ext4: import inodes chapter from wiki page

Import the chapter about inodes from the on-disk format wiki
page into the kernel documentation.
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>

Documentation/filesystems/ext4/ondisk/dynamic.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+Dynamic Structures
+==================
+
+Dynamic metadata are created on the fly when files and blocks are
+allocated to files.
+
+.. include:: inodes.rst

Documentation/filesystems/ext4/ondisk/index.rst

@@ -6,3 +6,4 @@ Data Structures and Algorithms
 .. include:: about.rst
 .. include:: overview.rst
 .. include:: globals.rst
+.. include:: dynamic.rst

Documentation/filesystems/ext4/ondisk/inodes.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+Index Nodes
+-----------
+
+In a regular UNIX filesystem, the inode stores all the metadata
+pertaining to the file (time stamps, block maps, extended attributes,
+etc), not the directory entry. To find the information associated with a
+file, one must traverse the directory files to find the directory entry
+associated with a file, then load the inode to find the metadata for
+that file. ext4 appears to cheat (for performance reasons) a little bit
+by storing a copy of the file type (normally stored in the inode) in the
+directory entry. (Compare all this to FAT, which stores all the file
+information directly in the directory entry, but does not support hard
+links and is in general more seek-happy than ext4 due to its simpler
+block allocator and extensive use of linked lists.)
+
+The inode table is a linear array of ``struct ext4_inode``. The table is
+sized to have enough blocks to store at least
+``sb.s_inode_size * sb.s_inodes_per_group`` bytes. The number of the
+block group containing an inode can be calculated as
+``(inode_number - 1) / sb.s_inodes_per_group``, and the offset into the
+group's table is ``(inode_number - 1) % sb.s_inodes_per_group``. There
+is no inode 0.
+
+The inode checksum is calculated against the FS UUID, the inode number,
+and the inode structure itself.
+
+The inode table entry is laid out in ``struct ext4_inode``.
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Size
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le16
+     - i\_mode
+     - File mode. See the table i_mode_ below.
+   * - 0x2
+     - \_\_le16
+     - i\_uid
+     - Lower 16-bits of Owner UID.
+   * - 0x4
+     - \_\_le32
+     - i\_size\_lo
+     - Lower 32-bits of size in bytes.
+   * - 0x8
+     - \_\_le32
+     - i\_atime
+     - Last access time, in seconds since the epoch. However, if the EA\_INODE
+       inode flag is set, this inode stores an extended attribute value and
+       this field contains the checksum of the value.
+   * - 0xC
+     - \_\_le32
+     - i\_ctime
+     - Last inode change time, in seconds since the epoch. However, if the
+       EA\_INODE inode flag is set, this inode stores an extended attribute
+       value and this field contains the lower 32 bits of the attribute value's
+       reference count.
+   * - 0x10
+     - \_\_le32
+     - i\_mtime
+     - Last data modification time, in seconds since the epoch. However, if the
+       EA\_INODE inode flag is set, this inode stores an extended attribute
+       value and this field contains the number of the inode that owns the
+       extended attribute.
+   * - 0x14
+     - \_\_le32
+     - i\_dtime
+     - Deletion Time, in seconds since the epoch.
+   * - 0x18
+     - \_\_le16
+     - i\_gid
+     - Lower 16-bits of GID.
+   * - 0x1A
+     - \_\_le16
+     - i\_links\_count
+     - Hard link count. Normally, ext4 does not permit an inode to have more
+       than 65,000 hard links. This applies to files as well as directories,
+       which means that there cannot be more than 64,998 subdirectories in a
+       directory (each subdirectory's '..' entry counts as a hard link, as does
+       the '.' entry in the directory itself). With the DIR\_NLINK feature
+       enabled, ext4 supports more than 64,998 subdirectories by setting this
+       field to 1 to indicate that the number of hard links is not known.
