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docs: Bring some order to filesystem documentation
Documentation/filesystems is, like much of the rest of the kernel's documentation, a jumble of unorganized information. Split the documentation into categories and try to bring some order to the top-level index.rst files. No text changes other than a few section-introductory blurbs; this is all just moving stuff around. Cc: linux-fsdevel@vger.kernel.org Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Jonathan Corbet
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5 changed files with 400 additions and 375 deletions
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Documentation/filesystems/api-summary.rst
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Documentation/filesystems/api-summary.rst
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=============================
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Linux Filesystems API summary
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=============================
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This section contains API-level documentation, mostly taken from the source
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code itself.
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The Linux VFS
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=============
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The Filesystem types
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--------------------
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.. kernel-doc:: include/linux/fs.h
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:internal:
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The Directory Cache
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-------------------
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.. kernel-doc:: fs/dcache.c
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:export:
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.. kernel-doc:: include/linux/dcache.h
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:internal:
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Inode Handling
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--------------
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.. kernel-doc:: fs/inode.c
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:export:
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.. kernel-doc:: fs/bad_inode.c
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:export:
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Registration and Superblocks
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----------------------------
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.. kernel-doc:: fs/super.c
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:export:
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File Locks
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----------
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.. kernel-doc:: fs/locks.c
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:export:
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.. kernel-doc:: fs/locks.c
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:internal:
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Other Functions
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---------------
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.. kernel-doc:: fs/mpage.c
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:export:
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.. kernel-doc:: fs/namei.c
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:export:
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.. kernel-doc:: fs/buffer.c
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:export:
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.. kernel-doc:: block/bio.c
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:export:
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.. kernel-doc:: fs/seq_file.c
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:export:
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.. kernel-doc:: fs/filesystems.c
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:export:
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.. kernel-doc:: fs/fs-writeback.c
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:export:
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.. kernel-doc:: fs/block_dev.c
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:export:
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.. kernel-doc:: fs/anon_inodes.c
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:export:
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.. kernel-doc:: fs/attr.c
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:export:
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.. kernel-doc:: fs/d_path.c
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:export:
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.. kernel-doc:: fs/dax.c
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:export:
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.. kernel-doc:: fs/direct-io.c
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:export:
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.. kernel-doc:: fs/file_table.c
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:export:
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.. kernel-doc:: fs/libfs.c
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:export:
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.. kernel-doc:: fs/posix_acl.c
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:export:
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.. kernel-doc:: fs/stat.c
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:export:
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.. kernel-doc:: fs/sync.c
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:export:
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.. kernel-doc:: fs/xattr.c
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:export:
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The proc filesystem
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===================
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sysctl interface
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----------------
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.. kernel-doc:: kernel/sysctl.c
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:export:
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proc filesystem interface
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-------------------------
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.. kernel-doc:: fs/proc/base.c
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:internal:
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Events based on file descriptors
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================================
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.. kernel-doc:: fs/eventfd.c
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:export:
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The Filesystem for Exporting Kernel Objects
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===========================================
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.. kernel-doc:: fs/sysfs/file.c
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:export:
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.. kernel-doc:: fs/sysfs/symlink.c
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:export:
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The debugfs filesystem
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======================
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debugfs interface
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-----------------
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.. kernel-doc:: fs/debugfs/inode.c
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:export:
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.. kernel-doc:: fs/debugfs/file.c
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:export:
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@ -1,389 +1,43 @@
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=====================
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===============================
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Linux Filesystems API
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Filesystems in the Linux kernel
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=====================
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===============================
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The Linux VFS
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This under-development manual will, some glorious day, provide
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=============
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comprehensive information on how the Linux virtual filesystem (VFS) layer
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works, along with the filesystems that sit below it. For now, what we have
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can be found below.
