Rebase master to b108

[zfs.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 2009 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#include <sys/zfs_context.h>
#include <sys/dmu.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>

static void dbuf_destroy(dmu_buf_impl_t *db);
static int 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);
static arc_done_func_t dbuf_write_ready;
static arc_done_func_t dbuf_write_done;
static zio_done_func_t dbuf_skip_write_ready;
static zio_done_func_t dbuf_skip_write_done;

/*
 * 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);
        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_impl_t *os = dn->dn_objset;
        uint64_t obj = dn->dn_object;
        uint64_t hv = DBUF_HASH(os, obj, level, blkid);
        uint64_t idx = hv & h->hash_table_mask;
        dmu_buf_impl_t *db;

        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_impl_t *os = db->db_objset;
        uint64_t obj = db->db.db_object;
        int level = db->db_level;
        uint64_t blkid = db->db_blkid;
        uint64_t hv = DBUF_HASH(os, obj, level, blkid);
        uint64_t idx = hv & h->hash_table_mask;
        dmu_buf_impl_t *dbf;

        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.  This operation will
 * fail if there are any existing holds on the db.
 */
static void
dbuf_hash_remove(dmu_buf_impl_t *db)
{
        dbuf_hash_table_t *h = &dbuf_hash_table;
        uint64_t hv = DBUF_HASH(db->db_objset, db->db.db_object,
            db->db_level, db->db_blkid);
        uint64_t idx = hv & h->hash_table_mask;
        dmu_buf_impl_t *dbf, **dbp;

        /*
         * We musn't hold db_mtx to maintin 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;
}

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;
        h->hash_table = kmem_zalloc(hsize * sizeof (void *), KM_NOSLEEP);
        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);
}

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

        for (i = 0; i < DBUF_MUTEXES; i++)
                mutex_destroy(&h->hash_mutexes[i]);
        kmem_free(h->hash_table, (h->hash_table_mask + 1) * sizeof (void *));
        kmem_cache_destroy(dbuf_cache);
}

/*
 * Other stuff.
 */

#ifdef ZFS_DEBUG
static void
dbuf_verify(dmu_buf_impl_t *db)
{
        dnode_t *dn = db->db_dnode;

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

        if (!(zfs_flags & ZFS_DEBUG_DBUF_VERIFY))
                return;

        ASSERT(db->db_objset != NULL);
        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 == DB_BONUS_BLKID ||
                    list_head(&dn->dn_dbufs));
        }
        if (db->db_blkid == DB_BONUS_BLKID) {
                ASSERT(dn != NULL);
                ASSERT3U(db->db.db_size, >=, dn->dn_bonuslen);
                ASSERT3U(db->db.db_offset, ==, DB_BONUS_BLKID);
        } else {
                ASSERT3U(db->db.db_offset, ==, db->db_blkid * db->db.db_size);
        }

        /*
         * 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) {
                dbuf_dirty_record_t *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 (db->db.db_object == DMU_META_DNODE_OBJECT)
                                ASSERT(db->db_parent == NULL);
                        else
                                ASSERT(db->db_parent != NULL);
                        ASSERT3P(db->db_blkptr, ==,
                            &dn->dn_phys->dn_blkptr[db->db_blkid]);
                } else {
                        /* db is pointed to by an indirect block */
                        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(&db->db_dnode->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.db_data && db->db_blkid != DB_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) {
                        uint64_t *buf = db->db.db_data;
                        int i;

                        for (i = 0; i < db->db.db_size >> 3; i++) {
                                ASSERT(buf[i] == 0);
                        }
                }
        }
}
#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));
        ASSERT(db->db_buf == NULL || !arc_has_callback(db->db_buf));
        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;
        }
}

