Don't activate metaslabs with weight 0

Signed-off-by: Paul Dagnelie <pcd@delphix.com>

module/zfs/metaslab.c

@@ -2485,6 +2485,18 @@ metaslab_activate(metaslab_t *msp, int allocator, uint64_t activation_weight)
 		return (0);
 	}
 
+	/*
+	 * If the metaslab has literally 0 space, it will have weight 0. In
+	 * that case, don't bother activating it. This can happen if the
+	 * metaslab had space during find_valid_metaslab, but another thread
+	 * loaded it and used all that space while we were waiting to grab the
+	 * lock.
+	 */
+	if (msp->ms_weight == 0) {
+		ASSERT0(range_tree_space(msp->ms_allocatable));
+		return (ENOSPC);
+	}
+
 	if ((error = metaslab_activate_allocator(msp->ms_group, msp,
 	    allocator, activation_weight)) != 0) {
 		return (error);