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diff --git a/Documentation/DocBook/journal-api.tmpl b/Documentation/DocBook/journal-api.tmpl deleted file mode 100644 index 2077f9a..0000000 --- a/Documentation/DocBook/journal-api.tmpl +++ /dev/null @@ -1,333 +0,0 @@ -<?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="LinuxJBDAPI"> - <bookinfo> - <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> - -<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="Overview"> - <title>Overview</title> - <sect1> - <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 journal_init_inode() call -is for journals stored in filesystem inodes, or the journal_init_dev() -call can be use 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 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, unless of course you haven't initialised it yet - in which case you -need to call journal_create(). -</para> - -<para> -Most of the time however your journal file will already have been created, but -before you load it you must call journal_wipe() to empty the journal file. -Hang on, you say , what if the filesystem wasn't cleanly umount()'d . Well, it is the -job of the client file system to detect this and skip the call to journal_wipe(). -</para> - -<para> -In either case the next call should be to journal_load() which prepares the -journal file for use. Note that journal_wipe(..,0) calls journal_skip_recovery() -for you if it detects any outstanding transactions in the journal and similarly -journal_load() will call journal_recover() if necessary. -I would advise reading fs/ext3/super.c for examples on this stage. -[RGG: Why is the journal_wipe() call necessary - doesn't this needlessly -complicate the API. Or isn't a good idea for the journal layer to hide -dirty mounts from the client fs] -</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 journal_start() which -returns a transaction handle. -</para> - -<para> -journal_start() -and its counterpart 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 journal_stop() the same number of times as -journal_start() before the transaction is completed (or more accurately -leaves the update phase). Ext3/VFS makes use of this feature to simplify -quota support. -</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 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 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 journal_forget() -in much the same way as you might have used bforget() in the past. -</para> - -<para> -A 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() (2.4) or write_super() (2.5) -you can then call 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 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 ext3) may be modified in a later syscall. -</para> - -<para> -The second case to bear in mind is that 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 journal_stop(). So to avoid -deadlocks you must treat journal_start/stop() as if they -were semaphores and include them in your semaphore ordering rules to prevent -deadlocks. Note that journal_extend() has similar blocking behaviour to -journal_start() so you can deadlock here just as easily as on 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 ext3_jbd.h to see the basis on which -ext3 uses to make these decisions. -</para> - -<para> -Another wriggle to watch out for is your on-disk block allocation strategy. -why? Because, if you undo a delete, you need to ensure you haven't reused any -of the freed blocks in a later transaction. One simple way of doing this -is make sure any blocks you allocate only have checkpointed transactions -listed against them. Ext3 does this in ext3_test_allocatable(). -</para> - -<para> -Lock is also providing through journal_{un,}lock_updates(), -ext3 uses this when it wants a window with a clean and stable fs for a moment. -eg. -</para> - -<programlisting> - - journal_lock_updates() //stop new stuff happening.. - journal_flush() // checkpoint everything. - ..do stuff on stable fs - 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> - -<para> -A new feature of jbd since 2.5.25 is commit callbacks with the new -journal_callback_set() function you can now ask the journalling layer -to call you back when the transaction is finally committed to disk, so that -you can do some of your own management. The key to this is the journal_callback -struct, this maintains the internal callback information but you can -extend it like this:- -</para> -<programlisting> - struct myfs_callback_s { - //Data structure element required by jbd.. - struct journal_callback for_jbd; - // Stuff for myfs allocated together. - myfs_inode* i_commited; - - } -</programlisting> - -<para> -this would be useful if you needed to know when data was committed to a -particular inode. -</para> - -</sect1> - -<sect1> -<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> - -<para> -Here is a some pseudo code to give you an idea of how it works, as -an example. -</para> - -<programlisting> - journal_t* my_jnrl = journal_create(); - journal_init_{dev,inode}(jnrl,...) - if (clean) journal_wipe(); - journal_load(); - - foreach(transaction) { /*transactions must be - completed before - a syscall returns to - userspace*/ - - handle_t * xct=journal_start(my_jnrl); - foreach(bh) { - journal_get_{create,write,undo}_access(xact,bh); - if ( myfs_modify(bh) ) { /* returns true - if makes changes */ - journal_dirty_{meta,}data(xact,bh); - } else { - journal_forget(bh); - } - } - journal_stop(xct); - } - journal_destroy(my_jrnl); -</programlisting> -</sect1> - -</chapter> - - <chapter id="adt"> - <title>Data Types</title> - <para> - The journalling layer uses typedefs to 'hide' the concrete definitions - of the structures used. As a client of the JBD 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> - <sect1><title>Structures</title> -!Iinclude/linux/jbd.h - </sect1> -</chapter> - - <chapter id="calls"> - <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> - <sect1><title>Journal Level</title> -!Efs/jbd/journal.c -!Ifs/jbd/recovery.c - </sect1> - <sect1><title>Transasction Level</title> -!Efs/jbd/transaction.c - </sect1> -</chapter> -<chapter> - <title>See also</title> - <para> - <citation> - <ulink url="ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/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> -</chapter> - -</book> |