Rebase master to b105

[mirror_zfs.git] / module / zfs / dnode.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 2008 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#include <sys/zfs_context.h>
#include <sys/dbuf.h>
#include <sys/dnode.h>
#include <sys/dmu.h>
#include <sys/dmu_impl.h>
#include <sys/dmu_tx.h>
#include <sys/dmu_objset.h>
#include <sys/dsl_dir.h>
#include <sys/dsl_dataset.h>
#include <sys/spa.h>
#include <sys/zio.h>
#include <sys/dmu_zfetch.h>

static int free_range_compar(const void *node1, const void *node2);

static kmem_cache_t *dnode_cache;

static dnode_phys_t dnode_phys_zero;

int zfs_default_bs = SPA_MINBLOCKSHIFT;
int zfs_default_ibs = DN_MAX_INDBLKSHIFT;

/* ARGSUSED */
static int
dnode_cons(void *arg, void *unused, int kmflag)
{
        int i;
        dnode_t *dn = arg;
        bzero(dn, sizeof (dnode_t));

        rw_init(&dn->dn_struct_rwlock, NULL, RW_DEFAULT, NULL);
        mutex_init(&dn->dn_mtx, NULL, MUTEX_DEFAULT, NULL);
        mutex_init(&dn->dn_dbufs_mtx, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&dn->dn_notxholds, NULL, CV_DEFAULT, NULL);

        refcount_create(&dn->dn_holds);
        refcount_create(&dn->dn_tx_holds);

        for (i = 0; i < TXG_SIZE; i++) {
                avl_create(&dn->dn_ranges[i], free_range_compar,
                    sizeof (free_range_t),
                    offsetof(struct free_range, fr_node));
                list_create(&dn->dn_dirty_records[i],
                    sizeof (dbuf_dirty_record_t),
                    offsetof(dbuf_dirty_record_t, dr_dirty_node));
        }

        list_create(&dn->dn_dbufs, sizeof (dmu_buf_impl_t),
            offsetof(dmu_buf_impl_t, db_link));

        return (0);
}

/* ARGSUSED */
static void
dnode_dest(void *arg, void *unused)
{
        int i;
        dnode_t *dn = arg;

        rw_destroy(&dn->dn_struct_rwlock);
        mutex_destroy(&dn->dn_mtx);
        mutex_destroy(&dn->dn_dbufs_mtx);
        cv_destroy(&dn->dn_notxholds);
        refcount_destroy(&dn->dn_holds);
        refcount_destroy(&dn->dn_tx_holds);

        for (i = 0; i < TXG_SIZE; i++) {
                avl_destroy(&dn->dn_ranges[i]);
                list_destroy(&dn->dn_dirty_records[i]);
        }

        list_destroy(&dn->dn_dbufs);
}

void
dnode_init(void)
{
        dnode_cache = kmem_cache_create("dnode_t",
            sizeof (dnode_t),
            0, dnode_cons, dnode_dest, NULL, NULL, NULL, 0);
}

void
dnode_fini(void)
{
        kmem_cache_destroy(dnode_cache);
}


#ifdef ZFS_DEBUG
void
dnode_verify(dnode_t *dn)
{
        int drop_struct_lock = FALSE;

        ASSERT(dn->dn_phys);
        ASSERT(dn->dn_objset);

        ASSERT(dn->dn_phys->dn_type < DMU_OT_NUMTYPES);

        if (!(zfs_flags & ZFS_DEBUG_DNODE_VERIFY))
                return;

        if (!RW_WRITE_HELD(&dn->dn_struct_rwlock)) {
                rw_enter(&dn->dn_struct_rwlock, RW_READER);
                drop_struct_lock = TRUE;
        }
        if (dn->dn_phys->dn_type != DMU_OT_NONE || dn->dn_allocated_txg != 0) {
                int i;
                ASSERT3U(dn->dn_indblkshift, >=, 0);
                ASSERT3U(dn->dn_indblkshift, <=, SPA_MAXBLOCKSHIFT);
                if (dn->dn_datablkshift) {
                        ASSERT3U(dn->dn_datablkshift, >=, SPA_MINBLOCKSHIFT);
                        ASSERT3U(dn->dn_datablkshift, <=, SPA_MAXBLOCKSHIFT);
                        ASSERT3U(1<<dn->dn_datablkshift, ==, dn->dn_datablksz);
                }
                ASSERT3U(dn->dn_nlevels, <=, 30);
                ASSERT3U(dn->dn_type, <=, DMU_OT_NUMTYPES);
                ASSERT3U(dn->dn_nblkptr, >=, 1);
                ASSERT3U(dn->dn_nblkptr, <=, DN_MAX_NBLKPTR);
                ASSERT3U(dn->dn_bonuslen, <=, DN_MAX_BONUSLEN);
                ASSERT3U(dn->dn_datablksz, ==,
                    dn->dn_datablkszsec << SPA_MINBLOCKSHIFT);
                ASSERT3U(ISP2(dn->dn_datablksz), ==, dn->dn_datablkshift != 0);
                ASSERT3U((dn->dn_nblkptr - 1) * sizeof (blkptr_t) +
                    dn->dn_bonuslen, <=, DN_MAX_BONUSLEN);
                for (i = 0; i < TXG_SIZE; i++) {
                        ASSERT3U(dn->dn_next_nlevels[i], <=, dn->dn_nlevels);
                }
        }
        if (dn->dn_phys->dn_type != DMU_OT_NONE)
                ASSERT3U(dn->dn_phys->dn_nlevels, <=, dn->dn_nlevels);
        ASSERT(dn->dn_object == DMU_META_DNODE_OBJECT || dn->dn_dbuf != NULL);
        if (dn->dn_dbuf != NULL) {
                ASSERT3P(dn->dn_phys, ==,
                    (dnode_phys_t *)dn->dn_dbuf->db.db_data +
                    (dn->dn_object % (dn->dn_dbuf->db.db_size >> DNODE_SHIFT)));
        }
        if (drop_struct_lock)
                rw_exit(&dn->dn_struct_rwlock);
}
#endif

void
dnode_byteswap(dnode_phys_t *dnp)
{
        uint64_t *buf64 = (void*)&dnp->dn_blkptr;
        int i;

        if (dnp->dn_type == DMU_OT_NONE) {
                bzero(dnp, sizeof (dnode_phys_t));
                return;
        }

        dnp->dn_datablkszsec = BSWAP_16(dnp->dn_datablkszsec);
        dnp->dn_bonuslen = BSWAP_16(dnp->dn_bonuslen);
        dnp->dn_maxblkid = BSWAP_64(dnp->dn_maxblkid);
        dnp->dn_used = BSWAP_64(dnp->dn_used);

