Imported Upstream version 0.6.4.2

[mirror_zfs-debian.git] / module / zfs / dbuf.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 2011 Nexenta Systems, Inc.  All rights reserved.
 * Copyright (c) 2012, 2014 by Delphix. All rights reserved.
 * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
 */

#include <sys/zfs_context.h>
#include <sys/arc.h>
#include <sys/dmu.h>
#include <sys/dmu_send.h>
#include <sys/dmu_impl.h>
#include <sys/dbuf.h>
#include <sys/dmu_objset.h>
#include <sys/dsl_dataset.h>
#include <sys/dsl_dir.h>
#include <sys/dmu_tx.h>
#include <sys/spa.h>
#include <sys/zio.h>
#include <sys/dmu_zfetch.h>
#include <sys/sa.h>
#include <sys/sa_impl.h>
#include <sys/zfeature.h>
#include <sys/blkptr.h>
#include <sys/range_tree.h>
#include <sys/trace_dbuf.h>

struct dbuf_hold_impl_data {
        /* Function arguments */
        dnode_t *dh_dn;
        uint8_t dh_level;
        uint64_t dh_blkid;
        int dh_fail_sparse;
        void *dh_tag;
        dmu_buf_impl_t **dh_dbp;
        /* Local variables */
        dmu_buf_impl_t *dh_db;
        dmu_buf_impl_t *dh_parent;
        blkptr_t *dh_bp;
        int dh_err;
        dbuf_dirty_record_t *dh_dr;
        arc_buf_contents_t dh_type;
        int dh_depth;
};

static void __dbuf_hold_impl_init(struct dbuf_hold_impl_data *dh,
    dnode_t *dn, uint8_t level, uint64_t blkid, int fail_sparse,
    void *tag, dmu_buf_impl_t **dbp, int depth);
static int __dbuf_hold_impl(struct dbuf_hold_impl_data *dh);

/*
 * Number of times that zfs_free_range() took the slow path while doing
 * a zfs receive.  A nonzero value indicates a potential performance problem.
 */
uint64_t zfs_free_range_recv_miss;

static void dbuf_destroy(dmu_buf_impl_t *db);
static boolean_t dbuf_undirty(dmu_buf_impl_t *db, dmu_tx_t *tx);
static void dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, dmu_tx_t *tx);

/*
 * Global data structures and functions for the dbuf cache.
 */
static kmem_cache_t *dbuf_cache;

/* ARGSUSED */
static int
dbuf_cons(void *vdb, void *unused, int kmflag)
{
        dmu_buf_impl_t *db = vdb;
        bzero(db, sizeof (dmu_buf_impl_t));

        mutex_init(&db->db_mtx, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&db->db_changed, NULL, CV_DEFAULT, NULL);
        refcount_create(&db->db_holds);
        list_link_init(&db->db_link);
        return (0);
}

/* ARGSUSED */
static void
dbuf_dest(void *vdb, void *unused)
{
        dmu_buf_impl_t *db = vdb;
        mutex_destroy(&db->db_mtx);
        cv_destroy(&db->db_changed);
        refcount_destroy(&db->db_holds);
}

/*
 * dbuf hash table routines
 */
static dbuf_hash_table_t dbuf_hash_table;

static uint64_t dbuf_hash_count;

static uint64_t
dbuf_hash(void *os, uint64_t obj, uint8_t lvl, uint64_t blkid)
{
        uintptr_t osv = (uintptr_t)os;
        uint64_t crc = -1ULL;

        ASSERT(zfs_crc64_table[128] == ZFS_CRC64_POLY);
        crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (lvl)) & 0xFF];
        crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (osv >> 6)) & 0xFF];
        crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (obj >> 0)) & 0xFF];
        crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (obj >> 8)) & 0xFF];
        crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (blkid >> 0)) & 0xFF];
        crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (blkid >> 8)) & 0xFF];

        crc ^= (osv>>14) ^ (obj>>16) ^ (blkid>>16);

        return (crc);
}

#define DBUF_HASH(os, obj, level, blkid) dbuf_hash(os, obj, level, blkid);

#define DBUF_EQUAL(dbuf, os, obj, level, blkid)         \
        ((dbuf)->db.db_object == (obj) &&               \
        (dbuf)->db_objset == (os) &&                    \
        (dbuf)->db_level == (level) &&                  \
        (dbuf)->db_blkid == (blkid))

dmu_buf_impl_t *
dbuf_find(dnode_t *dn, uint8_t level, uint64_t blkid)
{
        dbuf_hash_table_t *h = &dbuf_hash_table;
        objset_t *os = dn->dn_objset;
        uint64_t obj;
        uint64_t hv;
        uint64_t idx;
        dmu_buf_impl_t *db;

        obj = dn->dn_object;
        hv = DBUF_HASH(os, obj, level, blkid);
        idx = hv & h->hash_table_mask;

        mutex_enter(DBUF_HASH_MUTEX(h, idx));
        for (db = h->hash_table[idx]; db != NULL; db = db->db_hash_next) {
                if (DBUF_EQUAL(db, os, obj, level, blkid)) {
                        mutex_enter(&db->db_mtx);
                        if (db->db_state != DB_EVICTING) {
                                mutex_exit(DBUF_HASH_MUTEX(h, idx));
                                return (db);
                        }
                        mutex_exit(&db->db_mtx);
                }
        }
        mutex_exit(DBUF_HASH_MUTEX(h, idx));
        return (NULL);
}

/*
 * Insert an entry into the hash table.  If there is already an element
 * equal to elem in the hash table, then the already existing element
 * will be returned and the new element will not be inserted.
 * Otherwise returns NULL.
 */
static dmu_buf_impl_t *
dbuf_hash_insert(dmu_buf_impl_t *db)
{
        dbuf_hash_table_t *h = &dbuf_hash_table;
        objset_t *os = db->db_objset;
        uint64_t obj = db->db.db_object;
        int level = db->db_level;
        uint64_t blkid, hv, idx;
        dmu_buf_impl_t *dbf;

        blkid = db->db_blkid;
        hv = DBUF_HASH(os, obj, level, blkid);
        idx = hv & h->hash_table_mask;

        mutex_enter(DBUF_HASH_MUTEX(h, idx));
        for (dbf = h->hash_table[idx]; dbf != NULL; dbf = dbf->db_hash_next) {
                if (DBUF_EQUAL(dbf, os, obj, level, blkid)) {
                        mutex_enter(&dbf->db_mtx);
                        if (dbf->db_state != DB_EVICTING) {
                                mutex_exit(DBUF_HASH_MUTEX(h, idx));
                                return (dbf);
                        }
                        mutex_exit(&dbf->db_mtx);
                }
        }

        mutex_enter(&db->db_mtx);
        db->db_hash_next = h->hash_table[idx];
        h->hash_table[idx] = db;
        mutex_exit(DBUF_HASH_MUTEX(h, idx));
        atomic_add_64(&dbuf_hash_count, 1);

        return (NULL);
}

/*
 * Remove an entry from the hash table.  It must be in the EVICTING state.
 */
static void
dbuf_hash_remove(dmu_buf_impl_t *db)
{
        dbuf_hash_table_t *h = &dbuf_hash_table;
        uint64_t hv, idx;
        dmu_buf_impl_t *dbf, **dbp;

        hv = DBUF_HASH(db->db_objset, db->db.db_object,
            db->db_level, db->db_blkid);
        idx = hv & h->hash_table_mask;

        /*
         * We musn't hold db_mtx to maintain lock ordering:
         * DBUF_HASH_MUTEX > db_mtx.
         */
        ASSERT(refcount_is_zero(&db->db_holds));
        ASSERT(db->db_state == DB_EVICTING);
        ASSERT(!MUTEX_HELD(&db->db_mtx));

        mutex_enter(DBUF_HASH_MUTEX(h, idx));
        dbp = &h->hash_table[idx];
        while ((dbf = *dbp) != db) {
                dbp = &dbf->db_hash_next;
                ASSERT(dbf != NULL);
        }
        *dbp = db->db_hash_next;
        db->db_hash_next = NULL;
        mutex_exit(DBUF_HASH_MUTEX(h, idx));
        atomic_add_64(&dbuf_hash_count, -1);
}

static arc_evict_func_t dbuf_do_evict;

static void
dbuf_evict_user(dmu_buf_impl_t *db)
{
        ASSERT(MUTEX_HELD(&db->db_mtx));

        if (db->db_level != 0 || db->db_evict_func == NULL)
                return;

        if (db->db_user_data_ptr_ptr)
                *db->db_user_data_ptr_ptr = db->db.db_data;
        db->db_evict_func(&db->db, db->db_user_ptr);
        db->db_user_ptr = NULL;
        db->db_user_data_ptr_ptr = NULL;
        db->db_evict_func = NULL;
}

boolean_t
dbuf_is_metadata(dmu_buf_impl_t *db)
{
        /*
         * Consider indirect blocks and spill blocks to be meta data.
         */
        if (db->db_level > 0 || db->db_blkid == DMU_SPILL_BLKID) {
                return (B_TRUE);
        } else {
                boolean_t is_metadata;

                DB_DNODE_ENTER(db);
                is_metadata = DMU_OT_IS_METADATA(DB_DNODE(db)->dn_type);
                DB_DNODE_EXIT(db);

                return (is_metadata);
        }
}

void
dbuf_evict(dmu_buf_impl_t *db)
{
        ASSERT(MUTEX_HELD(&db->db_mtx));
        ASSERT(db->db_buf == NULL);
        ASSERT(db->db_data_pending == NULL);

        dbuf_clear(db);
        dbuf_destroy(db);
}

void
dbuf_init(void)
{
        uint64_t hsize = 1ULL << 16;
        dbuf_hash_table_t *h = &dbuf_hash_table;
        int i;

        /*
         * The hash table is big enough to fill all of physical memory
         * with an average 4K block size.  The table will take up
         * totalmem*sizeof(void*)/4K (i.e. 2MB/GB with 8-byte pointers).
         */
        while (hsize * 4096 < physmem * PAGESIZE)
                hsize <<= 1;

retry:
        h->hash_table_mask = hsize - 1;
#if defined(_KERNEL) && defined(HAVE_SPL)
        /*
         * Large allocations which do not require contiguous pages
         * should be using vmem_alloc() in the linux kernel
         */
        h->hash_table = vmem_zalloc(hsize * sizeof (void *), KM_SLEEP);
#else
        h->hash_table = kmem_zalloc(hsize * sizeof (void *), KM_NOSLEEP);
#endif
        if (h->hash_table == NULL) {
                /* XXX - we should really return an error instead of assert */
                ASSERT(hsize > (1ULL << 10));
                hsize >>= 1;
                goto retry;
        }

        dbuf_cache = kmem_cache_create("dmu_buf_impl_t",
            sizeof (dmu_buf_impl_t),
            0, dbuf_cons, dbuf_dest, NULL, NULL, NULL, 0);

        for (i = 0; i < DBUF_MUTEXES; i++)
                mutex_init(&h->hash_mutexes[i], NULL, MUTEX_DEFAULT, NULL);

        dbuf_stats_init(h);
}

void
dbuf_fini(void)
{
        dbuf_hash_table_t *h = &dbuf_hash_table;
        int i;

        dbuf_stats_destroy();

        for (i = 0; i < DBUF_MUTEXES; i++)
                mutex_destroy(&h->hash_mutexes[i]);
#if defined(_KERNEL) && defined(HAVE_SPL)
        /*
         * Large allocations which do not require contiguous pages
         * should be using vmem_free() in the linux kernel
         */
        vmem_free(h->hash_table, (h->hash_table_mask + 1) * sizeof (void *));
#else
        kmem_free(h->hash_table, (h->hash_table_mask + 1) * sizeof (void *));
#endif
        kmem_cache_destroy(dbuf_cache);
}

/*
 * Other stuff.
 */

#ifdef ZFS_DEBUG
static void
dbuf_verify(dmu_buf_impl_t *db)
{
        dnode_t *dn;
        dbuf_dirty_record_t *dr;

        ASSERT(MUTEX_HELD(&db->db_mtx));

        if (!(zfs_flags & ZFS_DEBUG_DBUF_VERIFY))
                return;

        ASSERT(db->db_objset != NULL);
        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        if (dn == NULL) {
                ASSERT(db->db_parent == NULL);
                ASSERT(db->db_blkptr == NULL);
        } else {
                ASSERT3U(db->db.db_object, ==, dn->dn_object);
                ASSERT3P(db->db_objset, ==, dn->dn_objset);
                ASSERT3U(db->db_level, <, dn->dn_nlevels);
                ASSERT(db->db_blkid == DMU_BONUS_BLKID ||
                    db->db_blkid == DMU_SPILL_BLKID ||
                    !list_is_empty(&dn->dn_dbufs));
        }
        if (db->db_blkid == DMU_BONUS_BLKID) {
                ASSERT(dn != NULL);
                ASSERT3U(db->db.db_size, >=, dn->dn_bonuslen);
                ASSERT3U(db->db.db_offset, ==, DMU_BONUS_BLKID);
        } else if (db->db_blkid == DMU_SPILL_BLKID) {
                ASSERT(dn != NULL);
                ASSERT3U(db->db.db_size, >=, dn->dn_bonuslen);
                ASSERT0(db->db.db_offset);
        } else {
                ASSERT3U(db->db.db_offset, ==, db->db_blkid * db->db.db_size);
        }

        for (dr = db->db_data_pending; dr != NULL; dr = dr->dr_next)
                ASSERT(dr->dr_dbuf == db);

        for (dr = db->db_last_dirty; dr != NULL; dr = dr->dr_next)
                ASSERT(dr->dr_dbuf == db);

        /*
         * We can't assert that db_size matches dn_datablksz because it
         * can be momentarily different when another thread is doing
         * dnode_set_blksz().
         */
        if (db->db_level == 0 && db->db.db_object == DMU_META_DNODE_OBJECT) {
                dr = db->db_data_pending;
                /*
                 * It should only be modified in syncing context, so
                 * make sure we only have one copy of the data.
                 */
                ASSERT(dr == NULL || dr->dt.dl.dr_data == db->db_buf);
        }

