modules/zfs/zfs.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 2006 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

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

#include <mdb/mdb_ctf.h>
#include <sys/zfs_context.h>
#include <sys/mdb_modapi.h>
#include <sys/dbuf.h>
#include <sys/dmu_objset.h>
#include <sys/dsl_dir.h>
#include <sys/dsl_pool.h>
#include <sys/metaslab_impl.h>
#include <sys/space_map.h>
#include <sys/list.h>
#include <sys/spa_impl.h>
#include <sys/vdev_impl.h>
#include <sys/zio_compress.h>

#ifndef _KERNEL
#include "../genunix/list.h"
#endif

#ifdef _KERNEL
#define	ZFS_OBJ_NAME	"zfs"
#else
#define	ZFS_OBJ_NAME	"libzpool.so.1"
#endif

static char *
local_strdup(const char *s)
{
	char *s1 = mdb_alloc(strlen(s) + 1, UM_SLEEP);

	(void) strcpy(s1, s);
	return (s1);
}

static int
getmember(uintptr_t addr, const char *type, mdb_ctf_id_t *idp,
    const char *member, int len, void *buf)
{
	mdb_ctf_id_t id;
	ulong_t off;
	char name[64];

	if (idp == NULL) {
		if (mdb_ctf_lookup_by_name(type, &id) == -1) {
			mdb_warn("couldn't find type %s", type);
			return (DCMD_ERR);
		}
		idp = &id;
	} else {
		type = name;
		mdb_ctf_type_name(*idp, name, sizeof (name));
	}

	if (mdb_ctf_offsetof(*idp, member, &off) == -1) {
		mdb_warn("couldn't find member %s of type %s\n", member, type);
		return (DCMD_ERR);
	}
	if (off % 8 != 0) {
		mdb_warn("member %s of type %s is unsupported bitfield",
		    member, type);
		return (DCMD_ERR);
	}
	off /= 8;

	if (mdb_vread(buf, len, addr + off) == -1) {
		mdb_warn("failed to read %s from %s at %p",
		    member, type, addr + off);
		return (DCMD_ERR);
	}
	/* mdb_warn("read %s from %s at %p+%llx\n", member, type, addr, off); */

	return (0);
}

#define	GETMEMB(addr, type, member, dest) \
	getmember(addr, #type, NULL, #member, sizeof (dest), &(dest))

#define	GETMEMBID(addr, ctfid, member, dest) \
	getmember(addr, NULL, ctfid, #member, sizeof (dest), &(dest))

static int
getrefcount(uintptr_t addr, mdb_ctf_id_t *id,
    const char *member, uint64_t *rc)
{
	static int gotid;
	static mdb_ctf_id_t rc_id;
	ulong_t off;

	if (!gotid) {
		if (mdb_ctf_lookup_by_name("struct refcount", &rc_id) == -1) {
			mdb_warn("couldn't find struct refcount");
			return (DCMD_ERR);
		}
		gotid = TRUE;
	}

	if (mdb_ctf_offsetof(*id, member, &off) == -1) {
		char name[64];
		mdb_ctf_type_name(*id, name, sizeof (name));
		mdb_warn("couldn't find member %s of type %s\n", member, name);
		return (DCMD_ERR);
	}
	off /= 8;

	return (GETMEMBID(addr + off, &rc_id, rc_count, *rc));
}

static int
read_symbol(char *sym_name, void **bufp)
{
	GElf_Sym sym;

	if (mdb_lookup_by_obj(MDB_TGT_OBJ_EVERY, sym_name, &sym)) {
		mdb_warn("can't find symbol %s", sym_name);
		return (DCMD_ERR);
	}

	*bufp = mdb_alloc(sym.st_size, UM_SLEEP);

	if (mdb_vread(*bufp, sym.st_size, sym.st_value) == -1) {
		mdb_warn("can't read data for symbol %s", sym_name);
		mdb_free(*bufp, sym.st_size);
		return (DCMD_ERR);
	}

	return (DCMD_OK);
}

static int verbose;

static int
freelist_walk_init(mdb_walk_state_t *wsp)
{
	if (wsp->walk_addr == NULL) {
		mdb_warn("must supply starting address\n");
		return (WALK_ERR);
	}

	wsp->walk_data = 0;  /* Index into the freelist */
	return (WALK_NEXT);
}

static int
freelist_walk_step(mdb_walk_state_t *wsp)
{
	uint64_t entry;
	uintptr_t number = (uintptr_t)wsp->walk_data;
	char *ddata[] = { "ALLOC", "FREE", "CONDENSE", "INVALID" };
	int mapshift = SPA_MINBLOCKSHIFT;

	if (mdb_vread(&entry, sizeof (entry), wsp->walk_addr) == -1) {
		mdb_warn("failed to read freelist entry %p", wsp->walk_addr);
		return (WALK_DONE);
	}
	wsp->walk_addr += sizeof (entry);
	wsp->walk_data = (void *)(number + 1);

	if (SM_DEBUG_DECODE(entry)) {
		mdb_printf("DEBUG: %3u  %10s: txg=%llu  pass=%llu\n",
		    number,
		    ddata[SM_DEBUG_ACTION_DECODE(entry)],
		    SM_DEBUG_TXG_DECODE(entry),
		    SM_DEBUG_SYNCPASS_DECODE(entry));
	} else {
		mdb_printf("Entry: %3u  offsets=%08llx-%08llx  type=%c  "
		    "size=%06llx", number,
		    SM_OFFSET_DECODE(entry) << mapshift,
		    (SM_OFFSET_DECODE(entry) + SM_RUN_DECODE(entry)) <<
		    mapshift,
		    SM_TYPE_DECODE(entry) == SM_ALLOC ? 'A' : 'F',
		    SM_RUN_DECODE(entry) << mapshift);
		if (verbose)
			mdb_printf("      (raw=%012llx)\n", entry);
		mdb_printf("\n");
	}
	return (WALK_NEXT);
}

/* ARGSUSED */
static void
freelist_walk_fini(mdb_walk_state_t *wsp)
{
}

typedef struct dbuf_walk_data {
	dbuf_hash_table_t ht;
	int64_t bucket;
	uintptr_t dbp;
	dmu_buf_impl_t db;
} dbuf_walk_data_t;

static int
dbuf_walk_init(mdb_walk_state_t *wsp)
{
	dbuf_walk_data_t *dwd;

	if (wsp->walk_addr != NULL) {
		mdb_warn("must supply starting address\n");
		return (WALK_ERR);
	}

	dwd = mdb_alloc(sizeof (dbuf_walk_data_t), UM_SLEEP);

	if (mdb_readvar(&dwd->ht, "dbuf_hash_table") == -1) {
		mdb_warn("failed to read 'dbuf_hash_table'");
		mdb_free(dwd, sizeof (dbuf_walk_data_t));
		return (WALK_ERR);
	}
	dwd->bucket = -1;
	dwd->dbp = 0;
	wsp->walk_data = dwd;
	return (WALK_NEXT);
}

