Use cached feature info in spa_add_feature_stats()

Avoid issuing I/O to the pool when retrieving feature flags information.
Trying to read the ZAPs from disk means that zpool clear would hang if
the pool is suspended and recovery would require a reboot. We need to
ensure the list of features enabled on the pool stays resident in
memory.

There already exists an nvlist for this purpose called
ZPOOL_CONFIG_ENABLED_FEAT in spa->spa_load_info. The features_for_write
features aren't added to the list, however, in the final import phase.
This patch updates spa_load_impl() so the features_for_read and
features_for_write features are both added to ZPOOL_CONFIG_ENABLED_FEAT
in each import phase. The full list of pool features and their initial
reference counts are now available for use in spa_add_feature_stats().

In spa_add_feature_stats() we iterate over the spa_feature_table[]
rather than the ZAP objects read from disk. We operate on
ZPOOL_CONFIG_ENABLED_FEAT in-place and refresh the reference counts for
the supported features from the values cached in the spa_t. This is
sufficient for 'zpool get all' to display correct values for the
unsupported@ and feature@ properties. For the unsupported features, if
the reference count is 0 is shows 'inactive' otherwise it shows
'read-only'. In either case, the unsupported reference counts could not
have changed after import, so it is safe to rely on the initial values.

Fixes openzfs#3082

Signed-off-by: Ned Bass <[email protected]>

module/zfs/spa.c

@@ -2395,11 +2395,10 @@ spa_load_impl(spa_t *spa, uint64_t pool_guid, nvlist_t *config,
 		    unsup_feat, enabled_feat))
 			missing_feat_read = B_TRUE;
 
-		if (spa_writeable(spa) || state == SPA_LOAD_TRYIMPORT) {
-			if (!spa_features_check(spa, B_TRUE,
-			    unsup_feat, enabled_feat)) {
-				missing_feat_write = B_TRUE;
-			}
+		if (!spa_features_check(spa, B_TRUE,
+		    unsup_feat, enabled_feat) &&
+		    (spa_writeable(spa) || state == SPA_LOAD_TRYIMPORT)) {
+			missing_feat_write = B_TRUE;
 		}
 
 		fnvlist_add_nvlist(spa->spa_load_info,
@@ -3205,41 +3204,24 @@ static void
 spa_add_feature_stats(spa_t *spa, nvlist_t *config)
 {
 	nvlist_t *features;
-	zap_cursor_t zc;
-	zap_attribute_t za;
+	int i;
 
 	ASSERT(spa_config_held(spa, SCL_CONFIG, RW_READER));
-	VERIFY(nvlist_alloc(&features, NV_UNIQUE_NAME, KM_SLEEP) == 0);
-
-	if (spa->spa_feat_for_read_obj != 0) {
-		for (zap_cursor_init(&zc, spa->spa_meta_objset,
-		    spa->spa_feat_for_read_obj);
-		    zap_cursor_retrieve(&zc, &za) == 0;
-		    zap_cursor_advance(&zc)) {
-			ASSERT(za.za_integer_length == sizeof (uint64_t) &&
-			    za.za_num_integers == 1);
-			VERIFY3U(0, ==, nvlist_add_uint64(features, za.za_name,
-			    za.za_first_integer));
-		}
-		zap_cursor_fini(&zc);
-	}
-
-	if (spa->spa_feat_for_write_obj != 0) {
-		for (zap_cursor_init(&zc, spa->spa_meta_objset,
-		    spa->spa_feat_for_write_obj);
-		    zap_cursor_retrieve(&zc, &za) == 0;
-		    zap_cursor_advance(&zc)) {
-			ASSERT(za.za_integer_length == sizeof (uint64_t) &&
-			    za.za_num_integers == 1);
-			VERIFY3U(0, ==, nvlist_add_uint64(features, za.za_name,
-			    za.za_first_integer));
-		}
-		zap_cursor_fini(&zc);
+
+	VERIFY0(nvlist_lookup_nvlist(spa->spa_load_info,
+	    ZPOOL_CONFIG_ENABLED_FEAT, &features));
+
+	for (i = 0; i < SPA_FEATURES; i++) {
+		zfeature_info_t feature = spa_feature_table[i];
+		uint64_t refcount;
+
+		if (feature_get_refcount(spa, &feature, &refcount) == 0)
+			VERIFY0(nvlist_add_uint64(features, feature.fi_guid,
+			    refcount));
 	}
 
-	VERIFY(nvlist_add_nvlist(config, ZPOOL_CONFIG_FEATURE_STATS,
-	    features) == 0);
-	nvlist_free(features);
+	VERIFY0(nvlist_add_nvlist(config, ZPOOL_CONFIG_FEATURE_STATS,
+	    features));
 }
 
 int

module/zfs/zfeature.c

@@ -228,7 +228,7 @@ spa_features_check(spa_t *spa, boolean_t for_write,
  *
  * Note: well-designed features will not need to use this; they should
  * use spa_feature_is_enabled() and spa_feature_is_active() instead.
- * However, this is non-static for zdb and zhack.
+ * However, this is non-static for zdb, zhack, and spa_add_feature_stats().
  */
 int
 feature_get_refcount(spa_t *spa, zfeature_info_t *feature, uint64_t *res)