Allow dsl_deadlist_open() return errors

[zfs.git] / module / zfs / space_map.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 2009 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */
/*
 * Copyright (c) 2012, 2019 by Delphix. All rights reserved.
 */

#include <sys/zfs_context.h>
#include <sys/spa.h>
#include <sys/dmu.h>
#include <sys/dmu_tx.h>
#include <sys/dnode.h>
#include <sys/dsl_pool.h>
#include <sys/zio.h>
#include <sys/space_map.h>
#include <sys/zfeature.h>

/*
 * Note on space map block size:
 *
 * The data for a given space map can be kept on blocks of any size.
 * Larger blocks entail fewer I/O operations, but they also cause the
 * DMU to keep more data in-core, and also to waste more I/O bandwidth
 * when only a few blocks have changed since the last transaction group.
 */

/*
 * Enabled whenever we want to stress test the use of double-word
 * space map entries.
 */
boolean_t zfs_force_some_double_word_sm_entries = B_FALSE;

/*
 * Override the default indirect block size of 128K, instead use 16K for
 * spacemaps (2^14 bytes).  This dramatically reduces write inflation since
 * appending to a spacemap typically has to write one data block (4KB) and one
 * or two indirect blocks (16K-32K, rather than 128K).
 */
int space_map_ibs = 14;

boolean_t
sm_entry_is_debug(uint64_t e)
{
        return (SM_PREFIX_DECODE(e) == SM_DEBUG_PREFIX);
}

boolean_t
sm_entry_is_single_word(uint64_t e)
{
        uint8_t prefix = SM_PREFIX_DECODE(e);
        return (prefix != SM_DEBUG_PREFIX && prefix != SM2_PREFIX);
}

boolean_t
sm_entry_is_double_word(uint64_t e)
{
        return (SM_PREFIX_DECODE(e) == SM2_PREFIX);
}

/*
 * Iterate through the space map, invoking the callback on each (non-debug)
 * space map entry. Stop after reading 'end' bytes of the space map.
 */
int
space_map_iterate(space_map_t *sm, uint64_t end, sm_cb_t callback, void *arg)
{
        uint64_t blksz = sm->sm_blksz;

        ASSERT3U(blksz, !=, 0);
        ASSERT3U(end, <=, space_map_length(sm));
        ASSERT0(P2PHASE(end, sizeof (uint64_t)));

        dmu_prefetch(sm->sm_os, space_map_object(sm), 0, 0, end,
            ZIO_PRIORITY_SYNC_READ);

        int error = 0;
        uint64_t txg = 0, sync_pass = 0;
        for (uint64_t block_base = 0; block_base < end && error == 0;
            block_base += blksz) {
                dmu_buf_t *db;
                error = dmu_buf_hold(sm->sm_os, space_map_object(sm),
                    block_base, FTAG, &db, DMU_READ_PREFETCH);
                if (error != 0)
                        return (error);

                uint64_t *block_start = db->db_data;
                uint64_t block_length = MIN(end - block_base, blksz);
                uint64_t *block_end = block_start +
                    (block_length / sizeof (uint64_t));

                VERIFY0(P2PHASE(block_length, sizeof (uint64_t)));
                VERIFY3U(block_length, !=, 0);
                ASSERT3U(blksz, ==, db->db_size);

                for (uint64_t *block_cursor = block_start;
                    block_cursor < block_end && error == 0; block_cursor++) {
                        uint64_t e = *block_cursor;

                        if (sm_entry_is_debug(e)) {
                                /*
                                 * Debug entries are only needed to record the
                                 * current TXG and sync pass if available.
                                 *
                                 * Note though that sometimes there can be
                                 * debug entries that are used as padding
                                 * at the end of space map blocks in-order
                                 * to not split a double-word entry in the
                                 * middle between two blocks. These entries
                                 * have their TXG field set to 0 and we
                                 * skip them without recording the TXG.
                                 * [see comment in space_map_write_seg()]
                                 */
                                uint64_t e_txg = SM_DEBUG_TXG_DECODE(e);
                                if (e_txg != 0) {
                                        txg = e_txg;
                                        sync_pass = SM_DEBUG_SYNCPASS_DECODE(e);
                                } else {
                                        ASSERT0(SM_DEBUG_SYNCPASS_DECODE(e));
                                }
                                continue;
                        }

                        uint64_t raw_offset, raw_run, vdev_id;
                        maptype_t type;
                        if (sm_entry_is_single_word(e)) {
                                type = SM_TYPE_DECODE(e);
                                vdev_id = SM_NO_VDEVID;
                                raw_offset = SM_OFFSET_DECODE(e);
                                raw_run = SM_RUN_DECODE(e);
                        } else {
                                /* it is a two-word entry */
                                ASSERT(sm_entry_is_double_word(e));
                                raw_run = SM2_RUN_DECODE(e);
                                vdev_id = SM2_VDEV_DECODE(e);

                                /* move on to the second word */
                                block_cursor++;
                                e = *block_cursor;
                                VERIFY3P(block_cursor, <=, block_end);

