MFC r268713: MFV r268702:

[FreeBSD/stable/10.git] / sys / cddl / contrib / opensolaris / uts / common / fs / 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 (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2012, 2014 by Delphix. All rights reserved.
 */

#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>
#include <sys/range_tree.h>

static kmem_cache_t *dnode_cache;
/*
 * Define DNODE_STATS to turn on statistic gathering. By default, it is only
 * turned on when DEBUG is also defined.
 */
#ifdef  DEBUG
#define DNODE_STATS
#endif  /* DEBUG */

#ifdef  DNODE_STATS
#define DNODE_STAT_ADD(stat)                    ((stat)++)
#else
#define DNODE_STAT_ADD(stat)                    /* nothing */
#endif  /* DNODE_STATS */

static dnode_phys_t dnode_phys_zero;

int zfs_default_bs = SPA_MINBLOCKSHIFT;
int zfs_default_ibs = DN_MAX_INDBLKSHIFT;

#ifdef sun
static kmem_cbrc_t dnode_move(void *, void *, size_t, void *);
#endif

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

        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);

        /*
         * Every dbuf has a reference, and dropping a tracked reference is
         * O(number of references), so don't track dn_holds.
         */
        refcount_create_untracked(&dn->dn_holds);
        refcount_create(&dn->dn_tx_holds);
        list_link_init(&dn->dn_link);

        bzero(&dn->dn_next_nblkptr[0], sizeof (dn->dn_next_nblkptr));
        bzero(&dn->dn_next_nlevels[0], sizeof (dn->dn_next_nlevels));
        bzero(&dn->dn_next_indblkshift[0], sizeof (dn->dn_next_indblkshift));
        bzero(&dn->dn_next_bonustype[0], sizeof (dn->dn_next_bonustype));
        bzero(&dn->dn_rm_spillblk[0], sizeof (dn->dn_rm_spillblk));
        bzero(&dn->dn_next_bonuslen[0], sizeof (dn->dn_next_bonuslen));
        bzero(&dn->dn_next_blksz[0], sizeof (dn->dn_next_blksz));

        for (i = 0; i < TXG_SIZE; i++) {
                list_link_init(&dn->dn_dirty_link[i]);
                dn->dn_free_ranges[i] = NULL;
                list_create(&dn->dn_dirty_records[i],
                    sizeof (dbuf_dirty_record_t),
                    offsetof(dbuf_dirty_record_t, dr_dirty_node));
        }

        dn->dn_allocated_txg = 0;
        dn->dn_free_txg = 0;
        dn->dn_assigned_txg = 0;
        dn->dn_dirtyctx = 0;
        dn->dn_dirtyctx_firstset = NULL;
        dn->dn_bonus = NULL;
        dn->dn_have_spill = B_FALSE;
        dn->dn_zio = NULL;
        dn->dn_oldused = 0;
        dn->dn_oldflags = 0;
        dn->dn_olduid = 0;
        dn->dn_oldgid = 0;
        dn->dn_newuid = 0;
        dn->dn_newgid = 0;
        dn->dn_id_flags = 0;

        dn->dn_dbufs_count = 0;
        dn->dn_unlisted_l0_blkid = 0;
        list_create(&dn->dn_dbufs, sizeof (dmu_buf_impl_t),
            offsetof(dmu_buf_impl_t, db_link));

        dn->dn_moved = 0;
        POINTER_INVALIDATE(&dn->dn_objset);
        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);
        ASSERT(!list_link_active(&dn->dn_link));

        for (i = 0; i < TXG_SIZE; i++) {
                ASSERT(!list_link_active(&dn->dn_dirty_link[i]));
                ASSERT3P(dn->dn_free_ranges[i], ==, NULL);
                list_destroy(&dn->dn_dirty_records[i]);
                ASSERT0(dn->dn_next_nblkptr[i]);
                ASSERT0(dn->dn_next_nlevels[i]);
                ASSERT0(dn->dn_next_indblkshift[i]);
                ASSERT0(dn->dn_next_bonustype[i]);
                ASSERT0(dn->dn_rm_spillblk[i]);
                ASSERT0(dn->dn_next_bonuslen[i]);
                ASSERT0(dn->dn_next_blksz[i]);
        }

        ASSERT0(dn->dn_allocated_txg);
        ASSERT0(dn->dn_free_txg);
        ASSERT0(dn->dn_assigned_txg);
        ASSERT0(dn->dn_dirtyctx);
        ASSERT3P(dn->dn_dirtyctx_firstset, ==, NULL);
        ASSERT3P(dn->dn_bonus, ==, NULL);
        ASSERT(!dn->dn_have_spill);
        ASSERT3P(dn->dn_zio, ==, NULL);
        ASSERT0(dn->dn_oldused);
        ASSERT0(dn->dn_oldflags);
        ASSERT0(dn->dn_olduid);
        ASSERT0(dn->dn_oldgid);
        ASSERT0(dn->dn_newuid);
        ASSERT0(dn->dn_newgid);
        ASSERT0(dn->dn_id_flags);

        ASSERT0(dn->dn_dbufs_count);
        ASSERT0(dn->dn_unlisted_l0_blkid);
        list_destroy(&dn->dn_dbufs);
}

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

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


#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_handle->dnh_dnode == dn);

