fs/zfs/dbuf.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2008 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#pragma ident	"%Z%%M%	%I%	%E% SMI"

#include <sys/zfs_context.h>
#include <sys/dmu.h>
#include <sys/dmu_impl.h>
#include <sys/dbuf.h>
#include <sys/dmu_objset.h>
#include <sys/dsl_dataset.h>
#include <sys/dsl_dir.h>
#include <sys/dmu_tx.h>
#include <sys/spa.h>
#include <sys/zio.h>
#include <sys/dmu_zfetch.h>

static void dbuf_destroy(dmu_buf_impl_t *db);
static int dbuf_undirty(dmu_buf_impl_t *db, dmu_tx_t *tx);
static void dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, int checksum,
    int compress, dmu_tx_t *tx);
static arc_done_func_t dbuf_write_ready;
static arc_done_func_t dbuf_write_done;

int zfs_mdcomp_disable = 0;

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

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

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

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

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

static uint64_t dbuf_hash_count;

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

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

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

	return (crc);
}

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

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

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

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

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

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

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

	return (NULL);
}

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

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

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

static arc_evict_func_t dbuf_do_evict;

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

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

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

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

	dbuf_clear(db);
	dbuf_destroy(db);
}

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

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

retry:
	h->hash_table_mask = hsize - 1;
	h->hash_table = kmem_zalloc(hsize * sizeof (void *), KM_NOSLEEP);
	if (h->hash_table == NULL) {
		/* XXX - we should really return an error instead of assert */
		ASSERT(hsize > (1ULL << 10));
		hsize >>= 1;
		goto retry;
	}

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

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

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

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

/*
 * Other stuff.
 */

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

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

	if (!(zfs_flags & ZFS_DEBUG_DBUF_VERIFY))
		return;

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

	if (db->db_level == 0) {
		/* we can be momentarily larger in dnode_set_blksz() */
		if (db->db_blkid != DB_BONUS_BLKID && dn) {
			ASSERT3U(db->db.db_size, >=, dn->dn_datablksz);
		}
		if (db->db.db_object == DMU_META_DNODE_OBJECT) {
			dbuf_dirty_record_t *dr = db->db_data_pending;
			/*
			 * it should only be modified in syncing
			 * context, so make sure we only have
			 * one copy of the data.
			 */
			ASSERT(dr == NULL || dr->dt.dl.dr_data == db->db_buf);
		}
	}

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

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

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

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

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

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

	mutex_enter(&db->db_mtx);
	ASSERT3U(db->db_state, ==, DB_READ);
	/*
	 * All reads are synchronous, so we must have a hold on the dbuf
	 */
	ASSERT(refcount_count(&db->db_holds) > 0);
	ASSERT(db->db_buf == NULL);
	ASSERT(db->db.db_data == NULL);
	if (db->db_level == 0 && db->db_freed_in_flight) {
		/* we were freed in flight; disregard any error */
		arc_release(buf, db);
		bzero(buf->b_data, db->db.db_size);
		arc_buf_freeze(buf);
		db->db_freed_in_flight = FALSE;
		dbuf_set_data(db, buf);
		db->db_state = DB_CACHED;
	} else if (zio == NULL || zio->io_error == 0) {
		dbuf_set_data(db, buf);
		db->db_state = DB_CACHED;
	} else {
		ASSERT(db->db_blkid != DB_BONUS_BLKID);
		ASSERT3P(db->db_buf, ==, NULL);
		VERIFY(arc_buf_remove_ref(buf, db) == 1);
		db->db_state = DB_UNCACHED;
	}
	cv_broadcast(&db->db_changed);
	mutex_exit(&db->db_mtx);
	dbuf_rele(db, NULL);
}

static void
dbuf_read_impl(dmu_buf_impl_t *db, zio_t *zio, uint32_t *flags)
{
	blkptr_t *bp;
	zbookmark_t zb;
	uint32_t aflags = ARC_NOWAIT;

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

	if (db->db_blkid == DB_BONUS_BLKID) {
		int bonuslen = db->db_dnode->dn_bonuslen;

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

	if (db->db_level == 0 && dnode_block_freed(db->db_dnode, db->db_blkid))
		bp = NULL;
	else
		bp = db->db_blkptr;

	if (bp == NULL)
		dprintf_dbuf(db, "blkptr: %s\n", "NULL");
	else
		dprintf_dbuf_bp(db, bp, "%s", "blkptr:");

	if (bp == NULL || BP_IS_HOLE(bp)) {
		arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);

		ASSERT(bp == NULL || BP_IS_HOLE(bp));
		dbuf_set_data(db, arc_buf_alloc(db->db_dnode->dn_objset->os_spa,
		    db->db.db_size, db, type));
		bzero(db->db.db_data, db->db.db_size);
		db->db_state = DB_CACHED;
		*flags |= DB_RF_CACHED;
		mutex_exit(&db->db_mtx);
		return;
	}

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

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

	dbuf_add_ref(db, NULL);
	/* ZIO_FLAG_CANFAIL callers have to check the parent zio's error */
	ASSERT3U(db->db_dnode->dn_type, <, DMU_OT_NUMTYPES);
	(void) arc_read(zio, db->db_dnode->dn_objset->os_spa, bp,
	    db->db_level > 0 ? byteswap_uint64_array :
	    dmu_ot[db->db_dnode->dn_type].ot_byteswap,
	    dbuf_read_done, db, ZIO_PRIORITY_SYNC_READ,
	    (*flags & DB_RF_CANFAIL) ? ZIO_FLAG_CANFAIL : ZIO_FLAG_MUSTSUCCEED,
	    &aflags, &zb);
	if (aflags & ARC_CACHED)
		*flags |= DB_RF_CACHED;
}