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The Filesystem types
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Core VFS documentation
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--------------------
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.. kernel-doc:: include/linux/fs.h
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:internal:
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The Directory Cache
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-------------------
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.. kernel-doc:: fs/dcache.c
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:export:
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.. kernel-doc:: include/linux/dcache.h
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:internal:
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Inode Handling
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--------------
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.. kernel-doc:: fs/inode.c
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:export:
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.. kernel-doc:: fs/bad_inode.c
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:export:
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Registration and Superblocks
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----------------------------
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.. kernel-doc:: fs/super.c
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:export:
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File Locks
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----------
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.. kernel-doc:: fs/locks.c
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:export:
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.. kernel-doc:: fs/locks.c
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:internal:
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Other Functions
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---------------
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.. kernel-doc:: fs/mpage.c
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:export:
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.. kernel-doc:: fs/namei.c
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:export:
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.. kernel-doc:: fs/buffer.c
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:export:
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.. kernel-doc:: block/bio.c
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:export:
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.. kernel-doc:: fs/seq_file.c
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:export:
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.. kernel-doc:: fs/filesystems.c
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:export:
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.. kernel-doc:: fs/fs-writeback.c
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:export:
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.. kernel-doc:: fs/block_dev.c
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:export:
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.. kernel-doc:: fs/anon_inodes.c
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:export:
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.. kernel-doc:: fs/attr.c
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:export:
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.. kernel-doc:: fs/d_path.c
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:export:
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.. kernel-doc:: fs/dax.c
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:export:
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.. kernel-doc:: fs/direct-io.c
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:export:
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.. kernel-doc:: fs/file_table.c
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:export:
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.. kernel-doc:: fs/libfs.c
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:export:
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.. kernel-doc:: fs/posix_acl.c
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:export:
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.. kernel-doc:: fs/stat.c
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:export:
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.. kernel-doc:: fs/sync.c
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:export:
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.. kernel-doc:: fs/xattr.c
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:export:
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The proc filesystem
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===================
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sysctl interface
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----------------
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.. kernel-doc:: kernel/sysctl.c
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:export:
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proc filesystem interface
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-------------------------
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.. kernel-doc:: fs/proc/base.c
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:internal:
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Events based on file descriptors
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================================
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.. kernel-doc:: fs/eventfd.c
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:export:
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The Filesystem for Exporting Kernel Objects
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===========================================
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.. kernel-doc:: fs/sysfs/file.c
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:export:
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.. kernel-doc:: fs/sysfs/symlink.c
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:export:
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The debugfs filesystem
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======================
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======================
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debugfs interface
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See these manuals for documentation about the VFS layer itself and how its
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-----------------
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algorithms work.
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.. kernel-doc:: fs/debugfs/inode.c
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:export:
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.. kernel-doc:: fs/debugfs/file.c
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:export:
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The Linux Journalling API
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=========================
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Overview
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--------
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Details
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~~~~~~~
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The journalling layer is easy to use. You need to first of all create a
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journal_t data structure. There are two calls to do this dependent on
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how you decide to allocate the physical media on which the journal
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resides. The :c:func:`jbd2_journal_init_inode` call is for journals stored in
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filesystem inodes, or the :c:func:`jbd2_journal_init_dev` call can be used
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for journal stored on a raw device (in a continuous range of blocks). A
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journal_t is a typedef for a struct pointer, so when you are finally
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finished make sure you call :c:func:`jbd2_journal_destroy` on it to free up
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any used kernel memory.
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Once you have got your journal_t object you need to 'mount' or load the
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journal file. The journalling layer expects the space for the journal
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was already allocated and initialized properly by the userspace tools.
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When loading the journal you must call :c:func:`jbd2_journal_load` to process
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journal contents. If the client file system detects the journal contents
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does not need to be processed (or even need not have valid contents), it
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may call :c:func:`jbd2_journal_wipe` to clear the journal contents before
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calling :c:func:`jbd2_journal_load`.
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Note that jbd2_journal_wipe(..,0) calls
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:c:func:`jbd2_journal_skip_recovery` for you if it detects any outstanding
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transactions in the journal and similarly :c:func:`jbd2_journal_load` will
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call :c:func:`jbd2_journal_recover` if necessary. I would advise reading
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:c:func:`ext4_load_journal` in fs/ext4/super.c for examples on this stage.
