fs/zfs/dmu_tx.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#pragma ident	"%Z%%M%	%I%	%E% SMI"

#include <sys/dmu.h>
#include <sys/dmu_impl.h>
#include <sys/dbuf.h>
#include <sys/dmu_tx.h>
#include <sys/dmu_objset.h>
#include <sys/dsl_dataset.h> /* for dsl_dataset_block_freeable() */
#include <sys/dsl_dir.h> /* for dsl_dir_tempreserve_*() */
#include <sys/dsl_pool.h>
#include <sys/zap_impl.h> /* for fzap_default_block_shift */
#include <sys/spa.h>
#include <sys/zfs_context.h>

typedef void (*dmu_tx_hold_func_t)(dmu_tx_t *tx, struct dnode *dn,
    uint64_t arg1, uint64_t arg2);


dmu_tx_t *
dmu_tx_create_dd(dsl_dir_t *dd)
{
	dmu_tx_t *tx = kmem_zalloc(sizeof (dmu_tx_t), KM_SLEEP);
	tx->tx_dir = dd;
	if (dd)
		tx->tx_pool = dd->dd_pool;
	list_create(&tx->tx_holds, sizeof (dmu_tx_hold_t),
	    offsetof(dmu_tx_hold_t, txh_node));
#ifdef ZFS_DEBUG
	refcount_create(&tx->tx_space_written);
	refcount_create(&tx->tx_space_freed);
#endif
	return (tx);
}

dmu_tx_t *
dmu_tx_create(objset_t *os)
{
	dmu_tx_t *tx = dmu_tx_create_dd(os->os->os_dsl_dataset->ds_dir);
	tx->tx_objset = os;
	tx->tx_lastsnap_txg = dsl_dataset_prev_snap_txg(os->os->os_dsl_dataset);
	return (tx);
}

dmu_tx_t *
dmu_tx_create_assigned(struct dsl_pool *dp, uint64_t txg)
{
	dmu_tx_t *tx = dmu_tx_create_dd(NULL);

	ASSERT3U(txg, <=, dp->dp_tx.tx_open_txg);
	tx->tx_pool = dp;
	tx->tx_txg = txg;
	tx->tx_anyobj = TRUE;

	return (tx);
}

int
dmu_tx_is_syncing(dmu_tx_t *tx)
{
	return (tx->tx_anyobj);
}

int
dmu_tx_private_ok(dmu_tx_t *tx)
{
	return (tx->tx_anyobj);
}

static dmu_tx_hold_t *
dmu_tx_hold_object_impl(dmu_tx_t *tx, objset_t *os, uint64_t object,
    enum dmu_tx_hold_type type, uint64_t arg1, uint64_t arg2)
{
	dmu_tx_hold_t *txh;
	dnode_t *dn = NULL;
	int err;

	if (object != DMU_NEW_OBJECT) {
		err = dnode_hold(os->os, object, tx, &dn);
		if (err) {
			tx->tx_err = err;
			return (NULL);
		}

		if (err == 0 && tx->tx_txg != 0) {
			mutex_enter(&dn->dn_mtx);
			/*
			 * dn->dn_assigned_txg == tx->tx_txg doesn't pose a
			 * problem, but there's no way for it to happen (for
			 * now, at least).
			 */
			ASSERT(dn->dn_assigned_txg == 0);
			dn->dn_assigned_txg = tx->tx_txg;
			(void) refcount_add(&dn->dn_tx_holds, tx);
			mutex_exit(&dn->dn_mtx);
		}
	}

	txh = kmem_zalloc(sizeof (dmu_tx_hold_t), KM_SLEEP);
	txh->txh_tx = tx;
	txh->txh_dnode = dn;
#ifdef ZFS_DEBUG
	txh->txh_type = type;
	txh->txh_arg1 = arg1;
	txh->txh_arg2 = arg2;
#endif
	list_insert_tail(&tx->tx_holds, txh);

	return (txh);
}

void
dmu_tx_add_new_object(dmu_tx_t *tx, objset_t *os, uint64_t object)
{
	/*
	 * If we're syncing, they can manipulate any object anyhow, and
	 * the hold on the dnode_t can cause problems.
	 */
	if (!dmu_tx_is_syncing(tx)) {
		(void) dmu_tx_hold_object_impl(tx, os,
		    object, THT_NEWOBJECT, 0, 0);
	}
}

