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     Roger

     Gammans

      rgammans@computer-surgery.co.uk

    Stephen

    Tweedie

      sct@redhat.com

   2002

   Roger Gammans

     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

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     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.

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

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     For more details see the file COPYING in the source

     distribution of Linux.

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.

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().

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().

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]

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 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.

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 the update phase). Ext3/VFS makes use of this feature to simplify

quota support.

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.

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

all your transactions.

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.

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.

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().

Try to reserve the right number of blocks the first time. ;-).

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().

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.

        journal_lock_updates() //stop new stuff happening..

        journal_flush()        // checkpoint everything.

        ..do stuff on stable fs

        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.

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.

Here is a some pseudo code to give you an idea of how it works, as

an example.

  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);

        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'.

!Iinclude/linux/jbd.h

        The functions here are split into two groups those that

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

        manage transactions

!Efs/jbd/journal.c

!Efs/jbd/recovery.c

!Efs/jbd/transaction.c

                Journaling the Linux ext2fs Filesystem,LinuxExpo 98, Stephen Tweedie

                Ext3 Journalling FileSystem , OLS 2000, Dr. Stephen Tweedie