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Now you can go ahead and start modifying the underlying filesystem.
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Almost.
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You still need to actually journal your filesystem changes, this is done
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by wrapping them into transactions. Additionally you also need to wrap
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the modification of each of the buffers with calls to the journal layer,
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so it knows what the modifications you are actually making are. To do
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this use :c:func:`jbd2_journal_start` which returns a transaction handle.
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:c:func:`jbd2_journal_start` and its counterpart :c:func:`jbd2_journal_stop`,
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which indicates the end of a transaction are nestable calls, so you can
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reenter a transaction if necessary, but remember you must call
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:c:func:`jbd2_journal_stop` the same number of times as
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:c:func:`jbd2_journal_start` before the transaction is completed (or more
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accurately leaves the update phase). Ext4/VFS makes use of this feature to
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simplify handling of inode dirtying, quota support, etc.
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Inside each transaction you need to wrap the modifications to the
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individual buffers (blocks). Before you start to modify a buffer you
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need to call :c:func:`jbd2_journal_get_create_access()` /
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:c:func:`jbd2_journal_get_write_access()` /
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:c:func:`jbd2_journal_get_undo_access()` as appropriate, this allows the
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journalling layer to copy the unmodified
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data if it needs to. After all the buffer may be part of a previously
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uncommitted transaction. At this point you are at last ready to modify a
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buffer, and once you are have done so you need to call
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:c:func:`jbd2_journal_dirty_metadata`. Or if you've asked for access to a
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buffer you now know is now longer required to be pushed back on the
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device you can call :c:func:`jbd2_journal_forget` in much the same way as you
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might have used :c:func:`bforget` in the past.
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A :c:func:`jbd2_journal_flush` may be called at any time to commit and
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checkpoint all your transactions.
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Then at umount time , in your :c:func:`put_super` you can then call
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:c:func:`jbd2_journal_destroy` to clean up your in-core journal object.
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Unfortunately there a couple of ways the journal layer can cause a
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deadlock. The first thing to note is that each task can only have a
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single outstanding transaction at any one time, remember nothing commits
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until the outermost :c:func:`jbd2_journal_stop`. This means you must complete
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the transaction at the end of each file/inode/address etc. operation you
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perform, so that the journalling system isn't re-entered on another
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journal. Since transactions can't be nested/batched across differing
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journals, and another filesystem other than yours (say ext4) may be
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modified in a later syscall.
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The second case to bear in mind is that :c:func:`jbd2_journal_start` can block
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if there isn't enough space in the journal for your transaction (based
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on the passed nblocks param) - when it blocks it merely(!) needs to wait
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for transactions to complete and be committed from other tasks, so
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essentially we are waiting for :c:func:`jbd2_journal_stop`. So to avoid
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deadlocks you must treat :c:func:`jbd2_journal_start` /
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:c:func:`jbd2_journal_stop` as if they were semaphores and include them in
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your semaphore ordering rules to prevent
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deadlocks. Note that :c:func:`jbd2_journal_extend` has similar blocking
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behaviour to :c:func:`jbd2_journal_start` so you can deadlock here just as
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easily as on :c:func:`jbd2_journal_start`.
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Try to reserve the right number of blocks the first time. ;-). This will
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be the maximum number of blocks you are going to touch in this
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transaction. I advise having a look at at least ext4_jbd.h to see the
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basis on which ext4 uses to make these decisions.
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Another wriggle to watch out for is your on-disk block allocation
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strategy. Why? Because, if you do a delete, you need to ensure you
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haven't reused any of the freed blocks until the transaction freeing
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these blocks commits. If you reused these blocks and crash happens,
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there is no way to restore the contents of the reallocated blocks at the
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end of the last fully committed transaction. One simple way of doing
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this is to mark blocks as free in internal in-memory block allocation
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structures only after the transaction freeing them commits. Ext4 uses
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journal commit callback for this purpose.