static int
dbuf_walk_step(mdb_walk_state_t *wsp)
{
	int status;
	dbuf_walk_data_t *dwd = wsp->walk_data;

	while (dwd->dbp == 0) {
		dwd->bucket++;
		if (dwd->bucket == dwd->ht.hash_table_mask+1)
			return (WALK_DONE);

		if (mdb_vread(&dwd->dbp, sizeof (void *),
		    (uintptr_t)(dwd->ht.hash_table+dwd->bucket)) == -1) {
			mdb_warn("failed to read hash bucket %u at %p",
			    dwd->bucket, dwd->ht.hash_table+dwd->bucket);
			return (WALK_DONE);
		}
	}

	wsp->walk_addr = dwd->dbp;
	if (mdb_vread(&dwd->db, sizeof (dmu_buf_impl_t),
	    wsp->walk_addr) == -1) {
		mdb_warn("failed to read dbuf at %p", wsp->walk_addr);
		return (WALK_DONE);
	}
	status = wsp->walk_callback(wsp->walk_addr, &dwd->db, wsp->walk_cbdata);

	dwd->dbp = (uintptr_t)dwd->db.db_hash_next;
	return (status);
}

static void
dbuf_walk_fini(mdb_walk_state_t *wsp)
{
	dbuf_walk_data_t *dwd = wsp->walk_data;
	mdb_free(dwd, sizeof (dbuf_walk_data_t));
}

static int
dataset_name(uintptr_t addr, char *buf)
{
	static int gotid;
	static mdb_ctf_id_t dd_id;
	uintptr_t dd_parent;
	char dd_myname[MAXNAMELEN];

	if (!gotid) {
		if (mdb_ctf_lookup_by_name("struct dsl_dir",
		    &dd_id) == -1) {
			mdb_warn("couldn't find struct dsl_dir");
			return (DCMD_ERR);
		}
		gotid = TRUE;
	}
	if (GETMEMBID(addr, &dd_id, dd_parent, dd_parent) ||
	    GETMEMBID(addr, &dd_id, dd_myname, dd_myname)) {
		return (DCMD_ERR);
	}

	if (dd_parent) {
		if (dataset_name(dd_parent, buf))
			return (DCMD_ERR);
		strcat(buf, "/");
	}

	if (dd_myname[0])
		strcat(buf, dd_myname);
	else
		strcat(buf, "???");

	return (0);
}

static int
objset_name(uintptr_t addr, char *buf)
{
	static int gotid;
	static mdb_ctf_id_t osi_id, ds_id;
	uintptr_t os_dsl_dataset;
	char ds_snapname[MAXNAMELEN];
	uintptr_t ds_dir;

	buf[0] = '\0';

	if (!gotid) {
		if (mdb_ctf_lookup_by_name("struct objset_impl",
		    &osi_id) == -1) {
			mdb_warn("couldn't find struct objset_impl");
			return (DCMD_ERR);
		}
		if (mdb_ctf_lookup_by_name("struct dsl_dataset",
		    &ds_id) == -1) {
			mdb_warn("couldn't find struct dsl_dataset");
			return (DCMD_ERR);
		}

		gotid = TRUE;
	}

	if (GETMEMBID(addr, &osi_id, os_dsl_dataset, os_dsl_dataset))
		return (DCMD_ERR);

	if (os_dsl_dataset == 0) {
		strcat(buf, "mos");
		return (0);
	}

	if (GETMEMBID(os_dsl_dataset, &ds_id, ds_snapname, ds_snapname) ||
	    GETMEMBID(os_dsl_dataset, &ds_id, ds_dir, ds_dir)) {
		return (DCMD_ERR);
	}

	if (ds_dir && dataset_name(ds_dir, buf))
		return (DCMD_ERR);

	if (ds_snapname[0]) {
		strcat(buf, "@");
		strcat(buf, ds_snapname);
	}
	return (0);
}

static void
enum_lookup(char *out, size_t size, mdb_ctf_id_t id, int val,
    const char *prefix)
{
	const char *cp;
	size_t len = strlen(prefix);

	if ((cp = mdb_ctf_enum_name(id, val)) != NULL) {
		if (strncmp(cp, prefix, len) == 0)
			cp += len;
		(void) strncpy(out, cp, size);
	} else {
		mdb_snprintf(out, size, "? (%d)", val);
	}
}

/* ARGSUSED */
static int
zio_pipeline(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
	mdb_ctf_id_t pipe_enum;
	int i;
	char stage[1024];

	if (mdb_ctf_lookup_by_name("enum zio_stage", &pipe_enum) == -1) {
		mdb_warn("Could not find enum zio_stage");
		return (DCMD_ERR);
	}

	for (i = 0; i < 32; i++) {
		if (addr & (1U << i)) {
			enum_lookup(stage, sizeof (stage), pipe_enum, i,
			    "ZIO_STAGE_");
			mdb_printf("    %s\n", stage);
		}
	}

	return (DCMD_OK);
}

/* ARGSUSED */
static int
blkptr(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
	blkptr_t bp;
	dmu_object_type_info_t *doti;
	zio_compress_info_t *zct;
	zio_checksum_info_t *zci;
	int i;
	char buf[MAXPATHLEN];

	if (mdb_vread(&bp, sizeof (blkptr_t), addr) == -1) {
		mdb_warn("failed to read blkptr_t");
		return (DCMD_ERR);
	}

	if (read_symbol("dmu_ot", (void **)&doti) != DCMD_OK)
		return (DCMD_ERR);
	for (i = 0; i < DMU_OT_NUMTYPES; i++) {
		mdb_readstr(buf, sizeof (buf), (uintptr_t)doti[i].ot_name);
		doti[i].ot_name = local_strdup(buf);
	}

	if (read_symbol("zio_checksum_table", (void **)&zci) != DCMD_OK)
		return (DCMD_ERR);
	for (i = 0; i < ZIO_CHECKSUM_FUNCTIONS; i++) {
		mdb_readstr(buf, sizeof (buf), (uintptr_t)zci[i].ci_name);
		zci[i].ci_name = local_strdup(buf);
	}

	if (read_symbol("zio_compress_table", (void **)&zct) != DCMD_OK)
		return (DCMD_ERR);
	for (i = 0; i < ZIO_COMPRESS_FUNCTIONS; i++) {
		mdb_readstr(buf, sizeof (buf), (uintptr_t)zct[i].ci_name);
		zct[i].ci_name = local_strdup(buf);
	}

