docs-rst: convert filesystems book to ReST

DOCBOOKS := z8530book.xml  \

	    networking.xml \

	    filesystems.xml lsm.xml \

	    lsm.xml \

	    libata.xml mtdnand.xml librs.xml rapidio.xml \

	    s390-drivers.xml scsi.xml \

	    sh.xml w1.xml

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"

	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>

<book id="Linux-filesystems-API">

 <bookinfo>

  <title>Linux Filesystems API</title>

  <legalnotice>

   <para>

     This documentation is free software; you can redistribute

     it and/or modify it under the terms of the GNU General Public

     License as published by the Free Software Foundation; either

     version 2 of the License, or (at your option) any later

     version.

   </para>

   <para>

     This program is distributed in the hope that it will be

     useful, but WITHOUT ANY WARRANTY; without even the implied

     warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

     See the GNU General Public License for more details.

   </para>

   <para>

     You should have received a copy of the GNU General Public

     License along with this program; if not, write to the Free

     Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,

     MA 02111-1307 USA

   </para>

   <para>

     For more details see the file COPYING in the source

     distribution of Linux.

   </para>

  </legalnotice>

 </bookinfo>

<toc></toc>

  <chapter id="vfs">

     <title>The Linux VFS</title>

     <sect1 id="the_filesystem_types"><title>The Filesystem types</title>

!Iinclude/linux/fs.h

     </sect1>

     <sect1 id="the_directory_cache"><title>The Directory Cache</title>

!Efs/dcache.c

!Iinclude/linux/dcache.h

     </sect1>

     <sect1 id="inode_handling"><title>Inode Handling</title>

!Efs/inode.c

!Efs/bad_inode.c

     </sect1>

     <sect1 id="registration_and_superblocks"><title>Registration and Superblocks</title>

!Efs/super.c

     </sect1>

     <sect1 id="file_locks"><title>File Locks</title>

!Efs/locks.c

!Ifs/locks.c

     </sect1>

     <sect1 id="other_functions"><title>Other Functions</title>

!Efs/mpage.c

!Efs/namei.c

!Efs/buffer.c

!Eblock/bio.c

!Efs/seq_file.c

!Efs/filesystems.c

!Efs/fs-writeback.c

!Efs/block_dev.c

     </sect1>

  </chapter>

  <chapter id="proc">

     <title>The proc filesystem</title>

     <sect1 id="sysctl_interface"><title>sysctl interface</title>

!Ekernel/sysctl.c

     </sect1>

     <sect1 id="proc_filesystem_interface"><title>proc filesystem interface</title>

!Ifs/proc/base.c

     </sect1>

  </chapter>

  <chapter id="fs_events">

     <title>Events based on file descriptors</title>

!Efs/eventfd.c

  </chapter>

  <chapter id="sysfs">

     <title>The Filesystem for Exporting Kernel Objects</title>

!Efs/sysfs/file.c

!Efs/sysfs/symlink.c

  </chapter>

  <chapter id="debugfs">

     <title>The debugfs filesystem</title>

     <sect1 id="debugfs_interface"><title>debugfs interface</title>

!Efs/debugfs/inode.c

!Efs/debugfs/file.c

     </sect1>

  </chapter>

  <chapter id="LinuxJDBAPI">

  <chapterinfo>

  <title>The Linux Journalling API</title>

  <authorgroup>

  <author>

     <firstname>Roger</firstname>

     <surname>Gammans</surname>

     <affiliation>

     <address>

      <email>rgammans@computer-surgery.co.uk</email>

     </address>

    </affiliation>

     </author>

  </authorgroup>

  <authorgroup>

   <author>

    <firstname>Stephen</firstname>

    <surname>Tweedie</surname>

    <affiliation>

     <address>

      <email>sct@redhat.com</email>

     </address>

    </affiliation>

   </author>

  </authorgroup>

  <copyright>

   <year>2002</year>

   <holder>Roger Gammans</holder>

  </copyright>

  </chapterinfo>

  <title>The Linux Journalling API</title>

    <sect1 id="journaling_overview">

     <title>Overview</title>

    <sect2 id="journaling_details">

     <title>Details</title>

<para>

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 jbd2_journal_init_inode() call

is for journals stored in filesystem inodes, or the 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 jbd2_journal_destroy() on it

to free up any used kernel memory.

</para>

<para>

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 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 jbd2_journal_wipe() to

clear the journal contents before calling jbd2_journal_load().

</para>

<para>

Note that jbd2_journal_wipe(..,0) calls jbd2_journal_skip_recovery() for you if

it detects any outstanding transactions in the journal and similarly

jbd2_journal_load() will call jbd2_journal_recover() if necessary.  I would

advise reading ext4_load_journal() in fs/ext4/super.c for examples on this

stage.

</para>

<para>

Now you can go ahead and start modifying the underlying

filesystem. Almost.

</para>

<para>

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 jbd2_journal_start() which

returns a transaction handle.

</para>

<para>

jbd2_journal_start()

and its counterpart 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 jbd2_journal_stop() the same number of times as

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.

</para>

<para>

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 jbd2_journal_get_{create,write,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 jbd2_journal_dirty_{meta,}data().

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 jbd2_journal_forget()

in much the same way as you might have used bforget() in the past.