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With journal commit callbacks you can ask the journalling layer to call
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a callback function when the transaction is finally committed to disk,
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so that you can do some of your own management. You ask the journalling
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layer for calling the callback by simply setting
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|
||||||
``journal->j_commit_callback`` function pointer and that function is
|
|
||||||
called after each transaction commit. You can also use
|
|
||||||
``transaction->t_private_list`` for attaching entries to a transaction
|
|
||||||
that need processing when the transaction commits.
|
|
||||||
|
|
||||||
JBD2 also provides a way to block all transaction updates via
|
|
||||||
:c:func:`jbd2_journal_lock_updates()` /
|
|
||||||
:c:func:`jbd2_journal_unlock_updates()`. Ext4 uses this when it wants a
|
|
||||||
window with a clean and stable fs for a moment. E.g.
|
|
||||||
|
|
||||||
::
|
|
||||||
|
|
||||||
|
|
||||||
jbd2_journal_lock_updates() //stop new stuff happening..
|
|
||||||
jbd2_journal_flush() // checkpoint everything.
|
|
||||||
..do stuff on stable fs
|
|
||||||
jbd2_journal_unlock_updates() // carry on with filesystem use.
|
|
||||||
|
|
||||||
The opportunities for abuse and DOS attacks with this should be obvious,
|
|
||||||
if you allow unprivileged userspace to trigger codepaths containing
|
|
||||||
these calls.
|
|
||||||
|
|
||||||
Summary
|
|
||||||
~~~~~~~
|
|
||||||
|
|
||||||
Using the journal is a matter of wrapping the different context changes,
|
|
||||||
being each mount, each modification (transaction) and each changed
|
|
||||||
buffer to tell the journalling layer about them.
|
|
||||||
|
|
||||||
Data Types
|
|
||||||
----------
|
|
||||||
|
|
||||||
The journalling layer uses typedefs to 'hide' the concrete definitions
|
|
||||||
of the structures used. As a client of the JBD2 layer you can just rely
|
|
||||||
on the using the pointer as a magic cookie of some sort. Obviously the
|
|
||||||
hiding is not enforced as this is 'C'.
|
|
||||||
|
|
||||||
Structures
|
|
||||||
~~~~~~~~~~
|
|
||||||
|
|
||||||
.. kernel-doc:: include/linux/jbd2.h
|
|
||||||
:internal:
|
|
||||||
|
|
||||||
Functions
|
|
||||||
---------
|
|
||||||
|
|
||||||
The functions here are split into two groups those that affect a journal
|
|
||||||
as a whole, and those which are used to manage transactions
|
|
||||||
|
|
||||||
Journal Level
|
|
||||||
~~~~~~~~~~~~~
|
|
||||||
|
|
||||||
.. kernel-doc:: fs/jbd2/journal.c
|
|
||||||
:export:
|
|
||||||
|
|
||||||
.. kernel-doc:: fs/jbd2/recovery.c
|
|
||||||
:internal:
|
|
||||||
|
|
||||||
Transasction Level
|
|
||||||
~~~~~~~~~~~~~~~~~~
|
|
||||||
|
|
||||||
.. kernel-doc:: fs/jbd2/transaction.c
|
|
||||||
|
|
||||||
See also
|
|
||||||
--------
|
|
||||||
|
|
||||||
`Journaling the Linux ext2fs Filesystem, LinuxExpo 98, Stephen
|
|
||||||
Tweedie <http://kernel.org/pub/linux/kernel/people/sct/ext3/journal-design.ps.gz>`__
|
|
||||||
|
|
||||||
`Ext3 Journalling FileSystem, OLS 2000, Dr. Stephen
|
|
||||||
Tweedie <http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html>`__
|
|
||||||
|
|
||||||
splice API
|
|
||||||
==========
|
|
||||||
|
|
||||||
splice is a method for moving blocks of data around inside the kernel,
|
|
||||||
without continually transferring them between the kernel and user space.
|
|
||||||
|
|
||||||
.. kernel-doc:: fs/splice.c
|
|
||||||
|
|
||||||
pipes API
|
|
||||||
=========
|
|
||||||
|
|
||||||
Pipe interfaces are all for in-kernel (builtin image) use. They are not
|
|
||||||
exported for use by modules.