        ASSERT(DMU_OT_IS_VALID(dn->dn_phys->dn_type));

        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);
                ASSERT(DMU_OT_IS_VALID(dn->dn_type));
                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(DMU_OBJECT_IS_SPECIAL(dn->dn_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;
                ASSERT(DMU_OT_IS_VALID(dnp->dn_bonustype));
                dmu_object_byteswap_t byteswap =
                    DMU_OT_BYTESWAP(dnp->dn_bonustype);
                dmu_ot_byteswap[byteswap].ob_func(dnp->dn_bonus + off, len);
        }

        /* Swap SPILL block if we have one */
        if (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR)
                byteswap_uint64_array(&dnp->dn_spill, sizeof (blkptr_t));

}

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++;
        }
}

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);
}

void
dnode_setbonus_type(dnode_t *dn, dmu_object_type_t newtype, dmu_tx_t *tx)
{
        ASSERT3U(refcount_count(&dn->dn_holds), >=, 1);
        dnode_setdirty(dn, tx);
        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
        dn->dn_bonustype = newtype;
        dn->dn_next_bonustype[tx->tx_txg & TXG_MASK] = dn->dn_bonustype;
        rw_exit(&dn->dn_struct_rwlock);
}

void
dnode_rm_spill(dnode_t *dn, dmu_tx_t *tx)
{
        ASSERT3U(refcount_count(&dn->dn_holds), >=, 1);
        ASSERT(RW_WRITE_HELD(&dn->dn_struct_rwlock));
        dnode_setdirty(dn, tx);
        dn->dn_rm_spillblk[tx->tx_txg&TXG_MASK] = DN_KILL_SPILLBLK;
        dn->dn_have_spill = B_FALSE;
}

static void
dnode_setdblksz(dnode_t *dn, int size)
{
        ASSERT0(P2PHASE(size, SPA_MINBLOCKSIZE));
        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) ? highbit64(size - 1) : 0;
}

static dnode_t *
dnode_create(objset_t *os, dnode_phys_t *dnp, dmu_buf_impl_t *db,
    uint64_t object, dnode_handle_t *dnh)
{
        dnode_t *dn = kmem_cache_alloc(dnode_cache, KM_SLEEP);

        ASSERT(!POINTER_IS_VALID(dn->dn_objset));
        dn->dn_moved = 0;

        /*
         * Defer setting dn_objset until the dnode is ready to be a candidate
         * for the dnode_move() callback.
         */
        dn->dn_object = object;
        dn->dn_dbuf = db;
        dn->dn_handle = dnh;
        dn->dn_phys = dnp;

        if (dnp->dn_datablkszsec) {
                dnode_setdblksz(dn, dnp->dn_datablkszsec << SPA_MINBLOCKSHIFT);
        } else {
                dn->dn_datablksz = 0;
                dn->dn_datablkszsec = 0;
                dn->dn_datablkshift = 0;
        }
        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;
        dn->dn_have_spill = ((dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR) != 0);
        dn->dn_id_flags = 0;

        dmu_zfetch_init(&dn->dn_zfetch, dn);

        ASSERT(DMU_OT_IS_VALID(dn->dn_phys->dn_type));

        mutex_enter(&os->os_lock);
        list_insert_head(&os->os_dnodes, dn);
        membar_producer();
        /*
         * Everything else must be valid before assigning dn_objset makes the
         * dnode eligible for dnode_move().
         */
        dn->dn_objset = os;
        mutex_exit(&os->os_lock);

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

/*
 * Caller must be holding the dnode handle, which is released upon return.
 */
static void
dnode_destroy(dnode_t *dn)
{
        objset_t *os = dn->dn_objset;

        ASSERT((dn->dn_id_flags & DN_ID_NEW_EXIST) == 0);

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

        /* the dnode can no longer move, so we can release the handle */
        zrl_remove(&dn->dn_handle->dnh_zrlock);

        dn->dn_allocated_txg = 0;
        dn->dn_free_txg = 0;
        dn->dn_assigned_txg = 0;

        dn->dn_dirtyctx = 0;
        if (dn->dn_dirtyctx_firstset != NULL) {
                kmem_free(dn->dn_dirtyctx_firstset, 1);
                dn->dn_dirtyctx_firstset = NULL;
        }
        if (dn->dn_bonus != NULL) {
                mutex_enter(&dn->dn_bonus->db_mtx);
                dbuf_evict(dn->dn_bonus);
                dn->dn_bonus = NULL;
        }
        dn->dn_zio = NULL;

        dn->dn_have_spill = B_FALSE;
        dn->dn_oldused = 0;
        dn->dn_oldflags = 0;
        dn->dn_olduid = 0;
        dn->dn_oldgid = 0;
        dn->dn_newuid = 0;
        dn->dn_newgid = 0;
        dn->dn_id_flags = 0;
        dn->dn_unlisted_l0_blkid = 0;

        dmu_zfetch_rele(&dn->dn_zfetch);
        kmem_cache_free(dnode_cache, dn);
        arc_space_return(sizeof (dnode_t), ARC_SPACE_OTHER);
}

