FreeBSD: Lock vnode in zfs_ioctl()

[zfs.git] / module / zfs / spa_errlog.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or https://opensource.org/licenses/CDDL-1.0.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright (c) 2006, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2013, 2014, Delphix. All rights reserved.
 * Copyright (c) 2019 Datto Inc.
 * Copyright (c) 2021, 2022, George Amanakis. All rights reserved.
 */

/*
 * Routines to manage the on-disk persistent error log.
 *
 * Each pool stores a log of all logical data errors seen during normal
 * operation.  This is actually the union of two distinct logs: the last log,
 * and the current log.  All errors seen are logged to the current log.  When a
 * scrub completes, the current log becomes the last log, the last log is thrown
 * out, and the current log is reinitialized.  This way, if an error is somehow
 * corrected, a new scrub will show that it no longer exists, and will be
 * deleted from the log when the scrub completes.
 *
 * The log is stored using a ZAP object whose key is a string form of the
 * zbookmark_phys tuple (objset, object, level, blkid), and whose contents is an
 * optional 'objset:object' human-readable string describing the data.  When an
 * error is first logged, this string will be empty, indicating that no name is
 * known.  This prevents us from having to issue a potentially large amount of
 * I/O to discover the object name during an error path.  Instead, we do the
 * calculation when the data is requested, storing the result so future queries
 * will be faster.
 *
 * If the head_errlog feature is enabled, a different on-disk format is used.
 * The error log of each head dataset is stored separately in the zap object
 * and keyed by the head id. This enables listing every dataset affected in
 * userland. In order to be able to track whether an error block has been
 * modified or added to snapshots since it was marked as an error, a new tuple
 * is introduced: zbookmark_err_phys_t. It allows the storage of the birth
 * transaction group of an error block on-disk. The birth transaction group is
 * used by check_filesystem() to assess whether this block was freed,
 * re-written or added to a snapshot since its marking as an error.
 *
 * This log is then shipped into an nvlist where the key is the dataset name and
 * the value is the object name.  Userland is then responsible for uniquifying
 * this list and displaying it to the user.
 */

#include <sys/dmu_tx.h>
#include <sys/spa.h>
#include <sys/spa_impl.h>
#include <sys/zap.h>
#include <sys/zio.h>
#include <sys/dsl_dir.h>
#include <sys/dmu_objset.h>
#include <sys/dbuf.h>
#include <sys/zfs_znode.h>

#define NAME_MAX_LEN 64

typedef struct clones {
        uint64_t clone_ds;
        list_node_t node;
} clones_t;

/*
 * spa_upgrade_errlog_limit : A zfs module parameter that controls the number
 *              of on-disk error log entries that will be converted to the new
 *              format when enabling head_errlog. Defaults to 0 which converts
 *              all log entries.
 */
static uint_t spa_upgrade_errlog_limit = 0;

/*
 * Convert a bookmark to a string.
 */
static void
bookmark_to_name(zbookmark_phys_t *zb, char *buf, size_t len)
{
        (void) snprintf(buf, len, "%llx:%llx:%llx:%llx",
            (u_longlong_t)zb->zb_objset, (u_longlong_t)zb->zb_object,
            (u_longlong_t)zb->zb_level, (u_longlong_t)zb->zb_blkid);
}

/*
 * Convert an err_phys to a string.
 */
static void
errphys_to_name(zbookmark_err_phys_t *zep, char *buf, size_t len)
{
        (void) snprintf(buf, len, "%llx:%llx:%llx:%llx",
            (u_longlong_t)zep->zb_object, (u_longlong_t)zep->zb_level,
            (u_longlong_t)zep->zb_blkid, (u_longlong_t)zep->zb_birth);
}

/*
 * Convert a string to a err_phys.
 */
void
name_to_errphys(char *buf, zbookmark_err_phys_t *zep)
{
        zep->zb_object = zfs_strtonum(buf, &buf);
        ASSERT(*buf == ':');
        zep->zb_level = (int)zfs_strtonum(buf + 1, &buf);
        ASSERT(*buf == ':');
        zep->zb_blkid = zfs_strtonum(buf + 1, &buf);
        ASSERT(*buf == ':');
        zep->zb_birth = zfs_strtonum(buf + 1, &buf);
        ASSERT(*buf == '\0');
}

/*
 * Convert a string to a bookmark.
 */
static void
name_to_bookmark(char *buf, zbookmark_phys_t *zb)
{
        zb->zb_objset = zfs_strtonum(buf, &buf);
        ASSERT(*buf == ':');
        zb->zb_object = zfs_strtonum(buf + 1, &buf);
        ASSERT(*buf == ':');
        zb->zb_level = (int)zfs_strtonum(buf + 1, &buf);
        ASSERT(*buf == ':');
        zb->zb_blkid = zfs_strtonum(buf + 1, &buf);
        ASSERT(*buf == '\0');
}

void
zep_to_zb(uint64_t dataset, zbookmark_err_phys_t *zep, zbookmark_phys_t *zb)
{
        zb->zb_objset = dataset;
        zb->zb_object = zep->zb_object;
        zb->zb_level = zep->zb_level;
        zb->zb_blkid = zep->zb_blkid;
}

static void
name_to_object(char *buf, uint64_t *obj)
{
        *obj = zfs_strtonum(buf, &buf);
        ASSERT(*buf == '\0');
}

/*
 * Retrieve the head filesystem.
 */
static int get_head_ds(spa_t *spa, uint64_t dsobj, uint64_t *head_ds)
{
        dsl_dataset_t *ds;
        int error = dsl_dataset_hold_obj_flags(spa->spa_dsl_pool,
            dsobj, DS_HOLD_FLAG_DECRYPT, FTAG, &ds);

        if (error != 0)
                return (error);

        ASSERT(head_ds);
        *head_ds = dsl_dir_phys(ds->ds_dir)->dd_head_dataset_obj;
        dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);

        return (error);
}

/*
 * Log an uncorrectable error to the persistent error log.  We add it to the
 * spa's list of pending errors.  The changes are actually synced out to disk
 * during spa_errlog_sync().
 */
void
spa_log_error(spa_t *spa, const zbookmark_phys_t *zb, const uint64_t birth)
{
        spa_error_entry_t search;
        spa_error_entry_t *new;
        avl_tree_t *tree;
        avl_index_t where;

        /*
         * If we are trying to import a pool, ignore any errors, as we won't be
         * writing to the pool any time soon.
         */
        if (spa_load_state(spa) == SPA_LOAD_TRYIMPORT)
                return;

        mutex_enter(&spa->spa_errlist_lock);