|
|
||||||
|
|
||||||
.. kernel-doc:: include/linux/pipe_fs_i.h
|
|
||||||
:internal:
|
|
||||||
|
|
||||||
.. kernel-doc:: fs/pipe.c
|
|
||||||
|
|
||||||
Encryption API
|
|
||||||
==============
|
|
||||||
|
|
||||||
A library which filesystems can hook into to support transparent
|
|
||||||
encryption of files and directories.
|
|
||||||
|
|
||||||
.. toctree::
|
|
||||||
:maxdepth: 2
|
|
||||||
|
|
||||||
fscrypt
|
|
||||||
|
|
||||||
Pathname lookup
|
|
||||||
===============
|
|
||||||
|
|
||||||
|
|
||||||
This write-up is based on three articles published at lwn.net:
|
|
||||||
|
|
||||||
- <https://lwn.net/Articles/649115/> Pathname lookup in Linux
|
|
||||||
- <https://lwn.net/Articles/649729/> RCU-walk: faster pathname lookup in Linux
|
|
||||||
- <https://lwn.net/Articles/650786/> A walk among the symlinks
|
|
||||||
|
|
||||||
Written by Neil Brown with help from Al Viro and Jon Corbet.
|
|
||||||
It has subsequently been updated to reflect changes in the kernel
|
|
||||||
including:
|
|
||||||
|
|
||||||
- per-directory parallel name lookup.
|
|
||||||
|
|
||||||
.. toctree::
|
.. toctree::
|
||||||
:maxdepth: 2
|
:maxdepth: 2
|
||||||
|
|
||||||
path-lookup.rst
|
path-lookup.rst
|
||||||
|
api-summary
|
||||||
|
splice
|
||||||
|
|
||||||
binderfs
|
Filesystem support layers
|
||||||
========
|
=========================
|
||||||
|
|
||||||
|
Documentation for the support code within the filesystem layer for use in
|
||||||
|
filesystem implementations.
|
||||||
|
|
||||||
.. toctree::
|
.. toctree::
|
||||||
|
:maxdepth: 2
|
||||||
|
|
||||||
|
journalling
|
||||||
|
fscrypt
|
||||||
|
|
||||||
|
Filesystem-specific documentation
|
||||||
|
=================================
|
||||||
|
|
||||||
|
Documentation for individual filesystem types can be found here.
|
||||||
|
|
||||||
|
.. toctree::
|
||||||
|
:maxdepth: 2
|
||||||
|
|
||||||
binderfs.rst
|
binderfs.rst
|
||||||
|
|
|
||||||
184
Documentation/filesystems/journalling.rst
Normal file
184
Documentation/filesystems/journalling.rst
Normal file
|
|
@ -0,0 +1,184 @@
|
||||||
|
The Linux Journalling API
|
||||||
|
=========================
|
||||||
|
|
||||||
|
Overview
|
||||||
|
--------
|
||||||
|
|
||||||
|
Details
|
||||||
|
~~~~~~~
|
||||||
|
|
||||||
|
The journalling layer is easy to use. You need to first of all create a
|
||||||
|
journal_t data structure. There are two calls to do this dependent on
|
||||||
|
how you decide to allocate the physical media on which the journal
|
||||||
|
resides. The :c:func:`jbd2_journal_init_inode` call is for journals stored in
|
||||||
|
filesystem inodes, or the :c:func:`jbd2_journal_init_dev` call can be used
|
||||||
|
for journal stored on a raw device (in a continuous range of blocks). A
|
||||||
|
journal_t is a typedef for a struct pointer, so when you are finally
|
||||||
|
finished make sure you call :c:func:`jbd2_journal_destroy` on it to free up
|
||||||
|
any used kernel memory.
|
||||||
|
|
||||||
|
Once you have got your journal_t object you need to 'mount' or load the
|
||||||
|
journal file. The journalling layer expects the space for the journal
|
||||||
|
was already allocated and initialized properly by the userspace tools.