        /* verify db->db_blkptr */
        if (db->db_blkptr) {
                if (db->db_parent == dn->dn_dbuf) {
                        /* db is pointed to by the dnode */
                        /* ASSERT3U(db->db_blkid, <, dn->dn_nblkptr); */
                        if (DMU_OBJECT_IS_SPECIAL(db->db.db_object))
                                ASSERT(db->db_parent == NULL);
                        else
                                ASSERT(db->db_parent != NULL);
                        if (db->db_blkid != DMU_SPILL_BLKID)
                                ASSERT3P(db->db_blkptr, ==,
                                    &dn->dn_phys->dn_blkptr[db->db_blkid]);
                } else {
                        /* db is pointed to by an indirect block */
                        ASSERTV(int epb = db->db_parent->db.db_size >>
                                SPA_BLKPTRSHIFT);
                        ASSERT3U(db->db_parent->db_level, ==, db->db_level+1);
                        ASSERT3U(db->db_parent->db.db_object, ==,
                            db->db.db_object);
                        /*
                         * dnode_grow_indblksz() can make this fail if we don't
                         * have the struct_rwlock.  XXX indblksz no longer
                         * grows.  safe to do this now?
                         */
                        if (RW_WRITE_HELD(&dn->dn_struct_rwlock)) {
                                ASSERT3P(db->db_blkptr, ==,
                                    ((blkptr_t *)db->db_parent->db.db_data +
                                    db->db_blkid % epb));
                        }
                }
        }
        if ((db->db_blkptr == NULL || BP_IS_HOLE(db->db_blkptr)) &&
            (db->db_buf == NULL || db->db_buf->b_data) &&
            db->db.db_data && db->db_blkid != DMU_BONUS_BLKID &&
            db->db_state != DB_FILL && !dn->dn_free_txg) {
                /*
                 * If the blkptr isn't set but they have nonzero data,
                 * it had better be dirty, otherwise we'll lose that
                 * data when we evict this buffer.
                 */
                if (db->db_dirtycnt == 0) {
                        ASSERTV(uint64_t *buf = db->db.db_data);
                        int i;

                        for (i = 0; i < db->db.db_size >> 3; i++) {
                                ASSERT(buf[i] == 0);
                        }
                }
        }
        DB_DNODE_EXIT(db);
}
#endif

static void
dbuf_update_data(dmu_buf_impl_t *db)
{
        ASSERT(MUTEX_HELD(&db->db_mtx));
        if (db->db_level == 0 && db->db_user_data_ptr_ptr) {
                ASSERT(!refcount_is_zero(&db->db_holds));
                *db->db_user_data_ptr_ptr = db->db.db_data;
        }
}

static void
dbuf_set_data(dmu_buf_impl_t *db, arc_buf_t *buf)
{
        ASSERT(MUTEX_HELD(&db->db_mtx));
        db->db_buf = buf;
        if (buf != NULL) {
                ASSERT(buf->b_data != NULL);
                db->db.db_data = buf->b_data;
                if (!arc_released(buf))
                        arc_set_callback(buf, dbuf_do_evict, db);
                dbuf_update_data(db);
        } else {
                dbuf_evict_user(db);
                db->db.db_data = NULL;
                if (db->db_state != DB_NOFILL)
                        db->db_state = DB_UNCACHED;
        }
}

/*
 * Loan out an arc_buf for read.  Return the loaned arc_buf.
 */
arc_buf_t *
dbuf_loan_arcbuf(dmu_buf_impl_t *db)
{
        arc_buf_t *abuf;

        mutex_enter(&db->db_mtx);
        if (arc_released(db->db_buf) || refcount_count(&db->db_holds) > 1) {
                int blksz = db->db.db_size;
                spa_t *spa = db->db_objset->os_spa;

                mutex_exit(&db->db_mtx);
                abuf = arc_loan_buf(spa, blksz);
                bcopy(db->db.db_data, abuf->b_data, blksz);
        } else {
                abuf = db->db_buf;
                arc_loan_inuse_buf(abuf, db);
                dbuf_set_data(db, NULL);
                mutex_exit(&db->db_mtx);
        }
        return (abuf);
}

uint64_t
dbuf_whichblock(dnode_t *dn, uint64_t offset)
{
        if (dn->dn_datablkshift) {
                return (offset >> dn->dn_datablkshift);
        } else {
                ASSERT3U(offset, <, dn->dn_datablksz);
                return (0);
        }
}

static void
dbuf_read_done(zio_t *zio, arc_buf_t *buf, void *vdb)
{
        dmu_buf_impl_t *db = vdb;

        mutex_enter(&db->db_mtx);
        ASSERT3U(db->db_state, ==, DB_READ);
        /*
         * All reads are synchronous, so we must have a hold on the dbuf
         */
        ASSERT(refcount_count(&db->db_holds) > 0);
        ASSERT(db->db_buf == NULL);
        ASSERT(db->db.db_data == NULL);
        if (db->db_level == 0 && db->db_freed_in_flight) {
                /* we were freed in flight; disregard any error */
                arc_release(buf, db);
                bzero(buf->b_data, db->db.db_size);
                arc_buf_freeze(buf);
                db->db_freed_in_flight = FALSE;
                dbuf_set_data(db, buf);
                db->db_state = DB_CACHED;
        } else if (zio == NULL || zio->io_error == 0) {
                dbuf_set_data(db, buf);
                db->db_state = DB_CACHED;
        } else {
                ASSERT(db->db_blkid != DMU_BONUS_BLKID);
                ASSERT3P(db->db_buf, ==, NULL);
                VERIFY(arc_buf_remove_ref(buf, db));
                db->db_state = DB_UNCACHED;
        }
        cv_broadcast(&db->db_changed);
        dbuf_rele_and_unlock(db, NULL);
}

static int
dbuf_read_impl(dmu_buf_impl_t *db, zio_t *zio, uint32_t *flags)
{
        dnode_t *dn;
        zbookmark_phys_t zb;
        uint32_t aflags = ARC_NOWAIT;
        int err;

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        ASSERT(!refcount_is_zero(&db->db_holds));
        /* We need the struct_rwlock to prevent db_blkptr from changing. */
        ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));
        ASSERT(MUTEX_HELD(&db->db_mtx));
        ASSERT(db->db_state == DB_UNCACHED);
        ASSERT(db->db_buf == NULL);

        if (db->db_blkid == DMU_BONUS_BLKID) {
                int bonuslen = MIN(dn->dn_bonuslen, dn->dn_phys->dn_bonuslen);

                ASSERT3U(bonuslen, <=, db->db.db_size);
                db->db.db_data = zio_buf_alloc(DN_MAX_BONUSLEN);
                arc_space_consume(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                if (bonuslen < DN_MAX_BONUSLEN)
                        bzero(db->db.db_data, DN_MAX_BONUSLEN);
                if (bonuslen)
                        bcopy(DN_BONUS(dn->dn_phys), db->db.db_data, bonuslen);
                DB_DNODE_EXIT(db);
                dbuf_update_data(db);
                db->db_state = DB_CACHED;
                mutex_exit(&db->db_mtx);
                return (0);
        }

        /*
         * Recheck BP_IS_HOLE() after dnode_block_freed() in case dnode_sync()
         * processes the delete record and clears the bp while we are waiting
         * for the dn_mtx (resulting in a "no" from block_freed).
         */
        if (db->db_blkptr == NULL || BP_IS_HOLE(db->db_blkptr) ||
            (db->db_level == 0 && (dnode_block_freed(dn, db->db_blkid) ||
            BP_IS_HOLE(db->db_blkptr)))) {
                arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);

                DB_DNODE_EXIT(db);
                dbuf_set_data(db, arc_buf_alloc(db->db_objset->os_spa,
                    db->db.db_size, db, type));
                bzero(db->db.db_data, db->db.db_size);
                db->db_state = DB_CACHED;
                *flags |= DB_RF_CACHED;
                mutex_exit(&db->db_mtx);
                return (0);
        }

        DB_DNODE_EXIT(db);

        db->db_state = DB_READ;
        mutex_exit(&db->db_mtx);

        if (DBUF_IS_L2CACHEABLE(db))
                aflags |= ARC_L2CACHE;
        if (DBUF_IS_L2COMPRESSIBLE(db))
                aflags |= ARC_L2COMPRESS;

        SET_BOOKMARK(&zb, db->db_objset->os_dsl_dataset ?
            db->db_objset->os_dsl_dataset->ds_object : DMU_META_OBJSET,
            db->db.db_object, db->db_level, db->db_blkid);

        dbuf_add_ref(db, NULL);

        err = arc_read(zio, db->db_objset->os_spa, db->db_blkptr,
            dbuf_read_done, db, ZIO_PRIORITY_SYNC_READ,
            (*flags & DB_RF_CANFAIL) ? ZIO_FLAG_CANFAIL : ZIO_FLAG_MUSTSUCCEED,
            &aflags, &zb);
        if (aflags & ARC_CACHED)
                *flags |= DB_RF_CACHED;

        return (SET_ERROR(err));
}

int
dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
{
        int err = 0;
        boolean_t havepzio = (zio != NULL);
        boolean_t prefetch;
        dnode_t *dn;

        /*
         * We don't have to hold the mutex to check db_state because it
         * can't be freed while we have a hold on the buffer.
         */
        ASSERT(!refcount_is_zero(&db->db_holds));

        if (db->db_state == DB_NOFILL)
                return (SET_ERROR(EIO));

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        if ((flags & DB_RF_HAVESTRUCT) == 0)
                rw_enter(&dn->dn_struct_rwlock, RW_READER);

        prefetch = db->db_level == 0 && db->db_blkid != DMU_BONUS_BLKID &&
            (flags & DB_RF_NOPREFETCH) == 0 && dn != NULL &&
            DBUF_IS_CACHEABLE(db);

        mutex_enter(&db->db_mtx);
        if (db->db_state == DB_CACHED) {
                mutex_exit(&db->db_mtx);
                if (prefetch)
                        dmu_zfetch(&dn->dn_zfetch, db->db.db_offset,
                            db->db.db_size, TRUE);
                if ((flags & DB_RF_HAVESTRUCT) == 0)
                        rw_exit(&dn->dn_struct_rwlock);
                DB_DNODE_EXIT(db);
        } else if (db->db_state == DB_UNCACHED) {
                spa_t *spa = dn->dn_objset->os_spa;

                if (zio == NULL)
                        zio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);

                err = dbuf_read_impl(db, zio, &flags);

                /* dbuf_read_impl has dropped db_mtx for us */

                if (!err && prefetch)
                        dmu_zfetch(&dn->dn_zfetch, db->db.db_offset,
                            db->db.db_size, flags & DB_RF_CACHED);

                if ((flags & DB_RF_HAVESTRUCT) == 0)
                        rw_exit(&dn->dn_struct_rwlock);
                DB_DNODE_EXIT(db);

                if (!err && !havepzio)
                        err = zio_wait(zio);
        } else {
                /*
                 * Another reader came in while the dbuf was in flight
                 * between UNCACHED and CACHED.  Either a writer will finish
                 * writing the buffer (sending the dbuf to CACHED) or the
                 * first reader's request will reach the read_done callback
                 * and send the dbuf to CACHED.  Otherwise, a failure
                 * occurred and the dbuf went to UNCACHED.
                 */
                mutex_exit(&db->db_mtx);
                if (prefetch)
                        dmu_zfetch(&dn->dn_zfetch, db->db.db_offset,
                            db->db.db_size, TRUE);
                if ((flags & DB_RF_HAVESTRUCT) == 0)
                        rw_exit(&dn->dn_struct_rwlock);
                DB_DNODE_EXIT(db);

                /* Skip the wait per the caller's request. */
                mutex_enter(&db->db_mtx);
                if ((flags & DB_RF_NEVERWAIT) == 0) {
                        while (db->db_state == DB_READ ||
                            db->db_state == DB_FILL) {
                                ASSERT(db->db_state == DB_READ ||
                                    (flags & DB_RF_HAVESTRUCT) == 0);
                                DTRACE_PROBE2(blocked__read, dmu_buf_impl_t *,
                                    db, zio_t *, zio);
                                cv_wait(&db->db_changed, &db->db_mtx);
                        }
                        if (db->db_state == DB_UNCACHED)
                                err = SET_ERROR(EIO);
                }
                mutex_exit(&db->db_mtx);
        }

        ASSERT(err || havepzio || db->db_state == DB_CACHED);
        return (err);
}

static void
dbuf_noread(dmu_buf_impl_t *db)
{
        ASSERT(!refcount_is_zero(&db->db_holds));
        ASSERT(db->db_blkid != DMU_BONUS_BLKID);
        mutex_enter(&db->db_mtx);
        while (db->db_state == DB_READ || db->db_state == DB_FILL)
                cv_wait(&db->db_changed, &db->db_mtx);
        if (db->db_state == DB_UNCACHED) {
                arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);
                spa_t *spa = db->db_objset->os_spa;

                ASSERT(db->db_buf == NULL);
                ASSERT(db->db.db_data == NULL);
                dbuf_set_data(db, arc_buf_alloc(spa, db->db.db_size, db, type));
                db->db_state = DB_FILL;
        } else if (db->db_state == DB_NOFILL) {
                dbuf_set_data(db, NULL);
        } else {
                ASSERT3U(db->db_state, ==, DB_CACHED);
        }
        mutex_exit(&db->db_mtx);
}

/*
 * This is our just-in-time copy function.  It makes a copy of
 * buffers, that have been modified in a previous transaction
 * group, before we modify them in the current active group.
 *
 * This function is used in two places: when we are dirtying a
 * buffer for the first time in a txg, and when we are freeing
 * a range in a dnode that includes this buffer.
 *
 * Note that when we are called from dbuf_free_range() we do
 * not put a hold on the buffer, we just traverse the active
 * dbuf list for the dnode.
 */
static void
dbuf_fix_old_data(dmu_buf_impl_t *db, uint64_t txg)
{
        dbuf_dirty_record_t *dr = db->db_last_dirty;

        ASSERT(MUTEX_HELD(&db->db_mtx));
        ASSERT(db->db.db_data != NULL);
        ASSERT(db->db_level == 0);
        ASSERT(db->db.db_object != DMU_META_DNODE_OBJECT);

        if (dr == NULL ||
            (dr->dt.dl.dr_data !=
            ((db->db_blkid  == DMU_BONUS_BLKID) ? db->db.db_data : db->db_buf)))
                return;

