Date:   Tue, 27 Feb 2018 21:34:01 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <chao@...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 02/27, Chao Yu wrote:
> Ping,
> 
> On 2018/2/13 15:34, Chao Yu wrote:
> > Hi Jaegeuk,
> > 
> > On 2018/2/10 10:52, Chao Yu wrote:
> >> 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?
> > 
> > Do you have time to explain more about the design of multiple version structure
> > for node block, I'm still be confused about two things:
> > 1. what will we do if we want to add one new field in node structure.
> > 2. how can we recognize which fields are valid and which ones are invalid.

Can we discuss this in LSF/MM, if we get an invitation letter? :P

> > 
> > Thanks,
> > 
> >>
> >> 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
> >>>>>>>
> >>>>>>> .
> >>>>>>>
> >>
> >> .
> >>
> > 

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