static int
dmu_tx_check_ioerr(zio_t *zio, dnode_t *dn, int level, uint64_t blkid)
{
	int err;
	dmu_buf_impl_t *db;

	rw_enter(&dn->dn_struct_rwlock, RW_READER);
	db = dbuf_hold_level(dn, level, blkid, FTAG);
	rw_exit(&dn->dn_struct_rwlock);
	if (db == NULL)
		return (EIO);
	err = dbuf_read(db, zio, DB_RF_CANFAIL);
	dbuf_rele(db, FTAG);
	return (err);
}

/* ARGSUSED */
static void
dmu_tx_count_write(dmu_tx_hold_t *txh, uint64_t off, uint64_t len)
{
	dnode_t *dn = txh->txh_dnode;
	uint64_t start, end, i;
	int min_bs, max_bs, min_ibs, max_ibs, epbs, bits;
	int err = 0;

	if (len == 0)
		return;

	min_bs = SPA_MINBLOCKSHIFT;
	max_bs = SPA_MAXBLOCKSHIFT;
	min_ibs = DN_MIN_INDBLKSHIFT;
	max_ibs = DN_MAX_INDBLKSHIFT;


	/*
	 * For i/o error checking, read the first and last level-0
	 * blocks (if they are not aligned), and all the level-1 blocks.
	 */

	if (dn) {
		if (dn->dn_maxblkid == 0) {
			err = dmu_tx_check_ioerr(NULL, dn, 0, 0);
			if (err)
				goto out;
		} else {
			zio_t *zio = zio_root(dn->dn_objset->os_spa,
			    NULL, NULL, ZIO_FLAG_CANFAIL);

			/* first level-0 block */
			start = off >> dn->dn_datablkshift;
			if (P2PHASE(off, dn->dn_datablksz) ||
			    len < dn->dn_datablksz) {
				err = dmu_tx_check_ioerr(zio, dn, 0, start);
				if (err)
					goto out;
			}

			/* last level-0 block */
			end = (off+len-1) >> dn->dn_datablkshift;
			if (end != start &&
			    P2PHASE(off+len, dn->dn_datablksz)) {
				err = dmu_tx_check_ioerr(zio, dn, 0, end);
				if (err)
					goto out;
			}

			/* level-1 blocks */
			if (dn->dn_nlevels > 1) {
				start >>= dn->dn_indblkshift - SPA_BLKPTRSHIFT;
				end >>= dn->dn_indblkshift - SPA_BLKPTRSHIFT;
				for (i = start+1; i < end; i++) {
					err = dmu_tx_check_ioerr(zio, dn, 1, i);
					if (err)
						goto out;
				}
			}

			err = zio_wait(zio);
			if (err)
				goto out;
		}
	}

	/*
	 * If there's more than one block, the blocksize can't change,
	 * so we can make a more precise estimate.  Alternatively,
	 * if the dnode's ibs is larger than max_ibs, always use that.
	 * This ensures that if we reduce DN_MAX_INDBLKSHIFT,
	 * the code will still work correctly on existing pools.
	 */
	if (dn && (dn->dn_maxblkid != 0 || dn->dn_indblkshift > max_ibs)) {
		min_ibs = max_ibs = dn->dn_indblkshift;
		if (dn->dn_datablkshift != 0)
			min_bs = max_bs = dn->dn_datablkshift;
	}

	/*
	 * 'end' is the last thing we will access, not one past.
	 * This way we won't overflow when accessing the last byte.
	 */
	start = P2ALIGN(off, 1ULL << max_bs);
	end = P2ROUNDUP(off + len, 1ULL << max_bs) - 1;
	txh->txh_space_towrite += end - start + 1;

	start >>= min_bs;
	end >>= min_bs;

	epbs = min_ibs - SPA_BLKPTRSHIFT;