                                type = SM2_TYPE_DECODE(e);
                                raw_offset = SM2_OFFSET_DECODE(e);
                        }

                        uint64_t entry_offset = (raw_offset << sm->sm_shift) +
                            sm->sm_start;
                        uint64_t entry_run = raw_run << sm->sm_shift;

                        VERIFY0(P2PHASE(entry_offset, 1ULL << sm->sm_shift));
                        VERIFY0(P2PHASE(entry_run, 1ULL << sm->sm_shift));
                        ASSERT3U(entry_offset, >=, sm->sm_start);
                        ASSERT3U(entry_offset, <, sm->sm_start + sm->sm_size);
                        ASSERT3U(entry_run, <=, sm->sm_size);
                        ASSERT3U(entry_offset + entry_run, <=,
                            sm->sm_start + sm->sm_size);

                        space_map_entry_t sme = {
                            .sme_type = type,
                            .sme_vdev = vdev_id,
                            .sme_offset = entry_offset,
                            .sme_run = entry_run,
                            .sme_txg = txg,
                            .sme_sync_pass = sync_pass
                        };
                        error = callback(&sme, arg);
                }
                dmu_buf_rele(db, FTAG);
        }
        return (error);
}

/*
 * Reads the entries from the last block of the space map into
 * buf in reverse order. Populates nwords with number of words
 * in the last block.
 *
 * Refer to block comment within space_map_incremental_destroy()
 * to understand why this function is needed.
 */
static int
space_map_reversed_last_block_entries(space_map_t *sm, uint64_t *buf,
    uint64_t bufsz, uint64_t *nwords)
{
        int error = 0;
        dmu_buf_t *db;

        /*
         * Find the offset of the last word in the space map and use
         * that to read the last block of the space map with
         * dmu_buf_hold().
         */
        uint64_t last_word_offset =
            sm->sm_phys->smp_length - sizeof (uint64_t);
        error = dmu_buf_hold(sm->sm_os, space_map_object(sm), last_word_offset,
            FTAG, &db, DMU_READ_NO_PREFETCH);
        if (error != 0)
                return (error);

        ASSERT3U(sm->sm_object, ==, db->db_object);
        ASSERT3U(sm->sm_blksz, ==, db->db_size);
        ASSERT3U(bufsz, >=, db->db_size);
        ASSERT(nwords != NULL);

        uint64_t *words = db->db_data;
        *nwords =
            (sm->sm_phys->smp_length - db->db_offset) / sizeof (uint64_t);

        ASSERT3U(*nwords, <=, bufsz / sizeof (uint64_t));

        uint64_t n = *nwords;
        uint64_t j = n - 1;
        for (uint64_t i = 0; i < n; i++) {
                uint64_t entry = words[i];
                if (sm_entry_is_double_word(entry)) {
                        /*
                         * Since we are populating the buffer backwards
                         * we have to be extra careful and add the two
                         * words of the double-word entry in the right
                         * order.
                         */
                        ASSERT3U(j, >, 0);
                        buf[j - 1] = entry;

                        i++;
                        ASSERT3U(i, <, n);
                        entry = words[i];
                        buf[j] = entry;
                        j -= 2;
                } else {
                        ASSERT(sm_entry_is_debug(entry) ||
                            sm_entry_is_single_word(entry));
                        buf[j] = entry;
                        j--;
                }
        }

        /*
         * Assert that we wrote backwards all the
         * way to the beginning of the buffer.
         */
        ASSERT3S(j, ==, -1);

        dmu_buf_rele(db, FTAG);
        return (error);
}

/*
 * Note: This function performs destructive actions - specifically
 * it deletes entries from the end of the space map. Thus, callers
 * should ensure that they are holding the appropriate locks for
 * the space map that they provide.
 */
int
space_map_incremental_destroy(space_map_t *sm, sm_cb_t callback, void *arg,
    dmu_tx_t *tx)
{
        uint64_t bufsz = MAX(sm->sm_blksz, SPA_MINBLOCKSIZE);
        uint64_t *buf = zio_buf_alloc(bufsz);

        dmu_buf_will_dirty(sm->sm_dbuf, tx);

        /*
         * Ideally we would want to iterate from the beginning of the
         * space map to the end in incremental steps. The issue with this
         * approach is that we don't have any field on-disk that points
         * us where to start between each step. We could try zeroing out
         * entries that we've destroyed, but this doesn't work either as
         * an entry that is 0 is a valid one (ALLOC for range [0x0:0x200]).
         *
         * As a result, we destroy its entries incrementally starting from
         * the end after applying the callback to each of them.
         *
         * The problem with this approach is that we cannot literally
         * iterate through the words in the space map backwards as we
         * can't distinguish two-word space map entries from their second
         * word. Thus we do the following:
         *
         * 1] We get all the entries from the last block of the space map
         *    and put them into a buffer in reverse order. This way the
         *    last entry comes first in the buffer, the second to last is
         *    second, etc.
         * 2] We iterate through the entries in the buffer and we apply
         *    the callback to each one. As we move from entry to entry we
         *    we decrease the size of the space map, deleting effectively
         *    each entry.
         * 3] If there are no more entries in the space map or the callback
         *    returns a value other than 0, we stop iterating over the
         *    space map. If there are entries remaining and the callback
         *    returned 0, we go back to step [1].
         */
        int error = 0;
        while (space_map_length(sm) > 0 && error == 0) {
                uint64_t nwords = 0;
                error = space_map_reversed_last_block_entries(sm, buf, bufsz,
                    &nwords);
                if (error != 0)
                        break;

