Reduce dirty records memory usage

[zfs.git] / module / zfs / dmu.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 https://opensource.org/licenses/CDDL-1.0.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2011, 2020 by Delphix. All rights reserved.
 * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
 * Copyright (c) 2013, Joyent, Inc. All rights reserved.
 * Copyright (c) 2016, Nexenta Systems, Inc. All rights reserved.
 * Copyright (c) 2015 by Chunwei Chen. All rights reserved.
 * Copyright (c) 2019 Datto Inc.
 * Copyright (c) 2019, 2023, Klara Inc.
 * Copyright (c) 2019, Allan Jude
 * Copyright (c) 2022 Hewlett Packard Enterprise Development LP.
 * Copyright (c) 2021, 2022 by Pawel Jakub Dawidek
 */

#include <sys/dmu.h>
#include <sys/dmu_impl.h>
#include <sys/dmu_tx.h>
#include <sys/dbuf.h>
#include <sys/dnode.h>
#include <sys/zfs_context.h>
#include <sys/dmu_objset.h>
#include <sys/dmu_traverse.h>
#include <sys/dsl_dataset.h>
#include <sys/dsl_dir.h>
#include <sys/dsl_pool.h>
#include <sys/dsl_synctask.h>
#include <sys/dsl_prop.h>
#include <sys/dmu_zfetch.h>
#include <sys/zfs_ioctl.h>
#include <sys/zap.h>
#include <sys/zio_checksum.h>
#include <sys/zio_compress.h>
#include <sys/sa.h>
#include <sys/zfeature.h>
#include <sys/abd.h>
#include <sys/brt.h>
#include <sys/trace_zfs.h>
#include <sys/zfs_racct.h>
#include <sys/zfs_rlock.h>
#ifdef _KERNEL
#include <sys/vmsystm.h>
#include <sys/zfs_znode.h>
#endif

/*
 * Enable/disable nopwrite feature.
 */
static int zfs_nopwrite_enabled = 1;

/*
 * Tunable to control percentage of dirtied L1 blocks from frees allowed into
 * one TXG. After this threshold is crossed, additional dirty blocks from frees
 * will wait until the next TXG.
 * A value of zero will disable this throttle.
 */
static uint_t zfs_per_txg_dirty_frees_percent = 30;

/*
 * Enable/disable forcing txg sync when dirty checking for holes with lseek().
 * By default this is enabled to ensure accurate hole reporting, it can result
 * in a significant performance penalty for lseek(SEEK_HOLE) heavy workloads.
 * Disabling this option will result in holes never being reported in dirty
 * files which is always safe.
 */
static int zfs_dmu_offset_next_sync = 1;

/*
 * Limit the amount we can prefetch with one call to this amount.  This
 * helps to limit the amount of memory that can be used by prefetching.
 * Larger objects should be prefetched a bit at a time.
 */
#ifdef _ILP32
uint_t dmu_prefetch_max = 8 * 1024 * 1024;
#else
uint_t dmu_prefetch_max = 8 * SPA_MAXBLOCKSIZE;
#endif

/*
 * Override copies= for dedup state objects. 0 means the traditional behaviour
 * (ie the default for the containing objset ie 3 for the MOS).
 */
uint_t dmu_ddt_copies = 0;

const dmu_object_type_info_t dmu_ot[DMU_OT_NUMTYPES] = {
        {DMU_BSWAP_UINT8,  TRUE,  FALSE, FALSE, "unallocated"           },
        {DMU_BSWAP_ZAP,    TRUE,  TRUE,  FALSE, "object directory"      },
        {DMU_BSWAP_UINT64, TRUE,  TRUE,  FALSE, "object array"          },
        {DMU_BSWAP_UINT8,  TRUE,  FALSE, FALSE, "packed nvlist"         },
        {DMU_BSWAP_UINT64, TRUE,  FALSE, FALSE, "packed nvlist size"    },
        {DMU_BSWAP_UINT64, TRUE,  FALSE, FALSE, "bpobj"                 },
        {DMU_BSWAP_UINT64, TRUE,  FALSE, FALSE, "bpobj header"          },
        {DMU_BSWAP_UINT64, TRUE,  FALSE, FALSE, "SPA space map header"  },
        {DMU_BSWAP_UINT64, TRUE,  FALSE, FALSE, "SPA space map"         },
        {DMU_BSWAP_UINT64, TRUE,  FALSE, TRUE,  "ZIL intent log"        },
        {DMU_BSWAP_DNODE,  TRUE,  FALSE, TRUE,  "DMU dnode"             },
        {DMU_BSWAP_OBJSET, TRUE,  TRUE,  FALSE, "DMU objset"            },
        {DMU_BSWAP_UINT64, TRUE,  TRUE,  FALSE, "DSL directory"         },
        {DMU_BSWAP_ZAP,    TRUE,  TRUE,  FALSE, "DSL directory child map"},
        {DMU_BSWAP_ZAP,    TRUE,  TRUE,  FALSE, "DSL dataset snap map"  },
        {DMU_BSWAP_ZAP,    TRUE,  TRUE,  FALSE, "DSL props"             },
        {DMU_BSWAP_UINT64, TRUE,  TRUE,  FALSE, "DSL dataset"           },
        {DMU_BSWAP_ZNODE,  TRUE,  FALSE, FALSE, "ZFS znode"             },
        {DMU_BSWAP_OLDACL, TRUE,  FALSE, TRUE,  "ZFS V0 ACL"            },
        {DMU_BSWAP_UINT8,  FALSE, FALSE, TRUE,  "ZFS plain file"        },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, TRUE,  "ZFS directory"         },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, FALSE, "ZFS master node"       },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, TRUE,  "ZFS delete queue"      },
        {DMU_BSWAP_UINT8,  FALSE, FALSE, TRUE,  "zvol object"           },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, FALSE, "zvol prop"             },
        {DMU_BSWAP_UINT8,  FALSE, FALSE, TRUE,  "other uint8[]"         },
        {DMU_BSWAP_UINT64, FALSE, FALSE, TRUE,  "other uint64[]"        },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, FALSE, "other ZAP"             },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, FALSE, "persistent error log"  },
        {DMU_BSWAP_UINT8,  TRUE,  FALSE, FALSE, "SPA history"           },
        {DMU_BSWAP_UINT64, TRUE,  FALSE, FALSE, "SPA history offsets"   },
        {DMU_BSWAP_ZAP,    TRUE,  TRUE,  FALSE, "Pool properties"       },
        {DMU_BSWAP_ZAP,    TRUE,  TRUE,  FALSE, "DSL permissions"       },
        {DMU_BSWAP_ACL,    TRUE,  FALSE, TRUE,  "ZFS ACL"               },
        {DMU_BSWAP_UINT8,  TRUE,  FALSE, TRUE,  "ZFS SYSACL"            },
        {DMU_BSWAP_UINT8,  TRUE,  FALSE, TRUE,  "FUID table"            },
        {DMU_BSWAP_UINT64, TRUE,  FALSE, FALSE, "FUID table size"       },
        {DMU_BSWAP_ZAP,    TRUE,  TRUE,  FALSE, "DSL dataset next clones"},
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, FALSE, "scan work queue"       },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, TRUE,  "ZFS user/group/project used" },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, TRUE,  "ZFS user/group/project quota"},
        {DMU_BSWAP_ZAP,    TRUE,  TRUE,  FALSE, "snapshot refcount tags"},
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, FALSE, "DDT ZAP algorithm"     },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, FALSE, "DDT statistics"        },
        {DMU_BSWAP_UINT8,  TRUE,  FALSE, TRUE,  "System attributes"     },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, TRUE,  "SA master node"        },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, TRUE,  "SA attr registration"  },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, TRUE,  "SA attr layouts"       },
        {DMU_BSWAP_ZAP,    TRUE,  FALSE, FALSE, "scan translations"     },
        {DMU_BSWAP_UINT8,  FALSE, FALSE, TRUE,  "deduplicated block"    },
        {DMU_BSWAP_ZAP,    TRUE,  TRUE,  FALSE, "DSL deadlist map"      },
        {DMU_BSWAP_UINT64, TRUE,  TRUE,  FALSE, "DSL deadlist map hdr"  },
        {DMU_BSWAP_ZAP,    TRUE,  TRUE,  FALSE, "DSL dir clones"        },
        {DMU_BSWAP_UINT64, TRUE,  FALSE, FALSE, "bpobj subobj"          }
};

dmu_object_byteswap_info_t dmu_ot_byteswap[DMU_BSWAP_NUMFUNCS] = {
        {       byteswap_uint8_array,   "uint8"         },
        {       byteswap_uint16_array,  "uint16"        },
        {       byteswap_uint32_array,  "uint32"        },
        {       byteswap_uint64_array,  "uint64"        },
        {       zap_byteswap,           "zap"           },
        {       dnode_buf_byteswap,     "dnode"         },
        {       dmu_objset_byteswap,    "objset"        },
        {       zfs_znode_byteswap,     "znode"         },
        {       zfs_oldacl_byteswap,    "oldacl"        },
        {       zfs_acl_byteswap,       "acl"           }
};

int
dmu_buf_hold_noread_by_dnode(dnode_t *dn, uint64_t offset,
    const void *tag, dmu_buf_t **dbp)
{
        uint64_t blkid;
        dmu_buf_impl_t *db;

        rw_enter(&dn->dn_struct_rwlock, RW_READER);
        blkid = dbuf_whichblock(dn, 0, offset);
        db = dbuf_hold(dn, blkid, tag);
        rw_exit(&dn->dn_struct_rwlock);

        if (db == NULL) {
                *dbp = NULL;
                return (SET_ERROR(EIO));
        }

        *dbp = &db->db;
        return (0);
}

int
dmu_buf_hold_noread(objset_t *os, uint64_t object, uint64_t offset,
    const void *tag, dmu_buf_t **dbp)
{
        dnode_t *dn;
        uint64_t blkid;
        dmu_buf_impl_t *db;
        int err;

        err = dnode_hold(os, object, FTAG, &dn);
        if (err)
                return (err);
        rw_enter(&dn->dn_struct_rwlock, RW_READER);
        blkid = dbuf_whichblock(dn, 0, offset);
        db = dbuf_hold(dn, blkid, tag);
        rw_exit(&dn->dn_struct_rwlock);
        dnode_rele(dn, FTAG);

        if (db == NULL) {
                *dbp = NULL;
                return (SET_ERROR(EIO));
        }

        *dbp = &db->db;
        return (err);
}

int
dmu_buf_hold_by_dnode(dnode_t *dn, uint64_t offset,
    const void *tag, dmu_buf_t **dbp, int flags)
{
        int err;
        int db_flags = DB_RF_CANFAIL;

        if (flags & DMU_READ_NO_PREFETCH)
                db_flags |= DB_RF_NOPREFETCH;
        if (flags & DMU_READ_NO_DECRYPT)
                db_flags |= DB_RF_NO_DECRYPT;

        err = dmu_buf_hold_noread_by_dnode(dn, offset, tag, dbp);
        if (err == 0) {
                dmu_buf_impl_t *db = (dmu_buf_impl_t *)(*dbp);
                err = dbuf_read(db, NULL, db_flags);
                if (err != 0) {
                        dbuf_rele(db, tag);
                        *dbp = NULL;
                }
        }

        return (err);
}

int
dmu_buf_hold(objset_t *os, uint64_t object, uint64_t offset,
    const void *tag, dmu_buf_t **dbp, int flags)
{
        int err;
        int db_flags = DB_RF_CANFAIL;

        if (flags & DMU_READ_NO_PREFETCH)
                db_flags |= DB_RF_NOPREFETCH;
        if (flags & DMU_READ_NO_DECRYPT)
                db_flags |= DB_RF_NO_DECRYPT;

        err = dmu_buf_hold_noread(os, object, offset, tag, dbp);
        if (err == 0) {
                dmu_buf_impl_t *db = (dmu_buf_impl_t *)(*dbp);
                err = dbuf_read(db, NULL, db_flags);
                if (err != 0) {
                        dbuf_rele(db, tag);
                        *dbp = NULL;
                }
        }

        return (err);
}

int
dmu_bonus_max(void)
{
        return (DN_OLD_MAX_BONUSLEN);
}

int
dmu_set_bonus(dmu_buf_t *db_fake, int newsize, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        dnode_t *dn;
        int error;

        if (newsize < 0 || newsize > db_fake->db_size)
                return (SET_ERROR(EINVAL));

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

        if (dn->dn_bonus != db) {
                error = SET_ERROR(EINVAL);
        } else {
                dnode_setbonuslen(dn, newsize, tx);
                error = 0;
        }

        DB_DNODE_EXIT(db);
        return (error);
}

int
dmu_set_bonustype(dmu_buf_t *db_fake, dmu_object_type_t type, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        dnode_t *dn;
        int error;

        if (!DMU_OT_IS_VALID(type))
                return (SET_ERROR(EINVAL));