        /*
         * dn_nblkptr is only one byte, so it's OK to read it in either
         * byte order.  We can't read dn_bouslen.
         */
        ASSERT(dnp->dn_indblkshift <= SPA_MAXBLOCKSHIFT);
        ASSERT(dnp->dn_nblkptr <= DN_MAX_NBLKPTR);
        for (i = 0; i < dnp->dn_nblkptr * sizeof (blkptr_t)/8; i++)
                buf64[i] = BSWAP_64(buf64[i]);

        /*
         * OK to check dn_bonuslen for zero, because it won't matter if
         * we have the wrong byte order.  This is necessary because the
         * dnode dnode is smaller than a regular dnode.
         */
        if (dnp->dn_bonuslen != 0) {
                /*
                 * Note that the bonus length calculated here may be
                 * longer than the actual bonus buffer.  This is because
                 * we always put the bonus buffer after the last block
                 * pointer (instead of packing it against the end of the
                 * dnode buffer).
                 */
                int off = (dnp->dn_nblkptr-1) * sizeof (blkptr_t);
                size_t len = DN_MAX_BONUSLEN - off;
                ASSERT3U(dnp->dn_bonustype, <, DMU_OT_NUMTYPES);
                dmu_ot[dnp->dn_bonustype].ot_byteswap(dnp->dn_bonus + off, len);
        }
}

void
dnode_buf_byteswap(void *vbuf, size_t size)
{
        dnode_phys_t *buf = vbuf;
        int i;

        ASSERT3U(sizeof (dnode_phys_t), ==, (1<<DNODE_SHIFT));
        ASSERT((size & (sizeof (dnode_phys_t)-1)) == 0);

        size >>= DNODE_SHIFT;
        for (i = 0; i < size; i++) {
                dnode_byteswap(buf);
                buf++;
        }
}

static int
free_range_compar(const void *node1, const void *node2)
{
        const free_range_t *rp1 = node1;
        const free_range_t *rp2 = node2;

        if (rp1->fr_blkid < rp2->fr_blkid)
                return (-1);
        else if (rp1->fr_blkid > rp2->fr_blkid)
                return (1);
        else return (0);
}

void
dnode_setbonuslen(dnode_t *dn, int newsize, dmu_tx_t *tx)
{
        ASSERT3U(refcount_count(&dn->dn_holds), >=, 1);

        dnode_setdirty(dn, tx);
        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
        ASSERT3U(newsize, <=, DN_MAX_BONUSLEN -
            (dn->dn_nblkptr-1) * sizeof (blkptr_t));
        dn->dn_bonuslen = newsize;
        if (newsize == 0)
                dn->dn_next_bonuslen[tx->tx_txg & TXG_MASK] = DN_ZERO_BONUSLEN;
        else
                dn->dn_next_bonuslen[tx->tx_txg & TXG_MASK] = dn->dn_bonuslen;
        rw_exit(&dn->dn_struct_rwlock);
}

static void
dnode_setdblksz(dnode_t *dn, int size)
{
        ASSERT3U(P2PHASE(size, SPA_MINBLOCKSIZE), ==, 0);
        ASSERT3U(size, <=, SPA_MAXBLOCKSIZE);
        ASSERT3U(size, >=, SPA_MINBLOCKSIZE);
        ASSERT3U(size >> SPA_MINBLOCKSHIFT, <,
            1<<(sizeof (dn->dn_phys->dn_datablkszsec) * 8));
        dn->dn_datablksz = size;
        dn->dn_datablkszsec = size >> SPA_MINBLOCKSHIFT;
        dn->dn_datablkshift = ISP2(size) ? highbit(size - 1) : 0;
}

static dnode_t *
dnode_create(objset_impl_t *os, dnode_phys_t *dnp, dmu_buf_impl_t *db,
    uint64_t object)
{
        dnode_t *dn = kmem_cache_alloc(dnode_cache, KM_SLEEP);
        (void) dnode_cons(dn, NULL, 0); /* XXX */

        dn->dn_objset = os;
        dn->dn_object = object;
        dn->dn_dbuf = db;
        dn->dn_phys = dnp;

        if (dnp->dn_datablkszsec)
                dnode_setdblksz(dn, dnp->dn_datablkszsec << SPA_MINBLOCKSHIFT);
        dn->dn_indblkshift = dnp->dn_indblkshift;
        dn->dn_nlevels = dnp->dn_nlevels;
        dn->dn_type = dnp->dn_type;
        dn->dn_nblkptr = dnp->dn_nblkptr;
        dn->dn_checksum = dnp->dn_checksum;
        dn->dn_compress = dnp->dn_compress;
        dn->dn_bonustype = dnp->dn_bonustype;
        dn->dn_bonuslen = dnp->dn_bonuslen;
        dn->dn_maxblkid = dnp->dn_maxblkid;

        dmu_zfetch_init(&dn->dn_zfetch, dn);

        ASSERT(dn->dn_phys->dn_type < DMU_OT_NUMTYPES);
        mutex_enter(&os->os_lock);
        list_insert_head(&os->os_dnodes, dn);
        mutex_exit(&os->os_lock);

        arc_space_consume(sizeof (dnode_t));
        return (dn);
}

static void
dnode_destroy(dnode_t *dn)
{
        objset_impl_t *os = dn->dn_objset;

#ifdef ZFS_DEBUG
        int i;

        for (i = 0; i < TXG_SIZE; i++) {
                ASSERT(!list_link_active(&dn->dn_dirty_link[i]));
                ASSERT(NULL == list_head(&dn->dn_dirty_records[i]));
                ASSERT(0 == avl_numnodes(&dn->dn_ranges[i]));
        }
        ASSERT(NULL == list_head(&dn->dn_dbufs));
#endif

        mutex_enter(&os->os_lock);
        list_remove(&os->os_dnodes, dn);
        mutex_exit(&os->os_lock);

        if (dn->dn_dirtyctx_firstset) {
                kmem_free(dn->dn_dirtyctx_firstset, 1);
                dn->dn_dirtyctx_firstset = NULL;
        }
        dmu_zfetch_rele(&dn->dn_zfetch);
        if (dn->dn_bonus) {
                mutex_enter(&dn->dn_bonus->db_mtx);
                dbuf_evict(dn->dn_bonus);
                dn->dn_bonus = NULL;
        }
        kmem_cache_free(dnode_cache, dn);
        arc_space_return(sizeof (dnode_t));
}

void
dnode_allocate(dnode_t *dn, dmu_object_type_t ot, int blocksize, int ibs,
    dmu_object_type_t bonustype, int bonuslen, dmu_tx_t *tx)
{
        int i;

        if (blocksize == 0)
                blocksize = 1 << zfs_default_bs;
        else if (blocksize > SPA_MAXBLOCKSIZE)
                blocksize = SPA_MAXBLOCKSIZE;
        else
                blocksize = P2ROUNDUP(blocksize, SPA_MINBLOCKSIZE);

        if (ibs == 0)
                ibs = zfs_default_ibs;

        ibs = MIN(MAX(ibs, DN_MIN_INDBLKSHIFT), DN_MAX_INDBLKSHIFT);

        dprintf("os=%p obj=%llu txg=%llu blocksize=%d ibs=%d\n", dn->dn_objset,
            dn->dn_object, tx->tx_txg, blocksize, ibs);