	/*
	 * Super-ick warning:  This code is also duplicated in
	 * cmd/zdb.c .   Yeah, I hate code replication, too.
	 */
	for (i = 0; i < BP_GET_NDVAS(&bp); i++) {
		dva_t *dva = &bp.blk_dva[i];

		mdb_printf("DVA[%d]: vdev_id %lld / %llx\n", i,
		    DVA_GET_VDEV(dva), DVA_GET_OFFSET(dva));
		mdb_printf("DVA[%d]:       GANG: %-5s  GRID:  %04x\t"
		    "ASIZE: %llx\n", i, DVA_GET_GANG(dva) ? "TRUE" : "FALSE",
		    DVA_GET_GRID(dva), DVA_GET_ASIZE(dva));
		mdb_printf("DVA[%d]: :%llu:%llx:%llx:%s%s%s%s\n", i,
		    DVA_GET_VDEV(dva), DVA_GET_OFFSET(dva), BP_GET_PSIZE(&bp),
		    BP_SHOULD_BYTESWAP(&bp) ? "e" : "",
		    !DVA_GET_GANG(dva) && BP_GET_LEVEL(&bp) != 0 ? "i" : "",
		    DVA_GET_GANG(dva) ? "g" : "",
		    BP_GET_COMPRESS(&bp) != 0 ? "d" : "");
	}
	mdb_printf("LSIZE:  %-16llx\t\tPSIZE: %llx\n",
	    BP_GET_LSIZE(&bp), BP_GET_PSIZE(&bp));
	mdb_printf("ENDIAN: %6s\t\t\t\t\tTYPE:  %s\n",
	    BP_GET_BYTEORDER(&bp) ? "LITTLE" : "BIG",
	    doti[BP_GET_TYPE(&bp)].ot_name);
	mdb_printf("BIRTH:  %-16llx   LEVEL: %-2d\tFILL:  %llx\n",
	    bp.blk_birth, BP_GET_LEVEL(&bp), bp.blk_fill);
	mdb_printf("CKFUNC: %-16s\t\tCOMP:  %s\n",
	    zci[BP_GET_CHECKSUM(&bp)].ci_name,
	    zct[BP_GET_COMPRESS(&bp)].ci_name);
	mdb_printf("CKSUM:  %llx:%llx:%llx:%llx\n",
	    bp.blk_cksum.zc_word[0],
	    bp.blk_cksum.zc_word[1],
	    bp.blk_cksum.zc_word[2],
	    bp.blk_cksum.zc_word[3]);

	return (DCMD_OK);
}

/* ARGSUSED */
static int
dbuf(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
	mdb_ctf_id_t id;
	dmu_buf_t db;
	uintptr_t objset;
	uint8_t level;
	uint64_t blkid;
	uint64_t holds;
	char objectname[32];
	char blkidname[32];
	char path[MAXNAMELEN];

	if (DCMD_HDRSPEC(flags)) {
		mdb_printf("        addr object lvl blkid holds os\n");
	}

	if (mdb_ctf_lookup_by_name("struct dmu_buf_impl", &id) == -1) {
		mdb_warn("couldn't find struct dmu_buf_impl_t");
		return (DCMD_ERR);
	}

	if (GETMEMBID(addr, &id, db_objset, objset) ||
	    GETMEMBID(addr, &id, db, db) ||
	    GETMEMBID(addr, &id, db_level, level) ||
	    GETMEMBID(addr, &id, db_blkid, blkid)) {
		return (WALK_ERR);
	}

	if (getrefcount(addr, &id, "db_holds", &holds)) {
		return (WALK_ERR);
	}

	if (db.db_object == DMU_META_DNODE_OBJECT)
		(void) strcpy(objectname, "mdn");
	else
		(void) mdb_snprintf(objectname, sizeof (objectname), "%llx",
		    (u_longlong_t)db.db_object);

	if (blkid == DB_BONUS_BLKID)
		(void) strcpy(blkidname, "bonus");
	else
		(void) mdb_snprintf(blkidname, sizeof (blkidname), "%llx",
		    (u_longlong_t)blkid);

	if (objset_name(objset, path)) {
		return (WALK_ERR);
	}

	mdb_printf("%p %8s %1u %9s %2llu %s\n",
	    addr, objectname, level, blkidname, holds, path);

	return (DCMD_OK);
}

/* ARGSUSED */
static int
dbuf_stats(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
#define	HISTOSZ 32
	uintptr_t dbp;
	dmu_buf_impl_t db;
	dbuf_hash_table_t ht;
	uint64_t bucket, ndbufs;
	uint64_t histo[HISTOSZ];
	uint64_t histo2[HISTOSZ];
	int i, maxidx;

	if (mdb_readvar(&ht, "dbuf_hash_table") == -1) {
		mdb_warn("failed to read 'dbuf_hash_table'");
		return (DCMD_ERR);
	}

	for (i = 0; i < HISTOSZ; i++) {
		histo[i] = 0;
		histo2[i] = 0;
	}

	ndbufs = 0;
	for (bucket = 0; bucket < ht.hash_table_mask+1; bucket++) {
		int len;

		if (mdb_vread(&dbp, sizeof (void *),
		    (uintptr_t)(ht.hash_table+bucket)) == -1) {
			mdb_warn("failed to read hash bucket %u at %p",
			    bucket, ht.hash_table+bucket);
			return (DCMD_ERR);
		}

		len = 0;
		while (dbp != 0) {
			if (mdb_vread(&db, sizeof (dmu_buf_impl_t),
			    dbp) == -1) {
				mdb_warn("failed to read dbuf at %p", dbp);
				return (DCMD_ERR);
			}
			dbp = (uintptr_t)db.db_hash_next;
			for (i = MIN(len, HISTOSZ - 1); i >= 0; i--)
				histo2[i]++;
			len++;
			ndbufs++;
		}

		if (len >= HISTOSZ)
			len = HISTOSZ-1;
		histo[len]++;
	}

	mdb_printf("hash table has %llu buckets, %llu dbufs "
	    "(avg %llu buckets/dbuf)\n",
	    ht.hash_table_mask+1, ndbufs,
	    (ht.hash_table_mask+1)/ndbufs);

	mdb_printf("\n");
	maxidx = 0;
	for (i = 0; i < HISTOSZ; i++)
		if (histo[i] > 0)
			maxidx = i;
	mdb_printf("hash chain length	number of buckets\n");
	for (i = 0; i <= maxidx; i++)
		mdb_printf("%u			%llu\n", i, histo[i]);

	mdb_printf("\n");
	maxidx = 0;
	for (i = 0; i < HISTOSZ; i++)
		if (histo2[i] > 0)
			maxidx = i;
	mdb_printf("hash chain depth	number of dbufs\n");
	for (i = 0; i <= maxidx; i++)
		mdb_printf("%u or more		%llu	%llu%%\n",
		    i, histo2[i], histo2[i]*100/ndbufs);


	return (DCMD_OK);
}

typedef struct dbufs_data {
	mdb_ctf_id_t id;
	uint64_t objset;
	uint64_t object;
	uint64_t level;
	uint64_t blkid;
	char *osname;
} dbufs_data_t;

#define	DBUFS_UNSET	(0xbaddcafedeadbeefULL)

