Fix activating large_microzap on receive

This ensures that the in-memory state of the feature is recorded and
that `dsl_dataset_activate_feature` is not called when the feature
is already active.

Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Austin Wise <AustinWise@gmail.com>
Closes #18143
Closes #18144

module/zfs/dmu_recv.c

@@ -997,24 +997,6 @@ dmu_recv_begin_sync(void *arg, dmu_tx_t *tx)
 		    numredactsnaps, tx);
 	}
 
-	if (featureflags & DMU_BACKUP_FEATURE_LARGE_MICROZAP) {
-		/*
-		 * The source has seen a large microzap at least once in its
-		 * life, so we activate the feature here to match. It's not
-		 * strictly necessary since a large microzap is usable without
-		 * the feature active, but if that object is sent on from here,
-		 * we need this info to know to add the stream feature.
-		 *
-		 * There may be no large microzap in the incoming stream, or
-		 * ever again, but this is a very niche feature and its very
-		 * difficult to spot a large microzap in the stream, so its
-		 * not worth the effort of trying harder to activate the
-		 * feature at first use.
-		 */
-		dsl_dataset_activate_feature(dsobj, SPA_FEATURE_LARGE_MICROZAP,
-		    (void *)B_TRUE, tx);
-	}
-
 	dmu_buf_will_dirty(newds->ds_dbuf, tx);
 	dsl_dataset_phys(newds)->ds_flags |= DS_FLAG_INCONSISTENT;
 
@@ -1044,6 +1026,26 @@ dmu_recv_begin_sync(void *arg, dmu_tx_t *tx)
 		newds->ds_feature[SPA_FEATURE_LONGNAME] = (void *)B_TRUE;
 	}
 
+	if (featureflags & DMU_BACKUP_FEATURE_LARGE_MICROZAP &&
+	    !dsl_dataset_feature_is_active(newds, SPA_FEATURE_LARGE_MICROZAP)) {
+		/*
+		 * The source has seen a large microzap at least once in its
+		 * life, so we activate the feature here to match. It's not
+		 * strictly necessary since a large microzap is usable without
+		 * the feature active, but if that object is sent on from here,
+		 * we need this info to know to add the stream feature.
+		 *
+		 * There may be no large microzap in the incoming stream, or
+		 * ever again, but this is a very niche feature and its very
+		 * difficult to spot a large microzap in the stream, so its
+		 * not worth the effort of trying harder to activate the
+		 * feature at first use.
+		 */
+		dsl_dataset_activate_feature(dsobj, SPA_FEATURE_LARGE_MICROZAP,
+		    (void *)B_TRUE, tx);
+		newds->ds_feature[SPA_FEATURE_LARGE_MICROZAP] = (void *)B_TRUE;
+	}
+
 	/*
 	 * If we actually created a non-clone, we need to create the objset
 	 * in our new dataset. If this is a raw send we postpone this until

tests/runfiles/common.run

@@ -998,6 +998,7 @@ tests = ['recv_dedup', 'recv_dedup_encrypted_zvol', 'rsend_001_pos',
     'send_spill_block', 'send_holds', 'send_hole_birth', 'send_mixed_raw',
     'send-wR_encrypted_zvol', 'send_partial_dataset', 'send_invalid',
     'send_large_blocks_incremental', 'send_large_blocks_initial',
+    'send_large_microzap_incremental', 'send_large_microzap_transitive',
     'send_doall', 'send_raw_spill_block', 'send_raw_ashift',
     'send_raw_large_blocks', 'send_leak_keymaps']
 tags = ['functional', 'rsend']

tests/zfs-tests/include/tunables.cfg

@@ -114,6 +114,7 @@ BCLONE_WAIT_DIRTY		bclone_wait_dirty		zfs_bclone_wait_dirty
 DIO_ENABLED			dio_enabled			zfs_dio_enabled
 DIO_STRICT			dio_strict			zfs_dio_strict
 XATTR_COMPAT			xattr_compat			zfs_xattr_compat
+ZAP_MICRO_MAX_SIZE		zap_micro_max_size		zap_micro_max_size
 ZEVENT_LEN_MAX			zevent.len_max			zfs_zevent_len_max
 ZEVENT_RETAIN_MAX		zevent.retain_max		zfs_zevent_retain_max
 ZIO_SLOW_IO_MS			zio.slow_io_ms			zio_slow_io_ms