+   * - 0x1C
+     - \_\_le32
+     - i\_blocks\_lo
+     - Lower 32-bits of “block” count. If the huge\_file feature flag is not
+       set on the filesystem, the file consumes ``i_blocks_lo`` 512-byte blocks
+       on disk. If huge\_file is set and EXT4\_HUGE\_FILE\_FL is NOT set in
+       ``inode.i_flags``, then the file consumes ``i_blocks_lo + (i_blocks_hi
+       << 32)`` 512-byte blocks on disk. If huge\_file is set and
+       EXT4\_HUGE\_FILE\_FL IS set in ``inode.i_flags``, then this file
+       consumes (``i_blocks_lo + i_blocks_hi`` << 32) filesystem blocks on
+       disk.
+   * - 0x20
+     - \_\_le32
+     - i\_flags
+     - Inode flags. See the table i_flags_ below.
+   * - 0x24
+     - 4 bytes
+     - i\_osd1
+     - See the table i_osd1_ for more details.
+   * - 0x28
+     - 60 bytes
+     - i\_block[EXT4\_N\_BLOCKS=15]
+     - Block map or extent tree. See the section “The Contents of inode.i\_block”.
+   * - 0x64
+     - \_\_le32
+     - i\_generation
+     - File version (for NFS).
+   * - 0x68
+     - \_\_le32
+     - i\_file\_acl\_lo
+     - Lower 32-bits of extended attribute block. ACLs are of course one of
+       many possible extended attributes; I think the name of this field is a
+       result of the first use of extended attributes being for ACLs.
+   * - 0x6C
+     - \_\_le32
+     - i\_size\_high / i\_dir\_acl
+     - Upper 32-bits of file/directory size. In ext2/3 this field was named
+       i\_dir\_acl, though it was usually set to zero and never used.
+   * - 0x70
+     - \_\_le32
+     - i\_obso\_faddr
+     - (Obsolete) fragment address.
+   * - 0x74
+     - 12 bytes
+     - i\_osd2
+     - See the table i_osd2_ for more details.
+   * - 0x80
+     - \_\_le16
+     - i\_extra\_isize
+     - Size of this inode - 128. Alternately, the size of the extended inode
+       fields beyond the original ext2 inode, including this field.
+   * - 0x82
+     - \_\_le16
+     - i\_checksum\_hi
+     - Upper 16-bits of the inode checksum.
+   * - 0x84
+     - \_\_le32
+     - i\_ctime\_extra
+     - Extra change time bits. This provides sub-second precision. See Inode
+       Timestamps section.
+   * - 0x88
+     - \_\_le32
+     - i\_mtime\_extra
+     - Extra modification time bits. This provides sub-second precision.
+   * - 0x8C
+     - \_\_le32
+     - i\_atime\_extra
+     - Extra access time bits. This provides sub-second precision.
+   * - 0x90
+     - \_\_le32
+     - i\_crtime
+     - File creation time, in seconds since the epoch.
+   * - 0x94
+     - \_\_le32
+     - i\_crtime\_extra
+     - Extra file creation time bits. This provides sub-second precision.
+   * - 0x98
+     - \_\_le32
+     - i\_version\_hi
+     - Upper 32-bits for version number.
+   * - 0x9C
+     - \_\_le32
+     - i\_projid
+     - Project ID.