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 != DB_BONUS_BLKID);
                ASSERT3P(db->db_buf, ==, NULL);
                VERIFY(arc_buf_remove_ref(buf, db) == 1);
                db->db_state = DB_UNCACHED;
        }
        cv_broadcast(&db->db_changed);
        mutex_exit(&db->db_mtx);
        dbuf_rele(db, NULL);
}

static void
dbuf_read_impl(dmu_buf_impl_t *db, zio_t *zio, uint32_t *flags)
{
        dnode_t *dn = db->db_dnode;
        zbookmark_t zb;
        uint32_t aflags = ARC_NOWAIT;
        arc_buf_t *pbuf;

        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 == DB_BONUS_BLKID) {
                int bonuslen = dn->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);
                bcopy(DN_BONUS(dn->dn_phys), db->db.db_data,
                    bonuslen);
                dbuf_update_data(db);
                db->db_state = DB_CACHED;
                mutex_exit(&db->db_mtx);
                return;
        }

        /*
         * 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);

                dbuf_set_data(db, arc_buf_alloc(dn->dn_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;
        }

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

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

        zb.zb_objset = db->db_objset->os_dsl_dataset ?
            db->db_objset->os_dsl_dataset->ds_object : 0;
        zb.zb_object = db->db.db_object;
        zb.zb_level = db->db_level;
        zb.zb_blkid = db->db_blkid;

        dbuf_add_ref(db, NULL);
        /* ZIO_FLAG_CANFAIL callers have to check the parent zio's error */

        if (db->db_parent)
                pbuf = db->db_parent->db_buf;
        else
                pbuf = db->db_objset->os_phys_buf;

        (void) arc_read(zio, dn->dn_objset->os_spa, db->db_blkptr, pbuf,
            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;
}

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

        /*
         * 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 (EIO);

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

        prefetch = db->db_level == 0 && db->db_blkid != DB_BONUS_BLKID &&
            (flags & DB_RF_NOPREFETCH) == 0 && db->db_dnode != 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(&db->db_dnode->dn_zfetch, db->db.db_offset,
                            db->db.db_size, TRUE);
                if ((flags & DB_RF_HAVESTRUCT) == 0)
                        rw_exit(&db->db_dnode->dn_struct_rwlock);
        } else if (db->db_state == DB_UNCACHED) {
                if (zio == NULL) {
                        zio = zio_root(db->db_dnode->dn_objset->os_spa,
                            NULL, NULL, ZIO_FLAG_CANFAIL);
                }
                dbuf_read_impl(db, zio, &flags);

                /* dbuf_read_impl has dropped db_mtx for us */

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

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

                if (!havepzio)
                        err = zio_wait(zio);
        } else {
                mutex_exit(&db->db_mtx);
                if (prefetch)
                        dmu_zfetch(&db->db_dnode->dn_zfetch, db->db.db_offset,
                            db->db.db_size, TRUE);
                if ((flags & DB_RF_HAVESTRUCT) == 0)
                        rw_exit(&db->db_dnode->dn_struct_rwlock);

                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);
                                cv_wait(&db->db_changed, &db->db_mtx);
                        }
                        if (db->db_state == DB_UNCACHED)
                                err = 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 != DB_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);

                ASSERT(db->db_buf == NULL);
                ASSERT(db->db.db_data == NULL);
                dbuf_set_data(db, arc_buf_alloc(db->db_dnode->dn_objset->os_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  == DB_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 == DB_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);
                dr->dt.dl.dr_data = arc_buf_alloc(
                    db->db_dnode->dn_objset->os_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;
        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 == DB_BONUS_BLKID ||
            dr->dt.dl.dr_override_state == DR_NOT_OVERRIDDEN)
                return;

