linux/Documentation/DocBook/filesystems.tmpl
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   1<?xml version="1.0" encoding="UTF-8"?>
   2<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
   3        "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
   4
   5<book id="Linux-filesystems-API">
   6 <bookinfo>
   7  <title>Linux Filesystems API</title>
   8
   9  <legalnotice>
  10   <para>
  11     This documentation is free software; you can redistribute
  12     it and/or modify it under the terms of the GNU General Public
  13     License as published by the Free Software Foundation; either
  14     version 2 of the License, or (at your option) any later
  15     version.
  16   </para>
  17
  18   <para>
  19     This program is distributed in the hope that it will be
  20     useful, but WITHOUT ANY WARRANTY; without even the implied
  21     warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  22     See the GNU General Public License for more details.
  23   </para>
  24
  25   <para>
  26     You should have received a copy of the GNU General Public
  27     License along with this program; if not, write to the Free
  28     Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
  29     MA 02111-1307 USA
  30   </para>
  31
  32   <para>
  33     For more details see the file COPYING in the source
  34     distribution of Linux.
  35   </para>
  36  </legalnotice>
  37 </bookinfo>
  38
  39<toc></toc>
  40
  41  <chapter id="vfs">
  42     <title>The Linux VFS</title>
  43     <sect1 id="the_filesystem_types"><title>The Filesystem types</title>
  44!Iinclude/linux/fs.h
  45     </sect1>
  46     <sect1 id="the_directory_cache"><title>The Directory Cache</title>
  47!Efs/dcache.c
  48!Iinclude/linux/dcache.h
  49     </sect1>
  50     <sect1 id="inode_handling"><title>Inode Handling</title>
  51!Efs/inode.c
  52!Efs/bad_inode.c
  53     </sect1>
  54     <sect1 id="registration_and_superblocks"><title>Registration and Superblocks</title>
  55!Efs/super.c
  56     </sect1>
  57     <sect1 id="file_locks"><title>File Locks</title>
  58!Efs/locks.c
  59!Ifs/locks.c
  60     </sect1>
  61     <sect1 id="other_functions"><title>Other Functions</title>
  62!Efs/mpage.c
  63!Efs/namei.c
  64!Efs/buffer.c
  65!Efs/bio.c
  66!Efs/seq_file.c
  67!Efs/filesystems.c
  68!Efs/fs-writeback.c
  69!Efs/block_dev.c
  70     </sect1>
  71  </chapter>
  72
  73  <chapter id="proc">
  74     <title>The proc filesystem</title>
  75
  76     <sect1 id="sysctl_interface"><title>sysctl interface</title>
  77!Ekernel/sysctl.c
  78     </sect1>
  79
  80     <sect1 id="proc_filesystem_interface"><title>proc filesystem interface</title>
  81!Ifs/proc/base.c
  82     </sect1>
  83  </chapter>
  84
  85  <chapter id="fs_events">
  86     <title>Events based on file descriptors</title>
  87!Efs/eventfd.c
  88  </chapter>
  89
  90  <chapter id="sysfs">
  91     <title>The Filesystem for Exporting Kernel Objects</title>
  92!Efs/sysfs/file.c
  93!Efs/sysfs/symlink.c
  94!Efs/sysfs/bin.c
  95  </chapter>
  96
  97  <chapter id="debugfs">
  98     <title>The debugfs filesystem</title>
  99
 100     <sect1 id="debugfs_interface"><title>debugfs interface</title>
 101!Efs/debugfs/inode.c
 102!Efs/debugfs/file.c
 103     </sect1>
 104  </chapter>
 105
 106  <chapter id="LinuxJDBAPI">
 107  <chapterinfo>
 108  <title>The Linux Journalling API</title>
 109
 110  <authorgroup>
 111  <author>
 112     <firstname>Roger</firstname>
 113     <surname>Gammans</surname>
 114     <affiliation>
 115     <address>
 116      <email>rgammans@computer-surgery.co.uk</email>
 117     </address>
 118    </affiliation>
 119     </author>
 120  </authorgroup>
 121
 122  <authorgroup>
 123   <author>
 124    <firstname>Stephen</firstname>
 125    <surname>Tweedie</surname>
 126    <affiliation>
 127     <address>
 128      <email>sct@redhat.com</email>
 129     </address>
 130    </affiliation>
 131   </author>
 132  </authorgroup>
 133
 134  <copyright>
 135   <year>2002</year>
 136   <holder>Roger Gammans</holder>
 137  </copyright>
 138  </chapterinfo>
 139
 140  <title>The Linux Journalling API</title>
 141
 142    <sect1 id="journaling_overview">
 143     <title>Overview</title>
 144    <sect2 id="journaling_details">
 145     <title>Details</title>
 146<para>
 147The journalling layer is  easy to use. You need to
 148first of all create a journal_t data structure. There are
 149two calls to do this dependent on how you decide to allocate the physical
 150media on which the journal resides. The journal_init_inode() call
 151is for journals stored in filesystem inodes, or the journal_init_dev()
 152call can be use for journal stored on a raw device (in a continuous range
 153of blocks). A journal_t is a typedef for a struct pointer, so when
 154you are finally finished make sure you call journal_destroy() on it
 155to free up any used kernel memory.
