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Linux/fs/cramfs/

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Name Size Last modified (GMT) Description
Back Parent directory 2024-10-29 14:06:53
File Kconfig 2148 bytes 2024-10-29 14:06:18
File Makefile 157 bytes 2024-10-29 14:06:18
File README 7687 bytes 2024-10-29 14:06:18
C file inode.c 26793 bytes 2024-10-29 14:06:18
C file internal.h 198 bytes 2024-10-29 14:06:18
C file uncompress.c 1807 bytes 2024-10-29 14:06:18

  1 Notes on Filesystem Layout
  2 --------------------------
  3 
  4 These notes describe what mkcramfs generates.  Kernel requirements are
  5 a bit looser, e.g. it doesn't care if the <file_data> items are
  6 swapped around (though it does care that directory entries (inodes) in
  7 a given directory are contiguous, as this is used by readdir).
  8 
  9 All data is currently in host-endian format; neither mkcramfs nor the
 10 kernel ever do swabbing.  (See section `Block Size' below.)
 11 
 12 <filesystem>:
 13         <superblock>
 14         <directory_structure>
 15         <data>
 16 
 17 <superblock>: struct cramfs_super (see cramfs_fs.h).
 18 
 19 <directory_structure>:
 20         For each file:
 21                 struct cramfs_inode (see cramfs_fs.h).
 22                 Filename.  Not generally null-terminated, but it is
 23                  null-padded to a multiple of 4 bytes.
 24 
 25 The order of inode traversal is described as "width-first" (not to be
 26 confused with breadth-first); i.e. like depth-first but listing all of
 27 a directory's entries before recursing down its subdirectories: the
 28 same order as `ls -AUR' (but without the /^\..*:$/ directory header
 29 lines); put another way, the same order as `find -type d -exec
 30 ls -AU1 {} \;'.
 31 
 32 Beginning in 2.4.7, directory entries are sorted.  This optimization
 33 allows cramfs_lookup to return more quickly when a filename does not
 34 exist, speeds up user-space directory sorts, etc.
 35 
 36 <data>:
 37         One <file_data> for each file that's either a symlink or a
 38          regular file of non-zero st_size.
 39 
 40 <file_data>:
 41         nblocks * <block_pointer>
 42          (where nblocks = (st_size - 1) / blksize + 1)
 43         nblocks * <block>
 44         padding to multiple of 4 bytes
 45 
 46 The i'th <block_pointer> for a file stores the byte offset of the
 47 *end* of the i'th <block> (i.e. one past the last byte, which is the
 48 same as the start of the (i+1)'th <block> if there is one).  The first
 49 <block> immediately follows the last <block_pointer> for the file.
 50 <block_pointer>s are each 32 bits long.
 51 
 52 When the CRAMFS_FLAG_EXT_BLOCK_POINTERS capability bit is set, each
 53 <block_pointer>'s top bits may contain special flags as follows:
 54 
 55 CRAMFS_BLK_FLAG_UNCOMPRESSED (bit 31):
 56         The block data is not compressed and should be copied verbatim.
 57 
 58 CRAMFS_BLK_FLAG_DIRECT_PTR (bit 30):
 59         The <block_pointer> stores the actual block start offset and not
 60         its end, shifted right by 2 bits. The block must therefore be
 61         aligned to a 4-byte boundary. The block size is either blksize
 62         if CRAMFS_BLK_FLAG_UNCOMPRESSED is also specified, otherwise
 63         the compressed data length is included in the first 2 bytes of
 64         the block data. This is used to allow discontiguous data layout
 65         and specific data block alignments e.g. for XIP applications.
 66 
 67 
 68 The order of <file_data>'s is a depth-first descent of the directory
 69 tree, i.e. the same order as `find -size +0 \( -type f -o -type l \)
 70 -print'.
 71 
 72 
 73 <block>: The i'th <block> is the output of zlib's compress function
 74 applied to the i'th blksize-sized chunk of the input data if the
 75 corresponding CRAMFS_BLK_FLAG_UNCOMPRESSED <block_ptr> bit is not set,
 76 otherwise it is the input data directly.
 77 (For the last <block> of the file, the input may of course be smaller.)
 78 Each <block> may be a different size.  (See <block_pointer> above.)
 79 
 80 <block>s are merely byte-aligned, not generally u32-aligned.
 81 
 82 When CRAMFS_BLK_FLAG_DIRECT_PTR is specified then the corresponding
 83 <block> may be located anywhere and not necessarily contiguous with
 84 the previous/next blocks. In that case it is minimally u32-aligned.
 85 If CRAMFS_BLK_FLAG_UNCOMPRESSED is also specified then the size is always
 86 blksize except for the last block which is limited by the file length.
 87 If CRAMFS_BLK_FLAG_DIRECT_PTR is set and CRAMFS_BLK_FLAG_UNCOMPRESSED
