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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;

/*
 * 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 < (uint64_t)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;
            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 = 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);
            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 ((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 {
            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_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);

      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_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) {
            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_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;
      }

      ASSERT3U(db->db_state, ==, DB_UNCACHED);
      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.
                   */
                  ASSERT3U(db->db_state, ==, DB_UNCACHED);
                  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;

      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 {
            ASSERT3U(db->db_state, ==, DB_CACHED);
      }
      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;
            if (dr->dr_dbuf->db_level != 0) {
                  list_destroy(&dr->dt.di.dr_children);
                  mutex_destroy(&dr->dt.di.dr_mtx);
            }
            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 (dn->dn_object != DMU_META_DNODE_OBJECT &&
          refcount_count(&db->db_holds) > 1 &&
          *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);
      }

      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 {
            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);

      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);
}

/* 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;
                  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 (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);
}

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