        ASSERT(dn->dn_type == DMU_OT_NONE);
        ASSERT(bcmp(dn->dn_phys, &dnode_phys_zero, sizeof (dnode_phys_t)) == 0);
        ASSERT(dn->dn_phys->dn_type == DMU_OT_NONE);
        ASSERT(ot != DMU_OT_NONE);
        ASSERT3U(ot, <, DMU_OT_NUMTYPES);
        ASSERT((bonustype == DMU_OT_NONE && bonuslen == 0) ||
            (bonustype != DMU_OT_NONE && bonuslen != 0));
        ASSERT3U(bonustype, <, DMU_OT_NUMTYPES);
        ASSERT3U(bonuslen, <=, DN_MAX_BONUSLEN);
        ASSERT(dn->dn_type == DMU_OT_NONE);
        ASSERT3U(dn->dn_maxblkid, ==, 0);
        ASSERT3U(dn->dn_allocated_txg, ==, 0);
        ASSERT3U(dn->dn_assigned_txg, ==, 0);
        ASSERT(refcount_is_zero(&dn->dn_tx_holds));
        ASSERT3U(refcount_count(&dn->dn_holds), <=, 1);
        ASSERT3P(list_head(&dn->dn_dbufs), ==, NULL);

        for (i = 0; i < TXG_SIZE; i++) {
                ASSERT3U(dn->dn_next_nlevels[i], ==, 0);
                ASSERT3U(dn->dn_next_indblkshift[i], ==, 0);
                ASSERT3U(dn->dn_next_bonuslen[i], ==, 0);
                ASSERT3U(dn->dn_next_blksz[i], ==, 0);
                ASSERT(!list_link_active(&dn->dn_dirty_link[i]));
                ASSERT3P(list_head(&dn->dn_dirty_records[i]), ==, NULL);
                ASSERT3U(avl_numnodes(&dn->dn_ranges[i]), ==, 0);
        }

        dn->dn_type = ot;
        dnode_setdblksz(dn, blocksize);
        dn->dn_indblkshift = ibs;
        dn->dn_nlevels = 1;
        dn->dn_nblkptr = 1 + ((DN_MAX_BONUSLEN - bonuslen) >> SPA_BLKPTRSHIFT);
        dn->dn_bonustype = bonustype;
        dn->dn_bonuslen = bonuslen;
        dn->dn_checksum = ZIO_CHECKSUM_INHERIT;
        dn->dn_compress = ZIO_COMPRESS_INHERIT;
        dn->dn_dirtyctx = 0;

        dn->dn_free_txg = 0;
        if (dn->dn_dirtyctx_firstset) {
                kmem_free(dn->dn_dirtyctx_firstset, 1);
                dn->dn_dirtyctx_firstset = NULL;
        }

        dn->dn_allocated_txg = tx->tx_txg;

        dnode_setdirty(dn, tx);
        dn->dn_next_indblkshift[tx->tx_txg & TXG_MASK] = ibs;
        dn->dn_next_bonuslen[tx->tx_txg & TXG_MASK] = dn->dn_bonuslen;
        dn->dn_next_blksz[tx->tx_txg & TXG_MASK] = dn->dn_datablksz;
}

void
dnode_reallocate(dnode_t *dn, dmu_object_type_t ot, int blocksize,
    dmu_object_type_t bonustype, int bonuslen, dmu_tx_t *tx)
{
        int i, old_nblkptr;
        dmu_buf_impl_t *db = NULL;

        ASSERT3U(blocksize, >=, SPA_MINBLOCKSIZE);
        ASSERT3U(blocksize, <=, SPA_MAXBLOCKSIZE);
        ASSERT3U(blocksize % SPA_MINBLOCKSIZE, ==, 0);
        ASSERT(dn->dn_object != DMU_META_DNODE_OBJECT || dmu_tx_private_ok(tx));
        ASSERT(tx->tx_txg != 0);
        ASSERT((bonustype == DMU_OT_NONE && bonuslen == 0) ||
            (bonustype != DMU_OT_NONE && bonuslen != 0));
        ASSERT3U(bonustype, <, DMU_OT_NUMTYPES);
        ASSERT3U(bonuslen, <=, DN_MAX_BONUSLEN);

        for (i = 0; i < TXG_SIZE; i++)
                ASSERT(!list_link_active(&dn->dn_dirty_link[i]));

        /* clean up any unreferenced dbufs */
        dnode_evict_dbufs(dn);
        ASSERT3P(list_head(&dn->dn_dbufs), ==, NULL);

        /*
         * XXX I should really have a generation number to tell if we
         * need to do this...
         */
        if (blocksize != dn->dn_datablksz ||
            dn->dn_bonustype != bonustype || dn->dn_bonuslen != bonuslen) {
                /* free all old data */
                dnode_free_range(dn, 0, -1ULL, tx);
        }

        /* change blocksize */
        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
        if (blocksize != dn->dn_datablksz &&
            (!BP_IS_HOLE(&dn->dn_phys->dn_blkptr[0]) ||
            list_head(&dn->dn_dbufs) != NULL)) {
                db = dbuf_hold(dn, 0, FTAG);
                dbuf_new_size(db, blocksize, tx);
        }
        dnode_setdblksz(dn, blocksize);
        dnode_setdirty(dn, tx);
        dn->dn_next_bonuslen[tx->tx_txg&TXG_MASK] = bonuslen;
        dn->dn_next_blksz[tx->tx_txg&TXG_MASK] = blocksize;
        rw_exit(&dn->dn_struct_rwlock);
        if (db)
                dbuf_rele(db, FTAG);

        /* change type */
        dn->dn_type = ot;

        /* change bonus size and type */
        mutex_enter(&dn->dn_mtx);
        old_nblkptr = dn->dn_nblkptr;
        dn->dn_bonustype = bonustype;
        dn->dn_bonuslen = bonuslen;
        dn->dn_nblkptr = 1 + ((DN_MAX_BONUSLEN - bonuslen) >> SPA_BLKPTRSHIFT);
        dn->dn_checksum = ZIO_CHECKSUM_INHERIT;
        dn->dn_compress = ZIO_COMPRESS_INHERIT;
        ASSERT3U(dn->dn_nblkptr, <=, DN_MAX_NBLKPTR);

        /* XXX - for now, we can't make nblkptr smaller */
        ASSERT3U(dn->dn_nblkptr, >=, old_nblkptr);