        /*
         * If the last dirty record for this dbuf has not yet synced
         * and its referencing the dbuf data, either:
         *      reset the reference to point to a new copy,
         * or (if there a no active holders)
         *      just null out the current db_data pointer.
         */
        ASSERT(dr->dr_txg >= txg - 2);
        if (db->db_blkid == DMU_BONUS_BLKID) {
                /* Note that the data bufs here are zio_bufs */
                dr->dt.dl.dr_data = zio_buf_alloc(DN_MAX_BONUSLEN);
                arc_space_consume(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                bcopy(db->db.db_data, dr->dt.dl.dr_data, DN_MAX_BONUSLEN);
        } else if (refcount_count(&db->db_holds) > db->db_dirtycnt) {
                int size = db->db.db_size;
                arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);
                spa_t *spa = db->db_objset->os_spa;

                dr->dt.dl.dr_data = arc_buf_alloc(spa, size, db, type);
                bcopy(db->db.db_data, dr->dt.dl.dr_data->b_data, size);
        } else {
                dbuf_set_data(db, NULL);
        }
}

void
dbuf_unoverride(dbuf_dirty_record_t *dr)
{
        dmu_buf_impl_t *db = dr->dr_dbuf;
        blkptr_t *bp = &dr->dt.dl.dr_overridden_by;
        uint64_t txg = dr->dr_txg;

        ASSERT(MUTEX_HELD(&db->db_mtx));
        ASSERT(dr->dt.dl.dr_override_state != DR_IN_DMU_SYNC);
        ASSERT(db->db_level == 0);

        if (db->db_blkid == DMU_BONUS_BLKID ||
            dr->dt.dl.dr_override_state == DR_NOT_OVERRIDDEN)
                return;

        ASSERT(db->db_data_pending != dr);

        /* free this block */
        if (!BP_IS_HOLE(bp) && !dr->dt.dl.dr_nopwrite)
                zio_free(db->db_objset->os_spa, txg, bp);

        dr->dt.dl.dr_override_state = DR_NOT_OVERRIDDEN;
        dr->dt.dl.dr_nopwrite = B_FALSE;

        /*
         * Release the already-written buffer, so we leave it in
         * a consistent dirty state.  Note that all callers are
         * modifying the buffer, so they will immediately do
         * another (redundant) arc_release().  Therefore, leave
         * the buf thawed to save the effort of freezing &
         * immediately re-thawing it.
         */
        arc_release(dr->dt.dl.dr_data, db);
}

/*
 * Evict (if its unreferenced) or clear (if its referenced) any level-0
 * data blocks in the free range, so that any future readers will find
 * empty blocks.
 *
 * This is a no-op if the dataset is in the middle of an incremental
 * receive; see comment below for details.
 */
void
dbuf_free_range(dnode_t *dn, uint64_t start, uint64_t end, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db, *db_next;
        uint64_t txg = tx->tx_txg;
        boolean_t freespill =
            (start == DMU_SPILL_BLKID || end == DMU_SPILL_BLKID);

        if (end > dn->dn_maxblkid && !freespill)
                end = dn->dn_maxblkid;
        dprintf_dnode(dn, "start=%llu end=%llu\n", start, end);

        mutex_enter(&dn->dn_dbufs_mtx);
        if (start >= dn->dn_unlisted_l0_blkid * dn->dn_datablksz &&
            !freespill) {
                /* There can't be any dbufs in this range; no need to search. */
                mutex_exit(&dn->dn_dbufs_mtx);
                return;
        } else if (dmu_objset_is_receiving(dn->dn_objset)) {
                /*
                 * If we are receiving, we expect there to be no dbufs in
                 * the range to be freed, because receive modifies each
                 * block at most once, and in offset order.  If this is
                 * not the case, it can lead to performance problems,
                 * so note that we unexpectedly took the slow path.
                 */
                atomic_inc_64(&zfs_free_range_recv_miss);
        }

        for (db = list_head(&dn->dn_dbufs); db != NULL; db = db_next) {
                db_next = list_next(&dn->dn_dbufs, db);
                ASSERT(db->db_blkid != DMU_BONUS_BLKID);

                /* Skip indirect blocks. */
                if (db->db_level != 0)
                        continue;
                /* Skip direct blocks outside the range. */
                if (!freespill && (db->db_blkid < start || db->db_blkid > end))
                        continue;
                /* Skip all direct blocks, only free spill blocks. */
                if (freespill && (db->db_blkid != DMU_SPILL_BLKID))
                        continue;

                /* found a level 0 buffer in the range */
                mutex_enter(&db->db_mtx);
                if (dbuf_undirty(db, tx)) {
                        /* mutex has been dropped and dbuf destroyed */
                        continue;
                }

                if (db->db_state == DB_UNCACHED ||
                    db->db_state == DB_NOFILL ||
                    db->db_state == DB_EVICTING) {
                        ASSERT(db->db.db_data == NULL);
                        mutex_exit(&db->db_mtx);
                        continue;
                }
                if (db->db_state == DB_READ || db->db_state == DB_FILL) {
                        /* will be handled in dbuf_read_done or dbuf_rele */
                        db->db_freed_in_flight = TRUE;
                        mutex_exit(&db->db_mtx);
                        continue;
                }
                if (refcount_count(&db->db_holds) == 0) {
                        ASSERT(db->db_buf);
                        dbuf_clear(db);
                        continue;
                }
                /* The dbuf is referenced */

                if (db->db_last_dirty != NULL) {
                        dbuf_dirty_record_t *dr = db->db_last_dirty;

                        if (dr->dr_txg == txg) {
                                /*
                                 * This buffer is "in-use", re-adjust the file
                                 * size to reflect that this buffer may
                                 * contain new data when we sync.
                                 */
                                if (db->db_blkid != DMU_SPILL_BLKID &&
                                    db->db_blkid > dn->dn_maxblkid)
                                        dn->dn_maxblkid = db->db_blkid;
                                dbuf_unoverride(dr);
                        } else {
                                /*
                                 * This dbuf is not dirty in the open context.
                                 * Either uncache it (if its not referenced in
                                 * the open context) or reset its contents to
                                 * empty.
                                 */
                                dbuf_fix_old_data(db, txg);
                        }
                }
                /* clear the contents if its cached */
                if (db->db_state == DB_CACHED) {
                        ASSERT(db->db.db_data != NULL);
                        arc_release(db->db_buf, db);
                        bzero(db->db.db_data, db->db.db_size);
                        arc_buf_freeze(db->db_buf);
                }

                mutex_exit(&db->db_mtx);
        }
        mutex_exit(&dn->dn_dbufs_mtx);
}

static int
dbuf_block_freeable(dmu_buf_impl_t *db)
{
        dsl_dataset_t *ds = db->db_objset->os_dsl_dataset;
        uint64_t birth_txg = 0;

        /*
         * We don't need any locking to protect db_blkptr:
         * If it's syncing, then db_last_dirty will be set
         * so we'll ignore db_blkptr.
         *
         * This logic ensures that only block births for
         * filled blocks are considered.
         */
        ASSERT(MUTEX_HELD(&db->db_mtx));
        if (db->db_last_dirty && (db->db_blkptr == NULL ||
            !BP_IS_HOLE(db->db_blkptr))) {
                birth_txg = db->db_last_dirty->dr_txg;
        } else if (db->db_blkptr != NULL && !BP_IS_HOLE(db->db_blkptr)) {
                birth_txg = db->db_blkptr->blk_birth;
        }

        /*
         * If this block don't exist or is in a snapshot, it can't be freed.
         * Don't pass the bp to dsl_dataset_block_freeable() since we
         * are holding the db_mtx lock and might deadlock if we are
         * prefetching a dedup-ed block.
         */
        if (birth_txg != 0)
                return (ds == NULL ||
                    dsl_dataset_block_freeable(ds, NULL, birth_txg));
        else
                return (B_FALSE);
}

void
dbuf_new_size(dmu_buf_impl_t *db, int size, dmu_tx_t *tx)
{
        arc_buf_t *buf, *obuf;
        int osize = db->db.db_size;
        arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);
        dnode_t *dn;

        ASSERT(db->db_blkid != DMU_BONUS_BLKID);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);

        /* XXX does *this* func really need the lock? */
        ASSERT(RW_WRITE_HELD(&dn->dn_struct_rwlock));

        /*
         * This call to dmu_buf_will_dirty() with the dn_struct_rwlock held
         * is OK, because there can be no other references to the db
         * when we are changing its size, so no concurrent DB_FILL can
         * be happening.
         */
        /*
         * XXX we should be doing a dbuf_read, checking the return
         * value and returning that up to our callers
         */
        dmu_buf_will_dirty(&db->db, tx);

        /* create the data buffer for the new block */
        buf = arc_buf_alloc(dn->dn_objset->os_spa, size, db, type);

        /* copy old block data to the new block */
        obuf = db->db_buf;
        bcopy(obuf->b_data, buf->b_data, MIN(osize, size));
        /* zero the remainder */
        if (size > osize)
                bzero((uint8_t *)buf->b_data + osize, size - osize);

        mutex_enter(&db->db_mtx);
        dbuf_set_data(db, buf);
        VERIFY(arc_buf_remove_ref(obuf, db));
        db->db.db_size = size;

        if (db->db_level == 0) {
                ASSERT3U(db->db_last_dirty->dr_txg, ==, tx->tx_txg);
                db->db_last_dirty->dt.dl.dr_data = buf;
        }
        mutex_exit(&db->db_mtx);

        dnode_willuse_space(dn, size-osize, tx);
        DB_DNODE_EXIT(db);
}

void
dbuf_release_bp(dmu_buf_impl_t *db)
{
        ASSERTV(objset_t *os = db->db_objset);

        ASSERT(dsl_pool_sync_context(dmu_objset_pool(os)));
        ASSERT(arc_released(os->os_phys_buf) ||
            list_link_active(&os->os_dsl_dataset->ds_synced_link));
        ASSERT(db->db_parent == NULL || arc_released(db->db_parent->db_buf));

        (void) arc_release(db->db_buf, db);
}

dbuf_dirty_record_t *
dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
{
        dnode_t *dn;
        objset_t *os;
        dbuf_dirty_record_t **drp, *dr;
        int drop_struct_lock = FALSE;
        boolean_t do_free_accounting = B_FALSE;
        int txgoff = tx->tx_txg & TXG_MASK;

        ASSERT(tx->tx_txg != 0);
        ASSERT(!refcount_is_zero(&db->db_holds));
        DMU_TX_DIRTY_BUF(tx, db);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        /*
         * Shouldn't dirty a regular buffer in syncing context.  Private
         * objects may be dirtied in syncing context, but only if they
         * were already pre-dirtied in open context.
         */
        ASSERT(!dmu_tx_is_syncing(tx) ||
            BP_IS_HOLE(dn->dn_objset->os_rootbp) ||
            DMU_OBJECT_IS_SPECIAL(dn->dn_object) ||
            dn->dn_objset->os_dsl_dataset == NULL);
        /*
         * We make this assert for private objects as well, but after we
         * check if we're already dirty.  They are allowed to re-dirty
         * in syncing context.
         */
        ASSERT(dn->dn_object == DMU_META_DNODE_OBJECT ||
            dn->dn_dirtyctx == DN_UNDIRTIED || dn->dn_dirtyctx ==
            (dmu_tx_is_syncing(tx) ? DN_DIRTY_SYNC : DN_DIRTY_OPEN));

        mutex_enter(&db->db_mtx);
        /*
         * XXX make this true for indirects too?  The problem is that
         * transactions created with dmu_tx_create_assigned() from
         * syncing context don't bother holding ahead.
         */
        ASSERT(db->db_level != 0 ||
            db->db_state == DB_CACHED || db->db_state == DB_FILL ||
            db->db_state == DB_NOFILL);

        mutex_enter(&dn->dn_mtx);
        /*
         * Don't set dirtyctx to SYNC if we're just modifying this as we
         * initialize the objset.
         */
        if (dn->dn_dirtyctx == DN_UNDIRTIED &&
            !BP_IS_HOLE(dn->dn_objset->os_rootbp)) {
                dn->dn_dirtyctx =
                    (dmu_tx_is_syncing(tx) ? DN_DIRTY_SYNC : DN_DIRTY_OPEN);
                ASSERT(dn->dn_dirtyctx_firstset == NULL);
                dn->dn_dirtyctx_firstset = kmem_alloc(1, KM_SLEEP);
        }
        mutex_exit(&dn->dn_mtx);

        if (db->db_blkid == DMU_SPILL_BLKID)
                dn->dn_have_spill = B_TRUE;

        /*
         * If this buffer is already dirty, we're done.
         */
        drp = &db->db_last_dirty;
        ASSERT(*drp == NULL || (*drp)->dr_txg <= tx->tx_txg ||
            db->db.db_object == DMU_META_DNODE_OBJECT);
        while ((dr = *drp) != NULL && dr->dr_txg > tx->tx_txg)
                drp = &dr->dr_next;
        if (dr && dr->dr_txg == tx->tx_txg) {
                DB_DNODE_EXIT(db);

                if (db->db_level == 0 && db->db_blkid != DMU_BONUS_BLKID) {
                        /*
                         * If this buffer has already been written out,
                         * we now need to reset its state.
                         */
                        dbuf_unoverride(dr);
                        if (db->db.db_object != DMU_META_DNODE_OBJECT &&
                            db->db_state != DB_NOFILL)
                                arc_buf_thaw(db->db_buf);
                }
                mutex_exit(&db->db_mtx);
                return (dr);
        }

        /*
         * Only valid if not already dirty.
         */
        ASSERT(dn->dn_object == 0 ||
            dn->dn_dirtyctx == DN_UNDIRTIED || dn->dn_dirtyctx ==
            (dmu_tx_is_syncing(tx) ? DN_DIRTY_SYNC : DN_DIRTY_OPEN));

        ASSERT3U(dn->dn_nlevels, >, db->db_level);
        ASSERT((dn->dn_phys->dn_nlevels == 0 && db->db_level == 0) ||
            dn->dn_phys->dn_nlevels > db->db_level ||
            dn->dn_next_nlevels[txgoff] > db->db_level ||
            dn->dn_next_nlevels[(tx->tx_txg-1) & TXG_MASK] > db->db_level ||
            dn->dn_next_nlevels[(tx->tx_txg-2) & TXG_MASK] > db->db_level);