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);
        ASSERT(DMU_OT_IS_VALID(ot));
        ASSERT((bonustype == DMU_OT_NONE && bonuslen == 0) ||
            (bonustype == DMU_OT_SA && bonuslen == 0) ||
            (bonustype != DMU_OT_NONE && bonuslen != 0));
        ASSERT(DMU_OT_IS_VALID(bonustype));
        ASSERT3U(bonuslen, <=, DN_MAX_BONUSLEN);
        ASSERT(dn->dn_type == DMU_OT_NONE);
        ASSERT0(dn->dn_maxblkid);
        ASSERT0(dn->dn_allocated_txg);
        ASSERT0(dn->dn_assigned_txg);
        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++) {
                ASSERT0(dn->dn_next_nblkptr[i]);
                ASSERT0(dn->dn_next_nlevels[i]);
                ASSERT0(dn->dn_next_indblkshift[i]);
                ASSERT0(dn->dn_next_bonuslen[i]);
                ASSERT0(dn->dn_next_bonustype[i]);
                ASSERT0(dn->dn_rm_spillblk[i]);
                ASSERT0(dn->dn_next_blksz[i]);
                ASSERT(!list_link_active(&dn->dn_dirty_link[i]));
                ASSERT3P(list_head(&dn->dn_dirty_records[i]), ==, NULL);
                ASSERT3P(dn->dn_free_ranges[i], ==, NULL);
        }

        dn->dn_type = ot;
        dnode_setdblksz(dn, blocksize);
        dn->dn_indblkshift = ibs;
        dn->dn_nlevels = 1;
        if (bonustype == DMU_OT_SA) /* Maximize bonus space for SA */
                dn->dn_nblkptr = 1;
        else
                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;
        dn->dn_id_flags = 0;

        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_bonustype[tx->tx_txg & TXG_MASK] = dn->dn_bonustype;
        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 nblkptr;

        ASSERT3U(blocksize, >=, SPA_MINBLOCKSIZE);
        ASSERT3U(blocksize, <=, SPA_MAXBLOCKSIZE);
        ASSERT0(blocksize % SPA_MINBLOCKSIZE);
        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) ||
            (bonustype == DMU_OT_SA && bonuslen == 0));
        ASSERT(DMU_OT_IS_VALID(bonustype));
        ASSERT3U(bonuslen, <=, DN_MAX_BONUSLEN);

        /* clean up any unreferenced dbufs */
        dnode_evict_dbufs(dn);

        dn->dn_id_flags = 0;

        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
        dnode_setdirty(dn, tx);
        if (dn->dn_datablksz != blocksize) {
                /* change blocksize */
                ASSERT(dn->dn_maxblkid == 0 &&
                    (BP_IS_HOLE(&dn->dn_phys->dn_blkptr[0]) ||
                    dnode_block_freed(dn, 0)));
                dnode_setdblksz(dn, blocksize);
                dn->dn_next_blksz[tx->tx_txg&TXG_MASK] = blocksize;
        }
        if (dn->dn_bonuslen != bonuslen)
                dn->dn_next_bonuslen[tx->tx_txg&TXG_MASK] = bonuslen;

        if (bonustype == DMU_OT_SA) /* Maximize bonus space for SA */
                nblkptr = 1;
        else
                nblkptr = 1 + ((DN_MAX_BONUSLEN - bonuslen) >> SPA_BLKPTRSHIFT);
        if (dn->dn_bonustype != bonustype)
                dn->dn_next_bonustype[tx->tx_txg&TXG_MASK] = bonustype;
        if (dn->dn_nblkptr != nblkptr)
                dn->dn_next_nblkptr[tx->tx_txg&TXG_MASK] = nblkptr;
        if (dn->dn_phys->dn_flags & DNODE_FLAG_SPILL_BLKPTR) {
                dbuf_rm_spill(dn, tx);
                dnode_rm_spill(dn, tx);
        }
        rw_exit(&dn->dn_struct_rwlock);

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

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

        /* fix up the bonus db_size */
        if (dn->dn_bonus) {
                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);
}

#ifdef  DNODE_STATS
static struct {
        uint64_t dms_dnode_invalid;
        uint64_t dms_dnode_recheck1;
        uint64_t dms_dnode_recheck2;
        uint64_t dms_dnode_special;
        uint64_t dms_dnode_handle;
        uint64_t dms_dnode_rwlock;
        uint64_t dms_dnode_active;
} dnode_move_stats;
#endif  /* DNODE_STATS */

static void
dnode_move_impl(dnode_t *odn, dnode_t *ndn)
{
        int i;

        ASSERT(!RW_LOCK_HELD(&odn->dn_struct_rwlock));
        ASSERT(MUTEX_NOT_HELD(&odn->dn_mtx));
        ASSERT(MUTEX_NOT_HELD(&odn->dn_dbufs_mtx));
        ASSERT(!RW_LOCK_HELD(&odn->dn_zfetch.zf_rwlock));

