objset,
				    ds->ds_phys->ds_userrefs_obj,
				    &ds->ds_userrefs);
			}
		}

		if (err == 0 && !dsl_dataset_is_snapshot(ds)) {
			/*
			 * In sync context, we're called with either no lock
			 * or with the write lock.  If we're not syncing,
			 * we're always called with the read lock held.
			 */
			boolean_t need_lock =
			    !RW_WRITE_HELD(&dp->dp_config_rwlock) &&
			    dsl_pool_sync_context(dp);

			if (need_lock)
				rw_enter(&dp->dp_config_rwlock, RW_READER);

			err = dsl_prop_get_ds(ds,
			    "refreservation", sizeof (uint64_t), 1,
			    &ds->ds_reserved, NULL);
			if (err == 0) {
				err = dsl_prop_get_ds(ds,
				    "refquota", sizeof (uint64_t), 1,
				    &ds->ds_quota, NULL);
			}

			if (need_lock)
				rw_exit(&dp->dp_config_rwlock);
		} else {
			ds->ds_reserved = ds->ds_quota = 0;
		}

		if (err == 0) {
			winner = dmu_buf_set_user_ie(dbuf, ds, &ds->ds_phys,
			    dsl_dataset_evict);
		}
		if (err || winner) {
			bplist_destroy(&ds->ds_pending_deadlist);
			dsl_deadlist_close(&ds->ds_deadlist);
			if (ds->ds_prev)
				dsl_dataset_drop_ref(ds->ds_prev, ds);
			dsl_dir_close(ds->ds_dir, ds);
			mutex_destroy(&ds->ds_lock);
			mutex_destroy(&ds->ds_recvlock);
			mutex_destroy(&ds->ds_opening_lock);
			rw_destroy(&ds->ds_rwlock);
			cv_destroy(&ds->ds_exclusive_cv);
			kmem_free(ds, sizeof (dsl_dataset_t));
			if (err) {
				dmu_buf_rele(dbuf, tag);
				return (err);
			}
			ds = winner;
		} else {
			ds->ds_fsid_guid =
			    unique_insert(ds->ds_phys->ds_fsid_guid);
		}
	}
	ASSERT3P(ds->ds_dbuf, ==, dbuf);
	ASSERT3P(ds->ds_phys, ==, dbuf->db_data);
	ASSERT(ds->ds_phys->ds_prev_snap_obj != 0 ||
	    spa_version(dp->dp_spa) < SPA_VERSION_ORIGIN ||
	    dp->dp_origin_snap == NULL || ds == dp->dp_origin_snap);
	mutex_enter(&ds->ds_lock);
	if (!dsl_pool_sync_context(dp) && DSL_DATASET_IS_DESTROYED(ds)) {
		mutex_exit(&ds->ds_lock);
		dmu_buf_rele(ds->ds_dbuf, tag);
		return (ENOENT);
	}
	mutex_exit(&ds->ds_lock);
	*dsp = ds;
	return (0);
}

static int
dsl_dataset_hold_ref(dsl_dataset_t *ds, void *tag)
{
	dsl_pool_t *dp = ds->ds_dir->dd_pool;

	/*
	 * In syncing context we don't want the rwlock lock: there
	 * may be an existing writer waiting for sync phase to
	 * finish.  We don't need to worry about such writers, since
	 * sync phase is single-threaded, so the writer can't be
	 * doing anything while we are active.
	 */
	if (dsl_pool_sync_context(dp)) {
		ASSERT(!DSL_DATASET_IS_DESTROYED(ds));
		return (0);
	}

	/*
	 * Normal users will hold the ds_rwlock as a READER until they
	 * are finished (i.e., call dsl_dataset_rele()).  "Owners" will
	 * drop their READER lock after they set the ds_owner field.
	 *
	 * If the dataset is being destroyed, the destroy thread will
	 * obtain a WRITER lock for exclusive access after it's done its
	 * open-context work and then change the ds_owner to
	 * dsl_reaper once destruction is assured.  So threads
	 * may block here temporarily, until the "destructability" of
	 * the dataset is determined.
	 */
	ASSERT(!RW_WRITE_HELD(&dp->dp_config_rwlock));
	mutex_enter(&ds->ds_lock);
	while (!rw_tryenter(&ds->ds_rwlock, RW_READER)) {
		rw_exit(&dp->dp_config_rwlock);
		cv_wait(&ds->ds_exclusive_cv, &ds->ds_lock);
		if (DSL_DATASET_IS_DESTROYED(ds)) {
			mutex_exit(&ds->ds_lock);
			dsl_dataset_drop_ref(ds, tag);
			rw_enter(&dp->dp_config_rwlock, RW_READER);
			return (ENOENT);
		}
		/*
		 * The dp_config_rwlock lives above the ds_lock. And
		 * we need to check DSL_DATASET_IS_DESTROYED() while
		 * holding the ds_lock, so we have to drop and reacquire
		 * the ds_lock here.
		 */
		mutex_exit(&ds->ds_lock);
		rw_enter(&dp->dp_config_rwlock, RW_READER);
		mutex_enter(&ds->ds_lock);
	}
	mutex_exit(&ds->ds_lock);
	return (0);
}

int
dsl_dataset_hold_obj(dsl_pool_t *dp, uint64_t dsobj, void *tag,
    dsl_dataset_t **dsp)
{
	int err = dsl_dataset_get_ref(dp, dsobj, tag, dsp);

	if (err)
		return (err);
	return (dsl_dataset_hold_ref(*dsp, tag));
}

int
dsl_dataset_own_obj(dsl_pool_t *dp, uint64_t dsobj, boolean_t inconsistentok,
    void *tag, dsl_dataset_t **dsp)
{
	int err = dsl_dataset_hold_obj(dp, dsobj, tag, dsp);
	if (err)
		return (err);
	if (!dsl_dataset_tryown(*dsp, inconsistentok, tag)) {
		dsl_dataset_rele(*dsp, tag);
		*dsp = NULL;
		return (EBUSY);
	}
	return (0);
}

int
dsl_dataset_hold(const char *name, void *tag, dsl_dataset_t **dsp)
{
	dsl_dir_t *dd;
	dsl_pool_t *dp;
	const char *snapname;
	uint64_t obj;
	int err = 0;

	err = dsl_dir_open_spa(NULL, name, FTAG, &dd, &snapname);
	if (err)
		return (err);

	dp = dd->dd_pool;
	obj = dd->dd_phys->dd_head_dataset_obj;
	rw_enter(&dp->dp_config_rwlock, RW_READER);
	if (obj)
		err = dsl_dataset_get_ref(dp, obj, tag, dsp);
	else
		err = ENOENT;
	if (err)
		goto out;

	err = dsl_dataset_hold_ref(*dsp, tag);

	/* we may be looking for a snapshot */
	if (err == 0 && snapname != NULL) {
		dsl_dataset_t *ds = NULL;

		if (*snapname++ != '@') {
			dsl_dataset_rele(*dsp, tag);
			err = ENOENT;
			goto out;
		}

		dprintf("looking for snapshot '%s'\n", snapname);
		err = dsl_dataset_snap_lookup(*dsp, snapname, &obj);
		if (err == 0)
			err = dsl_dataset_get_ref(dp, obj, tag, &ds);
		dsl_dataset_rele(*dsp, tag);

		ASSERT3U((err == 0), ==, (ds != NULL));

		if (ds) {
			mutex_enter(&ds->ds_lock);
			if (ds->ds_snapname[0] == 0)
				(void) strlcpy(ds->ds_snapname, snapname,
				    sizeof (ds->ds_snapname));
			mutex_exit(&ds->ds_lock);
			err = dsl_dataset_hold_ref(ds, tag);
			*dsp = err ? NULL : ds;
		}
	}
out:
	rw_exit(&dp->dp_config_rwlock);
	dsl_dir_close(dd, FTAG);
	return (err);
}

int
dsl_dataset_own(const char *name, boolean_t inconsistentok,
    void *tag, dsl_dataset_t **dsp)
{
	int err = dsl_dataset_hold(name, tag, dsp);
	if (err)
		return (err);
	if (!dsl_dataset_tryown(*dsp, inconsistentok, tag)) {
		dsl_dataset_rele(*dsp, tag);
		return (EBUSY);
	}
	return (0);
}

void
dsl_dataset_name(dsl_dataset_t *ds, char *name)
{
	if (ds == NULL) {
		(void) strcpy(name, "mos");
	} else {
		dsl_dir_name(ds->ds_dir, name);
		VERIFY(0 == dsl_dataset_get_snapname(ds));
		if (ds->ds_snapname[0]) {
			(void) strcat(name, "@");
			/*
			 * We use a "recursive" mutex so that we
			 * can call dprintf_ds() with ds_lock held.
			 */
			if (!MUTEX_HELD(&ds->ds_lock)) {
				mutex_enter(&ds->ds_lock);
				(void) strcat(name, ds->ds_snapname);
				mutex_exit(&ds->ds_lock);
			} else {
				(void) strcat(name, ds->ds_snapname);
			}
		}
	}
}

static int
dsl_dataset_namelen(dsl_dataset_t *ds)
{
	int result;

	if (ds == NULL) {
		result = 3;	/* "mos" */
	} else {
		result = dsl_dir_namelen(ds->ds_dir);
		VERIFY(0 == dsl_dataset_get_snapname(ds));
		if (ds->ds_snapname[0]) {
			++result;	/* adding one for the @-sign */
			if (!MUTEX_HELD(&ds->ds_lock)) {
				mutex_enter(&ds->ds_lock);
				result += strlen(ds->ds_snapname);
				mutex_exit(&ds->ds_lock);
			} else {
				result += strlen(ds->ds_snapname);
			}
		}
	}

	return (result);
}

void
dsl_dataset_drop_ref(dsl_dataset_t *ds, void *tag)
{
	dmu_buf_rele(ds->ds_dbuf, tag);
}

void
dsl_dataset_rele(dsl_dataset_t *ds, void *tag)
{
	if (!dsl_pool_sync_context(ds->ds_dir->dd_pool)) {
		rw_exit(&ds->ds_rwlock);
	}
	dsl_dataset_drop_ref(ds, tag);
}

void
dsl_dataset_disown(dsl_dataset_t *ds, void *tag)
{
	ASSERT((ds->ds_owner == tag && ds->ds_dbuf) ||
	    (DSL_DATASET_IS_DESTROYED(ds) && ds->ds_dbuf == NULL));

	mutex_enter(&ds->ds_lock);
	ds->ds_owner = NULL;
	if (RW_WRITE_HELD(&ds->ds_rwlock)) {
		rw_exit(&ds->ds_rwlock);
		cv_broadcast(&ds->ds_exclusive_cv);
	}
	mutex_exit(&ds->ds_lock);
	if (ds->ds_dbuf)
		dsl_dataset_drop_ref(ds, tag);
	else
		dsl_dataset_evict(NULL, ds);
}

boolean_t
dsl_dataset_tryown(dsl_dataset_t *ds, boolean_t inconsistentok, void *tag)
{
	boolean_t gotit = FALSE;

	mutex_enter(&ds->ds_lock);
	if (ds->ds_owner == NULL &&
	    (!DS_IS_INCONSISTENT(ds) || inconsistentok)) {
		ds->ds_owner = tag;
		if (!dsl_pool_sync_context(ds->ds_dir->dd_pool))
			rw_exit(&ds->ds_rwlock);
		gotit = TRUE;
	}
	mutex_exit(&ds->ds_lock);
	return (gotit);
}

void
dsl_dataset_make_exclusive(dsl_dataset_t *ds, void *owner)
{
	ASSERT3P(owner, ==, ds->ds_owner);
	if (!RW_WRITE_HELD(&ds->ds_rwlock))
		rw_enter(&ds->ds_rwlock, RW_WRITER);
}

uint64_t
dsl_dataset_create_sync_dd(dsl_dir_t *dd, dsl_dataset_t *origin,
    uint64_t flags, dmu_tx_t *tx)
{
	dsl_pool_t *dp = dd->dd_pool;
	dmu_buf_t *dbuf;
	dsl_dataset_phys_t *dsphys;
	uint64_t dsobj;
	objset_t *mos = dp->dp_meta_objset;

	if (origin == NULL)
		origin = dp->dp_origin_snap;

	ASSERT(origin == NULL || origin->ds_dir->dd_pool == dp);
	ASSERT(origin == NULL || origin->ds_phys->ds_num_children > 0);
	ASSERT(dmu_tx_is_syncing(tx));
	ASSERT(dd->dd_phys->dd_head_dataset_obj == 0);

	dsobj = dmu_object_alloc(mos, DMU_OT_DSL_DATASET, 0,
	    DMU_OT_DSL_DATASET, sizeof (dsl_dataset_phys_t), tx);
	VERIFY(0 == dmu_bonus_hold(mos, dsobj, FTAG, &dbuf));
	dmu_buf_will_dirty(dbuf, tx);
	dsphys = dbuf->db_data;
	bzero(dsphys, sizeof (dsl_dataset_phys_t));
	dsphys->ds_dir_obj = dd->dd_object;
	dsphys->ds_flags = flags;
	dsphys->ds_fsid_guid = unique_create();
	(void) random_get_pseudo_bytes((void*)&dsphys->ds_guid,
	    sizeof (dsphys->ds_guid));
	dsphys->ds_snapnames_zapobj =
	    zap_create_norm(mos, U8_TEXTPREP_TOUPPER, DMU_OT_DSL_DS_SNAP_MAP,
	    DMU_OT_NONE, 0, tx);
	dsphys->ds_creation_time = gethrestime_sec();
	dsphys->ds_creation_txg = tx->tx_txg == TXG_INITIAL ? 1 : tx->tx_txg;

	if (origin == NULL) {
		dsphys->ds_deadlist_obj = dsl_deadlist_alloc(mos, tx);
	} else {
		dsl_dataset_t *ohds;

		dsphys->ds_prev_snap_obj = origin->ds_object;
		dsphys->ds_prev_snap_txg =
		    origin->ds_phys->ds_creation_txg;
		dsphys->ds_used_bytes =
		    origin->ds_phys->ds_used_bytes;
		dsphys->ds_compressed_bytes =
		    origin->ds_phys->ds_compressed_bytes;
		dsphys->ds_uncompressed_bytes =
		    origin->ds_phys->ds_uncompressed_bytes;
		dsphys->ds_bp = origin->ds_phys->ds_bp;
		dsphys->ds_flags |= origin->ds_phys->ds_flags;

		dmu_buf_will_dirty(origin->ds_dbuf, tx);
		origin->ds_phys->ds_num_children++;

		VERIFY3U(0, ==, dsl_dataset_hold_obj(dp,
		    origin->ds_dir->dd_phys->dd_head_dataset_obj, FTAG, &ohds));
		dsphys->ds_deadlist_obj = dsl_deadlist_clone(&ohds->ds_deadlist,
		    dsphys->ds_prev_snap_txg, dsphys->ds_prev_snap_obj, tx);
		dsl_dataset_rele(ohds, FTAG);

		if (spa_version(dp->dp_spa) >= SPA_VERSION_NEXT_CLONES) {
			if (origin->ds_phys->ds_next_clones_obj == 0) {
				origin->ds_phys->ds_next_clones_obj =
				    zap_create(mos,
				    DMU_OT_NEXT_CLONES, DMU_OT_NONE, 0, tx);
			}
			VERIFY(0 == zap_add_int(mos,
			    origin->ds_phys->ds_next_clones_obj,
			    dsobj, tx));
		}

		dmu_buf_will_dirty(dd->dd_dbuf, tx);
		dd->dd_phys->dd_origin_obj = origin->ds_object;
		if (spa_version(dp->dp_spa) >= SPA_VERSION_DIR_CLONES) {
			if (origin->ds_dir->dd_phys->dd_clones == 0) {
				dmu_buf_will_dirty(origin->ds_dir->dd_dbuf, tx);
				origin->ds_dir->dd_phys->dd_clones =
				    zap_create(mos,
				    DMU_OT_DSL_CLONES, DMU_OT_NONE, 0, tx);
			}
			VERIFY3U(0, ==, zap_add_int(mos,
			    origin->ds_dir->dd_phys->dd_clones, dsobj, tx));
		}
	}

	if (spa_version(dp->dp_spa) >= SPA_VERSION_UNIQUE_ACCURATE)
		dsphys->ds_flags |= DS_FLAG_UNIQUE_ACCURATE;

	dmu_buf_rele(dbuf, FTAG);

	dmu_buf_will_dirty(dd->dd_dbuf, tx);
	dd->dd_phys->dd_head_dataset_obj = dsobj;

	return (dsobj);
}

uint64_t
dsl_dataset_create_sync(dsl_dir_t *pdd, const char *lastname,
    dsl_dataset_t *origin, uint64_t flags, cred_t *cr, dmu_tx_t *tx)
{
	dsl_pool_t *dp = pdd->dd_pool;
	uint64_t dsobj, ddobj;
	dsl_dir_t *dd;