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

        if (dn->dn_bonus != db) {
                error = SET_ERROR(EINVAL);
        } else {
                dnode_setbonus_type(dn, type, tx);
                error = 0;
        }

        DB_DNODE_EXIT(db);
        return (error);
}

dmu_object_type_t
dmu_get_bonustype(dmu_buf_t *db_fake)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        dmu_object_type_t type;

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

        return (type);
}

int
dmu_rm_spill(objset_t *os, uint64_t object, dmu_tx_t *tx)
{
        dnode_t *dn;
        int error;

        error = dnode_hold(os, object, FTAG, &dn);
        dbuf_rm_spill(dn, tx);
        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
        dnode_rm_spill(dn, tx);
        rw_exit(&dn->dn_struct_rwlock);
        dnode_rele(dn, FTAG);
        return (error);
}

/*
 * Lookup and hold the bonus buffer for the provided dnode.  If the dnode
 * has not yet been allocated a new bonus dbuf a will be allocated.
 * Returns ENOENT, EIO, or 0.
 */
int dmu_bonus_hold_by_dnode(dnode_t *dn, const void *tag, dmu_buf_t **dbp,
    uint32_t flags)
{
        dmu_buf_impl_t *db;
        int error;
        uint32_t db_flags = DB_RF_MUST_SUCCEED;

        if (flags & DMU_READ_NO_PREFETCH)
                db_flags |= DB_RF_NOPREFETCH;
        if (flags & DMU_READ_NO_DECRYPT)
                db_flags |= DB_RF_NO_DECRYPT;

        rw_enter(&dn->dn_struct_rwlock, RW_READER);
        if (dn->dn_bonus == NULL) {
                if (!rw_tryupgrade(&dn->dn_struct_rwlock)) {
                        rw_exit(&dn->dn_struct_rwlock);
                        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
                }
                if (dn->dn_bonus == NULL)
                        dbuf_create_bonus(dn);
        }
        db = dn->dn_bonus;

        /* as long as the bonus buf is held, the dnode will be held */
        if (zfs_refcount_add(&db->db_holds, tag) == 1) {
                VERIFY(dnode_add_ref(dn, db));
                atomic_inc_32(&dn->dn_dbufs_count);
        }

        /*
         * Wait to drop dn_struct_rwlock until after adding the bonus dbuf's
         * hold and incrementing the dbuf count to ensure that dnode_move() sees
         * a dnode hold for every dbuf.
         */
        rw_exit(&dn->dn_struct_rwlock);

        error = dbuf_read(db, NULL, db_flags);
        if (error) {
                dnode_evict_bonus(dn);
                dbuf_rele(db, tag);
                *dbp = NULL;
                return (error);
        }

        *dbp = &db->db;
        return (0);
}

int
dmu_bonus_hold(objset_t *os, uint64_t object, const void *tag, dmu_buf_t **dbp)
{
        dnode_t *dn;
        int error;

        error = dnode_hold(os, object, FTAG, &dn);
        if (error)
                return (error);

        error = dmu_bonus_hold_by_dnode(dn, tag, dbp, DMU_READ_NO_PREFETCH);
        dnode_rele(dn, FTAG);

        return (error);
}

/*
 * returns ENOENT, EIO, or 0.
 *
 * This interface will allocate a blank spill dbuf when a spill blk
 * doesn't already exist on the dnode.
 *
 * if you only want to find an already existing spill db, then
 * dmu_spill_hold_existing() should be used.
 */
int
dmu_spill_hold_by_dnode(dnode_t *dn, uint32_t flags, const void *tag,
    dmu_buf_t **dbp)
{
        dmu_buf_impl_t *db = NULL;
        int err;

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

        db = dbuf_hold(dn, DMU_SPILL_BLKID, tag);

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

        if (db == NULL) {
                *dbp = NULL;
                return (SET_ERROR(EIO));
        }
        err = dbuf_read(db, NULL, flags);
        if (err == 0)
                *dbp = &db->db;
        else {
                dbuf_rele(db, tag);
                *dbp = NULL;
        }
        return (err);
}

int
dmu_spill_hold_existing(dmu_buf_t *bonus, const void *tag, dmu_buf_t **dbp)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)bonus;
        dnode_t *dn;
        int err;

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

        if (spa_version(dn->dn_objset->os_spa) < SPA_VERSION_SA) {
                err = SET_ERROR(EINVAL);
        } else {
                rw_enter(&dn->dn_struct_rwlock, RW_READER);

                if (!dn->dn_have_spill) {
                        err = SET_ERROR(ENOENT);
                } else {
                        err = dmu_spill_hold_by_dnode(dn,
                            DB_RF_HAVESTRUCT | DB_RF_CANFAIL, tag, dbp);
                }

                rw_exit(&dn->dn_struct_rwlock);
        }

        DB_DNODE_EXIT(db);
        return (err);
}

int
dmu_spill_hold_by_bonus(dmu_buf_t *bonus, uint32_t flags, const void *tag,
    dmu_buf_t **dbp)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)bonus;
        int err;
        uint32_t db_flags = DB_RF_CANFAIL;

        if (flags & DMU_READ_NO_DECRYPT)
                db_flags |= DB_RF_NO_DECRYPT;

        DB_DNODE_ENTER(db);
        err = dmu_spill_hold_by_dnode(DB_DNODE(db), db_flags, tag, dbp);
        DB_DNODE_EXIT(db);

        return (err);
}

/*
 * Note: longer-term, we should modify all of the dmu_buf_*() interfaces
 * to take a held dnode rather than <os, object> -- the lookup is wasteful,
 * and can induce severe lock contention when writing to several files
 * whose dnodes are in the same block.
 */
int
dmu_buf_hold_array_by_dnode(dnode_t *dn, uint64_t offset, uint64_t length,
    boolean_t read, const void *tag, int *numbufsp, dmu_buf_t ***dbpp,
    uint32_t flags)
{
        dmu_buf_t **dbp;
        zstream_t *zs = NULL;
        uint64_t blkid, nblks, i;
        uint32_t dbuf_flags;
        int err;
        zio_t *zio = NULL;
        boolean_t missed = B_FALSE;

        ASSERT(!read || length <= DMU_MAX_ACCESS);

        /*
         * Note: We directly notify the prefetch code of this read, so that
         * we can tell it about the multi-block read.  dbuf_read() only knows
         * about the one block it is accessing.
         */
        dbuf_flags = DB_RF_CANFAIL | DB_RF_NEVERWAIT | DB_RF_HAVESTRUCT |
            DB_RF_NOPREFETCH;

        if ((flags & DMU_READ_NO_DECRYPT) != 0)
                dbuf_flags |= DB_RF_NO_DECRYPT;

        rw_enter(&dn->dn_struct_rwlock, RW_READER);
        if (dn->dn_datablkshift) {
                int blkshift = dn->dn_datablkshift;
                nblks = (P2ROUNDUP(offset + length, 1ULL << blkshift) -
                    P2ALIGN_TYPED(offset, 1ULL << blkshift, uint64_t))
                    >> blkshift;
        } else {
                if (offset + length > dn->dn_datablksz) {
                        zfs_panic_recover("zfs: accessing past end of object "
                            "%llx/%llx (size=%u access=%llu+%llu)",
                            (longlong_t)dn->dn_objset->
                            os_dsl_dataset->ds_object,
                            (longlong_t)dn->dn_object, dn->dn_datablksz,
                            (longlong_t)offset, (longlong_t)length);
                        rw_exit(&dn->dn_struct_rwlock);
                        return (SET_ERROR(EIO));
                }
                nblks = 1;
        }
        dbp = kmem_zalloc(sizeof (dmu_buf_t *) * nblks, KM_SLEEP);

        if (read)
                zio = zio_root(dn->dn_objset->os_spa, NULL, NULL,
                    ZIO_FLAG_CANFAIL);
        blkid = dbuf_whichblock(dn, 0, offset);
        if ((flags & DMU_READ_NO_PREFETCH) == 0) {
                /*
                 * Prepare the zfetch before initiating the demand reads, so
                 * that if multiple threads block on same indirect block, we
                 * base predictions on the original less racy request order.
                 */
                zs = dmu_zfetch_prepare(&dn->dn_zfetch, blkid, nblks, read,
                    B_TRUE);
        }
        for (i = 0; i < nblks; i++) {
                dmu_buf_impl_t *db = dbuf_hold(dn, blkid + i, tag);
                if (db == NULL) {
                        if (zs) {
                                dmu_zfetch_run(&dn->dn_zfetch, zs, missed,
                                    B_TRUE);
                        }
                        rw_exit(&dn->dn_struct_rwlock);
                        dmu_buf_rele_array(dbp, nblks, tag);
                        if (read)
                                zio_nowait(zio);
                        return (SET_ERROR(EIO));
                }

                /*
                 * Initiate async demand data read.
                 * We check the db_state after calling dbuf_read() because
                 * (1) dbuf_read() may change the state to CACHED due to a
                 * hit in the ARC, and (2) on a cache miss, a child will
                 * have been added to "zio" but not yet completed, so the
                 * state will not yet be CACHED.
                 */
                if (read) {
                        if (i == nblks - 1 && blkid + i < dn->dn_maxblkid &&
                            offset + length < db->db.db_offset +
                            db->db.db_size) {
                                if (offset <= db->db.db_offset)
                                        dbuf_flags |= DB_RF_PARTIAL_FIRST;
                                else
                                        dbuf_flags |= DB_RF_PARTIAL_MORE;
                        }
                        (void) dbuf_read(db, zio, dbuf_flags);
                        if (db->db_state != DB_CACHED)
                                missed = B_TRUE;
                }
                dbp[i] = &db->db;
        }

        /*
         * If we are doing O_DIRECT we still hold the dbufs, even for reads,
         * but we do not issue any reads here. We do not want to account for
         * writes in this case.
         *
         * O_DIRECT write/read accounting takes place in
         * dmu_{write/read}_abd().
         */
        if (!read && ((flags & DMU_DIRECTIO) == 0))
                zfs_racct_write(dn->dn_objset->os_spa, length, nblks, flags);

        if (zs)
                dmu_zfetch_run(&dn->dn_zfetch, zs, missed, B_TRUE);
        rw_exit(&dn->dn_struct_rwlock);

        if (read) {
                /* wait for async read i/o */
                err = zio_wait(zio);
                if (err) {
                        dmu_buf_rele_array(dbp, nblks, tag);
                        return (err);
                }

                /* wait for other io to complete */
                for (i = 0; i < nblks; i++) {
                        dmu_buf_impl_t *db = (dmu_buf_impl_t *)dbp[i];
                        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)
                                err = SET_ERROR(EIO);
                        mutex_exit(&db->db_mtx);
                        if (err) {
                                dmu_buf_rele_array(dbp, nblks, tag);
                                return (err);
                        }
                }
        }

        *numbufsp = nblks;
        *dbpp = dbp;
        return (0);
}

int
dmu_buf_hold_array(objset_t *os, uint64_t object, uint64_t offset,
    uint64_t length, int read, const void *tag, int *numbufsp,
    dmu_buf_t ***dbpp)
{
        dnode_t *dn;
        int err;

        err = dnode_hold(os, object, FTAG, &dn);
        if (err)
                return (err);

        err = dmu_buf_hold_array_by_dnode(dn, offset, length, read, tag,
            numbufsp, dbpp, DMU_READ_PREFETCH);

        dnode_rele(dn, FTAG);

        return (err);
}

int
dmu_buf_hold_array_by_bonus(dmu_buf_t *db_fake, uint64_t offset,
    uint64_t length, boolean_t read, const void *tag, int *numbufsp,
    dmu_buf_t ***dbpp)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        int err;

        DB_DNODE_ENTER(db);
        err = dmu_buf_hold_array_by_dnode(DB_DNODE(db), offset, length, read,
            tag, numbufsp, dbpp, DMU_READ_PREFETCH);
        DB_DNODE_EXIT(db);

        return (err);
}

void
dmu_buf_rele_array(dmu_buf_t **dbp_fake, int numbufs, const void *tag)
{
        int i;
        dmu_buf_impl_t **dbp = (dmu_buf_impl_t **)dbp_fake;

        if (numbufs == 0)
                return;

        for (i = 0; i < numbufs; i++) {
                if (dbp[i])
                        dbuf_rele(dbp[i], tag);
        }

        kmem_free(dbp, sizeof (dmu_buf_t *) * numbufs);
}

/*
 * Issue prefetch I/Os for the given blocks.  If level is greater than 0, the
 * indirect blocks prefetched will be those that point to the blocks containing
 * the data starting at offset, and continuing to offset + len.  If the range
 * is too long, prefetch the first dmu_prefetch_max bytes as requested, while
 * for the rest only a higher level, also fitting within dmu_prefetch_max.  It
 * should primarily help random reads, since for long sequential reads there is
 * a speculative prefetcher.
 *
 * Note that if the indirect blocks above the blocks being prefetched are not
 * in cache, they will be asynchronously read in.  Dnode read by dnode_hold()
 * is currently synchronous.
 */
void
dmu_prefetch(objset_t *os, uint64_t object, int64_t level, uint64_t offset,
    uint64_t len, zio_priority_t pri)
{
        dnode_t *dn;

        if (dmu_prefetch_max == 0 || len == 0) {
                dmu_prefetch_dnode(os, object, pri);
                return;
        }

        if (dnode_hold(os, object, FTAG, &dn) != 0)
                return;

        dmu_prefetch_by_dnode(dn, level, offset, len, pri);

        dnode_rele(dn, FTAG);
}

void
dmu_prefetch_by_dnode(dnode_t *dn, int64_t level, uint64_t offset,
    uint64_t len, zio_priority_t pri)
{
        int64_t level2 = level;
        uint64_t start, end, start2, end2;