/* ARGSUSED */
static int
dbufs_cb(uintptr_t addr, const void *unknown, void *arg)
{
	dbufs_data_t *data = arg;
	uintptr_t objset;
	dmu_buf_t db;
	uint8_t level;
	uint64_t blkid;
	char osname[MAXNAMELEN];

	if (GETMEMBID(addr, &data->id, db_objset, objset) ||
	    GETMEMBID(addr, &data->id, db, db) ||
	    GETMEMBID(addr, &data->id, db_level, level) ||
	    GETMEMBID(addr, &data->id, db_blkid, blkid)) {
		return (WALK_ERR);
	}

	if ((data->objset == DBUFS_UNSET || data->objset == objset) &&
	    (data->osname == NULL || (objset_name(objset, osname) == 0 &&
		strcmp(data->osname, osname) == 0)) &&
	    (data->object == DBUFS_UNSET || data->object == db.db_object) &&
	    (data->level == DBUFS_UNSET || data->level == level) &&
	    (data->blkid == DBUFS_UNSET || data->blkid == blkid)) {
		mdb_printf("%#lr\n", addr);
	}
	return (WALK_NEXT);
}

/* ARGSUSED */
static int
dbufs(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
	dbufs_data_t data;
	char *object = NULL;
	char *blkid = NULL;

	data.objset = data.object = data.level = data.blkid = DBUFS_UNSET;
	data.osname = NULL;

	if (mdb_getopts(argc, argv,
	    'O', MDB_OPT_UINT64, &data.objset,
	    'n', MDB_OPT_STR, &data.osname,
	    'o', MDB_OPT_STR, &object,
	    'l', MDB_OPT_UINT64, &data.level,
	    'b', MDB_OPT_STR, &blkid) != argc) {
		return (DCMD_USAGE);
	}

	if (object) {
		if (strcmp(object, "mdn") == 0) {
			data.object = DMU_META_DNODE_OBJECT;
		} else {
			data.object = mdb_strtoull(object);
		}
	}

	if (blkid) {
		if (strcmp(blkid, "bonus") == 0) {
			data.blkid = DB_BONUS_BLKID;
		} else {
			data.blkid = mdb_strtoull(blkid);
		}
	}

	if (mdb_ctf_lookup_by_name("struct dmu_buf_impl", &data.id) == -1) {
		mdb_warn("couldn't find struct dmu_buf_impl_t");
		return (DCMD_ERR);
	}

	if (mdb_pwalk("dbufs", dbufs_cb, &data, 0) != 0) {
		mdb_warn("can't walk dbufs");
		return (DCMD_ERR);
	}

	return (DCMD_OK);
}

typedef struct abuf_find_data {
	dva_t dva;
	mdb_ctf_id_t id;
} abuf_find_data_t;

/* ARGSUSED */
static int
abuf_find_cb(uintptr_t addr, const void *unknown, void *arg)
{
	abuf_find_data_t *data = arg;
	dva_t dva;

	if (GETMEMBID(addr, &data->id, b_dva, dva)) {
		return (WALK_ERR);
	}

	if (dva.dva_word[0] == data->dva.dva_word[0] &&
	    dva.dva_word[1] == data->dva.dva_word[1]) {
		mdb_printf("%#lr\n", addr);
	}
	return (WALK_NEXT);
}

/* ARGSUSED */
static int
abuf_find(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
	abuf_find_data_t data;
	GElf_Sym sym;
	int i;
	const char *syms[] = {
		"ARC_mru_top",
		"ARC_mru_bot",
		"ARC_mfu_top",
		"ARC_mfu_bot",
	};

	if (argc != 2)
		return (DCMD_USAGE);

	for (i = 0; i < 2; i ++) {
		switch (argv[i].a_type) {
		case MDB_TYPE_STRING:
			data.dva.dva_word[i] = mdb_strtoull(argv[i].a_un.a_str);
			break;
		case MDB_TYPE_IMMEDIATE:
			data.dva.dva_word[i] = argv[i].a_un.a_val;
			break;
		default:
			return (DCMD_USAGE);
		}
	}

	if (mdb_ctf_lookup_by_name("struct arc_buf_hdr", &data.id) == -1) {
		mdb_warn("couldn't find struct arc_buf_hdr");
		return (DCMD_ERR);
	}

	for (i = 0; i < sizeof (syms) / sizeof (syms[0]); i++) {
		if (mdb_lookup_by_name(syms[i], &sym)) {
			mdb_warn("can't find symbol %s", syms[i]);
			return (DCMD_ERR);
		}

		if (mdb_pwalk("list", abuf_find_cb, &data, sym.st_value) != 0) {
			mdb_warn("can't walk %s", syms[i]);
			return (DCMD_ERR);
		}
	}

	return (DCMD_OK);
}

void
abuf_help(void)
{
	mdb_printf("::abuf_find dva_word[0] dva_word[1]\n");
}

/*
 * ::spa
 *
 * 	-c	Print configuration information as well
 * 	-v	Print vdev state
 * 	-e	Print vdev error stats
 *
 * Print a summarized spa_t.  When given no arguments, prints out a table of all
 * active pools on the system.
 */
/* ARGSUSED */
static int
spa_print(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
	spa_t spa;
	char poolname[MAXNAMELEN];
	const char *statetab[] = { "ACTIVE", "EXPORTED", "DESTROYED",
		"UNINIT", "UNAVAIL" };
	const char *state;
	int config = FALSE;
	int vdevs = FALSE;
	int errors = FALSE;

	if (mdb_getopts(argc, argv,
	    'c', MDB_OPT_SETBITS, TRUE, &config,
	    'v', MDB_OPT_SETBITS, TRUE, &vdevs,
	    'e', MDB_OPT_SETBITS, TRUE, &errors,
	    NULL) != argc)
		return (DCMD_USAGE);

	if (!(flags & DCMD_ADDRSPEC)) {
		if (mdb_walk_dcmd("spa", "spa", argc, argv) == -1) {
			mdb_warn("can't walk spa");
			return (DCMD_ERR);
		}

		return (DCMD_OK);
	}

	if (flags & DCMD_PIPE_OUT) {
		mdb_printf("%#lr\n", addr);
		return (DCMD_OK);
	}

	if (DCMD_HDRSPEC(flags))
		mdb_printf("%<u>%-?s %9s %-*s%</u>\n", "ADDR", "STATE",
		    sizeof (uintptr_t) == 4 ? 60 : 52, "NAME");

	if (mdb_vread(&spa, sizeof (spa), addr) == -1) {
		mdb_warn("failed to read spa_t at %p", addr);
		return (DCMD_ERR);
	}

	if (mdb_readstr(poolname, sizeof (poolname), (uintptr_t)spa.spa_name)
	    == -1) {
		mdb_warn("failed to read pool name at %p", spa.spa_name);
		return (DCMD_ERR);
	}

	if (spa.spa_state < 0 || spa.spa_state > POOL_STATE_UNAVAIL)
		state = "UNKNOWN";
	else
		state = statetab[spa.spa_state];

	mdb_printf("%0?p %9s %s\n", addr, state, poolname);

	if (config) {
		mdb_printf("\n");
		mdb_inc_indent(4);
		if (mdb_call_dcmd("spa_config", addr, flags, 0,
		    NULL) != DCMD_OK)
			return (DCMD_ERR);
		mdb_dec_indent(4);
	}