        /* free this block */
        if (!BP_IS_HOLE(&dr->dt.dl.dr_overridden_by)) {
                /* XXX can get silent EIO here */
                (void) dsl_free(NULL,
                    spa_get_dsl(db->db_dnode->dn_objset->os_spa),
                    txg, &dr->dt.dl.dr_overridden_by, NULL, NULL, ARC_WAIT);
        }
        dr->dt.dl.dr_override_state = DR_NOT_OVERRIDDEN;
        /*
         * 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.  Also, if we happen accross any level-1 dbufs in the
 * range that have not already been marked dirty, mark them dirty so
 * they stay in memory.
 */
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;
        int epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;
        uint64_t first_l1 = start >> epbs;
        uint64_t last_l1 = end >> epbs;

        if (end > dn->dn_maxblkid) {
                end = dn->dn_maxblkid;
                last_l1 = end >> epbs;
        }
        dprintf_dnode(dn, "start=%llu end=%llu\n", start, end);
        mutex_enter(&dn->dn_dbufs_mtx);
        for (db = list_head(&dn->dn_dbufs); db; db = db_next) {
                db_next = list_next(&dn->dn_dbufs, db);
                ASSERT(db->db_blkid != DB_BONUS_BLKID);

                if (db->db_level == 1 &&
                    db->db_blkid >= first_l1 && db->db_blkid <= last_l1) {
                        mutex_enter(&db->db_mtx);
                        if (db->db_last_dirty &&
                            db->db_last_dirty->dr_txg < txg) {
                                dbuf_add_ref(db, FTAG);
                                mutex_exit(&db->db_mtx);
                                dbuf_will_dirty(db, tx);
                                dbuf_rele(db, FTAG);
                        } else {
                                mutex_exit(&db->db_mtx);
                        }
                }

                if (db->db_level != 0)
                        continue;
                dprintf_dbuf(db, "found buf %s\n", "");
                if (db->db_blkid < start || db->db_blkid > end)
                        continue;

                /* found a level 0 buffer in the range */
                if (dbuf_undirty(db, tx))
                        continue;

                mutex_enter(&db->db_mtx);
                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 > 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.
         */
        ASSERT(MUTEX_HELD(&db->db_mtx));
        if (db->db_last_dirty)
                birth_txg = db->db_last_dirty->dr_txg;
        else if (db->db_blkptr)
                birth_txg = db->db_blkptr->blk_birth;

        /* If we don't exist or are in a snapshot, we can't be freed */
        if (birth_txg)
                return (ds == NULL ||
                    dsl_dataset_block_freeable(ds, birth_txg));
        else
                return (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);

        ASSERT(db->db_blkid != DB_BONUS_BLKID);

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

        /*
         * This call to dbuf_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
         */
        dbuf_will_dirty(db, tx);

        /* create the data buffer for the new block */
        buf = arc_buf_alloc(db->db_dnode->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) == 1);
        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(db->db_dnode, size-osize, tx);
}

dbuf_dirty_record_t *
dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
{
        dnode_t *dn = db->db_dnode;
        objset_impl_t *os = dn->dn_objset;
        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);

        /*
         * 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.
         * XXX We may want to prohibit dirtying in syncing context even
         * if they did pre-dirty.
         */
        ASSERT(!dmu_tx_is_syncing(tx) ||
            BP_IS_HOLE(dn->dn_objset->os_rootbp) ||
            dn->dn_object == DMU_META_DNODE_OBJECT ||
            dn->dn_objset->os_dsl_dataset == NULL ||
            dsl_dir_is_private(dn->dn_objset->os_dsl_dataset->ds_dir));

        /*
         * 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 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) {
                if (db->db_level == 0 && db->db_blkid != DB_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)
                                arc_buf_thaw(db->db_buf);
                }
                mutex_exit(&db->db_mtx);
                return (dr);
        }