        /*
         * If we have had a request to rotate the log, log it to the next list
         * instead of the current one.
         */
        if (spa->spa_scrub_active || spa->spa_scrub_finished)
                tree = &spa->spa_errlist_scrub;
        else
                tree = &spa->spa_errlist_last;

        search.se_bookmark = *zb;
        if (avl_find(tree, &search, &where) != NULL) {
                mutex_exit(&spa->spa_errlist_lock);
                return;
        }

        new = kmem_zalloc(sizeof (spa_error_entry_t), KM_SLEEP);
        new->se_bookmark = *zb;

        /*
         * If the head_errlog feature is enabled, store the birth txg now. In
         * case the file is deleted before spa_errlog_sync() runs, we will not
         * be able to retrieve the birth txg.
         */
        if (spa_feature_is_enabled(spa, SPA_FEATURE_HEAD_ERRLOG)) {
                new->se_zep.zb_object = zb->zb_object;
                new->se_zep.zb_level = zb->zb_level;
                new->se_zep.zb_blkid = zb->zb_blkid;
                new->se_zep.zb_birth = birth;
        }

        avl_insert(tree, new, where);
        mutex_exit(&spa->spa_errlist_lock);
}

int
find_birth_txg(dsl_dataset_t *ds, zbookmark_err_phys_t *zep,
    uint64_t *birth_txg)
{
        objset_t *os;
        int error = dmu_objset_from_ds(ds, &os);
        if (error != 0)
                return (error);

        dnode_t *dn;
        blkptr_t bp;

        error = dnode_hold(os, zep->zb_object, FTAG, &dn);
        if (error != 0)
                return (error);

        rw_enter(&dn->dn_struct_rwlock, RW_READER);
        error = dbuf_dnode_findbp(dn, zep->zb_level, zep->zb_blkid, &bp, NULL,
            NULL);
        if (error == 0 && BP_IS_HOLE(&bp))
                error = SET_ERROR(ENOENT);

        *birth_txg = BP_GET_LOGICAL_BIRTH(&bp);
        rw_exit(&dn->dn_struct_rwlock);
        dnode_rele(dn, FTAG);
        return (error);
}

/*
 * This function finds the oldest affected filesystem containing an error
 * block.
 */
int
find_top_affected_fs(spa_t *spa, uint64_t head_ds, zbookmark_err_phys_t *zep,
    uint64_t *top_affected_fs)
{
        uint64_t oldest_dsobj;
        int error = dsl_dataset_oldest_snapshot(spa, head_ds, zep->zb_birth,
            &oldest_dsobj);
        if (error != 0)
                return (error);

        dsl_dataset_t *ds;
        error = dsl_dataset_hold_obj_flags(spa->spa_dsl_pool, oldest_dsobj,
            DS_HOLD_FLAG_DECRYPT, FTAG, &ds);
        if (error != 0)
                return (error);

        *top_affected_fs =
            dsl_dir_phys(ds->ds_dir)->dd_head_dataset_obj;
        dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);
        return (0);
}


#ifdef _KERNEL
/*
 * Copy the bookmark to the end of the user-space buffer which starts at
 * uaddr and has *count unused entries, and decrement *count by 1.
 */
static int
copyout_entry(const zbookmark_phys_t *zb, void *uaddr, uint64_t *count)
{
        if (*count == 0)
                return (SET_ERROR(ENOMEM));

        *count -= 1;
        if (copyout(zb, (char *)uaddr + (*count) * sizeof (zbookmark_phys_t),
            sizeof (zbookmark_phys_t)) != 0)
                return (SET_ERROR(EFAULT));
        return (0);
}

/*
 * Each time the error block is referenced by a snapshot or clone, add a
 * zbookmark_phys_t entry to the userspace array at uaddr. The array is
 * filled from the back and the in-out parameter *count is modified to be the
 * number of unused entries at the beginning of the array. The function
 * scrub_filesystem() is modelled after this one.
 */
static int
check_filesystem(spa_t *spa, uint64_t head_ds, zbookmark_err_phys_t *zep,
    void *uaddr, uint64_t *count, list_t *clones_list)
{
        dsl_dataset_t *ds;
        dsl_pool_t *dp = spa->spa_dsl_pool;

        int error = dsl_dataset_hold_obj_flags(dp, head_ds,
            DS_HOLD_FLAG_DECRYPT, FTAG, &ds);
        if (error != 0)
                return (error);

        uint64_t latest_txg;
        uint64_t txg_to_consider = spa->spa_syncing_txg;
        boolean_t check_snapshot = B_TRUE;
        error = find_birth_txg(ds, zep, &latest_txg);

        /*
         * If find_birth_txg() errors out otherwise, let txg_to_consider be
         * equal to the spa's syncing txg: if check_filesystem() errors out
         * then affected snapshots or clones will not be checked.
         */
        if (error == 0 && zep->zb_birth == latest_txg) {
                /* Block neither free nor rewritten. */
                zbookmark_phys_t zb;
                zep_to_zb(head_ds, zep, &zb);
                error = copyout_entry(&zb, uaddr, count);
                if (error != 0) {
                        dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);
                        return (error);
                }
                check_snapshot = B_FALSE;
        } else if (error == 0) {
                txg_to_consider = latest_txg;
        }

        /*
         * Retrieve the number of snapshots if the dataset is not a snapshot.
         */
        uint64_t snap_count = 0;
        if (dsl_dataset_phys(ds)->ds_snapnames_zapobj != 0) {

                error = zap_count(spa->spa_meta_objset,
                    dsl_dataset_phys(ds)->ds_snapnames_zapobj, &snap_count);

                if (error != 0) {
                        dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);
                        return (error);
                }
        }

        if (snap_count == 0) {
                /* Filesystem without snapshots. */
                dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);
                return (0);
        }

        uint64_t *snap_obj_array = kmem_zalloc(snap_count * sizeof (uint64_t),
            KM_SLEEP);

        int aff_snap_count = 0;
        uint64_t snap_obj = dsl_dataset_phys(ds)->ds_prev_snap_obj;
        uint64_t snap_obj_txg = dsl_dataset_phys(ds)->ds_prev_snap_txg;
        uint64_t zap_clone = dsl_dir_phys(ds->ds_dir)->dd_clones;

        dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);

        /* Check only snapshots created from this file system. */
        while (snap_obj != 0 && zep->zb_birth < snap_obj_txg &&
            snap_obj_txg <= txg_to_consider) {

                error = dsl_dataset_hold_obj_flags(dp, snap_obj,
                    DS_HOLD_FLAG_DECRYPT, FTAG, &ds);
                if (error != 0)
                        goto out;

                if (dsl_dir_phys(ds->ds_dir)->dd_head_dataset_obj != head_ds) {
                        snap_obj = dsl_dataset_phys(ds)->ds_prev_snap_obj;
                        snap_obj_txg = dsl_dataset_phys(ds)->ds_prev_snap_txg;
                        dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);
                        continue;
                }

                boolean_t affected = B_TRUE;
                if (check_snapshot) {
                        uint64_t blk_txg;
                        error = find_birth_txg(ds, zep, &blk_txg);
                        affected = (error == 0 && zep->zb_birth == blk_txg);
                }