</para>

<para>

A jbd2_journal_flush() may be called at any time to commit and checkpoint

all your transactions.

</para>

<para>

Then at umount time , in your put_super() you can then call jbd2_journal_destroy()

to clean up your in-core journal object.

</para>

<para>

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 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.

</para>

<para>

The second case to bear in mind is that 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 jbd2_journal_stop(). So to avoid

deadlocks you must treat jbd2_journal_start/stop() as if they

were semaphores and include them in your semaphore ordering rules to prevent

deadlocks. Note that jbd2_journal_extend() has similar blocking behaviour to

jbd2_journal_start() so you can deadlock here just as easily as on

jbd2_journal_start().

</para>

<para>

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.

</para>

<para>

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.

</para>

<para>

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.

</para>

<para>

JBD2 also provides a way to block all transaction updates via

jbd2_journal_{un,}lock_updates(). Ext4 uses this when it wants a window with a

clean and stable fs for a moment.  E.g.

</para>

<programlisting>

	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.

</programlisting>

<para>

The opportunities for abuse and DOS attacks with this should be obvious,

if you allow unprivileged userspace to trigger codepaths containing these

calls.

</para>

    </sect2>

    <sect2 id="jbd_summary">

     <title>Summary</title>

<para>

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.

</para>

    </sect2>

    </sect1>

    <sect1 id="data_types">

     <title>Data Types</title>

     <para>

	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'.

     </para>

	<sect2 id="structures"><title>Structures</title>

!Iinclude/linux/jbd2.h

	</sect2>

    </sect1>

    <sect1 id="functions">

     <title>Functions</title>

     <para>

	The functions here are split into two groups those that

	affect a journal as a whole, and those which are used to

	manage transactions

     </para>

	<sect2 id="journal_level"><title>Journal Level</title>

!Efs/jbd2/journal.c

!Ifs/jbd2/recovery.c

	</sect2>

	<sect2 id="transaction_level"><title>Transasction Level</title>

!Efs/jbd2/transaction.c

	</sect2>

    </sect1>

    <sect1 id="see_also">

     <title>See also</title>

	<para>

	  <citation>

	   <ulink url="http://kernel.org/pub/linux/kernel/people/sct/ext3/journal-design.ps.gz">

	   	Journaling the Linux ext2fs Filesystem, LinuxExpo 98, Stephen Tweedie

	   </ulink>

	  </citation>

	</para>

	<para>

	   <citation>

	   <ulink url="http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html">

	   	Ext3 Journalling FileSystem, OLS 2000, Dr. Stephen Tweedie

	   </ulink>

	   </citation>

	</para>

    </sect1>

  </chapter>

  <chapter id="splice">

      <title>splice API</title>

  <para>

	splice is a method for moving blocks of data around inside the

	kernel, without continually transferring them between the kernel

	and user space.

  </para>

!Ffs/splice.c

  </chapter>

  <chapter id="pipes">

      <title>pipes API</title>

  <para>

	Pipe interfaces are all for in-kernel (builtin image) use.

	They are not exported for use by modules.

  </para>

!Iinclude/linux/pipe_fs_i.h

!Ffs/pipe.c

  </chapter>

</book>

     'The kernel development community', 'manual'),

    ('driver-api/index', 'driver-api.tex', 'The kernel driver API manual',

     'The kernel development community', 'manual'),

    ('filesystems/index', 'filesystems.tex', 'Linux Filesystems API',

     'The kernel development community', 'manual'),

    ('gpu/index', 'gpu.tex', 'Linux GPU Driver Developer\'s Guide',

     'The kernel development community', 'manual'),

    ('input/index', 'linux-input.tex', 'The Linux input driver subsystem',

# -*- coding: utf-8; mode: python -*-

project = "Linux Filesystems API"

tags.add("subproject")

latex_documents = [

    ('index', 'filesystems.tex', project,

     'The kernel development community', 'manual'),

]

=====================

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:

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 jbd2_journal_init_inode() call is for journals stored in

filesystem inodes, or the 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 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 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 jbd2_journal_wipe() to clear the journal contents before

calling jbd2_journal_load().

Note that jbd2_journal_wipe(..,0) calls

jbd2_journal_skip_recovery() for you if it detects any outstanding

transactions in the journal and similarly jbd2_journal_load() will

call jbd2_journal_recover() if necessary. I would advise reading

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 jbd2_journal_start() which returns a transaction handle.

jbd2_journal_start() and its counterpart 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

jbd2_journal_stop() the same number of times as 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 jbd2_journal_get_{create,write,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

jbd2_journal_dirty_{meta,}data(). 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 jbd2_journal_forget() in much the same way as you

might have used bforget() in the past.

A jbd2_journal_flush() may be called at any time to commit and

checkpoint all your transactions.

Then at umount time , in your put_super() you can then call

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 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 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 jbd2_journal_stop(). So to avoid

deadlocks you must treat jbd2_journal_start/stop() as if they were

semaphores and include them in your semaphore ordering rules to prevent

deadlocks. Note that jbd2_journal_extend() has similar blocking

behaviour to jbd2_journal_start() so you can deadlock here just as

easily as on 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

jbd2_journal_{un,}lock_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

   :export:

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