        /*
         * Only valid if not already dirty.
         */
        ASSERT(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, a spa os, or we're initializing the os.  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.
         */
        ASSERT(!dmu_tx_is_syncing(tx) ||
            os->os_dsl_dataset == NULL ||
            !dsl_dir_is_private(os->os_dsl_dataset->ds_dir) ||
            !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 != DB_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_dasize() 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);
        if (db->db_level == 0) {
                void *data_old = db->db_buf;

                if (db->db_state != DB_NOFILL) {
                        if (db->db_blkid == DB_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));
        }
        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 != DB_BONUS_BLKID) {
                mutex_enter(&dn->dn_mtx);
                dnode_clear_range(dn, db->db_blkid, 1, tx);
                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 == DB_BONUS_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);
                return (dr);
        } else if (do_free_accounting) {
                blkptr_t *bp = db->db_blkptr;
                int64_t willfree = (bp && !BP_IS_HOLE(bp)) ?
                    bp_get_dasize(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.
                 */
                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);
                        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);
                /*  possible race with dbuf_undirty() */
                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 == db->db_dnode->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);
        return (dr);
}

static int
dbuf_undirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
{
        dnode_t *dn = db->db_dnode;
        uint64_t txg = tx->tx_txg;
        dbuf_dirty_record_t *dr, **drp;

        ASSERT(txg != 0);
        ASSERT(db->db_blkid != DB_BONUS_BLKID);

        mutex_enter(&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) {
                mutex_exit(&db->db_mtx);
                return (0);
        }
        ASSERT(dr->dr_txg == txg);

        /*
         * If this buffer is currently held, we cannot undirty
         * it, since one of the current holders may be in the
         * middle of an update.  Note that users of dbuf_undirty()
         * should not place a hold on the dbuf before the call.
         */
        if (refcount_count(&db->db_holds) > db->db_dirtycnt) {
                mutex_exit(&db->db_mtx);
                /* Make sure we don't toss this buffer at sync phase */
                mutex_enter(&dn->dn_mtx);
                dnode_clear_range(dn, db->db_blkid, 1, tx);
                mutex_exit(&dn->dn_mtx);
                return (0);
        }

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

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

        /* XXX would be nice to fix up dn_towrite_space[] */

        *drp = dr->dr_next;

        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_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);
        }

        if (db->db_level == 0) {
                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) == 1);
                }
        } else {
                ASSERT(db->db_buf != NULL);
                ASSERT(list_head(&dr->dt.di.dr_children) == NULL);
                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;

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

                ASSERT(arc_released(buf));
                dbuf_set_data(db, NULL);
                VERIFY(arc_buf_remove_ref(buf, db) == 1);
                dbuf_evict(db);
                return (1);
        }

        mutex_exit(&db->db_mtx);
        return (0);
}

#pragma weak dmu_buf_will_dirty = dbuf_will_dirty
void
dbuf_will_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
{
        int rf = DB_RF_MUST_SUCCEED | DB_RF_NOPREFETCH;

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

        if (RW_WRITE_HELD(&db->db_dnode->dn_struct_rwlock))
                rf |= DB_RF_HAVESTRUCT;
        (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 != DB_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 != DB_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);
}

/*
 * "Clear" the contents of this dbuf.  This will mark the dbuf
 * EVICTING and clear *most* of its references.  Unfortunetely,
 * 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_buf_evict()->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_buf_evict()
 *      ARC: dbuf_do_evict()->dbuf_destroy()
 */
void
dbuf_clear(dmu_buf_impl_t *db)
{
        dnode_t *dn = db->db_dnode;
        dmu_buf_impl_t *parent = db->db_parent;
        dmu_buf_impl_t *dndb = dn->dn_dbuf;
        int dbuf_gone = 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 == DB_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;

        if (db->db_blkid != DB_BONUS_BLKID && MUTEX_HELD(&dn->dn_dbufs_mtx)) {
                list_remove(&dn->dn_dbufs, db);
                dnode_rele(dn, db);
                db->db_dnode = NULL;
        }

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

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

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

static int
dbuf_findbp(dnode_t *dn, int level, uint64_t blkid, int fail_sparse,
    dmu_buf_impl_t **parentp, blkptr_t **bpp)
{
        int nlevels, epbs;

        *parentp = NULL;
        *bpp = NULL;

        ASSERT(blkid != DB_BONUS_BLKID);

