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

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Message-ID: <20180417033858.GB76077@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Mon, 16 Apr 2018 20:38:58 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     Chao Yu <chao@...nel.org>, 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 04/13, Chao Yu wrote:
> Ping again..
> 
> Do you have time to discuss this?

We may need a time to have a chat in person. Do you have any chance to visit
US?

> 
> On 2018/2/27 22:16, 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.
> >>
> >> 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
> >>>>>>>>
> >>>>>>>> .
> >>>>>>>>
> >>>
> >>> .
> >>>
> >>
> > 
> > .
> >