        /*
         * Depending on len we may do two prefetches: blocks [start, end) at
         * level, and following blocks [start2, end2) at higher level2.
         */
        rw_enter(&dn->dn_struct_rwlock, RW_READER);
        if (dn->dn_datablkshift != 0) {
                /*
                 * The object has multiple blocks.  Calculate the full range
                 * of blocks [start, end2) and then split it into two parts,
                 * so that the first [start, end) fits into dmu_prefetch_max.
                 */
                start = dbuf_whichblock(dn, level, offset);
                end2 = dbuf_whichblock(dn, level, offset + len - 1) + 1;
                uint8_t ibs = dn->dn_indblkshift;
                uint8_t bs = (level == 0) ? dn->dn_datablkshift : ibs;
                uint_t limit = P2ROUNDUP(dmu_prefetch_max, 1 << bs) >> bs;
                start2 = end = MIN(end2, start + limit);

                /*
                 * Find level2 where [start2, end2) fits into dmu_prefetch_max.
                 */
                uint8_t ibps = ibs - SPA_BLKPTRSHIFT;
                limit = P2ROUNDUP(dmu_prefetch_max, 1 << ibs) >> ibs;
                do {
                        level2++;
                        start2 = P2ROUNDUP(start2, 1 << ibps) >> ibps;
                        end2 = P2ROUNDUP(end2, 1 << ibps) >> ibps;
                } while (end2 - start2 > limit);
        } else {
                /* There is only one block.  Prefetch it or nothing. */
                start = start2 = end2 = 0;
                end = start + (level == 0 && offset < dn->dn_datablksz);
        }

        for (uint64_t i = start; i < end; i++)
                dbuf_prefetch(dn, level, i, pri, 0);
        for (uint64_t i = start2; i < end2; i++)
                dbuf_prefetch(dn, level2, i, pri, 0);
        rw_exit(&dn->dn_struct_rwlock);
}

typedef struct {
        kmutex_t        dpa_lock;
        kcondvar_t      dpa_cv;
        uint64_t        dpa_pending_io;
} dmu_prefetch_arg_t;

static void
dmu_prefetch_done(void *arg, uint64_t level, uint64_t blkid, boolean_t issued)
{
        (void) level; (void) blkid; (void)issued;
        dmu_prefetch_arg_t *dpa = arg;

        ASSERT0(level);

        mutex_enter(&dpa->dpa_lock);
        ASSERT3U(dpa->dpa_pending_io, >, 0);
        if (--dpa->dpa_pending_io == 0)
                cv_broadcast(&dpa->dpa_cv);
        mutex_exit(&dpa->dpa_lock);
}

static void
dmu_prefetch_wait_by_dnode(dnode_t *dn, uint64_t offset, uint64_t len)
{
        dmu_prefetch_arg_t dpa;

        mutex_init(&dpa.dpa_lock, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&dpa.dpa_cv, NULL, CV_DEFAULT, NULL);

        rw_enter(&dn->dn_struct_rwlock, RW_READER);

        uint64_t start = dbuf_whichblock(dn, 0, offset);
        uint64_t end = dbuf_whichblock(dn, 0, offset + len - 1) + 1;
        dpa.dpa_pending_io = end - start;

        for (uint64_t blk = start; blk < end; blk++) {
                (void) dbuf_prefetch_impl(dn, 0, blk, ZIO_PRIORITY_ASYNC_READ,
                    0, dmu_prefetch_done, &dpa);
        }

        rw_exit(&dn->dn_struct_rwlock);

        /* wait for prefetch L0 reads to finish */
        mutex_enter(&dpa.dpa_lock);
        while (dpa.dpa_pending_io > 0) {
                cv_wait(&dpa.dpa_cv, &dpa.dpa_lock);

        }
        mutex_exit(&dpa.dpa_lock);

        mutex_destroy(&dpa.dpa_lock);
        cv_destroy(&dpa.dpa_cv);
}

/*
 * Issue prefetch I/Os for the given L0 block range and wait for the I/O
 * to complete. This does not enforce dmu_prefetch_max and will prefetch
 * the entire range. The blocks are read from disk into the ARC but no
 * decompression occurs (i.e., the dbuf cache is not required).
 */
int
dmu_prefetch_wait(objset_t *os, uint64_t object, uint64_t offset, uint64_t size)
{
        dnode_t *dn;
        int err = 0;

        err = dnode_hold(os, object, FTAG, &dn);
        if (err != 0)
                return (err);

        /*
         * Chunk the requests (16 indirects worth) so that we can be interrupted
         */
        uint64_t chunksize;
        if (dn->dn_indblkshift) {
                uint64_t nbps = bp_span_in_blocks(dn->dn_indblkshift, 1);
                chunksize = (nbps * 16) << dn->dn_datablkshift;
        } else {
                chunksize = dn->dn_datablksz;
        }

        while (size > 0) {
                uint64_t mylen = MIN(size, chunksize);

                dmu_prefetch_wait_by_dnode(dn, offset, mylen);

                offset += mylen;
                size -= mylen;

                if (issig()) {
                        err = SET_ERROR(EINTR);
                        break;
                }
        }

        dnode_rele(dn, FTAG);

        return (err);
}

/*
 * Issue prefetch I/Os for the given object's dnode.
 */
void
dmu_prefetch_dnode(objset_t *os, uint64_t object, zio_priority_t pri)
{
        if (object == 0 || object >= DN_MAX_OBJECT)
                return;

        dnode_t *dn = DMU_META_DNODE(os);
        rw_enter(&dn->dn_struct_rwlock, RW_READER);
        uint64_t blkid = dbuf_whichblock(dn, 0, object * sizeof (dnode_phys_t));
        dbuf_prefetch(dn, 0, blkid, pri, 0);
        rw_exit(&dn->dn_struct_rwlock);
}

/*
 * Get the next "chunk" of file data to free.  We traverse the file from
 * the end so that the file gets shorter over time (if we crash in the
 * middle, this will leave us in a better state).  We find allocated file
 * data by simply searching the allocated level 1 indirects.
 *
 * On input, *start should be the first offset that does not need to be
 * freed (e.g. "offset + length").  On return, *start will be the first
 * offset that should be freed and l1blks is set to the number of level 1
 * indirect blocks found within the chunk.
 */
static int
get_next_chunk(dnode_t *dn, uint64_t *start, uint64_t minimum, uint64_t *l1blks)
{
        uint64_t blks;
        uint64_t maxblks = DMU_MAX_ACCESS >> (dn->dn_indblkshift + 1);
        /* bytes of data covered by a level-1 indirect block */
        uint64_t iblkrange = (uint64_t)dn->dn_datablksz *
            EPB(dn->dn_indblkshift, SPA_BLKPTRSHIFT);

        ASSERT3U(minimum, <=, *start);

        /* dn_nlevels == 1 means we don't have any L1 blocks */
        if (dn->dn_nlevels <= 1) {
                *l1blks = 0;
                *start = minimum;
                return (0);
        }

        /*
         * Check if we can free the entire range assuming that all of the
         * L1 blocks in this range have data. If we can, we use this
         * worst case value as an estimate so we can avoid having to look
         * at the object's actual data.
         */
        uint64_t total_l1blks =
            (roundup(*start, iblkrange) - (minimum / iblkrange * iblkrange)) /
            iblkrange;
        if (total_l1blks <= maxblks) {
                *l1blks = total_l1blks;
                *start = minimum;
                return (0);
        }
        ASSERT(ISP2(iblkrange));

        for (blks = 0; *start > minimum && blks < maxblks; blks++) {
                int err;

                /*
                 * dnode_next_offset(BACKWARDS) will find an allocated L1
                 * indirect block at or before the input offset.  We must
                 * decrement *start so that it is at the end of the region
                 * to search.
                 */
                (*start)--;

                err = dnode_next_offset(dn,
                    DNODE_FIND_BACKWARDS, start, 2, 1, 0);

                /* if there are no indirect blocks before start, we are done */
                if (err == ESRCH) {
                        *start = minimum;
                        break;
                } else if (err != 0) {
                        *l1blks = blks;
                        return (err);
                }

                /* set start to the beginning of this L1 indirect */
                *start = P2ALIGN_TYPED(*start, iblkrange, uint64_t);
        }
        if (*start < minimum)
                *start = minimum;
        *l1blks = blks;

        return (0);
}

/*
 * If this objset is of type OST_ZFS return true if vfs's unmounted flag is set,
 * otherwise return false.
 * Used below in dmu_free_long_range_impl() to enable abort when unmounting
 */
static boolean_t
dmu_objset_zfs_unmounting(objset_t *os)
{
#ifdef _KERNEL
        if (dmu_objset_type(os) == DMU_OST_ZFS)
                return (zfs_get_vfs_flag_unmounted(os));
#else
        (void) os;
#endif
        return (B_FALSE);
}

static int
dmu_free_long_range_impl(objset_t *os, dnode_t *dn, uint64_t offset,
    uint64_t length)
{
        uint64_t object_size;
        int err;
        uint64_t dirty_frees_threshold;
        dsl_pool_t *dp = dmu_objset_pool(os);

        if (dn == NULL)
                return (SET_ERROR(EINVAL));

        object_size = (dn->dn_maxblkid + 1) * dn->dn_datablksz;
        if (offset >= object_size)
                return (0);

        if (zfs_per_txg_dirty_frees_percent <= 100)
                dirty_frees_threshold =
                    zfs_per_txg_dirty_frees_percent * zfs_dirty_data_max / 100;
        else
                dirty_frees_threshold = zfs_dirty_data_max / 20;

        if (length == DMU_OBJECT_END || offset + length > object_size)
                length = object_size - offset;

        while (length != 0) {
                uint64_t chunk_end, chunk_begin, chunk_len;
                uint64_t l1blks;
                dmu_tx_t *tx;

                if (dmu_objset_zfs_unmounting(dn->dn_objset))
                        return (SET_ERROR(EINTR));

                chunk_end = chunk_begin = offset + length;

                /* move chunk_begin backwards to the beginning of this chunk */
                err = get_next_chunk(dn, &chunk_begin, offset, &l1blks);
                if (err)
                        return (err);
                ASSERT3U(chunk_begin, >=, offset);
                ASSERT3U(chunk_begin, <=, chunk_end);

                chunk_len = chunk_end - chunk_begin;

                tx = dmu_tx_create(os);
                dmu_tx_hold_free(tx, dn->dn_object, chunk_begin, chunk_len);

                /*
                 * Mark this transaction as typically resulting in a net
                 * reduction in space used.
                 */
                dmu_tx_mark_netfree(tx);
                err = dmu_tx_assign(tx, TXG_WAIT);
                if (err) {
                        dmu_tx_abort(tx);
                        return (err);
                }

                uint64_t txg = dmu_tx_get_txg(tx);

                mutex_enter(&dp->dp_lock);
                uint64_t long_free_dirty =
                    dp->dp_long_free_dirty_pertxg[txg & TXG_MASK];
                mutex_exit(&dp->dp_lock);

                /*
                 * To avoid filling up a TXG with just frees, wait for
                 * the next TXG to open before freeing more chunks if
                 * we have reached the threshold of frees.
                 */
                if (dirty_frees_threshold != 0 &&
                    long_free_dirty >= dirty_frees_threshold) {
                        DMU_TX_STAT_BUMP(dmu_tx_dirty_frees_delay);
                        dmu_tx_commit(tx);
                        txg_wait_open(dp, 0, B_TRUE);
                        continue;
                }