        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 (ENOENT);
        } else if (level < nlevels-1) {
                /* this block is referenced from an indirect block */
                int err = dbuf_hold_impl(dn, level+1,
                    blkid >> epbs, fail_sparse, NULL, parentp);
                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_impl_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 = dn;
        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 == DB_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 = DB_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 {
                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);
        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);

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

        return (db);
}

static int
dbuf_do_evict(void *private)
{
        arc_buf_t *buf = private;
        dmu_buf_impl_t *db = buf->b_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 != DB_BONUS_BLKID) {
                /*
                 * If this dbuf is still on the dn_dbufs list,
                 * remove it from that list.
                 */
                if (db->db_dnode) {
                        dnode_t *dn = db->db_dnode;

                        mutex_enter(&dn->dn_dbufs_mtx);
                        list_remove(&dn->dn_dbufs, db);
                        mutex_exit(&dn->dn_dbufs_mtx);

                        dnode_rele(dn, db);
                        db->db_dnode = 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)
{
        dmu_buf_impl_t *db = NULL;
        blkptr_t *bp = NULL;

        ASSERT(blkid != DB_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)) {
                if (refcount_count(&db->db_holds) > 0) {
                        /*
                         * This dbuf is active.  We assume that it is
                         * already CACHED, or else about to be either
                         * read or filled.
                         */
                        mutex_exit(&db->db_mtx);
                        return;
                }
                mutex_exit(&db->db_mtx);
                db = NULL;
        }

        if (dbuf_findbp(dn, 0, blkid, TRUE, &db, &bp) == 0) {
                if (bp && !BP_IS_HOLE(bp)) {
                        arc_buf_t *pbuf;
                        uint32_t aflags = ARC_NOWAIT | ARC_PREFETCH;
                        zbookmark_t zb;
                        zb.zb_objset = dn->dn_objset->os_dsl_dataset ?
                            dn->dn_objset->os_dsl_dataset->ds_object : 0;
                        zb.zb_object = dn->dn_object;
                        zb.zb_level = 0;
                        zb.zb_blkid = blkid;

                        if (db)
                                pbuf = db->db_buf;
                        else
                                pbuf = dn->dn_objset->os_phys_buf;

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

/*
 * Returns with db_holds incremented, and db_mtx not held.
 * Note: dn_struct_rwlock must be held.
 */
int
dbuf_hold_impl(dnode_t *dn, uint8_t level, uint64_t blkid, int fail_sparse,
    void *tag, dmu_buf_impl_t **dbp)
{
        dmu_buf_impl_t *db, *parent = NULL;

        ASSERT(blkid != DB_BONUS_BLKID);
        ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));
        ASSERT3U(dn->dn_nlevels, >, level);

        *dbp = NULL;
top:
        /* dbuf_find() returns with db_mtx held */
        db = dbuf_find(dn, level, blkid);

        if (db == NULL) {
                blkptr_t *bp = NULL;
                int err;

                ASSERT3P(parent, ==, NULL);
                err = dbuf_findbp(dn, level, blkid, fail_sparse, &parent, &bp);
                if (fail_sparse) {
                        if (err == 0 && bp && BP_IS_HOLE(bp))
                                err = ENOENT;
                        if (err) {
                                if (parent)
                                        dbuf_rele(parent, NULL);
                                return (err);
                        }
                }
                if (err && err != ENOENT)
                        return (err);
                db = dbuf_create(dn, level, blkid, parent, bp);
        }

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

        ASSERT(db->db_buf == NULL || arc_referenced(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 (db->db_level == 0 && db->db_blkid != DB_BONUS_BLKID &&
            dn->dn_object != DMU_META_DNODE_OBJECT &&
            db->db_state == DB_CACHED && db->db_data_pending) {
                dbuf_dirty_record_t *dr = db->db_data_pending;

                if (dr->dt.dl.dr_data == db->db_buf) {
                        arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);

