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dnode_sync.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/dbuf.h>
#include <sys/dnode.h>
#include <sys/dmu.h>
#include <sys/dmu_tx.h>
#include <sys/dmu_objset.h>
#include <sys/dsl_dataset.h>
#include <sys/spa.h>

static void
dnode_increase_indirection(dnode_t *dn, dmu_tx_t *tx)
{
      dmu_buf_impl_t *db;
      int txgoff = tx->tx_txg & TXG_MASK;
      int nblkptr = dn->dn_phys->dn_nblkptr;
      int old_toplvl = dn->dn_phys->dn_nlevels - 1;
      int new_level = dn->dn_next_nlevels[txgoff];
      int i;

      rw_enter(&dn->dn_struct_rwlock, RW_WRITER);

      /* this dnode can't be paged out because it's dirty */
      ASSERT(dn->dn_phys->dn_type != DMU_OT_NONE);
      ASSERT(RW_WRITE_HELD(&dn->dn_struct_rwlock));
      ASSERT(new_level > 1 && dn->dn_phys->dn_nlevels > 0);

      db = dbuf_hold_level(dn, dn->dn_phys->dn_nlevels, 0, FTAG);
      ASSERT(db != NULL);

      dn->dn_phys->dn_nlevels = new_level;
      dprintf("os=%p obj=%llu, increase to %d\n", dn->dn_objset,
          dn->dn_object, dn->dn_phys->dn_nlevels);

      /* check for existing blkptrs in the dnode */
      for (i = 0; i < nblkptr; i++)
            if (!BP_IS_HOLE(&dn->dn_phys->dn_blkptr[i]))
                  break;
      if (i != nblkptr) {
            /* transfer dnode's block pointers to new indirect block */
            (void) dbuf_read(db, NULL, DB_RF_MUST_SUCCEED|DB_RF_HAVESTRUCT);
            ASSERT(db->db.db_data);
            ASSERT(arc_released(db->db_buf));
            ASSERT3U(sizeof (blkptr_t) * nblkptr, <=, db->db.db_size);
            bcopy(dn->dn_phys->dn_blkptr, db->db.db_data,
                sizeof (blkptr_t) * nblkptr);
            arc_buf_freeze(db->db_buf);
      }

      /* set dbuf's parent pointers to new indirect buf */
      for (i = 0; i < nblkptr; i++) {
            dmu_buf_impl_t *child = dbuf_find(dn, old_toplvl, i);

            if (child == NULL)
                  continue;
            ASSERT3P(child->db_dnode, ==, dn);
            if (child->db_parent && child->db_parent != dn->dn_dbuf) {
                  ASSERT(child->db_parent->db_level == db->db_level);
                  ASSERT(child->db_blkptr !=
                      &dn->dn_phys->dn_blkptr[child->db_blkid]);
                  mutex_exit(&child->db_mtx);
                  continue;
            }
            ASSERT(child->db_parent == NULL ||
                child->db_parent == dn->dn_dbuf);

            child->db_parent = db;
            dbuf_add_ref(db, child);
            if (db->db.db_data)
                  child->db_blkptr = (blkptr_t *)db->db.db_data + i;
            else
                  child->db_blkptr = NULL;
            dprintf_dbuf_bp(child, child->db_blkptr,
                "changed db_blkptr to new indirect %s", "");

            mutex_exit(&child->db_mtx);
      }

      bzero(dn->dn_phys->dn_blkptr, sizeof (blkptr_t) * nblkptr);

      dbuf_rele(db, FTAG);

      rw_exit(&dn->dn_struct_rwlock);
}

static int
free_blocks(dnode_t *dn, blkptr_t *bp, int num, dmu_tx_t *tx)
{
      dsl_dataset_t *ds = dn->dn_objset->os_dsl_dataset;
      uint64_t bytesfreed = 0;
      int i, blocks_freed = 0;

      dprintf("ds=%p obj=%llx num=%d\n", ds, dn->dn_object, num);

      for (i = 0; i < num; i++, bp++) {
            if (BP_IS_HOLE(bp))
                  continue;

            bytesfreed += dsl_dataset_block_kill(ds, bp, dn->dn_zio, tx);
            ASSERT3U(bytesfreed, <=, DN_USED_BYTES(dn->dn_phys));
            bzero(bp, sizeof (blkptr_t));
            blocks_freed += 1;
      }
      dnode_diduse_space(dn, -bytesfreed);
      return (blocks_freed);
}