        /*
         * We should only be dirtying in syncing context if it's the
         * mos or we're initializing the os or it's a special object.
         * However, we are allowed to dirty in syncing context provided
         * we already dirtied it in open context.  Hence we must make
         * this assertion only if we're not already dirty.
         */
        os = dn->dn_objset;
        ASSERT(!dmu_tx_is_syncing(tx) || DMU_OBJECT_IS_SPECIAL(dn->dn_object) ||
            os->os_dsl_dataset == NULL || BP_IS_HOLE(os->os_rootbp));
        ASSERT(db->db.db_size != 0);

        dprintf_dbuf(db, "size=%llx\n", (u_longlong_t)db->db.db_size);

        if (db->db_blkid != DMU_BONUS_BLKID) {
                /*
                 * Update the accounting.
                 * Note: we delay "free accounting" until after we drop
                 * the db_mtx.  This keeps us from grabbing other locks
                 * (and possibly deadlocking) in bp_get_dsize() while
                 * also holding the db_mtx.
                 */
                dnode_willuse_space(dn, db->db.db_size, tx);
                do_free_accounting = dbuf_block_freeable(db);
        }

        /*
         * If this buffer is dirty in an old transaction group we need
         * to make a copy of it so that the changes we make in this
         * transaction group won't leak out when we sync the older txg.
         */
        dr = kmem_zalloc(sizeof (dbuf_dirty_record_t), KM_SLEEP);
        list_link_init(&dr->dr_dirty_node);
        if (db->db_level == 0) {
                void *data_old = db->db_buf;

                if (db->db_state != DB_NOFILL) {
                        if (db->db_blkid == DMU_BONUS_BLKID) {
                                dbuf_fix_old_data(db, tx->tx_txg);
                                data_old = db->db.db_data;
                        } else if (db->db.db_object != DMU_META_DNODE_OBJECT) {
                                /*
                                 * Release the data buffer from the cache so
                                 * that we can modify it without impacting
                                 * possible other users of this cached data
                                 * block.  Note that indirect blocks and
                                 * private objects are not released until the
                                 * syncing state (since they are only modified
                                 * then).
                                 */
                                arc_release(db->db_buf, db);
                                dbuf_fix_old_data(db, tx->tx_txg);
                                data_old = db->db_buf;
                        }
                        ASSERT(data_old != NULL);
                }
                dr->dt.dl.dr_data = data_old;
        } else {
                mutex_init(&dr->dt.di.dr_mtx, NULL, MUTEX_DEFAULT, NULL);
                list_create(&dr->dt.di.dr_children,
                    sizeof (dbuf_dirty_record_t),
                    offsetof(dbuf_dirty_record_t, dr_dirty_node));
        }
        if (db->db_blkid != DMU_BONUS_BLKID && os->os_dsl_dataset != NULL)
                dr->dr_accounted = db->db.db_size;
        dr->dr_dbuf = db;
        dr->dr_txg = tx->tx_txg;
        dr->dr_next = *drp;
        *drp = dr;

        /*
         * We could have been freed_in_flight between the dbuf_noread
         * and dbuf_dirty.  We win, as though the dbuf_noread() had
         * happened after the free.
         */
        if (db->db_level == 0 && db->db_blkid != DMU_BONUS_BLKID &&
            db->db_blkid != DMU_SPILL_BLKID) {
                mutex_enter(&dn->dn_mtx);
                if (dn->dn_free_ranges[txgoff] != NULL) {
                        range_tree_clear(dn->dn_free_ranges[txgoff],
                            db->db_blkid, 1);
                }
                mutex_exit(&dn->dn_mtx);
                db->db_freed_in_flight = FALSE;
        }

        /*
         * This buffer is now part of this txg
         */
        dbuf_add_ref(db, (void *)(uintptr_t)tx->tx_txg);
        db->db_dirtycnt += 1;
        ASSERT3U(db->db_dirtycnt, <=, 3);

        mutex_exit(&db->db_mtx);

        if (db->db_blkid == DMU_BONUS_BLKID ||
            db->db_blkid == DMU_SPILL_BLKID) {
                mutex_enter(&dn->dn_mtx);
                ASSERT(!list_link_active(&dr->dr_dirty_node));
                list_insert_tail(&dn->dn_dirty_records[txgoff], dr);
                mutex_exit(&dn->dn_mtx);
                dnode_setdirty(dn, tx);
                DB_DNODE_EXIT(db);
                return (dr);
        } else if (do_free_accounting) {
                blkptr_t *bp = db->db_blkptr;
                int64_t willfree = (bp && !BP_IS_HOLE(bp)) ?
                    bp_get_dsize(os->os_spa, bp) : db->db.db_size;
                /*
                 * This is only a guess -- if the dbuf is dirty
                 * in a previous txg, we don't know how much
                 * space it will use on disk yet.  We should
                 * really have the struct_rwlock to access
                 * db_blkptr, but since this is just a guess,
                 * it's OK if we get an odd answer.
                 */
                ddt_prefetch(os->os_spa, bp);
                dnode_willuse_space(dn, -willfree, tx);
        }

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

        if (db->db_level == 0) {
                dnode_new_blkid(dn, db->db_blkid, tx, drop_struct_lock);
                ASSERT(dn->dn_maxblkid >= db->db_blkid);
        }

        if (db->db_level+1 < dn->dn_nlevels) {
                dmu_buf_impl_t *parent = db->db_parent;
                dbuf_dirty_record_t *di;
                int parent_held = FALSE;

                if (db->db_parent == NULL || db->db_parent == dn->dn_dbuf) {
                        int epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;

                        parent = dbuf_hold_level(dn, db->db_level+1,
                            db->db_blkid >> epbs, FTAG);
                        ASSERT(parent != NULL);
                        parent_held = TRUE;
                }
                if (drop_struct_lock)
                        rw_exit(&dn->dn_struct_rwlock);
                ASSERT3U(db->db_level+1, ==, parent->db_level);
                di = dbuf_dirty(parent, tx);
                if (parent_held)
                        dbuf_rele(parent, FTAG);

                mutex_enter(&db->db_mtx);
                /*
                 * Since we've dropped the mutex, it's possible that
                 * dbuf_undirty() might have changed this out from under us.
                 */
                if (db->db_last_dirty == dr ||
                    dn->dn_object == DMU_META_DNODE_OBJECT) {
                        mutex_enter(&di->dt.di.dr_mtx);
                        ASSERT3U(di->dr_txg, ==, tx->tx_txg);
                        ASSERT(!list_link_active(&dr->dr_dirty_node));
                        list_insert_tail(&di->dt.di.dr_children, dr);
                        mutex_exit(&di->dt.di.dr_mtx);
                        dr->dr_parent = di;
                }
                mutex_exit(&db->db_mtx);
        } else {
                ASSERT(db->db_level+1 == dn->dn_nlevels);
                ASSERT(db->db_blkid < dn->dn_nblkptr);
                ASSERT(db->db_parent == NULL || db->db_parent == dn->dn_dbuf);
                mutex_enter(&dn->dn_mtx);
                ASSERT(!list_link_active(&dr->dr_dirty_node));
                list_insert_tail(&dn->dn_dirty_records[txgoff], dr);
                mutex_exit(&dn->dn_mtx);
                if (drop_struct_lock)
                        rw_exit(&dn->dn_struct_rwlock);
        }

        dnode_setdirty(dn, tx);
        DB_DNODE_EXIT(db);
        return (dr);
}

/*
 * Undirty a buffer in the transaction group referenced by the given
 * transaction.  Return whether this evicted the dbuf.
 */
static boolean_t
dbuf_undirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
{
        dnode_t *dn;
        uint64_t txg = tx->tx_txg;
        dbuf_dirty_record_t *dr, **drp;

        ASSERT(txg != 0);
        ASSERT(db->db_blkid != DMU_BONUS_BLKID);
        ASSERT0(db->db_level);
        ASSERT(MUTEX_HELD(&db->db_mtx));

        /*
         * If this buffer is not dirty, we're done.
         */
        for (drp = &db->db_last_dirty; (dr = *drp) != NULL; drp = &dr->dr_next)
                if (dr->dr_txg <= txg)
                        break;
        if (dr == NULL || dr->dr_txg < txg)
                return (B_FALSE);
        ASSERT(dr->dr_txg == txg);
        ASSERT(dr->dr_dbuf == db);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);

        dprintf_dbuf(db, "size=%llx\n", (u_longlong_t)db->db.db_size);

        ASSERT(db->db.db_size != 0);

        /*
         * Any space we accounted for in dp_dirty_* will be cleaned up by
         * dsl_pool_sync().  This is relatively rare so the discrepancy
         * is not a big deal.
         */

        *drp = dr->dr_next;

        /*
         * Note that there are three places in dbuf_dirty()
         * where this dirty record may be put on a list.
         * Make sure to do a list_remove corresponding to
         * every one of those list_insert calls.
         */
        if (dr->dr_parent) {
                mutex_enter(&dr->dr_parent->dt.di.dr_mtx);
                list_remove(&dr->dr_parent->dt.di.dr_children, dr);
                mutex_exit(&dr->dr_parent->dt.di.dr_mtx);
        } else if (db->db_blkid == DMU_SPILL_BLKID ||
            db->db_level+1 == dn->dn_nlevels) {
                ASSERT(db->db_blkptr == NULL || db->db_parent == dn->dn_dbuf);
                mutex_enter(&dn->dn_mtx);
                list_remove(&dn->dn_dirty_records[txg & TXG_MASK], dr);
                mutex_exit(&dn->dn_mtx);
        }
        DB_DNODE_EXIT(db);

        if (db->db_state != DB_NOFILL) {
                dbuf_unoverride(dr);

                ASSERT(db->db_buf != NULL);
                ASSERT(dr->dt.dl.dr_data != NULL);
                if (dr->dt.dl.dr_data != db->db_buf)
                        VERIFY(arc_buf_remove_ref(dr->dt.dl.dr_data, db));
        }
        kmem_free(dr, sizeof (dbuf_dirty_record_t));

        ASSERT(db->db_dirtycnt > 0);
        db->db_dirtycnt -= 1;

        if (refcount_remove(&db->db_holds, (void *)(uintptr_t)txg) == 0) {
                arc_buf_t *buf = db->db_buf;

                ASSERT(db->db_state == DB_NOFILL || arc_released(buf));
                dbuf_set_data(db, NULL);
                VERIFY(arc_buf_remove_ref(buf, db));
                dbuf_evict(db);
                return (B_TRUE);
        }

        return (B_FALSE);
}

void
dmu_buf_will_dirty(dmu_buf_t *db_fake, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        int rf = DB_RF_MUST_SUCCEED | DB_RF_NOPREFETCH;

        ASSERT(tx->tx_txg != 0);
        ASSERT(!refcount_is_zero(&db->db_holds));

        DB_DNODE_ENTER(db);
        if (RW_WRITE_HELD(&DB_DNODE(db)->dn_struct_rwlock))
                rf |= DB_RF_HAVESTRUCT;
        DB_DNODE_EXIT(db);
        (void) dbuf_read(db, NULL, rf);
        (void) dbuf_dirty(db, tx);
}

void
dmu_buf_will_not_fill(dmu_buf_t *db_fake, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;

        db->db_state = DB_NOFILL;

        dmu_buf_will_fill(db_fake, tx);
}

void
dmu_buf_will_fill(dmu_buf_t *db_fake, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;

        ASSERT(db->db_blkid != DMU_BONUS_BLKID);
        ASSERT(tx->tx_txg != 0);
        ASSERT(db->db_level == 0);
        ASSERT(!refcount_is_zero(&db->db_holds));

        ASSERT(db->db.db_object != DMU_META_DNODE_OBJECT ||
            dmu_tx_private_ok(tx));

        dbuf_noread(db);
        (void) dbuf_dirty(db, tx);
}

#pragma weak dmu_buf_fill_done = dbuf_fill_done
/* ARGSUSED */
void
dbuf_fill_done(dmu_buf_impl_t *db, dmu_tx_t *tx)
{
        mutex_enter(&db->db_mtx);
        DBUF_VERIFY(db);

        if (db->db_state == DB_FILL) {
                if (db->db_level == 0 && db->db_freed_in_flight) {
                        ASSERT(db->db_blkid != DMU_BONUS_BLKID);
                        /* we were freed while filling */
                        /* XXX dbuf_undirty? */
                        bzero(db->db.db_data, db->db.db_size);
                        db->db_freed_in_flight = FALSE;
                }
                db->db_state = DB_CACHED;
                cv_broadcast(&db->db_changed);
        }
        mutex_exit(&db->db_mtx);
}

void
dmu_buf_write_embedded(dmu_buf_t *dbuf, void *data,
    bp_embedded_type_t etype, enum zio_compress comp,
    int uncompressed_size, int compressed_size, int byteorder,
    dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)dbuf;
        struct dirty_leaf *dl;
        dmu_object_type_t type;

        DB_DNODE_ENTER(db);
        type = DB_DNODE(db)->dn_type;
        DB_DNODE_EXIT(db);

        ASSERT0(db->db_level);
        ASSERT(db->db_blkid != DMU_BONUS_BLKID);

        dmu_buf_will_not_fill(dbuf, tx);

        ASSERT3U(db->db_last_dirty->dr_txg, ==, tx->tx_txg);
        dl = &db->db_last_dirty->dt.dl;
        encode_embedded_bp_compressed(&dl->dr_overridden_by,
            data, comp, uncompressed_size, compressed_size);
        BPE_SET_ETYPE(&dl->dr_overridden_by, etype);
        BP_SET_TYPE(&dl->dr_overridden_by, type);
        BP_SET_LEVEL(&dl->dr_overridden_by, 0);
        BP_SET_BYTEORDER(&dl->dr_overridden_by, byteorder);

        dl->dr_override_state = DR_OVERRIDDEN;
        dl->dr_overridden_by.blk_birth = db->db_last_dirty->dr_txg;
}

/*
 * Directly assign a provided arc buf to a given dbuf if it's not referenced
 * by anybody except our caller. Otherwise copy arcbuf's contents to dbuf.
 */
void
dbuf_assign_arcbuf(dmu_buf_impl_t *db, arc_buf_t *buf, dmu_tx_t *tx)
{
        ASSERT(!refcount_is_zero(&db->db_holds));
        ASSERT(db->db_blkid != DMU_BONUS_BLKID);
        ASSERT(db->db_level == 0);
        ASSERT(DBUF_GET_BUFC_TYPE(db) == ARC_BUFC_DATA);
        ASSERT(buf != NULL);
        ASSERT(arc_buf_size(buf) == db->db.db_size);
        ASSERT(tx->tx_txg != 0);

        arc_return_buf(buf, db);
        ASSERT(arc_released(buf));

        mutex_enter(&db->db_mtx);

        while (db->db_state == DB_READ || db->db_state == DB_FILL)
                cv_wait(&db->db_changed, &db->db_mtx);