	/*
	 * The object contains at most 2^(64 - min_bs) blocks,
	 * and each indirect level maps 2^epbs.
	 */
	for (bits = 64 - min_bs; bits >= 0; bits -= epbs) {
		start >>= epbs;
		end >>= epbs;
		/*
		 * If we increase the number of levels of indirection,
		 * we'll need new blkid=0 indirect blocks.  If start == 0,
		 * we're already accounting for that blocks; and if end == 0,
		 * we can't increase the number of levels beyond that.
		 */
		if (start != 0 && end != 0)
			txh->txh_space_towrite += 1ULL << max_ibs;
		txh->txh_space_towrite += (end - start + 1) << max_ibs;
	}

	ASSERT(txh->txh_space_towrite < 2 * DMU_MAX_ACCESS);

out:
	if (err)
		txh->txh_tx->tx_err = err;
}

static void
dmu_tx_count_dnode(dmu_tx_hold_t *txh)
{
	dnode_t *dn = txh->txh_dnode;
	dnode_t *mdn = txh->txh_tx->tx_objset->os->os_meta_dnode;
	uint64_t space = mdn->dn_datablksz +
	    ((mdn->dn_nlevels-1) << mdn->dn_indblkshift);

	if (dn && dn->dn_dbuf->db_blkptr &&
	    dsl_dataset_block_freeable(dn->dn_objset->os_dsl_dataset,
	    dn->dn_dbuf->db_blkptr->blk_birth)) {
		txh->txh_space_tooverwrite += space;
	} else {
		txh->txh_space_towrite += space;
	}
}

void
dmu_tx_hold_write(dmu_tx_t *tx, uint64_t object, uint64_t off, int len)
{
	dmu_tx_hold_t *txh;

	ASSERT(tx->tx_txg == 0);
	ASSERT(len < DMU_MAX_ACCESS);
	ASSERT(len == 0 || UINT64_MAX - off >= len - 1);

	txh = dmu_tx_hold_object_impl(tx, tx->tx_objset,
	    object, THT_WRITE, off, len);
	if (txh == NULL)
		return;

	dmu_tx_count_write(txh, off, len);
	dmu_tx_count_dnode(txh);
}

static void
dmu_tx_count_free(dmu_tx_hold_t *txh, uint64_t off, uint64_t len)
{
	uint64_t blkid, nblks;
	uint64_t space = 0;
	dnode_t *dn = txh->txh_dnode;
	dsl_dataset_t *ds = dn->dn_objset->os_dsl_dataset;
	spa_t *spa = txh->txh_tx->tx_pool->dp_spa;
	int dirty;

	/*
	 * We don't need to use any locking to check for dirtyness
	 * because it's OK if we get stale data -- the dnode may become
	 * dirty immediately after our check anyway.  This is just a
	 * means to avoid the expensive count when we aren't sure we
	 * need it.  We need to be able to deal with a dirty dnode.
	 */
	dirty = list_link_active(&dn->dn_dirty_link[0]) |
	    list_link_active(&dn->dn_dirty_link[1]) |
	    list_link_active(&dn->dn_dirty_link[2]) |
	    list_link_active(&dn->dn_dirty_link[3]);
	if (dirty || dn->dn_assigned_txg || dn->dn_phys->dn_nlevels == 0)
		return;

	/*
	 * the struct_rwlock protects us against dn_phys->dn_nlevels
	 * changing, in case (against all odds) we manage to dirty &
	 * sync out the changes after we check for being dirty.
	 * also, dbuf_hold_impl() wants us to have the struct_rwlock.
	 *
	 * It's fine to use dn_datablkshift rather than the dn_phys
	 * equivalent because if it is changing, maxblkid==0 and we will
	 * bail.
	 */
	rw_enter(&dn->dn_struct_rwlock, RW_READER);
	if (dn->dn_phys->dn_maxblkid == 0) {
		if (off == 0 && len >= dn->dn_datablksz) {
			blkid = 0;
			nblks = 1;
		} else {
			rw_exit(&dn->dn_struct_rwlock);
			return;
		}
	} else {
		blkid = off >> dn->dn_datablkshift;
		nblks = (off + len) >> dn->dn_datablkshift;

		if (blkid >= dn->dn_phys->dn_maxblkid) {
			rw_exit(&dn->dn_struct_rwlock);
			return;
		}
		if (blkid + nblks > dn->dn_phys->dn_maxblkid)
			nblks = dn->dn_phys->dn_maxblkid - blkid;