	ASSERT(lastname[0] != '@');

	ddobj = dsl_dir_create_sync(dp, pdd, lastname, tx);
	VERIFY(0 == dsl_dir_open_obj(dp, ddobj, lastname, FTAG, &dd));

	dsobj = dsl_dataset_create_sync_dd(dd, origin, flags, tx);

	dsl_deleg_set_create_perms(dd, tx, cr);

	dsl_dir_close(dd, FTAG);

	/*
	 * If we are creating a clone, make sure we zero out any stale
	 * data from the origin snapshots zil header.
	 */
	if (origin != NULL) {
		dsl_dataset_t *ds;
		objset_t *os;

		VERIFY3U(0, ==, dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds));
		VERIFY3U(0, ==, dmu_objset_from_ds(ds, &os));
		bzero(&os->os_zil_header, sizeof (os->os_zil_header));
		dsl_dataset_dirty(ds, tx);
		dsl_dataset_rele(ds, FTAG);
	}

	return (dsobj);
}

struct destroyarg {
	dsl_sync_task_group_t *dstg;
	char *snapname;
	char *failed;
	boolean_t defer;
};

static int
dsl_snapshot_destroy_one(const char *name, void *arg)
{
	struct destroyarg *da = arg;
	dsl_dataset_t *ds;
	int err;
	char *dsname;

	dsname = kmem_asprintf("%s@%s", name, da->snapname);
	err = dsl_dataset_own(dsname, B_TRUE, da->dstg, &ds);
	strfree(dsname);
	if (err == 0) {
		struct dsl_ds_destroyarg *dsda;

		dsl_dataset_make_exclusive(ds, da->dstg);
		dsda = kmem_zalloc(sizeof (struct dsl_ds_destroyarg), KM_SLEEP);
		dsda->ds = ds;
		dsda->defer = da->defer;
		dsl_sync_task_create(da->dstg, dsl_dataset_destroy_check,
		    dsl_dataset_destroy_sync, dsda, da->dstg, 0);
	} else if (err == ENOENT) {
		err = 0;
	} else {
		(void) strcpy(da->failed, name);
	}
	return (err);
}

/*
 * Destroy 'snapname' in all descendants of 'fsname'.
 */
#pragma weak dmu_snapshots_destroy = dsl_snapshots_destroy
int
dsl_snapshots_destroy(char *fsname, char *snapname, boolean_t defer)
{
	int err;
	struct destroyarg da;
	dsl_sync_task_t *dst;
	spa_t *spa;

	err = spa_open(fsname, &spa, FTAG);
	if (err)
		return (err);
	da.dstg = dsl_sync_task_group_create(spa_get_dsl(spa));
	da.snapname = snapname;
	da.failed = fsname;
	da.defer = defer;

	err = dmu_objset_find(fsname,
	    dsl_snapshot_destroy_one, &da, DS_FIND_CHILDREN);

	if (err == 0)
		err = dsl_sync_task_group_wait(da.dstg);

	for (dst = list_head(&da.dstg->dstg_tasks); dst;
	    dst = list_next(&da.dstg->dstg_tasks, dst)) {
		struct dsl_ds_destroyarg *dsda = dst->dst_arg1;
		dsl_dataset_t *ds = dsda->ds;

		/*
		 * Return the file system name that triggered the error
		 */
		if (dst->dst_err) {
			dsl_dataset_name(ds, fsname);
			*strchr(fsname, '@') = '\0';
		}
		ASSERT3P(dsda->rm_origin, ==, NULL);
		dsl_dataset_disown(ds, da.dstg);
		kmem_free(dsda, sizeof (struct dsl_ds_destroyarg));
	}

	dsl_sync_task_group_destroy(da.dstg);
	spa_close(spa, FTAG);
	return (err);
}

static boolean_t
dsl_dataset_might_destroy_origin(dsl_dataset_t *ds)
{
	boolean_t might_destroy = B_FALSE;

	mutex_enter(&ds->ds_lock);
	if (ds->ds_phys->ds_num_children == 2 && ds->ds_userrefs == 0 &&
	    DS_IS_DEFER_DESTROY(ds))
		might_destroy = B_TRUE;
	mutex_exit(&ds->ds_lock);

	return (might_destroy);
}

/*
 * If we're removing a clone, and these three conditions are true:
 *	1) the clone's origin has no other children
 *	2) the clone's origin has no user references
 *	3) the clone's origin has been marked for deferred destruction
 * Then, prepare to remove the origin as part of this sync task group.
 */
static int
dsl_dataset_origin_rm_prep(struct dsl_ds_destroyarg *dsda, void *tag)
{
	dsl_dataset_t *ds = dsda->ds;
	dsl_dataset_t *origin = ds->ds_prev;

	if (dsl_dataset_might_destroy_origin(origin)) {
		char *name;
		int namelen;
		int error;

		namelen = dsl_dataset_namelen(origin) + 1;
		name = kmem_alloc(namelen, KM_SLEEP);
		dsl_dataset_name(origin, name);
#ifdef _KERNEL
		error = zfs_unmount_snap(name, NULL);
		if (error) {
			kmem_free(name, namelen);
			return (error);
		}
#endif
		error = dsl_dataset_own(name, B_TRUE, tag, &origin);
		kmem_free(name, namelen);
		if (error)
			return (error);
		dsda->rm_origin = origin;
		dsl_dataset_make_exclusive(origin, tag);
	}

	return (0);
}

/*
 * ds must be opened as OWNER.  On return (whether successful or not),
 * ds will be closed and caller can no longer dereference it.
 */
int
dsl_dataset_destroy(dsl_dataset_t *ds, void *tag, boolean_t defer)
{
	int err;
	dsl_sync_task_group_t *dstg;
	objset_t *os;
	dsl_dir_t *dd;
	uint64_t obj;
	struct dsl_ds_destroyarg dsda = { 0 };
	dsl_dataset_t dummy_ds = { 0 };

	dsda.ds = ds;

	if (dsl_dataset_is_snapshot(ds)) {
		/* Destroying a snapshot is simpler */
		dsl_dataset_make_exclusive(ds, tag);

		dsda.defer = defer;
		err = dsl_sync_task_do(ds->ds_dir->dd_pool,
		    dsl_dataset_destroy_check, dsl_dataset_destroy_sync,
		    &dsda, tag, 0);
		ASSERT3P(dsda.rm_origin, ==, NULL);
		goto out;
	} else if (defer) {
		err = EINVAL;
		goto out;
	}

	dd = ds->ds_dir;
	dummy_ds.ds_dir = dd;
	dummy_ds.ds_object = ds->ds_object;

	/*
	 * Check for errors and mark this ds as inconsistent, in
	 * case we crash while freeing the objects.
	 */
	err = dsl_sync_task_do(dd->dd_pool, dsl_dataset_destroy_begin_check,
	    dsl_dataset_destroy_begin_sync, ds, NULL, 0);
	if (err)
		goto out;

	err = dmu_objset_from_ds(ds, &os);
	if (err)
		goto out;

	/*
	 * remove the objects in open context, so that we won't
	 * have too much to do in syncing context.
	 */
	for (obj = 0; err == 0; err = dmu_object_next(os, &obj, FALSE,
	    ds->ds_phys->ds_prev_snap_txg)) {
		/*
		 * Ignore errors, if there is not enough disk space
		 * we will deal with it in dsl_dataset_destroy_sync().
		 */
		(void) dmu_free_object(os, obj);
	}
	if (err != ESRCH)
		goto out;

	/*
	 * Only the ZIL knows how to free log blocks.
	 */
	zil_destroy(dmu_objset_zil(os), B_FALSE);

	/*
	 * Sync out all in-flight IO.
	 */
	txg_wait_synced(dd->dd_pool, 0);

	/*
	 * If we managed to free all the objects in open
	 * context, the user space accounting should be zero.
	 */
	if (ds->ds_phys->ds_bp.blk_fill == 0 &&
	    dmu_objset_userused_enabled(os)) {
		uint64_t count;

		ASSERT(zap_count(os, DMU_USERUSED_OBJECT, &count) != 0 ||
		    count == 0);
		ASSERT(zap_count(os, DMU_GROUPUSED_OBJECT, &count) != 0 ||
		    count == 0);
	}

	rw_enter(&dd->dd_pool->dp_config_rwlock, RW_READER);
	err = dsl_dir_open_obj(dd->dd_pool, dd->dd_object, NULL, FTAG, &dd);
	rw_exit(&dd->dd_pool->dp_config_rwlock);

	if (err)
		goto out;

	/*
	 * Blow away the dsl_dir + head dataset.
	 */
	dsl_dataset_make_exclusive(ds, tag);
	/*
	 * If we're removing a clone, we might also need to remove its
	 * origin.
	 */
	do {
		dsda.need_prep = B_FALSE;
		if (dsl_dir_is_clone(dd)) {
			err = dsl_dataset_origin_rm_prep(&dsda, tag);
			if (err) {
				dsl_dir_close(dd, FTAG);
				goto out;
			}
		}

		dstg = dsl_sync_task_group_create(ds->ds_dir->dd_pool);
		dsl_sync_task_create(dstg, dsl_dataset_destroy_check,
		    dsl_dataset_destroy_sync, &dsda, tag, 0);
		dsl_sync_task_create(dstg, dsl_dir_destroy_check,
		    dsl_dir_destroy_sync, &dummy_ds, FTAG, 0);
		err = dsl_sync_task_group_wait(dstg);
		dsl_sync_task_group_destroy(dstg);

		/*
		 * We could be racing against 'zfs release' or 'zfs destroy -d'
		 * on the origin snap, in which case we can get EBUSY if we
		 * needed to destroy the origin snap but were not ready to
		 * do so.
		 */
		if (dsda.need_prep) {
			ASSERT(err == EBUSY);
			ASSERT(dsl_dir_is_clone(dd));
			ASSERT(dsda.rm_origin == NULL);
		}
	} while (dsda.need_prep);

	if (dsda.rm_origin != NULL)
		dsl_dataset_disown(dsda.rm_origin, tag);

	/* if it is successful, dsl_dir_destroy_sync will close the dd */
	if (err)
		dsl_dir_close(dd, FTAG);
out:
	dsl_dataset_disown(ds, tag);
	return (err);
}

blkptr_t *
dsl_dataset_get_blkptr(dsl_dataset_t *ds)
{
	return (&ds->ds_phys->ds_bp);
}

void
dsl_dataset_set_blkptr(dsl_dataset_t *ds, blkptr_t *bp, dmu_tx_t *tx)
{
	ASSERT(dmu_tx_is_syncing(tx));
	/* If it's the meta-objset, set dp_meta_rootbp */
	if (ds == NULL) {
		tx->tx_pool->dp_meta_rootbp = *bp;
	} else {
		dmu_buf_will_dirty(ds->ds_dbuf, tx);
		ds->ds_phys->ds_bp = *bp;
	}
}

spa_t *
dsl_dataset_get_spa(dsl_dataset_t *ds)
{
	return (ds->ds_dir->dd_pool->dp_spa);
}

void
dsl_dataset_dirty(dsl_dataset_t *ds, dmu_tx_t *tx)
{
	dsl_pool_t *dp;

	if (ds == NULL) /* this is the meta-objset */
		return;

	ASSERT(ds->ds_objset != NULL);

	if (ds->ds_phys->ds_next_snap_obj != 0)
		panic("dirtying snapshot!");

	dp = ds->ds_dir->dd_pool;

	if (txg_list_add(&dp->dp_dirty_datasets, ds, tx->tx_txg) == 0) {
		/* up the hold count until we can be written out */
		dmu_buf_add_ref(ds->ds_dbuf, ds);
	}
}

/*
 * The unique space in the head dataset can be calculated by subtracting
 * the space used in the most recent snapshot, that is still being used
 * in this file system, from the space currently in use.  To figure out
 * the space in the most recent snapshot still in use, we need to take
 * the total space used in the snapshot and subtract out the space that
 * has been freed up since the snapshot was taken.
 */
static void
dsl_dataset_recalc_head_uniq(dsl_dataset_t *ds)
{
	uint64_t mrs_used;
	uint64_t dlused, dlcomp, dluncomp;

	ASSERT(!dsl_dataset_is_snapshot(ds));

	if (ds->ds_phys->ds_prev_snap_obj != 0)
		mrs_used = ds->ds_prev->ds_phys->ds_used_bytes;
	else
		mrs_used = 0;

	dsl_deadlist_space(&ds->ds_deadlist, &dlused, &dlcomp, &dluncomp);

	ASSERT3U(dlused, <=, mrs_used);
	ds->ds_phys->ds_unique_bytes =
	    ds->ds_phys->ds_used_bytes - (mrs_used - dlused);

	if (spa_version(ds->ds_dir->dd_pool->dp_spa) >=
	    SPA_VERSION_UNIQUE_ACCURATE)
		ds->ds_phys->ds_flags |= DS_FLAG_UNIQUE_ACCURATE;
}

struct killarg {
	dsl_dataset_t *ds;
	dmu_tx_t *tx;
};

/* ARGSUSED */
static int
kill_blkptr(spa_t *spa, zilog_t *zilog, const blkptr_t *bp, arc_buf_t *pbuf,
    const zbookmark_t *zb, const dnode_phys_t *dnp, void *arg)
{
	struct killarg *ka = arg;
	dmu_tx_t *tx = ka->tx;

	if (bp == NULL)
		return (0);

	if (zb->zb_level == ZB_ZIL_LEVEL) {
		ASSERT(zilog != NULL);
		/*
		 * It's a block in the intent log.  It has no
		 * accounting, so just free it.
		 */
		dsl_free(ka->tx->tx_pool, ka->tx->tx_txg, bp);
	} else {
		ASSERT(zilog == NULL);
		ASSERT3U(bp->blk_birth, >, ka->ds->ds_phys->ds_prev_snap_txg);
		(void) dsl_dataset_block_kill(ka->ds, bp, tx, B_FALSE);
	}

	return (0);
}

/* ARGSUSED */
static int
dsl_dataset_destroy_begin_check(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	objset_t *mos = ds->ds_dir->dd_pool->dp_meta_objset;
	uint64_t count;
	int err;

	/*
	 * Can't delete a head dataset if there are snapshots of it.
	 * (Except if the only snapshots are from the branch we cloned
	 * from.)
	 */
	if (ds->ds_prev != NULL &&
	    ds->ds_prev->ds_phys->ds_next_snap_obj == ds->ds_object)
		return (EBUSY);

	/*
	 * This is really a dsl_dir thing, but check it here so that
	 * we'll be less likely to leave this dataset inconsistent &
	 * nearly destroyed.
	 */
	err = zap_count(mos, ds->ds_dir->dd_phys->dd_child_dir_zapobj, &count);
	if (err)
		return (err);
	if (count != 0)
		return (EEXIST);

	return (0);
}

/* ARGSUSED */
static void
dsl_dataset_destroy_begin_sync(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	dsl_pool_t *dp = ds->ds_dir->dd_pool;

	/* Mark it as inconsistent on-disk, in case we crash */
	dmu_buf_will_dirty(ds->ds_dbuf, tx);
	ds->ds_phys->ds_flags |= DS_FLAG_INCONSISTENT;

	spa_history_log_internal(LOG_DS_DESTROY_BEGIN, dp->dp_spa, tx,
	    "dataset = %llu", ds->ds_object);
}

static int
dsl_dataset_origin_check(struct dsl_ds_destroyarg *dsda, void *tag,
    dmu_tx_t *tx)
{
	dsl_dataset_t *ds = dsda->ds;
	dsl_dataset_t *ds_prev = ds->ds_prev;

	if (dsl_dataset_might_destroy_origin(ds_prev)) {
		struct dsl_ds_destroyarg ndsda = {0};

		/*
		 * If we're not prepared to remove the origin, don't remove
		 * the clone either.
		 */
		if (dsda->rm_origin == NULL) {
			dsda->need_prep = B_TRUE;
			return (EBUSY);
		}

		ndsda.ds = ds_prev;
		ndsda.is_origin_rm = B_TRUE;
		return (dsl_dataset_destroy_check(&ndsda, tag, tx));
	}

	/*
	 * If we're not going to remove the origin after all,
	 * undo the open context setup.
	 */
	if (dsda->rm_origin != NULL) {
		dsl_dataset_disown(dsda->rm_origin, tag);
		dsda->rm_origin = NULL;
	}

	return (0);
}

/*
 * If you add new checks here, you may need to add
 * additional checks to the "temporary" case in
 * snapshot_check() in dmu_objset.c.
 */
/* ARGSUSED */
int
dsl_dataset_destroy_check(void *arg1, void *arg2, dmu_tx_t *tx)
{
	struct dsl_ds_destroyarg *dsda = arg1;
	dsl_dataset_t *ds = dsda->ds;

