linux-kernel - Re: [PATCH 2/2] f2fs: support {d,id,did,x}node checksum

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Date:   Thu, 1 Feb 2018 22:20:42 +0800
From:   Chao Yu <chao@...nel.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] f2fs: support {d,id,did,x}node checksum



On 2018/2/1 6:15, Jaegeuk Kim wrote:
> On 01/31, Chao Yu wrote:
>> On 2018/1/31 10:02, Jaegeuk Kim wrote:
>>> What if we want to add more entries in addition to node_checksum? Do we have
>>> to add a new feature flag at every time? How about adding a layout value instead
>>
>> Hmm.. for previous implementation, IMO, we'd better add a new feature flag at
>> every time, otherwise, w/ extra_nsize only, in current image, we can know a
>> valid range of extended area in node block, but we don't know which
>> fields/features are valid/enabled or not.
>>
>> One more thing is that if we can add one feature flag for each field, we got one
>> more chance to disable it dynamically.
>>
>>> of extra_nsize? For example, layout #1 means node_checksum with extra_nsize=X?
>>>
>>>
>>> What does 1017 mean? We need to make this structure more flexibly for new
>>
>> Yes, using raw 1017 is not appropriate here.
>>
>>> entries. Like this?
>>> 		union {
>>> 			struct node_v1;
>>> 			struct node_v2;
>>> 			struct node_v3;
>>> 			...
>>> 			struct direct_node dn;
>>> 			struct indirect_node in;
>>> 		};
>>> 	};
>>>
>>> 	struct node_v1 {
>>> 		__le32 data[DEF_ADDRS_PER_BLOCK - V1_NSIZE=1];
>>> 		__le32 node_checksum;
>>> 	}
>>>
>>> 	struct node_v2 {
>>> 		__le32 data[DEF_ADDRS_PER_BLOCK - V2_NSIZE=500];
>>
>> Hmm.. If we only need to add one more 4 bytes field in struct node_v2, but
>> V2_NSIZE is defined as fixed 500, there must be 492 bytes wasted.
>>
>> Or we can define V2_NSIZE as 8, but if there comes more and more extended
>> fields, node version count can be a large number, it results in complicated
>> version recognization and handling.
>>
>> One more question is how can we control which fields are valid or not in
>> comp[Vx_NSIZE]?
>>
>>
>> Anyway, what I'm thinking is maybe we can restructure layout of node block like
>> the one used by f2fs_inode:
>>
>> struct f2fs_node {
>> 	union {
>> 		struct f2fs_inode i;
>> 		union {
>> 			struct {
>> 				__le32 node_checksum;
>> 				__le32 feature_field_1;
>> 				__le32 feature_field_2;
>> 				....
>> 				__le32 addr[];
>> 				
>> 			};
>> 			struct direct_node dn;
>> 			struct indirect_node in;
>> 		};
>> 	};
>> 	struct node_footer footer;
>> } __packed;
>>
>> Moving all extended fields to the head of f2fs_node, so we don't have to use
>> macro to indicate actual size of addr.
> 
> Thinking what'd be the best way. My concern is, once getting more entries, we

OK, I think we need more discussion.. ;)

> can't set each of features individually. Like the second entry should have

Oh, that will be hard. If we have to avoid that, we have to tag in somewhere
e.g. f2fs_inode::i_flags2 to indicate which new field in f2fs_node is valid, for
example:

#define F2FS_NODE_CHECKSUM	0x0001
#define F2FS_NODE_FIELD1	0x0002
#define F2FS_NODE_FIELD2	0x0004

	union {
		struct {
			__le32 node_checksum;
			__le32 field_1;
			__le32 field_2;
			....
			__le32 addr[];
		};
		struct direct_node dn;
		struct indirect_node in;
	};

f2fs_inode::i_flags2 = F2FS_NODE_CHECKSUM | F2FS_NODE_FIELD1
indicates that f2fs_node::node_checksum and f2fs_node::field_1 are valid;

f2fs_inode::i_flags2 = F2FS_NODE_FIELD1 | F2FS_NODE_FIELD2
indicates that f2fs_node::field_1 and f2fs_node::field_2 are valid.

Any thoughts?

Thanks,

> enabled node_checksum, which we may not want to do.
> 
>>
>> Thanks,
>>
>>> 		__le32 comp[V2_NSIZE];
>>> 	}
>>> 	...
>>>
>>>> +			};
>>>> +			struct direct_node dn;
>>>> +			struct indirect_node in;
>>>> +		};
>>>>  	};
>>>>  	struct node_footer footer;
>>>>  } __packed;
>>>> -- 
>>>> 2.15.0.55.gc2ece9dc4de6
>>>
>>> .
>>>