        ASSERT(db->db_state == DB_CACHED || db->db_state == DB_UNCACHED);

        if (db->db_state == DB_CACHED &&
            refcount_count(&db->db_holds) - 1 > db->db_dirtycnt) {
                mutex_exit(&db->db_mtx);
                (void) dbuf_dirty(db, tx);
                bcopy(buf->b_data, db->db.db_data, db->db.db_size);
                VERIFY(arc_buf_remove_ref(buf, db));
                xuio_stat_wbuf_copied();
                return;
        }

        xuio_stat_wbuf_nocopy();
        if (db->db_state == DB_CACHED) {
                dbuf_dirty_record_t *dr = db->db_last_dirty;

                ASSERT(db->db_buf != NULL);
                if (dr != NULL && dr->dr_txg == tx->tx_txg) {
                        ASSERT(dr->dt.dl.dr_data == db->db_buf);
                        if (!arc_released(db->db_buf)) {
                                ASSERT(dr->dt.dl.dr_override_state ==
                                    DR_OVERRIDDEN);
                                arc_release(db->db_buf, db);
                        }
                        dr->dt.dl.dr_data = buf;
                        VERIFY(arc_buf_remove_ref(db->db_buf, db));
                } else if (dr == NULL || dr->dt.dl.dr_data != db->db_buf) {
                        arc_release(db->db_buf, db);
                        VERIFY(arc_buf_remove_ref(db->db_buf, db));
                }
                db->db_buf = NULL;
        }
        ASSERT(db->db_buf == NULL);
        dbuf_set_data(db, buf);
        db->db_state = DB_FILL;
        mutex_exit(&db->db_mtx);
        (void) dbuf_dirty(db, tx);
        dmu_buf_fill_done(&db->db, tx);
}

/*
 * "Clear" the contents of this dbuf.  This will mark the dbuf
 * EVICTING and clear *most* of its references.  Unfortunately,
 * when we are not holding the dn_dbufs_mtx, we can't clear the
 * entry in the dn_dbufs list.  We have to wait until dbuf_destroy()
 * in this case.  For callers from the DMU we will usually see:
 *      dbuf_clear()->arc_clear_callback()->dbuf_do_evict()->dbuf_destroy()
 * For the arc callback, we will usually see:
 *      dbuf_do_evict()->dbuf_clear();dbuf_destroy()
 * Sometimes, though, we will get a mix of these two:
 *      DMU: dbuf_clear()->arc_clear_callback()
 *      ARC: dbuf_do_evict()->dbuf_destroy()
 *
 * This routine will dissociate the dbuf from the arc, by calling
 * arc_clear_callback(), but will not evict the data from the ARC.
 */
void
dbuf_clear(dmu_buf_impl_t *db)
{
        dnode_t *dn;
        dmu_buf_impl_t *parent = db->db_parent;
        dmu_buf_impl_t *dndb;
        boolean_t dbuf_gone = B_FALSE;

        ASSERT(MUTEX_HELD(&db->db_mtx));
        ASSERT(refcount_is_zero(&db->db_holds));

        dbuf_evict_user(db);

        if (db->db_state == DB_CACHED) {
                ASSERT(db->db.db_data != NULL);
                if (db->db_blkid == DMU_BONUS_BLKID) {
                        zio_buf_free(db->db.db_data, DN_MAX_BONUSLEN);
                        arc_space_return(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                }
                db->db.db_data = NULL;
                db->db_state = DB_UNCACHED;
        }

        ASSERT(db->db_state == DB_UNCACHED || db->db_state == DB_NOFILL);
        ASSERT(db->db_data_pending == NULL);

        db->db_state = DB_EVICTING;
        db->db_blkptr = NULL;

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        dndb = dn->dn_dbuf;
        if (db->db_blkid != DMU_BONUS_BLKID && MUTEX_HELD(&dn->dn_dbufs_mtx)) {
                list_remove(&dn->dn_dbufs, db);
                atomic_dec_32(&dn->dn_dbufs_count);
                membar_producer();
                DB_DNODE_EXIT(db);
                /*
                 * Decrementing the dbuf count means that the hold corresponding
                 * to the removed dbuf is no longer discounted in dnode_move(),
                 * so the dnode cannot be moved until after we release the hold.
                 * The membar_producer() ensures visibility of the decremented
                 * value in dnode_move(), since DB_DNODE_EXIT doesn't actually
                 * release any lock.
                 */
                dnode_rele(dn, db);
                db->db_dnode_handle = NULL;
        } else {
                DB_DNODE_EXIT(db);
        }

        if (db->db_buf)
                dbuf_gone = arc_clear_callback(db->db_buf);

        if (!dbuf_gone)
                mutex_exit(&db->db_mtx);

        /*
         * If this dbuf is referenced from an indirect dbuf,
         * decrement the ref count on the indirect dbuf.
         */
        if (parent && parent != dndb)
                dbuf_rele(parent, db);
}

__attribute__((always_inline))
static inline int
dbuf_findbp(dnode_t *dn, int level, uint64_t blkid, int fail_sparse,
    dmu_buf_impl_t **parentp, blkptr_t **bpp, struct dbuf_hold_impl_data *dh)
{
        int nlevels, epbs;

        *parentp = NULL;
        *bpp = NULL;

        ASSERT(blkid != DMU_BONUS_BLKID);

        if (blkid == DMU_SPILL_BLKID) {
                mutex_enter(&dn->dn_mtx);
                if (dn->dn_have_spill &&
                    (dn->dn_phys->dn_flags & DNODE_FLAG_SPILL_BLKPTR))
                        *bpp = &dn->dn_phys->dn_spill;
                else
                        *bpp = NULL;
                dbuf_add_ref(dn->dn_dbuf, NULL);
                *parentp = dn->dn_dbuf;
                mutex_exit(&dn->dn_mtx);
                return (0);
        }

        if (dn->dn_phys->dn_nlevels == 0)
                nlevels = 1;
        else
                nlevels = dn->dn_phys->dn_nlevels;

        epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;

        ASSERT3U(level * epbs, <, 64);
        ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));
        if (level >= nlevels ||
            (blkid > (dn->dn_phys->dn_maxblkid >> (level * epbs)))) {
                /* the buffer has no parent yet */
                return (SET_ERROR(ENOENT));
        } else if (level < nlevels-1) {
                /* this block is referenced from an indirect block */
                int err;
                if (dh == NULL) {
                        err = dbuf_hold_impl(dn, level+1, blkid >> epbs,
                                        fail_sparse, NULL, parentp);
                } else {
                        __dbuf_hold_impl_init(dh + 1, dn, dh->dh_level + 1,
                                        blkid >> epbs, fail_sparse, NULL,
                                        parentp, dh->dh_depth + 1);
                        err = __dbuf_hold_impl(dh + 1);
                }
                if (err)
                        return (err);
                err = dbuf_read(*parentp, NULL,
                    (DB_RF_HAVESTRUCT | DB_RF_NOPREFETCH | DB_RF_CANFAIL));
                if (err) {
                        dbuf_rele(*parentp, NULL);
                        *parentp = NULL;
                        return (err);
                }
                *bpp = ((blkptr_t *)(*parentp)->db.db_data) +
                    (blkid & ((1ULL << epbs) - 1));
                return (0);
        } else {
                /* the block is referenced from the dnode */
                ASSERT3U(level, ==, nlevels-1);
                ASSERT(dn->dn_phys->dn_nblkptr == 0 ||
                    blkid < dn->dn_phys->dn_nblkptr);
                if (dn->dn_dbuf) {
                        dbuf_add_ref(dn->dn_dbuf, NULL);
                        *parentp = dn->dn_dbuf;
                }
                *bpp = &dn->dn_phys->dn_blkptr[blkid];
                return (0);
        }
}

static dmu_buf_impl_t *
dbuf_create(dnode_t *dn, uint8_t level, uint64_t blkid,
    dmu_buf_impl_t *parent, blkptr_t *blkptr)
{
        objset_t *os = dn->dn_objset;
        dmu_buf_impl_t *db, *odb;

        ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));
        ASSERT(dn->dn_type != DMU_OT_NONE);

        db = kmem_cache_alloc(dbuf_cache, KM_SLEEP);

        db->db_objset = os;
        db->db.db_object = dn->dn_object;
        db->db_level = level;
        db->db_blkid = blkid;
        db->db_last_dirty = NULL;
        db->db_dirtycnt = 0;
        db->db_dnode_handle = dn->dn_handle;
        db->db_parent = parent;
        db->db_blkptr = blkptr;

        db->db_user_ptr = NULL;
        db->db_user_data_ptr_ptr = NULL;
        db->db_evict_func = NULL;
        db->db_immediate_evict = 0;
        db->db_freed_in_flight = 0;

        if (blkid == DMU_BONUS_BLKID) {
                ASSERT3P(parent, ==, dn->dn_dbuf);
                db->db.db_size = DN_MAX_BONUSLEN -
                    (dn->dn_nblkptr-1) * sizeof (blkptr_t);
                ASSERT3U(db->db.db_size, >=, dn->dn_bonuslen);
                db->db.db_offset = DMU_BONUS_BLKID;
                db->db_state = DB_UNCACHED;
                /* the bonus dbuf is not placed in the hash table */
                arc_space_consume(sizeof (dmu_buf_impl_t), ARC_SPACE_OTHER);
                return (db);
        } else if (blkid == DMU_SPILL_BLKID) {
                db->db.db_size = (blkptr != NULL) ?
                    BP_GET_LSIZE(blkptr) : SPA_MINBLOCKSIZE;
                db->db.db_offset = 0;
        } else {
                int blocksize =
                    db->db_level ? 1 << dn->dn_indblkshift : dn->dn_datablksz;
                db->db.db_size = blocksize;
                db->db.db_offset = db->db_blkid * blocksize;
        }

        /*
         * Hold the dn_dbufs_mtx while we get the new dbuf
         * in the hash table *and* added to the dbufs list.
         * This prevents a possible deadlock with someone
         * trying to look up this dbuf before its added to the
         * dn_dbufs list.
         */
        mutex_enter(&dn->dn_dbufs_mtx);
        db->db_state = DB_EVICTING;
        if ((odb = dbuf_hash_insert(db)) != NULL) {
                /* someone else inserted it first */
                kmem_cache_free(dbuf_cache, db);
                mutex_exit(&dn->dn_dbufs_mtx);
                return (odb);
        }
        list_insert_head(&dn->dn_dbufs, db);
        if (db->db_level == 0 && db->db_blkid >=
            dn->dn_unlisted_l0_blkid)
                dn->dn_unlisted_l0_blkid = db->db_blkid + 1;
        db->db_state = DB_UNCACHED;
        mutex_exit(&dn->dn_dbufs_mtx);
        arc_space_consume(sizeof (dmu_buf_impl_t), ARC_SPACE_OTHER);

        if (parent && parent != dn->dn_dbuf)
                dbuf_add_ref(parent, db);

        ASSERT(dn->dn_object == DMU_META_DNODE_OBJECT ||
            refcount_count(&dn->dn_holds) > 0);
        (void) refcount_add(&dn->dn_holds, db);
        atomic_inc_32(&dn->dn_dbufs_count);

        dprintf_dbuf(db, "db=%p\n", db);

        return (db);
}

static int
dbuf_do_evict(void *private)
{
        dmu_buf_impl_t *db = private;

        if (!MUTEX_HELD(&db->db_mtx))
                mutex_enter(&db->db_mtx);

        ASSERT(refcount_is_zero(&db->db_holds));

        if (db->db_state != DB_EVICTING) {
                ASSERT(db->db_state == DB_CACHED);
                DBUF_VERIFY(db);
                db->db_buf = NULL;
                dbuf_evict(db);
        } else {
                mutex_exit(&db->db_mtx);
                dbuf_destroy(db);
        }
        return (0);
}

static void
dbuf_destroy(dmu_buf_impl_t *db)
{
        ASSERT(refcount_is_zero(&db->db_holds));

        if (db->db_blkid != DMU_BONUS_BLKID) {
                /*
                 * If this dbuf is still on the dn_dbufs list,
                 * remove it from that list.
                 */
                if (db->db_dnode_handle != NULL) {
                        dnode_t *dn;

                        DB_DNODE_ENTER(db);
                        dn = DB_DNODE(db);
                        mutex_enter(&dn->dn_dbufs_mtx);
                        list_remove(&dn->dn_dbufs, db);
                        atomic_dec_32(&dn->dn_dbufs_count);
                        mutex_exit(&dn->dn_dbufs_mtx);
                        DB_DNODE_EXIT(db);
                        /*
                         * Decrementing the dbuf count means that the hold
                         * corresponding to the removed dbuf is no longer
                         * discounted in dnode_move(), so the dnode cannot be
                         * moved until after we release the hold.
                         */
                        dnode_rele(dn, db);
                        db->db_dnode_handle = NULL;
                }
                dbuf_hash_remove(db);
        }
        db->db_parent = NULL;
        db->db_buf = NULL;

        ASSERT(!list_link_active(&db->db_link));
        ASSERT(db->db.db_data == NULL);
        ASSERT(db->db_hash_next == NULL);
        ASSERT(db->db_blkptr == NULL);
        ASSERT(db->db_data_pending == NULL);

        kmem_cache_free(dbuf_cache, db);
        arc_space_return(sizeof (dmu_buf_impl_t), ARC_SPACE_OTHER);
}

void
dbuf_prefetch(dnode_t *dn, uint64_t blkid, zio_priority_t prio)
{
        dmu_buf_impl_t *db = NULL;
        blkptr_t *bp = NULL;

        ASSERT(blkid != DMU_BONUS_BLKID);
        ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));

        if (dnode_block_freed(dn, blkid))
                return;