 156</para>
 157
 158<para>
 159Once you have got your journal_t object you need to 'mount' or load the journal
 160file, unless of course you haven't initialised it yet - in which case you
 161need to call journal_create().
 162</para>
 163
 164<para>
 165Most of the time however your journal file will already have been created, but
 166before you load it you must call journal_wipe() to empty the journal file.
 167Hang on, you say , what if the filesystem wasn't cleanly umount()'d . Well, it is the
 168job of the client file system to detect this and skip the call to journal_wipe().
 169</para>
 170
 171<para>
 172In either case the next call should be to journal_load() which prepares the
 173journal file for use. Note that journal_wipe(..,0) calls journal_skip_recovery()
 174for you if it detects any outstanding transactions in the journal and similarly
 175journal_load() will call journal_recover() if necessary.
 176I would advise reading fs/ext3/super.c for examples on this stage.
 177[RGG: Why is the journal_wipe() call necessary - doesn't this needlessly
 178complicate the API. Or isn't a good idea for the journal layer to hide
 179dirty mounts from the client fs]
 180</para>
 181
 182<para>
 183Now you can go ahead and start modifying the underlying
 184filesystem. Almost.
 185</para>
 186
 187<para>
 188
 189You still need to actually journal your filesystem changes, this
 190is done by wrapping them into transactions. Additionally you
 191also need to wrap the modification of each of the buffers
 192with calls to the journal layer, so it knows what the modifications
 193you are actually making are. To do this use  journal_start() which
 194returns a transaction handle.
 195</para>
 196
 197<para>
 198journal_start()
 199and its counterpart journal_stop(), which indicates the end of a transaction
 200are nestable calls, so you can reenter a transaction if necessary,
 201but remember you must call journal_stop() the same number of times as
 202journal_start() before the transaction is completed (or more accurately
 203leaves the update phase). Ext3/VFS makes use of this feature to simplify
 204quota support.
 205</para>
 206
 207<para>
 208Inside each transaction you need to wrap the modifications to the
 209individual buffers (blocks). Before you start to modify a buffer you
 210need to call journal_get_{create,write,undo}_access() as appropriate,
 211this allows the journalling layer to copy the unmodified data if it
 212needs to. After all the buffer may be part of a previously uncommitted
 213transaction.
 214At this point you are at last ready to modify a buffer, and once
 215you are have done so you need to call journal_dirty_{meta,}data().
 216Or if you've asked for access to a buffer you now know is now longer
 217required to be pushed back on the device you can call journal_forget()
 218in much the same way as you might have used bforget() in the past.
 219</para>
 220
 221<para>
 222A journal_flush() may be called at any time to commit and checkpoint
 223all your transactions.
 224</para>
 225
 226<para>
 227Then at umount time , in your put_super() you can then call journal_destroy()
 228to clean up your in-core journal object.
 229</para>
 230
 231<para>
 232Unfortunately there a couple of ways the journal layer can cause a deadlock.
 233The first thing to note is that each task can only have
 234a single outstanding transaction at any one time, remember nothing
 235commits until the outermost journal_stop(). This means
 236you must complete the transaction at the end of each file/inode/address
 237etc. operation you perform, so that the journalling system isn't re-entered
 238on another journal. Since transactions can't be nested/batched
 239across differing journals, and another filesystem other than
 240yours (say ext3) may be modified in a later syscall.
 241</para>
 242
 243<para>
 244The second case to bear in mind is that journal_start() can
 245block if there isn't enough space in the journal for your transaction
 246(based on the passed nblocks param) - when it blocks it merely(!) needs to
 247wait for transactions to complete and be committed from other tasks,
 248so essentially we are waiting for journal_stop(). So to avoid
 249deadlocks you must treat journal_start/stop() as if they
 250were semaphores and include them in your semaphore ordering rules to prevent
 251deadlocks. Note that journal_extend() has similar blocking behaviour to
 252journal_start() so you can deadlock here just as easily as on journal_start().
 253</para>
 254
 255<para>
 256Try to reserve the right number of blocks the first time. ;-). This will
 257be the maximum number of blocks you are going to touch in this transaction.
 258I advise having a look at at least ext3_jbd.h to see the basis on which
 259ext3 uses to make these decisions.
 260</para>
 261
 262<para>
 263Another wriggle to watch out for is your on-disk block allocation strategy.
 264why? Because, if you undo a delete, you need to ensure you haven't reused any
 265of the freed blocks in a later transaction. One simple way of doing this
 266is make sure any blocks you allocate only have checkpointed transactions
 267listed against them. Ext3 does this in ext3_test_allocatable().
 268</para>
 269
 270<para>
 271Lock is also providing through journal_{un,}lock_updates(),
 272ext3 uses this when it wants a window with a clean and stable fs for a moment.