                /*
                 * In order to prevent unnecessary write throttling, for each
                 * TXG, we track the cumulative size of L1 blocks being dirtied
                 * in dnode_free_range() below. We compare this number to a
                 * tunable threshold, past which we prevent new L1 dirty freeing
                 * blocks from being added into the open TXG. See
                 * dmu_free_long_range_impl() for details. The threshold
                 * prevents write throttle activation due to dirty freeing L1
                 * blocks taking up a large percentage of zfs_dirty_data_max.
                 */
                mutex_enter(&dp->dp_lock);
                dp->dp_long_free_dirty_pertxg[txg & TXG_MASK] +=
                    l1blks << dn->dn_indblkshift;
                mutex_exit(&dp->dp_lock);
                DTRACE_PROBE3(free__long__range,
                    uint64_t, long_free_dirty, uint64_t, chunk_len,
                    uint64_t, txg);
                dnode_free_range(dn, chunk_begin, chunk_len, tx);

                dmu_tx_commit(tx);

                length -= chunk_len;
        }
        return (0);
}

int
dmu_free_long_range(objset_t *os, uint64_t object,
    uint64_t offset, uint64_t length)
{
        dnode_t *dn;
        int err;

        err = dnode_hold(os, object, FTAG, &dn);
        if (err != 0)
                return (err);
        err = dmu_free_long_range_impl(os, dn, offset, length);

        /*
         * It is important to zero out the maxblkid when freeing the entire
         * file, so that (a) subsequent calls to dmu_free_long_range_impl()
         * will take the fast path, and (b) dnode_reallocate() can verify
         * that the entire file has been freed.
         */
        if (err == 0 && offset == 0 && length == DMU_OBJECT_END)
                dn->dn_maxblkid = 0;

        dnode_rele(dn, FTAG);
        return (err);
}

int
dmu_free_long_object(objset_t *os, uint64_t object)
{
        dmu_tx_t *tx;
        int err;

        err = dmu_free_long_range(os, object, 0, DMU_OBJECT_END);
        if (err != 0)
                return (err);

        tx = dmu_tx_create(os);
        dmu_tx_hold_bonus(tx, object);
        dmu_tx_hold_free(tx, object, 0, DMU_OBJECT_END);
        dmu_tx_mark_netfree(tx);
        err = dmu_tx_assign(tx, TXG_WAIT);
        if (err == 0) {
                err = dmu_object_free(os, object, tx);
                dmu_tx_commit(tx);
        } else {
                dmu_tx_abort(tx);
        }

        return (err);
}

int
dmu_free_range(objset_t *os, uint64_t object, uint64_t offset,
    uint64_t size, dmu_tx_t *tx)
{
        dnode_t *dn;
        int err = dnode_hold(os, object, FTAG, &dn);
        if (err)
                return (err);
        ASSERT(offset < UINT64_MAX);
        ASSERT(size == DMU_OBJECT_END || size <= UINT64_MAX - offset);
        dnode_free_range(dn, offset, size, tx);
        dnode_rele(dn, FTAG);
        return (0);
}

static int
dmu_read_impl(dnode_t *dn, uint64_t offset, uint64_t size,
    void *buf, uint32_t flags)
{
        dmu_buf_t **dbp;
        int numbufs, err = 0;

        /*
         * Deal with odd block sizes, where there can't be data past the first
         * block. If we ever do the tail block optimization, we will need to
         * handle that here as well.
         */
        if (dn->dn_maxblkid == 0) {
                uint64_t newsz = offset > dn->dn_datablksz ? 0 :
                    MIN(size, dn->dn_datablksz - offset);
                memset((char *)buf + newsz, 0, size - newsz);
                size = newsz;
        }

        if (size == 0)
                return (0);

        /* Allow Direct I/O when requested and properly aligned */
        if ((flags & DMU_DIRECTIO) && zfs_dio_page_aligned(buf) &&
            zfs_dio_aligned(offset, size, PAGESIZE)) {
                abd_t *data = abd_get_from_buf(buf, size);
                err = dmu_read_abd(dn, offset, size, data, flags);
                abd_free(data);
                return (err);
        }

        while (size > 0) {
                uint64_t mylen = MIN(size, DMU_MAX_ACCESS / 2);
                int i;

                /*
                 * NB: we could do this block-at-a-time, but it's nice
                 * to be reading in parallel.
                 */
                err = dmu_buf_hold_array_by_dnode(dn, offset, mylen,
                    TRUE, FTAG, &numbufs, &dbp, flags);
                if (err)
                        break;

                for (i = 0; i < numbufs; i++) {
                        uint64_t tocpy;
                        int64_t bufoff;
                        dmu_buf_t *db = dbp[i];

                        ASSERT(size > 0);

                        bufoff = offset - db->db_offset;
                        tocpy = MIN(db->db_size - bufoff, size);

                        (void) memcpy(buf, (char *)db->db_data + bufoff, tocpy);

                        offset += tocpy;
                        size -= tocpy;
                        buf = (char *)buf + tocpy;
                }
                dmu_buf_rele_array(dbp, numbufs, FTAG);
        }
        return (err);
}

int
dmu_read(objset_t *os, uint64_t object, uint64_t offset, uint64_t size,
    void *buf, uint32_t flags)
{
        dnode_t *dn;
        int err;

        err = dnode_hold(os, object, FTAG, &dn);
        if (err != 0)
                return (err);

        err = dmu_read_impl(dn, offset, size, buf, flags);
        dnode_rele(dn, FTAG);
        return (err);
}

int
dmu_read_by_dnode(dnode_t *dn, uint64_t offset, uint64_t size, void *buf,
    uint32_t flags)
{
        return (dmu_read_impl(dn, offset, size, buf, flags));
}

static void
dmu_write_impl(dmu_buf_t **dbp, int numbufs, uint64_t offset, uint64_t size,
    const void *buf, dmu_tx_t *tx)
{
        int i;

        for (i = 0; i < numbufs; i++) {
                uint64_t tocpy;
                int64_t bufoff;
                dmu_buf_t *db = dbp[i];

                ASSERT(size > 0);

                bufoff = offset - db->db_offset;
                tocpy = MIN(db->db_size - bufoff, size);

                ASSERT(i == 0 || i == numbufs-1 || tocpy == db->db_size);

                if (tocpy == db->db_size)
                        dmu_buf_will_fill(db, tx, B_FALSE);
                else
                        dmu_buf_will_dirty(db, tx);

                (void) memcpy((char *)db->db_data + bufoff, buf, tocpy);

                if (tocpy == db->db_size)
                        dmu_buf_fill_done(db, tx, B_FALSE);

                offset += tocpy;
                size -= tocpy;
                buf = (char *)buf + tocpy;
        }
}

void
dmu_write(objset_t *os, uint64_t object, uint64_t offset, uint64_t size,
    const void *buf, dmu_tx_t *tx)
{
        dmu_buf_t **dbp;
        int numbufs;

        if (size == 0)
                return;

        VERIFY0(dmu_buf_hold_array(os, object, offset, size,
            FALSE, FTAG, &numbufs, &dbp));
        dmu_write_impl(dbp, numbufs, offset, size, buf, tx);
        dmu_buf_rele_array(dbp, numbufs, FTAG);
}

/*
 * This interface is not used internally by ZFS but is provided for
 * use by Lustre which is built on the DMU interfaces.
 */
int
dmu_write_by_dnode_flags(dnode_t *dn, uint64_t offset, uint64_t size,
    const void *buf, dmu_tx_t *tx, uint32_t flags)
{
        dmu_buf_t **dbp;
        int numbufs;
        int error;

        if (size == 0)
                return (0);

        /* Allow Direct I/O when requested and properly aligned */
        if ((flags & DMU_DIRECTIO) && zfs_dio_page_aligned((void *)buf) &&
            zfs_dio_aligned(offset, size, dn->dn_datablksz)) {
                abd_t *data = abd_get_from_buf((void *)buf, size);
                error = dmu_write_abd(dn, offset, size, data, DMU_DIRECTIO, tx);
                abd_free(data);
                return (error);
        }

        VERIFY0(dmu_buf_hold_array_by_dnode(dn, offset, size,
            FALSE, FTAG, &numbufs, &dbp, DMU_READ_PREFETCH));
        dmu_write_impl(dbp, numbufs, offset, size, buf, tx);
        dmu_buf_rele_array(dbp, numbufs, FTAG);
        return (0);
}

int
dmu_write_by_dnode(dnode_t *dn, uint64_t offset, uint64_t size,
    const void *buf, dmu_tx_t *tx)
{
        return (dmu_write_by_dnode_flags(dn, offset, size, buf, tx, 0));
}

void
dmu_prealloc(objset_t *os, uint64_t object, uint64_t offset, uint64_t size,
    dmu_tx_t *tx)
{
        dmu_buf_t **dbp;
        int numbufs, i;

        if (size == 0)
                return;

        VERIFY(0 == dmu_buf_hold_array(os, object, offset, size,
            FALSE, FTAG, &numbufs, &dbp));

        for (i = 0; i < numbufs; i++) {
                dmu_buf_t *db = dbp[i];

                dmu_buf_will_not_fill(db, tx);
        }
        dmu_buf_rele_array(dbp, numbufs, FTAG);
}

void
dmu_write_embedded(objset_t *os, uint64_t object, uint64_t offset,
    void *data, uint8_t etype, uint8_t comp, int uncompressed_size,
    int compressed_size, int byteorder, dmu_tx_t *tx)
{
        dmu_buf_t *db;

        ASSERT3U(etype, <, NUM_BP_EMBEDDED_TYPES);
        ASSERT3U(comp, <, ZIO_COMPRESS_FUNCTIONS);
        VERIFY0(dmu_buf_hold_noread(os, object, offset,
            FTAG, &db));

        dmu_buf_write_embedded(db,
            data, (bp_embedded_type_t)etype, (enum zio_compress)comp,
            uncompressed_size, compressed_size, byteorder, tx);

        dmu_buf_rele(db, FTAG);
}

void
dmu_redact(objset_t *os, uint64_t object, uint64_t offset, uint64_t size,
    dmu_tx_t *tx)
{
        int numbufs, i;
        dmu_buf_t **dbp;

        VERIFY0(dmu_buf_hold_array(os, object, offset, size, FALSE, FTAG,
            &numbufs, &dbp));
        for (i = 0; i < numbufs; i++)
                dmu_buf_redact(dbp[i], tx);
        dmu_buf_rele_array(dbp, numbufs, FTAG);
}

#ifdef _KERNEL
int
dmu_read_uio_dnode(dnode_t *dn, zfs_uio_t *uio, uint64_t size)
{
        dmu_buf_t **dbp;
        int numbufs, i, err;

        if (uio->uio_extflg & UIO_DIRECT)
                return (dmu_read_uio_direct(dn, uio, size));

        /*
         * NB: we could do this block-at-a-time, but it's nice
         * to be reading in parallel.
         */
        err = dmu_buf_hold_array_by_dnode(dn, zfs_uio_offset(uio), size,
            TRUE, FTAG, &numbufs, &dbp, 0);
        if (err)
                return (err);

        for (i = 0; i < numbufs; i++) {
                uint64_t tocpy;
                int64_t bufoff;
                dmu_buf_t *db = dbp[i];

                ASSERT(size > 0);

                bufoff = zfs_uio_offset(uio) - db->db_offset;
                tocpy = MIN(db->db_size - bufoff, size);

                err = zfs_uio_fault_move((char *)db->db_data + bufoff, tocpy,
                    UIO_READ, uio);

                if (err)
                        break;

                size -= tocpy;
        }
        dmu_buf_rele_array(dbp, numbufs, FTAG);

        return (err);
}

/*
 * Read 'size' bytes into the uio buffer.
 * From object zdb->db_object.
 * Starting at zfs_uio_offset(uio).
 *
 * If the caller already has a dbuf in the target object
 * (e.g. its bonus buffer), this routine is faster than dmu_read_uio(),
 * because we don't have to find the dnode_t for the object.
 */
int
dmu_read_uio_dbuf(dmu_buf_t *zdb, zfs_uio_t *uio, uint64_t size)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)zdb;
        int err;

        if (size == 0)
                return (0);

        DB_DNODE_ENTER(db);
        err = dmu_read_uio_dnode(DB_DNODE(db), uio, size);
        DB_DNODE_EXIT(db);

        return (err);
}

/*
 * Read 'size' bytes into the uio buffer.
 * From the specified object
 * Starting at offset zfs_uio_offset(uio).
 */
int
dmu_read_uio(objset_t *os, uint64_t object, zfs_uio_t *uio, uint64_t size)
{
        dnode_t *dn;
        int err;

        if (size == 0)
                return (0);

        err = dnode_hold(os, object, FTAG, &dn);
        if (err)
                return (err);

        err = dmu_read_uio_dnode(dn, uio, size);

        dnode_rele(dn, FTAG);

        return (err);
}

int
dmu_write_uio_dnode(dnode_t *dn, zfs_uio_t *uio, uint64_t size, dmu_tx_t *tx)
{
        dmu_buf_t **dbp;
        int numbufs;
        int err = 0;
        uint64_t write_size;

top:
        write_size = size;