	/* we have an owner hold, so noone else can destroy us */
	ASSERT(!DSL_DATASET_IS_DESTROYED(ds));

	/*
	 * Only allow deferred destroy on pools that support it.
	 * NOTE: deferred destroy is only supported on snapshots.
	 */
	if (dsda->defer) {
		if (spa_version(ds->ds_dir->dd_pool->dp_spa) <
		    SPA_VERSION_USERREFS)
			return (ENOTSUP);
		ASSERT(dsl_dataset_is_snapshot(ds));
		return (0);
	}

	/*
	 * Can't delete a head dataset if there are snapshots of it.
	 * (Except if the only snapshots are from the branch we cloned
	 * from.)
	 */
	if (ds->ds_prev != NULL &&
	    ds->ds_prev->ds_phys->ds_next_snap_obj == ds->ds_object)
		return (EBUSY);

	/*
	 * If we made changes this txg, traverse_dsl_dataset won't find
	 * them.  Try again.
	 */
	if (ds->ds_phys->ds_bp.blk_birth >= tx->tx_txg)
		return (EAGAIN);

	if (dsl_dataset_is_snapshot(ds)) {
		/*
		 * If this snapshot has an elevated user reference count,
		 * we can't destroy it yet.
		 */
		if (ds->ds_userrefs > 0 && !dsda->releasing)
			return (EBUSY);

		mutex_enter(&ds->ds_lock);
		/*
		 * Can't delete a branch point. However, if we're destroying
		 * a clone and removing its origin due to it having a user
		 * hold count of 0 and having been marked for deferred destroy,
		 * it's OK for the origin to have a single clone.
		 */
		if (ds->ds_phys->ds_num_children >
		    (dsda->is_origin_rm ? 2 : 1)) {
			mutex_exit(&ds->ds_lock);
			return (EEXIST);
		}
		mutex_exit(&ds->ds_lock);
	} else if (dsl_dir_is_clone(ds->ds_dir)) {
		return (dsl_dataset_origin_check(dsda, arg2, tx));
	}

	/* XXX we should do some i/o error checking... */
	return (0);
}

struct refsarg {
	kmutex_t lock;
	boolean_t gone;
	kcondvar_t cv;
};

/* ARGSUSED */
static void
dsl_dataset_refs_gone(dmu_buf_t *db, void *argv)
{
	struct refsarg *arg = argv;

	mutex_enter(&arg->lock);
	arg->gone = TRUE;
	cv_signal(&arg->cv);
	mutex_exit(&arg->lock);
}

static void
dsl_dataset_drain_refs(dsl_dataset_t *ds, void *tag)
{
	struct refsarg arg;

	mutex_init(&arg.lock, NULL, MUTEX_DEFAULT, NULL);
	cv_init(&arg.cv, NULL, CV_DEFAULT, NULL);
	arg.gone = FALSE;
	(void) dmu_buf_update_user(ds->ds_dbuf, ds, &arg, &ds->ds_phys,
	    dsl_dataset_refs_gone);
	dmu_buf_rele(ds->ds_dbuf, tag);
	mutex_enter(&arg.lock);
	while (!arg.gone)
		cv_wait(&arg.cv, &arg.lock);
	ASSERT(arg.gone);
	mutex_exit(&arg.lock);
	ds->ds_dbuf = NULL;
	ds->ds_phys = NULL;
	mutex_destroy(&arg.lock);
	cv_destroy(&arg.cv);
}

static void
remove_from_next_clones(dsl_dataset_t *ds, uint64_t obj, dmu_tx_t *tx)
{
	objset_t *mos = ds->ds_dir->dd_pool->dp_meta_objset;
	uint64_t count;
	int err;

	ASSERT(ds->ds_phys->ds_num_children >= 2);
	err = zap_remove_int(mos, ds->ds_phys->ds_next_clones_obj, obj, tx);
	/*
	 * The err should not be ENOENT, but a bug in a previous version
	 * of the code could cause upgrade_clones_cb() to not set
	 * ds_next_snap_obj when it should, leading to a missing entry.
	 * If we knew that the pool was created after
	 * SPA_VERSION_NEXT_CLONES, we could assert that it isn't
	 * ENOENT.  However, at least we can check that we don't have
	 * too many entries in the next_clones_obj even after failing to
	 * remove this one.
	 */
	if (err != ENOENT) {
		VERIFY3U(err, ==, 0);
	}
	ASSERT3U(0, ==, zap_count(mos, ds->ds_phys->ds_next_clones_obj,
	    &count));
	ASSERT3U(count, <=, ds->ds_phys->ds_num_children - 2);
}

static void
dsl_dataset_remove_clones_key(dsl_dataset_t *ds, uint64_t mintxg, dmu_tx_t *tx)
{
	objset_t *mos = ds->ds_dir->dd_pool->dp_meta_objset;
	zap_cursor_t zc;
	zap_attribute_t za;

	/*
	 * If it is the old version, dd_clones doesn't exist so we can't
	 * find the clones, but deadlist_remove_key() is a no-op so it
	 * doesn't matter.
	 */
	if (ds->ds_dir->dd_phys->dd_clones == 0)
		return;

	for (zap_cursor_init(&zc, mos, ds->ds_dir->dd_phys->dd_clones);
	    zap_cursor_retrieve(&zc, &za) == 0;
	    zap_cursor_advance(&zc)) {
		dsl_dataset_t *clone;

		VERIFY3U(0, ==, dsl_dataset_hold_obj(ds->ds_dir->dd_pool,
		    za.za_first_integer, FTAG, &clone));
		if (clone->ds_dir->dd_origin_txg > mintxg) {
			dsl_deadlist_remove_key(&clone->ds_deadlist,
			    mintxg, tx);
			dsl_dataset_remove_clones_key(clone, mintxg, tx);
		}
		dsl_dataset_rele(clone, FTAG);
	}
	zap_cursor_fini(&zc);
}

struct process_old_arg {
	dsl_dataset_t *ds;
	dsl_dataset_t *ds_prev;
	boolean_t after_branch_point;
	zio_t *pio;
	uint64_t used, comp, uncomp;
};

static int
process_old_cb(void *arg, const blkptr_t *bp, dmu_tx_t *tx)
{
	struct process_old_arg *poa = arg;
	dsl_pool_t *dp = poa->ds->ds_dir->dd_pool;

	if (bp->blk_birth <= poa->ds->ds_phys->ds_prev_snap_txg) {
		dsl_deadlist_insert(&poa->ds->ds_deadlist, bp, tx);
		if (poa->ds_prev && !poa->after_branch_point &&
		    bp->blk_birth >
		    poa->ds_prev->ds_phys->ds_prev_snap_txg) {
			poa->ds_prev->ds_phys->ds_unique_bytes +=
			    bp_get_dsize_sync(dp->dp_spa, bp);
		}
	} else {
		poa->used += bp_get_dsize_sync(dp->dp_spa, bp);
		poa->comp += BP_GET_PSIZE(bp);
		poa->uncomp += BP_GET_UCSIZE(bp);
		dsl_free_sync(poa->pio, dp, tx->tx_txg, bp);
	}
	return (0);
}

static void
process_old_deadlist(dsl_dataset_t *ds, dsl_dataset_t *ds_prev,
    dsl_dataset_t *ds_next, boolean_t after_branch_point, dmu_tx_t *tx)
{
	struct process_old_arg poa = { 0 };
	dsl_pool_t *dp = ds->ds_dir->dd_pool;
	objset_t *mos = dp->dp_meta_objset;

	ASSERT(ds->ds_deadlist.dl_oldfmt);
	ASSERT(ds_next->ds_deadlist.dl_oldfmt);

	poa.ds = ds;
	poa.ds_prev = ds_prev;
	poa.after_branch_point = after_branch_point;
	poa.pio = zio_root(dp->dp_spa, NULL, NULL, ZIO_FLAG_MUSTSUCCEED);
	VERIFY3U(0, ==, bpobj_iterate(&ds_next->ds_deadlist.dl_bpobj,
	    process_old_cb, &poa, tx));
	VERIFY3U(zio_wait(poa.pio), ==, 0);
	ASSERT3U(poa.used, ==, ds->ds_phys->ds_unique_bytes);

	/* change snapused */
	dsl_dir_diduse_space(ds->ds_dir, DD_USED_SNAP,
	    -poa.used, -poa.comp, -poa.uncomp, tx);

	/* swap next's deadlist to our deadlist */
	dsl_deadlist_close(&ds->ds_deadlist);
	dsl_deadlist_close(&ds_next->ds_deadlist);
	SWITCH64(ds_next->ds_phys->ds_deadlist_obj,
	    ds->ds_phys->ds_deadlist_obj);
	dsl_deadlist_open(&ds->ds_deadlist, mos, ds->ds_phys->ds_deadlist_obj);
	dsl_deadlist_open(&ds_next->ds_deadlist, mos,
	    ds_next->ds_phys->ds_deadlist_obj);
}

void
dsl_dataset_destroy_sync(void *arg1, void *tag, dmu_tx_t *tx)
{
	struct dsl_ds_destroyarg *dsda = arg1;
	dsl_dataset_t *ds = dsda->ds;
	int err;
	int after_branch_point = FALSE;
	dsl_pool_t *dp = ds->ds_dir->dd_pool;
	objset_t *mos = dp->dp_meta_objset;
	dsl_dataset_t *ds_prev = NULL;
	boolean_t wont_destroy;
	uint64_t obj;

	wont_destroy = (dsda->defer &&
	    (ds->ds_userrefs > 0 || ds->ds_phys->ds_num_children > 1));

	ASSERT(ds->ds_owner || wont_destroy);
	ASSERT(dsda->defer || ds->ds_phys->ds_num_children <= 1);
	ASSERT(ds->ds_prev == NULL ||
	    ds->ds_prev->ds_phys->ds_next_snap_obj != ds->ds_object);
	ASSERT3U(ds->ds_phys->ds_bp.blk_birth, <=, tx->tx_txg);

	if (wont_destroy) {
		ASSERT(spa_version(dp->dp_spa) >= SPA_VERSION_USERREFS);
		dmu_buf_will_dirty(ds->ds_dbuf, tx);
		ds->ds_phys->ds_flags |= DS_FLAG_DEFER_DESTROY;
		return;
	}

	/* signal any waiters that this dataset is going away */
	mutex_enter(&ds->ds_lock);
	ds->ds_owner = dsl_reaper;
	cv_broadcast(&ds->ds_exclusive_cv);
	mutex_exit(&ds->ds_lock);

	/* Remove our reservation */
	if (ds->ds_reserved != 0) {
		dsl_prop_setarg_t psa;
		uint64_t value = 0;

		dsl_prop_setarg_init_uint64(&psa, "refreservation",
		    (ZPROP_SRC_NONE | ZPROP_SRC_LOCAL | ZPROP_SRC_RECEIVED),
		    &value);
		psa.psa_effective_value = 0;	/* predict default value */

		dsl_dataset_set_reservation_sync(ds, &psa, tx);
		ASSERT3U(ds->ds_reserved, ==, 0);
	}

	ASSERT(RW_WRITE_HELD(&dp->dp_config_rwlock));

	dsl_scan_ds_destroyed(ds, tx);

	obj = ds->ds_object;

	if (ds->ds_phys->ds_prev_snap_obj != 0) {
		if (ds->ds_prev) {
			ds_prev = ds->ds_prev;
		} else {
			VERIFY(0 == dsl_dataset_hold_obj(dp,
			    ds->ds_phys->ds_prev_snap_obj, FTAG, &ds_prev));
		}
		after_branch_point =
		    (ds_prev->ds_phys->ds_next_snap_obj != obj);

		dmu_buf_will_dirty(ds_prev->ds_dbuf, tx);
		if (after_branch_point &&
		    ds_prev->ds_phys->ds_next_clones_obj != 0) {
			remove_from_next_clones(ds_prev, obj, tx);
			if (ds->ds_phys->ds_next_snap_obj != 0) {
				VERIFY(0 == zap_add_int(mos,
				    ds_prev->ds_phys->ds_next_clones_obj,
				    ds->ds_phys->ds_next_snap_obj, tx));
			}
		}
		if (after_branch_point &&
		    ds->ds_phys->ds_next_snap_obj == 0) {
			/* This clone is toast. */
			ASSERT(ds_prev->ds_phys->ds_num_children > 1);
			ds_prev->ds_phys->ds_num_children--;

			/*
			 * If the clone's origin has no other clones, no
			 * user holds, and has been marked for deferred
			 * deletion, then we should have done the necessary
			 * destroy setup for it.
			 */
			if (ds_prev->ds_phys->ds_num_children == 1 &&
			    ds_prev->ds_userrefs == 0 &&
			    DS_IS_DEFER_DESTROY(ds_prev)) {
				ASSERT3P(dsda->rm_origin, !=, NULL);
			} else {
				ASSERT3P(dsda->rm_origin, ==, NULL);
			}
		} else if (!after_branch_point) {
			ds_prev->ds_phys->ds_next_snap_obj =
			    ds->ds_phys->ds_next_snap_obj;
		}
	}

	if (dsl_dataset_is_snapshot(ds)) {
		dsl_dataset_t *ds_next;
		uint64_t old_unique;
		uint64_t used = 0, comp = 0, uncomp = 0;

		VERIFY(0 == dsl_dataset_hold_obj(dp,
		    ds->ds_phys->ds_next_snap_obj, FTAG, &ds_next));
		ASSERT3U(ds_next->ds_phys->ds_prev_snap_obj, ==, obj);

		old_unique = ds_next->ds_phys->ds_unique_bytes;

		dmu_buf_will_dirty(ds_next->ds_dbuf, tx);
		ds_next->ds_phys->ds_prev_snap_obj =
		    ds->ds_phys->ds_prev_snap_obj;
		ds_next->ds_phys->ds_prev_snap_txg =
		    ds->ds_phys->ds_prev_snap_txg;
		ASSERT3U(ds->ds_phys->ds_prev_snap_txg, ==,
		    ds_prev ? ds_prev->ds_phys->ds_creation_txg : 0);


		if (ds_next->ds_deadlist.dl_oldfmt) {
			process_old_deadlist(ds, ds_prev, ds_next,
			    after_branch_point, tx);
		} else {
			/* Adjust prev's unique space. */
			if (ds_prev && !after_branch_point) {
				dsl_deadlist_space_range(&ds_next->ds_deadlist,
				    ds_prev->ds_phys->ds_prev_snap_txg,
				    ds->ds_phys->ds_prev_snap_txg,
				    &used, &comp, &uncomp);
				ds_prev->ds_phys->ds_unique_bytes += used;
			}

			/* Adjust snapused. */
			dsl_deadlist_space_range(&ds_next->ds_deadlist,
			    ds->ds_phys->ds_prev_snap_txg, UINT64_MAX,
			    &used, &comp, &uncomp);
			dsl_dir_diduse_space(ds->ds_dir, DD_USED_SNAP,
			    -used, -comp, -uncomp, tx);

			/* Move blocks to be freed to pool's free list. */
			dsl_deadlist_move_bpobj(&ds_next->ds_deadlist,
			    &dp->dp_free_bpobj, ds->ds_phys->ds_prev_snap_txg,
			    tx);
			dsl_dir_diduse_space(tx->tx_pool->dp_free_dir,
			    DD_USED_HEAD, used, comp, uncomp, tx);
			dsl_dir_dirty(tx->tx_pool->dp_free_dir, tx);

			/* Merge our deadlist into next's and free it. */
			dsl_deadlist_merge(&ds_next->ds_deadlist,
			    ds->ds_phys->ds_deadlist_obj, tx);
		}
		dsl_deadlist_close(&ds->ds_deadlist);
		dsl_deadlist_free(mos, ds->ds_phys->ds_deadlist_obj, tx);

		/* Collapse range in clone heads */
		dsl_dataset_remove_clones_key(ds,
		    ds->ds_phys->ds_creation_txg, tx);

		if (dsl_dataset_is_snapshot(ds_next)) {
			dsl_dataset_t *ds_nextnext;

			/*
			 * Update next's unique to include blocks which
			 * were previously shared by only this snapshot
			 * and it.  Those blocks will be born after the
			 * prev snap and before this snap, and will have
			 * died after the next snap and before the one
			 * after that (ie. be on the snap after next's
			 * deadlist).
			 */
			VERIFY(0 == dsl_dataset_hold_obj(dp,
			    ds_next->ds_phys->ds_next_snap_obj,
			    FTAG, &ds_nextnext));
			dsl_deadlist_space_range(&ds_nextnext->ds_deadlist,
			    ds->ds_phys->ds_prev_snap_txg,
			    ds->ds_phys->ds_creation_txg,
			    &used, &comp, &uncomp);
			ds_next->ds_phys->ds_unique_bytes += used;
			dsl_dataset_rele(ds_nextnext, FTAG);
			ASSERT3P(ds_next->ds_prev, ==, NULL);