        /* fix up the bonus db_size if dn_nblkptr has changed */
        if (dn->dn_bonus && dn->dn_bonuslen != old_nblkptr) {
                dn->dn_bonus->db.db_size =
                    DN_MAX_BONUSLEN - (dn->dn_nblkptr-1) * sizeof (blkptr_t);
                ASSERT(dn->dn_bonuslen <= dn->dn_bonus->db.db_size);
        }

        dn->dn_allocated_txg = tx->tx_txg;
        mutex_exit(&dn->dn_mtx);
}

void
dnode_special_close(dnode_t *dn)
{
        /*
         * Wait for final references to the dnode to clear.  This can
         * only happen if the arc is asyncronously evicting state that
         * has a hold on this dnode while we are trying to evict this
         * dnode.
         */
        while (refcount_count(&dn->dn_holds) > 0)
                delay(1);
        dnode_destroy(dn);
}

dnode_t *
dnode_special_open(objset_impl_t *os, dnode_phys_t *dnp, uint64_t object)
{
        dnode_t *dn = dnode_create(os, dnp, NULL, object);
        DNODE_VERIFY(dn);
        return (dn);
}

static void
dnode_buf_pageout(dmu_buf_t *db, void *arg)
{
        dnode_t **children_dnodes = arg;
        int i;
        int epb = db->db_size >> DNODE_SHIFT;

        for (i = 0; i < epb; i++) {
                dnode_t *dn = children_dnodes[i];
                int n;

                if (dn == NULL)
                        continue;
#ifdef ZFS_DEBUG
                /*
                 * If there are holds on this dnode, then there should
                 * be holds on the dnode's containing dbuf as well; thus
                 * it wouldn't be eligable for eviction and this function
                 * would not have been called.
                 */
                ASSERT(refcount_is_zero(&dn->dn_holds));
                ASSERT(list_head(&dn->dn_dbufs) == NULL);
                ASSERT(refcount_is_zero(&dn->dn_tx_holds));

                for (n = 0; n < TXG_SIZE; n++)
                        ASSERT(!list_link_active(&dn->dn_dirty_link[n]));
#endif
                children_dnodes[i] = NULL;
                dnode_destroy(dn);
        }
        kmem_free(children_dnodes, epb * sizeof (dnode_t *));
}

/*
 * errors:
 * EINVAL - invalid object number.
 * EIO - i/o error.
 * succeeds even for free dnodes.
 */
int
dnode_hold_impl(objset_impl_t *os, uint64_t object, int flag,
    void *tag, dnode_t **dnp)
{
        int epb, idx, err;
        int drop_struct_lock = FALSE;
        int type;
        uint64_t blk;
        dnode_t *mdn, *dn;
        dmu_buf_impl_t *db;
        dnode_t **children_dnodes;

        /*
         * If you are holding the spa config lock as writer, you shouldn't
         * be asking the DMU to do *anything*.
         */
        ASSERT(spa_config_held(os->os_spa, SCL_ALL, RW_WRITER) == 0);

        if (object == 0 || object >= DN_MAX_OBJECT)
                return (EINVAL);

        mdn = os->os_meta_dnode;

        DNODE_VERIFY(mdn);

        if (!RW_WRITE_HELD(&mdn->dn_struct_rwlock)) {
                rw_enter(&mdn->dn_struct_rwlock, RW_READER);
                drop_struct_lock = TRUE;
        }

        blk = dbuf_whichblock(mdn, object * sizeof (dnode_phys_t));

        db = dbuf_hold(mdn, blk, FTAG);
        if (drop_struct_lock)
                rw_exit(&mdn->dn_struct_rwlock);
        if (db == NULL)
                return (EIO);
        err = dbuf_read(db, NULL, DB_RF_CANFAIL);
        if (err) {
                dbuf_rele(db, FTAG);
                return (err);
        }

        ASSERT3U(db->db.db_size, >=, 1<<DNODE_SHIFT);
        epb = db->db.db_size >> DNODE_SHIFT;

        idx = object & (epb-1);

        children_dnodes = dmu_buf_get_user(&db->db);
        if (children_dnodes == NULL) {
                dnode_t **winner;
                children_dnodes = kmem_zalloc(epb * sizeof (dnode_t *),
                    KM_SLEEP);
                if (winner = dmu_buf_set_user(&db->db, children_dnodes, NULL,
                    dnode_buf_pageout)) {
                        kmem_free(children_dnodes, epb * sizeof (dnode_t *));
                        children_dnodes = winner;
                }
        }

        if ((dn = children_dnodes[idx]) == NULL) {
                dnode_phys_t *dnp = (dnode_phys_t *)db->db.db_data+idx;
                dnode_t *winner;

                dn = dnode_create(os, dnp, db, object);
                winner = atomic_cas_ptr(&children_dnodes[idx], NULL, dn);
                if (winner != NULL) {
                        dnode_destroy(dn);
                        dn = winner;
                }
        }

        mutex_enter(&dn->dn_mtx);
        type = dn->dn_type;
        if (dn->dn_free_txg ||
            ((flag & DNODE_MUST_BE_ALLOCATED) && type == DMU_OT_NONE) ||
            ((flag & DNODE_MUST_BE_FREE) && type != DMU_OT_NONE)) {
                mutex_exit(&dn->dn_mtx);
                dbuf_rele(db, FTAG);
                return (type == DMU_OT_NONE ? ENOENT : EEXIST);
        }
        mutex_exit(&dn->dn_mtx);

        if (refcount_add(&dn->dn_holds, tag) == 1)
                dbuf_add_ref(db, dn);

        DNODE_VERIFY(dn);
        ASSERT3P(dn->dn_dbuf, ==, db);
        ASSERT3U(dn->dn_object, ==, object);
        dbuf_rele(db, FTAG);

        *dnp = dn;
        return (0);
}

/*
 * Return held dnode if the object is allocated, NULL if not.
 */
int
dnode_hold(objset_impl_t *os, uint64_t object, void *tag, dnode_t **dnp)
{
        return (dnode_hold_impl(os, object, DNODE_MUST_BE_ALLOCATED, tag, dnp));
}

/*
 * Can only add a reference if there is already at least one
 * reference on the dnode.  Returns FALSE if unable to add a
 * new reference.
 */
boolean_t
dnode_add_ref(dnode_t *dn, void *tag)
{
        mutex_enter(&dn->dn_mtx);
        if (refcount_is_zero(&dn->dn_holds)) {
                mutex_exit(&dn->dn_mtx);
                return (FALSE);
        }
        VERIFY(1 < refcount_add(&dn->dn_holds, tag));
        mutex_exit(&dn->dn_mtx);
        return (TRUE);
}

void
dnode_rele(dnode_t *dn, void *tag)
{
        uint64_t refs;

        mutex_enter(&dn->dn_mtx);
        refs = refcount_remove(&dn->dn_holds, tag);
        mutex_exit(&dn->dn_mtx);
        /* NOTE: the DNODE_DNODE does not have a dn_dbuf */
        if (refs == 0 && dn->dn_dbuf)
                dbuf_rele(dn->dn_dbuf, dn);
}

void
dnode_setdirty(dnode_t *dn, dmu_tx_t *tx)
{
        objset_impl_t *os = dn->dn_objset;
        uint64_t txg = tx->tx_txg;

        if (dn->dn_object == DMU_META_DNODE_OBJECT)
                return;