#ifdef ZFS_DEBUG
static void
free_verify(dmu_buf_impl_t *db, uint64_t start, uint64_t end, dmu_tx_t *tx)
{
      int off, num;
      int i, err, epbs;
      uint64_t txg = tx->tx_txg;

      epbs = db->db_dnode->dn_phys->dn_indblkshift - SPA_BLKPTRSHIFT;
      off = start - (db->db_blkid * 1<<epbs);
      num = end - start + 1;

      ASSERT3U(off, >=, 0);
      ASSERT3U(num, >=, 0);
      ASSERT3U(db->db_level, >, 0);
      ASSERT3U(db->db.db_size, ==, 1<<db->db_dnode->dn_phys->dn_indblkshift);
      ASSERT3U(off+num, <=, db->db.db_size >> SPA_BLKPTRSHIFT);
      ASSERT(db->db_blkptr != NULL);

      for (i = off; i < off+num; i++) {
            uint64_t *buf;
            dmu_buf_impl_t *child;
            dbuf_dirty_record_t *dr;
            int j;

            ASSERT(db->db_level == 1);

            rw_enter(&db->db_dnode->dn_struct_rwlock, RW_READER);
            err = dbuf_hold_impl(db->db_dnode, db->db_level-1,
                (db->db_blkid << epbs) + i, TRUE, FTAG, &child);
            rw_exit(&db->db_dnode->dn_struct_rwlock);
            if (err == ENOENT)
                  continue;
            ASSERT(err == 0);
            ASSERT(child->db_level == 0);
            dr = child->db_last_dirty;
            while (dr && dr->dr_txg > txg)
                  dr = dr->dr_next;
            ASSERT(dr == NULL || dr->dr_txg == txg);

            /* data_old better be zeroed */
            if (dr) {
                  buf = dr->dt.dl.dr_data->b_data;
                  for (j = 0; j < child->db.db_size >> 3; j++) {
                        if (buf[j] != 0) {
                              panic("freed data not zero: "
                                  "child=%p i=%d off=%d num=%d\n",
                                  (void *)child, i, off, num);
                        }
                  }
            }

            /*
             * db_data better be zeroed unless it's dirty in a
             * future txg.
             */
            mutex_enter(&child->db_mtx);
            buf = child->db.db_data;
            if (buf != NULL && child->db_state != DB_FILL &&
                child->db_last_dirty == NULL) {
                  for (j = 0; j < child->db.db_size >> 3; j++) {
                        if (buf[j] != 0) {
                              panic("freed data not zero: "
                                  "child=%p i=%d off=%d num=%d\n",
                                  (void *)child, i, off, num);
                        }
                  }
            }
            mutex_exit(&child->db_mtx);

            dbuf_rele(child, FTAG);
      }
}
#endif

#define     ALL -1

static int
free_children(dmu_buf_impl_t *db, uint64_t blkid, uint64_t nblks, int trunc,
    dmu_tx_t *tx)
{
      dnode_t *dn = db->db_dnode;
      blkptr_t *bp;
      dmu_buf_impl_t *subdb;
      uint64_t start, end, dbstart, dbend, i;
      int epbs, shift, err;
      int all = TRUE;
      int blocks_freed = 0;

      /*
       * There is a small possibility that this block will not be cached:
       *   1 - if level > 1 and there are no children with level <= 1
       *   2 - if we didn't get a dirty hold (because this block had just
       *     finished being written -- and so had no holds), and then this
       *     block got evicted before we got here.
       */
      if (db->db_state != DB_CACHED)
            (void) dbuf_read(db, NULL, DB_RF_MUST_SUCCEED);

      arc_release(db->db_buf, db);
      bp = (blkptr_t *)db->db.db_data;