                        dbuf_set_data(db,
                            arc_buf_alloc(db->db_dnode->dn_objset->os_spa,
                            db->db.db_size, db, type));
                        bcopy(dr->dt.dl.dr_data->b_data, db->db.db_data,
                            db->db.db_size);
                }
        }

        (void) refcount_add(&db->db_holds, tag);
        dbuf_update_data(db);
        DBUF_VERIFY(db);
        mutex_exit(&db->db_mtx);

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

        ASSERT3P(db->db_dnode, ==, dn);
        ASSERT3U(db->db_blkid, ==, blkid);
        ASSERT3U(db->db_level, ==, level);
        *dbp = db;

        return (0);
}

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, DB_BONUS_BLKID, dn->dn_dbuf, NULL);
}

#pragma weak dmu_buf_add_ref = dbuf_add_ref
void
dbuf_add_ref(dmu_buf_impl_t *db, void *tag)
{
        int64_t holds = refcount_add(&db->db_holds, tag);
        ASSERT(holds > 1);
}

#pragma weak dmu_buf_rele = dbuf_rele
void
dbuf_rele(dmu_buf_impl_t *db, void *tag)
{
        int64_t holds;

        mutex_enter(&db->db_mtx);
        DBUF_VERIFY(db);

        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 == DB_BONUS_BLKID) {
                        mutex_exit(&db->db_mtx);
                        dnode_rele(db->db_dnode, 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) == 1);
                        dbuf_evict(db);
                } else {
                        VERIFY(arc_buf_remove_ref(db->db_buf, db) == 0);
                        if (!DBUF_IS_CACHEABLE(db))
                                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);
}

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_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);
        }
}

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 = db->db_dnode;
        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);

        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);
        ASSERT3U(db->db.db_size, ==, 1<<dn->dn_phys->dn_indblkshift);
        ASSERT(db->db_buf != NULL);

        dbuf_check_blkptr(dn, db);

        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);
}

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 = db->db_dnode;
        objset_impl_t *os = dn->dn_objset;
        uint64_t txg = tx->tx_txg;
        int blksz;

        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);

        /*
         * 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 == DB_BONUS_BLKID) {
                dbuf_dirty_record_t **drp;

                ASSERT(*datap != NULL);
                ASSERT3U(db->db_level, ==, 0);
                ASSERT3U(dn->dn_phys->dn_bonuslen, <=, DN_MAX_BONUSLEN);
                bcopy(*datap, DN_BONUS(dn->dn_phys), dn->dn_phys->dn_bonuslen);
                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);
                *drp = dr->dr_next;
                kmem_free(dr, sizeof (dbuf_dirty_record_t));
                ASSERT(db->db_dirtycnt > 0);
                db->db_dirtycnt -= 1;
                mutex_exit(&db->db_mtx);
                dbuf_rele(db, (void *)(uintptr_t)txg);
                return;
        }

        /*
         * 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 immdiate 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 this dbuf has already been written out via an immediate write,
         * just complete the write by copying over the new block pointer and
         * updating the accounting via the write-completion functions.
         */
        if (dr->dt.dl.dr_override_state == DR_OVERRIDDEN) {
                zio_t zio_fake;

                zio_fake.io_private = &db;
                zio_fake.io_error = 0;
                zio_fake.io_bp = db->db_blkptr;
                zio_fake.io_bp_orig = *db->db_blkptr;
                zio_fake.io_txg = txg;
                zio_fake.io_flags = 0;

                *db->db_blkptr = dr->dt.dl.dr_overridden_by;
                dr->dt.dl.dr_override_state = DR_NOT_OVERRIDDEN;
                db->db_data_pending = dr;
                dr->dr_zio = &zio_fake;
                mutex_exit(&db->db_mtx);

