diff --git a/pydantic/fields.py b/pydantic/fields.py
--- a/pydantic/fields.py
+++ b/pydantic/fields.py
@@ -199,7 +199,8 @@ def update_from_config(self, from_config: Dict[str, Any]) -> None:
                 current_value = getattr(self, attr_name)
             except AttributeError:
                 # attr_name is not an attribute of FieldInfo, it should therefore be added to extra
-                self.extra[attr_name] = value
+                # (except if extra already has this value!)
+                self.extra.setdefault(attr_name, value)
             else:
                 if current_value is self.__field_constraints__.get(attr_name, None):
                     setattr(self, attr_name, value)

You wrote `Config.field` for model B instead of `Config.fields` hence the difference.
Now you're right the output is not the expected one. I'll open a fix shortly
diff --git a/tests/test_schema.py b/tests/test_schema.py
--- a/tests/test_schema.py
+++ b/tests/test_schema.py
@@ -3001,3 +3001,27 @@ class Model(BaseModel):
             },
         },
     }
+
+
+def test_extra_inheritance():
+    class A(BaseModel):
+        root: Optional[str]
+
+        class Config:
+            fields = {
+                'root': {'description': 'root path of data', 'level': 1},
+            }
+
+    class Model(A):
+        root: str = Field('asa', description='image height', level=3)
+
+    m = Model()
+    assert m.schema()['properties'] == {
+        'root': {
+            'title': 'Root',
+            'type': 'string',
+            'description': 'image height',
+            'default': 'asa',
+            'level': 3,
+        }
+    }

sub-model cannot always re-write field infos from the same BaseModel.
### Checks

* [-] I added a descriptive title to this issue
* [-] I have searched (google, github) for similar issues and couldn't find anything
* [-] I have read and followed [the docs](https://pydantic-docs.helpmanual.io/) and still think this is a bug

# Bug

Output of `python -c "import pydantic.utils; print(pydantic.utils.version_info())"`:
```
             pydantic version: 1.9.0
            pydantic compiled: True
                 install path: /home/zuobinhua/anaconda3/lib/python3.7/site-packages/pydantic
               python version: 3.7.6 (default, Jan  8 2020, 19:59:22)  [GCC 7.3.0]
                     platform: Linux-4.15.0-159-generic-x86_64-with-debian-buster-sid
     optional deps. installed: ['email-validator', 'typing-extensions']
```
When creating two sub models(A, B) from Pydantic BaseModel, with fields infos defined in Config. Then creating another two sub models(Model1, Model2) from A and B, trying to rewrite the extra keys (we used `level` in the following code) in field info. Only one of the sub models can re-write the extra value correctly. 
As shown in the following code, `extra` in m2.__fields__["root"].field_info got level=3, but for m1 it still got level=1.

```py
from typing import Optional
from pydantic import BaseModel
from pydantic import Field


class A(BaseModel):
    root: Optional[str]
    class Config:
        fields = {
            "root": {"description": "root path of data", "level": 1},
        }

class B(BaseModel):
    root: Optional[str]
    class Config:
        field = {
            "root": {"description": "root path of data", "level": 1},
        }

class Model1(A):
    root: str = Field("asa", description="image height", level=3)

class Model2(B):
    root: str = Field("asa", description="image height", level=3)

m1 = Model1()
print(m1.__fields__["root"].field_info)
# default='asa' description='image height' extra={'level': 1}

m2 = Model2()
print(m2.__fields__["root"].field_info)
# default='asa' description='image height' extra={'level': 3}

```


tests/test_schema.py::test_extra_inheritance