      epbs = db->db_dnode->dn_phys->dn_indblkshift - SPA_BLKPTRSHIFT;
      shift = (db->db_level - 1) * epbs;
      dbstart = db->db_blkid << epbs;
      start = blkid >> shift;
      if (dbstart < start) {
            bp += start - dbstart;
            all = FALSE;
      } else {
            start = dbstart;
      }
      dbend = ((db->db_blkid + 1) << epbs) - 1;
      end = (blkid + nblks - 1) >> shift;
      if (dbend <= end)
            end = dbend;
      else if (all)
            all = trunc;
      ASSERT3U(start, <=, end);

      if (db->db_level == 1) {
            FREE_VERIFY(db, start, end, tx);
            blocks_freed = free_blocks(dn, bp, end-start+1, tx);
            arc_buf_freeze(db->db_buf);
            ASSERT(all || blocks_freed == 0 || db->db_last_dirty);
            return (all ? ALL : blocks_freed);
      }

      for (i = start; i <= end; i++, bp++) {
            if (BP_IS_HOLE(bp))
                  continue;
            rw_enter(&dn->dn_struct_rwlock, RW_READER);
            err = dbuf_hold_impl(dn, db->db_level-1, i, TRUE, FTAG, &subdb);
            ASSERT3U(err, ==, 0);
            rw_exit(&dn->dn_struct_rwlock);

            if (free_children(subdb, blkid, nblks, trunc, tx) == ALL) {
                  ASSERT3P(subdb->db_blkptr, ==, bp);
                  blocks_freed += free_blocks(dn, bp, 1, tx);
            } else {
                  all = FALSE;
            }
            dbuf_rele(subdb, FTAG);
      }
      arc_buf_freeze(db->db_buf);
#ifdef ZFS_DEBUG
      bp -= (end-start)+1;
      for (i = start; i <= end; i++, bp++) {
            if (i == start && blkid != 0)
                  continue;
            else if (i == end && !trunc)
                  continue;
            ASSERT3U(bp->blk_birth, ==, 0);
      }
#endif
      ASSERT(all || blocks_freed == 0 || db->db_last_dirty);
      return (all ? ALL : blocks_freed);
}

/*
 * free_range: Traverse the indicated range of the provided file
 * and "free" all the blocks contained there.
 */
static void
dnode_sync_free_range(dnode_t *dn, uint64_t blkid, uint64_t nblks, dmu_tx_t *tx)
{
      blkptr_t *bp = dn->dn_phys->dn_blkptr;
      dmu_buf_impl_t *db;
      int trunc, start, end, shift, i, err;
      int dnlevel = dn->dn_phys->dn_nlevels;

      if (blkid > dn->dn_phys->dn_maxblkid)
            return;

      ASSERT(dn->dn_phys->dn_maxblkid < UINT64_MAX);
      trunc = blkid + nblks > dn->dn_phys->dn_maxblkid;
      if (trunc)
            nblks = dn->dn_phys->dn_maxblkid - blkid + 1;

      /* There are no indirect blocks in the object */
      if (dnlevel == 1) {
            if (blkid >= dn->dn_phys->dn_nblkptr) {
                  /* this range was never made persistent */
                  return;
            }
            ASSERT3U(blkid + nblks, <=, dn->dn_phys->dn_nblkptr);
            (void) free_blocks(dn, bp + blkid, nblks, tx);
            if (trunc) {
                  uint64_t off = (dn->dn_phys->dn_maxblkid + 1) *
                      (dn->dn_phys->dn_datablkszsec << SPA_MINBLOCKSHIFT);
                  dn->dn_phys->dn_maxblkid = (blkid ? blkid - 1 : 0);
                  ASSERT(off < dn->dn_phys->dn_maxblkid ||
                      dn->dn_phys->dn_maxblkid == 0 ||
                      dnode_next_offset(dn, 0, &off, 1, 1, 0) != 0);
            }
            return;
      }

      shift = (dnlevel - 1) * (dn->dn_phys->dn_indblkshift - SPA_BLKPTRSHIFT);
      start = blkid >> shift;
      ASSERT(start < dn->dn_phys->dn_nblkptr);
      end = (blkid + nblks - 1) >> shift;
      bp += start;
      for (i = start; i <= end; i++, bp++) {
            if (BP_IS_HOLE(bp))
                  continue;
            rw_enter(&dn->dn_struct_rwlock, RW_READER);
            err = dbuf_hold_impl(dn, dnlevel-1, i, TRUE, FTAG, &db);
            ASSERT3U(err, ==, 0);
            rw_exit(&dn->dn_struct_rwlock);