	if (vdevs || errors) {
		mdb_arg_t v;

		v.a_type = MDB_TYPE_STRING;
		v.a_un.a_str = "-e";

		mdb_printf("\n");
		mdb_inc_indent(4);
		if (mdb_call_dcmd("spa_vdevs", addr, flags, errors ? 1 : 0,
		    &v) != DCMD_OK)
			return (DCMD_ERR);
		mdb_dec_indent(4);
	}

	return (DCMD_OK);
}

/*
 * ::spa_config
 *
 * Given a spa_t, print the configuration information stored in spa_config.
 * Since it's just an nvlist, format it as an indented list of name=value pairs.
 * We simply read the value of spa_config and pass off to ::nvlist.
 */
/* ARGSUSED */
static int
spa_print_config(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
	spa_t spa;

	if (argc != 0 || !(flags & DCMD_ADDRSPEC))
		return (DCMD_USAGE);

	if (mdb_vread(&spa, sizeof (spa), addr) == -1) {
		mdb_warn("failed to read spa_t at %p", addr);
		return (DCMD_ERR);
	}

	if (spa.spa_config == NULL) {
		mdb_printf("(none)\n");
		return (DCMD_OK);
	}

	return (mdb_call_dcmd("nvlist", (uintptr_t)spa.spa_config, flags,
	    0, NULL));
}

void
vdev_help(void)
{
	mdb_printf("[vdev_t*]::vdev [-qr]\n"
		"\t-> -q display vdev_queue parameters\n"
		"\t-> -r recursive (visit all children)\n");
}

/*
 * ::vdev
 *
 * Print out a summarized vdev_t, in the following form:
 *
 * ADDR             STATE	AUX            DESC
 * fffffffbcde23df0 HEALTHY	-              /dev/dsk/c0t0d0
 *
 * or with "-q" to print out a vdev_t's vdev_queue parameters:
 *
 *  vdev_t: c26ae4c0
 *     c26ae73c min pending         0x2
 *     c26ae744 max pending         0x23
 *     c26ae74c agg limit           0x20000
 *     c26ae754 time shift          0x4
 *     c26ae75c ramp rate           0x2
 *
 * If '-r' is specified, recursively visit all children.
 *
 * With '-e', the statistics associated with the vdev are printed as well.
 */
static int
do_print_vdev(uintptr_t addr, int flags, int depth, int queue, int stats,
    int recursive)
{
	vdev_t vdev;
	char desc[MAXNAMELEN];
	int c, children;
	uintptr_t *child;
	const char *state, *aux;

	if (mdb_vread(&vdev, sizeof (vdev), (uintptr_t)addr) == -1) {
		mdb_warn("failed to read vdev_t at %p\n", (uintptr_t)addr);
		return (DCMD_ERR);
	}

	if (flags & DCMD_PIPE_OUT) {
		mdb_printf("%#lr", addr);
	} else {
		if (vdev.vdev_path != NULL) {
			if (mdb_readstr(desc, sizeof (desc),
			    (uintptr_t)vdev.vdev_path) == -1) {
				mdb_warn("failed to read vdev_path at %p\n",
				    vdev.vdev_path);
				return (DCMD_ERR);
			}
		} else if (vdev.vdev_ops != NULL) {
			vdev_ops_t ops;
			if (mdb_vread(&ops, sizeof (ops),
			    (uintptr_t)vdev.vdev_ops) == -1) {
				mdb_warn("failed to read vdev_ops at %p\n",
				    vdev.vdev_ops);
				return (DCMD_ERR);
			}
			(void) strcpy(desc, ops.vdev_op_type);
		} else {
			(void) strcpy(desc, "<unknown>");
		}

		if (depth == 0 && DCMD_HDRSPEC(flags))
			mdb_printf("%<u>%-?s %-9s %-12s %-*s%</u>\n",
			    "ADDR", "STATE", "AUX",
			    sizeof (uintptr_t) == 4 ? 43 : 35,
			    "DESCRIPTION");

		mdb_printf("%0?p ", addr);

		switch (vdev.vdev_state) {
		case VDEV_STATE_CLOSED:
		    state = "CLOSED";
		    break;
		case VDEV_STATE_OFFLINE:
		    state = "OFFLINE";
		    break;
		case VDEV_STATE_CANT_OPEN:
		    state = "CANT_OPEN";
		    break;
		case VDEV_STATE_DEGRADED:
		    state = "DEGRADED";
		    break;
		case VDEV_STATE_HEALTHY:
		    state = "HEALTHY";
		    break;
		default:
		    state = "UNKNOWN";
		    break;
		}

		switch (vdev.vdev_stat.vs_aux) {
		case VDEV_AUX_NONE:
			aux = "-";
			break;
		case VDEV_AUX_OPEN_FAILED:
			aux = "OPEN_FAILED";
			break;
		case VDEV_AUX_CORRUPT_DATA:
			aux = "CORRUPT_DATA";
			break;
		case VDEV_AUX_NO_REPLICAS:
			aux = "NO_REPLICAS";
			break;
		case VDEV_AUX_BAD_GUID_SUM:
			aux = "BAD_GUID_SUM";
			break;
		case VDEV_AUX_TOO_SMALL:
			aux = "TOO_SMALL";
			break;
		case VDEV_AUX_BAD_LABEL:
			aux = "BAD_LABEL";
			break;
		default:
			aux = "UNKNOWN";
			break;
		}

		mdb_printf("%-9s %-12s %*s%s\n", state, aux, depth, "", desc);

		if (queue) {
			mdb_inc_indent(4);
			mdb_printf("\n");
			mdb_printf("%p min pending		0x%llx\n",
			    (uintptr_t)(addr + offsetof(vdev_t,
			    vdev_queue.vq_min_pending)),
			    vdev.vdev_queue.vq_min_pending);
			mdb_printf("%p max pending		0x%llx\n",
			    (uintptr_t)(addr + offsetof(vdev_t,
			    vdev_queue.vq_max_pending)),
			    vdev.vdev_queue.vq_max_pending);
			mdb_printf("%p agg limit		0x%llx\n",
			    (uintptr_t)(addr + offsetof(vdev_t,
			    vdev_queue.vq_agg_limit)),
			    vdev.vdev_queue.vq_agg_limit);
			mdb_printf("%p time shift		0x%llx\n",
			    (uintptr_t)(addr + offsetof(vdev_t,
			    vdev_queue.vq_time_shift)),
			    vdev.vdev_queue.vq_time_shift);
			mdb_printf("%p ramp rate 		0x%llx\n",
			    (uintptr_t)(addr + offsetof(vdev_t,
			    vdev_queue.vq_ramp_rate)),
			    vdev.vdev_queue.vq_ramp_rate);
			mdb_dec_indent(4);
		}

		if (stats) {
			vdev_stat_t *vs = &vdev.vdev_stat;
			int i;