        /* Copy fields. */
        ndn->dn_objset = odn->dn_objset;
        ndn->dn_object = odn->dn_object;
        ndn->dn_dbuf = odn->dn_dbuf;
        ndn->dn_handle = odn->dn_handle;
        ndn->dn_phys = odn->dn_phys;
        ndn->dn_type = odn->dn_type;
        ndn->dn_bonuslen = odn->dn_bonuslen;
        ndn->dn_bonustype = odn->dn_bonustype;
        ndn->dn_nblkptr = odn->dn_nblkptr;
        ndn->dn_checksum = odn->dn_checksum;
        ndn->dn_compress = odn->dn_compress;
        ndn->dn_nlevels = odn->dn_nlevels;
        ndn->dn_indblkshift = odn->dn_indblkshift;
        ndn->dn_datablkshift = odn->dn_datablkshift;
        ndn->dn_datablkszsec = odn->dn_datablkszsec;
        ndn->dn_datablksz = odn->dn_datablksz;
        ndn->dn_maxblkid = odn->dn_maxblkid;
        bcopy(&odn->dn_next_nblkptr[0], &ndn->dn_next_nblkptr[0],
            sizeof (odn->dn_next_nblkptr));
        bcopy(&odn->dn_next_nlevels[0], &ndn->dn_next_nlevels[0],
            sizeof (odn->dn_next_nlevels));
        bcopy(&odn->dn_next_indblkshift[0], &ndn->dn_next_indblkshift[0],
            sizeof (odn->dn_next_indblkshift));
        bcopy(&odn->dn_next_bonustype[0], &ndn->dn_next_bonustype[0],
            sizeof (odn->dn_next_bonustype));
        bcopy(&odn->dn_rm_spillblk[0], &ndn->dn_rm_spillblk[0],
            sizeof (odn->dn_rm_spillblk));
        bcopy(&odn->dn_next_bonuslen[0], &ndn->dn_next_bonuslen[0],
            sizeof (odn->dn_next_bonuslen));
        bcopy(&odn->dn_next_blksz[0], &ndn->dn_next_blksz[0],
            sizeof (odn->dn_next_blksz));
        for (i = 0; i < TXG_SIZE; i++) {
                list_move_tail(&ndn->dn_dirty_records[i],
                    &odn->dn_dirty_records[i]);
        }
        bcopy(&odn->dn_free_ranges[0], &ndn->dn_free_ranges[0],
            sizeof (odn->dn_free_ranges));
        ndn->dn_allocated_txg = odn->dn_allocated_txg;
        ndn->dn_free_txg = odn->dn_free_txg;
        ndn->dn_assigned_txg = odn->dn_assigned_txg;
        ndn->dn_dirtyctx = odn->dn_dirtyctx;
        ndn->dn_dirtyctx_firstset = odn->dn_dirtyctx_firstset;
        ASSERT(refcount_count(&odn->dn_tx_holds) == 0);
        refcount_transfer(&ndn->dn_holds, &odn->dn_holds);
        ASSERT(list_is_empty(&ndn->dn_dbufs));
        list_move_tail(&ndn->dn_dbufs, &odn->dn_dbufs);
        ndn->dn_dbufs_count = odn->dn_dbufs_count;
        ndn->dn_unlisted_l0_blkid = odn->dn_unlisted_l0_blkid;
        ndn->dn_bonus = odn->dn_bonus;
        ndn->dn_have_spill = odn->dn_have_spill;
        ndn->dn_zio = odn->dn_zio;
        ndn->dn_oldused = odn->dn_oldused;
        ndn->dn_oldflags = odn->dn_oldflags;
        ndn->dn_olduid = odn->dn_olduid;
        ndn->dn_oldgid = odn->dn_oldgid;
        ndn->dn_newuid = odn->dn_newuid;
        ndn->dn_newgid = odn->dn_newgid;
        ndn->dn_id_flags = odn->dn_id_flags;
        dmu_zfetch_init(&ndn->dn_zfetch, NULL);
        list_move_tail(&ndn->dn_zfetch.zf_stream, &odn->dn_zfetch.zf_stream);
        ndn->dn_zfetch.zf_dnode = odn->dn_zfetch.zf_dnode;
        ndn->dn_zfetch.zf_stream_cnt = odn->dn_zfetch.zf_stream_cnt;
        ndn->dn_zfetch.zf_alloc_fail = odn->dn_zfetch.zf_alloc_fail;

        /*
         * Update back pointers. Updating the handle fixes the back pointer of
         * every descendant dbuf as well as the bonus dbuf.
         */
        ASSERT(ndn->dn_handle->dnh_dnode == odn);
        ndn->dn_handle->dnh_dnode = ndn;
        if (ndn->dn_zfetch.zf_dnode == odn) {
                ndn->dn_zfetch.zf_dnode = ndn;
        }

        /*
         * Invalidate the original dnode by clearing all of its back pointers.
         */
        odn->dn_dbuf = NULL;
        odn->dn_handle = NULL;
        list_create(&odn->dn_dbufs, sizeof (dmu_buf_impl_t),
            offsetof(dmu_buf_impl_t, db_link));
        odn->dn_dbufs_count = 0;
        odn->dn_unlisted_l0_blkid = 0;
        odn->dn_bonus = NULL;
        odn->dn_zfetch.zf_dnode = NULL;

        /*
         * Set the low bit of the objset pointer to ensure that dnode_move()
         * recognizes the dnode as invalid in any subsequent callback.
         */
        POINTER_INVALIDATE(&odn->dn_objset);