		/* don't bother after 128,000 blocks */
		nblks = MIN(nblks, 128*1024);
	}

	if (dn->dn_phys->dn_nlevels == 1) {
		int i;
		for (i = 0; i < nblks; i++) {
			blkptr_t *bp = dn->dn_phys->dn_blkptr;
			ASSERT3U(blkid + i, <, dn->dn_phys->dn_nblkptr);
			bp += blkid + i;
			if (dsl_dataset_block_freeable(ds, bp->blk_birth)) {
				dprintf_bp(bp, "can free old%s", "");
				space += bp_get_dasize(spa, bp);
			}
		}
		nblks = 0;
	}

	while (nblks) {
		dmu_buf_impl_t *dbuf;
		int err, epbs, blkoff, tochk;

		epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;
		blkoff = P2PHASE(blkid, 1<<epbs);
		tochk = MIN((1<<epbs) - blkoff, nblks);

		err = dbuf_hold_impl(dn, 1, blkid >> epbs, TRUE, FTAG, &dbuf);
		if (err == 0) {
			int i;
			blkptr_t *bp;

			err = dbuf_read(dbuf, NULL,
			    DB_RF_HAVESTRUCT | DB_RF_CANFAIL);
			if (err != 0) {
				txh->txh_tx->tx_err = err;
				dbuf_rele(dbuf, FTAG);
				break;
			}

			bp = dbuf->db.db_data;
			bp += blkoff;

			for (i = 0; i < tochk; i++) {
				if (dsl_dataset_block_freeable(ds,
				    bp[i].blk_birth)) {
					dprintf_bp(&bp[i],
					    "can free old%s", "");
					space += bp_get_dasize(spa, &bp[i]);
				}
			}
			dbuf_rele(dbuf, FTAG);
		}
		if (err && err != ENOENT) {
			txh->txh_tx->tx_err = err;
			break;
		}

		blkid += tochk;
		nblks -= tochk;
	}
	rw_exit(&dn->dn_struct_rwlock);

	txh->txh_space_tofree += space;
}

void
dmu_tx_hold_free(dmu_tx_t *tx, uint64_t object, uint64_t off, uint64_t len)
{
	dmu_tx_hold_t *txh;
	dnode_t *dn;
	uint64_t start, end, i;
	int err, shift;
	zio_t *zio;

	ASSERT(tx->tx_txg == 0);

	txh = dmu_tx_hold_object_impl(tx, tx->tx_objset,
	    object, THT_FREE, off, len);
	if (txh == NULL)
		return;
	dn = txh->txh_dnode;

	/* first block */
	if (off != 0)
		dmu_tx_count_write(txh, off, 1);
	/* last block */
	if (len != DMU_OBJECT_END)
		dmu_tx_count_write(txh, off+len, 1);

	if (off >= (dn->dn_maxblkid+1) * dn->dn_datablksz)
		return;
	if (len == DMU_OBJECT_END)
		len = (dn->dn_maxblkid+1) * dn->dn_datablksz - off;

	/*
	 * For i/o error checking, read the first and last level-0
	 * blocks, and all the level-1 blocks.  The above count_write's
	 * will take care of the level-0 blocks.
	 */
	if (dn->dn_nlevels > 1) {
		shift = dn->dn_datablkshift + dn->dn_indblkshift -
		    SPA_BLKPTRSHIFT;
		start = off >> shift;
		end = dn->dn_datablkshift ? ((off+len) >> shift) : 0;

		zio = zio_root(tx->tx_pool->dp_spa,
		    NULL, NULL, ZIO_FLAG_CANFAIL);
		for (i = start; i <= end; i++) {
			uint64_t ibyte = i << shift;
			err = dnode_next_offset(dn, FALSE, &ibyte, 2, 1, 0);
			i = ibyte >> shift;
			if (err == ESRCH)
				break;
			if (err) {
				tx->tx_err = err;
				return;
			}

			err = dmu_tx_check_ioerr(zio, dn, 1, i);
			if (err) {
				tx->tx_err = err;
				return;
			}
		}
		err = zio_wait(zio);
		if (err) {
			tx->tx_err = err;
			return;
		}
	}

	dmu_tx_count_dnode(txh);
	dmu_tx_count_free(txh, off, len);
}

void
dmu_tx_hold_zap(dmu_tx_t *tx, uint64_t object, int add, char *name)
{
	dmu_tx_hold_t *txh;
	dnode_t *dn;
	uint64_t nblocks;
	int epbs, err;