                /* Report errors in snapshots. */
                if (affected) {
                        snap_obj_array[aff_snap_count] = snap_obj;
                        aff_snap_count++;

                        zbookmark_phys_t zb;
                        zep_to_zb(snap_obj, zep, &zb);
                        error = copyout_entry(&zb, uaddr, count);
                        if (error != 0) {
                                dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT,
                                    FTAG);
                                goto out;
                        }
                }
                snap_obj = dsl_dataset_phys(ds)->ds_prev_snap_obj;
                snap_obj_txg = dsl_dataset_phys(ds)->ds_prev_snap_txg;
                dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);
        }

        if (zap_clone == 0 || aff_snap_count == 0) {
                error = 0;
                goto out;
        }

        /* Check clones. */
        zap_cursor_t *zc;
        zap_attribute_t *za;

        zc = kmem_zalloc(sizeof (zap_cursor_t), KM_SLEEP);
        za = zap_attribute_alloc();

        for (zap_cursor_init(zc, spa->spa_meta_objset, zap_clone);
            zap_cursor_retrieve(zc, za) == 0;
            zap_cursor_advance(zc)) {

                dsl_dataset_t *clone;
                error = dsl_dataset_hold_obj_flags(dp, za->za_first_integer,
                    DS_HOLD_FLAG_DECRYPT, FTAG, &clone);

                if (error != 0)
                        break;

                /*
                 * Only clones whose origins were affected could also
                 * have affected snapshots.
                 */
                boolean_t found = B_FALSE;
                for (int i = 0; i < snap_count; i++) {
                        if (dsl_dir_phys(clone->ds_dir)->dd_origin_obj
                            == snap_obj_array[i])
                                found = B_TRUE;
                }
                dsl_dataset_rele_flags(clone, DS_HOLD_FLAG_DECRYPT, FTAG);

                if (!found)
                        continue;

                clones_t *ct = kmem_zalloc(sizeof (*ct), KM_SLEEP);
                ct->clone_ds = za->za_first_integer;
                list_insert_tail(clones_list, ct);
        }

        zap_cursor_fini(zc);
        zap_attribute_free(za);
        kmem_free(zc, sizeof (*zc));

out:
        kmem_free(snap_obj_array, sizeof (*snap_obj_array));
        return (error);
}

static int
process_error_block(spa_t *spa, uint64_t head_ds, zbookmark_err_phys_t *zep,
    void *uaddr, uint64_t *count)
{
        /*
         * If zb_birth == 0 or head_ds == 0 it means we failed to retrieve the
         * birth txg or the head filesystem of the block pointer. This may
         * happen e.g. when an encrypted filesystem is not mounted or when
         * the key is not loaded. In this case do not proceed to
         * check_filesystem(), instead do the accounting here.
         */
        if (zep->zb_birth == 0 || head_ds == 0) {
                zbookmark_phys_t zb;
                zep_to_zb(head_ds, zep, &zb);
                int error = copyout_entry(&zb, uaddr, count);
                if (error != 0) {
                        return (error);
                }
                return (0);
        }

        uint64_t top_affected_fs;
        uint64_t init_count = *count;
        int error = find_top_affected_fs(spa, head_ds, zep, &top_affected_fs);
        if (error == 0) {
                clones_t *ct;
                list_t clones_list;

                list_create(&clones_list, sizeof (clones_t),
                    offsetof(clones_t, node));

                error = check_filesystem(spa, top_affected_fs, zep,
                    uaddr, count, &clones_list);

                while ((ct = list_remove_head(&clones_list)) != NULL) {
                        error = check_filesystem(spa, ct->clone_ds, zep,
                            uaddr, count, &clones_list);
                        kmem_free(ct, sizeof (*ct));

                        if (error) {
                                while (!list_is_empty(&clones_list)) {
                                        ct = list_remove_head(&clones_list);
                                        kmem_free(ct, sizeof (*ct));
                                }
                                break;
                        }
                }

                list_destroy(&clones_list);
        }
        if (error == 0 && init_count == *count) {
                /*
                 * If we reach this point, no errors have been detected
                 * in the checked filesystems/snapshots. Before returning mark
                 * the error block to be removed from the error lists and logs.
                 */
                zbookmark_phys_t zb;
                zep_to_zb(head_ds, zep, &zb);
                spa_remove_error(spa, &zb, zep->zb_birth);
        }

        return (error);
}
#endif

/* Return the number of errors in the error log */
uint64_t
spa_get_last_errlog_size(spa_t *spa)
{
        uint64_t total = 0, count;
        mutex_enter(&spa->spa_errlog_lock);

        if (spa->spa_errlog_last != 0 &&
            zap_count(spa->spa_meta_objset, spa->spa_errlog_last,
            &count) == 0)
                total += count;
        mutex_exit(&spa->spa_errlog_lock);
        return (total);
}

/*
 * If a healed bookmark matches an entry in the error log we stash it in a tree
 * so that we can later remove the related log entries in sync context.
 */
static void
spa_add_healed_error(spa_t *spa, uint64_t obj, zbookmark_phys_t *healed_zb,
    const uint64_t birth)
{
        char name[NAME_MAX_LEN];

        if (obj == 0)
                return;

        boolean_t held_list = B_FALSE;
        boolean_t held_log = B_FALSE;

        if (!spa_feature_is_enabled(spa, SPA_FEATURE_HEAD_ERRLOG)) {
                bookmark_to_name(healed_zb, name, sizeof (name));

                if (zap_contains(spa->spa_meta_objset, healed_zb->zb_objset,
                    name) == 0) {
                        if (!MUTEX_HELD(&spa->spa_errlog_lock)) {
                                mutex_enter(&spa->spa_errlog_lock);
                                held_log = B_TRUE;
                        }