|
||||||
|
When loading the journal you must call :c:func:`jbd2_journal_load` to process
|
||||||
|
journal contents. If the client file system detects the journal contents
|
||||||
|
does not need to be processed (or even need not have valid contents), it
|
||||||
|
may call :c:func:`jbd2_journal_wipe` to clear the journal contents before
|
||||||
|
calling :c:func:`jbd2_journal_load`.
|
||||||
|
|
||||||
|
Note that jbd2_journal_wipe(..,0) calls
|
||||||
|
:c:func:`jbd2_journal_skip_recovery` for you if it detects any outstanding
|
||||||
|
transactions in the journal and similarly :c:func:`jbd2_journal_load` will
|
||||||
|
call :c:func:`jbd2_journal_recover` if necessary. I would advise reading
|
||||||
|
:c:func:`ext4_load_journal` in fs/ext4/super.c for examples on this stage.
|
||||||
|
|
||||||
|
Now you can go ahead and start modifying the underlying filesystem.
|
||||||
|
Almost.
|
||||||
|
|
||||||
|
You still need to actually journal your filesystem changes, this is done
|
||||||
|
by wrapping them into transactions. Additionally you also need to wrap
|
||||||
|
the modification of each of the buffers with calls to the journal layer,
|
||||||
|
so it knows what the modifications you are actually making are. To do
|
||||||
|
this use :c:func:`jbd2_journal_start` which returns a transaction handle.
|
||||||
|
|
||||||
|
:c:func:`jbd2_journal_start` and its counterpart :c:func:`jbd2_journal_stop`,
|
||||||
|
which indicates the end of a transaction are nestable calls, so you can
|
||||||
|
reenter a transaction if necessary, but remember you must call
|
||||||
|
:c:func:`jbd2_journal_stop` the same number of times as
|
||||||
|
:c:func:`jbd2_journal_start` before the transaction is completed (or more
|
||||||
|
accurately leaves the update phase). Ext4/VFS makes use of this feature to
|
||||||
|
simplify handling of inode dirtying, quota support, etc.
|
||||||
|
|
||||||
|
Inside each transaction you need to wrap the modifications to the
|
||||||
|
individual buffers (blocks). Before you start to modify a buffer you
|
||||||
|
need to call :c:func:`jbd2_journal_get_create_access()` /
|
||||||
|
:c:func:`jbd2_journal_get_write_access()` /
|
||||||
|
:c:func:`jbd2_journal_get_undo_access()` as appropriate, this allows the
|
||||||
|
journalling layer to copy the unmodified
|
||||||
|
data if it needs to. After all the buffer may be part of a previously
|
||||||
|
uncommitted transaction. At this point you are at last ready to modify a
|
||||||
|
buffer, and once you are have done so you need to call
|
||||||
|
:c:func:`jbd2_journal_dirty_metadata`. Or if you've asked for access to a
|
||||||
|
buffer you now know is now longer required to be pushed back on the
|
||||||
|
device you can call :c:func:`jbd2_journal_forget` in much the same way as you
|
||||||
|
might have used :c:func:`bforget` in the past.
|
||||||
|
|
||||||
|
A :c:func:`jbd2_journal_flush` may be called at any time to commit and
|
||||||
|
checkpoint all your transactions.
|
||||||
|
|
||||||
|
Then at umount time , in your :c:func:`put_super` you can then call
|
||||||
|
:c:func:`jbd2_journal_destroy` to clean up your in-core journal object.
|
||||||
|
|
||||||
|
Unfortunately there a couple of ways the journal layer can cause a
|
||||||
|
deadlock. The first thing to note is that each task can only have a
|
||||||
|
single outstanding transaction at any one time, remember nothing commits
|
||||||
|
until the outermost :c:func:`jbd2_journal_stop`. This means you must complete
|
||||||
|
the transaction at the end of each file/inode/address etc. operation you
|
||||||
|
perform, so that the journalling system isn't re-entered on another
|
||||||
|
journal. Since transactions can't be nested/batched across differing
|
||||||
|
journals, and another filesystem other than yours (say ext4) may be
|
||||||
|
modified in a later syscall.