tests/test_schema.py::test_key tests/test_schema.py::test_by_alias tests/test_schema.py::test_ref_template tests/test_schema.py::test_by_alias_generator tests/test_schema.py::test_sub_model tests/test_schema.py::test_schema_class tests/test_schema.py::test_schema_repr tests/test_schema.py::test_schema_class_by_alias tests/test_schema.py::test_choices tests/test_schema.py::test_enum_modify_schema tests/test_schema.py::test_enum_schema_custom_field tests/test_schema.py::test_enum_and_model_have_same_behaviour tests/test_schema.py::test_enum_includes_extra_without_other_params tests/test_schema.py::test_list_enum_schema_extras tests/test_schema.py::test_json_schema tests/test_schema.py::test_list_sub_model tests/test_schema.py::test_optional tests/test_schema.py::test_any tests/test_schema.py::test_set tests/test_schema.py::test_const_str tests/test_schema.py::test_const_false tests/test_schema.py::test_tuple[tuple-extra_props0] tests/test_schema.py::test_tuple[field_type1-extra_props1] tests/test_schema.py::test_tuple[field_type2-extra_props2] tests/test_schema.py::test_tuple[field_type3-extra_props3] tests/test_schema.py::test_deque tests/test_schema.py::test_bool tests/test_schema.py::test_strict_bool tests/test_schema.py::test_dict tests/test_schema.py::test_list tests/test_schema.py::test_list_union_dict[field_type0-expected_schema0] tests/test_schema.py::test_list_union_dict[field_type1-expected_schema1] tests/test_schema.py::test_list_union_dict[field_type2-expected_schema2] tests/test_schema.py::test_list_union_dict[field_type3-expected_schema3] tests/test_schema.py::test_list_union_dict[field_type4-expected_schema4] tests/test_schema.py::test_date_types[datetime-expected_schema0] tests/test_schema.py::test_date_types[date-expected_schema1] tests/test_schema.py::test_date_types[time-expected_schema2] tests/test_schema.py::test_date_types[timedelta-expected_schema3] tests/test_schema.py::test_str_basic_types[field_type0-expected_schema0] tests/test_schema.py::test_str_basic_types[field_type1-expected_schema1] tests/test_schema.py::test_str_basic_types[field_type2-expected_schema2] tests/test_schema.py::test_str_basic_types[field_type3-expected_schema3] tests/test_schema.py::test_str_constrained_types[StrictStr-expected_schema0] tests/test_schema.py::test_str_constrained_types[ConstrainedStr-expected_schema1] tests/test_schema.py::test_str_constrained_types[ConstrainedStrValue-expected_schema2] tests/test_schema.py::test_special_str_types[AnyUrl-expected_schema0] tests/test_schema.py::test_special_str_types[UrlValue-expected_schema1] tests/test_schema.py::test_email_str_types[EmailStr-email] tests/test_schema.py::test_email_str_types[NameEmail-name-email] tests/test_schema.py::test_secret_types[SecretBytes-string] tests/test_schema.py::test_secret_types[SecretStr-string] tests/test_schema.py::test_special_int_types[ConstrainedInt-expected_schema0] tests/test_schema.py::test_special_int_types[ConstrainedIntValue-expected_schema1] tests/test_schema.py::test_special_int_types[ConstrainedIntValue-expected_schema2] tests/test_schema.py::test_special_int_types[ConstrainedIntValue-expected_schema3] tests/test_schema.py::test_special_int_types[PositiveInt-expected_schema4] tests/test_schema.py::test_special_int_types[NegativeInt-expected_schema5] tests/test_schema.py::test_special_int_types[NonNegativeInt-expected_schema6] tests/test_schema.py::test_special_int_types[NonPositiveInt-expected_schema7] tests/test_schema.py::test_special_float_types[ConstrainedFloat-expected_schema0] tests/test_schema.py::test_special_float_types[ConstrainedFloatValue-expected_schema1] tests/test_schema.py::test_special_float_types[ConstrainedFloatValue-expected_schema2] tests/test_schema.py::test_special_float_types[ConstrainedFloatValue-expected_schema3] tests/test_schema.py::test_special_float_types[PositiveFloat-expected_schema4] tests/test_schema.py::test_special_float_types[NegativeFloat-expected_schema5] tests/test_schema.py::test_special_float_types[NonNegativeFloat-expected_schema6] tests/test_schema.py::test_special_float_types[NonPositiveFloat-expected_schema7] tests/test_schema.py::test_special_float_types[ConstrainedDecimal-expected_schema8] tests/test_schema.py::test_special_float_types[ConstrainedDecimalValue-expected_schema9] tests/test_schema.py::test_special_float_types[ConstrainedDecimalValue-expected_schema10] tests/test_schema.py::test_special_float_types[ConstrainedDecimalValue-expected_schema11] tests/test_schema.py::test_uuid_types[UUID-uuid] tests/test_schema.py::test_uuid_types[UUID1-uuid1] tests/test_schema.py::test_uuid_types[UUID3-uuid3] tests/test_schema.py::test_uuid_types[UUID4-uuid4] tests/test_schema.py::test_uuid_types[UUID5-uuid5] tests/test_schema.py::test_path_types[FilePath-file-path] tests/test_schema.py::test_path_types[DirectoryPath-directory-path] tests/test_schema.py::test_path_types[Path-path] tests/test_schema.py::test_json_type tests/test_schema.py::test_ipv4address_type tests/test_schema.py::test_ipv6address_type tests/test_schema.py::test_ipvanyaddress_type tests/test_schema.py::test_ipv4interface_type tests/test_schema.py::test_ipv6interface_type tests/test_schema.py::test_ipvanyinterface_type tests/test_schema.py::test_ipv4network_type tests/test_schema.py::test_ipv6network_type tests/test_schema.py::test_ipvanynetwork_type tests/test_schema.py::test_callable_type[type_0-default_value0] tests/test_schema.py::test_callable_type[type_1-<lambda>] tests/test_schema.py::test_callable_type[type_2-default_value2] tests/test_schema.py::test_callable_type[type_3-<lambda>] tests/test_schema.py::test_error_non_supported_types tests/test_schema.py::test_flat_models_unique_models tests/test_schema.py::test_flat_models_with_submodels tests/test_schema.py::test_flat_models_with_submodels_from_sequence tests/test_schema.py::test_model_name_maps tests/test_schema.py::test_schema_overrides tests/test_schema.py::test_schema_overrides_w_union tests/test_schema.py::test_schema_from_models tests/test_schema.py::test_schema_with_refs[#/components/schemas/-None] tests/test_schema.py::test_schema_with_refs[None-#/components/schemas/{model}] tests/test_schema.py::test_schema_with_refs[#/components/schemas/-#/{model}/schemas/] tests/test_schema.py::test_schema_with_custom_ref_template tests/test_schema.py::test_schema_ref_template_key_error tests/test_schema.py::test_schema_no_definitions tests/test_schema.py::test_list_default tests/test_schema.py::test_enum_str_default tests/test_schema.py::test_enum_int_default tests/test_schema.py::test_dict_default tests/test_schema.py::test_constraints_schema[kwargs0-str-expected_extra0] tests/test_schema.py::test_constraints_schema[kwargs1-ConstrainedStrValue-expected_extra1] tests/test_schema.py::test_constraints_schema[kwargs2-str-expected_extra2] tests/test_schema.py::test_constraints_schema[kwargs3-bytes-expected_extra3] tests/test_schema.py::test_constraints_schema[kwargs4-str-expected_extra4] tests/test_schema.py::test_constraints_schema[kwargs5-int-expected_extra5] tests/test_schema.py::test_constraints_schema[kwargs6-int-expected_extra6] tests/test_schema.py::test_constraints_schema[kwargs7-int-expected_extra7] tests/test_schema.py::test_constraints_schema[kwargs8-int-expected_extra8] tests/test_schema.py::test_constraints_schema[kwargs9-int-expected_extra9] tests/test_schema.py::test_constraints_schema[kwargs10-float-expected_extra10] tests/test_schema.py::test_constraints_schema[kwargs11-float-expected_extra11] tests/test_schema.py::test_constraints_schema[kwargs12-float-expected_extra12] tests/test_schema.py::test_constraints_schema[kwargs13-float-expected_extra13] tests/test_schema.py::test_constraints_schema[kwargs14-float-expected_extra14] tests/test_schema.py::test_constraints_schema[kwargs15-float-