 88 is not set then the first 2 bytes of the block contains the size of the
 89 remaining block data as this cannot be determined from the placement of
 90 logically adjacent blocks.
 91 
 92 
 93 Holes
 94 -----
 95 
 96 This kernel supports cramfs holes (i.e. [efficient representation of]
 97 blocks in uncompressed data consisting entirely of NUL bytes), but by
 98 default mkcramfs doesn't test for & create holes, since cramfs in
 99 kernels up to at least 2.3.39 didn't support holes.  Run mkcramfs
100 with -z if you want it to create files that can have holes in them.
101 
102 
103 Tools
104 -----
105 
106 The cramfs user-space tools, including mkcramfs and cramfsck, are
107 located at <http://sourceforge.net/projects/cramfs/>.
108 
109 
110 Future Development
111 ==================
112 
113 Block Size
114 ----------
115 
116 (Block size in cramfs refers to the size of input data that is
117 compressed at a time.  It's intended to be somewhere around
118 PAGE_SIZE for cramfs_read_folio's convenience.)
119 
120 The superblock ought to indicate the block size that the fs was
121 written for, since comments in <linux/pagemap.h> indicate that
122 PAGE_SIZE may grow in future (if I interpret the comment
123 correctly).
124 
125 Currently, mkcramfs #define's PAGE_SIZE as 4096 and uses that
126 for blksize, whereas Linux-2.3.39 uses its PAGE_SIZE, which in
127 turn is defined as PAGE_SIZE (which can be as large as 32KB on arm).
128 This discrepancy is a bug, though it's not clear which should be
129 changed.
130 
131 One option is to change mkcramfs to take its PAGE_SIZE from
132 <asm/page.h>.  Personally I don't like this option, but it does
133 require the least amount of change: just change `#define
134 PAGE_SIZE (4096)' to `#include <asm/page.h>'.  The disadvantage
135 is that the generated cramfs cannot always be shared between different
136 kernels, not even necessarily kernels of the same architecture if
137 PAGE_SIZE is subject to change between kernel versions
138 (currently possible with arm and ia64).
139 
140 The remaining options try to make cramfs more sharable.
141 
142 One part of that is addressing endianness.  The two options here are
143 `always use little-endian' (like ext2fs) or `writer chooses
144 endianness; kernel adapts at runtime'.  Little-endian wins because of
145 code simplicity and little CPU overhead even on big-endian machines.
146 
147 The cost of swabbing is changing the code to use the le32_to_cpu
148 etc. macros as used by ext2fs.  We don't need to swab the compressed
149 data, only the superblock, inodes and block pointers.
150 
151 
152 The other part of making cramfs more sharable is choosing a block
153 size.  The options are:
154 
155   1. Always 4096 bytes.
156 
157   2. Writer chooses blocksize; kernel adapts but rejects blocksize >
158      PAGE_SIZE.
159 
160   3. Writer chooses blocksize; kernel adapts even to blocksize >
161      PAGE_SIZE.
162 
163 It's easy enough to change the kernel to use a smaller value than
164 PAGE_SIZE: just make cramfs_read_folio read multiple blocks.
165 
166 The cost of option 1 is that kernels with a larger PAGE_SIZE
167 value don't get as good compression as they can.
168 
169 The cost of option 2 relative to option 1 is that the code uses
170 variables instead of #define'd constants.  The gain is that people
171 with kernels having larger PAGE_SIZE can make use of that if
172 they don't mind their cramfs being inaccessible to kernels with
173 smaller PAGE_SIZE values.
174 
175 Option 3 is easy to implement if we don't mind being CPU-inefficient:
176 e.g. get read_folio to decompress to a buffer of size MAX_BLKSIZE (which
177 must be no larger than 32KB) and discard what it doesn't need.
178 Getting read_folio to read into all the covered pages is harder.
179 
180 The main advantage of option 3 over 1, 2, is better compression.  The
181 cost is greater complexity.  Probably not worth it, but I hope someone
182 will disagree.  (If it is implemented, then I'll re-use that code in
183 e2compr.)
184 
185 
186 Another cost of 2 and 3 over 1 is making mkcramfs use a different
187 block size, but that just means adding and parsing a -b option.
188 
189 
190 Inode Size
191 ----------
192 
193 Given that cramfs will probably be used for CDs etc. as well as just
194 silicon ROMs, it might make sense to expand the inode a little from
195 its current 12 bytes.  Inodes other than the root inode are followed
196 by filename, so the expansion doesn't even have to be a multiple of 4
197 bytes.

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