        /* dbuf_find() returns with db_mtx held */
        if ((db = dbuf_find(dn, 0, blkid))) {
                /*
                 * This dbuf is already in the cache.  We assume that
                 * it is already CACHED, or else about to be either
                 * read or filled.
                 */
                mutex_exit(&db->db_mtx);
                return;
        }

        if (dbuf_findbp(dn, 0, blkid, TRUE, &db, &bp, NULL) == 0) {
                if (bp && !BP_IS_HOLE(bp) && !BP_IS_EMBEDDED(bp)) {
                        dsl_dataset_t *ds = dn->dn_objset->os_dsl_dataset;
                        uint32_t aflags = ARC_NOWAIT | ARC_PREFETCH;
                        zbookmark_phys_t zb;

                        SET_BOOKMARK(&zb, ds ? ds->ds_object : DMU_META_OBJSET,
                            dn->dn_object, 0, blkid);

                        (void) arc_read(NULL, dn->dn_objset->os_spa,
                            bp, NULL, NULL, prio,
                            ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE,
                            &aflags, &zb);
                }
                if (db)
                        dbuf_rele(db, NULL);
        }
}

#define DBUF_HOLD_IMPL_MAX_DEPTH        20

/*
 * Returns with db_holds incremented, and db_mtx not held.
 * Note: dn_struct_rwlock must be held.
 */
static int
__dbuf_hold_impl(struct dbuf_hold_impl_data *dh)
{
        ASSERT3S(dh->dh_depth, <, DBUF_HOLD_IMPL_MAX_DEPTH);
        dh->dh_parent = NULL;

        ASSERT(dh->dh_blkid != DMU_BONUS_BLKID);
        ASSERT(RW_LOCK_HELD(&dh->dh_dn->dn_struct_rwlock));
        ASSERT3U(dh->dh_dn->dn_nlevels, >, dh->dh_level);

        *(dh->dh_dbp) = NULL;
top:
        /* dbuf_find() returns with db_mtx held */
        dh->dh_db = dbuf_find(dh->dh_dn, dh->dh_level, dh->dh_blkid);

        if (dh->dh_db == NULL) {
                dh->dh_bp = NULL;

                ASSERT3P(dh->dh_parent, ==, NULL);
                dh->dh_err = dbuf_findbp(dh->dh_dn, dh->dh_level, dh->dh_blkid,
                                        dh->dh_fail_sparse, &dh->dh_parent,
                                        &dh->dh_bp, dh);
                if (dh->dh_fail_sparse) {
                        if (dh->dh_err == 0 &&
                            dh->dh_bp && BP_IS_HOLE(dh->dh_bp))
                                dh->dh_err = SET_ERROR(ENOENT);
                        if (dh->dh_err) {
                                if (dh->dh_parent)
                                        dbuf_rele(dh->dh_parent, NULL);
                                return (dh->dh_err);
                        }
                }
                if (dh->dh_err && dh->dh_err != ENOENT)
                        return (dh->dh_err);
                dh->dh_db = dbuf_create(dh->dh_dn, dh->dh_level, dh->dh_blkid,
                                        dh->dh_parent, dh->dh_bp);
        }

        if (dh->dh_db->db_buf && refcount_is_zero(&dh->dh_db->db_holds)) {
                arc_buf_add_ref(dh->dh_db->db_buf, dh->dh_db);
                if (dh->dh_db->db_buf->b_data == NULL) {
                        dbuf_clear(dh->dh_db);
                        if (dh->dh_parent) {
                                dbuf_rele(dh->dh_parent, NULL);
                                dh->dh_parent = NULL;
                        }
                        goto top;
                }
                ASSERT3P(dh->dh_db->db.db_data, ==, dh->dh_db->db_buf->b_data);
        }

        ASSERT(dh->dh_db->db_buf == NULL || arc_referenced(dh->dh_db->db_buf));

        /*
         * If this buffer is currently syncing out, and we are are
         * still referencing it from db_data, we need to make a copy
         * of it in case we decide we want to dirty it again in this txg.
         */
        if (dh->dh_db->db_level == 0 &&
            dh->dh_db->db_blkid != DMU_BONUS_BLKID &&
            dh->dh_dn->dn_object != DMU_META_DNODE_OBJECT &&
            dh->dh_db->db_state == DB_CACHED && dh->dh_db->db_data_pending) {
                dh->dh_dr = dh->dh_db->db_data_pending;

                if (dh->dh_dr->dt.dl.dr_data == dh->dh_db->db_buf) {
                        dh->dh_type = DBUF_GET_BUFC_TYPE(dh->dh_db);

                        dbuf_set_data(dh->dh_db,
                            arc_buf_alloc(dh->dh_dn->dn_objset->os_spa,
                            dh->dh_db->db.db_size, dh->dh_db, dh->dh_type));
                        bcopy(dh->dh_dr->dt.dl.dr_data->b_data,
                            dh->dh_db->db.db_data, dh->dh_db->db.db_size);
                }
        }

        (void) refcount_add(&dh->dh_db->db_holds, dh->dh_tag);
        dbuf_update_data(dh->dh_db);
        DBUF_VERIFY(dh->dh_db);
        mutex_exit(&dh->dh_db->db_mtx);

        /* NOTE: we can't rele the parent until after we drop the db_mtx */
        if (dh->dh_parent)
                dbuf_rele(dh->dh_parent, NULL);

        ASSERT3P(DB_DNODE(dh->dh_db), ==, dh->dh_dn);
        ASSERT3U(dh->dh_db->db_blkid, ==, dh->dh_blkid);
        ASSERT3U(dh->dh_db->db_level, ==, dh->dh_level);
        *(dh->dh_dbp) = dh->dh_db;

        return (0);
}

/*
 * The following code preserves the recursive function dbuf_hold_impl()
 * but moves the local variables AND function arguments to the heap to
 * minimize the stack frame size.  Enough space is initially allocated
 * on the stack for 20 levels of recursion.
 */
int
dbuf_hold_impl(dnode_t *dn, uint8_t level, uint64_t blkid, int fail_sparse,
    void *tag, dmu_buf_impl_t **dbp)
{
        struct dbuf_hold_impl_data *dh;
        int error;

        dh = kmem_zalloc(sizeof (struct dbuf_hold_impl_data) *
            DBUF_HOLD_IMPL_MAX_DEPTH, KM_SLEEP);
        __dbuf_hold_impl_init(dh, dn, level, blkid, fail_sparse, tag, dbp, 0);

        error = __dbuf_hold_impl(dh);

        kmem_free(dh, sizeof (struct dbuf_hold_impl_data) *
            DBUF_HOLD_IMPL_MAX_DEPTH);

        return (error);
}

static void
__dbuf_hold_impl_init(struct dbuf_hold_impl_data *dh,
    dnode_t *dn, uint8_t level, uint64_t blkid, int fail_sparse,
    void *tag, dmu_buf_impl_t **dbp, int depth)
{
        dh->dh_dn = dn;
        dh->dh_level = level;
        dh->dh_blkid = blkid;
        dh->dh_fail_sparse = fail_sparse;
        dh->dh_tag = tag;
        dh->dh_dbp = dbp;
        dh->dh_depth = depth;
}

dmu_buf_impl_t *
dbuf_hold(dnode_t *dn, uint64_t blkid, void *tag)
{
        dmu_buf_impl_t *db;
        int err = dbuf_hold_impl(dn, 0, blkid, FALSE, tag, &db);
        return (err ? NULL : db);
}

dmu_buf_impl_t *
dbuf_hold_level(dnode_t *dn, int level, uint64_t blkid, void *tag)
{
        dmu_buf_impl_t *db;
        int err = dbuf_hold_impl(dn, level, blkid, FALSE, tag, &db);
        return (err ? NULL : db);
}

void
dbuf_create_bonus(dnode_t *dn)
{
        ASSERT(RW_WRITE_HELD(&dn->dn_struct_rwlock));

        ASSERT(dn->dn_bonus == NULL);
        dn->dn_bonus = dbuf_create(dn, 0, DMU_BONUS_BLKID, dn->dn_dbuf, NULL);
}

int
dbuf_spill_set_blksz(dmu_buf_t *db_fake, uint64_t blksz, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        dnode_t *dn;

        if (db->db_blkid != DMU_SPILL_BLKID)
                return (SET_ERROR(ENOTSUP));
        if (blksz == 0)
                blksz = SPA_MINBLOCKSIZE;
        if (blksz > SPA_MAXBLOCKSIZE)
                blksz = SPA_MAXBLOCKSIZE;
        else
                blksz = P2ROUNDUP(blksz, SPA_MINBLOCKSIZE);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
        dbuf_new_size(db, blksz, tx);
        rw_exit(&dn->dn_struct_rwlock);
        DB_DNODE_EXIT(db);

        return (0);
}

void
dbuf_rm_spill(dnode_t *dn, dmu_tx_t *tx)
{
        dbuf_free_range(dn, DMU_SPILL_BLKID, DMU_SPILL_BLKID, tx);
}

#pragma weak dmu_buf_add_ref = dbuf_add_ref
void
dbuf_add_ref(dmu_buf_impl_t *db, void *tag)
{
        VERIFY(refcount_add(&db->db_holds, tag) > 1);
}

/*
 * If you call dbuf_rele() you had better not be referencing the dnode handle
 * unless you have some other direct or indirect hold on the dnode. (An indirect
 * hold is a hold on one of the dnode's dbufs, including the bonus buffer.)
 * Without that, the dbuf_rele() could lead to a dnode_rele() followed by the
 * dnode's parent dbuf evicting its dnode handles.
 */
void
dbuf_rele(dmu_buf_impl_t *db, void *tag)
{
        mutex_enter(&db->db_mtx);
        dbuf_rele_and_unlock(db, tag);
}

void
dmu_buf_rele(dmu_buf_t *db, void *tag)
{
        dbuf_rele((dmu_buf_impl_t *)db, tag);
}

/*
 * dbuf_rele() for an already-locked dbuf.  This is necessary to allow
 * db_dirtycnt and db_holds to be updated atomically.
 */
void
dbuf_rele_and_unlock(dmu_buf_impl_t *db, void *tag)
{
        int64_t holds;

        ASSERT(MUTEX_HELD(&db->db_mtx));
        DBUF_VERIFY(db);

        /*
         * Remove the reference to the dbuf before removing its hold on the
         * dnode so we can guarantee in dnode_move() that a referenced bonus
         * buffer has a corresponding dnode hold.
         */
        holds = refcount_remove(&db->db_holds, tag);
        ASSERT(holds >= 0);

        /*
         * We can't freeze indirects if there is a possibility that they
         * may be modified in the current syncing context.
         */
        if (db->db_buf && holds == (db->db_level == 0 ? db->db_dirtycnt : 0))
                arc_buf_freeze(db->db_buf);

        if (holds == db->db_dirtycnt &&
            db->db_level == 0 && db->db_immediate_evict)
                dbuf_evict_user(db);

        if (holds == 0) {
                if (db->db_blkid == DMU_BONUS_BLKID) {
                        dnode_t *dn;

                        /*
                         * If the dnode moves here, we cannot cross this
                         * barrier until the move completes.
                         */
                        DB_DNODE_ENTER(db);

                        dn = DB_DNODE(db);
                        atomic_dec_32(&dn->dn_dbufs_count);

                        /*
                         * Decrementing the dbuf count means that the bonus
                         * buffer's dnode hold is no longer discounted in
                         * dnode_move(). The dnode cannot move until after
                         * the dnode_rele_and_unlock() below.
                         */
                        DB_DNODE_EXIT(db);

                        /*
                         * Do not reference db after its lock is dropped.
                         * Another thread may evict it.
                         */
                        mutex_exit(&db->db_mtx);

                        /*
                         * If the dnode has been freed, evict the bonus
                         * buffer immediately.  The data in the bonus
                         * buffer is no longer relevant and this prevents
                         * a stale bonus buffer from being associated
                         * with this dnode_t should the dnode_t be reused
                         * prior to being destroyed.
                         */
                        mutex_enter(&dn->dn_mtx);
                        if (dn->dn_type == DMU_OT_NONE ||
                            dn->dn_free_txg != 0) {
                                /*
                                 * Drop dn_mtx.  It is a leaf lock and
                                 * cannot be held when dnode_evict_bonus()
                                 * acquires other locks in order to
                                 * perform the eviction.
                                 *
                                 * Freed dnodes cannot be reused until the
                                 * last hold is released.  Since this bonus
                                 * buffer has a hold, the dnode will remain
                                 * in the free state, even without dn_mtx
                                 * held, until the dnode_rele_and_unlock()
                                 * below.
                                 */
                                mutex_exit(&dn->dn_mtx);
                                dnode_evict_bonus(dn);
                                mutex_enter(&dn->dn_mtx);
                        }
                        dnode_rele_and_unlock(dn, db);
                } else if (db->db_buf == NULL) {
                        /*
                         * This is a special case: we never associated this
                         * dbuf with any data allocated from the ARC.
                         */
                        ASSERT(db->db_state == DB_UNCACHED ||
                            db->db_state == DB_NOFILL);
                        dbuf_evict(db);
                } else if (arc_released(db->db_buf)) {
                        arc_buf_t *buf = db->db_buf;
                        /*
                         * This dbuf has anonymous data associated with it.
                         */
                        dbuf_set_data(db, NULL);
                        VERIFY(arc_buf_remove_ref(buf, db));
                        dbuf_evict(db);
                } else {
                        VERIFY(!arc_buf_remove_ref(db->db_buf, db));

                        /*
                         * A dbuf will be eligible for eviction if either the
                         * 'primarycache' property is set or a duplicate
                         * copy of this buffer is already cached in the arc.
                         *
                         * In the case of the 'primarycache' a buffer
                         * is considered for eviction if it matches the
                         * criteria set in the property.
                         *
                         * To decide if our buffer is considered a
                         * duplicate, we must call into the arc to determine
                         * if multiple buffers are referencing the same
                         * block on-disk. If so, then we simply evict
                         * ourselves.
                         */
                        if (!DBUF_IS_CACHEABLE(db)) {
                                if (db->db_blkptr != NULL &&
                                    !BP_IS_HOLE(db->db_blkptr) &&
                                    !BP_IS_EMBEDDED(db->db_blkptr)) {
                                        spa_t *spa =
                                            dmu_objset_spa(db->db_objset);
                                        blkptr_t bp = *db->db_blkptr;
                                        dbuf_clear(db);
                                        arc_freed(spa, &bp);
                                } else {
                                        dbuf_clear(db);
                                }
                        } else if (arc_buf_eviction_needed(db->db_buf)) {
                                dbuf_clear(db);
                        } else {
                                mutex_exit(&db->db_mtx);
                        }
                }
        } else {
                mutex_exit(&db->db_mtx);
        }
}