        DNODE_VERIFY(dn);

#ifdef ZFS_DEBUG
        mutex_enter(&dn->dn_mtx);
        ASSERT(dn->dn_phys->dn_type || dn->dn_allocated_txg);
        /* ASSERT(dn->dn_free_txg == 0 || dn->dn_free_txg >= txg); */
        mutex_exit(&dn->dn_mtx);
#endif

        mutex_enter(&os->os_lock);

        /*
         * If we are already marked dirty, we're done.
         */
        if (list_link_active(&dn->dn_dirty_link[txg & TXG_MASK])) {
                mutex_exit(&os->os_lock);
                return;
        }

        ASSERT(!refcount_is_zero(&dn->dn_holds) || list_head(&dn->dn_dbufs));
        ASSERT(dn->dn_datablksz != 0);
        ASSERT3U(dn->dn_next_bonuslen[txg&TXG_MASK], ==, 0);
        ASSERT3U(dn->dn_next_blksz[txg&TXG_MASK], ==, 0);

        dprintf_ds(os->os_dsl_dataset, "obj=%llu txg=%llu\n",
            dn->dn_object, txg);

        if (dn->dn_free_txg > 0 && dn->dn_free_txg <= txg) {
                list_insert_tail(&os->os_free_dnodes[txg&TXG_MASK], dn);
        } else {
                list_insert_tail(&os->os_dirty_dnodes[txg&TXG_MASK], dn);
        }

        mutex_exit(&os->os_lock);

        /*
         * The dnode maintains a hold on its containing dbuf as
         * long as there are holds on it.  Each instantiated child
         * dbuf maintaines a hold on the dnode.  When the last child
         * drops its hold, the dnode will drop its hold on the
         * containing dbuf. We add a "dirty hold" here so that the
         * dnode will hang around after we finish processing its
         * children.
         */
        VERIFY(dnode_add_ref(dn, (void *)(uintptr_t)tx->tx_txg));

        (void) dbuf_dirty(dn->dn_dbuf, tx);

        dsl_dataset_dirty(os->os_dsl_dataset, tx);
}

void
dnode_free(dnode_t *dn, dmu_tx_t *tx)
{
        int txgoff = tx->tx_txg & TXG_MASK;

        dprintf("dn=%p txg=%llu\n", dn, tx->tx_txg);

        /* we should be the only holder... hopefully */
        /* ASSERT3U(refcount_count(&dn->dn_holds), ==, 1); */

        mutex_enter(&dn->dn_mtx);
        if (dn->dn_type == DMU_OT_NONE || dn->dn_free_txg) {
                mutex_exit(&dn->dn_mtx);
                return;
        }
        dn->dn_free_txg = tx->tx_txg;
        mutex_exit(&dn->dn_mtx);

        /*
         * If the dnode is already dirty, it needs to be moved from
         * the dirty list to the free list.
         */
        mutex_enter(&dn->dn_objset->os_lock);
        if (list_link_active(&dn->dn_dirty_link[txgoff])) {
                list_remove(&dn->dn_objset->os_dirty_dnodes[txgoff], dn);
                list_insert_tail(&dn->dn_objset->os_free_dnodes[txgoff], dn);
                mutex_exit(&dn->dn_objset->os_lock);
        } else {
                mutex_exit(&dn->dn_objset->os_lock);
                dnode_setdirty(dn, tx);
        }
}

/*
 * Try to change the block size for the indicated dnode.  This can only
 * succeed if there are no blocks allocated or dirty beyond first block
 */
int
dnode_set_blksz(dnode_t *dn, uint64_t size, int ibs, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db, *db_next;
        int err;

        if (size == 0)
                size = SPA_MINBLOCKSIZE;
        if (size > SPA_MAXBLOCKSIZE)
                size = SPA_MAXBLOCKSIZE;
        else
                size = P2ROUNDUP(size, SPA_MINBLOCKSIZE);

        if (ibs == dn->dn_indblkshift)
                ibs = 0;

        if (size >> SPA_MINBLOCKSHIFT == dn->dn_datablkszsec && ibs == 0)
                return (0);

        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);

        /* Check for any allocated blocks beyond the first */
        if (dn->dn_phys->dn_maxblkid != 0)
                goto fail;

        mutex_enter(&dn->dn_dbufs_mtx);
        for (db = list_head(&dn->dn_dbufs); db; db = db_next) {
                db_next = list_next(&dn->dn_dbufs, db);

                if (db->db_blkid != 0 && db->db_blkid != DB_BONUS_BLKID) {
                        mutex_exit(&dn->dn_dbufs_mtx);
                        goto fail;
                }
        }
        mutex_exit(&dn->dn_dbufs_mtx);

        if (ibs && dn->dn_nlevels != 1)
                goto fail;

        /* resize the old block */
        err = dbuf_hold_impl(dn, 0, 0, TRUE, FTAG, &db);
        if (err == 0)
                dbuf_new_size(db, size, tx);
        else if (err != ENOENT)
                goto fail;

        dnode_setdblksz(dn, size);
        dnode_setdirty(dn, tx);
        dn->dn_next_blksz[tx->tx_txg&TXG_MASK] = size;
        if (ibs) {
                dn->dn_indblkshift = ibs;
                dn->dn_next_indblkshift[tx->tx_txg&TXG_MASK] = ibs;
        }
        /* rele after we have fixed the blocksize in the dnode */
        if (db)
                dbuf_rele(db, FTAG);

        rw_exit(&dn->dn_struct_rwlock);
        return (0);

fail:
        rw_exit(&dn->dn_struct_rwlock);
        return (ENOTSUP);
}

/* read-holding callers must not rely on the lock being continuously held */
void
dnode_new_blkid(dnode_t *dn, uint64_t blkid, dmu_tx_t *tx, boolean_t have_read)
{
        uint64_t txgoff = tx->tx_txg & TXG_MASK;
        int epbs, new_nlevels;
        uint64_t sz;

        ASSERT(blkid != DB_BONUS_BLKID);

        ASSERT(have_read ?
            RW_READ_HELD(&dn->dn_struct_rwlock) :
            RW_WRITE_HELD(&dn->dn_struct_rwlock));

        /*
         * if we have a read-lock, check to see if we need to do any work
         * before upgrading to a write-lock.
         */
        if (have_read) {
                if (blkid <= dn->dn_maxblkid)
                        return;

                if (!rw_tryupgrade(&dn->dn_struct_rwlock)) {
                        rw_exit(&dn->dn_struct_rwlock);
                        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
                }
        }

        if (blkid <= dn->dn_maxblkid)
                goto out;

        dn->dn_maxblkid = blkid;