        /*
         * We only allow Direct I/O writes to happen if we are block
         * sized aligned. Otherwise, we pass the write off to the ARC.
         */
        if ((uio->uio_extflg & UIO_DIRECT) &&
            (write_size >= dn->dn_datablksz)) {
                if (zfs_dio_aligned(zfs_uio_offset(uio), write_size,
                    dn->dn_datablksz)) {
                        return (dmu_write_uio_direct(dn, uio, size, tx));
                } else if (write_size > dn->dn_datablksz &&
                    zfs_dio_offset_aligned(zfs_uio_offset(uio),
                    dn->dn_datablksz)) {
                        write_size =
                            dn->dn_datablksz * (write_size / dn->dn_datablksz);
                        err = dmu_write_uio_direct(dn, uio, write_size, tx);
                        if (err == 0) {
                                size -= write_size;
                                goto top;
                        } else {
                                return (err);
                        }
                } else {
                        write_size =
                            P2PHASE(zfs_uio_offset(uio), dn->dn_datablksz);
                }
        }

        err = dmu_buf_hold_array_by_dnode(dn, zfs_uio_offset(uio), write_size,
            FALSE, FTAG, &numbufs, &dbp, DMU_READ_PREFETCH);
        if (err)
                return (err);

        for (int i = 0; i < numbufs; i++) {
                uint64_t tocpy;
                int64_t bufoff;
                dmu_buf_t *db = dbp[i];

                ASSERT(write_size > 0);

                offset_t off = zfs_uio_offset(uio);
                bufoff = off - db->db_offset;
                tocpy = MIN(db->db_size - bufoff, write_size);

                ASSERT(i == 0 || i == numbufs-1 || tocpy == db->db_size);

                if (tocpy == db->db_size)
                        dmu_buf_will_fill(db, tx, B_TRUE);
                else
                        dmu_buf_will_dirty(db, tx);

                err = zfs_uio_fault_move((char *)db->db_data + bufoff,
                    tocpy, UIO_WRITE, uio);

                if (tocpy == db->db_size && dmu_buf_fill_done(db, tx, err)) {
                        /* The fill was reverted.  Undo any uio progress. */
                        zfs_uio_advance(uio, off - zfs_uio_offset(uio));
                }

                if (err)
                        break;

                write_size -= tocpy;
                size -= tocpy;
        }

        IMPLY(err == 0, write_size == 0);

        dmu_buf_rele_array(dbp, numbufs, FTAG);

        if ((uio->uio_extflg & UIO_DIRECT) && size > 0) {
                goto top;
        }

        return (err);
}

/*
 * Write 'size' bytes from the uio buffer.
 * To object zdb->db_object.
 * Starting at offset zfs_uio_offset(uio).
 *
 * If the caller already has a dbuf in the target object
 * (e.g. its bonus buffer), this routine is faster than dmu_write_uio(),
 * because we don't have to find the dnode_t for the object.
 */
int
dmu_write_uio_dbuf(dmu_buf_t *zdb, zfs_uio_t *uio, uint64_t size,
    dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)zdb;
        int err;

        if (size == 0)
                return (0);

        DB_DNODE_ENTER(db);
        err = dmu_write_uio_dnode(DB_DNODE(db), uio, size, tx);
        DB_DNODE_EXIT(db);

        return (err);
}

/*
 * Write 'size' bytes from the uio buffer.
 * To the specified object.
 * Starting at offset zfs_uio_offset(uio).
 */
int
dmu_write_uio(objset_t *os, uint64_t object, zfs_uio_t *uio, uint64_t size,
    dmu_tx_t *tx)
{
        dnode_t *dn;
        int err;

        if (size == 0)
                return (0);

        err = dnode_hold(os, object, FTAG, &dn);
        if (err)
                return (err);

        err = dmu_write_uio_dnode(dn, uio, size, tx);

        dnode_rele(dn, FTAG);

        return (err);
}
#endif /* _KERNEL */

static void
dmu_cached_bps(spa_t *spa, blkptr_t *bps, uint_t nbps,
    uint64_t *l1sz, uint64_t *l2sz)
{
        int cached_flags;

        if (bps == NULL)
                return;

        for (size_t blk_off = 0; blk_off < nbps; blk_off++) {
                blkptr_t *bp = &bps[blk_off];

                if (BP_IS_HOLE(bp))
                        continue;

                cached_flags = arc_cached(spa, bp);
                if (cached_flags == 0)
                        continue;

                if ((cached_flags & (ARC_CACHED_IN_L1 | ARC_CACHED_IN_L2)) ==
                    ARC_CACHED_IN_L2)
                        *l2sz += BP_GET_LSIZE(bp);
                else
                        *l1sz += BP_GET_LSIZE(bp);
        }
}

/*
 * Estimate DMU object cached size.
 */
int
dmu_object_cached_size(objset_t *os, uint64_t object,
    uint64_t *l1sz, uint64_t *l2sz)
{
        dnode_t *dn;
        dmu_object_info_t doi;
        int err = 0;

        *l1sz = *l2sz = 0;

        if (dnode_hold(os, object, FTAG, &dn) != 0)
                return (0);

        if (dn->dn_nlevels < 2) {
                dnode_rele(dn, FTAG);
                return (0);
        }

        dmu_object_info_from_dnode(dn, &doi);

        for (uint64_t off = 0; off < doi.doi_max_offset;
            off += dmu_prefetch_max) {
                /* dbuf_read doesn't prefetch L1 blocks. */
                dmu_prefetch_by_dnode(dn, 1, off,
                    dmu_prefetch_max, ZIO_PRIORITY_SYNC_READ);
        }

        /*
         * Hold all valid L1 blocks, asking ARC the status of each BP
         * contained in each such L1 block.
         */
        uint_t nbps = bp_span_in_blocks(dn->dn_indblkshift, 1);
        uint64_t l1blks = 1 + (dn->dn_maxblkid / nbps);

        rw_enter(&dn->dn_struct_rwlock, RW_READER);
        for (uint64_t blk = 0; blk < l1blks; blk++) {
                dmu_buf_impl_t *db = NULL;

                if (issig()) {
                        /*
                         * On interrupt, get out, and bubble up EINTR
                         */
                        err = EINTR;
                        break;
                }

                /*
                 * If we get an i/o error here, the L1 can't be read,
                 * and nothing under it could be cached, so we just
                 * continue. Ignoring the error from dbuf_hold_impl
                 * or from dbuf_read is then a reasonable choice.
                 */
                err = dbuf_hold_impl(dn, 1, blk, B_TRUE, B_FALSE, FTAG, &db);
                if (err != 0) {
                        /*
                         * ignore error and continue
                         */
                        err = 0;
                        continue;
                }

                err = dbuf_read(db, NULL, DB_RF_CANFAIL);
                if (err == 0) {
                        dmu_cached_bps(dmu_objset_spa(os), db->db.db_data,
                            nbps, l1sz, l2sz);
                }
                /*
                 * error may be ignored, and we continue
                 */
                err = 0;
                dbuf_rele(db, FTAG);
        }
        rw_exit(&dn->dn_struct_rwlock);

        dnode_rele(dn, FTAG);
        return (err);
}

/*
 * Allocate a loaned anonymous arc buffer.
 */
arc_buf_t *
dmu_request_arcbuf(dmu_buf_t *handle, int size)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)handle;

        return (arc_loan_buf(db->db_objset->os_spa, B_FALSE, size));
}

/*
 * Free a loaned arc buffer.
 */
void
dmu_return_arcbuf(arc_buf_t *buf)
{
        arc_return_buf(buf, FTAG);
        arc_buf_destroy(buf, FTAG);
}

/*
 * A "lightweight" write is faster than a regular write (e.g.
 * dmu_write_by_dnode() or dmu_assign_arcbuf_by_dnode()), because it avoids the
 * CPU cost of creating a dmu_buf_impl_t and arc_buf_[hdr_]_t.  However, the
 * data can not be read or overwritten until the transaction's txg has been
 * synced.  This makes it appropriate for workloads that are known to be
 * (temporarily) write-only, like "zfs receive".
 *
 * A single block is written, starting at the specified offset in bytes.  If
 * the call is successful, it returns 0 and the provided abd has been
 * consumed (the caller should not free it).
 */
int
dmu_lightweight_write_by_dnode(dnode_t *dn, uint64_t offset, abd_t *abd,
    const zio_prop_t *zp, zio_flag_t flags, dmu_tx_t *tx)
{
        dbuf_dirty_record_t *dr =
            dbuf_dirty_lightweight(dn, dbuf_whichblock(dn, 0, offset), tx);
        if (dr == NULL)
                return (SET_ERROR(EIO));
        dr->dt.dll.dr_abd = abd;
        dr->dt.dll.dr_props = *zp;
        dr->dt.dll.dr_flags = flags;
        return (0);
}

/*
 * When possible directly assign passed loaned arc buffer to a dbuf.
 * If this is not possible copy the contents of passed arc buf via
 * dmu_write().
 */
int
dmu_assign_arcbuf_by_dnode(dnode_t *dn, uint64_t offset, arc_buf_t *buf,
    dmu_tx_t *tx)
{
        dmu_buf_impl_t *db;
        objset_t *os = dn->dn_objset;
        uint64_t object = dn->dn_object;
        uint32_t blksz = (uint32_t)arc_buf_lsize(buf);
        uint64_t blkid;

        rw_enter(&dn->dn_struct_rwlock, RW_READER);
        blkid = dbuf_whichblock(dn, 0, offset);
        db = dbuf_hold(dn, blkid, FTAG);
        rw_exit(&dn->dn_struct_rwlock);
        if (db == NULL)
                return (SET_ERROR(EIO));

        /*
         * We can only assign if the offset is aligned and the arc buf is the
         * same size as the dbuf.
         */
        if (offset == db->db.db_offset && blksz == db->db.db_size) {
                zfs_racct_write(os->os_spa, blksz, 1, 0);
                dbuf_assign_arcbuf(db, buf, tx);
                dbuf_rele(db, FTAG);
        } else {
                /* compressed bufs must always be assignable to their dbuf */
                ASSERT3U(arc_get_compression(buf), ==, ZIO_COMPRESS_OFF);
                ASSERT(!(buf->b_flags & ARC_BUF_FLAG_COMPRESSED));

                dbuf_rele(db, FTAG);
                dmu_write(os, object, offset, blksz, buf->b_data, tx);
                dmu_return_arcbuf(buf);
        }

        return (0);
}

int
dmu_assign_arcbuf_by_dbuf(dmu_buf_t *handle, uint64_t offset, arc_buf_t *buf,
    dmu_tx_t *tx)
{
        int err;
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)handle;

        DB_DNODE_ENTER(db);
        err = dmu_assign_arcbuf_by_dnode(DB_DNODE(db), offset, buf, tx);
        DB_DNODE_EXIT(db);

        return (err);
}

void
dmu_sync_ready(zio_t *zio, arc_buf_t *buf, void *varg)
{
        (void) buf;
        dmu_sync_arg_t *dsa = varg;

        if (zio->io_error == 0) {
                dbuf_dirty_record_t *dr = dsa->dsa_dr;
                blkptr_t *bp = zio->io_bp;

                if (BP_IS_HOLE(bp)) {
                        dmu_buf_t *db = NULL;
                        if (dr)
                                db = &(dr->dr_dbuf->db);
                        else
                                db = dsa->dsa_zgd->zgd_db;
                        /*
                         * A block of zeros may compress to a hole, but the
                         * block size still needs to be known for replay.
                         */
                        BP_SET_LSIZE(bp, db->db_size);
                } else if (!BP_IS_EMBEDDED(bp)) {
                        ASSERT(BP_GET_LEVEL(bp) == 0);
                        BP_SET_FILL(bp, 1);
                }
        }
}

static void
dmu_sync_late_arrival_ready(zio_t *zio)
{
        dmu_sync_ready(zio, NULL, zio->io_private);
}

void
dmu_sync_done(zio_t *zio, arc_buf_t *buf, void *varg)
{
        (void) buf;
        dmu_sync_arg_t *dsa = varg;
        dbuf_dirty_record_t *dr = dsa->dsa_dr;
        dmu_buf_impl_t *db = dr->dr_dbuf;
        zgd_t *zgd = dsa->dsa_zgd;

        /*
         * Record the vdev(s) backing this blkptr so they can be flushed after
         * the writes for the lwb have completed.
         */
        if (zgd && zio->io_error == 0) {
                zil_lwb_add_block(zgd->zgd_lwb, zgd->zgd_bp);
        }

        mutex_enter(&db->db_mtx);
        ASSERT(dr->dt.dl.dr_override_state == DR_IN_DMU_SYNC);
        if (zio->io_error == 0) {
                ASSERT0(dr->dt.dl.dr_has_raw_params);
                dr->dt.dl.dr_nopwrite = !!(zio->io_flags & ZIO_FLAG_NOPWRITE);
                if (dr->dt.dl.dr_nopwrite) {
                        blkptr_t *bp = zio->io_bp;
                        blkptr_t *bp_orig = &zio->io_bp_orig;
                        uint8_t chksum = BP_GET_CHECKSUM(bp_orig);

