bcachefs: Ensure bch_sb_field_ext always exists

This makes bch_sb_field_ext more consistent with the rest of -o
nochanges - we don't want to be varying other codepaths based on -o
nochanges, since it's used for testing in dry run mode; also fixes some
potential null ptr derefs.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/recovery.c

@@ -592,16 +592,9 @@ int bch2_fs_recovery(struct bch_fs *c)
 
 	if (!c->opts.nochanges) {
 		mutex_lock(&c->sb_lock);
+		struct bch_sb_field_ext *ext = bch2_sb_field_get(c->disk_sb.sb, ext);
 		bool write_sb = false;
 
-		struct bch_sb_field_ext *ext =
-			bch2_sb_field_get_minsize(&c->disk_sb, ext, sizeof(*ext) / sizeof(u64));
-		if (!ext) {
-			ret = -BCH_ERR_ENOSPC_sb;
-			mutex_unlock(&c->sb_lock);
-			goto err;
-		}
-
 		if (BCH_SB_HAS_TOPOLOGY_ERRORS(c->disk_sb.sb)) {
 			ext->recovery_passes_required[0] |=
 				cpu_to_le64(bch2_recovery_passes_to_stable(BIT_ULL(BCH_RECOVERY_PASS_check_topology)));
@@ -832,6 +825,7 @@ int bch2_fs_recovery(struct bch_fs *c)
 	}
 
 	mutex_lock(&c->sb_lock);
+	struct bch_sb_field_ext *ext = bch2_sb_field_get(c->disk_sb.sb, ext);
 	bool write_sb = false;
 
 	if (BCH_SB_VERSION_UPGRADE_COMPLETE(c->disk_sb.sb) != le16_to_cpu(c->disk_sb.sb->version)) {
@@ -845,16 +839,13 @@ int bch2_fs_recovery(struct bch_fs *c)
 		write_sb = true;
 	}
 
-	if (!test_bit(BCH_FS_error, &c->flags)) {
-		struct bch_sb_field_ext *ext = bch2_sb_field_get(c->disk_sb.sb, ext);
-		if (ext &&
+	if (!test_bit(BCH_FS_error, &c->flags) &&
 	    (!bch2_is_zero(ext->recovery_passes_required, sizeof(ext->recovery_passes_required)) ||
 	     !bch2_is_zero(ext->errors_silent, sizeof(ext->errors_silent)))) {
 		memset(ext->recovery_passes_required, 0, sizeof(ext->recovery_passes_required));
 		memset(ext->errors_silent, 0, sizeof(ext->errors_silent));
 		write_sb = true;
 	}
-	}
 
 	if (c->opts.fsck &&
 	    !test_bit(BCH_FS_error, &c->flags) &&

fs/bcachefs/super.c

@@ -1015,8 +1015,16 @@ int bch2_fs_start(struct bch_fs *c)
 	for_each_online_member(c, ca)
 		bch2_members_v2_get_mut(c->disk_sb.sb, ca->dev_idx)->last_mount = cpu_to_le64(now);
 
+	struct bch_sb_field_ext *ext =
+		bch2_sb_field_get_minsize(&c->disk_sb, ext, sizeof(*ext) / sizeof(u64));
 	mutex_unlock(&c->sb_lock);
 
+	if (!ext) {
+		bch_err(c, "insufficient space in superblock for sb_field_ext");
+		ret = -BCH_ERR_ENOSPC_sb;
+		goto err;
+	}
+
 	for_each_rw_member(c, ca)
 		bch2_dev_allocator_add(c, ca);
 	bch2_recalc_capacity(c);