#pragma weak dmu_buf_refcount = dbuf_refcount
uint64_t
dbuf_refcount(dmu_buf_impl_t *db)
{
        return (refcount_count(&db->db_holds));
}

void *
dmu_buf_set_user(dmu_buf_t *db_fake, void *user_ptr, void *user_data_ptr_ptr,
    dmu_buf_evict_func_t *evict_func)
{
        return (dmu_buf_update_user(db_fake, NULL, user_ptr,
            user_data_ptr_ptr, evict_func));
}

void *
dmu_buf_set_user_ie(dmu_buf_t *db_fake, void *user_ptr, void *user_data_ptr_ptr,
    dmu_buf_evict_func_t *evict_func)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;

        db->db_immediate_evict = TRUE;
        return (dmu_buf_update_user(db_fake, NULL, user_ptr,
            user_data_ptr_ptr, evict_func));
}

void *
dmu_buf_update_user(dmu_buf_t *db_fake, void *old_user_ptr, void *user_ptr,
    void *user_data_ptr_ptr, dmu_buf_evict_func_t *evict_func)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        ASSERT(db->db_level == 0);

        ASSERT((user_ptr == NULL) == (evict_func == NULL));

        mutex_enter(&db->db_mtx);

        if (db->db_user_ptr == old_user_ptr) {
                db->db_user_ptr = user_ptr;
                db->db_user_data_ptr_ptr = user_data_ptr_ptr;
                db->db_evict_func = evict_func;

                dbuf_update_data(db);
        } else {
                old_user_ptr = db->db_user_ptr;
        }

        mutex_exit(&db->db_mtx);
        return (old_user_ptr);
}

void *
dmu_buf_get_user(dmu_buf_t *db_fake)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        ASSERT(!refcount_is_zero(&db->db_holds));

        return (db->db_user_ptr);
}

boolean_t
dmu_buf_freeable(dmu_buf_t *dbuf)
{
        boolean_t res = B_FALSE;
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)dbuf;

        if (db->db_blkptr)
                res = dsl_dataset_block_freeable(db->db_objset->os_dsl_dataset,
                    db->db_blkptr, db->db_blkptr->blk_birth);

        return (res);
}

blkptr_t *
dmu_buf_get_blkptr(dmu_buf_t *db)
{
        dmu_buf_impl_t *dbi = (dmu_buf_impl_t *)db;
        return (dbi->db_blkptr);
}

static void
dbuf_check_blkptr(dnode_t *dn, dmu_buf_impl_t *db)
{
        /* ASSERT(dmu_tx_is_syncing(tx) */
        ASSERT(MUTEX_HELD(&db->db_mtx));

        if (db->db_blkptr != NULL)
                return;

        if (db->db_blkid == DMU_SPILL_BLKID) {
                db->db_blkptr = &dn->dn_phys->dn_spill;
                BP_ZERO(db->db_blkptr);
                return;
        }
        if (db->db_level == dn->dn_phys->dn_nlevels-1) {
                /*
                 * This buffer was allocated at a time when there was
                 * no available blkptrs from the dnode, or it was
                 * inappropriate to hook it in (i.e., nlevels mis-match).
                 */
                ASSERT(db->db_blkid < dn->dn_phys->dn_nblkptr);
                ASSERT(db->db_parent == NULL);
                db->db_parent = dn->dn_dbuf;
                db->db_blkptr = &dn->dn_phys->dn_blkptr[db->db_blkid];
                DBUF_VERIFY(db);
        } else {
                dmu_buf_impl_t *parent = db->db_parent;
                int epbs = dn->dn_phys->dn_indblkshift - SPA_BLKPTRSHIFT;

                ASSERT(dn->dn_phys->dn_nlevels > 1);
                if (parent == NULL) {
                        mutex_exit(&db->db_mtx);
                        rw_enter(&dn->dn_struct_rwlock, RW_READER);
                        (void) dbuf_hold_impl(dn, db->db_level+1,
                            db->db_blkid >> epbs, FALSE, db, &parent);
                        rw_exit(&dn->dn_struct_rwlock);
                        mutex_enter(&db->db_mtx);
                        db->db_parent = parent;
                }
                db->db_blkptr = (blkptr_t *)parent->db.db_data +
                    (db->db_blkid & ((1ULL << epbs) - 1));
                DBUF_VERIFY(db);
        }
}

/*
 * dbuf_sync_indirect() is called recursively from dbuf_sync_list() so it
 * is critical the we not allow the compiler to inline this function in to
 * dbuf_sync_list() thereby drastically bloating the stack usage.
 */
noinline static void
dbuf_sync_indirect(dbuf_dirty_record_t *dr, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = dr->dr_dbuf;
        dnode_t *dn;
        zio_t *zio;

        ASSERT(dmu_tx_is_syncing(tx));

        dprintf_dbuf_bp(db, db->db_blkptr, "blkptr=%p", db->db_blkptr);

        mutex_enter(&db->db_mtx);

        ASSERT(db->db_level > 0);
        DBUF_VERIFY(db);

        /* Read the block if it hasn't been read yet. */
        if (db->db_buf == NULL) {
                mutex_exit(&db->db_mtx);
                (void) dbuf_read(db, NULL, DB_RF_MUST_SUCCEED);
                mutex_enter(&db->db_mtx);
        }
        ASSERT3U(db->db_state, ==, DB_CACHED);
        ASSERT(db->db_buf != NULL);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        /* Indirect block size must match what the dnode thinks it is. */
        ASSERT3U(db->db.db_size, ==, 1<<dn->dn_phys->dn_indblkshift);
        dbuf_check_blkptr(dn, db);
        DB_DNODE_EXIT(db);

        /* Provide the pending dirty record to child dbufs */
        db->db_data_pending = dr;

        mutex_exit(&db->db_mtx);
        dbuf_write(dr, db->db_buf, tx);

        zio = dr->dr_zio;
        mutex_enter(&dr->dt.di.dr_mtx);
        dbuf_sync_list(&dr->dt.di.dr_children, tx);
        ASSERT(list_head(&dr->dt.di.dr_children) == NULL);
        mutex_exit(&dr->dt.di.dr_mtx);
        zio_nowait(zio);
}

/*
 * dbuf_sync_leaf() is called recursively from dbuf_sync_list() so it is
 * critical the we not allow the compiler to inline this function in to
 * dbuf_sync_list() thereby drastically bloating the stack usage.
 */
noinline static void
dbuf_sync_leaf(dbuf_dirty_record_t *dr, dmu_tx_t *tx)
{
        arc_buf_t **datap = &dr->dt.dl.dr_data;
        dmu_buf_impl_t *db = dr->dr_dbuf;
        dnode_t *dn;
        objset_t *os;
        uint64_t txg = tx->tx_txg;

        ASSERT(dmu_tx_is_syncing(tx));

        dprintf_dbuf_bp(db, db->db_blkptr, "blkptr=%p", db->db_blkptr);

        mutex_enter(&db->db_mtx);
        /*
         * To be synced, we must be dirtied.  But we
         * might have been freed after the dirty.
         */
        if (db->db_state == DB_UNCACHED) {
                /* This buffer has been freed since it was dirtied */
                ASSERT(db->db.db_data == NULL);
        } else if (db->db_state == DB_FILL) {
                /* This buffer was freed and is now being re-filled */
                ASSERT(db->db.db_data != dr->dt.dl.dr_data);
        } else {
                ASSERT(db->db_state == DB_CACHED || db->db_state == DB_NOFILL);
        }
        DBUF_VERIFY(db);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);

        if (db->db_blkid == DMU_SPILL_BLKID) {
                mutex_enter(&dn->dn_mtx);
                dn->dn_phys->dn_flags |= DNODE_FLAG_SPILL_BLKPTR;
                mutex_exit(&dn->dn_mtx);
        }

        /*
         * If this is a bonus buffer, simply copy the bonus data into the
         * dnode.  It will be written out when the dnode is synced (and it
         * will be synced, since it must have been dirty for dbuf_sync to
         * be called).
         */
        if (db->db_blkid == DMU_BONUS_BLKID) {
                dbuf_dirty_record_t **drp;

                ASSERT(*datap != NULL);
                ASSERT0(db->db_level);
                ASSERT3U(dn->dn_phys->dn_bonuslen, <=, DN_MAX_BONUSLEN);
                bcopy(*datap, DN_BONUS(dn->dn_phys), dn->dn_phys->dn_bonuslen);
                DB_DNODE_EXIT(db);

                if (*datap != db->db.db_data) {
                        zio_buf_free(*datap, DN_MAX_BONUSLEN);
                        arc_space_return(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                }
                db->db_data_pending = NULL;
                drp = &db->db_last_dirty;
                while (*drp != dr)
                        drp = &(*drp)->dr_next;
                ASSERT(dr->dr_next == NULL);
                ASSERT(dr->dr_dbuf == db);
                *drp = dr->dr_next;
                if (dr->dr_dbuf->db_level != 0) {
                        mutex_destroy(&dr->dt.di.dr_mtx);
                        list_destroy(&dr->dt.di.dr_children);
                }
                kmem_free(dr, sizeof (dbuf_dirty_record_t));
                ASSERT(db->db_dirtycnt > 0);
                db->db_dirtycnt -= 1;
                dbuf_rele_and_unlock(db, (void *)(uintptr_t)txg);
                return;
        }

        os = dn->dn_objset;

        /*
         * This function may have dropped the db_mtx lock allowing a dmu_sync
         * operation to sneak in. As a result, we need to ensure that we
         * don't check the dr_override_state until we have returned from
         * dbuf_check_blkptr.
         */
        dbuf_check_blkptr(dn, db);

        /*
         * If this buffer is in the middle of an immediate write,
         * wait for the synchronous IO to complete.
         */
        while (dr->dt.dl.dr_override_state == DR_IN_DMU_SYNC) {
                ASSERT(dn->dn_object != DMU_META_DNODE_OBJECT);
                cv_wait(&db->db_changed, &db->db_mtx);
                ASSERT(dr->dt.dl.dr_override_state != DR_NOT_OVERRIDDEN);
        }

        if (db->db_state != DB_NOFILL &&
            dn->dn_object != DMU_META_DNODE_OBJECT &&
            refcount_count(&db->db_holds) > 1 &&
            dr->dt.dl.dr_override_state != DR_OVERRIDDEN &&
            *datap == db->db_buf) {
                /*
                 * If this buffer is currently "in use" (i.e., there
                 * are active holds and db_data still references it),
                 * then make a copy before we start the write so that
                 * any modifications from the open txg will not leak
                 * into this write.
                 *
                 * NOTE: this copy does not need to be made for
                 * objects only modified in the syncing context (e.g.
                 * DNONE_DNODE blocks).
                 */
                int blksz = arc_buf_size(*datap);
                arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);
                *datap = arc_buf_alloc(os->os_spa, blksz, db, type);
                bcopy(db->db.db_data, (*datap)->b_data, blksz);
        }
        db->db_data_pending = dr;

        mutex_exit(&db->db_mtx);

        dbuf_write(dr, *datap, tx);

        ASSERT(!list_link_active(&dr->dr_dirty_node));
        if (dn->dn_object == DMU_META_DNODE_OBJECT) {
                list_insert_tail(&dn->dn_dirty_records[txg&TXG_MASK], dr);
                DB_DNODE_EXIT(db);
        } else {
                /*
                 * Although zio_nowait() does not "wait for an IO", it does
                 * initiate the IO. If this is an empty write it seems plausible
                 * that the IO could actually be completed before the nowait
                 * returns. We need to DB_DNODE_EXIT() first in case
                 * zio_nowait() invalidates the dbuf.
                 */
                DB_DNODE_EXIT(db);
                zio_nowait(dr->dr_zio);
        }
}

void
dbuf_sync_list(list_t *list, dmu_tx_t *tx)
{
        dbuf_dirty_record_t *dr;

        while ((dr = list_head(list))) {
                if (dr->dr_zio != NULL) {
                        /*
                         * If we find an already initialized zio then we
                         * are processing the meta-dnode, and we have finished.
                         * The dbufs for all dnodes are put back on the list
                         * during processing, so that we can zio_wait()
                         * these IOs after initiating all child IOs.
                         */
                        ASSERT3U(dr->dr_dbuf->db.db_object, ==,
                            DMU_META_DNODE_OBJECT);
                        break;
                }
                list_remove(list, dr);
                if (dr->dr_dbuf->db_level > 0)
                        dbuf_sync_indirect(dr, tx);
                else
                        dbuf_sync_leaf(dr, tx);
        }
}

/* ARGSUSED */
static void
dbuf_write_ready(zio_t *zio, arc_buf_t *buf, void *vdb)
{
        dmu_buf_impl_t *db = vdb;
        dnode_t *dn;
        blkptr_t *bp = zio->io_bp;
        blkptr_t *bp_orig = &zio->io_bp_orig;
        spa_t *spa = zio->io_spa;
        int64_t delta;
        uint64_t fill = 0;
        int i;

        ASSERT3P(db->db_blkptr, ==, bp);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        delta = bp_get_dsize_sync(spa, bp) - bp_get_dsize_sync(spa, bp_orig);
        dnode_diduse_space(dn, delta - zio->io_prev_space_delta);
        zio->io_prev_space_delta = delta;

        if (bp->blk_birth != 0) {
                ASSERT((db->db_blkid != DMU_SPILL_BLKID &&
                    BP_GET_TYPE(bp) == dn->dn_type) ||
                    (db->db_blkid == DMU_SPILL_BLKID &&
                    BP_GET_TYPE(bp) == dn->dn_bonustype) ||
                    BP_IS_EMBEDDED(bp));
                ASSERT(BP_GET_LEVEL(bp) == db->db_level);
        }

        mutex_enter(&db->db_mtx);