                ASSERT3U(nwords, <=, bufsz / sizeof (uint64_t));

                for (uint64_t i = 0; i < nwords; i++) {
                        uint64_t e = buf[i];

                        if (sm_entry_is_debug(e)) {
                                sm->sm_phys->smp_length -= sizeof (uint64_t);
                                continue;
                        }

                        int words = 1;
                        uint64_t raw_offset, raw_run, vdev_id;
                        maptype_t type;
                        if (sm_entry_is_single_word(e)) {
                                type = SM_TYPE_DECODE(e);
                                vdev_id = SM_NO_VDEVID;
                                raw_offset = SM_OFFSET_DECODE(e);
                                raw_run = SM_RUN_DECODE(e);
                        } else {
                                ASSERT(sm_entry_is_double_word(e));
                                words = 2;

                                raw_run = SM2_RUN_DECODE(e);
                                vdev_id = SM2_VDEV_DECODE(e);

                                /* move to the second word */
                                i++;
                                e = buf[i];

                                ASSERT3P(i, <=, nwords);

                                type = SM2_TYPE_DECODE(e);
                                raw_offset = SM2_OFFSET_DECODE(e);
                        }

                        uint64_t entry_offset =
                            (raw_offset << sm->sm_shift) + sm->sm_start;
                        uint64_t entry_run = raw_run << sm->sm_shift;

                        VERIFY0(P2PHASE(entry_offset, 1ULL << sm->sm_shift));
                        VERIFY0(P2PHASE(entry_run, 1ULL << sm->sm_shift));
                        VERIFY3U(entry_offset, >=, sm->sm_start);
                        VERIFY3U(entry_offset, <, sm->sm_start + sm->sm_size);
                        VERIFY3U(entry_run, <=, sm->sm_size);
                        VERIFY3U(entry_offset + entry_run, <=,
                            sm->sm_start + sm->sm_size);

                        space_map_entry_t sme = {
                            .sme_type = type,
                            .sme_vdev = vdev_id,
                            .sme_offset = entry_offset,
                            .sme_run = entry_run
                        };
                        error = callback(&sme, arg);
                        if (error != 0)
                                break;

                        if (type == SM_ALLOC)
                                sm->sm_phys->smp_alloc -= entry_run;
                        else
                                sm->sm_phys->smp_alloc += entry_run;
                        sm->sm_phys->smp_length -= words * sizeof (uint64_t);
                }
        }

        if (space_map_length(sm) == 0) {
                ASSERT0(error);
                ASSERT0(space_map_allocated(sm));
        }

        zio_buf_free(buf, bufsz);
        return (error);
}

typedef struct space_map_load_arg {
        space_map_t     *smla_sm;
        range_tree_t    *smla_rt;
        maptype_t       smla_type;
} space_map_load_arg_t;

static int
space_map_load_callback(space_map_entry_t *sme, void *arg)
{
        space_map_load_arg_t *smla = arg;
        if (sme->sme_type == smla->smla_type) {
                VERIFY3U(range_tree_space(smla->smla_rt) + sme->sme_run, <=,
                    smla->smla_sm->sm_size);
                range_tree_add(smla->smla_rt, sme->sme_offset, sme->sme_run);
        } else {
                range_tree_remove(smla->smla_rt, sme->sme_offset, sme->sme_run);
        }

        return (0);
}

/*
 * Load the spacemap into the rangetree, like space_map_load. But only
 * read the first 'length' bytes of the spacemap.
 */
int
space_map_load_length(space_map_t *sm, range_tree_t *rt, maptype_t maptype,
    uint64_t length)
{
        space_map_load_arg_t smla;