            if (free_children(db, blkid, nblks, trunc, tx) == ALL) {
                  ASSERT3P(db->db_blkptr, ==, bp);
                  (void) free_blocks(dn, bp, 1, tx);
            }
            dbuf_rele(db, FTAG);
      }
      if (trunc) {
            uint64_t off = (dn->dn_phys->dn_maxblkid + 1) *
                (dn->dn_phys->dn_datablkszsec << SPA_MINBLOCKSHIFT);
            dn->dn_phys->dn_maxblkid = (blkid ? blkid - 1 : 0);
            ASSERT(off < dn->dn_phys->dn_maxblkid ||
                dn->dn_phys->dn_maxblkid == 0 ||
                dnode_next_offset(dn, 0, &off, 1, 1, 0) != 0);
      }
}

/*
 * Try to kick all the dnodes dbufs out of the cache...
 */
void
dnode_evict_dbufs(dnode_t *dn)
{
      int progress;
      int pass = 0;

      do {
            dmu_buf_impl_t *db, marker;
            int evicting = FALSE;

            progress = FALSE;
            mutex_enter(&dn->dn_dbufs_mtx);
            list_insert_tail(&dn->dn_dbufs, &marker);
            db = list_head(&dn->dn_dbufs);
            for (; db != &marker; db = list_head(&dn->dn_dbufs)) {
                  list_remove(&dn->dn_dbufs, db);
                  list_insert_tail(&dn->dn_dbufs, db);
                  ASSERT3P(db->db_dnode, ==, dn);

                  mutex_enter(&db->db_mtx);
                  if (db->db_state == DB_EVICTING) {
                        progress = TRUE;
                        evicting = TRUE;
                        mutex_exit(&db->db_mtx);
                  } else if (refcount_is_zero(&db->db_holds)) {
                        progress = TRUE;
                        dbuf_clear(db); /* exits db_mtx for us */
                  } else {
                        mutex_exit(&db->db_mtx);
                  }

            }
            list_remove(&dn->dn_dbufs, &marker);
            /*
             * NB: we need to drop dn_dbufs_mtx between passes so
             * that any DB_EVICTING dbufs can make progress.
             * Ideally, we would have some cv we could wait on, but
             * since we don't, just wait a bit to give the other
             * thread a chance to run.
             */
            mutex_exit(&dn->dn_dbufs_mtx);
            if (evicting)
                  delay(1);
            pass++;
            ASSERT(pass < 100); /* sanity check */
      } while (progress);

      rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
      if (dn->dn_bonus && refcount_is_zero(&dn->dn_bonus->db_holds)) {
            mutex_enter(&dn->dn_bonus->db_mtx);
            dbuf_evict(dn->dn_bonus);
            dn->dn_bonus = NULL;
      }
      rw_exit(&dn->dn_struct_rwlock);
}

static void
dnode_undirty_dbufs(list_t *list)
{
      dbuf_dirty_record_t *dr;

      while (dr = list_head(list)) {
            dmu_buf_impl_t *db = dr->dr_dbuf;
            uint64_t txg = dr->dr_txg;

            mutex_enter(&db->db_mtx);
            /* XXX - use dbuf_undirty()? */
            list_remove(list, dr);
            ASSERT(db->db_last_dirty == dr);
            db->db_last_dirty = NULL;
            db->db_dirtycnt -= 1;
            if (db->db_level == 0) {
                  ASSERT(db->db_blkid == DB_BONUS_BLKID ||
                      dr->dt.dl.dr_data == db->db_buf);
                  dbuf_unoverride(dr);
                  mutex_exit(&db->db_mtx);
            } else {
                  mutex_exit(&db->db_mtx);
                  dnode_undirty_dbufs(&dr->dt.di.dr_children);
                  list_destroy(&dr->dt.di.dr_children);
                  mutex_destroy(&dr->dt.di.dr_mtx);
            }
            kmem_free(dr, sizeof (dbuf_dirty_record_t));
            dbuf_rele(db, (void *)(uintptr_t)txg);
      }
}