        /*
         * Satisfy the destructor.
         */
        for (i = 0; i < TXG_SIZE; i++) {
                list_create(&odn->dn_dirty_records[i],
                    sizeof (dbuf_dirty_record_t),
                    offsetof(dbuf_dirty_record_t, dr_dirty_node));
                odn->dn_free_ranges[i] = NULL;
                odn->dn_next_nlevels[i] = 0;
                odn->dn_next_indblkshift[i] = 0;
                odn->dn_next_bonustype[i] = 0;
                odn->dn_rm_spillblk[i] = 0;
                odn->dn_next_bonuslen[i] = 0;
                odn->dn_next_blksz[i] = 0;
        }
        odn->dn_allocated_txg = 0;
        odn->dn_free_txg = 0;
        odn->dn_assigned_txg = 0;
        odn->dn_dirtyctx = 0;
        odn->dn_dirtyctx_firstset = NULL;
        odn->dn_have_spill = B_FALSE;
        odn->dn_zio = NULL;
        odn->dn_oldused = 0;
        odn->dn_oldflags = 0;
        odn->dn_olduid = 0;
        odn->dn_oldgid = 0;
        odn->dn_newuid = 0;
        odn->dn_newgid = 0;
        odn->dn_id_flags = 0;

        /*
         * Mark the dnode.
         */
        ndn->dn_moved = 1;
        odn->dn_moved = (uint8_t)-1;
}

#ifdef sun
#ifdef  _KERNEL
/*ARGSUSED*/
static kmem_cbrc_t
dnode_move(void *buf, void *newbuf, size_t size, void *arg)
{
        dnode_t *odn = buf, *ndn = newbuf;
        objset_t *os;
        int64_t refcount;
        uint32_t dbufs;

        /*
         * The dnode is on the objset's list of known dnodes if the objset
         * pointer is valid. We set the low bit of the objset pointer when
         * freeing the dnode to invalidate it, and the memory patterns written
         * by kmem (baddcafe and deadbeef) set at least one of the two low bits.
         * A newly created dnode sets the objset pointer last of all to indicate
         * that the dnode is known and in a valid state to be moved by this
         * function.
         */
        os = odn->dn_objset;
        if (!POINTER_IS_VALID(os)) {
                DNODE_STAT_ADD(dnode_move_stats.dms_dnode_invalid);
                return (KMEM_CBRC_DONT_KNOW);
        }

        /*
         * Ensure that the objset does not go away during the move.
         */
        rw_enter(&os_lock, RW_WRITER);
        if (os != odn->dn_objset) {
                rw_exit(&os_lock);
                DNODE_STAT_ADD(dnode_move_stats.dms_dnode_recheck1);
                return (KMEM_CBRC_DONT_KNOW);
        }

        /*
         * If the dnode is still valid, then so is the objset. We know that no
         * valid objset can be freed while we hold os_lock, so we can safely
         * ensure that the objset remains in use.
         */
        mutex_enter(&os->os_lock);

        /*
         * Recheck the objset pointer in case the dnode was removed just before
         * acquiring the lock.
         */
        if (os != odn->dn_objset) {
                mutex_exit(&os->os_lock);
                rw_exit(&os_lock);
                DNODE_STAT_ADD(dnode_move_stats.dms_dnode_recheck2);
                return (KMEM_CBRC_DONT_KNOW);
        }

        /*
         * At this point we know that as long as we hold os->os_lock, the dnode
         * cannot be freed and fields within the dnode can be safely accessed.
         * The objset listing this dnode cannot go away as long as this dnode is
         * on its list.
         */
        rw_exit(&os_lock);
        if (DMU_OBJECT_IS_SPECIAL(odn->dn_object)) {
                mutex_exit(&os->os_lock);
                DNODE_STAT_ADD(dnode_move_stats.dms_dnode_special);
                return (KMEM_CBRC_NO);
        }
        ASSERT(odn->dn_dbuf != NULL); /* only "special" dnodes have no parent */

        /*
         * Lock the dnode handle to prevent the dnode from obtaining any new
         * holds. This also prevents the descendant dbufs and the bonus dbuf
         * from accessing the dnode, so that we can discount their holds. The
         * handle is safe to access because we know that while the dnode cannot
         * go away, neither can its handle. Once we hold dnh_zrlock, we can
         * safely move any dnode referenced only by dbufs.
         */
        if (!zrl_tryenter(&odn->dn_handle->dnh_zrlock)) {
                mutex_exit(&os->os_lock);
                DNODE_STAT_ADD(dnode_move_stats.dms_dnode_handle);
                return (KMEM_CBRC_LATER);
        }

        /*
         * Ensure a consistent view of the dnode's holds and the dnode's dbufs.
         * We need to guarantee that there is a hold for every dbuf in order to
         * determine whether the dnode is actively referenced. Falsely matching
         * a dbuf to an active hold would lead to an unsafe move. It's possible
         * that a thread already having an active dnode hold is about to add a
         * dbuf, and we can't compare hold and dbuf counts while the add is in
         * progress.
         */
        if (!rw_tryenter(&odn->dn_struct_rwlock, RW_WRITER)) {
                zrl_exit(&odn->dn_handle->dnh_zrlock);
                mutex_exit(&os->os_lock);
                DNODE_STAT_ADD(dnode_move_stats.dms_dnode_rwlock);
                return (KMEM_CBRC_LATER);
        }