tests/zfs-tests/tests/Makefile.am

@@ -2067,6 +2067,8 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/rsend/send_invalid.ksh \
 	functional/rsend/send_large_blocks_incremental.ksh \
 	functional/rsend/send_large_blocks_initial.ksh \
+	functional/rsend/send_large_microzap_incremental.ksh \
+	functional/rsend/send_large_microzap_transitive.ksh \
 	functional/rsend/send_leak_keymaps.ksh \
 	functional/rsend/send-L_toggle.ksh \
 	functional/rsend/send_mixed_raw.ksh \

tests/zfs-tests/tests/functional/rsend/send_large_microzap_incremental.ksh

@@ -0,0 +1,91 @@
+#!/bin/ksh -p
+# SPDX-License-Identifier: CDDL-1.0
+
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+
+#
+# Copyright (c) 2026 by Austin Wise. All rights reserved.
+#
+
+. $STF_SUITE/tests/functional/rsend/rsend.kshlib
+. $STF_SUITE/include/properties.shlib
+
+#
+# Description:
+#   Ensure that it is possible to receive an incremental send of a snapshot that has the large microzap
+#   feature active into a pool where the feature is already active.
+#	Regression test for https://github.com/openzfs/zfs/issues/18143
+#
+# Strategy:
+#   1. Activate the large microzap feature in the source dataset.
+#   2. Create a snapshot.
+#   3. Send the snapshot from the first pool to a second pool.
+#   4. Create a second snapshot.
+#   5. Send the second snapshot incrementally to the second pool.
+#
+
+verify_runnable "both"
+
+log_assert "Verify incremental receive handles inactive large_blocks feature correctly."
+
+function cleanup
+{
+	restore_tunable ZAP_MICRO_MAX_SIZE
+	cleanup_pool $POOL
+	cleanup_pool $POOL2
+}
+
+function assert_feature_state {
+	typeset pool=$1
+	typeset expected_state=$2
+
+	typeset actual_state=$(zpool get -H -o value feature@large_microzap $pool)
+	log_note "Zpool $pool feature@large_microzap=$actual_state"
+	if [[ "$actual_state" != "$expected_state" ]]; then
+		log_fail "pool $pool feature@large_microzap=$actual_state (expected '$expected_state')"
+	fi
+}
+
+typeset src=$POOL/src
+typeset second=$POOL2/second
+
+log_onexit cleanup
+
+# Allow micro ZAPs to grow beyond SPA_OLD_MAXBLOCKSIZE.
+set_tunable64 ZAP_MICRO_MAX_SIZE 1048576
+
+# Create a dataset with a large recordsize (1MB)
+log_must zfs create -o recordsize=1M $src
+typeset mntpnt=$(get_prop mountpoint $src)
+
+# Activate the large_microzap feature by creating a micro ZAP that is larger than SPA_OLD_MAXBLOCKSIZE (128k)
+# but smaller than MZAP_MAX_SIZE (1MB). Each micro ZAP entry is 64 bytes (MZAP_ENT_LEN),
+# so 4096 files is about 256k.
+log_must eval "seq 1 4096 | xargs -I REPLACE_ME touch $mntpnt/REPLACE_ME"
+log_must zpool sync $POOL
+
+# Assert initial state of pools
+assert_feature_state $POOL "active"
+assert_feature_state $POOL2 "enabled"
+
+# Create initial snapshot and send to second pool.
+log_must zfs snapshot $src@snap
+log_must eval "zfs send -p -L $src@snap | zfs receive $second"
+log_must zpool sync $POOL2
+assert_feature_state $POOL2 "active"
+
+# Create a second snapshot and send incrementally.
+# This ensures that the feature is not activated a second time, which would cause a panic.
+log_must zfs snapshot $src@snap2
+log_must eval "zfs send -L -i $src@snap $src@snap2 | zfs receive -F $second"
+
+log_pass "Feature activation propagated successfully."