	ASSERT(tx->tx_txg == 0);

	txh = dmu_tx_hold_object_impl(tx, tx->tx_objset,
	    object, THT_ZAP, add, (uintptr_t)name);
	if (txh == NULL)
		return;
	dn = txh->txh_dnode;

	dmu_tx_count_dnode(txh);

	if (dn == NULL) {
		/*
		 * We will be able to fit a new object's entries into one leaf
		 * block.  So there will be at most 2 blocks total,
		 * including the header block.
		 */
		dmu_tx_count_write(txh, 0, 2 << fzap_default_block_shift);
		return;
	}

	ASSERT3P(dmu_ot[dn->dn_type].ot_byteswap, ==, zap_byteswap);

	if (dn->dn_maxblkid == 0 && !add) {
		/*
		 * If there is only one block  (i.e. this is a micro-zap)
		 * and we are not adding anything, the accounting is simple.
		 */
		err = dmu_tx_check_ioerr(NULL, dn, 0, 0);
		if (err) {
			tx->tx_err = err;
			return;
		}

		if (dsl_dataset_block_freeable(dn->dn_objset->os_dsl_dataset,
		    dn->dn_phys->dn_blkptr[0].blk_birth))
			txh->txh_space_tooverwrite += dn->dn_datablksz;
		else
			txh->txh_space_towrite += dn->dn_datablksz;
		return;
	}

	if (dn->dn_maxblkid > 0 && name) {
		/*
		 * access the name in this fat-zap so that we'll check
		 * for i/o errors to the leaf blocks, etc.
		 */
		err = zap_lookup(&dn->dn_objset->os, dn->dn_object, name,
		    8, 0, NULL);
		if (err == EIO) {
			tx->tx_err = err;
			return;
		}
	}

	/*
	 * 3 blocks overwritten: target leaf, ptrtbl block, header block
	 * 3 new blocks written if adding: new split leaf, 2 grown ptrtbl blocks
	 */
	dmu_tx_count_write(txh, dn->dn_maxblkid * dn->dn_datablksz,
	    (3 + add ? 3 : 0) << dn->dn_datablkshift);

	/*
	 * If the modified blocks are scattered to the four winds,
	 * we'll have to modify an indirect twig for each.
	 */
	epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;
	for (nblocks = dn->dn_maxblkid >> epbs; nblocks != 0; nblocks >>= epbs)
		txh->txh_space_towrite += 3 << dn->dn_indblkshift;
}

void
dmu_tx_hold_bonus(dmu_tx_t *tx, uint64_t object)
{
	dmu_tx_hold_t *txh;

	ASSERT(tx->tx_txg == 0);

	txh = dmu_tx_hold_object_impl(tx, tx->tx_objset,
	    object, THT_BONUS, 0, 0);
	if (txh)
		dmu_tx_count_dnode(txh);
}

void
dmu_tx_hold_space(dmu_tx_t *tx, uint64_t space)
{
	dmu_tx_hold_t *txh;
	ASSERT(tx->tx_txg == 0);

	txh = dmu_tx_hold_object_impl(tx, tx->tx_objset,
	    DMU_NEW_OBJECT, THT_SPACE, space, 0);

	txh->txh_space_towrite += space;
}

int
dmu_tx_holds(dmu_tx_t *tx, uint64_t object)
{
	dmu_tx_hold_t *txh;
	int holds = 0;

	/*
	 * By asserting that the tx is assigned, we're counting the
	 * number of dn_tx_holds, which is the same as the number of
	 * dn_holds.  Otherwise, we'd be counting dn_holds, but
	 * dn_tx_holds could be 0.
	 */
	ASSERT(tx->tx_txg != 0);

	/* if (tx->tx_anyobj == TRUE) */
		/* return (0); */

	for (txh = list_head(&tx->tx_holds); txh;
	    txh = list_next(&tx->tx_holds, txh)) {
		if (txh->txh_dnode && txh->txh_dnode->dn_object == object)
			holds++;
	}

	return (holds);
}

#ifdef ZFS_DEBUG
void
dmu_tx_dirty_buf(dmu_tx_t *tx, dmu_buf_impl_t *db)
{
	dmu_tx_hold_t *txh;
	int match_object = FALSE, match_offset = FALSE;
	dnode_t *dn = db->db_dnode;