        /*
         * A dbuf may be removed (evicted) without an active dnode hold. In that
         * case, the dbuf count is decremented under the handle lock before the
         * dbuf's hold is released. This order ensures that if we count the hold
         * after the dbuf is removed but before its hold is released, we will
         * treat the unmatched hold as active and exit safely. If we count the
         * hold before the dbuf is removed, the hold is discounted, and the
         * removal is blocked until the move completes.
         */
        refcount = refcount_count(&odn->dn_holds);
        ASSERT(refcount >= 0);
        dbufs = odn->dn_dbufs_count;

        /* We can't have more dbufs than dnode holds. */
        ASSERT3U(dbufs, <=, refcount);
        DTRACE_PROBE3(dnode__move, dnode_t *, odn, int64_t, refcount,
            uint32_t, dbufs);

        if (refcount > dbufs) {
                rw_exit(&odn->dn_struct_rwlock);
                zrl_exit(&odn->dn_handle->dnh_zrlock);
                mutex_exit(&os->os_lock);
                DNODE_STAT_ADD(dnode_move_stats.dms_dnode_active);
                return (KMEM_CBRC_LATER);
        }

        rw_exit(&odn->dn_struct_rwlock);

        /*
         * At this point we know that anyone with a hold on the dnode is not
         * actively referencing it. The dnode is known and in a valid state to
         * move. We're holding the locks needed to execute the critical section.
         */
        dnode_move_impl(odn, ndn);

        list_link_replace(&odn->dn_link, &ndn->dn_link);
        /* If the dnode was safe to move, the refcount cannot have changed. */
        ASSERT(refcount == refcount_count(&ndn->dn_holds));
        ASSERT(dbufs == ndn->dn_dbufs_count);
        zrl_exit(&ndn->dn_handle->dnh_zrlock); /* handle has moved */
        mutex_exit(&os->os_lock);

        return (KMEM_CBRC_YES);
}
#endif  /* _KERNEL */
#endif  /* sun */

void
dnode_special_close(dnode_handle_t *dnh)
{
        dnode_t *dn = dnh->dnh_dnode;

        /*
         * 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);
        zrl_add(&dnh->dnh_zrlock);
        dnode_destroy(dn); /* implicit zrl_remove() */
        zrl_destroy(&dnh->dnh_zrlock);
        dnh->dnh_dnode = NULL;
}

dnode_t *
dnode_special_open(objset_t *os, dnode_phys_t *dnp, uint64_t object,
    dnode_handle_t *dnh)
{
        dnode_t *dn = dnode_create(os, dnp, NULL, object, dnh);
        dnh->dnh_dnode = dn;
        zrl_init(&dnh->dnh_zrlock);
        DNODE_VERIFY(dn);
        return (dn);
}

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

        ASSERT(epb == children_dnodes->dnc_count);

        for (i = 0; i < epb; i++) {
                dnode_handle_t *dnh = &children_dnodes->dnc_children[i];
                dnode_t *dn;

                /*
                 * The dnode handle lock guards against the dnode moving to
                 * another valid address, so there is no need here to guard
                 * against changes to or from NULL.
                 */
                if (dnh->dnh_dnode == NULL) {
                        zrl_destroy(&dnh->dnh_zrlock);
                        continue;
                }

                zrl_add(&dnh->dnh_zrlock);
                dn = dnh->dnh_dnode;
                /*
                 * 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 eligible for eviction and this function
                 * would not have been called.
                 */
                ASSERT(refcount_is_zero(&dn->dn_holds));
                ASSERT(refcount_is_zero(&dn->dn_tx_holds));

                dnode_destroy(dn); /* implicit zrl_remove() */
                zrl_destroy(&dnh->dnh_zrlock);
                dnh->dnh_dnode = NULL;
        }
        kmem_free(children_dnodes, sizeof (dnode_children_t) +
            (epb - 1) * sizeof (dnode_handle_t));
}

/*
 * errors:
 * EINVAL - invalid object number.
 * EIO - i/o error.
 * succeeds even for free dnodes.
 */
int
dnode_hold_impl(objset_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_children_t *children_dnodes;
        dnode_handle_t *dnh;

        /*
         * If you are holding the spa config lock as writer, you shouldn't
         * be asking the DMU to do *anything* unless it's the root pool
         * which may require us to read from the root filesystem while
         * holding some (not all) of the locks as writer.
         */
        ASSERT(spa_config_held(os->os_spa, SCL_ALL, RW_WRITER) == 0 ||
            (spa_is_root(os->os_spa) &&
            spa_config_held(os->os_spa, SCL_STATE, RW_WRITER)));

        if (object == DMU_USERUSED_OBJECT || object == DMU_GROUPUSED_OBJECT) {
                dn = (object == DMU_USERUSED_OBJECT) ?
                    DMU_USERUSED_DNODE(os) : DMU_GROUPUSED_DNODE(os);
                if (dn == NULL)
                        return (SET_ERROR(ENOENT));
                type = dn->dn_type;
                if ((flag & DNODE_MUST_BE_ALLOCATED) && type == DMU_OT_NONE)
                        return (SET_ERROR(ENOENT));
                if ((flag & DNODE_MUST_BE_FREE) && type != DMU_OT_NONE)
                        return (SET_ERROR(EEXIST));
                DNODE_VERIFY(dn);
                (void) refcount_add(&dn->dn_holds, tag);
                *dnp = dn;
                return (0);
        }