                ASSERT(!DVA_EQUAL(BP_IDENTITY(zio_fake.io_bp),
                    BP_IDENTITY(&zio_fake.io_bp_orig)) ||
                    BP_IS_HOLE(zio_fake.io_bp));

                if (BP_IS_OLDER(&zio_fake.io_bp_orig, txg))
                        (void) dsl_dataset_block_kill(os->os_dsl_dataset,
                            &zio_fake.io_bp_orig, dn->dn_zio, tx);

                dbuf_write_ready(&zio_fake, db->db_buf, db);
                dbuf_write_done(&zio_fake, db->db_buf, db);

                return;
        }

        if (db->db_state != DB_NOFILL) {
                blksz = arc_buf_size(*datap);

                if (dn->dn_object != DMU_META_DNODE_OBJECT) {
                        /*
                         * 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).
                         */
                        if (refcount_count(&db->db_holds) > 1 &&
                            *datap == db->db_buf) {
                                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);
                        }
                }

                ASSERT(*datap != NULL);
        }
        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);
        else
                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);
        }
}

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 = db->db_dnode;
        objset_impl_t *os = dn->dn_objset;
        dmu_buf_impl_t *parent = db->db_parent;
        uint64_t txg = tx->tx_txg;
        zbookmark_t zb;
        writeprops_t wp = { 0 };
        zio_t *zio;

        if (!BP_IS_HOLE(db->db_blkptr) &&
            (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.
                 */
                arc_release(data, db);
        } else if (db->db_state != DB_NOFILL) {
                ASSERT(arc_released(data));
                /* XXX why do we need to thaw here? */
                arc_buf_thaw(data);
        }

        if (parent != dn->dn_dbuf) {
                ASSERT(parent && parent->db_data_pending);
                ASSERT(db->db_level == parent->db_level-1);
                ASSERT(arc_released(parent->db_buf));
                zio = parent->db_data_pending->dr_zio;
        } else {
                ASSERT(db->db_level == dn->dn_phys->dn_nlevels-1);
                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);

        zb.zb_objset = os->os_dsl_dataset ? os->os_dsl_dataset->ds_object : 0;
        zb.zb_object = db->db.db_object;
        zb.zb_level = db->db_level;
        zb.zb_blkid = db->db_blkid;

        wp.wp_type = dn->dn_type;
        wp.wp_level = db->db_level;
        wp.wp_copies = os->os_copies;
        wp.wp_dncompress = dn->dn_compress;
        wp.wp_oscompress = os->os_compress;
        wp.wp_dnchecksum = dn->dn_checksum;
        wp.wp_oschecksum = os->os_checksum;

        if (BP_IS_OLDER(db->db_blkptr, txg))
                (void) dsl_dataset_block_kill(
                    os->os_dsl_dataset, db->db_blkptr, zio, tx);

        if (db->db_state == DB_NOFILL) {
                zio_prop_t zp = { 0 };

                write_policy(os->os_spa, &wp, &zp);
                dr->dr_zio = zio_write(zio, os->os_spa,
                    txg, db->db_blkptr, NULL,
                    db->db.db_size, &zp, dbuf_skip_write_ready,
                    dbuf_skip_write_done, db, ZIO_PRIORITY_ASYNC_WRITE,
                    ZIO_FLAG_MUSTSUCCEED, &zb);
        } else {
                dr->dr_zio = arc_write(zio, os->os_spa, &wp,
                    DBUF_IS_L2CACHEABLE(db), txg, db->db_blkptr,
                    data, dbuf_write_ready, dbuf_write_done, db,
                    ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_MUSTSUCCEED, &zb);
        }
}

/* wrapper function for dbuf_write_ready bypassing ARC */
static void
dbuf_skip_write_ready(zio_t *zio)
{
        blkptr_t *bp = zio->io_bp;

        if (!BP_IS_GANG(bp))
                zio_skip_write(zio);

        dbuf_write_ready(zio, NULL, zio->io_private);
}

/* wrapper function for dbuf_write_done bypassing ARC */
static void
dbuf_skip_write_done(zio_t *zio)
{
        dbuf_write_done(zio, NULL, zio->io_private);
}

