8023 Panic destroying a metaslab deferred range tree

Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>

We don't want to dirty any data when we're in the final txgs of the pool
export logic. This change introduces checks to make sure that no data is
dirtied after a certain point. It also addresses the culprit of this
specific bug – the space map cannot be upgraded when we're in final
stages of pool export. If we encounter a space map that wants to be
upgraded in this phase, then we simply ignore the request as it will get
retried the next time we set the fragmentation metric on that metaslab.

Closes #327

usr/src/uts/common/fs/zfs/dbuf.c

@@ -1552,6 +1552,7 @@ dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
 	 * this assertion only if we're not already dirty.
 	 */
 	os = dn->dn_objset;
+	VERIFY3U(tx->tx_txg, <=, spa_final_dirty_txg(os->os_spa));
 #ifdef DEBUG
 	if (dn->dn_objset->os_dsl_dataset != NULL)
 		rrw_enter(&os->os_dsl_dataset->ds_bp_rwlock, RW_READER, FTAG);

usr/src/uts/common/fs/zfs/metaslab.c

@@ -1563,11 +1563,19 @@ metaslab_set_fragmentation(metaslab_t *msp)
 		uint64_t txg = spa_syncing_txg(spa);
 		vdev_t *vd = msp->ms_group->mg_vd;
 
-		if (spa_writeable(spa)) {
+		/*
+		 * If we've reached the final dirty txg, then we must
+		 * be shutting down the pool. We don't want to dirty
+		 * any data past this point so skip setting the condense
+		 * flag. We can retry this action the next time the pool
+		 * is imported.
+		 */
+		if (spa_writeable(spa) && txg < spa_final_dirty_txg(spa)) {
 			msp->ms_condense_wanted = B_TRUE;
 			vdev_dirty(vd, VDD_METASLAB, msp, txg + 1);
 			spa_dbgmsg(spa, "txg %llu, requesting force condense: "
-			    "msp %p, vd %p", txg, msp, vd);
+			    "ms_id %llu, vdev_id %llu", txg, msp->ms_id,
+			    vd->vdev_id);
 		}
 		msp->ms_fragmentation = ZFS_FRAG_INVALID;
 		return;
@@ -2189,13 +2197,17 @@ metaslab_sync(metaslab_t *msp, uint64_t txg)
 	/*
 	 * Normally, we don't want to process a metaslab if there
 	 * are no allocations or frees to perform. However, if the metaslab
-	 * is being forced to condense we need to let it through.
+	 * is being forced to condense and it's loaded, we need to let it
+	 * through.
 	 */
 	if (range_tree_space(alloctree) == 0 &&
 	    range_tree_space(msp->ms_freeingtree) == 0 &&
-	    !msp->ms_condense_wanted)
+	    !(msp->ms_loaded && msp->ms_condense_wanted))
 		return;
 
+
+	VERIFY(txg <= spa_final_dirty_txg(spa));
+
 	/*
 	 * The only state that can actually be changing concurrently with
 	 * metaslab_sync() is the metaslab's ms_tree.  No other thread can

usr/src/uts/common/fs/zfs/spa_misc.c

@@ -1621,6 +1621,16 @@ spa_syncing_txg(spa_t *spa)
 	return (spa->spa_syncing_txg);
 }
 
+/*
+ * Return the last txg where data can be dirtied. The final txgs
+ * will be used to just clear out any deferred frees that remain.
+ */
+uint64_t
+spa_final_dirty_txg(spa_t *spa)
+{
+	return (spa->spa_final_txg - TXG_DEFER_SIZE);
+}
+
 pool_state_t
 spa_state(spa_t *spa)
 {

usr/src/uts/common/fs/zfs/space_map.c

@@ -23,7 +23,7 @@
  * Use is subject to license terms.
  */
 /*
- * Copyright (c) 2012, 2014 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2016 by Delphix. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
@@ -403,6 +403,7 @@ space_map_truncate(space_map_t *sm, dmu_tx_t *tx)
 
 	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);
 
@@ -417,9 +418,10 @@ space_map_truncate(space_map_t *sm, dmu_tx_t *tx)
 	if ((spa_feature_is_enabled(spa, SPA_FEATURE_SPACEMAP_HISTOGRAM) &&
 	    doi.doi_bonus_size != sizeof (space_map_phys_t)) ||
 	    doi.doi_data_block_size != space_map_blksz) {
-		zfs_dbgmsg("txg %llu, spa %s, reallocating: "
-		    "old bonus %u, old blocksz %u", dmu_tx_get_txg(tx),
-		    spa_name(spa), doi.doi_bonus_size, doi.doi_data_block_size);
+		zfs_dbgmsg("txg %llu, spa %s, sm %p, reallocating "
+		    "object[%llu]: old bonus %u, old blocksz %u",
+		    dmu_tx_get_txg(tx), spa_name(spa), sm, sm->sm_object,
+		    doi.doi_bonus_size, doi.doi_data_block_size);
 
 		space_map_free(sm, tx);
 		dmu_buf_rele(sm->sm_dbuf, sm);

usr/src/uts/common/fs/zfs/sys/spa.h

@@ -771,6 +771,7 @@ extern uint64_t spa_load_guid(spa_t *spa);
 extern uint64_t spa_last_synced_txg(spa_t *spa);
 extern uint64_t spa_first_txg(spa_t *spa);
 extern uint64_t spa_syncing_txg(spa_t *spa);
+extern uint64_t spa_final_dirty_txg(spa_t *spa);
 extern uint64_t spa_version(spa_t *spa);
 extern pool_state_t spa_state(spa_t *spa);
 extern spa_load_state_t spa_load_state(spa_t *spa);