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

        mdn = DMU_META_DNODE(os);
        ASSERT(mdn->dn_object == DMU_META_DNODE_OBJECT);

        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 (SET_ERROR(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);

        ASSERT(DB_DNODE(db)->dn_type == DMU_OT_DNODE);
        children_dnodes = dmu_buf_get_user(&db->db);
        if (children_dnodes == NULL) {
                int i;
                dnode_children_t *winner;
                children_dnodes = kmem_zalloc(sizeof (dnode_children_t) +
                    (epb - 1) * sizeof (dnode_handle_t), KM_SLEEP);
                children_dnodes->dnc_count = epb;
                dnh = &children_dnodes->dnc_children[0];
                for (i = 0; i < epb; i++) {
                        zrl_init(&dnh[i].dnh_zrlock);
                        dnh[i].dnh_dnode = NULL;
                }
                if (winner = dmu_buf_set_user(&db->db, children_dnodes, NULL,
                    dnode_buf_pageout)) {

                        for (i = 0; i < epb; i++) {
                                zrl_destroy(&dnh[i].dnh_zrlock);
                        }

                        kmem_free(children_dnodes, sizeof (dnode_children_t) +
                            (epb - 1) * sizeof (dnode_handle_t));
                        children_dnodes = winner;
                }
        }
        ASSERT(children_dnodes->dnc_count == epb);

        dnh = &children_dnodes->dnc_children[idx];
        zrl_add(&dnh->dnh_zrlock);
        if ((dn = dnh->dnh_dnode) == NULL) {
                dnode_phys_t *phys = (dnode_phys_t *)db->db.db_data+idx;
                dnode_t *winner;

                dn = dnode_create(os, phys, db, object, dnh);
                winner = atomic_cas_ptr(&dnh->dnh_dnode, NULL, dn);
                if (winner != NULL) {
                        zrl_add(&dnh->dnh_zrlock);
                        dnode_destroy(dn); /* implicit zrl_remove() */
                        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 || !refcount_is_zero(&dn->dn_holds)))) {
                mutex_exit(&dn->dn_mtx);
                zrl_remove(&dnh->dnh_zrlock);
                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, dnh);
        /* Now we can rely on the hold to prevent the dnode from moving. */
        zrl_remove(&dnh->dnh_zrlock);

        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_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;
        /* Get while the hold prevents the dnode from moving. */
        dmu_buf_impl_t *db = dn->dn_dbuf;
        dnode_handle_t *dnh = dn->dn_handle;

        mutex_enter(&dn->dn_mtx);
        refs = refcount_remove(&dn->dn_holds, tag);
        mutex_exit(&dn->dn_mtx);

        /*
         * It's unsafe to release the last hold on a dnode by dnode_rele() or
         * indirectly by dbuf_rele() while relying on the dnode handle to
         * prevent the dnode from moving, since releasing the last hold could
         * result in the dnode's parent dbuf evicting its dnode handles. For
         * that reason anyone calling dnode_rele() or dbuf_rele() without some
         * other direct or indirect hold on the dnode must first drop the dnode
         * handle.
         */
        ASSERT(refs > 0 || dnh->dnh_zrlock.zr_owner != curthread);

        /* NOTE: the DNODE_DNODE does not have a dn_dbuf */
        if (refs == 0 && db != NULL) {
                /*
                 * Another thread could add a hold to the dnode handle in
                 * dnode_hold_impl() while holding the parent dbuf. Since the
                 * hold on the parent dbuf prevents the handle from being
                 * destroyed, the hold on the handle is OK. We can't yet assert
                 * that the handle has zero references, but that will be
                 * asserted anyway when the handle gets destroyed.
                 */
                dbuf_rele(db, dnh);
        }
}

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

        if (DMU_OBJECT_IS_SPECIAL(dn->dn_object)) {
                dsl_dataset_dirty(os->os_dsl_dataset, tx);
                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

        /*
         * Determine old uid/gid when necessary
         */
        dmu_objset_userquota_get_ids(dn, B_TRUE, tx);

        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);
        ASSERT0(dn->dn_next_bonuslen[txg&TXG_MASK]);
        ASSERT0(dn->dn_next_blksz[txg&TXG_MASK]);
        ASSERT0(dn->dn_next_bonustype[txg&TXG_MASK]);

        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 maintains 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_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 != DMU_BONUS_BLKID &&
                    db->db_blkid != DMU_SPILL_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 (SET_ERROR(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 != DMU_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);
                ASSERT(db != NULL);
                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 != DMU_BONUS_BLKID &&
                            dr->dr_dbuf->db_blkid != DMU_SPILL_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_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 == DMU_OBJECT_END) {
                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.
                         * Note that we won't dirty any indirect blocks,
                         * which is fine because we will be freeing the entire
                         * file and thus all indirect blocks will be freed
                         * by free_children().
                         */
                        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);
                                dmu_buf_will_dirty(&db->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);

        ASSERT0(P2PHASE(off, blksz));
        /* 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);
                                dmu_buf_will_dirty(&db->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;