        /*
         * Compute the number of levels necessary to support the new maxblkid.
         */
        new_nlevels = 1;
        epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;
        for (sz = dn->dn_nblkptr;
            sz <= blkid && sz >= dn->dn_nblkptr; sz <<= epbs)
                new_nlevels++;

        if (new_nlevels > dn->dn_nlevels) {
                int old_nlevels = dn->dn_nlevels;
                dmu_buf_impl_t *db;
                list_t *list;
                dbuf_dirty_record_t *new, *dr, *dr_next;

                dn->dn_nlevels = new_nlevels;

                ASSERT3U(new_nlevels, >, dn->dn_next_nlevels[txgoff]);
                dn->dn_next_nlevels[txgoff] = new_nlevels;

                /* dirty the left indirects */
                db = dbuf_hold_level(dn, old_nlevels, 0, FTAG);
                new = dbuf_dirty(db, tx);
                dbuf_rele(db, FTAG);

                /* transfer the dirty records to the new indirect */
                mutex_enter(&dn->dn_mtx);
                mutex_enter(&new->dt.di.dr_mtx);
                list = &dn->dn_dirty_records[txgoff];
                for (dr = list_head(list); dr; dr = dr_next) {
                        dr_next = list_next(&dn->dn_dirty_records[txgoff], dr);
                        if (dr->dr_dbuf->db_level != new_nlevels-1 &&
                            dr->dr_dbuf->db_blkid != DB_BONUS_BLKID) {
                                ASSERT(dr->dr_dbuf->db_level == old_nlevels-1);
                                list_remove(&dn->dn_dirty_records[txgoff], dr);
                                list_insert_tail(&new->dt.di.dr_children, dr);
                                dr->dr_parent = new;
                        }
                }
                mutex_exit(&new->dt.di.dr_mtx);
                mutex_exit(&dn->dn_mtx);
        }

out:
        if (have_read)
                rw_downgrade(&dn->dn_struct_rwlock);
}

void
dnode_clear_range(dnode_t *dn, uint64_t blkid, uint64_t nblks, dmu_tx_t *tx)
{
        avl_tree_t *tree = &dn->dn_ranges[tx->tx_txg&TXG_MASK];
        avl_index_t where;
        free_range_t *rp;
        free_range_t rp_tofind;
        uint64_t endblk = blkid + nblks;

        ASSERT(MUTEX_HELD(&dn->dn_mtx));
        ASSERT(nblks <= UINT64_MAX - blkid); /* no overflow */

        dprintf_dnode(dn, "blkid=%llu nblks=%llu txg=%llu\n",
            blkid, nblks, tx->tx_txg);
        rp_tofind.fr_blkid = blkid;
        rp = avl_find(tree, &rp_tofind, &where);
        if (rp == NULL)
                rp = avl_nearest(tree, where, AVL_BEFORE);
        if (rp == NULL)
                rp = avl_nearest(tree, where, AVL_AFTER);

        while (rp && (rp->fr_blkid <= blkid + nblks)) {
                uint64_t fr_endblk = rp->fr_blkid + rp->fr_nblks;
                free_range_t *nrp = AVL_NEXT(tree, rp);

                if (blkid <= rp->fr_blkid && endblk >= fr_endblk) {
                        /* clear this entire range */
                        avl_remove(tree, rp);
                        kmem_free(rp, sizeof (free_range_t));
                } else if (blkid <= rp->fr_blkid &&
                    endblk > rp->fr_blkid && endblk < fr_endblk) {
                        /* clear the beginning of this range */
                        rp->fr_blkid = endblk;
                        rp->fr_nblks = fr_endblk - endblk;
                } else if (blkid > rp->fr_blkid && blkid < fr_endblk &&
                    endblk >= fr_endblk) {
                        /* clear the end of this range */
                        rp->fr_nblks = blkid - rp->fr_blkid;
                } else if (blkid > rp->fr_blkid && endblk < fr_endblk) {
                        /* clear a chunk out of this range */
                        free_range_t *new_rp =
                            kmem_alloc(sizeof (free_range_t), KM_SLEEP);

                        new_rp->fr_blkid = endblk;
                        new_rp->fr_nblks = fr_endblk - endblk;
                        avl_insert_here(tree, new_rp, rp, AVL_AFTER);
                        rp->fr_nblks = blkid - rp->fr_blkid;
                }
                /* there may be no overlap */
                rp = nrp;
        }
}

void
dnode_free_range(dnode_t *dn, uint64_t off, uint64_t len, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db;
        uint64_t blkoff, blkid, nblks;
        int blksz, blkshift, head, tail;
        int trunc = FALSE;
        int epbs;

        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
        blksz = dn->dn_datablksz;
        blkshift = dn->dn_datablkshift;
        epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;

        if (len == -1ULL) {
                len = UINT64_MAX - off;
                trunc = TRUE;
        }

        /*
         * First, block align the region to free:
         */
        if (ISP2(blksz)) {
                head = P2NPHASE(off, blksz);
                blkoff = P2PHASE(off, blksz);
                if ((off >> blkshift) > dn->dn_maxblkid)
                        goto out;
        } else {
                ASSERT(dn->dn_maxblkid == 0);
                if (off == 0 && len >= blksz) {
                        /* Freeing the whole block; fast-track this request */
                        blkid = 0;
                        nblks = 1;
                        goto done;
                } else if (off >= blksz) {
                        /* Freeing past end-of-data */
                        goto out;
                } else {
                        /* Freeing part of the block. */
                        head = blksz - off;
                        ASSERT3U(head, >, 0);
                }
                blkoff = off;
        }
        /* zero out any partial block data at the start of the range */
        if (head) {
                ASSERT3U(blkoff + head, ==, blksz);
                if (len < head)
                        head = len;
                if (dbuf_hold_impl(dn, 0, dbuf_whichblock(dn, off), TRUE,
                    FTAG, &db) == 0) {
                        caddr_t data;

                        /* don't dirty if it isn't on disk and isn't dirty */
                        if (db->db_last_dirty ||
                            (db->db_blkptr && !BP_IS_HOLE(db->db_blkptr))) {
                                rw_exit(&dn->dn_struct_rwlock);
                                dbuf_will_dirty(db, tx);
                                rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
                                data = db->db.db_data;
                                bzero(data + blkoff, head);
                        }
                        dbuf_rele(db, FTAG);
                }
                off += head;
                len -= head;
        }

        /* If the range was less than one block, we're done */
        if (len == 0)
                goto out;

        /* If the remaining range is past end of file, we're done */
        if ((off >> blkshift) > dn->dn_maxblkid)
                goto out;

        ASSERT(ISP2(blksz));
        if (trunc)
                tail = 0;
        else
                tail = P2PHASE(len, blksz);