                        /*
                         * Found an error matching healed zb, add zb to our
                         * tree of healed errors
                         */
                        avl_tree_t *tree = &spa->spa_errlist_healed;
                        spa_error_entry_t search;
                        spa_error_entry_t *new;
                        avl_index_t where;
                        search.se_bookmark = *healed_zb;
                        if (!MUTEX_HELD(&spa->spa_errlist_lock)) {
                                mutex_enter(&spa->spa_errlist_lock);
                                held_list = B_TRUE;
                        }
                        if (avl_find(tree, &search, &where) != NULL) {
                                if (held_list)
                                        mutex_exit(&spa->spa_errlist_lock);
                                if (held_log)
                                        mutex_exit(&spa->spa_errlog_lock);
                                return;
                        }
                        new = kmem_zalloc(sizeof (spa_error_entry_t), KM_SLEEP);
                        new->se_bookmark = *healed_zb;
                        avl_insert(tree, new, where);
                        if (held_list)
                                mutex_exit(&spa->spa_errlist_lock);
                        if (held_log)
                                mutex_exit(&spa->spa_errlog_lock);
                }
                return;
        }

        zbookmark_err_phys_t healed_zep;
        healed_zep.zb_object = healed_zb->zb_object;
        healed_zep.zb_level = healed_zb->zb_level;
        healed_zep.zb_blkid = healed_zb->zb_blkid;
        healed_zep.zb_birth = birth;

        errphys_to_name(&healed_zep, name, sizeof (name));

        zap_cursor_t zc;
        zap_attribute_t *za = zap_attribute_alloc();
        for (zap_cursor_init(&zc, spa->spa_meta_objset, spa->spa_errlog_last);
            zap_cursor_retrieve(&zc, za) == 0; zap_cursor_advance(&zc)) {
                if (zap_contains(spa->spa_meta_objset, za->za_first_integer,
                    name) == 0) {
                        if (!MUTEX_HELD(&spa->spa_errlog_lock)) {
                                mutex_enter(&spa->spa_errlog_lock);
                                held_log = B_TRUE;
                        }

                        avl_tree_t *tree = &spa->spa_errlist_healed;
                        spa_error_entry_t search;
                        spa_error_entry_t *new;
                        avl_index_t where;
                        search.se_bookmark = *healed_zb;

                        if (!MUTEX_HELD(&spa->spa_errlist_lock)) {
                                mutex_enter(&spa->spa_errlist_lock);
                                held_list = B_TRUE;
                        }

                        if (avl_find(tree, &search, &where) != NULL) {
                                if (held_list)
                                        mutex_exit(&spa->spa_errlist_lock);
                                if (held_log)
                                        mutex_exit(&spa->spa_errlog_lock);
                                continue;
                        }
                        new = kmem_zalloc(sizeof (spa_error_entry_t), KM_SLEEP);
                        new->se_bookmark = *healed_zb;
                        new->se_zep = healed_zep;
                        avl_insert(tree, new, where);

                        if (held_list)
                                mutex_exit(&spa->spa_errlist_lock);
                        if (held_log)
                                mutex_exit(&spa->spa_errlog_lock);
                }
        }
        zap_cursor_fini(&zc);
        zap_attribute_free(za);
}

/*
 * If this error exists in the given tree remove it.
 */
static void
remove_error_from_list(spa_t *spa, avl_tree_t *t, const zbookmark_phys_t *zb)
{
        spa_error_entry_t search, *found;
        avl_index_t where;

        mutex_enter(&spa->spa_errlist_lock);
        search.se_bookmark = *zb;
        if ((found = avl_find(t, &search, &where)) != NULL) {
                avl_remove(t, found);
                kmem_free(found, sizeof (spa_error_entry_t));
        }
        mutex_exit(&spa->spa_errlist_lock);
}


/*
 * Removes all of the recv healed errors from both on-disk error logs
 */
static void
spa_remove_healed_errors(spa_t *spa, avl_tree_t *s, avl_tree_t *l, dmu_tx_t *tx)
{
        char name[NAME_MAX_LEN];
        spa_error_entry_t *se;
        void *cookie = NULL;

        ASSERT(MUTEX_HELD(&spa->spa_errlog_lock));

        while ((se = avl_destroy_nodes(&spa->spa_errlist_healed,
            &cookie)) != NULL) {
                remove_error_from_list(spa, s, &se->se_bookmark);
                remove_error_from_list(spa, l, &se->se_bookmark);

                if (!spa_feature_is_enabled(spa, SPA_FEATURE_HEAD_ERRLOG)) {
                        bookmark_to_name(&se->se_bookmark, name, sizeof (name));
                        (void) zap_remove(spa->spa_meta_objset,
                            spa->spa_errlog_last, name, tx);
                        (void) zap_remove(spa->spa_meta_objset,
                            spa->spa_errlog_scrub, name, tx);
                } else {
                        errphys_to_name(&se->se_zep, name, sizeof (name));
                        zap_cursor_t zc;
                        zap_attribute_t *za = zap_attribute_alloc();
                        for (zap_cursor_init(&zc, spa->spa_meta_objset,
                            spa->spa_errlog_last);
                            zap_cursor_retrieve(&zc, za) == 0;
                            zap_cursor_advance(&zc)) {
                                zap_remove(spa->spa_meta_objset,
                                    za->za_first_integer, name, tx);
                        }
                        zap_cursor_fini(&zc);

                        for (zap_cursor_init(&zc, spa->spa_meta_objset,
                            spa->spa_errlog_scrub);
                            zap_cursor_retrieve(&zc, za) == 0;
                            zap_cursor_advance(&zc)) {
                                zap_remove(spa->spa_meta_objset,
                                    za->za_first_integer, name, tx);
                        }
                        zap_cursor_fini(&zc);
                        zap_attribute_free(za);
                }
                kmem_free(se, sizeof (spa_error_entry_t));
        }
}

/*
 * Stash away healed bookmarks to remove them from the on-disk error logs
 * later in spa_remove_healed_errors().
 */
void
spa_remove_error(spa_t *spa, zbookmark_phys_t *zb, uint64_t birth)
{
        spa_add_healed_error(spa, spa->spa_errlog_last, zb, birth);
        spa_add_healed_error(spa, spa->spa_errlog_scrub, zb, birth);
}

static uint64_t
approx_errlog_size_impl(spa_t *spa, uint64_t spa_err_obj)
{
        if (spa_err_obj == 0)
                return (0);
        uint64_t total = 0;

        zap_cursor_t zc;
        zap_attribute_t *za = zap_attribute_alloc();
        for (zap_cursor_init(&zc, spa->spa_meta_objset, spa_err_obj);
            zap_cursor_retrieve(&zc, za) == 0; zap_cursor_advance(&zc)) {
                uint64_t count;
                if (zap_count(spa->spa_meta_objset, za->za_first_integer,
                    &count) == 0)
                        total += count;
        }
        zap_cursor_fini(&zc);
        zap_attribute_free(za);
        return (total);
}