expected_extra15] tests/test_schema.py::test_constraints_schema[kwargs16-float-expected_extra16] tests/test_schema.py::test_constraints_schema[kwargs17-float-expected_extra17] tests/test_schema.py::test_constraints_schema[kwargs18-float-expected_extra18] tests/test_schema.py::test_constraints_schema[kwargs19-Decimal-expected_extra19] tests/test_schema.py::test_constraints_schema[kwargs20-Decimal-expected_extra20] tests/test_schema.py::test_constraints_schema[kwargs21-Decimal-expected_extra21] tests/test_schema.py::test_constraints_schema[kwargs22-Decimal-expected_extra22] tests/test_schema.py::test_constraints_schema[kwargs23-Decimal-expected_extra23] tests/test_schema.py::test_unenforced_constraints_schema[kwargs0-int] tests/test_schema.py::test_unenforced_constraints_schema[kwargs1-float] tests/test_schema.py::test_unenforced_constraints_schema[kwargs2-Decimal] tests/test_schema.py::test_unenforced_constraints_schema[kwargs3-bool] tests/test_schema.py::test_unenforced_constraints_schema[kwargs4-int] tests/test_schema.py::test_unenforced_constraints_schema[kwargs5-str] tests/test_schema.py::test_unenforced_constraints_schema[kwargs6-bytes] tests/test_schema.py::test_unenforced_constraints_schema[kwargs7-str] tests/test_schema.py::test_unenforced_constraints_schema[kwargs8-bool] tests/test_schema.py::test_unenforced_constraints_schema[kwargs9-type_9] tests/test_schema.py::test_unenforced_constraints_schema[kwargs10-type_10] tests/test_schema.py::test_unenforced_constraints_schema[kwargs11-ConstrainedListValue] tests/test_schema.py::test_unenforced_constraints_schema[kwargs12-ConstrainedSetValue] tests/test_schema.py::test_unenforced_constraints_schema[kwargs13-ConstrainedFrozenSetValue] tests/test_schema.py::test_constraints_schema_validation[kwargs0-str-foo] tests/test_schema.py::test_constraints_schema_validation[kwargs1-str-foo] tests/test_schema.py::test_constraints_schema_validation[kwargs2-bytes-foo] tests/test_schema.py::test_constraints_schema_validation[kwargs3-str-foo] tests/test_schema.py::test_constraints_schema_validation[kwargs4-int-3] tests/test_schema.py::test_constraints_schema_validation[kwargs5-int-3] tests/test_schema.py::test_constraints_schema_validation[kwargs6-int-3] tests/test_schema.py::test_constraints_schema_validation[kwargs7-int-2] tests/test_schema.py::test_constraints_schema_validation[kwargs8-int-3] tests/test_schema.py::test_constraints_schema_validation[kwargs9-int-3] tests/test_schema.py::test_constraints_schema_validation[kwargs10-int-5] tests/test_schema.py::test_constraints_schema_validation[kwargs11-float-3.0] tests/test_schema.py::test_constraints_schema_validation[kwargs12-float-2.1] tests/test_schema.py::test_constraints_schema_validation[kwargs13-float-3.0] tests/test_schema.py::test_constraints_schema_validation[kwargs14-float-4.9] tests/test_schema.py::test_constraints_schema_validation[kwargs15-float-3.0] tests/test_schema.py::test_constraints_schema_validation[kwargs16-float-2.0] tests/test_schema.py::test_constraints_schema_validation[kwargs17-float-3.0] tests/test_schema.py::test_constraints_schema_validation[kwargs18-float-5.0] tests/test_schema.py::test_constraints_schema_validation[kwargs19-float-3] tests/test_schema.py::test_constraints_schema_validation[kwargs20-float-3] tests/test_schema.py::test_constraints_schema_validation[kwargs21-Decimal-value21] tests/test_schema.py::test_constraints_schema_validation[kwargs22-Decimal-value22] tests/test_schema.py::test_constraints_schema_validation[kwargs23-Decimal-value23] tests/test_schema.py::test_constraints_schema_validation[kwargs24-Decimal-value24] tests/test_schema.py::test_constraints_schema_validation[kwargs25-Decimal-value25] tests/test_schema.py::test_constraints_schema_validation[kwargs26-Decimal-value26] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs0-str-foobar] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs1-str-f] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs2-str-bar] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs3-int-2] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs4-int-5] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs5-int-1] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs6-int-6] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs7-float-2.0] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs8-float-5.0] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs9-float-1.9] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs10-float-5.1] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs11-Decimal-value11] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs12-Decimal-value12] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs13-Decimal-value13] tests/test_schema.py::test_constraints_schema_validation_raises[kwargs14-Decimal-value14] tests/test_schema.py::test_schema_kwargs tests/test_schema.py::test_schema_dict_constr tests/test_schema.py::test_bytes_constrained_types[ConstrainedBytes-expected_schema0] tests/test_schema.py::test_bytes_constrained_types[ConstrainedBytesValue-expected_schema1] tests/test_schema.py::test_optional_dict tests/test_schema.py::test_optional_validator tests/test_schema.py::test_field_with_validator tests/test_schema.py::test_unparameterized_schema_generation tests/test_schema.py::test_known_model_optimization tests/test_schema.py::test_root tests/test_schema.py::test_root_list tests/test_schema.py::test_root_nested_model tests/test_schema.py::test_new_type_schema tests/test_schema.py::test_literal_schema tests/test_schema.py::test_literal_enum tests/test_schema.py::test_color_type tests/test_schema.py::test_model_with_schema_extra tests/test_schema.py::test_model_with_schema_extra_callable tests/test_schema.py::test_model_with_schema_extra_callable_no_model_class tests/test_schema.py::test_model_with_schema_extra_callable_classmethod tests/test_schema.py::test_model_with_schema_extra_callable_instance_method tests/test_schema.py::test_model_with_extra_forbidden tests/test_schema.py::test_enforced_constraints[int-kwargs0-field_schema0] tests/test_schema.py::test_enforced_constraints[annotation1-kwargs1-field_schema1] tests/test_schema.py::test_enforced_constraints[annotation2-kwargs2-field_schema2] tests/test_schema.py::test_enforced_constraints[annotation3-kwargs3-field_schema3] tests/test_schema.py::test_enforced_constraints[annotation4-kwargs4-field_schema4] tests/test_schema.py::test_enforced_constraints[annotation5-kwargs5-field_schema5] tests/test_schema.py::test_enforced_constraints[annotation6-kwargs6-field_schema6] tests/test_schema.py::test_enforced_constraints[annotation7-kwargs7-field_schema7] tests/test_schema.py::test_real_vs_phony_constraints tests/test_schema.py::test_subfield_field_info tests/test_schema.py::test_dataclass tests/test_schema.py::test_schema_attributes tests/test_schema.py::test_model_process_schema_enum tests/test_schema.py::test_path_modify_schema tests/test_schema.py::test_frozen_set tests/test_schema.py::test_iterable tests/test_schema.py::test_new_type tests/test_schema.py::test_multiple_models_with_same_name tests/test_schema.py::test_multiple_enums_with_same_name tests/test_schema.py::test_schema_for_generic_field tests/test_schema.py::test_namedtuple_default tests/test_schema.py::test_advanced_generic_schema tests/test_schema.py::test_nested_generic tests/test_schema.py::test_nested_generic_model tests/test_schema.py::test_complex_nested_generic tests/test_schema.py::test_schema_with_field_parameter tests/test_schema.py::test_discriminated_union tests/test_schema.py::test_discriminated_annotated_union tests/test_schema.py::test_alias_same tests/test_schema.py::test_nested_python_dataclasses tests/test_schema.py::test_discriminated_union_in_list