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

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

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

	prefetch = db->db_level == 0 && db->db_blkid != DB_BONUS_BLKID &&
	    (flags & DB_RF_NOPREFETCH) == 0 && db->db_dnode != NULL;

	mutex_enter(&db->db_mtx);
	if (db->db_state == DB_CACHED) {
		mutex_exit(&db->db_mtx);
		if (prefetch)
			dmu_zfetch(&db->db_dnode->dn_zfetch, db->db.db_offset,
			    db->db.db_size, TRUE);
		if ((flags & DB_RF_HAVESTRUCT) == 0)
			rw_exit(&db->db_dnode->dn_struct_rwlock);
	} else if (db->db_state == DB_UNCACHED) {
		if (zio == NULL) {
			zio = zio_root(db->db_dnode->dn_objset->os_spa,
			    NULL, NULL, ZIO_FLAG_CANFAIL);
		}
		dbuf_read_impl(db, zio, &flags);

		/* dbuf_read_impl has dropped db_mtx for us */

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

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

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

		mutex_enter(&db->db_mtx);
		if ((flags & DB_RF_NEVERWAIT) == 0) {
			while (db->db_state == DB_READ ||
			    db->db_state == DB_FILL) {
				ASSERT(db->db_state == DB_READ ||
				    (flags & DB_RF_HAVESTRUCT) == 0);
				cv_wait(&db->db_changed, &db->db_mtx);
			}
			if (db->db_state == DB_UNCACHED)
				err = EIO;
		}
		mutex_exit(&db->db_mtx);
	}

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

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

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

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

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

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

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

void
dbuf_unoverride(dbuf_dirty_record_t *dr)
{
	dmu_buf_impl_t *db = dr->dr_dbuf;
	uint64_t txg = dr->dr_txg;

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

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

	/* free this block */
	if (!BP_IS_HOLE(&dr->dt.dl.dr_overridden_by)) {
		/* XXX can get silent EIO here */
		(void) arc_free(NULL, db->db_dnode->dn_objset->os_spa,
		    txg, &dr->dt.dl.dr_overridden_by, NULL, NULL, ARC_WAIT);
	}
	dr->dt.dl.dr_override_state = DR_NOT_OVERRIDDEN;
	/*
	 * Release the already-written buffer, so we leave it in
	 * a consistent dirty state.  Note that all callers are
	 * modifying the buffer, so they will immediately do
	 * another (redundant) arc_release().  Therefore, leave
	 * the buf thawed to save the effort of freezing &
	 * immediately re-thawing it.
	 */
	arc_release(dr->dt.dl.dr_data, db);
}

void
dbuf_free_range(dnode_t *dn, uint64_t blkid, uint64_t nblks, dmu_tx_t *tx)
{
	dmu_buf_impl_t *db, *db_next;
	uint64_t txg = tx->tx_txg;

	dprintf_dnode(dn, "blkid=%llu nblks=%llu\n", blkid, nblks);
	mutex_enter(&dn->dn_dbufs_mtx);
	for (db = list_head(&dn->dn_dbufs); db; db = db_next) {
		db_next = list_next(&dn->dn_dbufs, db);
		ASSERT(db->db_blkid != DB_BONUS_BLKID);
		if (db->db_level != 0)
			continue;
		dprintf_dbuf(db, "found buf %s\n", "");
		if (db->db_blkid < blkid ||
		    db->db_blkid >= blkid+nblks)
			continue;

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

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

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

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

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

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

	/*
	 * We don't need any locking to protect db_blkptr:
	 * If it's syncing, then db_last_dirty will be set
	 * so we'll ignore db_blkptr.
	 */
	ASSERT(MUTEX_HELD(&db->db_mtx));
	if (db->db_last_dirty)
		birth_txg = db->db_last_dirty->dr_txg;
	else if (db->db_blkptr)
		birth_txg = db->db_blkptr->blk_birth;

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

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

	ASSERT(db->db_blkid != DB_BONUS_BLKID);

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

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

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

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

	mutex_enter(&db->db_mtx);
	dbuf_set_data(db, buf);
	VERIFY(arc_buf_remove_ref(obuf, db) == 1);
	db->db.db_size = size;

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

	dnode_willuse_space(db->db_dnode, size-osize, tx);
}

dbuf_dirty_record_t *
dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
{
	dnode_t *dn = db->db_dnode;
	objset_impl_t *os = dn->dn_objset;
	dbuf_dirty_record_t **drp, *dr;
	int drop_struct_lock = FALSE;
	int txgoff = tx->tx_txg & TXG_MASK;

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

	/*
	 * Shouldn't dirty a regular buffer in syncing context.  Private
	 * objects may be dirtied in syncing context, but only if they
	 * were already pre-dirtied in open context.
	 * XXX We may want to prohibit dirtying in syncing context even
	 * if they did pre-dirty.
	 */
	ASSERT(!dmu_tx_is_syncing(tx) ||
	    BP_IS_HOLE(dn->dn_objset->os_rootbp) ||
	    dn->dn_object == DMU_META_DNODE_OBJECT ||
	    dn->dn_objset->os_dsl_dataset == NULL ||
	    dsl_dir_is_private(dn->dn_objset->os_dsl_dataset->ds_dir));

	/*
	 * We make this assert for private objects as well, but after we
	 * check if we're already dirty.  They are allowed to re-dirty
	 * in syncing context.
	 */
	ASSERT(dn->dn_object == DMU_META_DNODE_OBJECT ||
	    dn->dn_dirtyctx == DN_UNDIRTIED || dn->dn_dirtyctx ==
	    (dmu_tx_is_syncing(tx) ? DN_DIRTY_SYNC : DN_DIRTY_OPEN));

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