tests/zfs-tests/tests/functional/rsend/send_large_microzap_transitive.ksh

@@ -0,0 +1,100 @@
+#!/bin/ksh -p
+# SPDX-License-Identifier: CDDL-1.0
+
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+
+#
+# Copyright (c) 2026 by Austin Wise. All rights reserved.
+#
+
+. $STF_SUITE/tests/functional/rsend/rsend.kshlib
+. $STF_SUITE/include/properties.shlib
+
+#
+# Description:
+#   Ensures that sending a snapshot with the large microzap feature active propagates
+#   the feature activation to the receiving pool. Also checks that the received snapshot also
+#   correctly propagates the feature activation when sent to a third third pool.
+#	Regression test for https://github.com/openzfs/zfs/issues/18143
+#
+# Strategy:
+#   1. Enable the large microzap feature in the source dataset.
+#   2. Create a snapshot.
+#   3. Send the snapshot from the first pool to a second pool.
+#   4. Send the snapshot from the second pool to a third pool.
+#   5. Verify that all pools have the large microzap feature active.
+#
+
+verify_runnable "both"
+verify_disk_count "$DISKS" 3
+
+log_assert "Verify incremental receive handles inactive large_blocks feature correctly."
+
+function cleanup
+{
+	restore_tunable ZAP_MICRO_MAX_SIZE
+	cleanup_pool $POOL
+	cleanup_pool $POOL2
+	cleanup_pool $POOL3
+}
+
+function assert_feature_state {
+	typeset pool=$1
+	typeset expected_state=$2
+
+	typeset actual_state=$(zpool get -H -o value feature@large_microzap $pool)
+	log_note "Zpool $pool feature@large_microzap=$actual_state"
+	if [[ "$actual_state" != "$expected_state" ]]; then
+		log_fail "pool $pool feature@large_microzap=$actual_state (expected '$expected_state')"
+	fi
+}
+
+typeset src=$POOL/src
+typeset second=$POOL2/second
+typeset third=$POOL3/third
+
+log_onexit cleanup
+
+# Allow micro ZAPs to grow beyond SPA_OLD_MAXBLOCKSIZE.
+set_tunable64 ZAP_MICRO_MAX_SIZE 1048576
+
+# Ensure the third pool exists.
+datasetexists $POOL3 || log_must zpool create $POOL3 $DISK3
+
+# Create a dataset with a large recordsize (1MB)
+log_must zfs create -o recordsize=1M $src
+typeset mntpnt=$(get_prop mountpoint $src)
+
+# Activate the large_microzap feature by creating a micro ZAP that is larger than SPA_OLD_MAXBLOCKSIZE (128k)
+# but smaller than MZAP_MAX_SIZE (1MB). Each micro ZAP entry is 64 bytes (MZAP_ENT_LEN),
+# so 4096 files is about 256k.
+log_must eval "seq 1 4096 | xargs -I REPLACE_ME touch $mntpnt/REPLACE_ME"
+log_must zpool sync $POOL
+
+# Assert initial state of pools
+assert_feature_state $POOL "active"
+assert_feature_state $POOL2 "enabled"
+assert_feature_state $POOL3 "enabled"
+
+# Create initial snapshot and send to second pool.
+log_must zfs snapshot $src@snap
+log_must eval "zfs send -p -L $src@snap | zfs receive $second"
+log_must zpool sync $POOL2
+assert_feature_state $POOL2 "active"
+
+# Send to third pool from second. This ensures that the second pool correctly recorded that that
+# large_microzap feature was active when it received the first send stream.
+log_must eval "zfs send -p -L $second@snap | zfs receive $third"
+log_must zpool sync $POOL3
+assert_feature_state $POOL3 "active"
+
+log_pass "Feature activation propagated successfully."