btrfs: make btrfs_ref::real_root optional

Now that real_root is only used in ref-verify core gate it behind
CONFIG_BTRFS_FS_REF_VERIFY ifdef. This shrinks the size of pending
delayed refs by 8 bytes per ref, of which we can have many at any one
time depending on intensity of the workload. Also change the comment
about the member as it no longer deals with qgroups.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/delayed-ref.h

@@ -231,17 +231,10 @@ struct btrfs_ref {
 	 */
 	bool skip_qgroup;
 
-	/*
-	 * Optional. For which root is this modification.
-	 * Mostly used for qgroup optimization.
-	 *
-	 * When unset, data/tree ref init code will populate it.
-	 * In certain cases, we're modifying reference for a different root.
-	 * E.g. COW fs tree blocks for balance.
-	 * In that case, tree_ref::root will be fs tree, but we're doing this
-	 * for reloc tree, then we should set @real_root to reloc tree.
-	 */
+#ifdef CONFIG_BTRFS_FS_REF_VERIFY
+	/* Through which root is this modification. */
 	u64 real_root;
+#endif
 	u64 bytenr;
 	u64 len;
 
@@ -273,9 +266,10 @@ static inline void btrfs_init_generic_ref(struct btrfs_ref *generic_ref,
 static inline void btrfs_init_tree_ref(struct btrfs_ref *generic_ref,
 				int level, u64 root, u64 mod_root, bool skip_qgroup)
 {
+#ifdef CONFIG_BTRFS_FS_REF_VERIFY
 	/* If @real_root not set, use @root as fallback */
-	if (!generic_ref->real_root)
-		generic_ref->real_root = root;
+	generic_ref->real_root = mod_root ?: root;
+#endif
 	generic_ref->tree_ref.level = level;
 	generic_ref->tree_ref.owning_root = root;
 	generic_ref->type = BTRFS_REF_METADATA;
@@ -291,9 +285,10 @@ static inline void btrfs_init_data_ref(struct btrfs_ref *generic_ref,
 				u64 ref_root, u64 ino, u64 offset, u64 mod_root,
 				bool skip_qgroup)
 {
+#ifdef CONFIG_BTRFS_FS_REF_VERIFY
 	/* If @real_root not set, use @root as fallback */
-	if (!generic_ref->real_root)
-		generic_ref->real_root = ref_root;
+	generic_ref->real_root = mod_root ?: ref_root;
+#endif
 	generic_ref->data_ref.owning_root = ref_root;
 	generic_ref->data_ref.ino = ino;
 	generic_ref->data_ref.offset = offset;