382 lines
12 KiB
XML
382 lines
12 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
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<book id="Linux-filesystems-API">
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<bookinfo>
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<title>Linux Filesystems API</title>
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<legalnotice>
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<para>
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This documentation is free software; you can redistribute
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it and/or modify it under the terms of the GNU General Public
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License as published by the Free Software Foundation; either
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version 2 of the License, or (at your option) any later
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version.
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</para>
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<para>
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This program is distributed in the hope that it will be
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useful, but WITHOUT ANY WARRANTY; without even the implied
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warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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See the GNU General Public License for more details.
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</para>
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<para>
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You should have received a copy of the GNU General Public
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License along with this program; if not, write to the Free
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Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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MA 02111-1307 USA
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</para>
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<para>
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For more details see the file COPYING in the source
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distribution of Linux.
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</para>
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</legalnotice>
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</bookinfo>
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<toc></toc>
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<chapter id="vfs">
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<title>The Linux VFS</title>
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<sect1 id="the_filesystem_types"><title>The Filesystem types</title>
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!Iinclude/linux/fs.h
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</sect1>
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<sect1 id="the_directory_cache"><title>The Directory Cache</title>
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!Efs/dcache.c
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!Iinclude/linux/dcache.h
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</sect1>
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<sect1 id="inode_handling"><title>Inode Handling</title>
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!Efs/inode.c
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!Efs/bad_inode.c
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</sect1>
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<sect1 id="registration_and_superblocks"><title>Registration and Superblocks</title>
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!Efs/super.c
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</sect1>
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<sect1 id="file_locks"><title>File Locks</title>
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!Efs/locks.c
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!Ifs/locks.c
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</sect1>
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<sect1 id="other_functions"><title>Other Functions</title>
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!Efs/mpage.c
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!Efs/namei.c
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!Efs/buffer.c
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!Eblock/bio.c
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!Efs/seq_file.c
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!Efs/filesystems.c
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!Efs/fs-writeback.c
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!Efs/block_dev.c
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</sect1>
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</chapter>
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<chapter id="proc">
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<title>The proc filesystem</title>
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<sect1 id="sysctl_interface"><title>sysctl interface</title>
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!Ekernel/sysctl.c
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</sect1>
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<sect1 id="proc_filesystem_interface"><title>proc filesystem interface</title>
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!Ifs/proc/base.c
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</sect1>
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</chapter>
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<chapter id="fs_events">
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<title>Events based on file descriptors</title>
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!Efs/eventfd.c
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</chapter>
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<chapter id="sysfs">
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<title>The Filesystem for Exporting Kernel Objects</title>
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!Efs/sysfs/file.c
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!Efs/sysfs/symlink.c
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</chapter>
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<chapter id="debugfs">
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<title>The debugfs filesystem</title>
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<sect1 id="debugfs_interface"><title>debugfs interface</title>
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!Efs/debugfs/inode.c
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!Efs/debugfs/file.c
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</sect1>
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</chapter>
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<chapter id="LinuxJDBAPI">
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<chapterinfo>
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<title>The Linux Journalling API</title>
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<authorgroup>
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<author>
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<firstname>Roger</firstname>
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<surname>Gammans</surname>
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<affiliation>
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<address>
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<email>rgammans@computer-surgery.co.uk</email>
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</address>
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</affiliation>
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</author>
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</authorgroup>
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<authorgroup>
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<author>
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<firstname>Stephen</firstname>
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<surname>Tweedie</surname>
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<affiliation>
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<address>
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<email>sct@redhat.com</email>
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</address>
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</affiliation>
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</author>
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</authorgroup>
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<copyright>
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<year>2002</year>
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<holder>Roger Gammans</holder>
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</copyright>
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</chapterinfo>
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<title>The Linux Journalling API</title>
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<sect1 id="journaling_overview">
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<title>Overview</title>
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<sect2 id="journaling_details">
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<title>Details</title>
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<para>
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The journalling layer is easy to use. You need to
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first of all create a journal_t data structure. There are
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two calls to do this dependent on how you decide to allocate the physical
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media on which the journal resides. The jbd2_journal_init_inode() call
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is for journals stored in filesystem inodes, or the jbd2_journal_init_dev()
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call can be used for journal stored on a raw device (in a continuous range
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of blocks). A journal_t is a typedef for a struct pointer, so when
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you are finally finished make sure you call jbd2_journal_destroy() on it
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to free up any used kernel memory.
