[07/13] ext4: import bitmaps chapter from wiki page

Message ID 153124304859.17949.6601344635597452820.stgit@magnolia
State Awaiting Upstream
Headers show
Series
  • ext4: major documentation surgery
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Commit Message

Darrick J. Wong July 10, 2018, 5:17 p.m.
From: Darrick J. Wong <darrick.wong@oracle.com>

Import the chapter about bitmaps from the on-disk format wiki
page into the kernel documentation.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 Documentation/filesystems/ext4/ondisk/bitmaps.rst |   28 +++++++++++++++++++++
 Documentation/filesystems/ext4/ondisk/globals.rst |    1 +
 2 files changed, 29 insertions(+)
 create mode 100644 Documentation/filesystems/ext4/ondisk/bitmaps.rst

Patch

diff --git a/Documentation/filesystems/ext4/ondisk/bitmaps.rst b/Documentation/filesystems/ext4/ondisk/bitmaps.rst
new file mode 100644
index 000000000000..c7546dbc197a
--- /dev/null
+++ b/Documentation/filesystems/ext4/ondisk/bitmaps.rst
@@ -0,0 +1,28 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+
+Block and inode Bitmaps
+-----------------------
+
+The data block bitmap tracks the usage of data blocks within the block
+group.
+
+The inode bitmap records which entries in the inode table are in use.
+
+As with most bitmaps, one bit represents the usage status of one data
+block or inode table entry. This implies a block group size of 8 \*
+number\_of\_bytes\_in\_a\_logical\_block.
+
+NOTE: If ``BLOCK_UNINIT`` is set for a given block group, various parts
+of the kernel and e2fsprogs code pretends that the block bitmap contains
+zeros (i.e. all blocks in the group are free). However, it is not
+necessarily the case that no blocks are in use -- if ``meta_bg`` is set,
+the bitmaps and group descriptor live inside the group. Unfortunately,
+ext2fs\_test\_block\_bitmap2() will return '0' for those locations,
+which produces confusing debugfs output.
+
+Inode Table
+-----------
+Inode tables are statically allocated at mkfs time.  Each block group
+descriptor points to the start of the table, and the superblock records
+the number of inodes per group.  See the section on inodes for more
+information.
diff --git a/Documentation/filesystems/ext4/ondisk/globals.rst b/Documentation/filesystems/ext4/ondisk/globals.rst
index c0dea6297e7f..5891ee137749 100644
--- a/Documentation/filesystems/ext4/ondisk/globals.rst
+++ b/Documentation/filesystems/ext4/ondisk/globals.rst
@@ -8,3 +8,4 @@  have static metadata at fixed locations.
 
 .. include:: super.rst
 .. include:: group_descr.rst
+.. include:: bitmaps.rst