			/* Collapse range in this head. */
			dsl_dataset_t *hds;
			VERIFY3U(0, ==, dsl_dataset_hold_obj(dp,
			    ds->ds_dir->dd_phys->dd_head_dataset_obj,
			    FTAG, &hds));
			dsl_deadlist_remove_key(&hds->ds_deadlist,
			    ds->ds_phys->ds_creation_txg, tx);
			dsl_dataset_rele(hds, FTAG);

		} else {
			ASSERT3P(ds_next->ds_prev, ==, ds);
			dsl_dataset_drop_ref(ds_next->ds_prev, ds_next);
			ds_next->ds_prev = NULL;
			if (ds_prev) {
				VERIFY(0 == dsl_dataset_get_ref(dp,
				    ds->ds_phys->ds_prev_snap_obj,
				    ds_next, &ds_next->ds_prev));
			}

			dsl_dataset_recalc_head_uniq(ds_next);

			/*
			 * Reduce the amount of our unconsmed refreservation
			 * being charged to our parent by the amount of
			 * new unique data we have gained.
			 */
			if (old_unique < ds_next->ds_reserved) {
				int64_t mrsdelta;
				uint64_t new_unique =
				    ds_next->ds_phys->ds_unique_bytes;

				ASSERT(old_unique <= new_unique);
				mrsdelta = MIN(new_unique - old_unique,
				    ds_next->ds_reserved - old_unique);
				dsl_dir_diduse_space(ds->ds_dir,
				    DD_USED_REFRSRV, -mrsdelta, 0, 0, tx);
			}
		}
		dsl_dataset_rele(ds_next, FTAG);
	} else {
		/*
		 * There's no next snapshot, so this is a head dataset.
		 * Destroy the deadlist.  Unless it's a clone, the
		 * deadlist should be empty.  (If it's a clone, it's
		 * safe to ignore the deadlist contents.)
		 */
		struct killarg ka;

		dsl_deadlist_close(&ds->ds_deadlist);
		dsl_deadlist_free(mos, ds->ds_phys->ds_deadlist_obj, tx);
		ds->ds_phys->ds_deadlist_obj = 0;

		/*
		 * Free everything that we point to (that's born after
		 * the previous snapshot, if we are a clone)
		 *
		 * NB: this should be very quick, because we already
		 * freed all the objects in open context.
		 */
		ka.ds = ds;
		ka.tx = tx;
		err = traverse_dataset(ds, ds->ds_phys->ds_prev_snap_txg,
		    TRAVERSE_POST, kill_blkptr, &ka);
		ASSERT3U(err, ==, 0);
		ASSERT(!DS_UNIQUE_IS_ACCURATE(ds) ||
		    ds->ds_phys->ds_unique_bytes == 0);

		if (ds->ds_prev != NULL) {
			if (spa_version(dp->dp_spa) >= SPA_VERSION_DIR_CLONES) {
				VERIFY3U(0, ==, zap_remove_int(mos,
				    ds->ds_prev->ds_dir->dd_phys->dd_clones,
				    ds->ds_object, tx));
			}
			dsl_dataset_rele(ds->ds_prev, ds);
			ds->ds_prev = ds_prev = NULL;
		}
	}

	/*
	 * This must be done after the dsl_traverse(), because it will
	 * re-open the objset.
	 */
	if (ds->ds_objset) {
		dmu_objset_evict(ds->ds_objset);
		ds->ds_objset = NULL;
	}

	if (ds->ds_dir->dd_phys->dd_head_dataset_obj == ds->ds_object) {
		/* Erase the link in the dir */
		dmu_buf_will_dirty(ds->ds_dir->dd_dbuf, tx);
		ds->ds_dir->dd_phys->dd_head_dataset_obj = 0;
		ASSERT(ds->ds_phys->ds_snapnames_zapobj != 0);
		err = zap_destroy(mos, ds->ds_phys->ds_snapnames_zapobj, tx);
		ASSERT(err == 0);
	} else {
		/* remove from snapshot namespace */
		dsl_dataset_t *ds_head;
		ASSERT(ds->ds_phys->ds_snapnames_zapobj == 0);
		VERIFY(0 == dsl_dataset_hold_obj(dp,
		    ds->ds_dir->dd_phys->dd_head_dataset_obj, FTAG, &ds_head));
		VERIFY(0 == dsl_dataset_get_snapname(ds));
#ifdef ZFS_DEBUG
		{
			uint64_t val;

			err = dsl_dataset_snap_lookup(ds_head,
			    ds->ds_snapname, &val);
			ASSERT3U(err, ==, 0);
			ASSERT3U(val, ==, obj);
		}
#endif
		err = dsl_dataset_snap_remove(ds_head, ds->ds_snapname, tx);
		ASSERT(err == 0);
		dsl_dataset_rele(ds_head, FTAG);
	}

	if (ds_prev && ds->ds_prev != ds_prev)
		dsl_dataset_rele(ds_prev, FTAG);

	spa_prop_clear_bootfs(dp->dp_spa, ds->ds_object, tx);
	spa_history_log_internal(LOG_DS_DESTROY, dp->dp_spa, tx,
	    "dataset = %llu", ds->ds_object);

	if (ds->ds_phys->ds_next_clones_obj != 0) {
		uint64_t count;
		ASSERT(0 == zap_count(mos,
		    ds->ds_phys->ds_next_clones_obj, &count) && count == 0);
		VERIFY(0 == dmu_object_free(mos,
		    ds->ds_phys->ds_next_clones_obj, tx));
	}
	if (ds->ds_phys->ds_props_obj != 0)
		VERIFY(0 == zap_destroy(mos, ds->ds_phys->ds_props_obj, tx));
	if (ds->ds_phys->ds_userrefs_obj != 0)
		VERIFY(0 == zap_destroy(mos, ds->ds_phys->ds_userrefs_obj, tx));
	dsl_dir_close(ds->ds_dir, ds);
	ds->ds_dir = NULL;
	dsl_dataset_drain_refs(ds, tag);
	VERIFY(0 == dmu_object_free(mos, obj, tx));

	if (dsda->rm_origin) {
		/*
		 * Remove the origin of the clone we just destroyed.
		 */
		struct dsl_ds_destroyarg ndsda = {0};

		ndsda.ds = dsda->rm_origin;
		dsl_dataset_destroy_sync(&ndsda, tag, tx);
	}
}

static int
dsl_dataset_snapshot_reserve_space(dsl_dataset_t *ds, dmu_tx_t *tx)
{
	uint64_t asize;

	if (!dmu_tx_is_syncing(tx))
		return (0);

	/*
	 * If there's an fs-only reservation, any blocks that might become
	 * owned by the snapshot dataset must be accommodated by space
	 * outside of the reservation.
	 */
	ASSERT(ds->ds_reserved == 0 || DS_UNIQUE_IS_ACCURATE(ds));
	asize = MIN(ds->ds_phys->ds_unique_bytes, ds->ds_reserved);
	if (asize > dsl_dir_space_available(ds->ds_dir, NULL, 0, TRUE))
		return (ENOSPC);

	/*
	 * Propogate any reserved space for this snapshot to other
	 * snapshot checks in this sync group.
	 */
	if (asize > 0)
		dsl_dir_willuse_space(ds->ds_dir, asize, tx);

	return (0);
}

int
dsl_dataset_snapshot_check(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	const char *snapname = arg2;
	int err;
	uint64_t value;

	/*
	 * We don't allow multiple snapshots of the same txg.  If there
	 * is already one, try again.
	 */
	if (ds->ds_phys->ds_prev_snap_txg >= tx->tx_txg)
		return (EAGAIN);

	/*
	 * Check for conflicting name snapshot name.
	 */
	err = dsl_dataset_snap_lookup(ds, snapname, &value);
	if (err == 0)
		return (EEXIST);
	if (err != ENOENT)
		return (err);

	/*
	 * Check that the dataset's name is not too long.  Name consists
	 * of the dataset's length + 1 for the @-sign + snapshot name's length
	 */
	if (dsl_dataset_namelen(ds) + 1 + strlen(snapname) >= MAXNAMELEN)
		return (ENAMETOOLONG);

	err = dsl_dataset_snapshot_reserve_space(ds, tx);
	if (err)
		return (err);

	ds->ds_trysnap_txg = tx->tx_txg;
	return (0);
}

void
dsl_dataset_snapshot_sync(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	const char *snapname = arg2;
	dsl_pool_t *dp = ds->ds_dir->dd_pool;
	dmu_buf_t *dbuf;
	dsl_dataset_phys_t *dsphys;
	uint64_t dsobj, crtxg;
	objset_t *mos = dp->dp_meta_objset;
	int err;

	ASSERT(RW_WRITE_HELD(&dp->dp_config_rwlock));

	/*
	 * The origin's ds_creation_txg has to be < TXG_INITIAL
	 */
	if (strcmp(snapname, ORIGIN_DIR_NAME) == 0)
		crtxg = 1;
	else
		crtxg = tx->tx_txg;

	dsobj = dmu_object_alloc(mos, DMU_OT_DSL_DATASET, 0,
	    DMU_OT_DSL_DATASET, sizeof (dsl_dataset_phys_t), tx);
	VERIFY(0 == dmu_bonus_hold(mos, dsobj, FTAG, &dbuf));
	dmu_buf_will_dirty(dbuf, tx);
	dsphys = dbuf->db_data;
	bzero(dsphys, sizeof (dsl_dataset_phys_t));
	dsphys->ds_dir_obj = ds->ds_dir->dd_object;
	dsphys->ds_fsid_guid = unique_create();
	(void) random_get_pseudo_bytes((void*)&dsphys->ds_guid,
	    sizeof (dsphys->ds_guid));
	dsphys->ds_prev_snap_obj = ds->ds_phys->ds_prev_snap_obj;
	dsphys->ds_prev_snap_txg = ds->ds_phys->ds_prev_snap_txg;
	dsphys->ds_next_snap_obj = ds->ds_object;
	dsphys->ds_num_children = 1;
	dsphys->ds_creation_time = gethrestime_sec();
	dsphys->ds_creation_txg = crtxg;
	dsphys->ds_deadlist_obj = ds->ds_phys->ds_deadlist_obj;
	dsphys->ds_used_bytes = ds->ds_phys->ds_used_bytes;
	dsphys->ds_compressed_bytes = ds->ds_phys->ds_compressed_bytes;
	dsphys->ds_uncompressed_bytes = ds->ds_phys->ds_uncompressed_bytes;
	dsphys->ds_flags = ds->ds_phys->ds_flags;
	dsphys->ds_bp = ds->ds_phys->ds_bp;
	dmu_buf_rele(dbuf, FTAG);

	ASSERT3U(ds->ds_prev != 0, ==, ds->ds_phys->ds_prev_snap_obj != 0);
	if (ds->ds_prev) {
		uint64_t next_clones_obj =
		    ds->ds_prev->ds_phys->ds_next_clones_obj;
		ASSERT(ds->ds_prev->ds_phys->ds_next_snap_obj ==
		    ds->ds_object ||
		    ds->ds_prev->ds_phys->ds_num_children > 1);
		if (ds->ds_prev->ds_phys->ds_next_snap_obj == ds->ds_object) {
			dmu_buf_will_dirty(ds->ds_prev->ds_dbuf, tx);
			ASSERT3U(ds->ds_phys->ds_prev_snap_txg, ==,
			    ds->ds_prev->ds_phys->ds_creation_txg);
			ds->ds_prev->ds_phys->ds_next_snap_obj = dsobj;
		} else if (next_clones_obj != 0) {
			remove_from_next_clones(ds->ds_prev,
			    dsphys->ds_next_snap_obj, tx);
			VERIFY3U(0, ==, zap_add_int(mos,
			    next_clones_obj, dsobj, tx));
		}
	}

	/*
	 * If we have a reference-reservation on this dataset, we will
	 * need to increase the amount of refreservation being charged
	 * since our unique space is going to zero.
	 */
	if (ds->ds_reserved) {
		int64_t delta;
		ASSERT(DS_UNIQUE_IS_ACCURATE(ds));
		delta = MIN(ds->ds_phys->ds_unique_bytes, ds->ds_reserved);
		dsl_dir_diduse_space(ds->ds_dir, DD_USED_REFRSRV,
		    delta, 0, 0, tx);
	}

	dmu_buf_will_dirty(ds->ds_dbuf, tx);
	zfs_dbgmsg("taking snapshot %s@%s/%llu; newkey=%llu",
	    ds->ds_dir->dd_myname, snapname, dsobj,
	    ds->ds_phys->ds_prev_snap_txg);
	ds->ds_phys->ds_deadlist_obj = dsl_deadlist_clone(&ds->ds_deadlist,
	    UINT64_MAX, ds->ds_phys->ds_prev_snap_obj, tx);
	dsl_deadlist_close(&ds->ds_deadlist);
	dsl_deadlist_open(&ds->ds_deadlist, mos, ds->ds_phys->ds_deadlist_obj);
	dsl_deadlist_add_key(&ds->ds_deadlist,
	    ds->ds_phys->ds_prev_snap_txg, tx);

	ASSERT3U(ds->ds_phys->ds_prev_snap_txg, <, tx->tx_txg);
	ds->ds_phys->ds_prev_snap_obj = dsobj;
	ds->ds_phys->ds_prev_snap_txg = crtxg;
	ds->ds_phys->ds_unique_bytes = 0;
	if (spa_version(dp->dp_spa) >= SPA_VERSION_UNIQUE_ACCURATE)
		ds->ds_phys->ds_flags |= DS_FLAG_UNIQUE_ACCURATE;

	err = zap_add(mos, ds->ds_phys->ds_snapnames_zapobj,
	    snapname, 8, 1, &dsobj, tx);
	ASSERT(err == 0);

	if (ds->ds_prev)
		dsl_dataset_drop_ref(ds->ds_prev, ds);
	VERIFY(0 == dsl_dataset_get_ref(dp,
	    ds->ds_phys->ds_prev_snap_obj, ds, &ds->ds_prev));

	dsl_scan_ds_snapshotted(ds, tx);

	dsl_dir_snap_cmtime_update(ds->ds_dir);

	spa_history_log_internal(LOG_DS_SNAPSHOT, dp->dp_spa, tx,
	    "dataset = %llu", dsobj);
}

void
dsl_dataset_sync(dsl_dataset_t *ds, zio_t *zio, dmu_tx_t *tx)
{
	ASSERT(dmu_tx_is_syncing(tx));
	ASSERT(ds->ds_objset != NULL);
	ASSERT(ds->ds_phys->ds_next_snap_obj == 0);

	/*
	 * in case we had to change ds_fsid_guid when we opened it,
	 * sync it out now.
	 */
	dmu_buf_will_dirty(ds->ds_dbuf, tx);
	ds->ds_phys->ds_fsid_guid = ds->ds_fsid_guid;

	dsl_dir_dirty(ds->ds_dir, tx);
	dmu_objset_sync(ds->ds_objset, zio, tx);
}

void
dsl_dataset_stats(dsl_dataset_t *ds, nvlist_t *nv)
{
	uint64_t refd, avail, uobjs, aobjs, ratio;

	dsl_dir_stats(ds->ds_dir, nv);

	dsl_dataset_space(ds, &refd, &avail, &uobjs, &aobjs);
	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_AVAILABLE, avail);
	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_REFERENCED, refd);

	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_CREATION,
	    ds->ds_phys->ds_creation_time);
	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_CREATETXG,
	    ds->ds_phys->ds_creation_txg);
	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_REFQUOTA,
	    ds->ds_quota);
	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_REFRESERVATION,
	    ds->ds_reserved);
	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_GUID,
	    ds->ds_phys->ds_guid);
	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_UNIQUE,
	    ds->ds_phys->ds_unique_bytes);
	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_OBJSETID,
	    ds->ds_object);
	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_USERREFS,
	    ds->ds_userrefs);
	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_DEFER_DESTROY,
	    DS_IS_DEFER_DESTROY(ds) ? 1 : 0);

	ratio = ds->ds_phys->ds_compressed_bytes == 0 ? 100 :
	    (ds->ds_phys->ds_uncompressed_bytes * 100 /
	    ds->ds_phys->ds_compressed_bytes);
	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_REFRATIO, ratio);

	if (ds->ds_phys->ds_next_snap_obj) {
		/*
		 * This is a snapshot; override the dd's space used with
		 * our unique space and compression ratio.
		 */
		dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_USED,
		    ds->ds_phys->ds_unique_bytes);
		dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_COMPRESSRATIO, ratio);
	}
}

void
dsl_dataset_fast_stat(dsl_dataset_t *ds, dmu_objset_stats_t *stat)
{
	stat->dds_creation_txg = ds->ds_phys->ds_creation_txg;
	stat->dds_inconsistent = ds->ds_phys->ds_flags & DS_FLAG_INCONSISTENT;
	stat->dds_guid = ds->ds_phys->ds_guid;
	if (ds->ds_phys->ds_next_snap_obj) {
		stat->dds_is_snapshot = B_TRUE;
		stat->dds_num_clones = ds->ds_phys->ds_num_children - 1;
	} else {
		stat->dds_is_snapshot = B_FALSE;
		stat->dds_num_clones = 0;
	}