        VERIFY0(range_tree_space(rt));

        if (maptype == SM_FREE)
                range_tree_add(rt, sm->sm_start, sm->sm_size);

        smla.smla_rt = rt;
        smla.smla_sm = sm;
        smla.smla_type = maptype;
        int err = space_map_iterate(sm, length,
            space_map_load_callback, &smla);

        if (err != 0)
                range_tree_vacate(rt, NULL, NULL);

        return (err);
}

/*
 * Load the space map disk into the specified range tree. Segments of maptype
 * are added to the range tree, other segment types are removed.
 */
int
space_map_load(space_map_t *sm, range_tree_t *rt, maptype_t maptype)
{
        return (space_map_load_length(sm, rt, maptype, space_map_length(sm)));
}

void
space_map_histogram_clear(space_map_t *sm)
{
        if (sm->sm_dbuf->db_size != sizeof (space_map_phys_t))
                return;

        memset(sm->sm_phys->smp_histogram, 0,
            sizeof (sm->sm_phys->smp_histogram));
}

boolean_t
space_map_histogram_verify(space_map_t *sm, range_tree_t *rt)
{
        /*
         * Verify that the in-core range tree does not have any
         * ranges smaller than our sm_shift size.
         */
        for (int i = 0; i < sm->sm_shift; i++) {
                if (rt->rt_histogram[i] != 0)
                        return (B_FALSE);
        }
        return (B_TRUE);
}

void
space_map_histogram_add(space_map_t *sm, range_tree_t *rt, dmu_tx_t *tx)
{
        int idx = 0;

        ASSERT(dmu_tx_is_syncing(tx));
        VERIFY3U(space_map_object(sm), !=, 0);

        if (sm->sm_dbuf->db_size != sizeof (space_map_phys_t))
                return;

        dmu_buf_will_dirty(sm->sm_dbuf, tx);

        ASSERT(space_map_histogram_verify(sm, rt));
        /*
         * Transfer the content of the range tree histogram to the space
         * map histogram. The space map histogram contains 32 buckets ranging
         * between 2^sm_shift to 2^(32+sm_shift-1). The range tree,
         * however, can represent ranges from 2^0 to 2^63. Since the space
         * map only cares about allocatable blocks (minimum of sm_shift) we
         * can safely ignore all ranges in the range tree smaller than sm_shift.
         */
        for (int i = sm->sm_shift; i < RANGE_TREE_HISTOGRAM_SIZE; i++) {

                /*
                 * Since the largest histogram bucket in the space map is
                 * 2^(32+sm_shift-1), we need to normalize the values in
                 * the range tree for any bucket larger than that size. For
                 * example given an sm_shift of 9, ranges larger than 2^40
                 * would get normalized as if they were 1TB ranges. Assume
                 * the range tree had a count of 5 in the 2^44 (16TB) bucket,
                 * the calculation below would normalize this to 5 * 2^4 (16).
                 */
                ASSERT3U(i, >=, idx + sm->sm_shift);
                sm->sm_phys->smp_histogram[idx] +=
                    rt->rt_histogram[i] << (i - idx - sm->sm_shift);

                /*
                 * Increment the space map's index as long as we haven't
                 * reached the maximum bucket size. Accumulate all ranges
                 * larger than the max bucket size into the last bucket.
                 */
                if (idx < SPACE_MAP_HISTOGRAM_SIZE - 1) {
                        ASSERT3U(idx + sm->sm_shift, ==, i);
                        idx++;
                        ASSERT3U(idx, <, SPACE_MAP_HISTOGRAM_SIZE);
                }
        }
}

static void
space_map_write_intro_debug(space_map_t *sm, maptype_t maptype, dmu_tx_t *tx)
{
        dmu_buf_will_dirty(sm->sm_dbuf, tx);

        uint64_t dentry = SM_PREFIX_ENCODE(SM_DEBUG_PREFIX) |
            SM_DEBUG_ACTION_ENCODE(maptype) |
            SM_DEBUG_SYNCPASS_ENCODE(spa_sync_pass(tx->tx_pool->dp_spa)) |
            SM_DEBUG_TXG_ENCODE(dmu_tx_get_txg(tx));

        dmu_write(sm->sm_os, space_map_object(sm), sm->sm_phys->smp_length,
            sizeof (dentry), &dentry, tx);

        sm->sm_phys->smp_length += sizeof (dentry);
}

/*
 * Writes one or more entries given a segment.
 *
 * Note: The function may release the dbuf from the pointer initially
 * passed to it, and return a different dbuf. Also, the space map's
 * dbuf must be dirty for the changes in sm_phys to take effect.
 */
static void
space_map_write_seg(space_map_t *sm, uint64_t rstart, uint64_t rend,
    maptype_t maptype, uint64_t vdev_id, uint8_t words, dmu_buf_t **dbp,
    const void *tag, dmu_tx_t *tx)
{
        ASSERT3U(words, !=, 0);
        ASSERT3U(words, <=, 2);

        /* ensure the vdev_id can be represented by the space map */
        ASSERT3U(vdev_id, <=, SM_NO_VDEVID);

        /*
         * if this is a single word entry, ensure that no vdev was
         * specified.
         */
        IMPLY(words == 1, vdev_id == SM_NO_VDEVID);

        dmu_buf_t *db = *dbp;
        ASSERT3U(db->db_size, ==, sm->sm_blksz);

        uint64_t *block_base = db->db_data;
        uint64_t *block_end = block_base + (sm->sm_blksz / sizeof (uint64_t));
        uint64_t *block_cursor = block_base +
            (sm->sm_phys->smp_length - db->db_offset) / sizeof (uint64_t);

