ext4: documentation fixes

This commit aims to fix the following issues in ext4 documentation:
- Flexible block group docs said that the aim was to group block
  metadata together instead of block group metadata.
- The documentation consistly uses "location" instead of "block number".
  It is easy to confuse location to be an absolute offset on disk. Added
  a line to clarify all location values are in terms of block numbers.
- Dirent2 docs said that the rec_len field is shortened instead of the
  name_len field.
- Typo in bg_checksum description.
- Inode size is 160 bytes now, and hence i_extra_isize is now 32.
- Cluster size formula was incorrect, it did not include the +10 to
  s_log_cluster_size value.
- Typo: there were two s_wtime_hi in the superblock struct.
- Superblock struct was outdated, added the new fields which were part
  of s_reserved earlier.
- Multiple mount protection seems to be implemented in fs/ext4/mmp.c.

Signed-off-by: Ayush Ranjan <ayushr2@illinois.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>

Documentation/filesystems/ext4/blockgroup.rst

@@ -71,11 +71,11 @@ if the flex\_bg size is 4, then group 0 will contain (in order) the
 superblock, group descriptors, data block bitmaps for groups 0-3, inode
 bitmaps for groups 0-3, inode tables for groups 0-3, and the remaining
 space in group 0 is for file data. The effect of this is to group the
-block metadata close together for faster loading, and to enable large
-files to be continuous on disk. Backup copies of the superblock and
-group descriptors are always at the beginning of block groups, even if
-flex\_bg is enabled. The number of block groups that make up a flex\_bg
-is given by 2 ^ ``sb.s_log_groups_per_flex``.
+block group metadata close together for faster loading, and to enable
+large files to be continuous on disk. Backup copies of the superblock
+and group descriptors are always at the beginning of block groups, even
+if flex\_bg is enabled. The number of block groups that make up a
+flex\_bg is given by 2 ^ ``sb.s_log_groups_per_flex``.
 
 Meta Block Groups
 -----------------

Documentation/filesystems/ext4/blocks.rst

@@ -10,7 +10,9 @@ block groups. Block size is specified at mkfs time and typically is
 4KiB. You may experience mounting problems if block size is greater than
 page size (i.e. 64KiB blocks on a i386 which only has 4KiB memory
 pages). By default a filesystem can contain 2^32 blocks; if the '64bit'
-feature is enabled, then a filesystem can have 2^64 blocks.
+feature is enabled, then a filesystem can have 2^64 blocks. The location
+of structures is stored in terms of the block number the structure lives
+in and not the absolute offset on disk.
 
 For 32-bit filesystems, limits are as follows:
 

Documentation/filesystems/ext4/directory.rst

@@ -59,7 +59,7 @@ is at most 263 bytes long, though on disk you'll need to reference
      - File name.
 
 Since file names cannot be longer than 255 bytes, the new directory
-entry format shortens the rec\_len field and uses the space for a file
+entry format shortens the name\_len field and uses the space for a file
 type flag, probably to avoid having to load every inode during directory
 tree traversal. This format is ``ext4_dir_entry_2``, which is at most
 263 bytes long, though on disk you'll need to reference

Documentation/filesystems/ext4/group_descr.rst

@@ -99,9 +99,12 @@ The block group descriptor is laid out in ``struct ext4_group_desc``.
    * - 0x1E
      - \_\_le16
      - bg\_checksum
-     - Group descriptor checksum; crc16(sb\_uuid+group+desc) if the
-       RO\_COMPAT\_GDT\_CSUM feature is set, or crc32c(sb\_uuid+group\_desc) &
-       0xFFFF if the RO\_COMPAT\_METADATA\_CSUM feature is set.
+     - Group descriptor checksum; crc16(sb\_uuid+group\_num+bg\_desc) if the
+       RO\_COMPAT\_GDT\_CSUM feature is set, or
+       crc32c(sb\_uuid+group\_num+bg\_desc) & 0xFFFF if the
+       RO\_COMPAT\_METADATA\_CSUM feature is set.  The bg\_checksum
+       field in bg\_desc is skipped when calculating crc16 checksum,
+       and set to zero if crc32c checksum is used.
    * -
      -
      -

Documentation/filesystems/ext4/inodes.rst

@@ -470,8 +470,8 @@ inode, which allows struct ext4\_inode to grow for a new kernel without
 having to upgrade all of the on-disk inodes. Access to fields beyond
 EXT2\_GOOD\_OLD\_INODE\_SIZE should be verified to be within
 ``i_extra_isize``. By default, ext4 inode records are 256 bytes, and (as
-of October 2013) the inode structure is 156 bytes
-(``i_extra_isize = 28``). The extra space between the end of the inode
+of August 2019) the inode structure is 160 bytes
+(``i_extra_isize = 32``). The extra space between the end of the inode
 structure and the end of the inode record can be used to store extended
 attributes. Each inode record can be as large as the filesystem block
 size, though this is not terribly efficient.

Documentation/filesystems/ext4/super.rst

@@ -58,7 +58,7 @@ The ext4 superblock is laid out as follows in
    * - 0x1C
      - \_\_le32
      - s\_log\_cluster\_size
-     - Cluster size is (2 ^ s\_log\_cluster\_size) blocks if bigalloc is
+     - Cluster size is 2 ^ (10 + s\_log\_cluster\_size) blocks if bigalloc is
        enabled. Otherwise s\_log\_cluster\_size must equal s\_log\_block\_size.
    * - 0x20
      - \_\_le32
@@ -447,7 +447,7 @@ The ext4 superblock is laid out as follows in
      - Upper 8 bits of the s_wtime field.
    * - 0x275
      - \_\_u8
-     - s\_wtime_hi
+     - s\_mtime_hi
      - Upper 8 bits of the s_mtime field.
    * - 0x276
      - \_\_u8
@@ -466,12 +466,20 @@ The ext4 superblock is laid out as follows in
      - s\_last_error_time_hi
      - Upper 8 bits of the s_last_error_time_hi field.
    * - 0x27A
-     - \_\_u8[2]
-     - s\_pad
+     - \_\_u8
+     - s\_pad[2]
      - Zero padding.
    * - 0x27C
+     - \_\_le16
+     - s\_encoding
+     - Filename charset encoding.
+   * - 0x27E
+     - \_\_le16
+     - s\_encoding_flags
+     - Filename charset encoding flags.
+   * - 0x280
      - \_\_le32
-     - s\_reserved[96]
+     - s\_reserved[95]
      - Padding to the end of the block.
    * - 0x3FC
      - \_\_le32
@@ -617,7 +625,7 @@ following:
    * - 0x80
      - Enable a filesystem size of 2^64 blocks (INCOMPAT\_64BIT).
    * - 0x100
-     - Multiple mount protection. Not implemented (INCOMPAT\_MMP).
+     - Multiple mount protection (INCOMPAT\_MMP).
    * - 0x200
      - Flexible block groups. See the earlier discussion of this feature
        (INCOMPAT\_FLEX\_BG).