	/* clone origin is really a dsl_dir thing... */
	rw_enter(&ds->ds_dir->dd_pool->dp_config_rwlock, RW_READER);
	if (dsl_dir_is_clone(ds->ds_dir)) {
		dsl_dataset_t *ods;

		VERIFY(0 == dsl_dataset_get_ref(ds->ds_dir->dd_pool,
		    ds->ds_dir->dd_phys->dd_origin_obj, FTAG, &ods));
		dsl_dataset_name(ods, stat->dds_origin);
		dsl_dataset_drop_ref(ods, FTAG);
	} else {
		stat->dds_origin[0] = '\0';
	}
	rw_exit(&ds->ds_dir->dd_pool->dp_config_rwlock);
}

uint64_t
dsl_dataset_fsid_guid(dsl_dataset_t *ds)
{
	return (ds->ds_fsid_guid);
}

void
dsl_dataset_space(dsl_dataset_t *ds,
    uint64_t *refdbytesp, uint64_t *availbytesp,
    uint64_t *usedobjsp, uint64_t *availobjsp)
{
	*refdbytesp = ds->ds_phys->ds_used_bytes;
	*availbytesp = dsl_dir_space_available(ds->ds_dir, NULL, 0, TRUE);
	if (ds->ds_reserved > ds->ds_phys->ds_unique_bytes)
		*availbytesp += ds->ds_reserved - ds->ds_phys->ds_unique_bytes;
	if (ds->ds_quota != 0) {
		/*
		 * Adjust available bytes according to refquota
		 */
		if (*refdbytesp < ds->ds_quota)
			*availbytesp = MIN(*availbytesp,
			    ds->ds_quota - *refdbytesp);
		else
			*availbytesp = 0;
	}
	*usedobjsp = ds->ds_phys->ds_bp.blk_fill;
	*availobjsp = DN_MAX_OBJECT - *usedobjsp;
}

boolean_t
dsl_dataset_modified_since_lastsnap(dsl_dataset_t *ds)
{
	dsl_pool_t *dp = ds->ds_dir->dd_pool;

	ASSERT(RW_LOCK_HELD(&dp->dp_config_rwlock) ||
	    dsl_pool_sync_context(dp));
	if (ds->ds_prev == NULL)
		return (B_FALSE);
	if (ds->ds_phys->ds_bp.blk_birth >
	    ds->ds_prev->ds_phys->ds_creation_txg) {
		objset_t *os, *os_prev;
		/*
		 * It may be that only the ZIL differs, because it was
		 * reset in the head.  Don't count that as being
		 * modified.
		 */
		if (dmu_objset_from_ds(ds, &os) != 0)
			return (B_TRUE);
		if (dmu_objset_from_ds(ds->ds_prev, &os_prev) != 0)
			return (B_TRUE);
		return (bcmp(&os->os_phys->os_meta_dnode,
		    &os_prev->os_phys->os_meta_dnode,
		    sizeof (os->os_phys->os_meta_dnode)) != 0);
	}
	return (B_FALSE);
}

/* ARGSUSED */
static int
dsl_dataset_snapshot_rename_check(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	char *newsnapname = arg2;
	dsl_dir_t *dd = ds->ds_dir;
	dsl_dataset_t *hds;
	uint64_t val;
	int err;

	err = dsl_dataset_hold_obj(dd->dd_pool,
	    dd->dd_phys->dd_head_dataset_obj, FTAG, &hds);
	if (err)
		return (err);

	/* new name better not be in use */
	err = dsl_dataset_snap_lookup(hds, newsnapname, &val);
	dsl_dataset_rele(hds, FTAG);

	if (err == 0)
		err = EEXIST;
	else if (err == ENOENT)
		err = 0;

	/* dataset name + 1 for the "@" + the new snapshot name must fit */
	if (dsl_dir_namelen(ds->ds_dir) + 1 + strlen(newsnapname) >= MAXNAMELEN)
		err = ENAMETOOLONG;

	return (err);
}

static void
dsl_dataset_snapshot_rename_sync(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	const char *newsnapname = arg2;
	dsl_dir_t *dd = ds->ds_dir;
	objset_t *mos = dd->dd_pool->dp_meta_objset;
	dsl_dataset_t *hds;
	int err;

	ASSERT(ds->ds_phys->ds_next_snap_obj != 0);

	VERIFY(0 == dsl_dataset_hold_obj(dd->dd_pool,
	    dd->dd_phys->dd_head_dataset_obj, FTAG, &hds));

	VERIFY(0 == dsl_dataset_get_snapname(ds));
	err = dsl_dataset_snap_remove(hds, ds->ds_snapname, tx);
	ASSERT3U(err, ==, 0);
	mutex_enter(&ds->ds_lock);
	(void) strcpy(ds->ds_snapname, newsnapname);
	mutex_exit(&ds->ds_lock);
	err = zap_add(mos, hds->ds_phys->ds_snapnames_zapobj,
	    ds->ds_snapname, 8, 1, &ds->ds_object, tx);
	ASSERT3U(err, ==, 0);

	spa_history_log_internal(LOG_DS_RENAME, dd->dd_pool->dp_spa, tx,
	    "dataset = %llu", ds->ds_object);
	dsl_dataset_rele(hds, FTAG);
}

struct renamesnaparg {
	dsl_sync_task_group_t *dstg;
	char failed[MAXPATHLEN];
	char *oldsnap;
	char *newsnap;
};

static int
dsl_snapshot_rename_one(const char *name, void *arg)
{
	struct renamesnaparg *ra = arg;
	dsl_dataset_t *ds = NULL;
	char *snapname;
	int err;

	snapname = kmem_asprintf("%s@%s", name, ra->oldsnap);
	(void) strlcpy(ra->failed, snapname, sizeof (ra->failed));

	/*
	 * For recursive snapshot renames the parent won't be changing
	 * so we just pass name for both the to/from argument.
	 */
	err = zfs_secpolicy_rename_perms(snapname, snapname, CRED());
	if (err != 0) {
		strfree(snapname);
		return (err == ENOENT ? 0 : err);
	}

#ifdef _KERNEL
	/*
	 * For all filesystems undergoing rename, we'll need to unmount it.
	 */
	(void) zfs_unmount_snap(snapname, NULL);
#endif
	err = dsl_dataset_hold(snapname, ra->dstg, &ds);
	strfree(snapname);
	if (err != 0)
		return (err == ENOENT ? 0 : err);

	dsl_sync_task_create(ra->dstg, dsl_dataset_snapshot_rename_check,
	    dsl_dataset_snapshot_rename_sync, ds, ra->newsnap, 0);

	return (0);
}

static int
dsl_recursive_rename(char *oldname, const char *newname)
{
	int err;
	struct renamesnaparg *ra;
	dsl_sync_task_t *dst;
	spa_t *spa;
	char *cp, *fsname = spa_strdup(oldname);
	int len = strlen(oldname) + 1;

	/* truncate the snapshot name to get the fsname */
	cp = strchr(fsname, '@');
	*cp = '\0';

	err = spa_open(fsname, &spa, FTAG);
	if (err) {
		kmem_free(fsname, len);
		return (err);
	}
	ra = kmem_alloc(sizeof (struct renamesnaparg), KM_SLEEP);
	ra->dstg = dsl_sync_task_group_create(spa_get_dsl(spa));

	ra->oldsnap = strchr(oldname, '@') + 1;
	ra->newsnap = strchr(newname, '@') + 1;
	*ra->failed = '\0';

	err = dmu_objset_find(fsname, dsl_snapshot_rename_one, ra,
	    DS_FIND_CHILDREN);
	kmem_free(fsname, len);

	if (err == 0) {
		err = dsl_sync_task_group_wait(ra->dstg);
	}

	for (dst = list_head(&ra->dstg->dstg_tasks); dst;
	    dst = list_next(&ra->dstg->dstg_tasks, dst)) {
		dsl_dataset_t *ds = dst->dst_arg1;
		if (dst->dst_err) {
			dsl_dir_name(ds->ds_dir, ra->failed);
			(void) strlcat(ra->failed, "@", sizeof (ra->failed));
			(void) strlcat(ra->failed, ra->newsnap,
			    sizeof (ra->failed));
		}
		dsl_dataset_rele(ds, ra->dstg);
	}

	if (err)
		(void) strlcpy(oldname, ra->failed, sizeof (ra->failed));

	dsl_sync_task_group_destroy(ra->dstg);
	kmem_free(ra, sizeof (struct renamesnaparg));
	spa_close(spa, FTAG);
	return (err);
}

static int
dsl_valid_rename(const char *oldname, void *arg)
{
	int delta = *(int *)arg;

	if (strlen(oldname) + delta >= MAXNAMELEN)
		return (ENAMETOOLONG);

	return (0);
}

#pragma weak dmu_objset_rename = dsl_dataset_rename
int
dsl_dataset_rename(char *oldname, const char *newname, boolean_t recursive)
{
	dsl_dir_t *dd;
	dsl_dataset_t *ds;
	const char *tail;
	int err;

	err = dsl_dir_open(oldname, FTAG, &dd, &tail);
	if (err)
		return (err);

	if (tail == NULL) {
		int delta = strlen(newname) - strlen(oldname);

		/* if we're growing, validate child name lengths */
		if (delta > 0)
			err = dmu_objset_find(oldname, dsl_valid_rename,
			    &delta, DS_FIND_CHILDREN | DS_FIND_SNAPSHOTS);

		if (err == 0)
			err = dsl_dir_rename(dd, newname);
		dsl_dir_close(dd, FTAG);
		return (err);
	}

	if (tail[0] != '@') {
		/* the name ended in a nonexistent component */
		dsl_dir_close(dd, FTAG);
		return (ENOENT);
	}

	dsl_dir_close(dd, FTAG);

	/* new name must be snapshot in same filesystem */
	tail = strchr(newname, '@');
	if (tail == NULL)
		return (EINVAL);
	tail++;
	if (strncmp(oldname, newname, tail - newname) != 0)
		return (EXDEV);

	if (recursive) {
		err = dsl_recursive_rename(oldname, newname);
	} else {
		err = dsl_dataset_hold(oldname, FTAG, &ds);
		if (err)
			return (err);

		err = dsl_sync_task_do(ds->ds_dir->dd_pool,
		    dsl_dataset_snapshot_rename_check,
		    dsl_dataset_snapshot_rename_sync, ds, (char *)tail, 1);

		dsl_dataset_rele(ds, FTAG);
	}

	return (err);
}

struct promotenode {
	list_node_t link;
	dsl_dataset_t *ds;
};

struct promotearg {
	list_t shared_snaps, origin_snaps, clone_snaps;
	dsl_dataset_t *origin_origin;
	uint64_t used, comp, uncomp, unique, cloneusedsnap, originusedsnap;
	char *err_ds;
};

static int snaplist_space(list_t *l, uint64_t mintxg, uint64_t *spacep);
static boolean_t snaplist_unstable(list_t *l);

static int
dsl_dataset_promote_check(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *hds = arg1;
	struct promotearg *pa = arg2;
	struct promotenode *snap = list_head(&pa->shared_snaps);
	dsl_dataset_t *origin_ds = snap->ds;
	int err;
	uint64_t unused;

	/* Check that it is a real clone */
	if (!dsl_dir_is_clone(hds->ds_dir))
		return (EINVAL);

	/* Since this is so expensive, don't do the preliminary check */
	if (!dmu_tx_is_syncing(tx))
		return (0);

	if (hds->ds_phys->ds_flags & DS_FLAG_NOPROMOTE)
		return (EXDEV);

	/* compute origin's new unique space */
	snap = list_tail(&pa->clone_snaps);
	ASSERT3U(snap->ds->ds_phys->ds_prev_snap_obj, ==, origin_ds->ds_object);
	dsl_deadlist_space_range(&snap->ds->ds_deadlist,
	    origin_ds->ds_phys->ds_prev_snap_txg, UINT64_MAX,
	    &pa->unique, &unused, &unused);

	/*
	 * Walk the snapshots that we are moving
	 *
	 * Compute space to transfer.  Consider the incremental changes
	 * to used for each snapshot:
	 * (my used) = (prev's used) + (blocks born) - (blocks killed)
	 * So each snapshot gave birth to:
	 * (blocks born) = (my used) - (prev's used) + (blocks killed)
	 * So a sequence would look like:
	 * (uN - u(N-1) + kN) + ... + (u1 - u0 + k1) + (u0 - 0 + k0)
	 * Which simplifies to:
	 * uN + kN + kN-1 + ... + k1 + k0
	 * Note however, if we stop before we reach the ORIGIN we get:
	 * uN + kN + kN-1 + ... + kM - uM-1
	 */
	pa->used = origin_ds->ds_phys->ds_used_bytes;
	pa->comp = origin_ds->ds_phys->ds_compressed_bytes;
	pa->uncomp = origin_ds->ds_phys->ds_uncompressed_bytes;
	for (snap = list_head(&pa->shared_snaps); snap;
	    snap = list_next(&pa->shared_snaps, snap)) {
		uint64_t val, dlused, dlcomp, dluncomp;
		dsl_dataset_t *ds = snap->ds;

		/* Check that the snapshot name does not conflict */
		VERIFY(0 == dsl_dataset_get_snapname(ds));
		err = dsl_dataset_snap_lookup(hds, ds->ds_snapname, &val);
		if (err == 0) {
			err = EEXIST;
			goto out;
		}
		if (err != ENOENT)
			goto out;

		/* The very first snapshot does not have a deadlist */
		if (ds->ds_phys->ds_prev_snap_obj == 0)
			continue;

		dsl_deadlist_space(&ds->ds_deadlist,
		    &dlused, &dlcomp, &dluncomp);
		pa->used += dlused;
		pa->comp += dlcomp;
		pa->uncomp += dluncomp;
	}

	/*
	 * If we are a clone of a clone then we never reached ORIGIN,
	 * so we need to subtract out the clone origin's used space.
	 */
	if (pa->origin_origin) {
		pa->used -= pa->origin_origin->ds_phys->ds_used_bytes;
		pa->comp -= pa->origin_origin->ds_phys->ds_compressed_bytes;
		pa->uncomp -= pa->origin_origin->ds_phys->ds_uncompressed_bytes;
	}

	/* Check that there is enough space here */
	err = dsl_dir_transfer_possible(origin_ds->ds_dir, hds->ds_dir,
	    pa->used);
	if (err)
		return (err);

	/*
	 * Compute the amounts of space that will be used by snapshots
	 * after the promotion (for both origin and clone).  For each,
	 * it is the amount of space that will be on all of their
	 * deadlists (that was not born before their new origin).
	 */
	if (hds->ds_dir->dd_phys->dd_flags & DD_FLAG_USED_BREAKDOWN) {
		uint64_t space;

		/*
		 * Note, typically this will not be a clone of a clone,
		 * so dd_origin_txg will be < TXG_INITIAL, so
		 * these snaplist_space() -> dsl_deadlist_space_range()
		 * calls will be fast because they do not have to
		 * iterate over all bps.
		 */
		snap = list_head(&pa->origin_snaps);
		err = snaplist_space(&pa->shared_snaps,
		    snap->ds->ds_dir->dd_origin_txg, &pa->cloneusedsnap);
		if (err)
			return (err);

		err = snaplist_space(&pa->clone_snaps,
		    snap->ds->ds_dir->dd_origin_txg, &space);
		if (err)
			return (err);
		pa->cloneusedsnap += space;
	}
	if (origin_ds->ds_dir->dd_phys->dd_flags & DD_FLAG_USED_BREAKDOWN) {
		err = snaplist_space(&pa->origin_snaps,
		    origin_ds->ds_phys->ds_creation_txg, &pa->originusedsnap);
		if (err)
			return (err);
	}

	return (0);
out:
	pa->err_ds =  snap->ds->ds_snapname;
	return (err);
}

static void
dsl_dataset_promote_sync(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *hds = arg1;
	struct promotearg *pa = arg2;
	struct promotenode *snap = list_head(&pa->shared_snaps);
	dsl_dataset_t *origin_ds = snap->ds;
	dsl_dataset_t *origin_head;
	dsl_dir_t *dd = hds->ds_dir;
	dsl_pool_t *dp = hds->ds_dir->dd_pool;
	dsl_dir_t *odd = NULL;
	uint64_t oldnext_obj;
	int64_t delta;