/*
 * Return the approximate number of errors currently in the error log.  This
 * will be nonzero if there are some errors, but otherwise it may be more
 * or less than the number of entries returned by spa_get_errlog().
 */
uint64_t
spa_approx_errlog_size(spa_t *spa)
{
        uint64_t total = 0;

        if (!spa_feature_is_enabled(spa, SPA_FEATURE_HEAD_ERRLOG)) {
                mutex_enter(&spa->spa_errlog_lock);
                uint64_t count;
                if (spa->spa_errlog_scrub != 0 &&
                    zap_count(spa->spa_meta_objset, spa->spa_errlog_scrub,
                    &count) == 0)
                        total += count;

                if (spa->spa_errlog_last != 0 && !spa->spa_scrub_finished &&
                    zap_count(spa->spa_meta_objset, spa->spa_errlog_last,
                    &count) == 0)
                        total += count;
                mutex_exit(&spa->spa_errlog_lock);

        } else {
                mutex_enter(&spa->spa_errlog_lock);
                total += approx_errlog_size_impl(spa, spa->spa_errlog_last);
                total += approx_errlog_size_impl(spa, spa->spa_errlog_scrub);
                mutex_exit(&spa->spa_errlog_lock);
        }
        mutex_enter(&spa->spa_errlist_lock);
        total += avl_numnodes(&spa->spa_errlist_last);
        total += avl_numnodes(&spa->spa_errlist_scrub);
        mutex_exit(&spa->spa_errlist_lock);
        return (total);
}

/*
 * This function sweeps through an on-disk error log and stores all bookmarks
 * as error bookmarks in a new ZAP object. At the end we discard the old one,
 * and spa_update_errlog() will set the spa's on-disk error log to new ZAP
 * object.
 */
static void
sync_upgrade_errlog(spa_t *spa, uint64_t spa_err_obj, uint64_t *newobj,
    dmu_tx_t *tx)
{
        zap_cursor_t zc;
        zap_attribute_t *za;
        zbookmark_phys_t zb;
        uint64_t count;

        *newobj = zap_create(spa->spa_meta_objset, DMU_OT_ERROR_LOG,
            DMU_OT_NONE, 0, tx);

        /*
         * If we cannnot perform the upgrade we should clear the old on-disk
         * error logs.
         */
        if (zap_count(spa->spa_meta_objset, spa_err_obj, &count) != 0) {
                VERIFY0(dmu_object_free(spa->spa_meta_objset, spa_err_obj, tx));
                return;
        }

        za = zap_attribute_alloc();
        for (zap_cursor_init(&zc, spa->spa_meta_objset, spa_err_obj);
            zap_cursor_retrieve(&zc, za) == 0;
            zap_cursor_advance(&zc)) {
                if (spa_upgrade_errlog_limit != 0 &&
                    zc.zc_cd == spa_upgrade_errlog_limit)
                        break;

                name_to_bookmark(za->za_name, &zb);

                zbookmark_err_phys_t zep;
                zep.zb_object = zb.zb_object;
                zep.zb_level = zb.zb_level;
                zep.zb_blkid = zb.zb_blkid;
                zep.zb_birth = 0;

                /*
                 * In case of an error we should simply continue instead of
                 * returning prematurely. See the next comment.
                 */
                uint64_t head_ds;
                dsl_pool_t *dp = spa->spa_dsl_pool;
                dsl_dataset_t *ds;
                objset_t *os;

                int error = dsl_dataset_hold_obj_flags(dp, zb.zb_objset,
                    DS_HOLD_FLAG_DECRYPT, FTAG, &ds);
                if (error != 0)
                        continue;

                head_ds = dsl_dir_phys(ds->ds_dir)->dd_head_dataset_obj;

                /*
                 * The objset and the dnode are required for getting the block
                 * pointer, which is used to determine if BP_IS_HOLE(). If
                 * getting the objset or the dnode fails, do not create a
                 * zap entry (presuming we know the dataset) as this may create
                 * spurious errors that we cannot ever resolve. If an error is
                 * truly persistent, it should re-appear after a scan.
                 */
                if (dmu_objset_from_ds(ds, &os) != 0) {
                        dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);
                        continue;
                }

                dnode_t *dn;
                blkptr_t bp;

                if (dnode_hold(os, zep.zb_object, FTAG, &dn) != 0) {
                        dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);
                        continue;
                }

                rw_enter(&dn->dn_struct_rwlock, RW_READER);
                error = dbuf_dnode_findbp(dn, zep.zb_level, zep.zb_blkid, &bp,
                    NULL, NULL);
                if (error == EACCES)
                        error = 0;
                else if (!error)
                        zep.zb_birth = BP_GET_LOGICAL_BIRTH(&bp);

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

                if (error != 0 || BP_IS_HOLE(&bp))
                        continue;

                uint64_t err_obj;
                error = zap_lookup_int_key(spa->spa_meta_objset, *newobj,
                    head_ds, &err_obj);

                if (error == ENOENT) {
                        err_obj = zap_create(spa->spa_meta_objset,
                            DMU_OT_ERROR_LOG, DMU_OT_NONE, 0, tx);

                        (void) zap_update_int_key(spa->spa_meta_objset,
                            *newobj, head_ds, err_obj, tx);
                }

                char buf[64];
                errphys_to_name(&zep, buf, sizeof (buf));

                const char *name = "";
                (void) zap_update(spa->spa_meta_objset, err_obj,
                    buf, 1, strlen(name) + 1, name, tx);
        }
        zap_cursor_fini(&zc);
        zap_attribute_free(za);

        VERIFY0(dmu_object_free(spa->spa_meta_objset, spa_err_obj, tx));
}

void
spa_upgrade_errlog(spa_t *spa, dmu_tx_t *tx)
{
        uint64_t newobj = 0;

        mutex_enter(&spa->spa_errlog_lock);
        if (spa->spa_errlog_last != 0) {
                sync_upgrade_errlog(spa, spa->spa_errlog_last, &newobj, tx);
                spa->spa_errlog_last = newobj;

                (void) zap_update(spa->spa_meta_objset,
                    DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ERRLOG_LAST,
                    sizeof (uint64_t), 1, &spa->spa_errlog_last, tx);
        }

        if (spa->spa_errlog_scrub != 0) {
                sync_upgrade_errlog(spa, spa->spa_errlog_scrub, &newobj, tx);
                spa->spa_errlog_scrub = newobj;