|
||||||
|
|
||||||
|
The second case to bear in mind is that :c:func:`jbd2_journal_start` can block
|
||||||
|
if there isn't enough space in the journal for your transaction (based
|
||||||
|
on the passed nblocks param) - when it blocks it merely(!) needs to wait
|
||||||
|
for transactions to complete and be committed from other tasks, so
|
||||||
|
essentially we are waiting for :c:func:`jbd2_journal_stop`. So to avoid
|
||||||
|
deadlocks you must treat :c:func:`jbd2_journal_start` /
|
||||||
|
:c:func:`jbd2_journal_stop` as if they were semaphores and include them in
|
||||||
|
your semaphore ordering rules to prevent
|
||||||
|
deadlocks. Note that :c:func:`jbd2_journal_extend` has similar blocking
|
||||||
|
behaviour to :c:func:`jbd2_journal_start` so you can deadlock here just as
|
||||||
|
easily as on :c:func:`jbd2_journal_start`.
|
||||||
|
|
||||||
|
Try to reserve the right number of blocks the first time. ;-). This will
|
||||||
|
be the maximum number of blocks you are going to touch in this
|
||||||
|
transaction. I advise having a look at at least ext4_jbd.h to see the
|
||||||
|
basis on which ext4 uses to make these decisions.
|
||||||
|
|
||||||
|
Another wriggle to watch out for is your on-disk block allocation
|
||||||
|
strategy. Why? Because, if you do a delete, you need to ensure you
|
||||||
|
haven't reused any of the freed blocks until the transaction freeing
|
||||||
|
these blocks commits. If you reused these blocks and crash happens,
|
||||||
|
there is no way to restore the contents of the reallocated blocks at the
|
||||||
|
end of the last fully committed transaction. One simple way of doing
|
||||||
|
this is to mark blocks as free in internal in-memory block allocation
|
||||||
|
structures only after the transaction freeing them commits. Ext4 uses
|
||||||
|
journal commit callback for this purpose.
|
||||||
|
|
||||||
|
With journal commit callbacks you can ask the journalling layer to call
|
||||||
|
a callback function when the transaction is finally committed to disk,
|
||||||
|
so that you can do some of your own management. You ask the journalling
|
||||||
|
layer for calling the callback by simply setting
|
||||||
|
``journal->j_commit_callback`` function pointer and that function is
|
||||||
|
called after each transaction commit. You can also use
|
||||||
|
``transaction->t_private_list`` for attaching entries to a transaction
|
||||||
|
that need processing when the transaction commits.
|
||||||
|
|
||||||
|
JBD2 also provides a way to block all transaction updates via
|
||||||
|
:c:func:`jbd2_journal_lock_updates()` /
|
||||||
|
:c:func:`jbd2_journal_unlock_updates()`. Ext4 uses this when it wants a
|
||||||
|
window with a clean and stable fs for a moment. E.g.
|
||||||
|
|
||||||
|
::
|
||||||
|
|
||||||
|
|
||||||
|
jbd2_journal_lock_updates() //stop new stuff happening..
|
||||||
|
jbd2_journal_flush() // checkpoint everything.
|
||||||
|
..do stuff on stable fs
|
||||||
|
jbd2_journal_unlock_updates() // carry on with filesystem use.
|
||||||
|
|
||||||
|
The opportunities for abuse and DOS attacks with this should be obvious,
|
||||||
|
if you allow unprivileged userspace to trigger codepaths containing
|
||||||
|
these calls.
|
||||||
|
|
||||||
|
Summary
|
||||||
|
~~~~~~~
|
||||||
|
|
||||||
|
Using the journal is a matter of wrapping the different context changes,
|
||||||
|
being each mount, each modification (transaction) and each changed
|
||||||
|
buffer to tell the journalling layer about them.