	ASSERT(0 == (hds->ds_phys->ds_flags & DS_FLAG_NOPROMOTE));

	snap = list_head(&pa->origin_snaps);
	origin_head = snap->ds;

	/*
	 * We need to explicitly open odd, since origin_ds's dd will be
	 * changing.
	 */
	VERIFY(0 == dsl_dir_open_obj(dp, origin_ds->ds_dir->dd_object,
	    NULL, FTAG, &odd));

	/* change origin's next snap */
	dmu_buf_will_dirty(origin_ds->ds_dbuf, tx);
	oldnext_obj = origin_ds->ds_phys->ds_next_snap_obj;
	snap = list_tail(&pa->clone_snaps);
	ASSERT3U(snap->ds->ds_phys->ds_prev_snap_obj, ==, origin_ds->ds_object);
	origin_ds->ds_phys->ds_next_snap_obj = snap->ds->ds_object;

	/* change the origin's next clone */
	if (origin_ds->ds_phys->ds_next_clones_obj) {
		remove_from_next_clones(origin_ds, snap->ds->ds_object, tx);
		VERIFY3U(0, ==, zap_add_int(dp->dp_meta_objset,
		    origin_ds->ds_phys->ds_next_clones_obj,
		    oldnext_obj, tx));
	}

	/* change origin */
	dmu_buf_will_dirty(dd->dd_dbuf, tx);
	ASSERT3U(dd->dd_phys->dd_origin_obj, ==, origin_ds->ds_object);
	dd->dd_phys->dd_origin_obj = odd->dd_phys->dd_origin_obj;
	dd->dd_origin_txg = origin_head->ds_dir->dd_origin_txg;
	dmu_buf_will_dirty(odd->dd_dbuf, tx);
	odd->dd_phys->dd_origin_obj = origin_ds->ds_object;
	origin_head->ds_dir->dd_origin_txg =
	    origin_ds->ds_phys->ds_creation_txg;

	/* change dd_clone entries */
	if (spa_version(dp->dp_spa) >= SPA_VERSION_DIR_CLONES) {
		VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
		    odd->dd_phys->dd_clones, hds->ds_object, tx));
		VERIFY3U(0, ==, zap_add_int(dp->dp_meta_objset,
		    pa->origin_origin->ds_dir->dd_phys->dd_clones,
		    hds->ds_object, tx));

		VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
		    pa->origin_origin->ds_dir->dd_phys->dd_clones,
		    origin_head->ds_object, tx));
		if (dd->dd_phys->dd_clones == 0) {
			dd->dd_phys->dd_clones = zap_create(dp->dp_meta_objset,
			    DMU_OT_DSL_CLONES, DMU_OT_NONE, 0, tx);
		}
		VERIFY3U(0, ==, zap_add_int(dp->dp_meta_objset,
		    dd->dd_phys->dd_clones, origin_head->ds_object, tx));

	}

	/* move snapshots to this dir */
	for (snap = list_head(&pa->shared_snaps); snap;
	    snap = list_next(&pa->shared_snaps, snap)) {
		dsl_dataset_t *ds = snap->ds;

		/* unregister props as dsl_dir is changing */
		if (ds->ds_objset) {
			dmu_objset_evict(ds->ds_objset);
			ds->ds_objset = NULL;
		}
		/* move snap name entry */
		VERIFY(0 == dsl_dataset_get_snapname(ds));
		VERIFY(0 == dsl_dataset_snap_remove(origin_head,
		    ds->ds_snapname, tx));
		VERIFY(0 == zap_add(dp->dp_meta_objset,
		    hds->ds_phys->ds_snapnames_zapobj, ds->ds_snapname,
		    8, 1, &ds->ds_object, tx));

		/* change containing dsl_dir */
		dmu_buf_will_dirty(ds->ds_dbuf, tx);
		ASSERT3U(ds->ds_phys->ds_dir_obj, ==, odd->dd_object);
		ds->ds_phys->ds_dir_obj = dd->dd_object;
		ASSERT3P(ds->ds_dir, ==, odd);
		dsl_dir_close(ds->ds_dir, ds);
		VERIFY(0 == dsl_dir_open_obj(dp, dd->dd_object,
		    NULL, ds, &ds->ds_dir));

		/* move any clone references */
		if (ds->ds_phys->ds_next_clones_obj &&
		    spa_version(dp->dp_spa) >= SPA_VERSION_DIR_CLONES) {
			zap_cursor_t zc;
			zap_attribute_t za;

			for (zap_cursor_init(&zc, dp->dp_meta_objset,
			    ds->ds_phys->ds_next_clones_obj);
			    zap_cursor_retrieve(&zc, &za) == 0;
			    zap_cursor_advance(&zc)) {
				dsl_dataset_t *cnds;
				uint64_t o;

				if (za.za_first_integer == oldnext_obj) {
					/*
					 * We've already moved the
					 * origin's reference.
					 */
					continue;
				}

				VERIFY3U(0, ==, dsl_dataset_hold_obj(dp,
				    za.za_first_integer, FTAG, &cnds));
				o = cnds->ds_dir->dd_phys->dd_head_dataset_obj;

				VERIFY3U(zap_remove_int(dp->dp_meta_objset,
				    odd->dd_phys->dd_clones, o, tx), ==, 0);
				VERIFY3U(zap_add_int(dp->dp_meta_objset,
				    dd->dd_phys->dd_clones, o, tx), ==, 0);
				dsl_dataset_rele(cnds, FTAG);
			}
			zap_cursor_fini(&zc);
		}

		ASSERT3U(dsl_prop_numcb(ds), ==, 0);
	}

	/*
	 * Change space accounting.
	 * Note, pa->*usedsnap and dd_used_breakdown[SNAP] will either
	 * both be valid, or both be 0 (resulting in delta == 0).  This
	 * is true for each of {clone,origin} independently.
	 */

	delta = pa->cloneusedsnap -
	    dd->dd_phys->dd_used_breakdown[DD_USED_SNAP];
	ASSERT3S(delta, >=, 0);
	ASSERT3U(pa->used, >=, delta);
	dsl_dir_diduse_space(dd, DD_USED_SNAP, delta, 0, 0, tx);
	dsl_dir_diduse_space(dd, DD_USED_HEAD,
	    pa->used - delta, pa->comp, pa->uncomp, tx);

	delta = pa->originusedsnap -
	    odd->dd_phys->dd_used_breakdown[DD_USED_SNAP];
	ASSERT3S(delta, <=, 0);
	ASSERT3U(pa->used, >=, -delta);
	dsl_dir_diduse_space(odd, DD_USED_SNAP, delta, 0, 0, tx);
	dsl_dir_diduse_space(odd, DD_USED_HEAD,
	    -pa->used - delta, -pa->comp, -pa->uncomp, tx);

	origin_ds->ds_phys->ds_unique_bytes = pa->unique;

	/* log history record */
	spa_history_log_internal(LOG_DS_PROMOTE, dd->dd_pool->dp_spa, tx,
	    "dataset = %llu", hds->ds_object);

	dsl_dir_close(odd, FTAG);
}

static char *snaplist_tag = "snaplist";
/*
 * Make a list of dsl_dataset_t's for the snapshots between first_obj
 * (exclusive) and last_obj (inclusive).  The list will be in reverse
 * order (last_obj will be the list_head()).  If first_obj == 0, do all
 * snapshots back to this dataset's origin.
 */
static int
snaplist_make(dsl_pool_t *dp, boolean_t own,
    uint64_t first_obj, uint64_t last_obj, list_t *l)
{
	uint64_t obj = last_obj;

	ASSERT(RW_LOCK_HELD(&dp->dp_config_rwlock));

	list_create(l, sizeof (struct promotenode),
	    offsetof(struct promotenode, link));

	while (obj != first_obj) {
		dsl_dataset_t *ds;
		struct promotenode *snap;
		int err;

		if (own) {
			err = dsl_dataset_own_obj(dp, obj,
			    0, snaplist_tag, &ds);
			if (err == 0)
				dsl_dataset_make_exclusive(ds, snaplist_tag);
		} else {
			err = dsl_dataset_hold_obj(dp, obj, snaplist_tag, &ds);
		}
		if (err == ENOENT) {
			/* lost race with snapshot destroy */
			struct promotenode *last = list_tail(l);
			ASSERT(obj != last->ds->ds_phys->ds_prev_snap_obj);
			obj = last->ds->ds_phys->ds_prev_snap_obj;
			continue;
		} else if (err) {
			return (err);
		}

		if (first_obj == 0)
			first_obj = ds->ds_dir->dd_phys->dd_origin_obj;

		snap = kmem_alloc(sizeof (struct promotenode), KM_SLEEP);
		snap->ds = ds;
		list_insert_tail(l, snap);
		obj = ds->ds_phys->ds_prev_snap_obj;
	}

	return (0);
}

static int
snaplist_space(list_t *l, uint64_t mintxg, uint64_t *spacep)
{
	struct promotenode *snap;

	*spacep = 0;
	for (snap = list_head(l); snap; snap = list_next(l, snap)) {
		uint64_t used, comp, uncomp;
		dsl_deadlist_space_range(&snap->ds->ds_deadlist,
		    mintxg, UINT64_MAX, &used, &comp, &uncomp);
		*spacep += used;
	}
	return (0);
}

static void
snaplist_destroy(list_t *l, boolean_t own)
{
	struct promotenode *snap;

	if (!l || !list_link_active(&l->list_head))
		return;

	while ((snap = list_tail(l)) != NULL) {
		list_remove(l, snap);
		if (own)
			dsl_dataset_disown(snap->ds, snaplist_tag);
		else
			dsl_dataset_rele(snap->ds, snaplist_tag);
		kmem_free(snap, sizeof (struct promotenode));
	}
	list_destroy(l);
}

/*
 * Promote a clone.  Nomenclature note:
 * "clone" or "cds": the original clone which is being promoted
 * "origin" or "ods": the snapshot which is originally clone's origin
 * "origin head" or "ohds": the dataset which is the head
 * (filesystem/volume) for the origin
 * "origin origin": the origin of the origin's filesystem (typically
 * NULL, indicating that the clone is not a clone of a clone).
 */
int
dsl_dataset_promote(const char *name, char *conflsnap)
{
	dsl_dataset_t *ds;
	dsl_dir_t *dd;
	dsl_pool_t *dp;
	dmu_object_info_t doi;
	struct promotearg pa = { 0 };
	struct promotenode *snap;
	int err;

	err = dsl_dataset_hold(name, FTAG, &ds);
	if (err)
		return (err);
	dd = ds->ds_dir;
	dp = dd->dd_pool;

	err = dmu_object_info(dp->dp_meta_objset,
	    ds->ds_phys->ds_snapnames_zapobj, &doi);
	if (err) {
		dsl_dataset_rele(ds, FTAG);
		return (err);
	}

	if (dsl_dataset_is_snapshot(ds) || dd->dd_phys->dd_origin_obj == 0) {
		dsl_dataset_rele(ds, FTAG);
		return (EINVAL);
	}

	/*
	 * We are going to inherit all the snapshots taken before our
	 * origin (i.e., our new origin will be our parent's origin).
	 * Take ownership of them so that we can rename them into our
	 * namespace.
	 */
	rw_enter(&dp->dp_config_rwlock, RW_READER);

	err = snaplist_make(dp, B_TRUE, 0, dd->dd_phys->dd_origin_obj,
	    &pa.shared_snaps);
	if (err != 0)
		goto out;

	err = snaplist_make(dp, B_FALSE, 0, ds->ds_object, &pa.clone_snaps);
	if (err != 0)
		goto out;

	snap = list_head(&pa.shared_snaps);
	ASSERT3U(snap->ds->ds_object, ==, dd->dd_phys->dd_origin_obj);
	err = snaplist_make(dp, B_FALSE, dd->dd_phys->dd_origin_obj,
	    snap->ds->ds_dir->dd_phys->dd_head_dataset_obj, &pa.origin_snaps);
	if (err != 0)
		goto out;

	if (snap->ds->ds_dir->dd_phys->dd_origin_obj != 0) {
		err = dsl_dataset_hold_obj(dp,
		    snap->ds->ds_dir->dd_phys->dd_origin_obj,
		    FTAG, &pa.origin_origin);
		if (err != 0)
			goto out;
	}

out:
	rw_exit(&dp->dp_config_rwlock);

	/*
	 * Add in 128x the snapnames zapobj size, since we will be moving
	 * a bunch of snapnames to the promoted ds, and dirtying their
	 * bonus buffers.
	 */
	if (err == 0) {
		err = dsl_sync_task_do(dp, dsl_dataset_promote_check,
		    dsl_dataset_promote_sync, ds, &pa,
		    2 + 2 * doi.doi_physical_blocks_512);
		if (err && pa.err_ds && conflsnap)
			(void) strncpy(conflsnap, pa.err_ds, MAXNAMELEN);
	}

	snaplist_destroy(&pa.shared_snaps, B_TRUE);
	snaplist_destroy(&pa.clone_snaps, B_FALSE);
	snaplist_destroy(&pa.origin_snaps, B_FALSE);
	if (pa.origin_origin)
		dsl_dataset_rele(pa.origin_origin, FTAG);
	dsl_dataset_rele(ds, FTAG);
	return (err);
}

struct cloneswaparg {
	dsl_dataset_t *cds; /* clone dataset */
	dsl_dataset_t *ohds; /* origin's head dataset */
	boolean_t force;
	int64_t unused_refres_delta; /* change in unconsumed refreservation */
};

/* ARGSUSED */
static int
dsl_dataset_clone_swap_check(void *arg1, void *arg2, dmu_tx_t *tx)
{
	struct cloneswaparg *csa = arg1;

	/* they should both be heads */
	if (dsl_dataset_is_snapshot(csa->cds) ||
	    dsl_dataset_is_snapshot(csa->ohds))
		return (EINVAL);

	/* the branch point should be just before them */
	if (csa->cds->ds_prev != csa->ohds->ds_prev)
		return (EINVAL);

	/* cds should be the clone (unless they are unrelated) */
	if (csa->cds->ds_prev != NULL &&
	    csa->cds->ds_prev != csa->cds->ds_dir->dd_pool->dp_origin_snap &&
	    csa->ohds->ds_object !=
	    csa->cds->ds_prev->ds_phys->ds_next_snap_obj)
		return (EINVAL);

	/* the clone should be a child of the origin */
	if (csa->cds->ds_dir->dd_parent != csa->ohds->ds_dir)
		return (EINVAL);

	/* ohds shouldn't be modified unless 'force' */
	if (!csa->force && dsl_dataset_modified_since_lastsnap(csa->ohds))
		return (ETXTBSY);

	/* adjust amount of any unconsumed refreservation */
	csa->unused_refres_delta =
	    (int64_t)MIN(csa->ohds->ds_reserved,
	    csa->ohds->ds_phys->ds_unique_bytes) -
	    (int64_t)MIN(csa->ohds->ds_reserved,
	    csa->cds->ds_phys->ds_unique_bytes);

	if (csa->unused_refres_delta > 0 &&
	    csa->unused_refres_delta >
	    dsl_dir_space_available(csa->ohds->ds_dir, NULL, 0, TRUE))
		return (ENOSPC);

	if (csa->ohds->ds_quota != 0 &&
	    csa->cds->ds_phys->ds_unique_bytes > csa->ohds->ds_quota)
		return (EDQUOT);

	return (0);
}

/* ARGSUSED */
static void
dsl_dataset_clone_swap_sync(void *arg1, void *arg2, dmu_tx_t *tx)
{
	struct cloneswaparg *csa = arg1;
	dsl_pool_t *dp = csa->cds->ds_dir->dd_pool;

	ASSERT(csa->cds->ds_reserved == 0);
	ASSERT(csa->ohds->ds_quota == 0 ||
	    csa->cds->ds_phys->ds_unique_bytes <= csa->ohds->ds_quota);

	dmu_buf_will_dirty(csa->cds->ds_dbuf, tx);
	dmu_buf_will_dirty(csa->ohds->ds_dbuf, tx);

	if (csa->cds->ds_objset != NULL) {
		dmu_objset_evict(csa->cds->ds_objset);
		csa->cds->ds_objset = NULL;
	}

	if (csa->ohds->ds_objset != NULL) {
		dmu_objset_evict(csa->ohds->ds_objset);
		csa->ohds->ds_objset = NULL;
	}