        ASSERT3P(block_cursor, <=, block_end);

        uint64_t size = (rend - rstart) >> sm->sm_shift;
        uint64_t start = (rstart - sm->sm_start) >> sm->sm_shift;
        uint64_t run_max = (words == 2) ? SM2_RUN_MAX : SM_RUN_MAX;

        ASSERT3U(rstart, >=, sm->sm_start);
        ASSERT3U(rstart, <, sm->sm_start + sm->sm_size);
        ASSERT3U(rend - rstart, <=, sm->sm_size);
        ASSERT3U(rend, <=, sm->sm_start + sm->sm_size);

        while (size != 0) {
                ASSERT3P(block_cursor, <=, block_end);

                /*
                 * If we are at the end of this block, flush it and start
                 * writing again from the beginning.
                 */
                if (block_cursor == block_end) {
                        dmu_buf_rele(db, tag);

                        uint64_t next_word_offset = sm->sm_phys->smp_length;
                        VERIFY0(dmu_buf_hold(sm->sm_os,
                            space_map_object(sm), next_word_offset,
                            tag, &db, DMU_READ_PREFETCH));
                        dmu_buf_will_dirty(db, tx);

                        /* update caller's dbuf */
                        *dbp = db;

                        ASSERT3U(db->db_size, ==, sm->sm_blksz);

                        block_base = db->db_data;
                        block_cursor = block_base;
                        block_end = block_base +
                            (db->db_size / sizeof (uint64_t));
                }

                /*
                 * If we are writing a two-word entry and we only have one
                 * word left on this block, just pad it with an empty debug
                 * entry and write the two-word entry in the next block.
                 */
                uint64_t *next_entry = block_cursor + 1;
                if (next_entry == block_end && words > 1) {
                        ASSERT3U(words, ==, 2);
                        *block_cursor = SM_PREFIX_ENCODE(SM_DEBUG_PREFIX) |
                            SM_DEBUG_ACTION_ENCODE(0) |
                            SM_DEBUG_SYNCPASS_ENCODE(0) |
                            SM_DEBUG_TXG_ENCODE(0);
                        block_cursor++;
                        sm->sm_phys->smp_length += sizeof (uint64_t);
                        ASSERT3P(block_cursor, ==, block_end);
                        continue;
                }

                uint64_t run_len = MIN(size, run_max);
                switch (words) {
                case 1:
                        *block_cursor = SM_OFFSET_ENCODE(start) |
                            SM_TYPE_ENCODE(maptype) |
                            SM_RUN_ENCODE(run_len);
                        block_cursor++;
                        break;
                case 2:
                        /* write the first word of the entry */
                        *block_cursor = SM_PREFIX_ENCODE(SM2_PREFIX) |
                            SM2_RUN_ENCODE(run_len) |
                            SM2_VDEV_ENCODE(vdev_id);
                        block_cursor++;

                        /* move on to the second word of the entry */
                        ASSERT3P(block_cursor, <, block_end);
                        *block_cursor = SM2_TYPE_ENCODE(maptype) |
                            SM2_OFFSET_ENCODE(start);
                        block_cursor++;
                        break;
                default:
                        panic("%d-word space map entries are not supported",
                            words);
                        break;
                }
                sm->sm_phys->smp_length += words * sizeof (uint64_t);

                start += run_len;
                size -= run_len;
        }
        ASSERT0(size);

}

/*
 * Note: The space map's dbuf must be dirty for the changes in sm_phys to
 * take effect.
 */
static void
space_map_write_impl(space_map_t *sm, range_tree_t *rt, maptype_t maptype,
    uint64_t vdev_id, dmu_tx_t *tx)
{
        spa_t *spa = tx->tx_pool->dp_spa;
        dmu_buf_t *db;

        space_map_write_intro_debug(sm, maptype, tx);

#ifdef ZFS_DEBUG
        /*
         * We do this right after we write the intro debug entry
         * because the estimate does not take it into account.
         */
        uint64_t initial_objsize = sm->sm_phys->smp_length;
        uint64_t estimated_growth =
            space_map_estimate_optimal_size(sm, rt, SM_NO_VDEVID);
        uint64_t estimated_final_objsize = initial_objsize + estimated_growth;
#endif

        /*
         * Find the offset right after the last word in the space map
         * and use that to get a hold of the last block, so we can
         * start appending to it.
         */
        uint64_t next_word_offset = sm->sm_phys->smp_length;
        VERIFY0(dmu_buf_hold(sm->sm_os, space_map_object(sm),
            next_word_offset, FTAG, &db, DMU_READ_PREFETCH));
        ASSERT3U(db->db_size, ==, sm->sm_blksz);

        dmu_buf_will_dirty(db, tx);

        zfs_btree_t *t = &rt->rt_root;
        zfs_btree_index_t where;
        for (range_seg_t *rs = zfs_btree_first(t, &where); rs != NULL;
            rs = zfs_btree_next(t, &where, &where)) {
                uint64_t offset = (rs_get_start(rs, rt) - sm->sm_start) >>
                    sm->sm_shift;
                uint64_t length = (rs_get_end(rs, rt) - rs_get_start(rs, rt)) >>
                    sm->sm_shift;
                uint8_t words = 1;