#ifdef ZFS_DEBUG
        if (db->db_blkid == DMU_SPILL_BLKID) {
                ASSERT(dn->dn_phys->dn_flags & DNODE_FLAG_SPILL_BLKPTR);
                ASSERT(!(BP_IS_HOLE(db->db_blkptr)) &&
                    db->db_blkptr == &dn->dn_phys->dn_spill);
        }
#endif

        if (db->db_level == 0) {
                mutex_enter(&dn->dn_mtx);
                if (db->db_blkid > dn->dn_phys->dn_maxblkid &&
                    db->db_blkid != DMU_SPILL_BLKID)
                        dn->dn_phys->dn_maxblkid = db->db_blkid;
                mutex_exit(&dn->dn_mtx);

                if (dn->dn_type == DMU_OT_DNODE) {
                        dnode_phys_t *dnp = db->db.db_data;
                        for (i = db->db.db_size >> DNODE_SHIFT; i > 0;
                            i--, dnp++) {
                                if (dnp->dn_type != DMU_OT_NONE)
                                        fill++;
                        }
                } else {
                        if (BP_IS_HOLE(bp)) {
                                fill = 0;
                        } else {
                                fill = 1;
                        }
                }
        } else {
                blkptr_t *ibp = db->db.db_data;
                ASSERT3U(db->db.db_size, ==, 1<<dn->dn_phys->dn_indblkshift);
                for (i = db->db.db_size >> SPA_BLKPTRSHIFT; i > 0; i--, ibp++) {
                        if (BP_IS_HOLE(ibp))
                                continue;
                        fill += BP_GET_FILL(ibp);
                }
        }
        DB_DNODE_EXIT(db);

        if (!BP_IS_EMBEDDED(bp))
                bp->blk_fill = fill;

        mutex_exit(&db->db_mtx);
}

/*
 * The SPA will call this callback several times for each zio - once
 * for every physical child i/o (zio->io_phys_children times).  This
 * allows the DMU to monitor the progress of each logical i/o.  For example,
 * there may be 2 copies of an indirect block, or many fragments of a RAID-Z
 * block.  There may be a long delay before all copies/fragments are completed,
 * so this callback allows us to retire dirty space gradually, as the physical
 * i/os complete.
 */
/* ARGSUSED */
static void
dbuf_write_physdone(zio_t *zio, arc_buf_t *buf, void *arg)
{
        dmu_buf_impl_t *db = arg;
        objset_t *os = db->db_objset;
        dsl_pool_t *dp = dmu_objset_pool(os);
        dbuf_dirty_record_t *dr;
        int delta = 0;

        dr = db->db_data_pending;
        ASSERT3U(dr->dr_txg, ==, zio->io_txg);

        /*
         * The callback will be called io_phys_children times.  Retire one
         * portion of our dirty space each time we are called.  Any rounding
         * error will be cleaned up by dsl_pool_sync()'s call to
         * dsl_pool_undirty_space().
         */
        delta = dr->dr_accounted / zio->io_phys_children;
        dsl_pool_undirty_space(dp, delta, zio->io_txg);
}

/* ARGSUSED */
static void
dbuf_write_done(zio_t *zio, arc_buf_t *buf, void *vdb)
{
        dmu_buf_impl_t *db = vdb;
        blkptr_t *bp_orig = &zio->io_bp_orig;
        blkptr_t *bp = db->db_blkptr;
        objset_t *os = db->db_objset;
        dmu_tx_t *tx = os->os_synctx;
        dbuf_dirty_record_t **drp, *dr;

        ASSERT0(zio->io_error);
        ASSERT(db->db_blkptr == bp);

        /*
         * For nopwrites and rewrites we ensure that the bp matches our
         * original and bypass all the accounting.
         */
        if (zio->io_flags & (ZIO_FLAG_IO_REWRITE | ZIO_FLAG_NOPWRITE)) {
                ASSERT(BP_EQUAL(bp, bp_orig));
        } else {
                dsl_dataset_t *ds = os->os_dsl_dataset;
                (void) dsl_dataset_block_kill(ds, bp_orig, tx, B_TRUE);
                dsl_dataset_block_born(ds, bp, tx);
        }

        mutex_enter(&db->db_mtx);

        DBUF_VERIFY(db);

        drp = &db->db_last_dirty;
        while ((dr = *drp) != db->db_data_pending)
                drp = &dr->dr_next;
        ASSERT(!list_link_active(&dr->dr_dirty_node));
        ASSERT(dr->dr_dbuf == db);
        ASSERT(dr->dr_next == NULL);
        *drp = dr->dr_next;

#ifdef ZFS_DEBUG
        if (db->db_blkid == DMU_SPILL_BLKID) {
                dnode_t *dn;

                DB_DNODE_ENTER(db);
                dn = DB_DNODE(db);
                ASSERT(dn->dn_phys->dn_flags & DNODE_FLAG_SPILL_BLKPTR);
                ASSERT(!(BP_IS_HOLE(db->db_blkptr)) &&
                    db->db_blkptr == &dn->dn_phys->dn_spill);
                DB_DNODE_EXIT(db);
        }
#endif

        if (db->db_level == 0) {
                ASSERT(db->db_blkid != DMU_BONUS_BLKID);
                ASSERT(dr->dt.dl.dr_override_state == DR_NOT_OVERRIDDEN);
                if (db->db_state != DB_NOFILL) {
                        if (dr->dt.dl.dr_data != db->db_buf)
                                VERIFY(arc_buf_remove_ref(dr->dt.dl.dr_data,
                                    db));
                        else if (!arc_released(db->db_buf))
                                arc_set_callback(db->db_buf, dbuf_do_evict, db);
                }
        } else {
                dnode_t *dn;

                DB_DNODE_ENTER(db);
                dn = DB_DNODE(db);
                ASSERT(list_head(&dr->dt.di.dr_children) == NULL);
                ASSERT3U(db->db.db_size, ==, 1 << dn->dn_phys->dn_indblkshift);
                if (!BP_IS_HOLE(db->db_blkptr)) {
                        ASSERTV(int epbs = dn->dn_phys->dn_indblkshift -
                            SPA_BLKPTRSHIFT);
                        ASSERT3U(db->db_blkid, <=,
                            dn->dn_phys->dn_maxblkid >> (db->db_level * epbs));
                        ASSERT3U(BP_GET_LSIZE(db->db_blkptr), ==,
                            db->db.db_size);
                        if (!arc_released(db->db_buf))
                                arc_set_callback(db->db_buf, dbuf_do_evict, db);
                }
                DB_DNODE_EXIT(db);
                mutex_destroy(&dr->dt.di.dr_mtx);
                list_destroy(&dr->dt.di.dr_children);
        }
        kmem_free(dr, sizeof (dbuf_dirty_record_t));

        cv_broadcast(&db->db_changed);
        ASSERT(db->db_dirtycnt > 0);
        db->db_dirtycnt -= 1;
        db->db_data_pending = NULL;
        dbuf_rele_and_unlock(db, (void *)(uintptr_t)tx->tx_txg);
}

static void
dbuf_write_nofill_ready(zio_t *zio)
{
        dbuf_write_ready(zio, NULL, zio->io_private);
}

static void
dbuf_write_nofill_done(zio_t *zio)
{
        dbuf_write_done(zio, NULL, zio->io_private);
}

static void
dbuf_write_override_ready(zio_t *zio)
{
        dbuf_dirty_record_t *dr = zio->io_private;
        dmu_buf_impl_t *db = dr->dr_dbuf;

        dbuf_write_ready(zio, NULL, db);
}

static void
dbuf_write_override_done(zio_t *zio)
{
        dbuf_dirty_record_t *dr = zio->io_private;
        dmu_buf_impl_t *db = dr->dr_dbuf;
        blkptr_t *obp = &dr->dt.dl.dr_overridden_by;

        mutex_enter(&db->db_mtx);
        if (!BP_EQUAL(zio->io_bp, obp)) {
                if (!BP_IS_HOLE(obp))
                        dsl_free(spa_get_dsl(zio->io_spa), zio->io_txg, obp);
                arc_release(dr->dt.dl.dr_data, db);
        }
        mutex_exit(&db->db_mtx);

        dbuf_write_done(zio, NULL, db);
}

/* Issue I/O to commit a dirty buffer to disk. */
static void
dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = dr->dr_dbuf;
        dnode_t *dn;
        objset_t *os;
        dmu_buf_impl_t *parent = db->db_parent;
        uint64_t txg = tx->tx_txg;
        zbookmark_phys_t zb;
        zio_prop_t zp;
        zio_t *zio;
        int wp_flag = 0;

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        os = dn->dn_objset;

        if (db->db_state != DB_NOFILL) {
                if (db->db_level > 0 || dn->dn_type == DMU_OT_DNODE) {
                        /*
                         * Private object buffers are released here rather
                         * than in dbuf_dirty() since they are only modified
                         * in the syncing context and we don't want the
                         * overhead of making multiple copies of the data.
                         */
                        if (BP_IS_HOLE(db->db_blkptr)) {
                                arc_buf_thaw(data);
                        } else {
                                dbuf_release_bp(db);
                        }
                }
        }

        if (parent != dn->dn_dbuf) {
                /* Our parent is an indirect block. */
                /* We have a dirty parent that has been scheduled for write. */
                ASSERT(parent && parent->db_data_pending);
                /* Our parent's buffer is one level closer to the dnode. */
                ASSERT(db->db_level == parent->db_level-1);
                /*
                 * We're about to modify our parent's db_data by modifying
                 * our block pointer, so the parent must be released.
                 */
                ASSERT(arc_released(parent->db_buf));
                zio = parent->db_data_pending->dr_zio;
        } else {
                /* Our parent is the dnode itself. */
                ASSERT((db->db_level == dn->dn_phys->dn_nlevels-1 &&
                    db->db_blkid != DMU_SPILL_BLKID) ||
                    (db->db_blkid == DMU_SPILL_BLKID && db->db_level == 0));
                if (db->db_blkid != DMU_SPILL_BLKID)
                        ASSERT3P(db->db_blkptr, ==,
                            &dn->dn_phys->dn_blkptr[db->db_blkid]);
                zio = dn->dn_zio;
        }

        ASSERT(db->db_level == 0 || data == db->db_buf);
        ASSERT3U(db->db_blkptr->blk_birth, <=, txg);
        ASSERT(zio);

        SET_BOOKMARK(&zb, os->os_dsl_dataset ?
            os->os_dsl_dataset->ds_object : DMU_META_OBJSET,
            db->db.db_object, db->db_level, db->db_blkid);

        if (db->db_blkid == DMU_SPILL_BLKID)
                wp_flag = WP_SPILL;
        wp_flag |= (db->db_state == DB_NOFILL) ? WP_NOFILL : 0;

        dmu_write_policy(os, dn, db->db_level, wp_flag, &zp);
        DB_DNODE_EXIT(db);

        if (db->db_level == 0 &&
            dr->dt.dl.dr_override_state == DR_OVERRIDDEN) {
                /*
                 * The BP for this block has been provided by open context
                 * (by dmu_sync() or dmu_buf_write_embedded()).
                 */
                void *contents = (data != NULL) ? data->b_data : NULL;

                dr->dr_zio = zio_write(zio, os->os_spa, txg,
                    db->db_blkptr, contents, db->db.db_size, &zp,
                    dbuf_write_override_ready, NULL, dbuf_write_override_done,
                    dr, ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_MUSTSUCCEED, &zb);
                mutex_enter(&db->db_mtx);
                dr->dt.dl.dr_override_state = DR_NOT_OVERRIDDEN;
                zio_write_override(dr->dr_zio, &dr->dt.dl.dr_overridden_by,
                    dr->dt.dl.dr_copies, dr->dt.dl.dr_nopwrite);
                mutex_exit(&db->db_mtx);
        } else if (db->db_state == DB_NOFILL) {
                ASSERT(zp.zp_checksum == ZIO_CHECKSUM_OFF);
                dr->dr_zio = zio_write(zio, os->os_spa, txg,
                    db->db_blkptr, NULL, db->db.db_size, &zp,
                    dbuf_write_nofill_ready, NULL, dbuf_write_nofill_done, db,
                    ZIO_PRIORITY_ASYNC_WRITE,
                    ZIO_FLAG_MUSTSUCCEED | ZIO_FLAG_NODATA, &zb);
        } else {
                ASSERT(arc_released(data));
                dr->dr_zio = arc_write(zio, os->os_spa, txg,
                    db->db_blkptr, data, DBUF_IS_L2CACHEABLE(db),
                    DBUF_IS_L2COMPRESSIBLE(db), &zp, dbuf_write_ready,
                    dbuf_write_physdone, dbuf_write_done, db,
                    ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_MUSTSUCCEED, &zb);
        }
}

#if defined(_KERNEL) && defined(HAVE_SPL)
EXPORT_SYMBOL(dbuf_find);
EXPORT_SYMBOL(dbuf_is_metadata);
EXPORT_SYMBOL(dbuf_evict);
EXPORT_SYMBOL(dbuf_loan_arcbuf);
EXPORT_SYMBOL(dbuf_whichblock);
EXPORT_SYMBOL(dbuf_read);
EXPORT_SYMBOL(dbuf_unoverride);
EXPORT_SYMBOL(dbuf_free_range);
EXPORT_SYMBOL(dbuf_new_size);
EXPORT_SYMBOL(dbuf_release_bp);
EXPORT_SYMBOL(dbuf_dirty);
EXPORT_SYMBOL(dmu_buf_will_dirty);
EXPORT_SYMBOL(dmu_buf_will_not_fill);
EXPORT_SYMBOL(dmu_buf_will_fill);
EXPORT_SYMBOL(dmu_buf_fill_done);
EXPORT_SYMBOL(dmu_buf_rele);
EXPORT_SYMBOL(dbuf_assign_arcbuf);
EXPORT_SYMBOL(dbuf_clear);
EXPORT_SYMBOL(dbuf_prefetch);
EXPORT_SYMBOL(dbuf_hold_impl);
EXPORT_SYMBOL(dbuf_hold);
EXPORT_SYMBOL(dbuf_hold_level);
EXPORT_SYMBOL(dbuf_create_bonus);
EXPORT_SYMBOL(dbuf_spill_set_blksz);
EXPORT_SYMBOL(dbuf_rm_spill);
EXPORT_SYMBOL(dbuf_add_ref);
EXPORT_SYMBOL(dbuf_rele);
EXPORT_SYMBOL(dbuf_rele_and_unlock);
EXPORT_SYMBOL(dbuf_refcount);
EXPORT_SYMBOL(dbuf_sync_list);
EXPORT_SYMBOL(dmu_buf_set_user);
EXPORT_SYMBOL(dmu_buf_set_user_ie);
EXPORT_SYMBOL(dmu_buf_update_user);
EXPORT_SYMBOL(dmu_buf_get_user);
EXPORT_SYMBOL(dmu_buf_freeable);
EXPORT_SYMBOL(dmu_buf_get_blkptr);
#endif