Documentation/filesystems/ext4/super.rst

Source file repositories/reference/linux-study-clean/Documentation/filesystems/ext4/super.rst

File Facts

System: Linux kernel
Corpus path: Documentation/filesystems/ext4/super.rst
Extension: .rst
Size: 23789 bytes
Lines: 842
Domain: Support Tooling And Documentation
Bucket: Documentation
Inferred role: Support Tooling And Documentation: documentation
Status: atlas-only

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.
Defines or uses C structs; map object ownership, embedded links, reference counts, and lock ownership.

Dependency Surface

No C-style include directives detected by the generator.

Detected Declarations

No top-level syscall, struct, function, initcall, or export declaration detected by the generator.

Annotated Snippet

.. SPDX-License-Identifier: GPL-2.0

Super Block
-----------

The superblock records various information about the enclosing
filesystem, such as block counts, inode counts, supported features,
maintenance information, and more.

If the sparse_super feature flag is set, redundant copies of the
superblock and group descriptors are kept only in the groups whose group
number is either 0 or a power of 3, 5, or 7. If the flag is not set,
redundant copies are kept in all groups.

The superblock checksum is calculated against the superblock structure,
which includes the FS UUID.

The ext4 superblock is laid out as follows in
``struct ext4_super_block``:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Size
     - Name
     - Description
   * - 0x0
     - __le32
     - s_inodes_count
     - Total inode count.
   * - 0x4
     - __le32
     - s_blocks_count_lo
     - Total block count.
   * - 0x8
     - __le32
     - s_r_blocks_count_lo
     - This number of blocks can only be allocated by the super-user.
   * - 0xC
     - __le32
     - s_free_blocks_count_lo
     - Free block count.
   * - 0x10
     - __le32
     - s_free_inodes_count
     - Free inode count.
   * - 0x14
     - __le32
     - s_first_data_block
     - First data block. This must be at least 1 for 1k-block filesystems and
       is typically 0 for all other block sizes.
   * - 0x18
     - __le32
     - s_log_block_size
     - Block size is 2 ^ (10 + s_log_block_size).
   * - 0x1C
     - __le32
     - s_log_cluster_size
     - 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
     - s_blocks_per_group
     - Blocks per group.
   * - 0x24
     - __le32
     - s_clusters_per_group
     - Clusters per group, if bigalloc is enabled. Otherwise

Annotation

Atlas domain: Support Tooling And Documentation / Documentation.
Implementation status: atlas-only.

Implementation Notes

This generated page is the file-by-file coverage layer; curated subsystem chapters should link here when they synthesize a multi-file control flow.
Core OS pages should be promoted from atlas-only to deep-reviewed when they explain data structures, invariants, locking, lifecycle, and C implementation snippets.
Driver-family pages are intentionally pattern-oriented unless they are part of the selected PCIe/NVMe representative device path.