+
+.. _i_mode:
+
+The ``i_mode`` value is a combination of the following flags:
+
+.. list-table::
+   :widths: 1 79
+   :header-rows: 1
+
+   * - Value
+     - Description
+   * - 0x1
+     - S\_IXOTH (Others may execute)
+   * - 0x2
+     - S\_IWOTH (Others may write)
+   * - 0x4
+     - S\_IROTH (Others may read)
+   * - 0x8
+     - S\_IXGRP (Group members may execute)
+   * - 0x10
+     - S\_IWGRP (Group members may write)
+   * - 0x20
+     - S\_IRGRP (Group members may read)
+   * - 0x40
+     - S\_IXUSR (Owner may execute)
+   * - 0x80
+     - S\_IWUSR (Owner may write)
+   * - 0x100
+     - S\_IRUSR (Owner may read)
+   * - 0x200
+     - S\_ISVTX (Sticky bit)
+   * - 0x400
+     - S\_ISGID (Set GID)
+   * - 0x800
+     - S\_ISUID (Set UID)
+   * -
+     - These are mutually-exclusive file types:
+   * - 0x1000
+     - S\_IFIFO (FIFO)
+   * - 0x2000
+     - S\_IFCHR (Character device)
+   * - 0x4000
+     - S\_IFDIR (Directory)
+   * - 0x6000
+     - S\_IFBLK (Block device)
+   * - 0x8000
+     - S\_IFREG (Regular file)
+   * - 0xA000
+     - S\_IFLNK (Symbolic link)
+   * - 0xC000
+     - S\_IFSOCK (Socket)
+
+.. _i_flags:
+
+The ``i_flags`` field is a combination of these values:
+
+.. list-table::
+   :widths: 1 79
+   :header-rows: 1
+
+   * - Value
+     - Description
+   * - 0x1
+     - This file requires secure deletion (EXT4\_SECRM\_FL). (not implemented)
+   * - 0x2
+     - This file should be preserved, should undeletion be desired
+       (EXT4\_UNRM\_FL). (not implemented)
+   * - 0x4
+     - File is compressed (EXT4\_COMPR\_FL). (not really implemented)
+   * - 0x8
+     - All writes to the file must be synchronous (EXT4\_SYNC\_FL).
+   * - 0x10
+     - File is immutable (EXT4\_IMMUTABLE\_FL).
+   * - 0x20
+     - File can only be appended (EXT4\_APPEND\_FL).
+   * - 0x40
+     - The dump(1) utility should not dump this file (EXT4\_NODUMP\_FL).
+   * - 0x80
+     - Do not update access time (EXT4\_NOATIME\_FL).
+   * - 0x100
+     - Dirty compressed file (EXT4\_DIRTY\_FL). (not used)
+   * - 0x200
+     - File has one or more compressed clusters (EXT4\_COMPRBLK\_FL). (not used)
+   * - 0x400
+     - Do not compress file (EXT4\_NOCOMPR\_FL). (not used)
+   * - 0x800
+     - Encrypted inode (EXT4\_ENCRYPT\_FL). This bit value previously was
+       EXT4\_ECOMPR\_FL (compression error), which was never used.
+   * - 0x1000
+     - Directory has hashed indexes (EXT4\_INDEX\_FL).
+   * - 0x2000
+     - AFS magic directory (EXT4\_IMAGIC\_FL).
+   * - 0x4000
+     - File data must always be written through the journal
+       (EXT4\_JOURNAL\_DATA\_FL).
+   * - 0x8000
+     - File tail should not be merged (EXT4\_NOTAIL\_FL). (not used by ext4)
+   * - 0x10000
+     - All directory entry data should be written synchronously (see
+       ``dirsync``) (EXT4\_DIRSYNC\_FL).
+   * - 0x20000
+     - Top of directory hierarchy (EXT4\_TOPDIR\_FL).
+   * - 0x40000
+     - This is a huge file (EXT4\_HUGE\_FILE\_FL).
+   * - 0x80000
+     - Inode uses extents (EXT4\_EXTENTS\_FL).
+   * - 0x200000
+     - Inode stores a large extended attribute value in its data blocks
+       (EXT4\_EA\_INODE\_FL).
+   * - 0x400000
+     - This file has blocks allocated past EOF (EXT4\_EOFBLOCKS\_FL).
+       (deprecated)
+   * - 0x01000000
+     - Inode is a snapshot (``EXT4_SNAPFILE_FL``). (not in mainline)
+   * - 0x04000000
+     - Snapshot is being deleted (``EXT4_SNAPFILE_DELETED_FL``). (not in
+       mainline)
+   * - 0x08000000
+     - Snapshot shrink has completed (``EXT4_SNAPFILE_SHRUNK_FL``). (not in
+       mainline)
+   * - 0x10000000
+     - Inode has inline data (EXT4\_INLINE\_DATA\_FL).
