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

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Date:   Sat, 10 Feb 2018 10:52:18 +0800
From:   Chao Yu <chao@...nel.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     Chao Yu <yuchao0@...wei.com>,
        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 2018/2/10 9:41, Jaegeuk Kim wrote:
> On 02/01, Chao Yu wrote:
>>
>>
>> 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.
> 
> So, that's why I thought we may need a sort of each formats.

Hmm.. if we have two new added fields, there are (2 << 2) combinations
of all formats, as:

struct original {
	__le32 data[DEF_ADDRS_PER_BLOCK];
}

struct node_v1 {
	__le32 data[DEF_ADDRS_PER_BLOCK - V1_NSIZE=1];
	__le32 field_1;
}

struct node_v2 {
	__le32 data[DEF_ADDRS_PER_BLOCK - V2_NSIZE=1];
	__le32 field_2;
}

struct node_v2 {
	__le32 data[DEF_ADDRS_PER_BLOCK - V3_NSIZE=2];
	__le32 field_1;
	__le32 field_2;
}

If we add more new fields, the node version will increase sharply due
to there is (n << 2) combination with n fields. Right? Any thoughts to
reduce maintaining overhead on those node versions structures?

Thanks,

> 
>>
>> 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
>>>>>
>>>>> .
>>>>>