                (void) zap_update(spa->spa_meta_objset,
                    DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ERRLOG_SCRUB,
                    sizeof (uint64_t), 1, &spa->spa_errlog_scrub, tx);
        }

        mutex_exit(&spa->spa_errlog_lock);
}

#ifdef _KERNEL
/*
 * If an error block is shared by two datasets it will be counted twice.
 */
static int
process_error_log(spa_t *spa, uint64_t obj, void *uaddr, uint64_t *count)
{
        if (obj == 0)
                return (0);

        zap_cursor_t *zc;
        zap_attribute_t *za;

        zc = kmem_zalloc(sizeof (zap_cursor_t), KM_SLEEP);
        za = zap_attribute_alloc();

        if (!spa_feature_is_enabled(spa, SPA_FEATURE_HEAD_ERRLOG)) {
                for (zap_cursor_init(zc, spa->spa_meta_objset, obj);
                    zap_cursor_retrieve(zc, za) == 0;
                    zap_cursor_advance(zc)) {
                        if (*count == 0) {
                                zap_cursor_fini(zc);
                                kmem_free(zc, sizeof (*zc));
                                zap_attribute_free(za);
                                return (SET_ERROR(ENOMEM));
                        }

                        zbookmark_phys_t zb;
                        name_to_bookmark(za->za_name, &zb);

                        int error = copyout_entry(&zb, uaddr, count);
                        if (error != 0) {
                                zap_cursor_fini(zc);
                                kmem_free(zc, sizeof (*zc));
                                zap_attribute_free(za);
                                return (error);
                        }
                }
                zap_cursor_fini(zc);
                kmem_free(zc, sizeof (*zc));
                zap_attribute_free(za);
                return (0);
        }

        for (zap_cursor_init(zc, spa->spa_meta_objset, obj);
            zap_cursor_retrieve(zc, za) == 0;
            zap_cursor_advance(zc)) {

                zap_cursor_t *head_ds_cursor;
                zap_attribute_t *head_ds_attr;

                head_ds_cursor = kmem_zalloc(sizeof (zap_cursor_t), KM_SLEEP);
                head_ds_attr = zap_attribute_alloc();

                uint64_t head_ds_err_obj = za->za_first_integer;
                uint64_t head_ds;
                name_to_object(za->za_name, &head_ds);
                for (zap_cursor_init(head_ds_cursor, spa->spa_meta_objset,
                    head_ds_err_obj); zap_cursor_retrieve(head_ds_cursor,
                    head_ds_attr) == 0; zap_cursor_advance(head_ds_cursor)) {

                        zbookmark_err_phys_t head_ds_block;
                        name_to_errphys(head_ds_attr->za_name, &head_ds_block);
                        int error = process_error_block(spa, head_ds,
                            &head_ds_block, uaddr, count);

                        if (error != 0) {
                                zap_cursor_fini(head_ds_cursor);
                                kmem_free(head_ds_cursor,
                                    sizeof (*head_ds_cursor));
                                zap_attribute_free(head_ds_attr);

                                zap_cursor_fini(zc);
                                zap_attribute_free(za);
                                kmem_free(zc, sizeof (*zc));
                                return (error);
                        }
                }
                zap_cursor_fini(head_ds_cursor);
                kmem_free(head_ds_cursor, sizeof (*head_ds_cursor));
                zap_attribute_free(head_ds_attr);
        }
        zap_cursor_fini(zc);
        zap_attribute_free(za);
        kmem_free(zc, sizeof (*zc));
        return (0);
}

static int
process_error_list(spa_t *spa, avl_tree_t *list, void *uaddr, uint64_t *count)
{
        spa_error_entry_t *se;

        if (!spa_feature_is_enabled(spa, SPA_FEATURE_HEAD_ERRLOG)) {
                for (se = avl_first(list); se != NULL;
                    se = AVL_NEXT(list, se)) {
                        int error =
                            copyout_entry(&se->se_bookmark, uaddr, count);
                        if (error != 0) {
                                return (error);
                        }
                }
                return (0);
        }

        for (se = avl_first(list); se != NULL; se = AVL_NEXT(list, se)) {
                uint64_t head_ds = 0;
                int error = get_head_ds(spa, se->se_bookmark.zb_objset,
                    &head_ds);

                /*
                 * If get_head_ds() errors out, set the head filesystem
                 * to the filesystem stored in the bookmark of the
                 * error block.
                 */
                if (error != 0)
                        head_ds = se->se_bookmark.zb_objset;

                error = process_error_block(spa, head_ds,
                    &se->se_zep, uaddr, count);
                if (error != 0)
                        return (error);
        }
        return (0);
}
#endif

/*
 * Copy all known errors to userland as an array of bookmarks.  This is
 * actually a union of the on-disk last log and current log, as well as any
 * pending error requests.
 *
 * Because the act of reading the on-disk log could cause errors to be
 * generated, we have two separate locks: one for the error log and one for the
 * in-core error lists.  We only need the error list lock to log and error, so
 * we grab the error log lock while we read the on-disk logs, and only pick up
 * the error list lock when we are finished.
 */
int
spa_get_errlog(spa_t *spa, void *uaddr, uint64_t *count)
{
        int ret = 0;

#ifdef _KERNEL
        /*
         * The pool config lock is needed to hold a dataset_t via (among other
         * places) process_error_list() -> process_error_block()->
         * find_top_affected_fs(), and lock ordering requires that we get it
         * before the spa_errlog_lock.
         */
        dsl_pool_config_enter(spa->spa_dsl_pool, FTAG);
        mutex_enter(&spa->spa_errlog_lock);

        ret = process_error_log(spa, spa->spa_errlog_scrub, uaddr, count);

        if (!ret && !spa->spa_scrub_finished)
                ret = process_error_log(spa, spa->spa_errlog_last, uaddr,
                    count);

        mutex_enter(&spa->spa_errlist_lock);
        if (!ret)
                ret = process_error_list(spa, &spa->spa_errlist_scrub, uaddr,
                    count);
        if (!ret)
                ret = process_error_list(spa, &spa->spa_errlist_last, uaddr,
                    count);
        mutex_exit(&spa->spa_errlist_lock);

        mutex_exit(&spa->spa_errlog_lock);
        dsl_pool_config_exit(spa->spa_dsl_pool, FTAG);
#else
        (void) spa, (void) uaddr, (void) count;
#endif

        return (ret);
}

/*
 * Called when a scrub completes.  This simply set a bit which tells which AVL
 * tree to add new errors.  spa_errlog_sync() is responsible for actually
 * syncing the changes to the underlying objects.
 */
void
spa_errlog_rotate(spa_t *spa)
{
        mutex_enter(&spa->spa_errlist_lock);
        spa->spa_scrub_finished = B_TRUE;
        mutex_exit(&spa->spa_errlist_lock);
}