+   * - 0x20000000
+     - Create children with the same project ID (EXT4\_PROJINHERIT\_FL).
+   * - 0x80000000
+     - Reserved for ext4 library (EXT4\_RESERVED\_FL).
+   * -
+     - Aggregate flags:
+   * - 0x4BDFFF
+     - User-visible flags.
+   * - 0x4B80FF
+     - User-modifiable flags. Note that while EXT4\_JOURNAL\_DATA\_FL and
+       EXT4\_EXTENTS\_FL can be set with setattr, they are not in the kernel's
+       EXT4\_FL\_USER\_MODIFIABLE mask, since it needs to handle the setting of
+       these flags in a special manner and they are masked out of the set of
+       flags that are saved directly to i\_flags.
+
+.. _i_osd1:
+
+The ``osd1`` field has multiple meanings depending on the creator:
+
+Linux:
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Size
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le32
+     - l\_i\_version
+     - Inode version. However, if the EA\_INODE inode flag is set, this inode
+       stores an extended attribute value and this field contains the upper 32
+       bits of the attribute value's reference count.
+
+Hurd:
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Size
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le32
+     - h\_i\_translator
+     - ??
+
+Masix:
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Size
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le32
+     - m\_i\_reserved
+     - ??
+
+.. _i_osd2:
+
+The ``osd2`` field has multiple meanings depending on the filesystem creator:
+
+Linux:
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Size
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le16
+     - l\_i\_blocks\_high
+     - Upper 16-bits of the block count. Please see the note attached to
+       i\_blocks\_lo.
+   * - 0x2
+     - \_\_le16
+     - l\_i\_file\_acl\_high
+     - Upper 16-bits of the extended attribute block (historically, the file
+       ACL location). See the Extended Attributes section below.
+   * - 0x4
+     - \_\_le16
+     - l\_i\_uid\_high
+     - Upper 16-bits of the Owner UID.
+   * - 0x6
+     - \_\_le16
+     - l\_i\_gid\_high
+     - Upper 16-bits of the GID.
+   * - 0x8
+     - \_\_le16
+     - l\_i\_checksum\_lo
+     - Lower 16-bits of the inode checksum.
+   * - 0xA
+     - \_\_le16
+     - l\_i\_reserved
+     - Unused.
+
+Hurd:
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Size
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le16
+     - h\_i\_reserved1
+     - ??
+   * - 0x2
+     - \_\_u16
+     - h\_i\_mode\_high
+     - Upper 16-bits of the file mode.
+   * - 0x4
+     - \_\_le16
+     - h\_i\_uid\_high
+     - Upper 16-bits of the Owner UID.
+   * - 0x6
+     - \_\_le16
+     - h\_i\_gid\_high
+     - Upper 16-bits of the GID.
+   * - 0x8
+     - \_\_u32
+     - h\_i\_author
+     - Author code?
+
+Masix:
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Size
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le16
+     - h\_i\_reserved1
+     - ??
+   * - 0x2
+     - \_\_u16
+     - m\_i\_file\_acl\_high
+     - Upper 16-bits of the extended attribute block (historically, the file
+       ACL location).
+   * - 0x4
+     - \_\_u32
+     - m\_i\_reserved2[2]
+     - ??
+
+Inode Size
+~~~~~~~~~~
+
+In ext2 and ext3, the inode structure size was fixed at 128 bytes
+(``EXT2_GOOD_OLD_INODE_SIZE``) and each inode had a disk record size of
+128 bytes. Starting with ext4, it is possible to allocate a larger
+on-disk inode at format time for all inodes in the filesystem to provide
+space beyond the end of the original ext2 inode. The on-disk inode
+record size is recorded in the superblock as ``s_inode_size``. The
+number of bytes actually used by struct ext4\_inode beyond the original
+128-byte ext2 inode is recorded in the ``i_extra_isize`` field for each
+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
+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.