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

      ASSERT(dmu_tx_is_syncing(tx));

      /*
       * Our contents should have been freed in dnode_sync() by the
       * free range record inserted by the caller of dnode_free().
       */
      ASSERT3U(DN_USED_BYTES(dn->dn_phys), ==, 0);
      ASSERT(BP_IS_HOLE(dn->dn_phys->dn_blkptr));

      dnode_undirty_dbufs(&dn->dn_dirty_records[txgoff]);
      dnode_evict_dbufs(dn);
      ASSERT3P(list_head(&dn->dn_dbufs), ==, NULL);

      /*
       * XXX - It would be nice to assert this, but we may still
       * have residual holds from async evictions from the arc...
       *
       * zfs_obj_to_path() also depends on this being
       * commented out.
       *
       * ASSERT3U(refcount_count(&dn->dn_holds), ==, 1);
       */

      /* Undirty next bits */
      dn->dn_next_nlevels[txgoff] = 0;
      dn->dn_next_indblkshift[txgoff] = 0;
      dn->dn_next_blksz[txgoff] = 0;

      /* ASSERT(blkptrs are zero); */
      ASSERT(dn->dn_phys->dn_type != DMU_OT_NONE);
      ASSERT(dn->dn_type != DMU_OT_NONE);

      ASSERT(dn->dn_free_txg > 0);
      if (dn->dn_allocated_txg != dn->dn_free_txg)
            dbuf_will_dirty(dn->dn_dbuf, tx);
      bzero(dn->dn_phys, sizeof (dnode_phys_t));

      mutex_enter(&dn->dn_mtx);
      dn->dn_type = DMU_OT_NONE;
      dn->dn_maxblkid = 0;
      dn->dn_allocated_txg = 0;
      dn->dn_free_txg = 0;
      mutex_exit(&dn->dn_mtx);

      ASSERT(dn->dn_object != DMU_META_DNODE_OBJECT);

      dnode_rele(dn, (void *)(uintptr_t)tx->tx_txg);
      /*
       * Now that we've released our hold, the dnode may
       * be evicted, so we musn't access it.
       */
}

/*
 * Write out the dnode's dirty buffers.
 *
 * NOTE: The dnode is kept in memory by being dirty.  Once the
 * dirty bit is cleared, it may be evicted.  Beware of this!
 */
void
dnode_sync(dnode_t *dn, dmu_tx_t *tx)
{
      free_range_t *rp;
      dnode_phys_t *dnp = dn->dn_phys;
      int txgoff = tx->tx_txg & TXG_MASK;
      list_t *list = &dn->dn_dirty_records[txgoff];

      ASSERT(dmu_tx_is_syncing(tx));
      ASSERT(dnp->dn_type != DMU_OT_NONE || dn->dn_allocated_txg);
      DNODE_VERIFY(dn);

      ASSERT(dn->dn_dbuf == NULL || arc_released(dn->dn_dbuf->db_buf));

      mutex_enter(&dn->dn_mtx);
      if (dn->dn_allocated_txg == tx->tx_txg) {
            /* The dnode is newly allocated or reallocated */
            if (dnp->dn_type == DMU_OT_NONE) {
                  /* this is a first alloc, not a realloc */
                  /* XXX shouldn't the phys already be zeroed? */
                  bzero(dnp, DNODE_CORE_SIZE);
                  dnp->dn_nlevels = 1;
                  dnp->dn_nblkptr = dn->dn_nblkptr;
            }

            dnp->dn_type = dn->dn_type;
            dnp->dn_bonustype = dn->dn_bonustype;
            dnp->dn_bonuslen = dn->dn_bonuslen;
      }

