mirror of
https://github.com/torvalds/linux.git
synced 2024-11-19 18:41:48 +00:00
b7d3f17fa7
People have gone to all the effort of writing kernel-doc for these functions; the least we can do is put them in the "Other functions" part of the VFS documentation. Signed-off-by: Matthew Wilcox <willy@infradead.org> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
362 lines
11 KiB
ReStructuredText
362 lines
11 KiB
ReStructuredText
=====================
|
|
Linux Filesystems API
|
|
=====================
|
|
|
|
The Linux VFS
|
|
=============
|
|
|
|
The Filesystem types
|
|
--------------------
|
|
|
|
.. kernel-doc:: include/linux/fs.h
|
|
:internal:
|
|
|
|
The Directory Cache
|
|
-------------------
|
|
|
|
.. kernel-doc:: fs/dcache.c
|
|
:export:
|
|
|
|
.. kernel-doc:: include/linux/dcache.h
|
|
:internal:
|
|
|
|
Inode Handling
|
|
--------------
|
|
|
|
.. kernel-doc:: fs/inode.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/bad_inode.c
|
|
:export:
|
|
|
|
Registration and Superblocks
|
|
----------------------------
|
|
|
|
.. kernel-doc:: fs/super.c
|
|
:export:
|
|
|
|
File Locks
|
|
----------
|
|
|
|
.. kernel-doc:: fs/locks.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/locks.c
|
|
:internal:
|
|
|
|
Other Functions
|
|
---------------
|
|
|
|
.. kernel-doc:: fs/mpage.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/namei.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/buffer.c
|
|
:export:
|
|
|
|
.. kernel-doc:: block/bio.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/seq_file.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/filesystems.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/fs-writeback.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/block_dev.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/anon_inodes.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/attr.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/d_path.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/dax.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/direct-io.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/file_table.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/libfs.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/posix_acl.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/stat.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/sync.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/xattr.c
|
|
:export:
|
|
|
|
The proc filesystem
|
|
===================
|
|
|
|
sysctl interface
|
|
----------------
|
|
|
|
.. kernel-doc:: kernel/sysctl.c
|
|
:export:
|
|
|
|
proc filesystem interface
|
|
-------------------------
|
|
|
|
.. kernel-doc:: fs/proc/base.c
|
|
:internal:
|
|
|
|
Events based on file descriptors
|
|
================================
|
|
|
|
.. kernel-doc:: fs/eventfd.c
|
|
:export:
|
|
|
|
The Filesystem for Exporting Kernel Objects
|
|
===========================================
|
|
|
|
.. kernel-doc:: fs/sysfs/file.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/sysfs/symlink.c
|
|
:export:
|
|
|
|
The debugfs filesystem
|
|
======================
|
|
|
|
debugfs interface
|
|
-----------------
|
|
|
|
.. kernel-doc:: fs/debugfs/inode.c
|
|
:export:
|
|
|
|
.. kernel-doc:: fs/debugfs/file.c
|
|
:export:
|
|
|
|
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>`__
|
|
|
|
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
|