        /*
         * Dirty the first and last indirect blocks, as they (and/or their
         * parents) will need to be written out if they were only
         * partially freed.  Interior indirect blocks will be themselves freed,
         * by free_children(), so they need not be dirtied.  Note that these
         * interior blocks have already been prefetched by dmu_tx_hold_free().
         */
        if (dn->dn_nlevels > 1) {
                uint64_t first, last;

                first = blkid >> epbs;
                if (db = dbuf_hold_level(dn, 1, first, FTAG)) {
                        dmu_buf_will_dirty(&db->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))) {
                        dmu_buf_will_dirty(&db->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);
        int txgoff = tx->tx_txg & TXG_MASK;
        if (dn->dn_free_ranges[txgoff] == NULL) {
                dn->dn_free_ranges[txgoff] =
                    range_tree_create(NULL, NULL, &dn->dn_mtx);
        }
        range_tree_clear(dn->dn_free_ranges[txgoff], blkid, nblks);
        range_tree_add(dn->dn_free_ranges[txgoff], blkid, nblks);
        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:

        rw_exit(&dn->dn_struct_rwlock);
}

static boolean_t
dnode_spill_freed(dnode_t *dn)
{
        int i;

        mutex_enter(&dn->dn_mtx);
        for (i = 0; i < TXG_SIZE; i++) {
                if (dn->dn_rm_spillblk[i] == DN_KILL_SPILLBLK)
                        break;
        }
        mutex_exit(&dn->dn_mtx);
        return (i < TXG_SIZE);
}

/* 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)
{
        void *dp = spa_get_dsl(dn->dn_objset->os_spa);
        int i;

        if (blkid == DMU_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 (blkid == DMU_SPILL_BLKID)
                return (dnode_spill_freed(dn));

        mutex_enter(&dn->dn_mtx);
        for (i = 0; i < TXG_SIZE; i++) {
                if (dn->dn_free_ranges[i] != NULL &&
                    range_tree_contains(dn->dn_free_ranges[i], blkid, 1))
                        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);
                ASSERT0(P2PHASE(space, 1<<DEV_BSHIFT));
                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 to track
 * the amount of memory in use by the currently open txg.
 */
void
dnode_willuse_space(dnode_t *dn, int64_t space, dmu_tx_t *tx)
{
        objset_t *os = dn->dn_objset;
        dsl_dataset_t *ds = os->os_dsl_dataset;
        int64_t aspace = spa_get_asize(os->os_spa, space);

        if (ds != NULL) {
                dsl_dir_willuse_space(ds->ds_dir, aspace, tx);
                dsl_pool_dirty_space(dmu_tx_pool(tx), space, tx);
        }

        dmu_tx_willuse_space(tx, aspace);
}

/*
 * Scans a block at the indicated "level" looking for a hole or data,
 * depending on 'flags'.
 *
 * If level > 0, then we are scanning an indirect block looking at its
 * pointers.  If level == 0, then we are looking at a block of dnodes.
 *
 * If we don't find what we are looking for in the block, we return ESRCH.
 * Otherwise, return with *offset pointing to the beginning (if searching
 * forwards) or end (if searching backwards) of the range covered by the
 * block pointer we matched on (or dnode).
 *
 * The basic search algorithm used below by dnode_next_offset() is to
 * use this function to search up the block tree (widen the search) until
 * we find something (i.e., we don't return ESRCH) and then search back
 * down the tree (narrow the search) until we reach our original search
 * level.
 */
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) != 0);
        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 (SET_ERROR(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 != NULL && txg != 0 && (db->db_blkptr == NULL ||
            db->db_blkptr->blk_birth <= txg ||
            BP_IS_HOLE(db->db_blkptr))) {
                /*
                 * This can only happen when we are searching up the tree
                 * and these conditions mean that we need to keep climbing.
                 */
                error = SET_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) {
                        if ((dnp[i].dn_type == DMU_OT_NONE) == hole)
                                break;
                        *offset += (1ULL << span) * inc;
                }
                if (i < 0 || i == blkfill)
                        error = SET_ERROR(ESRCH);
        } else {
                blkptr_t *bp = data;
                uint64_t start = *offset;
                span = (lvl - 1) * epbs + dn->dn_datablkshift;
                minfill = 0;
                maxfill = blkfill << ((lvl - 1) * epbs);

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

                *offset = *offset >> span;
                for (i = BF64_GET(*offset, 0, epbs);
                    i >= 0 && i < epb; i += inc) {
                        if (BP_GET_FILL(&bp[i]) >= minfill &&
                            BP_GET_FILL(&bp[i]) <= maxfill &&
                            (hole || bp[i].blk_birth > txg))
                                break;
                        if (inc > 0 || *offset > 0)
                                *offset += inc;
                }
                *offset = *offset << span;
                if (inc < 0) {
                        /* traversing backwards; position offset at the end */
                        ASSERT3U(*offset, <=, start);
                        *offset = MIN(*offset + (1ULL << span) - 1, start);
                } else if (*offset < start) {
                        *offset = start;
                }
                if (i < 0 || i >= epb)
                        error = SET_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 = SET_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 = SET_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 = SET_ERROR(ESRCH);
out:
        if (!(flags & DNODE_FIND_HAVELOCK))
                rw_exit(&dn->dn_struct_rwlock);

        return (error);
}