/*
 * Discard any pending errors from the spa_t.  Called when unloading a faulted
 * pool, as the errors encountered during the open cannot be synced to disk.
 */
void
spa_errlog_drain(spa_t *spa)
{
        spa_error_entry_t *se;
        void *cookie;

        mutex_enter(&spa->spa_errlist_lock);

        cookie = NULL;
        while ((se = avl_destroy_nodes(&spa->spa_errlist_last,
            &cookie)) != NULL)
                kmem_free(se, sizeof (spa_error_entry_t));
        cookie = NULL;
        while ((se = avl_destroy_nodes(&spa->spa_errlist_scrub,
            &cookie)) != NULL)
                kmem_free(se, sizeof (spa_error_entry_t));

        mutex_exit(&spa->spa_errlist_lock);
}

/*
 * Process a list of errors into the current on-disk log.
 */
void
sync_error_list(spa_t *spa, avl_tree_t *t, uint64_t *obj, dmu_tx_t *tx)
{
        spa_error_entry_t *se;
        char buf[NAME_MAX_LEN];
        void *cookie;

        if (avl_numnodes(t) == 0)
                return;

        /* create log if necessary */
        if (*obj == 0)
                *obj = zap_create(spa->spa_meta_objset, DMU_OT_ERROR_LOG,
                    DMU_OT_NONE, 0, tx);

        /* add errors to the current log */
        if (!spa_feature_is_enabled(spa, SPA_FEATURE_HEAD_ERRLOG)) {
                for (se = avl_first(t); se != NULL; se = AVL_NEXT(t, se)) {
                        bookmark_to_name(&se->se_bookmark, buf, sizeof (buf));

                        const char *name = se->se_name ? se->se_name : "";
                        (void) zap_update(spa->spa_meta_objset, *obj, buf, 1,
                            strlen(name) + 1, name, tx);
                }
        } else {
                for (se = avl_first(t); se != NULL; se = AVL_NEXT(t, se)) {
                        zbookmark_err_phys_t zep;
                        zep.zb_object = se->se_zep.zb_object;
                        zep.zb_level = se->se_zep.zb_level;
                        zep.zb_blkid = se->se_zep.zb_blkid;
                        zep.zb_birth = se->se_zep.zb_birth;

                        uint64_t head_ds = 0;
                        int error = get_head_ds(spa, se->se_bookmark.zb_objset,
                            &head_ds);

                        /*
                         * If get_head_ds() errors out, set the head filesystem
                         * to the filesystem stored in the bookmark of the
                         * error block.
                         */
                        if (error != 0)
                                head_ds = se->se_bookmark.zb_objset;

                        uint64_t err_obj;
                        error = zap_lookup_int_key(spa->spa_meta_objset,
                            *obj, head_ds, &err_obj);

                        if (error == ENOENT) {
                                err_obj = zap_create(spa->spa_meta_objset,
                                    DMU_OT_ERROR_LOG, DMU_OT_NONE, 0, tx);

                                (void) zap_update_int_key(spa->spa_meta_objset,
                                    *obj, head_ds, err_obj, tx);
                        }
                        errphys_to_name(&zep, buf, sizeof (buf));

                        const char *name = se->se_name ? se->se_name : "";
                        (void) zap_update(spa->spa_meta_objset,
                            err_obj, buf, 1, strlen(name) + 1, name, tx);
                }
        }
        /* purge the error list */
        cookie = NULL;
        while ((se = avl_destroy_nodes(t, &cookie)) != NULL)
                kmem_free(se, sizeof (spa_error_entry_t));
}

static void
delete_errlog(spa_t *spa, uint64_t spa_err_obj, dmu_tx_t *tx)
{
        if (spa_feature_is_enabled(spa, SPA_FEATURE_HEAD_ERRLOG)) {
                zap_cursor_t zc;
                zap_attribute_t *za = zap_attribute_alloc();
                for (zap_cursor_init(&zc, spa->spa_meta_objset, spa_err_obj);
                    zap_cursor_retrieve(&zc, za) == 0;
                    zap_cursor_advance(&zc)) {
                        VERIFY0(dmu_object_free(spa->spa_meta_objset,
                            za->za_first_integer, tx));
                }
                zap_cursor_fini(&zc);
                zap_attribute_free(za);
        }
        VERIFY0(dmu_object_free(spa->spa_meta_objset, spa_err_obj, tx));
}

/*
 * Sync the error log out to disk.  This is a little tricky because the act of
 * writing the error log requires the spa_errlist_lock.  So, we need to lock the
 * error lists, take a copy of the lists, and then reinitialize them.  Then, we
 * drop the error list lock and take the error log lock, at which point we
 * do the errlog processing.  Then, if we encounter an I/O error during this
 * process, we can successfully add the error to the list.  Note that this will
 * result in the perpetual recycling of errors, but it is an unlikely situation
 * and not a performance critical operation.
 */
void
spa_errlog_sync(spa_t *spa, uint64_t txg)
{
        dmu_tx_t *tx;
        avl_tree_t scrub, last;
        int scrub_finished;

        mutex_enter(&spa->spa_errlist_lock);

        /*
         * Bail out early under normal circumstances.
         */
        if (avl_numnodes(&spa->spa_errlist_scrub) == 0 &&
            avl_numnodes(&spa->spa_errlist_last) == 0 &&
            avl_numnodes(&spa->spa_errlist_healed) == 0 &&
            !spa->spa_scrub_finished) {
                mutex_exit(&spa->spa_errlist_lock);
                return;
        }

        spa_get_errlists(spa, &last, &scrub);
        scrub_finished = spa->spa_scrub_finished;
        spa->spa_scrub_finished = B_FALSE;

        mutex_exit(&spa->spa_errlist_lock);

        /*
         * The pool config lock is needed to hold a dataset_t via
         * sync_error_list() -> get_head_ds(), and lock ordering
         * requires that we get it before the spa_errlog_lock.
         */
        dsl_pool_config_enter(spa->spa_dsl_pool, FTAG);
        mutex_enter(&spa->spa_errlog_lock);

        tx = dmu_tx_create_assigned(spa->spa_dsl_pool, txg);

        /*
         * Remove healed errors from errors.
         */
        spa_remove_healed_errors(spa, &last, &scrub, tx);

        /*
         * Sync out the current list of errors.
         */
        sync_error_list(spa, &last, &spa->spa_errlog_last, tx);

        /*
         * Rotate the log if necessary.
         */
        if (scrub_finished) {
                if (spa->spa_errlog_last != 0)
                        delete_errlog(spa, spa->spa_errlog_last, tx);
                spa->spa_errlog_last = spa->spa_errlog_scrub;
                spa->spa_errlog_scrub = 0;

                sync_error_list(spa, &scrub, &spa->spa_errlog_last, tx);
        }

        /*
         * Sync out any pending scrub errors.
         */
        sync_error_list(spa, &scrub, &spa->spa_errlog_scrub, tx);