                /*
                 * We only write two-word entries when both of the following
                 * are true:
                 *
                 * [1] The feature is enabled.
                 * [2] The offset or run is too big for a single-word entry,
                 *      or the vdev_id is set (meaning not equal to
                 *      SM_NO_VDEVID).
                 *
                 * Note that for purposes of testing we've added the case that
                 * we write two-word entries occasionally when the feature is
                 * enabled and zfs_force_some_double_word_sm_entries has been
                 * set.
                 */
                if (spa_feature_is_active(spa, SPA_FEATURE_SPACEMAP_V2) &&
                    (offset >= (1ULL << SM_OFFSET_BITS) ||
                    length > SM_RUN_MAX ||
                    vdev_id != SM_NO_VDEVID ||
                    (zfs_force_some_double_word_sm_entries &&
                    random_in_range(100) == 0)))
                        words = 2;

                space_map_write_seg(sm, rs_get_start(rs, rt), rs_get_end(rs,
                    rt), maptype, vdev_id, words, &db, FTAG, tx);
        }

        dmu_buf_rele(db, FTAG);

#ifdef ZFS_DEBUG
        /*
         * We expect our estimation to be based on the worst case
         * scenario [see comment in space_map_estimate_optimal_size()].
         * Therefore we expect the actual objsize to be equal or less
         * than whatever we estimated it to be.
         */
        ASSERT3U(estimated_final_objsize, >=, sm->sm_phys->smp_length);
#endif
}

/*
 * Note: This function manipulates the state of the given space map but
 * does not hold any locks implicitly. Thus the caller is responsible
 * for synchronizing writes to the space map.
 */
void
space_map_write(space_map_t *sm, range_tree_t *rt, maptype_t maptype,
    uint64_t vdev_id, dmu_tx_t *tx)
{
        ASSERT(dsl_pool_sync_context(dmu_objset_pool(sm->sm_os)));
        VERIFY3U(space_map_object(sm), !=, 0);

        dmu_buf_will_dirty(sm->sm_dbuf, tx);

        /*
         * This field is no longer necessary since the in-core space map
         * now contains the object number but is maintained for backwards
         * compatibility.
         */
        sm->sm_phys->smp_object = sm->sm_object;

        if (range_tree_is_empty(rt)) {
                VERIFY3U(sm->sm_object, ==, sm->sm_phys->smp_object);
                return;
        }

        if (maptype == SM_ALLOC)
                sm->sm_phys->smp_alloc += range_tree_space(rt);
        else
                sm->sm_phys->smp_alloc -= range_tree_space(rt);

        uint64_t nodes = zfs_btree_numnodes(&rt->rt_root);
        uint64_t rt_space = range_tree_space(rt);

        space_map_write_impl(sm, rt, maptype, vdev_id, tx);

        /*
         * Ensure that the space_map's accounting wasn't changed
         * while we were in the middle of writing it out.
         */
        VERIFY3U(nodes, ==, zfs_btree_numnodes(&rt->rt_root));
        VERIFY3U(range_tree_space(rt), ==, rt_space);
}

static int
space_map_open_impl(space_map_t *sm)
{
        int error;
        u_longlong_t blocks;

        error = dmu_bonus_hold(sm->sm_os, sm->sm_object, sm, &sm->sm_dbuf);
        if (error)
                return (error);

        dmu_object_size_from_db(sm->sm_dbuf, &sm->sm_blksz, &blocks);
        sm->sm_phys = sm->sm_dbuf->db_data;
        return (0);
}

int
space_map_open(space_map_t **smp, objset_t *os, uint64_t object,
    uint64_t start, uint64_t size, uint8_t shift)
{
        space_map_t *sm;
        int error;

        ASSERT(*smp == NULL);
        ASSERT(os != NULL);
        ASSERT(object != 0);

        sm = kmem_alloc(sizeof (space_map_t), KM_SLEEP);

        sm->sm_start = start;
        sm->sm_size = size;
        sm->sm_shift = shift;
        sm->sm_os = os;
        sm->sm_object = object;
        sm->sm_blksz = 0;
        sm->sm_dbuf = NULL;
        sm->sm_phys = NULL;

        error = space_map_open_impl(sm);
        if (error != 0) {
                space_map_close(sm);
                return (error);
        }
        *smp = sm;

        return (0);
}

void
space_map_close(space_map_t *sm)
{
        if (sm == NULL)
                return;

        if (sm->sm_dbuf != NULL)
                dmu_buf_rele(sm->sm_dbuf, sm);
        sm->sm_dbuf = NULL;
        sm->sm_phys = NULL;

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

void
space_map_truncate(space_map_t *sm, int blocksize, dmu_tx_t *tx)
{
        objset_t *os = sm->sm_os;
        spa_t *spa = dmu_objset_spa(os);
        dmu_object_info_t doi;