|
||||||
|
|
||||||
|
Data Types
|
||||||
|
----------
|
||||||
|
|
||||||
|
The journalling layer uses typedefs to 'hide' the concrete definitions
|
||||||
|
of the structures used. As a client of the JBD2 layer you can just rely
|
||||||
|
on the using the pointer as a magic cookie of some sort. Obviously the
|
||||||
|
hiding is not enforced as this is 'C'.
|
||||||
|
|
||||||
|
Structures
|
||||||
|
~~~~~~~~~~
|
||||||
|
|
||||||
|
.. kernel-doc:: include/linux/jbd2.h
|
||||||
|
:internal:
|
||||||
|
|
||||||
|
Functions
|
||||||
|
---------
|
||||||
|
|
||||||
|
The functions here are split into two groups those that affect a journal
|
||||||
|
as a whole, and those which are used to manage transactions
|
||||||
|
|
||||||
|
Journal Level
|
||||||
|
~~~~~~~~~~~~~
|
||||||
|
|
||||||
|
.. kernel-doc:: fs/jbd2/journal.c
|
||||||
|
:export:
|
||||||
|
|
||||||
|
.. kernel-doc:: fs/jbd2/recovery.c
|
||||||
|
:internal:
|
||||||
|
|
||||||
|
Transasction Level
|
||||||
|
~~~~~~~~~~~~~~~~~~
|
||||||
|
|
||||||
|
.. kernel-doc:: fs/jbd2/transaction.c
|
||||||
|
|
||||||
|
See also
|
||||||
|
--------
|
||||||
|
|
||||||
|
`Journaling the Linux ext2fs Filesystem, LinuxExpo 98, Stephen
|
||||||
|
Tweedie <http://kernel.org/pub/linux/kernel/people/sct/ext3/journal-design.ps.gz>`__
|
||||||
|
|
||||||
|
`Ext3 Journalling FileSystem, OLS 2000, Dr. Stephen
|
||||||
|
Tweedie <http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html>`__
|
||||||
|
|
||||||
|
|
@ -1,3 +1,18 @@
|
||||||
|
===============
|
||||||
|
Pathname lookup
|
||||||
|
===============
|
||||||
|
|
||||||
|
This write-up is based on three articles published at lwn.net:
|
||||||
|
|
||||||
|
- <https://lwn.net/Articles/649115/> Pathname lookup in Linux
|
||||||
|
- <https://lwn.net/Articles/649729/> RCU-walk: faster pathname lookup in Linux
|
||||||
|
- <https://lwn.net/Articles/650786/> A walk among the symlinks
|
||||||
|
|
||||||
|
Written by Neil Brown with help from Al Viro and Jon Corbet.
|
||||||
|
It has subsequently been updated to reflect changes in the kernel
|
||||||
|
including:
|
||||||
|
|
||||||
|
- per-directory parallel name lookup.
|
||||||
|
|
||||||
Introduction to pathname lookup
|
Introduction to pathname lookup
|
||||||
===============================
|
===============================
|
||||||
|
|
|
||||||
22
Documentation/filesystems/splice.rst
Normal file
22
Documentation/filesystems/splice.rst
Normal file
|
|
@ -0,0 +1,22 @@
|
||||||
|
================
|
||||||
|
splice and pipes
|
||||||
|
================
|
||||||
|
|
||||||
|
splice API
|
||||||
|
==========
|
||||||
|
|
||||||
|
splice is a method for moving blocks of data around inside the kernel,
|
||||||
|
without continually transferring them between the kernel and user space.
|
||||||
|
|
||||||
|
.. kernel-doc:: fs/splice.c
|
||||||
|
|
||||||
|
pipes API
|
||||||
|
=========
|
||||||
|
|
||||||
|
Pipe interfaces are all for in-kernel (builtin image) use. They are not
|
||||||
|
exported for use by modules.
|
||||||
|
|
||||||
|
.. kernel-doc:: include/linux/pipe_fs_i.h
|
||||||
|
:internal:
|
||||||
|
|
||||||
|
.. kernel-doc:: fs/pipe.c
|
||||||
Loading…
Add table
Reference in a new issue