                        ASSERT(BP_EQUAL(bp, bp_orig));
                        VERIFY(BP_EQUAL(bp, db->db_blkptr));
                        ASSERT(zio->io_prop.zp_compress != ZIO_COMPRESS_OFF);
                        VERIFY(zio_checksum_table[chksum].ci_flags &
                            ZCHECKSUM_FLAG_NOPWRITE);
                }
                dr->dt.dl.dr_overridden_by = *zio->io_bp;
                dr->dt.dl.dr_override_state = DR_OVERRIDDEN;
                dr->dt.dl.dr_copies = zio->io_prop.zp_copies;

                /*
                 * Old style holes are filled with all zeros, whereas
                 * new-style holes maintain their lsize, type, level,
                 * and birth time (see zio_write_compress). While we
                 * need to reset the BP_SET_LSIZE() call that happened
                 * in dmu_sync_ready for old style holes, we do *not*
                 * want to wipe out the information contained in new
                 * style holes. Thus, only zero out the block pointer if
                 * it's an old style hole.
                 */
                if (BP_IS_HOLE(&dr->dt.dl.dr_overridden_by) &&
                    BP_GET_LOGICAL_BIRTH(&dr->dt.dl.dr_overridden_by) == 0)
                        BP_ZERO(&dr->dt.dl.dr_overridden_by);
        } else {
                dr->dt.dl.dr_override_state = DR_NOT_OVERRIDDEN;
        }

        cv_broadcast(&db->db_changed);
        mutex_exit(&db->db_mtx);

        if (dsa->dsa_done)
                dsa->dsa_done(dsa->dsa_zgd, zio->io_error);

        kmem_free(dsa, sizeof (*dsa));
}

static void
dmu_sync_late_arrival_done(zio_t *zio)
{
        blkptr_t *bp = zio->io_bp;
        dmu_sync_arg_t *dsa = zio->io_private;
        zgd_t *zgd = dsa->dsa_zgd;

        if (zio->io_error == 0) {
                /*
                 * Record the vdev(s) backing this blkptr so they can be
                 * flushed after the writes for the lwb have completed.
                 */
                zil_lwb_add_block(zgd->zgd_lwb, zgd->zgd_bp);

                if (!BP_IS_HOLE(bp)) {
                        blkptr_t *bp_orig __maybe_unused = &zio->io_bp_orig;
                        ASSERT(!(zio->io_flags & ZIO_FLAG_NOPWRITE));
                        ASSERT(BP_IS_HOLE(bp_orig) || !BP_EQUAL(bp, bp_orig));
                        ASSERT(BP_GET_LOGICAL_BIRTH(zio->io_bp) == zio->io_txg);
                        ASSERT(zio->io_txg > spa_syncing_txg(zio->io_spa));
                        zio_free(zio->io_spa, zio->io_txg, zio->io_bp);
                }
        }

        dmu_tx_commit(dsa->dsa_tx);

        dsa->dsa_done(dsa->dsa_zgd, zio->io_error);

        abd_free(zio->io_abd);
        kmem_free(dsa, sizeof (*dsa));
}

static int
dmu_sync_late_arrival(zio_t *pio, objset_t *os, dmu_sync_cb_t *done, zgd_t *zgd,
    zio_prop_t *zp, zbookmark_phys_t *zb)
{
        dmu_sync_arg_t *dsa;
        dmu_tx_t *tx;
        int error;

        error = dbuf_read((dmu_buf_impl_t *)zgd->zgd_db, NULL,
            DB_RF_CANFAIL | DB_RF_NOPREFETCH);
        if (error != 0)
                return (error);

        tx = dmu_tx_create(os);
        dmu_tx_hold_space(tx, zgd->zgd_db->db_size);
        /*
         * This transaction does not produce any dirty data or log blocks, so
         * it should not be throttled.  All other cases wait for TXG sync, by
         * which time the log block we are writing will be obsolete, so we can
         * skip waiting and just return error here instead.
         */
        if (dmu_tx_assign(tx, TXG_NOWAIT | TXG_NOTHROTTLE) != 0) {
                dmu_tx_abort(tx);
                /* Make zl_get_data do txg_waited_synced() */
                return (SET_ERROR(EIO));
        }

        /*
         * In order to prevent the zgd's lwb from being free'd prior to
         * dmu_sync_late_arrival_done() being called, we have to ensure
         * the lwb's "max txg" takes this tx's txg into account.
         */
        zil_lwb_add_txg(zgd->zgd_lwb, dmu_tx_get_txg(tx));

        dsa = kmem_alloc(sizeof (dmu_sync_arg_t), KM_SLEEP);
        dsa->dsa_dr = NULL;
        dsa->dsa_done = done;
        dsa->dsa_zgd = zgd;
        dsa->dsa_tx = tx;

        /*
         * Since we are currently syncing this txg, it's nontrivial to
         * determine what BP to nopwrite against, so we disable nopwrite.
         *
         * When syncing, the db_blkptr is initially the BP of the previous
         * txg.  We can not nopwrite against it because it will be changed
         * (this is similar to the non-late-arrival case where the dbuf is
         * dirty in a future txg).
         *
         * Then dbuf_write_ready() sets bp_blkptr to the location we will write.
         * We can not nopwrite against it because although the BP will not
         * (typically) be changed, the data has not yet been persisted to this
         * location.
         *
         * Finally, when dbuf_write_done() is called, it is theoretically
         * possible to always nopwrite, because the data that was written in
         * this txg is the same data that we are trying to write.  However we
         * would need to check that this dbuf is not dirty in any future
         * txg's (as we do in the normal dmu_sync() path). For simplicity, we
         * don't nopwrite in this case.
         */
        zp->zp_nopwrite = B_FALSE;

        zio_nowait(zio_write(pio, os->os_spa, dmu_tx_get_txg(tx), zgd->zgd_bp,
            abd_get_from_buf(zgd->zgd_db->db_data, zgd->zgd_db->db_size),
            zgd->zgd_db->db_size, zgd->zgd_db->db_size, zp,
            dmu_sync_late_arrival_ready, NULL, dmu_sync_late_arrival_done,
            dsa, ZIO_PRIORITY_SYNC_WRITE, ZIO_FLAG_CANFAIL, zb));

        return (0);
}

/*
 * Intent log support: sync the block associated with db to disk.
 * N.B. and XXX: the caller is responsible for making sure that the
 * data isn't changing while dmu_sync() is writing it.
 *
 * Return values:
 *
 *      EEXIST: this txg has already been synced, so there's nothing to do.
 *              The caller should not log the write.
 *
 *      ENOENT: the block was dbuf_free_range()'d, so there's nothing to do.
 *              The caller should not log the write.
 *
 *      EALREADY: this block is already in the process of being synced.
 *              The caller should track its progress (somehow).
 *
 *      EIO: could not do the I/O.
 *              The caller should do a txg_wait_synced().
 *
 *      0: the I/O has been initiated.
 *              The caller should log this blkptr in the done callback.
 *              It is possible that the I/O will fail, in which case
 *              the error will be reported to the done callback and
 *              propagated to pio from zio_done().
 */
int
dmu_sync(zio_t *pio, uint64_t txg, dmu_sync_cb_t *done, zgd_t *zgd)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)zgd->zgd_db;
        objset_t *os = db->db_objset;
        dsl_dataset_t *ds = os->os_dsl_dataset;
        dbuf_dirty_record_t *dr, *dr_next;
        dmu_sync_arg_t *dsa;
        zbookmark_phys_t zb;
        zio_prop_t zp;

        ASSERT(pio != NULL);
        ASSERT(txg != 0);

        SET_BOOKMARK(&zb, ds->ds_object,
            db->db.db_object, db->db_level, db->db_blkid);

        DB_DNODE_ENTER(db);
        dmu_write_policy(os, DB_DNODE(db), db->db_level, WP_DMU_SYNC, &zp);
        DB_DNODE_EXIT(db);

        /*
         * If we're frozen (running ziltest), we always need to generate a bp.
         */
        if (txg > spa_freeze_txg(os->os_spa))
                return (dmu_sync_late_arrival(pio, os, done, zgd, &zp, &zb));

        /*
         * Grabbing db_mtx now provides a barrier between dbuf_sync_leaf()
         * and us.  If we determine that this txg is not yet syncing,
         * but it begins to sync a moment later, that's OK because the
         * sync thread will block in dbuf_sync_leaf() until we drop db_mtx.
         */
        mutex_enter(&db->db_mtx);

        if (txg <= spa_last_synced_txg(os->os_spa)) {
                /*
                 * This txg has already synced.  There's nothing to do.
                 */
                mutex_exit(&db->db_mtx);
                return (SET_ERROR(EEXIST));
        }

        if (txg <= spa_syncing_txg(os->os_spa)) {
                /*
                 * This txg is currently syncing, so we can't mess with
                 * the dirty record anymore; just write a new log block.
                 */
                mutex_exit(&db->db_mtx);
                return (dmu_sync_late_arrival(pio, os, done, zgd, &zp, &zb));
        }

        dr = dbuf_find_dirty_eq(db, txg);

        if (dr == NULL) {
                /*
                 * There's no dr for this dbuf, so it must have been freed.
                 * There's no need to log writes to freed blocks, so we're done.
                 */
                mutex_exit(&db->db_mtx);
                return (SET_ERROR(ENOENT));
        }

        dr_next = list_next(&db->db_dirty_records, dr);
        ASSERT(dr_next == NULL || dr_next->dr_txg < txg);

        if (db->db_blkptr != NULL) {
                /*
                 * We need to fill in zgd_bp with the current blkptr so that
                 * the nopwrite code can check if we're writing the same
                 * data that's already on disk.  We can only nopwrite if we
                 * are sure that after making the copy, db_blkptr will not
                 * change until our i/o completes.  We ensure this by
                 * holding the db_mtx, and only allowing nopwrite if the
                 * block is not already dirty (see below).  This is verified
                 * by dmu_sync_done(), which VERIFYs that the db_blkptr has
                 * not changed.
                 */
                *zgd->zgd_bp = *db->db_blkptr;
        }

        /*
         * Assume the on-disk data is X, the current syncing data (in
         * txg - 1) is Y, and the current in-memory data is Z (currently
         * in dmu_sync).
         *
         * We usually want to perform a nopwrite if X and Z are the
         * same.  However, if Y is different (i.e. the BP is going to
         * change before this write takes effect), then a nopwrite will
         * be incorrect - we would override with X, which could have
         * been freed when Y was written.
         *
         * (Note that this is not a concern when we are nop-writing from
         * syncing context, because X and Y must be identical, because
         * all previous txgs have been synced.)
         *
         * Therefore, we disable nopwrite if the current BP could change
         * before this TXG.  There are two ways it could change: by
         * being dirty (dr_next is non-NULL), or by being freed
         * (dnode_block_freed()).  This behavior is verified by
         * zio_done(), which VERIFYs that the override BP is identical
         * to the on-disk BP.
         */
        if (dr_next != NULL) {
                zp.zp_nopwrite = B_FALSE;
        } else {
                DB_DNODE_ENTER(db);
                if (dnode_block_freed(DB_DNODE(db), db->db_blkid))
                        zp.zp_nopwrite = B_FALSE;
                DB_DNODE_EXIT(db);
        }

        ASSERT(dr->dr_txg == txg);
        if (dr->dt.dl.dr_override_state == DR_IN_DMU_SYNC ||
            dr->dt.dl.dr_override_state == DR_OVERRIDDEN) {
                /*
                 * We have already issued a sync write for this buffer,
                 * or this buffer has already been synced.  It could not
                 * have been dirtied since, or we would have cleared the state.
                 */
                mutex_exit(&db->db_mtx);
                return (SET_ERROR(EALREADY));
        }

        ASSERT0(dr->dt.dl.dr_has_raw_params);
        ASSERT(dr->dt.dl.dr_override_state == DR_NOT_OVERRIDDEN);
        dr->dt.dl.dr_override_state = DR_IN_DMU_SYNC;
        mutex_exit(&db->db_mtx);

        dsa = kmem_alloc(sizeof (dmu_sync_arg_t), KM_SLEEP);
        dsa->dsa_dr = dr;
        dsa->dsa_done = done;
        dsa->dsa_zgd = zgd;
        dsa->dsa_tx = NULL;

        zio_nowait(arc_write(pio, os->os_spa, txg, zgd->zgd_bp,
            dr->dt.dl.dr_data, !DBUF_IS_CACHEABLE(db),
            dbuf_is_l2cacheable(db, NULL), &zp, dmu_sync_ready, NULL,
            dmu_sync_done, dsa, ZIO_PRIORITY_SYNC_WRITE, ZIO_FLAG_CANFAIL,
            &zb));

        return (0);
}

int
dmu_object_set_nlevels(objset_t *os, uint64_t object, int nlevels, dmu_tx_t *tx)
{
        dnode_t *dn;
        int err;

        err = dnode_hold(os, object, FTAG, &dn);
        if (err)
                return (err);
        err = dnode_set_nlevels(dn, nlevels, tx);
        dnode_rele(dn, FTAG);
        return (err);
}

int
dmu_object_set_blocksize(objset_t *os, uint64_t object, uint64_t size, int ibs,
    dmu_tx_t *tx)
{
        dnode_t *dn;
        int err;

        err = dnode_hold(os, object, FTAG, &dn);
        if (err)
                return (err);
        err = dnode_set_blksz(dn, size, ibs, tx);
        dnode_rele(dn, FTAG);
        return (err);
}

int
dmu_object_set_maxblkid(objset_t *os, uint64_t object, uint64_t maxblkid,
    dmu_tx_t *tx)
{
        dnode_t *dn;
        int err;

        err = dnode_hold(os, object, FTAG, &dn);
        if (err)
                return (err);
        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
        dnode_new_blkid(dn, maxblkid, tx, B_FALSE, B_TRUE);
        rw_exit(&dn->dn_struct_rwlock);
        dnode_rele(dn, FTAG);
        return (0);
}

void
dmu_object_set_checksum(objset_t *os, uint64_t object, uint8_t checksum,
    dmu_tx_t *tx)
{
        dnode_t *dn;