	ASSERT(tx->tx_txg != 0);
	ASSERT(tx->tx_objset == NULL || dn->dn_objset == tx->tx_objset->os);
	ASSERT3U(dn->dn_object, ==, db->db.db_object);

	if (tx->tx_anyobj)
		return;

	/* XXX No checking on the meta dnode for now */
	if (db->db.db_object == DMU_META_DNODE_OBJECT)
		return;

	for (txh = list_head(&tx->tx_holds); txh;
	    txh = list_next(&tx->tx_holds, txh)) {
		ASSERT(dn == NULL || dn->dn_assigned_txg == tx->tx_txg);
		if (txh->txh_dnode == dn && txh->txh_type != THT_NEWOBJECT)
			match_object = TRUE;
		if (txh->txh_dnode == NULL || txh->txh_dnode == dn) {
			int datablkshift = dn->dn_datablkshift ?
			    dn->dn_datablkshift : SPA_MAXBLOCKSHIFT;
			int epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;
			int shift = datablkshift + epbs * db->db_level;
			uint64_t beginblk = shift >= 64 ? 0 :
			    (txh->txh_arg1 >> shift);
			uint64_t endblk = shift >= 64 ? 0 :
			    ((txh->txh_arg1 + txh->txh_arg2 - 1) >> shift);
			uint64_t blkid = db->db_blkid;

			/* XXX txh_arg2 better not be zero... */

			dprintf("found txh type %x beginblk=%llx endblk=%llx\n",
			    txh->txh_type, beginblk, endblk);

			switch (txh->txh_type) {
			case THT_WRITE:
				if (blkid >= beginblk && blkid <= endblk)
					match_offset = TRUE;
				/*
				 * We will let this hold work for the bonus
				 * buffer so that we don't need to hold it
				 * when creating a new object.
				 */
				if (blkid == DB_BONUS_BLKID)
					match_offset = TRUE;
				/*
				 * They might have to increase nlevels,
				 * thus dirtying the new TLIBs.  Or the
				 * might have to change the block size,
				 * thus dirying the new lvl=0 blk=0.
				 */
				if (blkid == 0)
					match_offset = TRUE;
				break;
			case THT_FREE:
				if (blkid == beginblk &&
				    (txh->txh_arg1 != 0 ||
				    dn->dn_maxblkid == 0))
					match_offset = TRUE;
				if (blkid == endblk &&
				    txh->txh_arg2 != DMU_OBJECT_END)
					match_offset = TRUE;
				break;
			case THT_BONUS:
				if (blkid == DB_BONUS_BLKID)
					match_offset = TRUE;
				break;
			case THT_ZAP:
				match_offset = TRUE;
				break;
			case THT_NEWOBJECT:
				match_object = TRUE;
				break;
			default:
				ASSERT(!"bad txh_type");
			}
		}
		if (match_object && match_offset)
			return;
	}
	panic("dirtying dbuf obj=%llx lvl=%u blkid=%llx but not tx_held\n",
	    (u_longlong_t)db->db.db_object, db->db_level,
	    (u_longlong_t)db->db_blkid);
}
#endif

static int
dmu_tx_try_assign(dmu_tx_t *tx, uint64_t txg_how)
{
	dmu_tx_hold_t *txh;
	uint64_t lsize, asize, fsize, towrite, tofree, tooverwrite;

	ASSERT3U(tx->tx_txg, ==, 0);
	if (tx->tx_err)
		return (tx->tx_err);

	tx->tx_txg = txg_hold_open(tx->tx_pool, &tx->tx_txgh);
	tx->tx_needassign_txh = NULL;

