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

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Message-ID: <20180210014140.GB1885@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Fri, 9 Feb 2018 17:41:40 -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 02/01, 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.

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