/* ARGSUSED */
static void
dbuf_write_ready(zio_t *zio, arc_buf_t *buf, void *vdb)
{
        dmu_buf_impl_t *db = vdb;
        dnode_t *dn = db->db_dnode;
        objset_impl_t *os = dn->dn_objset;
        blkptr_t *bp = zio->io_bp;
        blkptr_t *bp_orig = &zio->io_bp_orig;
        uint64_t fill = 0;
        int old_size, new_size, i;

        ASSERT(db->db_blkptr == bp);

        dprintf_dbuf_bp(db, bp_orig, "bp_orig: %s", "");

        old_size = bp_get_dasize(os->os_spa, bp_orig);
        new_size = bp_get_dasize(os->os_spa, bp);

        dnode_diduse_space(dn, new_size - old_size);

        if (BP_IS_HOLE(bp)) {
                dsl_dataset_t *ds = os->os_dsl_dataset;
                dmu_tx_t *tx = os->os_synctx;

                if (bp_orig->blk_birth == tx->tx_txg)
                        (void) dsl_dataset_block_kill(ds, bp_orig, zio, tx);
                ASSERT3U(bp->blk_fill, ==, 0);
                return;
        }

        ASSERT(BP_GET_TYPE(bp) == dn->dn_type);
        ASSERT(BP_GET_LEVEL(bp) == db->db_level);

        mutex_enter(&db->db_mtx);

        if (db->db_level == 0) {
                mutex_enter(&dn->dn_mtx);
                if (db->db_blkid > dn->dn_phys->dn_maxblkid)
                        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 {
                        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;
                        ASSERT3U(BP_GET_LSIZE(ibp), ==,
                            db->db_level == 1 ? dn->dn_datablksz :
                            (1<<dn->dn_phys->dn_indblkshift));
                        fill += ibp->blk_fill;
                }
        }

        bp->blk_fill = fill;

        mutex_exit(&db->db_mtx);

        if (zio->io_flags & ZIO_FLAG_IO_REWRITE) {
                ASSERT(DVA_EQUAL(BP_IDENTITY(bp), BP_IDENTITY(bp_orig)));
        } else {
                dsl_dataset_t *ds = os->os_dsl_dataset;
                dmu_tx_t *tx = os->os_synctx;

                if (bp_orig->blk_birth == tx->tx_txg)
                        (void) dsl_dataset_block_kill(ds, bp_orig, zio, tx);
                dsl_dataset_block_born(ds, bp, tx);
        }
}

/* ARGSUSED */
static void
dbuf_write_done(zio_t *zio, arc_buf_t *buf, void *vdb)
{
        dmu_buf_impl_t *db = vdb;
        uint64_t txg = zio->io_txg;
        dbuf_dirty_record_t **drp, *dr;

        ASSERT3U(zio->io_error, ==, 0);

        mutex_enter(&db->db_mtx);

        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_txg == txg);
        ASSERT(dr->dr_next == NULL);
        *drp = dr->dr_next;

        if (db->db_level == 0) {
                ASSERT(db->db_blkid != DB_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) == 1);
                        else if (!BP_IS_HOLE(db->db_blkptr))
                                arc_set_callback(db->db_buf, dbuf_do_evict, db);
                        else
                                ASSERT(arc_released(db->db_buf));
                }
        } else {
                dnode_t *dn = db->db_dnode;

                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)) {
                        int epbs =
                            dn->dn_phys->dn_indblkshift - SPA_BLKPTRSHIFT;
                        ASSERT3U(BP_GET_LSIZE(db->db_blkptr), ==,
                            db->db.db_size);
                        ASSERT3U(dn->dn_phys->dn_maxblkid
                            >> (db->db_level * epbs), >=, db->db_blkid);
                        arc_set_callback(db->db_buf, dbuf_do_evict, 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;
        mutex_exit(&db->db_mtx);

        dprintf_dbuf_bp(db, zio->io_bp, "bp: %s", "");

        dbuf_rele(db, (void *)(uintptr_t)txg);
}