	/*
	 * NB: No error returns are allowed after txg_hold_open, but
	 * before processing the dnode holds, due to the
	 * dmu_tx_unassign() logic.
	 */

	towrite = tofree = tooverwrite = 0;
	for (txh = list_head(&tx->tx_holds); txh;
	    txh = list_next(&tx->tx_holds, txh)) {
		dnode_t *dn = txh->txh_dnode;
		if (dn != NULL) {
			mutex_enter(&dn->dn_mtx);
			if (dn->dn_assigned_txg == tx->tx_txg - 1) {
				mutex_exit(&dn->dn_mtx);
				tx->tx_needassign_txh = txh;
				return (ERESTART);
			}
			if (dn->dn_assigned_txg == 0)
				dn->dn_assigned_txg = tx->tx_txg;
			ASSERT3U(dn->dn_assigned_txg, ==, tx->tx_txg);
			(void) refcount_add(&dn->dn_tx_holds, tx);
			mutex_exit(&dn->dn_mtx);
		}
		towrite += txh->txh_space_towrite;
		tofree += txh->txh_space_tofree;
		tooverwrite += txh->txh_space_tooverwrite;
	}

	/*
	 * NB: This check must be after we've held the dnodes, so that
	 * the dmu_tx_unassign() logic will work properly
	 */
	if (txg_how >= TXG_INITIAL && txg_how != tx->tx_txg)
		return (ERESTART);

	/*
	 * If a snapshot has been taken since we made our estimates,
	 * assume that we won't be able to free or overwrite anything.
	 */
	if (tx->tx_objset &&
	    dsl_dataset_prev_snap_txg(tx->tx_objset->os->os_dsl_dataset) >
	    tx->tx_lastsnap_txg) {
		towrite += tooverwrite;
		tooverwrite = tofree = 0;
	}

	/*
	 * Convert logical size to worst-case allocated size.
	 */
	fsize = spa_get_asize(tx->tx_pool->dp_spa, tooverwrite) + tofree;
	lsize = towrite + tooverwrite;
	asize = spa_get_asize(tx->tx_pool->dp_spa, lsize);

#ifdef ZFS_DEBUG
	tx->tx_space_towrite = asize;
	tx->tx_space_tofree = tofree;
	tx->tx_space_tooverwrite = tooverwrite;
#endif

	if (tx->tx_dir && asize != 0) {
		int err = dsl_dir_tempreserve_space(tx->tx_dir,
		    lsize, asize, fsize, &tx->tx_tempreserve_cookie, tx);
		if (err)
			return (err);
	}

	return (0);
}

static void
dmu_tx_unassign(dmu_tx_t *tx)
{
	dmu_tx_hold_t *txh;

	if (tx->tx_txg == 0)
		return;

	txg_rele_to_quiesce(&tx->tx_txgh);

	for (txh = list_head(&tx->tx_holds); txh != tx->tx_needassign_txh;
	    txh = list_next(&tx->tx_holds, txh)) {
		dnode_t *dn = txh->txh_dnode;

		if (dn == NULL)
			continue;
		mutex_enter(&dn->dn_mtx);
		ASSERT3U(dn->dn_assigned_txg, ==, tx->tx_txg);

		if (refcount_remove(&dn->dn_tx_holds, tx) == 0) {
			dn->dn_assigned_txg = 0;
			cv_broadcast(&dn->dn_notxholds);
		}
		mutex_exit(&dn->dn_mtx);
	}

	txg_rele_to_sync(&tx->tx_txgh);

	tx->tx_lasttried_txg = tx->tx_txg;
	tx->tx_txg = 0;
}

/*
 * Assign tx to a transaction group.  txg_how can be one of:
 *
 * (1)	TXG_WAIT.  If the current open txg is full, waits until there's
 *	a new one.  This should be used when you're not holding locks.
 *	If will only fail if we're truly out of space (or over quota).
 *
 * (2)	TXG_NOWAIT.  If we can't assign into the current open txg without
 *	blocking, returns immediately with ERESTART.  This should be used
 *	whenever you're holding locks.  On an ERESTART error, the caller
 *	should drop locks, do a dmu_tx_wait(tx), and try again.
 *
 * (3)	A specific txg.  Use this if you need to ensure that multiple
 *	transactions all sync in the same txg.  Like TXG_NOWAIT, it
 *	returns ERESTART if it can't assign you into the requested txg.
 */
int
dmu_tx_assign(dmu_tx_t *tx, uint64_t txg_how)
{
	int err;