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</para>
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<para>
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Once you have got your journal_t object you need to 'mount' or load the journal
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file. The journalling layer expects the space for the journal was already
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allocated and initialized properly by the userspace tools. When loading the
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journal you must call jbd2_journal_load() to process journal contents. If the
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client file system detects the journal contents does not need to be processed
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(or even need not have valid contents), it may call jbd2_journal_wipe() to
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clear the journal contents before calling jbd2_journal_load().
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</para>
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<para>
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Note that jbd2_journal_wipe(..,0) calls jbd2_journal_skip_recovery() for you if
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it detects any outstanding transactions in the journal and similarly
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jbd2_journal_load() will call jbd2_journal_recover() if necessary. I would
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advise reading ext4_load_journal() in fs/ext4/super.c for examples on this
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stage.
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</para>
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<para>
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Now you can go ahead and start modifying the underlying
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filesystem. Almost.
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</para>
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<para>
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You still need to actually journal your filesystem changes, this
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is done by wrapping them into transactions. Additionally you
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also need to wrap the modification of each of the buffers
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with calls to the journal layer, so it knows what the modifications
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you are actually making are. To do this use jbd2_journal_start() which
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returns a transaction handle.
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</para>
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<para>
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jbd2_journal_start()
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and its counterpart jbd2_journal_stop(), which indicates the end of a
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transaction are nestable calls, so you can reenter a transaction if necessary,
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but remember you must call jbd2_journal_stop() the same number of times as
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jbd2_journal_start() before the transaction is completed (or more accurately
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leaves the update phase). Ext4/VFS makes use of this feature to simplify
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handling of inode dirtying, quota support, etc.
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</para>
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<para>
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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 jbd2_journal_get_{create,write,undo}_access() as appropriate,
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this allows the journalling layer to copy the unmodified data if it
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needs to. After all the buffer may be part of a previously uncommitted
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transaction.
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At this point you are at last ready to modify a buffer, and once
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you are have done so you need to call jbd2_journal_dirty_{meta,}data().
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Or if you've asked for access to a buffer you now know is now longer
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required to be pushed back on the device you can call jbd2_journal_forget()
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in much the same way as you might have used bforget() in the past.
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</para>
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<para>
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A jbd2_journal_flush() may be called at any time to commit and checkpoint
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all your transactions.
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</para>
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<para>
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Then at umount time , in your put_super() you can then call jbd2_journal_destroy()
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to clean up your in-core journal object.
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</para>
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<para>
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Unfortunately there a couple of ways the journal layer can cause a deadlock.
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The first thing to note is that each task can only have
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a single outstanding transaction at any one time, remember nothing
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commits until the outermost jbd2_journal_stop(). This means
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you must complete the transaction at the end of each file/inode/address
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etc. operation you perform, so that the journalling system isn't re-entered
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on another journal. Since transactions can't be nested/batched
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across differing journals, and another filesystem other than
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yours (say ext4) may be modified in a later syscall.
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</para>
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<para>
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The second case to bear in mind is that jbd2_journal_start() can
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block if there isn't enough space in the journal for your transaction
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(based on the passed nblocks param) - when it blocks it merely(!) needs to
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wait for transactions to complete and be committed from other tasks,
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so essentially we are waiting for jbd2_journal_stop(). So to avoid
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deadlocks you must treat jbd2_journal_start/stop() as if they
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were semaphores and include them in your semaphore ordering rules to prevent
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deadlocks. Note that jbd2_journal_extend() has similar blocking behaviour to
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jbd2_journal_start() so you can deadlock here just as easily as on
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jbd2_journal_start().
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</para>
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<para>
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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 transaction.