			mdb_inc_indent(4);
			mdb_printf("\n");
			mdb_printf("%<u>       %12s %12s %12s %12s "
			    "%12s%</u>\n", "READ", "WRITE", "FREE", "CLAIM",
			    "IOCTL");
			mdb_printf("OPS     ");
			for (i = 1; i < ZIO_TYPES; i++)
				mdb_printf("%11#llx%s", vs->vs_ops[i],
				    i == ZIO_TYPES - 1 ? "" : "  ");
			mdb_printf("\n");
			mdb_printf("BYTES   ");
			for (i = 1; i < ZIO_TYPES; i++)
				mdb_printf("%11#llx%s", vs->vs_bytes[i],
				    i == ZIO_TYPES - 1 ? "" : "  ");


			mdb_printf("\n");
			mdb_printf("EREAD    %10#llx\n", vs->vs_read_errors);
			mdb_printf("EWRITE   %10#llx\n", vs->vs_write_errors);
			mdb_printf("ECKSUM   %10#llx\n",
			    vs->vs_checksum_errors);
			mdb_dec_indent(4);
		}

		if (queue || stats)
			mdb_printf("\n");
	}

	children = vdev.vdev_children;

	if (children == 0 || !recursive)
		return (DCMD_OK);

	child = mdb_alloc(children * sizeof (void *), UM_SLEEP | UM_GC);
	if (mdb_vread(child, children * sizeof (void *),
	    (uintptr_t)vdev.vdev_child) == -1) {
		mdb_warn("failed to read vdev children at %p", vdev.vdev_child);
		return (DCMD_ERR);
	}

	for (c = 0; c < children; c++) {
		if (do_print_vdev(child[c], flags, depth + 2, queue, stats,
		    recursive))
			return (DCMD_ERR);
	}

	return (DCMD_OK);
}

static int
vdev_print(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
	int print_queue = FALSE;
	int recursive = FALSE;
	int stats = FALSE;

	if (mdb_getopts(argc, argv,
	    'q', MDB_OPT_SETBITS, TRUE, &print_queue,
	    'r', MDB_OPT_SETBITS, TRUE, &recursive,
	    'e', MDB_OPT_SETBITS, TRUE, &stats,
	    NULL) != argc)
		return (DCMD_USAGE);

	if (!(flags & DCMD_ADDRSPEC)) {
		mdb_warn("no vdev_t address given\n");
		return (DCMD_ERR);
	}

	return (do_print_vdev(addr, flags, 0, print_queue, stats, recursive));
}

typedef struct mdb_spa {
	uintptr_t spa_dsl_pool;
	uintptr_t spa_root_vdev;
} mdb_spa_t;

typedef struct mdb_dsl_dir {
	uintptr_t dd_phys;
	uint64_t dd_used_bytes;
	int64_t dd_space_towrite[TXG_SIZE];
} mdb_dsl_dir_t;

typedef struct mdb_dsl_dir_phys {
	uint64_t dd_used_bytes;
	uint64_t dd_compressed_bytes;
	uint64_t dd_uncompressed_bytes;
} mdb_dsl_dir_phys_t;

typedef struct mdb_vdev {
	uintptr_t vdev_parent;
	uintptr_t vdev_ms;
	uint64_t vdev_ms_count;
	vdev_stat_t vdev_stat;
} mdb_vdev_t;

typedef struct mdb_metaslab {
	space_map_t ms_allocmap[TXG_SIZE];
	space_map_t ms_freemap[TXG_SIZE];
	space_map_t ms_map;
	space_map_obj_t ms_smo;
} mdb_metaslab_t;

/*
 * ::spa_space [-b]
 *
 * Given a spa_t, print out it's on-disk space usage and in-core
 * estimates of future usage.  If -b is given, print space in bytes.
 * Otherwise print in megabytes.
 */
/* ARGSUSED */
static int
spa_space(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
	mdb_spa_t spa;
	uintptr_t dp_root_dir;
	mdb_dsl_dir_t dd;
	mdb_dsl_dir_phys_t dsp;
	uint64_t children;
	uintptr_t childaddr;
	uintptr_t *child;
	uint64_t ms_allocmap[TXG_SIZE] = {0, 0, 0, 0};
	uint64_t ms_freemap[TXG_SIZE] = {0, 0, 0, 0};
	uint64_t ms_map = 0;
	uint64_t avail = 0;
	int i, j;
	int havecompressed = TRUE;
	int shift = 20;
	char *suffix = "M";
	int bits = FALSE;

	if (mdb_getopts(argc, argv, 'b', MDB_OPT_SETBITS, TRUE, &bits, NULL) !=
	    argc)
		return (DCMD_USAGE);
	if (!(flags & DCMD_ADDRSPEC))
		return (DCMD_USAGE);

	if (bits) {
		shift = 0;
		suffix = "";
	}

	if (GETMEMB(addr, struct spa, spa_dsl_pool, spa.spa_dsl_pool) ||
	    GETMEMB(addr, struct spa, spa_root_vdev, spa.spa_root_vdev) ||
	    GETMEMB(spa.spa_root_vdev, struct vdev, vdev_children, children) ||
	    GETMEMB(spa.spa_root_vdev, struct vdev, vdev_child, childaddr) ||
	    GETMEMB(spa.spa_dsl_pool, struct dsl_pool,
	    dp_root_dir, dp_root_dir) ||
	    GETMEMB(dp_root_dir, struct dsl_dir, dd_phys, dd.dd_phys) ||
	    GETMEMB(dp_root_dir, struct dsl_dir,
	    dd_used_bytes, dd.dd_used_bytes) ||
	    GETMEMB(dp_root_dir, struct dsl_dir,
	    dd_space_towrite, dd.dd_space_towrite) ||
	    GETMEMB(dd.dd_phys, struct dsl_dir_phys,
	    dd_used_bytes, dsp.dd_used_bytes)) {
		return (DCMD_ERR);
	}

	if (GETMEMB(dd.dd_phys, struct dsl_dir_phys,
	    dd_compressed_bytes, dsp.dd_compressed_bytes) ||
	    GETMEMB(dd.dd_phys, struct dsl_dir_phys,
	    dd_uncompressed_bytes, dsp.dd_uncompressed_bytes)) {
		havecompressed = FALSE;
	}

	child = mdb_alloc(children * sizeof (void *), UM_SLEEP | UM_GC);
	if (mdb_vread(child, children * sizeof (void *), childaddr) == -1) {
		mdb_warn("failed to read root vdev children at %p", childaddr);
		return (DCMD_ERR);
	}

	mdb_printf("dd_space_towrite = %llu%s %llu%s %llu%s %llu%s\n",
	    dd.dd_space_towrite[0] >> shift, suffix,
	    dd.dd_space_towrite[1] >> shift, suffix,
	    dd.dd_space_towrite[2] >> shift, suffix,
	    dd.dd_space_towrite[3] >> shift, suffix);
	mdb_printf("dd_used_bytes = %llu%s\n",
	    dd.dd_used_bytes >> shift, suffix);

	mdb_printf("dd_phys.dd_used_bytes = %llu%s\n",
	    dsp.dd_used_bytes >> shift, suffix);
	if (havecompressed) {
		mdb_printf("dd_phys.dd_compressed_bytes = %llu%s\n",
		    dsp.dd_compressed_bytes >> shift, suffix);
		mdb_printf("dd_phys.dd_uncompressed_bytes = %llu%s\n",
		    dsp.dd_uncompressed_bytes >> shift, suffix);
	}

	for (i = 0; i < children; i++) {
		mdb_vdev_t vd;
		uintptr_t *vdev_ms;

		if (GETMEMB(child[i], struct vdev,
		    vdev_parent, vd.vdev_parent) ||
		    GETMEMB(child[i], struct vdev,
		    vdev_stat, vd.vdev_stat) ||
		    GETMEMB(child[i], struct vdev, vdev_ms, vd.vdev_ms) ||
		    GETMEMB(child[i], struct vdev,
		    vdev_ms_count, vd.vdev_ms_count)) {
			return (DCMD_ERR);
		}