        /*
         * Send streams include each object's checksum function.  This
         * check ensures that the receiving system can understand the
         * checksum function transmitted.
         */
        ASSERT3U(checksum, <, ZIO_CHECKSUM_LEGACY_FUNCTIONS);

        VERIFY0(dnode_hold(os, object, FTAG, &dn));
        ASSERT3U(checksum, <, ZIO_CHECKSUM_FUNCTIONS);
        dn->dn_checksum = checksum;
        dnode_setdirty(dn, tx);
        dnode_rele(dn, FTAG);
}

void
dmu_object_set_compress(objset_t *os, uint64_t object, uint8_t compress,
    dmu_tx_t *tx)
{
        dnode_t *dn;

        /*
         * Send streams include each object's compression function.  This
         * check ensures that the receiving system can understand the
         * compression function transmitted.
         */
        ASSERT3U(compress, <, ZIO_COMPRESS_LEGACY_FUNCTIONS);

        VERIFY0(dnode_hold(os, object, FTAG, &dn));
        dn->dn_compress = compress;
        dnode_setdirty(dn, tx);
        dnode_rele(dn, FTAG);
}

/*
 * When the "redundant_metadata" property is set to "most", only indirect
 * blocks of this level and higher will have an additional ditto block.
 */
static const int zfs_redundant_metadata_most_ditto_level = 2;

void
dmu_write_policy(objset_t *os, dnode_t *dn, int level, int wp, zio_prop_t *zp)
{
        dmu_object_type_t type = dn ? dn->dn_type : DMU_OT_OBJSET;
        boolean_t ismd = (level > 0 || DMU_OT_IS_METADATA(type) ||
            (wp & WP_SPILL));
        enum zio_checksum checksum = os->os_checksum;
        enum zio_compress compress = os->os_compress;
        uint8_t complevel = os->os_complevel;
        enum zio_checksum dedup_checksum = os->os_dedup_checksum;
        boolean_t dedup = B_FALSE;
        boolean_t nopwrite = B_FALSE;
        boolean_t dedup_verify = os->os_dedup_verify;
        boolean_t encrypt = B_FALSE;
        int copies = os->os_copies;

        /*
         * We maintain different write policies for each of the following
         * types of data:
         *       1. metadata
         *       2. preallocated blocks (i.e. level-0 blocks of a dump device)
         *       3. all other level 0 blocks
         */
        if (ismd) {
                /*
                 * XXX -- we should design a compression algorithm
                 * that specializes in arrays of bps.
                 */
                compress = zio_compress_select(os->os_spa,
                    ZIO_COMPRESS_ON, ZIO_COMPRESS_ON);

                /*
                 * Metadata always gets checksummed.  If the data
                 * checksum is multi-bit correctable, and it's not a
                 * ZBT-style checksum, then it's suitable for metadata
                 * as well.  Otherwise, the metadata checksum defaults
                 * to fletcher4.
                 */
                if (!(zio_checksum_table[checksum].ci_flags &
                    ZCHECKSUM_FLAG_METADATA) ||
                    (zio_checksum_table[checksum].ci_flags &
                    ZCHECKSUM_FLAG_EMBEDDED))
                        checksum = ZIO_CHECKSUM_FLETCHER_4;

                switch (os->os_redundant_metadata) {
                case ZFS_REDUNDANT_METADATA_ALL:
                        copies++;
                        break;
                case ZFS_REDUNDANT_METADATA_MOST:
                        if (level >= zfs_redundant_metadata_most_ditto_level ||
                            DMU_OT_IS_METADATA(type) || (wp & WP_SPILL))
                                copies++;
                        break;
                case ZFS_REDUNDANT_METADATA_SOME:
                        if (DMU_OT_IS_CRITICAL(type))
                                copies++;
                        break;
                case ZFS_REDUNDANT_METADATA_NONE:
                        break;
                }

                if (dmu_ddt_copies > 0) {
                        /*
                         * If this tuneable is set, and this is a write for a
                         * dedup entry store (zap or log), then we treat it
                         * something like ZFS_REDUNDANT_METADATA_MOST on a
                         * regular dataset: this many copies, and one more for
                         * "higher" indirect blocks. This specific exception is
                         * necessary because dedup objects are stored in the
                         * MOS, which always has the highest possible copies.
                         */
                        dmu_object_type_t stype =
                            dn ? dn->dn_storage_type : DMU_OT_NONE;
                        if (stype == DMU_OT_NONE)
                                stype = type;
                        if (stype == DMU_OT_DDT_ZAP) {
                                copies = dmu_ddt_copies;
                                if (level >=
                                    zfs_redundant_metadata_most_ditto_level)
                                        copies++;
                        }
                }
        } else if (wp & WP_NOFILL) {
                ASSERT(level == 0);

                /*
                 * If we're writing preallocated blocks, we aren't actually
                 * writing them so don't set any policy properties.  These
                 * blocks are currently only used by an external subsystem
                 * outside of zfs (i.e. dump) and not written by the zio
                 * pipeline.
                 */
                compress = ZIO_COMPRESS_OFF;
                checksum = ZIO_CHECKSUM_OFF;
        } else {
                compress = zio_compress_select(os->os_spa, dn->dn_compress,
                    compress);
                complevel = zio_complevel_select(os->os_spa, compress,
                    complevel, complevel);

                checksum = (dedup_checksum == ZIO_CHECKSUM_OFF) ?
                    zio_checksum_select(dn->dn_checksum, checksum) :
                    dedup_checksum;

                /*
                 * Determine dedup setting.  If we are in dmu_sync(),
                 * we won't actually dedup now because that's all
                 * done in syncing context; but we do want to use the
                 * dedup checksum.  If the checksum is not strong
                 * enough to ensure unique signatures, force
                 * dedup_verify.
                 */
                if (dedup_checksum != ZIO_CHECKSUM_OFF) {
                        dedup = (wp & WP_DMU_SYNC) ? B_FALSE : B_TRUE;
                        if (!(zio_checksum_table[checksum].ci_flags &
                            ZCHECKSUM_FLAG_DEDUP))
                                dedup_verify = B_TRUE;
                }

                /*
                 * Enable nopwrite if we have secure enough checksum
                 * algorithm (see comment in zio_nop_write) and
                 * compression is enabled.  We don't enable nopwrite if
                 * dedup is enabled as the two features are mutually
                 * exclusive.
                 */
                nopwrite = (!dedup && (zio_checksum_table[checksum].ci_flags &
                    ZCHECKSUM_FLAG_NOPWRITE) &&
                    compress != ZIO_COMPRESS_OFF && zfs_nopwrite_enabled);
        }

        /*
         * All objects in an encrypted objset are protected from modification
         * via a MAC. Encrypted objects store their IV and salt in the last DVA
         * in the bp, so we cannot use all copies. Encrypted objects are also
         * not subject to nopwrite since writing the same data will still
         * result in a new ciphertext. Only encrypted blocks can be dedup'd
         * to avoid ambiguity in the dedup code since the DDT does not store
         * object types.
         */
        if (os->os_encrypted && (wp & WP_NOFILL) == 0) {
                encrypt = B_TRUE;

                if (DMU_OT_IS_ENCRYPTED(type)) {
                        copies = MIN(copies, SPA_DVAS_PER_BP - 1);
                        nopwrite = B_FALSE;
                } else {
                        dedup = B_FALSE;
                }

                if (level <= 0 &&
                    (type == DMU_OT_DNODE || type == DMU_OT_OBJSET)) {
                        compress = ZIO_COMPRESS_EMPTY;
                }
        }

        zp->zp_compress = compress;
        zp->zp_complevel = complevel;
        zp->zp_checksum = checksum;
        zp->zp_type = (wp & WP_SPILL) ? dn->dn_bonustype : type;
        zp->zp_level = level;
        zp->zp_copies = MIN(copies, spa_max_replication(os->os_spa));
        zp->zp_dedup = dedup;
        zp->zp_dedup_verify = dedup && dedup_verify;
        zp->zp_nopwrite = nopwrite;
        zp->zp_encrypt = encrypt;
        zp->zp_byteorder = ZFS_HOST_BYTEORDER;
        zp->zp_direct_write = (wp & WP_DIRECT_WR) ? B_TRUE : B_FALSE;
        memset(zp->zp_salt, 0, ZIO_DATA_SALT_LEN);
        memset(zp->zp_iv, 0, ZIO_DATA_IV_LEN);
        memset(zp->zp_mac, 0, ZIO_DATA_MAC_LEN);
        zp->zp_zpl_smallblk = DMU_OT_IS_FILE(zp->zp_type) ?
            os->os_zpl_special_smallblock : 0;
        zp->zp_storage_type = dn ? dn->dn_storage_type : DMU_OT_NONE;

        ASSERT3U(zp->zp_compress, !=, ZIO_COMPRESS_INHERIT);
}

/*
 * Reports the location of data and holes in an object.  In order to
 * accurately report holes all dirty data must be synced to disk.  This
 * causes extremely poor performance when seeking for holes in a dirty file.
 * As a compromise, only provide hole data when the dnode is clean.  When
 * a dnode is dirty report the dnode as having no holes by returning EBUSY
 * which is always safe to do.
 */
int
dmu_offset_next(objset_t *os, uint64_t object, boolean_t hole, uint64_t *off)
{
        dnode_t *dn;
        int restarted = 0, err;

restart:
        err = dnode_hold(os, object, FTAG, &dn);
        if (err)
                return (err);

        rw_enter(&dn->dn_struct_rwlock, RW_READER);

        if (dnode_is_dirty(dn)) {
                /*
                 * If the zfs_dmu_offset_next_sync module option is enabled
                 * then hole reporting has been requested.  Dirty dnodes
                 * must be synced to disk to accurately report holes.
                 *
                 * Provided a RL_READER rangelock spanning 0-UINT64_MAX is
                 * held by the caller only a single restart will be required.
                 * We tolerate callers which do not hold the rangelock by
                 * returning EBUSY and not reporting holes after one restart.
                 */
                if (zfs_dmu_offset_next_sync) {
                        rw_exit(&dn->dn_struct_rwlock);
                        dnode_rele(dn, FTAG);

                        if (restarted)
                                return (SET_ERROR(EBUSY));

                        txg_wait_synced(dmu_objset_pool(os), 0);
                        restarted = 1;
                        goto restart;
                }

                err = SET_ERROR(EBUSY);
        } else {
                err = dnode_next_offset(dn, DNODE_FIND_HAVELOCK |
                    (hole ? DNODE_FIND_HOLE : 0), off, 1, 1, 0);
        }

        rw_exit(&dn->dn_struct_rwlock);
        dnode_rele(dn, FTAG);

        return (err);
}

int
dmu_read_l0_bps(objset_t *os, uint64_t object, uint64_t offset, uint64_t length,
    blkptr_t *bps, size_t *nbpsp)
{
        dmu_buf_t **dbp, *dbuf;
        dmu_buf_impl_t *db;
        blkptr_t *bp;
        int error, numbufs;

        error = dmu_buf_hold_array(os, object, offset, length, FALSE, FTAG,
            &numbufs, &dbp);
        if (error != 0) {
                if (error == ESRCH) {
                        error = SET_ERROR(ENXIO);
                }
                return (error);
        }

        ASSERT3U(numbufs, <=, *nbpsp);

        for (int i = 0; i < numbufs; i++) {
                dbuf = dbp[i];
                db = (dmu_buf_impl_t *)dbuf;

                mutex_enter(&db->db_mtx);

                if (!list_is_empty(&db->db_dirty_records)) {
                        dbuf_dirty_record_t *dr;