	ASSERT(tx->tx_txg == 0);
	ASSERT(txg_how != 0);
	ASSERT(!dsl_pool_sync_context(tx->tx_pool));

	while ((err = dmu_tx_try_assign(tx, txg_how)) != 0) {
		dmu_tx_unassign(tx);

		if (err != ERESTART || txg_how != TXG_WAIT)
			return (err);

		dmu_tx_wait(tx);
	}

	txg_rele_to_quiesce(&tx->tx_txgh);

	return (0);
}

void
dmu_tx_wait(dmu_tx_t *tx)
{
	ASSERT(tx->tx_txg == 0);
	ASSERT(tx->tx_lasttried_txg != 0);

	if (tx->tx_needassign_txh) {
		dnode_t *dn = tx->tx_needassign_txh->txh_dnode;

		mutex_enter(&dn->dn_mtx);
		while (dn->dn_assigned_txg == tx->tx_lasttried_txg - 1)
			cv_wait(&dn->dn_notxholds, &dn->dn_mtx);
		mutex_exit(&dn->dn_mtx);
		tx->tx_needassign_txh = NULL;
	} else {
		txg_wait_open(tx->tx_pool, tx->tx_lasttried_txg + 1);
	}
}

void
dmu_tx_willuse_space(dmu_tx_t *tx, int64_t delta)
{
#ifdef ZFS_DEBUG
	if (tx->tx_dir == NULL || delta == 0)
		return;

	if (delta > 0) {
		ASSERT3U(refcount_count(&tx->tx_space_written) + delta, <=,
		    tx->tx_space_towrite);
		(void) refcount_add_many(&tx->tx_space_written, delta, NULL);
	} else {
		(void) refcount_add_many(&tx->tx_space_freed, -delta, NULL);
	}
#endif
}

void
dmu_tx_commit(dmu_tx_t *tx)
{
	dmu_tx_hold_t *txh;

	ASSERT(tx->tx_txg != 0);

	while (txh = list_head(&tx->tx_holds)) {
		dnode_t *dn = txh->txh_dnode;

		list_remove(&tx->tx_holds, txh);
		kmem_free(txh, sizeof (dmu_tx_hold_t));
		if (dn == NULL)
			continue;
		mutex_enter(&dn->dn_mtx);
		ASSERT3U(dn->dn_assigned_txg, ==, tx->tx_txg);

		if (refcount_remove(&dn->dn_tx_holds, tx) == 0) {
			dn->dn_assigned_txg = 0;
			cv_broadcast(&dn->dn_notxholds);
		}
		mutex_exit(&dn->dn_mtx);
		dnode_rele(dn, tx);
	}

	if (tx->tx_tempreserve_cookie)
		dsl_dir_tempreserve_clear(tx->tx_tempreserve_cookie, tx);

	if (tx->tx_anyobj == FALSE)
		txg_rele_to_sync(&tx->tx_txgh);
#ifdef ZFS_DEBUG
	dprintf("towrite=%llu written=%llu tofree=%llu freed=%llu\n",
	    tx->tx_space_towrite, refcount_count(&tx->tx_space_written),
	    tx->tx_space_tofree, refcount_count(&tx->tx_space_freed));
	refcount_destroy_many(&tx->tx_space_written,
	    refcount_count(&tx->tx_space_written));
	refcount_destroy_many(&tx->tx_space_freed,
	    refcount_count(&tx->tx_space_freed));
#endif
	kmem_free(tx, sizeof (dmu_tx_t));
}

void
dmu_tx_abort(dmu_tx_t *tx)
{
	dmu_tx_hold_t *txh;

	ASSERT(tx->tx_txg == 0);

	while (txh = list_head(&tx->tx_holds)) {
		dnode_t *dn = txh->txh_dnode;

		list_remove(&tx->tx_holds, txh);
		kmem_free(txh, sizeof (dmu_tx_hold_t));
		if (dn != NULL)
			dnode_rele(dn, tx);
	}
#ifdef ZFS_DEBUG
	refcount_destroy_many(&tx->tx_space_written,
	    refcount_count(&tx->tx_space_written));
	refcount_destroy_many(&tx->tx_space_freed,
	    refcount_count(&tx->tx_space_freed));
#endif
	kmem_free(tx, sizeof (dmu_tx_t));
}

uint64_t
dmu_tx_get_txg(dmu_tx_t *tx)
{
	ASSERT(tx->tx_txg != 0);
	return (tx->tx_txg);
}