		/*
		 * If this is the root vdev, its stats are the pool-wide stats.
		 */
		if (vd.vdev_parent == NULL) {
			mdb_printf("pool_alloc = %llu%s\n",
			    vd.vdev_stat.vs_alloc >> shift, suffix);
			mdb_printf("pool_space = %llu%s\n",
			    vd.vdev_stat.vs_space >> shift, suffix);
		}

		/*
		 * If this is not a top-level vdev, it doesn't have space.
		 */
		if (vd.vdev_parent != spa.spa_root_vdev)
			continue;

		vdev_ms = mdb_alloc(vd.vdev_ms_count * sizeof (void*),
		    UM_SLEEP | UM_GC);
		if (mdb_vread(vdev_ms, vd.vdev_ms_count * sizeof (void*),
		    (uintptr_t)vd.vdev_ms) == -1) {
			mdb_warn("failed to read vdev_ms at %p", vd.vdev_ms);
			return (DCMD_ERR);
		}

		for (j = 0; j < vd.vdev_ms_count; j++) {
			mdb_metaslab_t ms;

			if (GETMEMB(vdev_ms[j], struct metaslab,
			    ms_allocmap, ms.ms_allocmap) ||
			    GETMEMB(vdev_ms[j], struct metaslab,
			    ms_freemap, ms.ms_freemap) ||
			    GETMEMB(vdev_ms[j], struct metaslab,
			    ms_map, ms.ms_map) ||
			    GETMEMB(vdev_ms[j], struct metaslab,
			    ms_smo, ms.ms_smo)) {
				return (DCMD_ERR);
			}

			ms_allocmap[0] += ms.ms_allocmap[0].sm_space;
			ms_allocmap[1] += ms.ms_allocmap[1].sm_space;
			ms_allocmap[2] += ms.ms_allocmap[2].sm_space;
			ms_allocmap[3] += ms.ms_allocmap[3].sm_space;
			ms_freemap[0] += ms.ms_freemap[0].sm_space;
			ms_freemap[1] += ms.ms_freemap[1].sm_space;
			ms_freemap[2] += ms.ms_freemap[2].sm_space;
			ms_freemap[3] += ms.ms_freemap[3].sm_space;
			ms_map += ms.ms_map.sm_space;
			avail += ms.ms_map.sm_size - ms.ms_smo.smo_alloc;
		}
	}

	mdb_printf("ms_allocmap = %llu%s %llu%s %llu%s %llu%s\n",
	    ms_allocmap[0] >> shift, suffix,
	    ms_allocmap[1] >> shift, suffix,
	    ms_allocmap[2] >> shift, suffix,
	    ms_allocmap[3] >> shift, suffix);
	mdb_printf("ms_freemap = %llu%s %llu%s %llu%s %llu%s\n",
	    ms_freemap[0] >> shift, suffix,
	    ms_freemap[1] >> shift, suffix,
	    ms_freemap[2] >> shift, suffix,
	    ms_freemap[3] >> shift, suffix);
	mdb_printf("ms_map = %llu%s\n", ms_map >> shift, suffix);
	mdb_printf("avail = %llu%s\n", avail >> shift, suffix);

	return (DCMD_OK);
}

/*
 * ::spa_verify
 *
 * Given a spa_t, verify that that the pool is self-consistent.
 * Currently, it only checks to make sure that the vdev tree exists.
 */
/* ARGSUSED */
static int
spa_verify(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
	spa_t spa;

	if (argc != 0 || !(flags & DCMD_ADDRSPEC))
		return (DCMD_USAGE);

	if (mdb_vread(&spa, sizeof (spa), addr) == -1) {
		mdb_warn("failed to read spa_t at %p", addr);
		return (DCMD_ERR);
	}

	if (spa.spa_root_vdev == NULL) {
		mdb_printf("no vdev tree present\n");
		return (DCMD_OK);
	}

	return (DCMD_OK);
}

/*
 * ::spa_vdevs
 *
 * 	-e	Include error stats
 *
 * Print out a summarized list of vdevs for the given spa_t.
 * This is accomplished by invoking "::vdev -re" on the root vdev.
 */
/* ARGSUSED */
static int
spa_vdevs(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
	spa_t spa;
	mdb_arg_t v;
	int errors = FALSE;

	if (mdb_getopts(argc, argv,
	    'e', MDB_OPT_SETBITS, TRUE, &errors,
	    NULL) != argc)
		return (DCMD_USAGE);

	if (!(flags & DCMD_ADDRSPEC))
		return (DCMD_USAGE);

	if (mdb_vread(&spa, sizeof (spa), addr) == -1) {
		mdb_warn("failed to read spa_t at %p", addr);
		return (DCMD_ERR);
	}

	/*
	 * Unitialized spa_t structures can have a NULL root vdev.
	 */
	if (spa.spa_root_vdev == NULL) {
		mdb_printf("no associated vdevs\n");
		return (DCMD_OK);
	}

	v.a_type = MDB_TYPE_STRING;
	v.a_un.a_str = errors ? "-re" : "-r";

	return (mdb_call_dcmd("vdev", (uintptr_t)spa.spa_root_vdev,
	    flags, 1, &v));
}

typedef struct txg_list_walk_data {
	uintptr_t lw_head[TXG_SIZE];
	int	lw_txgoff;
	int	lw_maxoff;
	size_t	lw_offset;
	void	*lw_obj;
} txg_list_walk_data_t;

static int
txg_list_walk_init_common(mdb_walk_state_t *wsp, int txg, int maxoff)
{
	txg_list_walk_data_t *lwd;
	txg_list_t list;
	int i;

	lwd = mdb_alloc(sizeof (txg_list_walk_data_t), UM_SLEEP | UM_GC);
	if (mdb_vread(&list, sizeof (txg_list_t), wsp->walk_addr) == -1) {
		mdb_warn("failed to read txg_list_t at %#lx", wsp->walk_addr);
		return (WALK_ERR);
	}