                        dr = list_head(&db->db_dirty_records);
                        if (dr->dt.dl.dr_brtwrite) {
                                /*
                                 * This is very special case where we clone a
                                 * block and in the same transaction group we
                                 * read its BP (most likely to clone the clone).
                                 */
                                bp = &dr->dt.dl.dr_overridden_by;
                        } else {
                                /*
                                 * The block was modified in the same
                                 * transaction group.
                                 */
                                mutex_exit(&db->db_mtx);
                                error = SET_ERROR(EAGAIN);
                                goto out;
                        }
                } else {
                        bp = db->db_blkptr;
                }

                mutex_exit(&db->db_mtx);

                if (bp == NULL) {
                        /*
                         * The file size was increased, but the block was never
                         * written, otherwise we would either have the block
                         * pointer or the dirty record and would not get here.
                         * It is effectively a hole, so report it as such.
                         */
                        BP_ZERO(&bps[i]);
                        continue;
                }
                /*
                 * Make sure we clone only data blocks.
                 */
                if (BP_IS_METADATA(bp) && !BP_IS_HOLE(bp)) {
                        error = SET_ERROR(EINVAL);
                        goto out;
                }

                /*
                 * If the block was allocated in transaction group that is not
                 * yet synced, we could clone it, but we couldn't write this
                 * operation into ZIL, or it may be impossible to replay, since
                 * the block may appear not yet allocated at that point.
                 */
                if (BP_GET_BIRTH(bp) > spa_freeze_txg(os->os_spa)) {
                        error = SET_ERROR(EINVAL);
                        goto out;
                }
                if (BP_GET_BIRTH(bp) > spa_last_synced_txg(os->os_spa)) {
                        error = SET_ERROR(EAGAIN);
                        goto out;
                }

                bps[i] = *bp;
        }

        *nbpsp = numbufs;
out:
        dmu_buf_rele_array(dbp, numbufs, FTAG);

        return (error);
}

int
dmu_brt_clone(objset_t *os, uint64_t object, uint64_t offset, uint64_t length,
    dmu_tx_t *tx, const blkptr_t *bps, size_t nbps)
{
        spa_t *spa;
        dmu_buf_t **dbp, *dbuf;
        dmu_buf_impl_t *db;
        struct dirty_leaf *dl;
        dbuf_dirty_record_t *dr;
        const blkptr_t *bp;
        int error = 0, i, numbufs;

        spa = os->os_spa;

        VERIFY0(dmu_buf_hold_array(os, object, offset, length, FALSE, FTAG,
            &numbufs, &dbp));
        ASSERT3U(nbps, ==, numbufs);

        /*
         * Before we start cloning make sure that the dbufs sizes match new BPs
         * sizes. If they don't, that's a no-go, as we are not able to shrink
         * dbufs.
         */
        for (i = 0; i < numbufs; i++) {
                dbuf = dbp[i];
                db = (dmu_buf_impl_t *)dbuf;
                bp = &bps[i];

                ASSERT3U(db->db.db_object, !=, DMU_META_DNODE_OBJECT);
                ASSERT0(db->db_level);
                ASSERT(db->db_blkid != DMU_BONUS_BLKID);
                ASSERT(db->db_blkid != DMU_SPILL_BLKID);

                if (!BP_IS_HOLE(bp) && BP_GET_LSIZE(bp) != dbuf->db_size) {
                        error = SET_ERROR(EXDEV);
                        goto out;
                }
        }

        for (i = 0; i < numbufs; i++) {
                dbuf = dbp[i];
                db = (dmu_buf_impl_t *)dbuf;
                bp = &bps[i];

                dmu_buf_will_clone_or_dio(dbuf, tx);

                mutex_enter(&db->db_mtx);

                dr = list_head(&db->db_dirty_records);
                VERIFY(dr != NULL);
                ASSERT3U(dr->dr_txg, ==, tx->tx_txg);
                dl = &dr->dt.dl;
                ASSERT0(dl->dr_has_raw_params);
                dl->dr_overridden_by = *bp;
                if (!BP_IS_HOLE(bp) || BP_GET_LOGICAL_BIRTH(bp) != 0) {
                        if (!BP_IS_EMBEDDED(bp)) {
                                BP_SET_BIRTH(&dl->dr_overridden_by, dr->dr_txg,
                                    BP_GET_BIRTH(bp));
                        } else {
                                BP_SET_LOGICAL_BIRTH(&dl->dr_overridden_by,
                                    dr->dr_txg);
                        }
                }
                dl->dr_brtwrite = B_TRUE;
                dl->dr_override_state = DR_OVERRIDDEN;

                mutex_exit(&db->db_mtx);

                /*
                 * When data in embedded into BP there is no need to create
                 * BRT entry as there is no data block. Just copy the BP as
                 * it contains the data.
                 */
                if (!BP_IS_HOLE(bp) && !BP_IS_EMBEDDED(bp)) {
                        brt_pending_add(spa, bp, tx);
                }
        }
out:
        dmu_buf_rele_array(dbp, numbufs, FTAG);

        return (error);
}

void
__dmu_object_info_from_dnode(dnode_t *dn, dmu_object_info_t *doi)
{
        dnode_phys_t *dnp = dn->dn_phys;

        doi->doi_data_block_size = dn->dn_datablksz;
        doi->doi_metadata_block_size = dn->dn_indblkshift ?
            1ULL << dn->dn_indblkshift : 0;
        doi->doi_type = dn->dn_type;
        doi->doi_bonus_type = dn->dn_bonustype;
        doi->doi_bonus_size = dn->dn_bonuslen;
        doi->doi_dnodesize = dn->dn_num_slots << DNODE_SHIFT;
        doi->doi_indirection = dn->dn_nlevels;
        doi->doi_checksum = dn->dn_checksum;
        doi->doi_compress = dn->dn_compress;
        doi->doi_nblkptr = dn->dn_nblkptr;
        doi->doi_physical_blocks_512 = (DN_USED_BYTES(dnp) + 256) >> 9;
        doi->doi_max_offset = (dn->dn_maxblkid + 1) * dn->dn_datablksz;
        doi->doi_fill_count = 0;
        for (int i = 0; i < dnp->dn_nblkptr; i++)
                doi->doi_fill_count += BP_GET_FILL(&dnp->dn_blkptr[i]);
}

void
dmu_object_info_from_dnode(dnode_t *dn, dmu_object_info_t *doi)
{
        rw_enter(&dn->dn_struct_rwlock, RW_READER);
        mutex_enter(&dn->dn_mtx);

        __dmu_object_info_from_dnode(dn, doi);

        mutex_exit(&dn->dn_mtx);
        rw_exit(&dn->dn_struct_rwlock);
}

/*
 * Get information on a DMU object.
 * If doi is NULL, just indicates whether the object exists.
 */
int
dmu_object_info(objset_t *os, uint64_t object, dmu_object_info_t *doi)
{
        dnode_t *dn;
        int err = dnode_hold(os, object, FTAG, &dn);

        if (err)
                return (err);

        if (doi != NULL)
                dmu_object_info_from_dnode(dn, doi);

        dnode_rele(dn, FTAG);
        return (0);
}

/*
 * As above, but faster; can be used when you have a held dbuf in hand.
 */
void
dmu_object_info_from_db(dmu_buf_t *db_fake, dmu_object_info_t *doi)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;

        DB_DNODE_ENTER(db);
        dmu_object_info_from_dnode(DB_DNODE(db), doi);
        DB_DNODE_EXIT(db);
}

/*
 * Faster still when you only care about the size.
 */
void
dmu_object_size_from_db(dmu_buf_t *db_fake, uint32_t *blksize,
    u_longlong_t *nblk512)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        dnode_t *dn;

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

        *blksize = dn->dn_datablksz;
        /* add in number of slots used for the dnode itself */
        *nblk512 = ((DN_USED_BYTES(dn->dn_phys) + SPA_MINBLOCKSIZE/2) >>
            SPA_MINBLOCKSHIFT) + dn->dn_num_slots;
        DB_DNODE_EXIT(db);
}

void
dmu_object_dnsize_from_db(dmu_buf_t *db_fake, int *dnsize)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;

        DB_DNODE_ENTER(db);
        *dnsize = DB_DNODE(db)->dn_num_slots << DNODE_SHIFT;
        DB_DNODE_EXIT(db);
}

void
byteswap_uint64_array(void *vbuf, size_t size)
{
        uint64_t *buf = vbuf;
        size_t count = size >> 3;
        int i;

        ASSERT((size & 7) == 0);

        for (i = 0; i < count; i++)
                buf[i] = BSWAP_64(buf[i]);
}

void
byteswap_uint32_array(void *vbuf, size_t size)
{
        uint32_t *buf = vbuf;
        size_t count = size >> 2;
        int i;

        ASSERT((size & 3) == 0);

        for (i = 0; i < count; i++)
                buf[i] = BSWAP_32(buf[i]);
}

void
byteswap_uint16_array(void *vbuf, size_t size)
{
        uint16_t *buf = vbuf;
        size_t count = size >> 1;
        int i;

        ASSERT((size & 1) == 0);

        for (i = 0; i < count; i++)
                buf[i] = BSWAP_16(buf[i]);
}

void
byteswap_uint8_array(void *vbuf, size_t size)
{
        (void) vbuf, (void) size;
}

void
dmu_init(void)
{
        abd_init();
        zfs_dbgmsg_init();
        sa_cache_init();
        dmu_objset_init();
        dnode_init();
        zfetch_init();
        dmu_tx_init();
        l2arc_init();
        arc_init();
        dbuf_init();
}

void
dmu_fini(void)
{
        arc_fini(); /* arc depends on l2arc, so arc must go first */
        l2arc_fini();
        dmu_tx_fini();
        zfetch_fini();
        dbuf_fini();
        dnode_fini();
        dmu_objset_fini();
        sa_cache_fini();
        zfs_dbgmsg_fini();
        abd_fini();
}

EXPORT_SYMBOL(dmu_bonus_hold);
EXPORT_SYMBOL(dmu_bonus_hold_by_dnode);
EXPORT_SYMBOL(dmu_buf_hold_array_by_bonus);
EXPORT_SYMBOL(dmu_buf_rele_array);
EXPORT_SYMBOL(dmu_prefetch);
EXPORT_SYMBOL(dmu_prefetch_by_dnode);
EXPORT_SYMBOL(dmu_prefetch_dnode);
EXPORT_SYMBOL(dmu_free_range);
EXPORT_SYMBOL(dmu_free_long_range);
EXPORT_SYMBOL(dmu_free_long_object);
EXPORT_SYMBOL(dmu_read);
EXPORT_SYMBOL(dmu_read_by_dnode);
EXPORT_SYMBOL(dmu_read_uio);
EXPORT_SYMBOL(dmu_read_uio_dbuf);
EXPORT_SYMBOL(dmu_read_uio_dnode);
EXPORT_SYMBOL(dmu_write);
EXPORT_SYMBOL(dmu_write_by_dnode);
EXPORT_SYMBOL(dmu_write_by_dnode_flags);
EXPORT_SYMBOL(dmu_write_uio);
EXPORT_SYMBOL(dmu_write_uio_dbuf);
EXPORT_SYMBOL(dmu_write_uio_dnode);
EXPORT_SYMBOL(dmu_prealloc);
EXPORT_SYMBOL(dmu_object_info);
EXPORT_SYMBOL(dmu_object_info_from_dnode);
EXPORT_SYMBOL(dmu_object_info_from_db);
EXPORT_SYMBOL(dmu_object_size_from_db);
EXPORT_SYMBOL(dmu_object_dnsize_from_db);
EXPORT_SYMBOL(dmu_object_set_nlevels);
EXPORT_SYMBOL(dmu_object_set_blocksize);
EXPORT_SYMBOL(dmu_object_set_maxblkid);
EXPORT_SYMBOL(dmu_object_set_checksum);
EXPORT_SYMBOL(dmu_object_set_compress);
EXPORT_SYMBOL(dmu_offset_next);
EXPORT_SYMBOL(dmu_write_policy);
EXPORT_SYMBOL(dmu_sync);
EXPORT_SYMBOL(dmu_request_arcbuf);
EXPORT_SYMBOL(dmu_return_arcbuf);
EXPORT_SYMBOL(dmu_assign_arcbuf_by_dnode);
EXPORT_SYMBOL(dmu_assign_arcbuf_by_dbuf);
EXPORT_SYMBOL(dmu_buf_hold);
EXPORT_SYMBOL(dmu_ot);

ZFS_MODULE_PARAM(zfs, zfs_, nopwrite_enabled, INT, ZMOD_RW,
        "Enable NOP writes");

ZFS_MODULE_PARAM(zfs, zfs_, per_txg_dirty_frees_percent, UINT, ZMOD_RW,
        "Percentage of dirtied blocks from frees in one TXG");

ZFS_MODULE_PARAM(zfs, zfs_, dmu_offset_next_sync, INT, ZMOD_RW,
        "Enable forcing txg sync to find holes");

/* CSTYLED */
ZFS_MODULE_PARAM(zfs, , dmu_prefetch_max, UINT, ZMOD_RW,
        "Limit one prefetch call to this size");

/* CSTYLED */
ZFS_MODULE_PARAM(zfs, , dmu_ddt_copies, UINT, ZMOD_RW,
        "Override copies= for dedup objects");