+
+Finding an Inode
+~~~~~~~~~~~~~~~~
+
+Each block group contains ``sb->s_inodes_per_group`` inodes. Because
+inode 0 is defined not to exist, this formula can be used to find the
+block group that an inode lives in:
+``bg = (inode_num - 1) / sb->s_inodes_per_group``. The particular inode
+can be found within the block group's inode table at
+``index = (inode_num - 1) % sb->s_inodes_per_group``. To get the byte
+address within the inode table, use
+``offset = index * sb->s_inode_size``.
+
+Inode Timestamps
+~~~~~~~~~~~~~~~~
+
+Four timestamps are recorded in the lower 128 bytes of the inode
+structure -- inode change time (ctime), access time (atime), data
+modification time (mtime), and deletion time (dtime). The four fields
+are 32-bit signed integers that represent seconds since the Unix epoch
+(1970-01-01 00:00:00 GMT), which means that the fields will overflow in
+January 2038. For inodes that are not linked from any directory but are
+still open (orphan inodes), the dtime field is overloaded for use with
+the orphan list. The superblock field ``s_last_orphan`` points to the
+first inode in the orphan list; dtime is then the number of the next
+orphaned inode, or zero if there are no more orphans.
+
+If the inode structure size ``sb->s_inode_size`` is larger than 128
+bytes and the ``i_inode_extra`` field is large enough to encompass the
+respective ``i_[cma]time_extra`` field, the ctime, atime, and mtime
+inode fields are widened to 64 bits. Within this “extra” 32-bit field,
+the lower two bits are used to extend the 32-bit seconds field to be 34
+bit wide; the upper 30 bits are used to provide nanosecond timestamp
+accuracy. Therefore, timestamps should not overflow until May 2446.
+dtime was not widened. There is also a fifth timestamp to record inode
+creation time (crtime); this field is 64-bits wide and decoded in the
+same manner as 64-bit [cma]time. Neither crtime nor dtime are accessible
+through the regular stat() interface, though debugfs will report them.
+
+We use the 32-bit signed time value plus (2^32 \* (extra epoch bits)).
+In other words:
+
+.. list-table::
+   :widths: 20 20 20 20 20
+   :header-rows: 1
+
+   * - Extra epoch bits
+     - MSB of 32-bit time
+     - Adjustment for signed 32-bit to 64-bit tv\_sec
+     - Decoded 64-bit tv\_sec
+     - valid time range
+   * - 0 0
+     - 1
+     - 0
+     - ``-0x80000000 - -0x00000001``
+     - 1901-12-13 to 1969-12-31
+   * - 0 0
+     - 0
+     - 0
+     - ``0x000000000 - 0x07fffffff``
+     - 1970-01-01 to 2038-01-19
+   * - 0 1
+     - 1
+     - 0x100000000
+     - ``0x080000000 - 0x0ffffffff``
+     - 2038-01-19 to 2106-02-07
+   * - 0 1
+     - 0
+     - 0x100000000
+     - ``0x100000000 - 0x17fffffff``
+     - 2106-02-07 to 2174-02-25
+   * - 1 0
+     - 1
+     - 0x200000000
+     - ``0x180000000 - 0x1ffffffff``
+     - 2174-02-25 to 2242-03-16
+   * - 1 0
+     - 0
+     - 0x200000000
+     - ``0x200000000 - 0x27fffffff``
+     - 2242-03-16 to 2310-04-04
+   * - 1 1
+     - 1
+     - 0x300000000
+     - ``0x280000000 - 0x2ffffffff``
+     - 2310-04-04 to 2378-04-22
+   * - 1 1
+     - 0
+     - 0x300000000
+     - ``0x300000000 - 0x37fffffff``
+     - 2378-04-22 to 2446-05-10
+
+This is a somewhat odd encoding since there are effectively seven times
+as many positive values as negative values. There have also been
+long-standing bugs decoding and encoding dates beyond 2038, which don't
+seem to be fixed as of kernel 3.12 and e2fsprogs 1.42.8. 64-bit kernels
+incorrectly use the extra epoch bits 1,1 for dates between 1901 and
+1970. At some point the kernel will be fixed and e2fsck will fix this
+situation, assuming that it is run before 2310.