        ASSERT3U(P2PHASE(off, blksz), ==, 0);
        /* zero out any partial block data at the end of the range */
        if (tail) {
                if (len < tail)
                        tail = len;
                if (dbuf_hold_impl(dn, 0, dbuf_whichblock(dn, off+len),
                    TRUE, FTAG, &db) == 0) {
                        /* don't dirty if not on disk and not dirty */
                        if (db->db_last_dirty ||
                            (db->db_blkptr && !BP_IS_HOLE(db->db_blkptr))) {
                                rw_exit(&dn->dn_struct_rwlock);
                                dbuf_will_dirty(db, tx);
                                rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
                                bzero(db->db.db_data, tail);
                        }
                        dbuf_rele(db, FTAG);
                }
                len -= tail;
        }

        /* If the range did not include a full block, we are done */
        if (len == 0)
                goto out;

        ASSERT(IS_P2ALIGNED(off, blksz));
        ASSERT(trunc || IS_P2ALIGNED(len, blksz));
        blkid = off >> blkshift;
        nblks = len >> blkshift;
        if (trunc)
                nblks += 1;

        /*
         * Read in and mark all the level-1 indirects dirty,
         * so that they will stay in memory until syncing phase.
         * Always dirty the first and last indirect to make sure
         * we dirty all the partial indirects.
         */
        if (dn->dn_nlevels > 1) {
                uint64_t i, first, last;
                int shift = epbs + dn->dn_datablkshift;

                first = blkid >> epbs;
                if (db = dbuf_hold_level(dn, 1, first, FTAG)) {
                        dbuf_will_dirty(db, tx);
                        dbuf_rele(db, FTAG);
                }
                if (trunc)
                        last = dn->dn_maxblkid >> epbs;
                else
                        last = (blkid + nblks - 1) >> epbs;
                if (last > first && (db = dbuf_hold_level(dn, 1, last, FTAG))) {
                        dbuf_will_dirty(db, tx);
                        dbuf_rele(db, FTAG);
                }
                for (i = first + 1; i < last; i++) {
                        uint64_t ibyte = i << shift;
                        int err;

                        err = dnode_next_offset(dn,
                            DNODE_FIND_HAVELOCK, &ibyte, 1, 1, 0);
                        i = ibyte >> shift;
                        if (err == ESRCH || i >= last)
                                break;
                        ASSERT(err == 0);
                        db = dbuf_hold_level(dn, 1, i, FTAG);
                        if (db) {
                                dbuf_will_dirty(db, tx);
                                dbuf_rele(db, FTAG);
                        }
                }
        }
done:
        /*
         * Add this range to the dnode range list.
         * We will finish up this free operation in the syncing phase.
         */
        mutex_enter(&dn->dn_mtx);
        dnode_clear_range(dn, blkid, nblks, tx);
        {
                free_range_t *rp, *found;
                avl_index_t where;
                avl_tree_t *tree = &dn->dn_ranges[tx->tx_txg&TXG_MASK];

                /* Add new range to dn_ranges */
                rp = kmem_alloc(sizeof (free_range_t), KM_SLEEP);
                rp->fr_blkid = blkid;
                rp->fr_nblks = nblks;
                found = avl_find(tree, rp, &where);
                ASSERT(found == NULL);
                avl_insert(tree, rp, where);
                dprintf_dnode(dn, "blkid=%llu nblks=%llu txg=%llu\n",
                    blkid, nblks, tx->tx_txg);
        }
        mutex_exit(&dn->dn_mtx);

        dbuf_free_range(dn, blkid, blkid + nblks - 1, tx);
        dnode_setdirty(dn, tx);
out:
        if (trunc && dn->dn_maxblkid >= (off >> blkshift))
                dn->dn_maxblkid = (off >> blkshift ? (off >> blkshift) - 1 : 0);

        rw_exit(&dn->dn_struct_rwlock);
}

/* return TRUE if this blkid was freed in a recent txg, or FALSE if it wasn't */
uint64_t
dnode_block_freed(dnode_t *dn, uint64_t blkid)
{
        free_range_t range_tofind;
        void *dp = spa_get_dsl(dn->dn_objset->os_spa);
        int i;

        if (blkid == DB_BONUS_BLKID)
                return (FALSE);

        /*
         * If we're in the process of opening the pool, dp will not be
         * set yet, but there shouldn't be anything dirty.
         */
        if (dp == NULL)
                return (FALSE);

        if (dn->dn_free_txg)
                return (TRUE);

        /*
         * If dn_datablkshift is not set, then there's only a single
         * block, in which case there will never be a free range so it
         * won't matter.
         */
        range_tofind.fr_blkid = blkid;
        mutex_enter(&dn->dn_mtx);
        for (i = 0; i < TXG_SIZE; i++) {
                free_range_t *range_found;
                avl_index_t idx;

                range_found = avl_find(&dn->dn_ranges[i], &range_tofind, &idx);
                if (range_found) {
                        ASSERT(range_found->fr_nblks > 0);
                        break;
                }
                range_found = avl_nearest(&dn->dn_ranges[i], idx, AVL_BEFORE);
                if (range_found &&
                    range_found->fr_blkid + range_found->fr_nblks > blkid)
                        break;
        }
        mutex_exit(&dn->dn_mtx);
        return (i < TXG_SIZE);
}

/* call from syncing context when we actually write/free space for this dnode */
void
dnode_diduse_space(dnode_t *dn, int64_t delta)
{
        uint64_t space;
        dprintf_dnode(dn, "dn=%p dnp=%p used=%llu delta=%lld\n",
            dn, dn->dn_phys,
            (u_longlong_t)dn->dn_phys->dn_used,
            (longlong_t)delta);

        mutex_enter(&dn->dn_mtx);
        space = DN_USED_BYTES(dn->dn_phys);
        if (delta > 0) {
                ASSERT3U(space + delta, >=, space); /* no overflow */
        } else {
                ASSERT3U(space, >=, -delta); /* no underflow */
        }
        space += delta;
        if (spa_version(dn->dn_objset->os_spa) < SPA_VERSION_DNODE_BYTES) {
                ASSERT((dn->dn_phys->dn_flags & DNODE_FLAG_USED_BYTES) == 0);
                ASSERT3U(P2PHASE(space, 1<<DEV_BSHIFT), ==, 0);
                dn->dn_phys->dn_used = space >> DEV_BSHIFT;
        } else {
                dn->dn_phys->dn_used = space;
                dn->dn_phys->dn_flags |= DNODE_FLAG_USED_BYTES;
        }
        mutex_exit(&dn->dn_mtx);
}