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I advise having a look at at least ext4_jbd.h to see the basis on which
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ext4 uses to make these decisions.
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</para>
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<para>
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Another wriggle to watch out for is your on-disk block allocation strategy.
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Why? Because, if you do a delete, you need to ensure you haven't reused any
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of the freed blocks until the transaction freeing these blocks commits. If you
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reused these blocks and crash happens, there is no way to restore the contents
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of the reallocated blocks at the end of the last fully committed transaction.
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One simple way of doing this is to mark blocks as free in internal in-memory
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block allocation structures only after the transaction freeing them commits.
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Ext4 uses journal commit callback for this purpose.
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</para>
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<para>
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With journal commit callbacks you can ask the journalling layer to call a
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callback function when the transaction is finally committed to disk, so that
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you can do some of your own management. You ask the journalling layer for
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calling the callback by simply setting journal->j_commit_callback function
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pointer and that function is called after each transaction commit. You can also
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use transaction->t_private_list for attaching entries to a transaction that
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need processing when the transaction commits.
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</para>
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<para>
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JBD2 also provides a way to block all transaction updates via
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jbd2_journal_{un,}lock_updates(). Ext4 uses this when it wants a window with a
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clean and stable fs for a moment. E.g.
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</para>
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<programlisting>
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jbd2_journal_lock_updates() //stop new stuff happening..
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jbd2_journal_flush() // checkpoint everything.
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..do stuff on stable fs
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jbd2_journal_unlock_updates() // carry on with filesystem use.
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</programlisting>
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<para>
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The opportunities for abuse and DOS attacks with this should be obvious,
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if you allow unprivileged userspace to trigger codepaths containing these
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calls.
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</para>
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</sect2>
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<sect2 id="jbd_summary">
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<title>Summary</title>
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<para>
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Using the journal is a matter of wrapping the different context changes,
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being each mount, each modification (transaction) and each changed buffer
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to tell the journalling layer about them.
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</para>
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</sect2>
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</sect1>
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<sect1 id="data_types">
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<title>Data Types</title>
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<para>
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The journalling layer uses typedefs to 'hide' the concrete definitions
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of the structures used. As a client of the JBD2 layer you can
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just rely on the using the pointer as a magic cookie of some sort.
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Obviously the hiding is not enforced as this is 'C'.
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</para>
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<sect2 id="structures"><title>Structures</title>
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!Iinclude/linux/jbd2.h
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</sect2>
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</sect1>
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<sect1 id="functions">
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<title>Functions</title>
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<para>
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The functions here are split into two groups those that
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affect a journal as a whole, and those which are used to
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manage transactions
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</para>
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<sect2 id="journal_level"><title>Journal Level</title>
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!Efs/jbd2/journal.c
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!Ifs/jbd2/recovery.c
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</sect2>
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<sect2 id="transaction_level"><title>Transasction Level</title>
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!Efs/jbd2/transaction.c
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</sect2>
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</sect1>
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<sect1 id="see_also">
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<title>See also</title>
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<para>
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<citation>
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<ulink url="http://kernel.org/pub/linux/kernel/people/sct/ext3/journal-design.ps.gz">
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Journaling the Linux ext2fs Filesystem, LinuxExpo 98, Stephen Tweedie
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</ulink>
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</citation>
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</para>
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<para>
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<citation>
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<ulink url="http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html">
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Ext3 Journalling FileSystem, OLS 2000, Dr. Stephen Tweedie
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</ulink>
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</citation>
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</para>
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</sect1>
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</chapter>
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<chapter id="splice">
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<title>splice API</title>
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<para>
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splice is a method for moving blocks of data around inside the
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kernel, without continually transferring them between the kernel
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and user space.
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</para>
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!Ffs/splice.c
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</chapter>
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<chapter id="pipes">
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<title>pipes API</title>
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<para>
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Pipe interfaces are all for in-kernel (builtin image) use.
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They are not exported for use by modules.
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</para>
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!Iinclude/linux/pipe_fs_i.h
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!Ffs/pipe.c
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</chapter>
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</book>
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