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doc: Update doc about journalling layer 

Documentation of journalling layer in
Documentation/DocBook/filesystems.tmpl speaks about JBD layer. Since
that is going away, update the documentation to speak about JBD2. Also
update the parts that have changed since someone last touched the
document and remove some parts which are just misleading and outdated.

Signed-off-by: Jan Kara <jack@suse.com>
This commit is contained in:
Jan Kara 2015-07-14 14:55:05 +02:00
parent acc84b05b1
commit 82ff50b222
1 changed files with 67 additions and 111 deletions
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178
Documentation/DocBook/filesystems.tmpl
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@ -146,36 +146,30 @@
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
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 journal_destroy() on it
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, unless of course you haven't initialised it yet - in which case you
need to call journal_create().
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>
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]
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>
@ -189,41 +183,41 @@ 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
you are actually making are. To do this use jbd2_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.
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 journal_get_{create,write,undo}_access() as appropriate,
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 journal_dirty_{meta,}data().
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 journal_forget()
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 journal_flush() may be called at any time to commit and checkpoint
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 journal_destroy()
Then at umount time , in your put_super() you can then call jbd2_journal_destroy()
to clean up your in-core journal object.
</para>
@ -231,82 +225,74 @@ 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
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 ext3) may be modified in a later syscall.
yours (say ext4) may be modified in a later syscall.
</para>
<para>
The second case to bear in mind is that journal_start() can
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 journal_stop(). So to avoid
deadlocks you must treat journal_start/stop() as if they
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 journal_extend() has similar blocking behaviour to
journal_start() so you can deadlock here just as easily as on journal_start().
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 ext3_jbd.h to see the basis on which
ext3 uses to make these decisions.
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 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().
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>
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.
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>
journal_lock_updates() //stop new stuff happening..
journal_flush() // checkpoint everything.
jbd2_journal_lock_updates() //stop new stuff happening..
jbd2_journal_flush() // checkpoint everything.
..do stuff on stable fs
journal_unlock_updates() // carry on with filesystem use.
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>
<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>
</sect2>
@ -319,36 +305,6 @@ 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>
</sect2>
</sect1>
@ -357,13 +313,13 @@ an example.
<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
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/jbd.h
!Iinclude/linux/jbd2.h
</sect2>
</sect1>
@ -375,11 +331,11 @@ an example.
manage transactions
</para>
<sect2 id="journal_level"><title>Journal Level</title>
!Efs/jbd/journal.c
!Ifs/jbd/recovery.c
!Efs/jbd2/journal.c
!Ifs/jbd2/recovery.c
</sect2>
<sect2 id="transaction_level"><title>Transasction Level</title>
!Efs/jbd/transaction.c
!Efs/jbd2/transaction.c
</sect2>
</sect1>
<sect1 id="see_also">