        /*
         * Update the MOS to reflect the new values.
         */
        (void) zap_update(spa->spa_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_ERRLOG_LAST, sizeof (uint64_t), 1,
            &spa->spa_errlog_last, tx);
        (void) zap_update(spa->spa_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_ERRLOG_SCRUB, sizeof (uint64_t), 1,
            &spa->spa_errlog_scrub, tx);

        dmu_tx_commit(tx);

        mutex_exit(&spa->spa_errlog_lock);
        dsl_pool_config_exit(spa->spa_dsl_pool, FTAG);
}

static void
delete_dataset_errlog(spa_t *spa, uint64_t spa_err_obj, uint64_t ds,
    dmu_tx_t *tx)
{
        if (spa_err_obj == 0)
                return;

        zap_cursor_t zc;
        zap_attribute_t *za = zap_attribute_alloc();
        for (zap_cursor_init(&zc, spa->spa_meta_objset, spa_err_obj);
            zap_cursor_retrieve(&zc, za) == 0; zap_cursor_advance(&zc)) {
                uint64_t head_ds;
                name_to_object(za->za_name, &head_ds);
                if (head_ds == ds) {
                        (void) zap_remove(spa->spa_meta_objset, spa_err_obj,
                            za->za_name, tx);
                        VERIFY0(dmu_object_free(spa->spa_meta_objset,
                            za->za_first_integer, tx));
                        break;
                }
        }
        zap_cursor_fini(&zc);
        zap_attribute_free(za);
}

void
spa_delete_dataset_errlog(spa_t *spa, uint64_t ds, dmu_tx_t *tx)
{
        mutex_enter(&spa->spa_errlog_lock);
        delete_dataset_errlog(spa, spa->spa_errlog_scrub, ds, tx);
        delete_dataset_errlog(spa, spa->spa_errlog_last, ds, tx);
        mutex_exit(&spa->spa_errlog_lock);
}

static int
find_txg_ancestor_snapshot(spa_t *spa, uint64_t new_head, uint64_t old_head,
    uint64_t *txg)
{
        dsl_dataset_t *ds;
        dsl_pool_t *dp = spa->spa_dsl_pool;

        int error = dsl_dataset_hold_obj_flags(dp, old_head,
            DS_HOLD_FLAG_DECRYPT, FTAG, &ds);
        if (error != 0)
                return (error);

        uint64_t prev_obj = dsl_dataset_phys(ds)->ds_prev_snap_obj;
        uint64_t prev_obj_txg = dsl_dataset_phys(ds)->ds_prev_snap_txg;

        while (prev_obj != 0) {
                dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);
                if ((error = dsl_dataset_hold_obj_flags(dp, prev_obj,
                    DS_HOLD_FLAG_DECRYPT, FTAG, &ds)) == 0 &&
                    dsl_dir_phys(ds->ds_dir)->dd_head_dataset_obj == new_head)
                        break;

                if (error != 0)
                        return (error);

                prev_obj_txg = dsl_dataset_phys(ds)->ds_prev_snap_txg;
                prev_obj = dsl_dataset_phys(ds)->ds_prev_snap_obj;
        }
        dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);
        ASSERT(prev_obj != 0);
        *txg = prev_obj_txg;
        return (0);
}

static void
swap_errlog(spa_t *spa, uint64_t spa_err_obj, uint64_t new_head, uint64_t
    old_head, dmu_tx_t *tx)
{
        if (spa_err_obj == 0)
                return;

        uint64_t old_head_errlog;
        int error = zap_lookup_int_key(spa->spa_meta_objset, spa_err_obj,
            old_head, &old_head_errlog);

        /* If no error log, then there is nothing to do. */
        if (error != 0)
                return;

        uint64_t txg;
        error = find_txg_ancestor_snapshot(spa, new_head, old_head, &txg);
        if (error != 0)
                return;

        /*
         * Create an error log if the file system being promoted does not
         * already have one.
         */
        uint64_t new_head_errlog;
        error = zap_lookup_int_key(spa->spa_meta_objset, spa_err_obj, new_head,
            &new_head_errlog);

        if (error != 0) {
                new_head_errlog = zap_create(spa->spa_meta_objset,
                    DMU_OT_ERROR_LOG, DMU_OT_NONE, 0, tx);

                (void) zap_update_int_key(spa->spa_meta_objset, spa_err_obj,
                    new_head, new_head_errlog, tx);
        }

        zap_cursor_t zc;
        zap_attribute_t *za = zap_attribute_alloc();
        zbookmark_err_phys_t err_block;
        for (zap_cursor_init(&zc, spa->spa_meta_objset, old_head_errlog);
            zap_cursor_retrieve(&zc, za) == 0; zap_cursor_advance(&zc)) {

                const char *name = "";
                name_to_errphys(za->za_name, &err_block);
                if (err_block.zb_birth < txg) {
                        (void) zap_update(spa->spa_meta_objset, new_head_errlog,
                            za->za_name, 1, strlen(name) + 1, name, tx);

                        (void) zap_remove(spa->spa_meta_objset, old_head_errlog,
                            za->za_name, tx);
                }
        }
        zap_cursor_fini(&zc);
        zap_attribute_free(za);
}

void
spa_swap_errlog(spa_t *spa, uint64_t new_head_ds, uint64_t old_head_ds,
    dmu_tx_t *tx)
{
        mutex_enter(&spa->spa_errlog_lock);
        swap_errlog(spa, spa->spa_errlog_scrub, new_head_ds, old_head_ds, tx);
        swap_errlog(spa, spa->spa_errlog_last, new_head_ds, old_head_ds, tx);
        mutex_exit(&spa->spa_errlog_lock);
}

#if defined(_KERNEL)
/* error handling */
EXPORT_SYMBOL(spa_log_error);
EXPORT_SYMBOL(spa_approx_errlog_size);
EXPORT_SYMBOL(spa_get_last_errlog_size);
EXPORT_SYMBOL(spa_get_errlog);
EXPORT_SYMBOL(spa_errlog_rotate);
EXPORT_SYMBOL(spa_errlog_drain);
EXPORT_SYMBOL(spa_errlog_sync);
EXPORT_SYMBOL(spa_get_errlists);
EXPORT_SYMBOL(spa_delete_dataset_errlog);
EXPORT_SYMBOL(spa_swap_errlog);
EXPORT_SYMBOL(sync_error_list);
EXPORT_SYMBOL(spa_upgrade_errlog);
EXPORT_SYMBOL(find_top_affected_fs);
EXPORT_SYMBOL(find_birth_txg);
EXPORT_SYMBOL(zep_to_zb);
EXPORT_SYMBOL(name_to_errphys);
#endif

/* BEGIN CSTYLED */
ZFS_MODULE_PARAM(zfs_spa, spa_, upgrade_errlog_limit, UINT, ZMOD_RW,
        "Limit the number of errors which will be upgraded to the new "
        "on-disk error log when enabling head_errlog");
/* END CSTYLED */