	/*
	 * Reset origin's unique bytes, if it exists.
	 */
	if (csa->cds->ds_prev) {
		dsl_dataset_t *origin = csa->cds->ds_prev;
		uint64_t comp, uncomp;

		dmu_buf_will_dirty(origin->ds_dbuf, tx);
		dsl_deadlist_space_range(&csa->cds->ds_deadlist,
		    origin->ds_phys->ds_prev_snap_txg, UINT64_MAX,
		    &origin->ds_phys->ds_unique_bytes, &comp, &uncomp);
	}

	/* swap blkptrs */
	{
		blkptr_t tmp;
		tmp = csa->ohds->ds_phys->ds_bp;
		csa->ohds->ds_phys->ds_bp = csa->cds->ds_phys->ds_bp;
		csa->cds->ds_phys->ds_bp = tmp;
	}

	/* set dd_*_bytes */
	{
		int64_t dused, dcomp, duncomp;
		uint64_t cdl_used, cdl_comp, cdl_uncomp;
		uint64_t odl_used, odl_comp, odl_uncomp;

		ASSERT3U(csa->cds->ds_dir->dd_phys->
		    dd_used_breakdown[DD_USED_SNAP], ==, 0);

		dsl_deadlist_space(&csa->cds->ds_deadlist,
		    &cdl_used, &cdl_comp, &cdl_uncomp);
		dsl_deadlist_space(&csa->ohds->ds_deadlist,
		    &odl_used, &odl_comp, &odl_uncomp);

		dused = csa->cds->ds_phys->ds_used_bytes + cdl_used -
		    (csa->ohds->ds_phys->ds_used_bytes + odl_used);
		dcomp = csa->cds->ds_phys->ds_compressed_bytes + cdl_comp -
		    (csa->ohds->ds_phys->ds_compressed_bytes + odl_comp);
		duncomp = csa->cds->ds_phys->ds_uncompressed_bytes +
		    cdl_uncomp -
		    (csa->ohds->ds_phys->ds_uncompressed_bytes + odl_uncomp);

		dsl_dir_diduse_space(csa->ohds->ds_dir, DD_USED_HEAD,
		    dused, dcomp, duncomp, tx);
		dsl_dir_diduse_space(csa->cds->ds_dir, DD_USED_HEAD,
		    -dused, -dcomp, -duncomp, tx);

		/*
		 * The difference in the space used by snapshots is the
		 * difference in snapshot space due to the head's
		 * deadlist (since that's the only thing that's
		 * changing that affects the snapused).
		 */
		dsl_deadlist_space_range(&csa->cds->ds_deadlist,
		    csa->ohds->ds_dir->dd_origin_txg, UINT64_MAX,
		    &cdl_used, &cdl_comp, &cdl_uncomp);
		dsl_deadlist_space_range(&csa->ohds->ds_deadlist,
		    csa->ohds->ds_dir->dd_origin_txg, UINT64_MAX,
		    &odl_used, &odl_comp, &odl_uncomp);
		dsl_dir_transfer_space(csa->ohds->ds_dir, cdl_used - odl_used,
		    DD_USED_HEAD, DD_USED_SNAP, tx);
	}

	/* swap ds_*_bytes */
	SWITCH64(csa->ohds->ds_phys->ds_used_bytes,
	    csa->cds->ds_phys->ds_used_bytes);
	SWITCH64(csa->ohds->ds_phys->ds_compressed_bytes,
	    csa->cds->ds_phys->ds_compressed_bytes);
	SWITCH64(csa->ohds->ds_phys->ds_uncompressed_bytes,
	    csa->cds->ds_phys->ds_uncompressed_bytes);
	SWITCH64(csa->ohds->ds_phys->ds_unique_bytes,
	    csa->cds->ds_phys->ds_unique_bytes);

	/* apply any parent delta for change in unconsumed refreservation */
	dsl_dir_diduse_space(csa->ohds->ds_dir, DD_USED_REFRSRV,
	    csa->unused_refres_delta, 0, 0, tx);

	/*
	 * Swap deadlists.
	 */
	dsl_deadlist_close(&csa->cds->ds_deadlist);
	dsl_deadlist_close(&csa->ohds->ds_deadlist);
	SWITCH64(csa->ohds->ds_phys->ds_deadlist_obj,
	    csa->cds->ds_phys->ds_deadlist_obj);
	dsl_deadlist_open(&csa->cds->ds_deadlist, dp->dp_meta_objset,
	    csa->cds->ds_phys->ds_deadlist_obj);
	dsl_deadlist_open(&csa->ohds->ds_deadlist, dp->dp_meta_objset,
	    csa->ohds->ds_phys->ds_deadlist_obj);

	dsl_scan_ds_clone_swapped(csa->ohds, csa->cds, tx);
}

/*
 * Swap 'clone' with its origin head datasets.  Used at the end of "zfs
 * recv" into an existing fs to swizzle the file system to the new
 * version, and by "zfs rollback".  Can also be used to swap two
 * independent head datasets if neither has any snapshots.
 */
int
dsl_dataset_clone_swap(dsl_dataset_t *clone, dsl_dataset_t *origin_head,
    boolean_t force)
{
	struct cloneswaparg csa;
	int error;

	ASSERT(clone->ds_owner);
	ASSERT(origin_head->ds_owner);
retry:
	/*
	 * Need exclusive access for the swap. If we're swapping these
	 * datasets back after an error, we already hold the locks.
	 */
	if (!RW_WRITE_HELD(&clone->ds_rwlock))
		rw_enter(&clone->ds_rwlock, RW_WRITER);
	if (!RW_WRITE_HELD(&origin_head->ds_rwlock) &&
	    !rw_tryenter(&origin_head->ds_rwlock, RW_WRITER)) {
		rw_exit(&clone->ds_rwlock);
		rw_enter(&origin_head->ds_rwlock, RW_WRITER);
		if (!rw_tryenter(&clone->ds_rwlock, RW_WRITER)) {
			rw_exit(&origin_head->ds_rwlock);
			goto retry;
		}
	}
	csa.cds = clone;
	csa.ohds = origin_head;
	csa.force = force;
	error = dsl_sync_task_do(clone->ds_dir->dd_pool,
	    dsl_dataset_clone_swap_check,
	    dsl_dataset_clone_swap_sync, &csa, NULL, 9);
	return (error);
}

/*
 * Given a pool name and a dataset object number in that pool,
 * return the name of that dataset.
 */
int
dsl_dsobj_to_dsname(char *pname, uint64_t obj, char *buf)
{
	spa_t *spa;
	dsl_pool_t *dp;
	dsl_dataset_t *ds;
	int error;

	if ((error = spa_open(pname, &spa, FTAG)) != 0)
		return (error);
	dp = spa_get_dsl(spa);
	rw_enter(&dp->dp_config_rwlock, RW_READER);
	if ((error = dsl_dataset_hold_obj(dp, obj, FTAG, &ds)) == 0) {
		dsl_dataset_name(ds, buf);
		dsl_dataset_rele(ds, FTAG);
	}
	rw_exit(&dp->dp_config_rwlock);
	spa_close(spa, FTAG);

	return (error);
}

int
dsl_dataset_check_quota(dsl_dataset_t *ds, boolean_t check_quota,
    uint64_t asize, uint64_t inflight, uint64_t *used, uint64_t *ref_rsrv)
{
	int error = 0;

	ASSERT3S(asize, >, 0);

	/*
	 * *ref_rsrv is the portion of asize that will come from any
	 * unconsumed refreservation space.
	 */
	*ref_rsrv = 0;

	mutex_enter(&ds->ds_lock);
	/*
	 * Make a space adjustment for reserved bytes.
	 */
	if (ds->ds_reserved > ds->ds_phys->ds_unique_bytes) {
		ASSERT3U(*used, >=,
		    ds->ds_reserved - ds->ds_phys->ds_unique_bytes);
		*used -= (ds->ds_reserved - ds->ds_phys->ds_unique_bytes);
		*ref_rsrv =
		    asize - MIN(asize, parent_delta(ds, asize + inflight));
	}

	if (!check_quota || ds->ds_quota == 0) {
		mutex_exit(&ds->ds_lock);
		return (0);
	}
	/*
	 * If they are requesting more space, and our current estimate
	 * is over quota, they get to try again unless the actual
	 * on-disk is over quota and there are no pending changes (which
	 * may free up space for us).
	 */
	if (ds->ds_phys->ds_used_bytes + inflight >= ds->ds_quota) {
		if (inflight > 0 || ds->ds_phys->ds_used_bytes < ds->ds_quota)
			error = ERESTART;
		else
			error = EDQUOT;
	}
	mutex_exit(&ds->ds_lock);

	return (error);
}

/* ARGSUSED */
static int
dsl_dataset_set_quota_check(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	dsl_prop_setarg_t *psa = arg2;
	int err;

	if (spa_version(ds->ds_dir->dd_pool->dp_spa) < SPA_VERSION_REFQUOTA)
		return (ENOTSUP);

	if ((err = dsl_prop_predict_sync(ds->ds_dir, psa)) != 0)
		return (err);

	if (psa->psa_effective_value == 0)
		return (0);

	if (psa->psa_effective_value < ds->ds_phys->ds_used_bytes ||
	    psa->psa_effective_value < ds->ds_reserved)
		return (ENOSPC);

	return (0);
}

extern void dsl_prop_set_sync(void *, void *, dmu_tx_t *);

void
dsl_dataset_set_quota_sync(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	dsl_prop_setarg_t *psa = arg2;
	uint64_t effective_value = psa->psa_effective_value;

	dsl_prop_set_sync(ds, psa, tx);
	DSL_PROP_CHECK_PREDICTION(ds->ds_dir, psa);

	if (ds->ds_quota != effective_value) {
		dmu_buf_will_dirty(ds->ds_dbuf, tx);
		ds->ds_quota = effective_value;

		spa_history_log_internal(LOG_DS_REFQUOTA,
		    ds->ds_dir->dd_pool->dp_spa, tx, "%lld dataset = %llu ",
		    (longlong_t)ds->ds_quota, ds->ds_object);
	}
}

int
dsl_dataset_set_quota(const char *dsname, zprop_source_t source, uint64_t quota)
{
	dsl_dataset_t *ds;
	dsl_prop_setarg_t psa;
	int err;

	dsl_prop_setarg_init_uint64(&psa, "refquota", source, &quota);

	err = dsl_dataset_hold(dsname, FTAG, &ds);
	if (err)
		return (err);

	/*
	 * If someone removes a file, then tries to set the quota, we
	 * want to make sure the file freeing takes effect.
	 */
	txg_wait_open(ds->ds_dir->dd_pool, 0);

	err = dsl_sync_task_do(ds->ds_dir->dd_pool,
	    dsl_dataset_set_quota_check, dsl_dataset_set_quota_sync,
	    ds, &psa, 0);

	dsl_dataset_rele(ds, FTAG);
	return (err);
}

static int
dsl_dataset_set_reservation_check(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	dsl_prop_setarg_t *psa = arg2;
	uint64_t effective_value;
	uint64_t unique;
	int err;

	if (spa_version(ds->ds_dir->dd_pool->dp_spa) <
	    SPA_VERSION_REFRESERVATION)
		return (ENOTSUP);

	if (dsl_dataset_is_snapshot(ds))
		return (EINVAL);

	if ((err = dsl_prop_predict_sync(ds->ds_dir, psa)) != 0)
		return (err);

	effective_value = psa->psa_effective_value;

	/*
	 * If we are doing the preliminary check in open context, the
	 * space estimates may be inaccurate.
	 */
	if (!dmu_tx_is_syncing(tx))
		return (0);

	mutex_enter(&ds->ds_lock);
	if (!DS_UNIQUE_IS_ACCURATE(ds))
		dsl_dataset_recalc_head_uniq(ds);
	unique = ds->ds_phys->ds_unique_bytes;
	mutex_exit(&ds->ds_lock);

	if (MAX(unique, effective_value) > MAX(unique, ds->ds_reserved)) {
		uint64_t delta = MAX(unique, effective_value) -
		    MAX(unique, ds->ds_reserved);

		if (delta > dsl_dir_space_available(ds->ds_dir, NULL, 0, TRUE))
			return (ENOSPC);
		if (ds->ds_quota > 0 &&
		    effective_value > ds->ds_quota)
			return (ENOSPC);
	}

	return (0);
}

static void
dsl_dataset_set_reservation_sync(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	dsl_prop_setarg_t *psa = arg2;
	uint64_t effective_value = psa->psa_effective_value;
	uint64_t unique;
	int64_t delta;

	dsl_prop_set_sync(ds, psa, tx);
	DSL_PROP_CHECK_PREDICTION(ds->ds_dir, psa);

	dmu_buf_will_dirty(ds->ds_dbuf, tx);

	mutex_enter(&ds->ds_dir->dd_lock);
	mutex_enter(&ds->ds_lock);
	ASSERT(DS_UNIQUE_IS_ACCURATE(ds));
	unique = ds->ds_phys->ds_unique_bytes;
	delta = MAX(0, (int64_t)(effective_value - unique)) -
	    MAX(0, (int64_t)(ds->ds_reserved - unique));
	ds->ds_reserved = effective_value;
	mutex_exit(&ds->ds_lock);

	dsl_dir_diduse_space(ds->ds_dir, DD_USED_REFRSRV, delta, 0, 0, tx);
	mutex_exit(&ds->ds_dir->dd_lock);

	spa_history_log_internal(LOG_DS_REFRESERV,
	    ds->ds_dir->dd_pool->dp_spa, tx, "%lld dataset = %llu",
	    (longlong_t)effective_value, ds->ds_object);
}

int
dsl_dataset_set_reservation(const char *dsname, zprop_source_t source,
    uint64_t reservation)
{
	dsl_dataset_t *ds;
	dsl_prop_setarg_t psa;
	int err;

	dsl_prop_setarg_init_uint64(&psa, "refreservation", source,
	    &reservation);

	err = dsl_dataset_hold(dsname, FTAG, &ds);
	if (err)
		return (err);

	err = dsl_sync_task_do(ds->ds_dir->dd_pool,
	    dsl_dataset_set_reservation_check,
	    dsl_dataset_set_reservation_sync, ds, &psa, 0);

	dsl_dataset_rele(ds, FTAG);
	return (err);
}

typedef struct zfs_hold_cleanup_arg {
	dsl_pool_t *dp;
	uint64_t dsobj;
	char htag[MAXNAMELEN];
} zfs_hold_cleanup_arg_t;

static void
dsl_dataset_user_release_onexit(void *arg)
{
	zfs_hold_cleanup_arg_t *ca = arg;

	(void) dsl_dataset_user_release_tmp(ca->dp, ca->dsobj, ca->htag,
	    B_TRUE);
	kmem_free(ca, sizeof (zfs_hold_cleanup_arg_t));
}

void
dsl_register_onexit_hold_cleanup(dsl_dataset_t *ds, const char *htag,
    minor_t minor)
{
	zfs_hold_cleanup_arg_t *ca;

	ca = kmem_alloc(sizeof (zfs_hold_cleanup_arg_t), KM_SLEEP);
	ca->dp = ds->ds_dir->dd_pool;
	ca->dsobj = ds->ds_object;
	(void) strlcpy(ca->htag, htag, sizeof (ca->htag));
	VERIFY3U(0, ==, zfs_onexit_add_cb(minor,
	    dsl_dataset_user_release_onexit, ca, NULL));
}

/*
 * If you add new checks here, you may need to add
 * additional checks to the "temporary" case in
 * snapshot_check() in dmu_objset.c.
 */
static int
dsl_dataset_user_hold_check(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	struct dsl_ds_holdarg *ha = arg2;
	char *htag = ha->htag;
	objset_t *mos = ds->ds_dir->dd_pool->dp_meta_objset;
	int error = 0;

	if (spa_version(ds->ds_dir->dd_pool->dp_spa) < SPA_VERSION_USERREFS)
		return (ENOTSUP);

	if (!dsl_dataset_is_snapshot(ds))
		return (EINVAL);

	/* tags must be unique */
	mutex_enter(&ds->ds_lock);
	if (ds->ds_phys->ds_userrefs_obj) {
		error = zap_lookup(mos, ds->ds_phys->ds_userrefs_obj, htag,
		    8, 1, tx);
		if (error == 0)
			error = EEXIST;
		else if (error == ENOENT)
			error = 0;
	}
	mutex_exit(&ds->ds_lock);

	if (error == 0 && ha->temphold &&
	    strlen(htag) + MAX_TAG_PREFIX_LEN >= MAXNAMELEN)
		error = E2BIG;

	return (error);
}

void
dsl_dataset_user_hold_sync(void *arg1, void *arg2, dmu_tx_t *tx)
{
	dsl_dataset_t *ds = arg1;
	struct dsl_ds_holdarg *ha = arg2;
	char *htag = ha->htag;
	dsl_pool_t *dp = ds->ds_dir->dd_pool;
	objset_t *mos = dp->dp_meta_objset;
	uint64_t now = gethrestime_sec();
	uint64_t zapobj;

	mutex_enter(&ds->ds_lock);
	if (ds->ds_phys->ds_userrefs_obj == 0) {
		/*
		 * This is the first user hold for this dataset.  Create
		 * the userrefs zap object.
		 */
		dmu_buf_will_dirty(ds->ds_dbuf, tx);
		zapobj = ds->ds_phys->ds_userrefs_obj =
		    zap_create(mos, DMU_OT_USERREFS, DMU_OT_NONE, 0, tx);
	} else {
		zapobj = ds->ds_phys->ds_userrefs_obj;
	}
	ds->ds_userrefs++;
	mutex_exit(&ds->ds_lock);