	for (i = 0; i < TXG_SIZE; i++)
		lwd->lw_head[i] = (uintptr_t)list.tl_head[i];
	lwd->lw_offset = list.tl_offset;
	lwd->lw_obj = mdb_alloc(lwd->lw_offset + sizeof (txg_node_t),
	    UM_SLEEP | UM_GC);
	lwd->lw_txgoff = txg;
	lwd->lw_maxoff = maxoff;

	wsp->walk_addr = lwd->lw_head[lwd->lw_txgoff];
	wsp->walk_data = lwd;

	return (WALK_NEXT);
}

static int
txg_list_walk_init(mdb_walk_state_t *wsp)
{
	return (txg_list_walk_init_common(wsp, 0, TXG_SIZE-1));
}

static int
txg_list0_walk_init(mdb_walk_state_t *wsp)
{
	return (txg_list_walk_init_common(wsp, 0, 0));
}

static int
txg_list1_walk_init(mdb_walk_state_t *wsp)
{
	return (txg_list_walk_init_common(wsp, 1, 1));
}

static int
txg_list2_walk_init(mdb_walk_state_t *wsp)
{
	return (txg_list_walk_init_common(wsp, 2, 2));
}

static int
txg_list3_walk_init(mdb_walk_state_t *wsp)
{
	return (txg_list_walk_init_common(wsp, 3, 3));
}

static int
txg_list_walk_step(mdb_walk_state_t *wsp)
{
	txg_list_walk_data_t *lwd = wsp->walk_data;
	uintptr_t addr;
	txg_node_t *node;
	int status;

	while (wsp->walk_addr == NULL && lwd->lw_txgoff < lwd->lw_maxoff) {
		lwd->lw_txgoff++;
		wsp->walk_addr = lwd->lw_head[lwd->lw_txgoff];
	}

	if (wsp->walk_addr == NULL)
		return (WALK_DONE);

	addr = wsp->walk_addr - lwd->lw_offset;

	if (mdb_vread(lwd->lw_obj,
	    lwd->lw_offset + sizeof (txg_node_t), addr) == -1) {
		mdb_warn("failed to read list element at %#lx", addr);
		return (WALK_ERR);
	}

	status = wsp->walk_callback(addr, lwd->lw_obj, wsp->walk_cbdata);
	node = (txg_node_t *)((uintptr_t)lwd->lw_obj + lwd->lw_offset);
	wsp->walk_addr = (uintptr_t)node->tn_next[lwd->lw_txgoff];

	return (status);
}

/* ARGSUSED */
static void
txg_list_walk_fini(mdb_walk_state_t *wsp)
{
}

/*
 * ::walk spa
 *
 * Walk all named spa_t structures in the namespace.  This is nothing more than
 * a layered avl walk.
 */
static int
spa_walk_init(mdb_walk_state_t *wsp)
{
	GElf_Sym sym;

	if (wsp->walk_addr != NULL) {
		mdb_warn("spa walk only supports global walks\n");
		return (WALK_ERR);
	}

	if (mdb_lookup_by_obj(ZFS_OBJ_NAME, "spa_namespace_avl", &sym) == -1) {
		mdb_warn("failed to find symbol 'spa_namespace_avl'");
		return (WALK_ERR);
	}

	wsp->walk_addr = (uintptr_t)sym.st_value;

	if (mdb_layered_walk("avl", wsp) == -1) {
		mdb_warn("failed to walk 'avl'\n");
		return (WALK_ERR);
	}

	return (WALK_NEXT);
}

static int
spa_walk_step(mdb_walk_state_t *wsp)
{
	spa_t	spa;

	if (mdb_vread(&spa, sizeof (spa), wsp->walk_addr) == -1) {
		mdb_warn("failed to read spa_t at %p", wsp->walk_addr);
		return (WALK_ERR);
	}

	return (wsp->walk_callback(wsp->walk_addr, &spa, wsp->walk_cbdata));
}

/*
 * MDB module linkage information:
 *
 * We declare a list of structures describing our dcmds, and a function
 * named _mdb_init to return a pointer to our module information.
 */

static const mdb_dcmd_t dcmds[] = {
	{ "blkptr", ":", "print blkptr_t", blkptr },
	{ "dbuf", ":", "print dmu_buf_impl_t", dbuf },
	{ "dbuf_stats", ":", "dbuf stats", dbuf_stats },
	{ "dbufs",
	"\t[-O objset_t*] [-n objset_name | \"mos\"] [-o object | \"mdn\"] \n"
	"\t[-l level] [-b blkid | \"bonus\"]",
	"find dmu_buf_impl_t's that meet criterion", dbufs },
	{ "abuf_find", "dva_word[0] dva_word[1]",
	"find arc_buf_hdr_t of a specified DVA",
	abuf_find },
	{ "spa", "?[-cv]", "spa_t summary", spa_print },
	{ "spa_config", ":", "print spa_t configuration", spa_print_config },
	{ "spa_verify", ":", "verify spa_t consistency", spa_verify },
	{ "spa_space", ":[-b]", "print spa_t on-disk space usage", spa_space },
	{ "spa_vdevs", ":", "given a spa_t, print vdev summary", spa_vdevs },
	{ "vdev", ":[-qre]", "vdev_t summary", vdev_print },
	{ "zio_pipeline", ":", "decode a zio pipeline", zio_pipeline },
	{ NULL }
};

static const mdb_walker_t walkers[] = {
	/*
	 * In userland, there is no generic provider of list_t walkers, so we
	 * need to add it.
	 */
#ifndef _KERNEL
	{ LIST_WALK_NAME, LIST_WALK_DESC,
		list_walk_init, list_walk_step, list_walk_fini },
#endif
	{ "dbufs", "walk cached ZFS dbufs",
		dbuf_walk_init, dbuf_walk_step, dbuf_walk_fini },
	{ "zms_freelist", "walk ZFS metaslab freelist",
		freelist_walk_init, freelist_walk_step, freelist_walk_fini },
	{ "txg_list", "given any txg_list_t *, walk all entries in all txgs",
		txg_list_walk_init, txg_list_walk_step, txg_list_walk_fini },
	{ "txg_list0", "given any txg_list_t *, walk all entries in txg 0",
		txg_list0_walk_init, txg_list_walk_step, txg_list_walk_fini },
	{ "txg_list1", "given any txg_list_t *, walk all entries in txg 1",
		txg_list1_walk_init, txg_list_walk_step, txg_list_walk_fini },
	{ "txg_list2", "given any txg_list_t *, walk all entries in txg 2",
		txg_list2_walk_init, txg_list_walk_step, txg_list_walk_fini },
	{ "txg_list3", "given any txg_list_t *, walk all entries in txg 3",
		txg_list3_walk_init, txg_list_walk_step, txg_list_walk_fini },
	{ "spa", "walk all spa_t entries in the namespace",
		spa_walk_init, spa_walk_step, NULL },
	{ NULL }
};

static const mdb_modinfo_t modinfo = {
	MDB_API_VERSION, dcmds, walkers
};

const mdb_modinfo_t *
_mdb_init(void)
{
	return (&modinfo);
}