        ASSERT(dsl_pool_sync_context(dmu_objset_pool(os)));
        ASSERT(dmu_tx_is_syncing(tx));
        VERIFY3U(dmu_tx_get_txg(tx), <=, spa_final_dirty_txg(spa));

        dmu_object_info_from_db(sm->sm_dbuf, &doi);

        /*
         * If the space map has the wrong bonus size (because
         * SPA_FEATURE_SPACEMAP_HISTOGRAM has recently been enabled), or
         * the wrong block size (because space_map_blksz has changed),
         * free and re-allocate its object with the updated sizes.
         *
         * Otherwise, just truncate the current object.
         */
        if ((spa_feature_is_enabled(spa, SPA_FEATURE_SPACEMAP_HISTOGRAM) &&
            doi.doi_bonus_size != sizeof (space_map_phys_t)) ||
            doi.doi_data_block_size != blocksize ||
            doi.doi_metadata_block_size != 1 << space_map_ibs) {
                zfs_dbgmsg("txg %llu, spa %s, sm %px, reallocating "
                    "object[%llu]: old bonus %llu, old blocksz %u",
                    (u_longlong_t)dmu_tx_get_txg(tx), spa_name(spa), sm,
                    (u_longlong_t)sm->sm_object,
                    (u_longlong_t)doi.doi_bonus_size,
                    doi.doi_data_block_size);

                space_map_free(sm, tx);
                dmu_buf_rele(sm->sm_dbuf, sm);

                sm->sm_object = space_map_alloc(sm->sm_os, blocksize, tx);
                VERIFY0(space_map_open_impl(sm));
        } else {
                VERIFY0(dmu_free_range(os, space_map_object(sm), 0, -1ULL, tx));

                /*
                 * If the spacemap is reallocated, its histogram
                 * will be reset.  Do the same in the common case so that
                 * bugs related to the uncommon case do not go unnoticed.
                 */
                memset(sm->sm_phys->smp_histogram, 0,
                    sizeof (sm->sm_phys->smp_histogram));
        }

        dmu_buf_will_dirty(sm->sm_dbuf, tx);
        sm->sm_phys->smp_length = 0;
        sm->sm_phys->smp_alloc = 0;
}

uint64_t
space_map_alloc(objset_t *os, int blocksize, dmu_tx_t *tx)
{
        spa_t *spa = dmu_objset_spa(os);
        uint64_t object;
        int bonuslen;

        if (spa_feature_is_enabled(spa, SPA_FEATURE_SPACEMAP_HISTOGRAM)) {
                spa_feature_incr(spa, SPA_FEATURE_SPACEMAP_HISTOGRAM, tx);
                bonuslen = sizeof (space_map_phys_t);
                ASSERT3U(bonuslen, <=, dmu_bonus_max());
        } else {
                bonuslen = SPACE_MAP_SIZE_V0;
        }

        object = dmu_object_alloc_ibs(os, DMU_OT_SPACE_MAP, blocksize,
            space_map_ibs, DMU_OT_SPACE_MAP_HEADER, bonuslen, tx);

        return (object);
}

void
space_map_free_obj(objset_t *os, uint64_t smobj, dmu_tx_t *tx)
{
        spa_t *spa = dmu_objset_spa(os);
        if (spa_feature_is_enabled(spa, SPA_FEATURE_SPACEMAP_HISTOGRAM)) {
                dmu_object_info_t doi;

                VERIFY0(dmu_object_info(os, smobj, &doi));
                if (doi.doi_bonus_size != SPACE_MAP_SIZE_V0) {
                        spa_feature_decr(spa,
                            SPA_FEATURE_SPACEMAP_HISTOGRAM, tx);
                }
        }

        VERIFY0(dmu_object_free(os, smobj, tx));
}

void
space_map_free(space_map_t *sm, dmu_tx_t *tx)
{
        if (sm == NULL)
                return;

        space_map_free_obj(sm->sm_os, space_map_object(sm), tx);
        sm->sm_object = 0;
}

/*
 * Given a range tree, it makes a worst-case estimate of how much
 * space would the tree's segments take if they were written to
 * the given space map.
 */
uint64_t
space_map_estimate_optimal_size(space_map_t *sm, range_tree_t *rt,
    uint64_t vdev_id)
{
        spa_t *spa = dmu_objset_spa(sm->sm_os);
        uint64_t shift = sm->sm_shift;
        uint64_t *histogram = rt->rt_histogram;
        uint64_t entries_for_seg = 0;