	VERIFY(0 == zap_add(mos, zapobj, htag, 8, 1, &now, tx));

	if (ha->temphold) {
		VERIFY(0 == dsl_pool_user_hold(dp, ds->ds_object,
		    htag, &now, tx));
	}

	spa_history_log_internal(LOG_DS_USER_HOLD,
	    dp->dp_spa, tx, "<%s> temp = %d dataset = %llu", htag,
	    (int)ha->temphold, ds->ds_object);
}

static int
dsl_dataset_user_hold_one(const char *dsname, void *arg)
{
	struct dsl_ds_holdarg *ha = arg;
	dsl_dataset_t *ds;
	int error;
	char *name;

	/* alloc a buffer to hold dsname@snapname plus terminating NULL */
	name = kmem_asprintf("%s@%s", dsname, ha->snapname);
	error = dsl_dataset_hold(name, ha->dstg, &ds);
	strfree(name);
	if (error == 0) {
		ha->gotone = B_TRUE;
		dsl_sync_task_create(ha->dstg, dsl_dataset_user_hold_check,
		    dsl_dataset_user_hold_sync, ds, ha, 0);
	} else if (error == ENOENT && ha->recursive) {
		error = 0;
	} else {
		(void) strlcpy(ha->failed, dsname, sizeof (ha->failed));
	}
	return (error);
}

int
dsl_dataset_user_hold_for_send(dsl_dataset_t *ds, char *htag,
    boolean_t temphold)
{
	struct dsl_ds_holdarg *ha;
	int error;

	ha = kmem_zalloc(sizeof (struct dsl_ds_holdarg), KM_SLEEP);
	ha->htag = htag;
	ha->temphold = temphold;
	error = dsl_sync_task_do(ds->ds_dir->dd_pool,
	    dsl_dataset_user_hold_check, dsl_dataset_user_hold_sync,
	    ds, ha, 0);
	kmem_free(ha, sizeof (struct dsl_ds_holdarg));

	return (error);
}

int
dsl_dataset_user_hold(char *dsname, char *snapname, char *htag,
    boolean_t recursive, boolean_t temphold, int cleanup_fd)
{
	struct dsl_ds_holdarg *ha;
	dsl_sync_task_t *dst;
	spa_t *spa;
	int error;
	minor_t minor = 0;

	if (cleanup_fd != -1) {
		/* Currently we only support cleanup-on-exit of tempholds. */
		if (!temphold)
			return (EINVAL);
		error = zfs_onexit_fd_hold(cleanup_fd, &minor);
		if (error)
			return (error);
	}

	ha = kmem_zalloc(sizeof (struct dsl_ds_holdarg), KM_SLEEP);

	(void) strlcpy(ha->failed, dsname, sizeof (ha->failed));

	error = spa_open(dsname, &spa, FTAG);
	if (error) {
		kmem_free(ha, sizeof (struct dsl_ds_holdarg));
		if (cleanup_fd != -1)
			zfs_onexit_fd_rele(cleanup_fd);
		return (error);
	}

	ha->dstg = dsl_sync_task_group_create(spa_get_dsl(spa));
	ha->htag = htag;
	ha->snapname = snapname;
	ha->recursive = recursive;
	ha->temphold = temphold;

	if (recursive) {
		error = dmu_objset_find(dsname, dsl_dataset_user_hold_one,
		    ha, DS_FIND_CHILDREN);
	} else {
		error = dsl_dataset_user_hold_one(dsname, ha);
	}
	if (error == 0)
		error = dsl_sync_task_group_wait(ha->dstg);

	for (dst = list_head(&ha->dstg->dstg_tasks); dst;
	    dst = list_next(&ha->dstg->dstg_tasks, dst)) {
		dsl_dataset_t *ds = dst->dst_arg1;

		if (dst->dst_err) {
			dsl_dataset_name(ds, ha->failed);
			*strchr(ha->failed, '@') = '\0';
		} else if (error == 0 && minor != 0 && temphold) {
			/*
			 * If this hold is to be released upon process exit,
			 * register that action now.
			 */
			dsl_register_onexit_hold_cleanup(ds, htag, minor);
		}
		dsl_dataset_rele(ds, ha->dstg);
	}

	if (error == 0 && recursive && !ha->gotone)
		error = ENOENT;

	if (error)
		(void) strlcpy(dsname, ha->failed, sizeof (ha->failed));

	dsl_sync_task_group_destroy(ha->dstg);

	kmem_free(ha, sizeof (struct dsl_ds_holdarg));
	spa_close(spa, FTAG);
	if (cleanup_fd != -1)
		zfs_onexit_fd_rele(cleanup_fd);
	return (error);
}

struct dsl_ds_releasearg {
	dsl_dataset_t *ds;
	const char *htag;
	boolean_t own;		/* do we own or just hold ds? */
};

static int
dsl_dataset_release_might_destroy(dsl_dataset_t *ds, const char *htag,
    boolean_t *might_destroy)
{
	objset_t *mos = ds->ds_dir->dd_pool->dp_meta_objset;
	uint64_t zapobj;
	uint64_t tmp;
	int error;

	*might_destroy = B_FALSE;

	mutex_enter(&ds->ds_lock);
	zapobj = ds->ds_phys->ds_userrefs_obj;
	if (zapobj == 0) {
		/* The tag can't possibly exist */
		mutex_exit(&ds->ds_lock);
		return (ESRCH);
	}

	/* Make sure the tag exists */
	error = zap_lookup(mos, zapobj, htag, 8, 1, &tmp);
	if (error) {
		mutex_exit(&ds->ds_lock);
		if (error == ENOENT)
			error = ESRCH;
		return (error);
	}

	if (ds->ds_userrefs == 1 && ds->ds_phys->ds_num_children == 1 &&
	    DS_IS_DEFER_DESTROY(ds))
		*might_destroy = B_TRUE;

	mutex_exit(&ds->ds_lock);
	return (0);
}

static int
dsl_dataset_user_release_check(void *arg1, void *tag, dmu_tx_t *tx)
{
	struct dsl_ds_releasearg *ra = arg1;
	dsl_dataset_t *ds = ra->ds;
	boolean_t might_destroy;
	int error;

	if (spa_version(ds->ds_dir->dd_pool->dp_spa) < SPA_VERSION_USERREFS)
		return (ENOTSUP);

	error = dsl_dataset_release_might_destroy(ds, ra->htag, &might_destroy);
	if (error)
		return (error);

	if (might_destroy) {
		struct dsl_ds_destroyarg dsda = {0};

		if (dmu_tx_is_syncing(tx)) {
			/*
			 * If we're not prepared to remove the snapshot,
			 * we can't allow the release to happen right now.
			 */
			if (!ra->own)
				return (EBUSY);
		}
		dsda.ds = ds;
		dsda.releasing = B_TRUE;
		return (dsl_dataset_destroy_check(&dsda, tag, tx));
	}

	return (0);
}

static void
dsl_dataset_user_release_sync(void *arg1, void *tag, dmu_tx_t *tx)
{
	struct dsl_ds_releasearg *ra = arg1;
	dsl_dataset_t *ds = ra->ds;
	dsl_pool_t *dp = ds->ds_dir->dd_pool;
	objset_t *mos = dp->dp_meta_objset;
	uint64_t zapobj;
	uint64_t dsobj = ds->ds_object;
	uint64_t refs;
	int error;

	mutex_enter(&ds->ds_lock);
	ds->ds_userrefs--;
	refs = ds->ds_userrefs;
	mutex_exit(&ds->ds_lock);
	error = dsl_pool_user_release(dp, ds->ds_object, ra->htag, tx);
	VERIFY(error == 0 || error == ENOENT);
	zapobj = ds->ds_phys->ds_userrefs_obj;
	VERIFY(0 == zap_remove(mos, zapobj, ra->htag, tx));
	if (ds->ds_userrefs == 0 && ds->ds_phys->ds_num_children == 1 &&
	    DS_IS_DEFER_DESTROY(ds)) {
		struct dsl_ds_destroyarg dsda = {0};

		ASSERT(ra->own);
		dsda.ds = ds;
		dsda.releasing = B_TRUE;
		/* We already did the destroy_check */
		dsl_dataset_destroy_sync(&dsda, tag, tx);
	}

	spa_history_log_internal(LOG_DS_USER_RELEASE,
	    dp->dp_spa, tx, "<%s> %lld dataset = %llu",
	    ra->htag, (longlong_t)refs, dsobj);
}

static int
dsl_dataset_user_release_one(const char *dsname, void *arg)
{
	struct dsl_ds_holdarg *ha = arg;
	struct dsl_ds_releasearg *ra;
	dsl_dataset_t *ds;
	int error;
	void *dtag = ha->dstg;
	char *name;
	boolean_t own = B_FALSE;
	boolean_t might_destroy;

	/* alloc a buffer to hold dsname@snapname, plus the terminating NULL */
	name = kmem_asprintf("%s@%s", dsname, ha->snapname);
	error = dsl_dataset_hold(name, dtag, &ds);
	strfree(name);
	if (error == ENOENT && ha->recursive)
		return (0);
	(void) strlcpy(ha->failed, dsname, sizeof (ha->failed));
	if (error)
		return (error);

	ha->gotone = B_TRUE;

	ASSERT(dsl_dataset_is_snapshot(ds));

	error = dsl_dataset_release_might_destroy(ds, ha->htag, &might_destroy);
	if (error) {
		dsl_dataset_rele(ds, dtag);
		return (error);
	}

	if (might_destroy) {
#ifdef _KERNEL
		name = kmem_asprintf("%s@%s", dsname, ha->snapname);
		error = zfs_unmount_snap(name, NULL);
		strfree(name);
		if (error) {
			dsl_dataset_rele(ds, dtag);
			return (error);
		}
#endif
		if (!dsl_dataset_tryown(ds, B_TRUE, dtag)) {
			dsl_dataset_rele(ds, dtag);
			return (EBUSY);
		} else {
			own = B_TRUE;
			dsl_dataset_make_exclusive(ds, dtag);
		}
	}

	ra = kmem_alloc(sizeof (struct dsl_ds_releasearg), KM_SLEEP);
	ra->ds = ds;
	ra->htag = ha->htag;
	ra->own = own;
	dsl_sync_task_create(ha->dstg, dsl_dataset_user_release_check,
	    dsl_dataset_user_release_sync, ra, dtag, 0);

	return (0);
}

int
dsl_dataset_user_release(char *dsname, char *snapname, char *htag,
    boolean_t recursive)
{
	struct dsl_ds_holdarg *ha;
	dsl_sync_task_t *dst;
	spa_t *spa;
	int error;

top:
	ha = kmem_zalloc(sizeof (struct dsl_ds_holdarg), KM_SLEEP);

	(void) strlcpy(ha->failed, dsname, sizeof (ha->failed));

	error = spa_open(dsname, &spa, FTAG);
	if (error) {
		kmem_free(ha, sizeof (struct dsl_ds_holdarg));
		return (error);
	}

	ha->dstg = dsl_sync_task_group_create(spa_get_dsl(spa));
	ha->htag = htag;
	ha->snapname = snapname;
	ha->recursive = recursive;
	if (recursive) {
		error = dmu_objset_find(dsname, dsl_dataset_user_release_one,
		    ha, DS_FIND_CHILDREN);
	} else {
		error = dsl_dataset_user_release_one(dsname, ha);
	}
	if (error == 0)
		error = dsl_sync_task_group_wait(ha->dstg);

	for (dst = list_head(&ha->dstg->dstg_tasks); dst;
	    dst = list_next(&ha->dstg->dstg_tasks, dst)) {
		struct dsl_ds_releasearg *ra = dst->dst_arg1;
		dsl_dataset_t *ds = ra->ds;

		if (dst->dst_err)
			dsl_dataset_name(ds, ha->failed);

		if (ra->own)
			dsl_dataset_disown(ds, ha->dstg);
		else
			dsl_dataset_rele(ds, ha->dstg);

		kmem_free(ra, sizeof (struct dsl_ds_releasearg));
	}

	if (error == 0 && recursive && !ha->gotone)
		error = ENOENT;

	if (error && error != EBUSY)
		(void) strlcpy(dsname, ha->failed, sizeof (ha->failed));

	dsl_sync_task_group_destroy(ha->dstg);
	kmem_free(ha, sizeof (struct dsl_ds_holdarg));
	spa_close(spa, FTAG);

	/*
	 * We can get EBUSY if we were racing with deferred destroy and
	 * dsl_dataset_user_release_check() hadn't done the necessary
	 * open context setup.  We can also get EBUSY if we're racing
	 * with destroy and that thread is the ds_owner.  Either way
	 * the busy condition should be transient, and we should retry
	 * the release operation.
	 */
	if (error == EBUSY)
		goto top;

	return (error);
}

/*
 * Called at spa_load time (with retry == B_FALSE) to release a stale
 * temporary user hold. Also called by the onexit code (with retry == B_TRUE).
 */
int
dsl_dataset_user_release_tmp(dsl_pool_t *dp, uint64_t dsobj, char *htag,
    boolean_t retry)
{
	dsl_dataset_t *ds;
	char *snap;
	char *name;
	int namelen;
	int error;

	do {
		rw_enter(&dp->dp_config_rwlock, RW_READER);
		error = dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds);
		rw_exit(&dp->dp_config_rwlock);
		if (error)
			return (error);
		namelen = dsl_dataset_namelen(ds)+1;
		name = kmem_alloc(namelen, KM_SLEEP);
		dsl_dataset_name(ds, name);
		dsl_dataset_rele(ds, FTAG);

		snap = strchr(name, '@');
		*snap = '\0';
		++snap;
		error = dsl_dataset_user_release(name, snap, htag, B_FALSE);
		kmem_free(name, namelen);

		/*
		 * The object can't have been destroyed because we have a hold,
		 * but it might have been renamed, resulting in ENOENT.  Retry
		 * if we've been requested to do so.
		 *
		 * It would be nice if we could use the dsobj all the way
		 * through and avoid ENOENT entirely.  But we might need to
		 * unmount the snapshot, and there's currently no way to lookup
		 * a vfsp using a ZFS object id.
		 */
	} while ((error == ENOENT) && retry);

	return (error);
}

int
dsl_dataset_get_holds(const char *dsname, nvlist_t **nvp)
{
	dsl_dataset_t *ds;
	int err;

	err = dsl_dataset_hold(dsname, FTAG, &ds);
	if (err)
		return (err);

	VERIFY(0 == nvlist_alloc(nvp, NV_UNIQUE_NAME, KM_SLEEP));
	if (ds->ds_phys->ds_userrefs_obj != 0) {
		zap_attribute_t *za;
		zap_cursor_t zc;

		za = kmem_alloc(sizeof (zap_attribute_t), KM_SLEEP);
		for (zap_cursor_init(&zc, ds->ds_dir->dd_pool->dp_meta_objset,
		    ds->ds_phys->ds_userrefs_obj);
		    zap_cursor_retrieve(&zc, za) == 0;
		    zap_cursor_advance(&zc)) {
			VERIFY(0 == nvlist_add_uint64(*nvp, za->za_name,
			    za->za_first_integer));
		}
		zap_cursor_fini(&zc);
		kmem_free(za, sizeof (zap_attribute_t));
	}
	dsl_dataset_rele(ds, FTAG);
	return (0);
}

/*
 * Note, this fuction is used as the callback for dmu_objset_find().  We
 * always return 0 so that we will continue to find and process
 * inconsistent datasets, even if we encounter an error trying to
 * process one of them.
 */
/* ARGSUSED */
int
dsl_destroy_inconsistent(const char *dsname, void *arg)
{
	dsl_dataset_t *ds;

	if (dsl_dataset_own(dsname, B_TRUE, FTAG, &ds) == 0) {
		if (DS_IS_INCONSISTENT(ds))
			(void) dsl_dataset_destroy(ds, FTAG, B_FALSE);
		else
			dsl_dataset_disown(ds, FTAG);
	}
	return (0);
}