      ASSERT(dnp->dn_nlevels > 1 ||
          BP_IS_HOLE(&dnp->dn_blkptr[0]) ||
          BP_GET_LSIZE(&dnp->dn_blkptr[0]) ==
          dnp->dn_datablkszsec << SPA_MINBLOCKSHIFT);

      if (dn->dn_next_blksz[txgoff]) {
            ASSERT(P2PHASE(dn->dn_next_blksz[txgoff],
                SPA_MINBLOCKSIZE) == 0);
            ASSERT(BP_IS_HOLE(&dnp->dn_blkptr[0]) ||
                dn->dn_maxblkid == 0 || list_head(list) != NULL ||
                dn->dn_next_blksz[txgoff] >> SPA_MINBLOCKSHIFT ==
                dnp->dn_datablkszsec);
            dnp->dn_datablkszsec =
                dn->dn_next_blksz[txgoff] >> SPA_MINBLOCKSHIFT;
            dn->dn_next_blksz[txgoff] = 0;
      }

      if (dn->dn_next_bonuslen[txgoff]) {
            if (dn->dn_next_bonuslen[txgoff] == DN_ZERO_BONUSLEN)
                  dnp->dn_bonuslen = 0;
            else
                  dnp->dn_bonuslen = dn->dn_next_bonuslen[txgoff];
            ASSERT(dnp->dn_bonuslen <= DN_MAX_BONUSLEN);
            dn->dn_next_bonuslen[txgoff] = 0;
      }

      if (dn->dn_next_indblkshift[txgoff]) {
            ASSERT(dnp->dn_nlevels == 1);
            dnp->dn_indblkshift = dn->dn_next_indblkshift[txgoff];
            dn->dn_next_indblkshift[txgoff] = 0;
      }

      /*
       * Just take the live (open-context) values for checksum and compress.
       * Strictly speaking it's a future leak, but nothing bad happens if we
       * start using the new checksum or compress algorithm a little early.
       */
      dnp->dn_checksum = dn->dn_checksum;
      dnp->dn_compress = dn->dn_compress;

      mutex_exit(&dn->dn_mtx);

      /* process all the "freed" ranges in the file */
      while (rp = avl_last(&dn->dn_ranges[txgoff])) {
            dnode_sync_free_range(dn, rp->fr_blkid, rp->fr_nblks, tx);
            /* grab the mutex so we don't race with dnode_block_freed() */
            mutex_enter(&dn->dn_mtx);
            avl_remove(&dn->dn_ranges[txgoff], rp);
            mutex_exit(&dn->dn_mtx);
            kmem_free(rp, sizeof (free_range_t));
      }

      if (dn->dn_free_txg > 0 && dn->dn_free_txg <= tx->tx_txg) {
            dnode_sync_free(dn, tx);
            return;
      }

      if (dn->dn_next_nblkptr[txgoff]) {
            /* this should only happen on a realloc */
            ASSERT(dn->dn_allocated_txg == tx->tx_txg);
            if (dn->dn_next_nblkptr[txgoff] > dnp->dn_nblkptr) {
                  /* zero the new blkptrs we are gaining */
                  bzero(dnp->dn_blkptr + dnp->dn_nblkptr,
                      sizeof (blkptr_t) *
                      (dn->dn_next_nblkptr[txgoff] - dnp->dn_nblkptr));
#ifdef ZFS_DEBUG
            } else {
                  int i;
                  ASSERT(dn->dn_next_nblkptr[txgoff] < dnp->dn_nblkptr);
                  /* the blkptrs we are losing better be unallocated */
                  for (i = dn->dn_next_nblkptr[txgoff];
                      i < dnp->dn_nblkptr; i++)
                        ASSERT(BP_IS_HOLE(&dnp->dn_blkptr[i]));
#endif
            }
            mutex_enter(&dn->dn_mtx);
            dnp->dn_nblkptr = dn->dn_next_nblkptr[txgoff];
            dn->dn_next_nblkptr[txgoff] = 0;
            mutex_exit(&dn->dn_mtx);
      }

      if (dn->dn_next_nlevels[txgoff]) {
            dnode_increase_indirection(dn, tx);
            dn->dn_next_nlevels[txgoff] = 0;
      }

      dbuf_sync_list(list, tx);

      if (dn->dn_object != DMU_META_DNODE_OBJECT) {
            ASSERT3P(list_head(list), ==, NULL);
            dnode_rele(dn, (void *)(uintptr_t)tx->tx_txg);
      }

      /*
       * Although we have dropped our reference to the dnode, it
       * can't be evicted until its written, and we haven't yet
       * initiated the IO for the dnode's dbuf.
       */
}

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