 273eg.
 274</para>
 275
 276<programlisting>
 277
 278        journal_lock_updates() //stop new stuff happening..
 279        journal_flush()        // checkpoint everything.
 280        ..do stuff on stable fs
 281        journal_unlock_updates() // carry on with filesystem use.
 282</programlisting>
 283
 284<para>
 285The opportunities for abuse and DOS attacks with this should be obvious,
 286if you allow unprivileged userspace to trigger codepaths containing these
 287calls.
 288</para>
 289
 290<para>
 291A new feature of jbd since 2.5.25 is commit callbacks with the new
 292journal_callback_set() function you can now ask the journalling layer
 293to call you back when the transaction is finally committed to disk, so that
 294you can do some of your own management. The key to this is the journal_callback
 295struct, this maintains the internal callback information but you can
 296extend it like this:-
 297</para>
 298<programlisting>
 299        struct  myfs_callback_s {
 300                //Data structure element required by jbd..
 301                struct journal_callback for_jbd;
 302                // Stuff for myfs allocated together.
 303                myfs_inode*    i_commited;
 304
 305        }
 306</programlisting>
 307
 308<para>
 309this would be useful if you needed to know when data was committed to a
 310particular inode.
 311</para>
 312
 313    </sect2>
 314
 315    <sect2 id="jbd_summary">
 316     <title>Summary</title>
 317<para>
 318Using the journal is a matter of wrapping the different context changes,
 319being each mount, each modification (transaction) and each changed buffer
 320to tell the journalling layer about them.
 321</para>
 322
 323<para>
 324Here is a some pseudo code to give you an idea of how it works, as
 325an example.
 326</para>
 327
 328<programlisting>
 329  journal_t* my_jnrl = journal_create();
 330  journal_init_{dev,inode}(jnrl,...)
 331  if (clean) journal_wipe();
 332  journal_load();
 333
 334   foreach(transaction) { /*transactions must be
 335                            completed before
 336                            a syscall returns to
 337                            userspace*/
 338
 339          handle_t * xct=journal_start(my_jnrl);
 340          foreach(bh) {
 341                journal_get_{create,write,undo}_access(xact,bh);
 342                if ( myfs_modify(bh) ) { /* returns true
 343                                        if makes changes */
 344                           journal_dirty_{meta,}data(xact,bh);
 345                } else {
 346                           journal_forget(bh);
 347                }
 348          }
 349          journal_stop(xct);
 350   }
 351   journal_destroy(my_jrnl);
 352</programlisting>
 353    </sect2>
 354
 355    </sect1>
 356
 357    <sect1 id="data_types">
 358     <title>Data Types</title>
 359     <para>
 360        The journalling layer uses typedefs to 'hide' the concrete definitions
 361        of the structures used. As a client of the JBD layer you can
 362        just rely on the using the pointer as a magic cookie  of some sort.
 363
 364        Obviously the hiding is not enforced as this is 'C'.
 365     </para>
 366        <sect2 id="structures"><title>Structures</title>
 367!Iinclude/linux/jbd.h
 368        </sect2>
 369    </sect1>
 370
 371    <sect1 id="functions">
 372     <title>Functions</title>
 373     <para>
 374        The functions here are split into two groups those that
 375        affect a journal as a whole, and those which are used to
 376        manage transactions
 377     </para>
 378        <sect2 id="journal_level"><title>Journal Level</title>
 379!Efs/jbd/journal.c
 380!Ifs/jbd/recovery.c
 381        </sect2>
 382        <sect2 id="transaction_level"><title>Transasction Level</title>
 383!Efs/jbd/transaction.c
 384        </sect2>
 385    </sect1>
 386    <sect1 id="see_also">
 387     <title>See also</title>
 388        <para>
 389          <citation>
 390           <ulink url="http://kernel.org/pub/linux/kernel/people/sct/ext3/journal-design.ps.gz">
 391                Journaling the Linux ext2fs Filesystem, LinuxExpo 98, Stephen Tweedie
 392           </ulink>
 393          </citation>
 394        </para>
 395        <para>
 396           <citation>
 397           <ulink url="http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html">
 398                Ext3 Journalling FileSystem, OLS 2000, Dr. Stephen Tweedie
 399           </ulink>
 400           </citation>
 401        </para>
 402    </sect1>
 403
 404  </chapter>
 405
 406  <chapter id="splice">
 407      <title>splice API</title>
 408  <para>
 409        splice is a method for moving blocks of data around inside the
 410        kernel, without continually transferring them between the kernel
 411        and user space.
 412  </para>
 413!Ffs/splice.c
 414  </chapter>
 415
 416  <chapter id="pipes">
 417      <title>pipes API</title>
 418  <para>
 419        Pipe interfaces are all for in-kernel (builtin image) use.
 420        They are not exported for use by modules.
 421  </para>
 422!Iinclude/linux/pipe_fs_i.h
 423!Ffs/pipe.c
 424  </chapter>
 425
 426</book>
 427
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