/*
 * Call when we think we're going to write/free space in open context.
 * Be conservative (ie. OK to write less than this or free more than
 * this, but don't write more or free less).
 */
void
dnode_willuse_space(dnode_t *dn, int64_t space, dmu_tx_t *tx)
{
        objset_impl_t *os = dn->dn_objset;
        dsl_dataset_t *ds = os->os_dsl_dataset;

        if (space > 0)
                space = spa_get_asize(os->os_spa, space);

        if (ds)
                dsl_dir_willuse_space(ds->ds_dir, space, tx);

        dmu_tx_willuse_space(tx, space);
}

static int
dnode_next_offset_level(dnode_t *dn, int flags, uint64_t *offset,
        int lvl, uint64_t blkfill, uint64_t txg)
{
        dmu_buf_impl_t *db = NULL;
        void *data = NULL;
        uint64_t epbs = dn->dn_phys->dn_indblkshift - SPA_BLKPTRSHIFT;
        uint64_t epb = 1ULL << epbs;
        uint64_t minfill, maxfill;
        boolean_t hole;
        int i, inc, error, span;

        dprintf("probing object %llu offset %llx level %d of %u\n",
            dn->dn_object, *offset, lvl, dn->dn_phys->dn_nlevels);

        hole = flags & DNODE_FIND_HOLE;
        inc = (flags & DNODE_FIND_BACKWARDS) ? -1 : 1;
        ASSERT(txg == 0 || !hole);

        if (lvl == dn->dn_phys->dn_nlevels) {
                error = 0;
                epb = dn->dn_phys->dn_nblkptr;
                data = dn->dn_phys->dn_blkptr;
        } else {
                uint64_t blkid = dbuf_whichblock(dn, *offset) >> (epbs * lvl);
                error = dbuf_hold_impl(dn, lvl, blkid, TRUE, FTAG, &db);
                if (error) {
                        if (error != ENOENT)
                                return (error);
                        if (hole)
                                return (0);
                        /*
                         * This can only happen when we are searching up
                         * the block tree for data.  We don't really need to
                         * adjust the offset, as we will just end up looking
                         * at the pointer to this block in its parent, and its
                         * going to be unallocated, so we will skip over it.
                         */
                        return (ESRCH);
                }
                error = dbuf_read(db, NULL, DB_RF_CANFAIL | DB_RF_HAVESTRUCT);
                if (error) {
                        dbuf_rele(db, FTAG);
                        return (error);
                }
                data = db->db.db_data;
        }

        if (db && txg &&
            (db->db_blkptr == NULL || db->db_blkptr->blk_birth <= txg)) {
                /*
                 * This can only happen when we are searching up the tree
                 * and these conditions mean that we need to keep climbing.
                 */
                error = ESRCH;
        } else if (lvl == 0) {
                dnode_phys_t *dnp = data;
                span = DNODE_SHIFT;
                ASSERT(dn->dn_type == DMU_OT_DNODE);

                for (i = (*offset >> span) & (blkfill - 1);
                    i >= 0 && i < blkfill; i += inc) {
                        boolean_t newcontents = B_TRUE;
                        if (txg) {
                                int j;
                                newcontents = B_FALSE;
                                for (j = 0; j < dnp[i].dn_nblkptr; j++) {
                                        if (dnp[i].dn_blkptr[j].blk_birth > txg)
                                                newcontents = B_TRUE;
                                }
                        }
                        if (!dnp[i].dn_type == hole && newcontents)
                                break;
                        *offset += (1ULL << span) * inc;
                }
                if (i < 0 || i == blkfill)
                        error = ESRCH;
        } else {
                blkptr_t *bp = data;
                span = (lvl - 1) * epbs + dn->dn_datablkshift;
                minfill = 0;
                maxfill = blkfill << ((lvl - 1) * epbs);

                if (hole)
                        maxfill--;
                else
                        minfill++;

                for (i = (*offset >> span) & ((1ULL << epbs) - 1);
                    i >= 0 && i < epb; i += inc) {
                        if (bp[i].blk_fill >= minfill &&
                            bp[i].blk_fill <= maxfill &&
                            (hole || bp[i].blk_birth > txg))
                                break;
                        if (inc < 0 && *offset < (1ULL << span))
                                *offset = 0;
                        else
                                *offset += (1ULL << span) * inc;
                }
                if (i < 0 || i == epb)
                        error = ESRCH;
        }

        if (db)
                dbuf_rele(db, FTAG);

        return (error);
}

/*
 * Find the next hole, data, or sparse region at or after *offset.
 * The value 'blkfill' tells us how many items we expect to find
 * in an L0 data block; this value is 1 for normal objects,
 * DNODES_PER_BLOCK for the meta dnode, and some fraction of
 * DNODES_PER_BLOCK when searching for sparse regions thereof.
 *
 * Examples:
 *
 * dnode_next_offset(dn, flags, offset, 1, 1, 0);
 *      Finds the next/previous hole/data in a file.
 *      Used in dmu_offset_next().
 *
 * dnode_next_offset(mdn, flags, offset, 0, DNODES_PER_BLOCK, txg);
 *      Finds the next free/allocated dnode an objset's meta-dnode.
 *      Only finds objects that have new contents since txg (ie.
 *      bonus buffer changes and content removal are ignored).
 *      Used in dmu_object_next().
 *
 * dnode_next_offset(mdn, DNODE_FIND_HOLE, offset, 2, DNODES_PER_BLOCK >> 2, 0);
 *      Finds the next L2 meta-dnode bp that's at most 1/4 full.
 *      Used in dmu_object_alloc().
 */
int
dnode_next_offset(dnode_t *dn, int flags, uint64_t *offset,
    int minlvl, uint64_t blkfill, uint64_t txg)
{
        uint64_t initial_offset = *offset;
        int lvl, maxlvl;
        int error = 0;

        if (!(flags & DNODE_FIND_HAVELOCK))
                rw_enter(&dn->dn_struct_rwlock, RW_READER);

        if (dn->dn_phys->dn_nlevels == 0) {
                error = ESRCH;
                goto out;
        }

        if (dn->dn_datablkshift == 0) {
                if (*offset < dn->dn_datablksz) {
                        if (flags & DNODE_FIND_HOLE)
                                *offset = dn->dn_datablksz;
                } else {
                        error = ESRCH;
                }
                goto out;
        }

        maxlvl = dn->dn_phys->dn_nlevels;

        for (lvl = minlvl; lvl <= maxlvl; lvl++) {
                error = dnode_next_offset_level(dn,
                    flags, offset, lvl, blkfill, txg);
                if (error != ESRCH)
                        break;
        }

        while (error == 0 && --lvl >= minlvl) {
                error = dnode_next_offset_level(dn,
                    flags, offset, lvl, blkfill, txg);
        }

        if (error == 0 && (flags & DNODE_FIND_BACKWARDS ?
            initial_offset < *offset : initial_offset > *offset))
                error = ESRCH;
out:
        if (!(flags & DNODE_FIND_HAVELOCK))
                rw_exit(&dn->dn_struct_rwlock);

        return (error);
}