        /*
         * In order to get a quick estimate of the optimal size that this
         * range tree would have on-disk as a space map, we iterate through
         * its histogram buckets instead of iterating through its nodes.
         *
         * Note that this is a highest-bound/worst-case estimate for the
         * following reasons:
         *
         * 1] We assume that we always add a debug padding for each block
         *    we write and we also assume that we start at the last word
         *    of a block attempting to write a two-word entry.
         * 2] Rounding up errors due to the way segments are distributed
         *    in the buckets of the range tree's histogram.
         * 3] The activation of zfs_force_some_double_word_sm_entries
         *    (tunable) when testing.
         *
         * = Math and Rounding Errors =
         *
         * rt_histogram[i] bucket of a range tree represents the number
         * of entries in [2^i, (2^(i+1))-1] of that range_tree. Given
         * that, we want to divide the buckets into groups: Buckets that
         * can be represented using a single-word entry, ones that can
         * be represented with a double-word entry, and ones that can
         * only be represented with multiple two-word entries.
         *
         * [Note that if the new encoding feature is not enabled there
         * are only two groups: single-word entry buckets and multiple
         * single-word entry buckets. The information below assumes
         * two-word entries enabled, but it can easily applied when
         * the feature is not enabled]
         *
         * To find the highest bucket that can be represented with a
         * single-word entry we look at the maximum run that such entry
         * can have, which is 2^(SM_RUN_BITS + sm_shift) [remember that
         * the run of a space map entry is shifted by sm_shift, thus we
         * add it to the exponent]. This way, excluding the value of the
         * maximum run that can be represented by a single-word entry,
         * all runs that are smaller exist in buckets 0 to
         * SM_RUN_BITS + shift - 1.
         *
         * To find the highest bucket that can be represented with a
         * double-word entry, we follow the same approach. Finally, any
         * bucket higher than that are represented with multiple two-word
         * entries. To be more specific, if the highest bucket whose
         * segments can be represented with a single two-word entry is X,
         * then bucket X+1 will need 2 two-word entries for each of its
         * segments, X+2 will need 4, X+3 will need 8, ...etc.
         *
         * With all of the above we make our estimation based on bucket
         * groups. There is a rounding error though. As we mentioned in
         * the example with the one-word entry, the maximum run that can
         * be represented in a one-word entry 2^(SM_RUN_BITS + shift) is
         * not part of bucket SM_RUN_BITS + shift - 1. Thus, segments of
         * that length fall into the next bucket (and bucket group) where
         * we start counting two-word entries and this is one more reason
         * why the estimated size may end up being bigger than the actual
         * size written.
         */
        uint64_t size = 0;
        uint64_t idx = 0;

        if (!spa_feature_is_enabled(spa, SPA_FEATURE_SPACEMAP_V2) ||
            (vdev_id == SM_NO_VDEVID && sm->sm_size < SM_OFFSET_MAX)) {

                /*
                 * If we are trying to force some double word entries just
                 * assume the worst-case of every single word entry being
                 * written as a double word entry.
                 */
                uint64_t entry_size =
                    (spa_feature_is_enabled(spa, SPA_FEATURE_SPACEMAP_V2) &&
                    zfs_force_some_double_word_sm_entries) ?
                    (2 * sizeof (uint64_t)) : sizeof (uint64_t);

                uint64_t single_entry_max_bucket = SM_RUN_BITS + shift - 1;
                for (; idx <= single_entry_max_bucket; idx++)
                        size += histogram[idx] * entry_size;

                if (!spa_feature_is_enabled(spa, SPA_FEATURE_SPACEMAP_V2)) {
                        for (; idx < RANGE_TREE_HISTOGRAM_SIZE; idx++) {
                                ASSERT3U(idx, >=, single_entry_max_bucket);
                                entries_for_seg =
                                    1ULL << (idx - single_entry_max_bucket);
                                size += histogram[idx] *
                                    entries_for_seg * entry_size;
                        }
                        return (size);
                }
        }

        ASSERT(spa_feature_is_enabled(spa, SPA_FEATURE_SPACEMAP_V2));

        uint64_t double_entry_max_bucket = SM2_RUN_BITS + shift - 1;
        for (; idx <= double_entry_max_bucket; idx++)
                size += histogram[idx] * 2 * sizeof (uint64_t);

        for (; idx < RANGE_TREE_HISTOGRAM_SIZE; idx++) {
                ASSERT3U(idx, >=, double_entry_max_bucket);
                entries_for_seg = 1ULL << (idx - double_entry_max_bucket);
                size += histogram[idx] *
                    entries_for_seg * 2 * sizeof (uint64_t);
        }

        /*
         * Assume the worst case where we start with the padding at the end
         * of the current block and we add an extra padding entry at the end
         * of all subsequent blocks.
         */
        size += ((size / sm->sm_blksz) + 1) * sizeof (uint64_t);

        return (size);
}

uint64_t
space_map_object(space_map_t *sm)
{
        return (sm != NULL ? sm->sm_object : 0);
}

int64_t
space_map_allocated(space_map_t *sm)
{
        return (sm != NULL ? sm->sm_phys->smp_alloc : 0);
}

uint64_t
space_map_length(space_map_t *sm)
{
        return (sm != NULL ? sm->sm_phys->smp_length : 0);
}

uint64_t
space_map_nblocks(space_map_t *sm)
{
        if (sm == NULL)
                return (0);
        return (DIV_ROUND_UP(space_map_length(sm), sm->sm_blksz));
}