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transaction.c

/*
 * linux/fs/transaction.c
 * 
 * Written by Stephen C. Tweedie <sct@redhat.com>, 1998
 *
 * Copyright 1998 Red Hat corp --- All Rights Reserved
 *
 * This file is part of the Linux kernel and is made available under
 * the terms of the GNU General Public License, version 2, or at your
 * option, any later version, incorporated herein by reference.
 *
 * Generic filesystem transaction handling code; part of the ext2fs
 * journaling system.  
 *
 * This file manages transactions (compound commits managed by the
 * journaling code) and handles (individual atomic operations by the
 * filesystem).
 */

#include <linux/time.h>
#include <linux/fs.h>
#include <linux/jbd.h>
#include <linux/errno.h>
#include <linux/slab.h>
#include <linux/timer.h>
#include <linux/smp_lock.h>
#include <linux/mm.h>
#include <linux/highmem.h>

/*
 * get_transaction: obtain a new transaction_t object.
 *
 * Simply allocate and initialise a new transaction.  Create it in
 * RUNNING state and add it to the current journal (which should not
 * have an existing running transaction: we only make a new transaction
 * once we have started to commit the old one).
 *
 * Preconditions:
 *    The journal MUST be locked.  We don't perform atomic mallocs on the
 *    new transaction   and we can't block without protecting against other
 *    processes trying to touch the journal while it is in transition.
 *
 * Called under j_state_lock
 */

static transaction_t *
get_transaction(journal_t *journal, transaction_t *transaction)
{
      transaction->t_journal = journal;
      transaction->t_state = T_RUNNING;
      transaction->t_tid = journal->j_transaction_sequence++;
      transaction->t_expires = jiffies + journal->j_commit_interval;
      spin_lock_init(&transaction->t_handle_lock);

      /* Set up the commit timer for the new transaction. */
      journal->j_commit_timer.expires = transaction->t_expires;
      add_timer(&journal->j_commit_timer);

      J_ASSERT(journal->j_running_transaction == NULL);
      journal->j_running_transaction = transaction;

      return transaction;
}

/*
 * Handle management.
 *
 * A handle_t is an object which represents a single atomic update to a
 * filesystem, and which tracks all of the modifications which form part
 * of that one update.
 */

/*
 * start_this_handle: Given a handle, deal with any locking or stalling
 * needed to make sure that there is enough journal space for the handle
 * to begin.  Attach the handle to a transaction and set up the
 * transaction's buffer credits.  
 */

static int start_this_handle(journal_t *journal, handle_t *handle)
{
      transaction_t *transaction;
      int needed;
      int nblocks = handle->h_buffer_credits;
      transaction_t *new_transaction = NULL;
      int ret = 0;

      if (nblocks > journal->j_max_transaction_buffers) {
            printk(KERN_ERR "JBD: %s wants too many credits (%d > %d)\n",
                   current->comm, nblocks,
                   journal->j_max_transaction_buffers);
            ret = -ENOSPC;
            goto out;
      }

alloc_transaction:
      if (!journal->j_running_transaction) {
            new_transaction = jbd_kmalloc(sizeof(*new_transaction),
                                    GFP_NOFS);
            if (!new_transaction) {
                  ret = -ENOMEM;
                  goto out;
            }
            memset(new_transaction, 0, sizeof(*new_transaction));
      }

      jbd_debug(3, "New handle %p going live.\n", handle);

repeat:

      /*
       * We need to hold j_state_lock until t_updates has been incremented,
       * for proper journal barrier handling
       */
      spin_lock(&journal->j_state_lock);
repeat_locked:
      if (is_journal_aborted(journal) ||
          (journal->j_errno != 0 && !(journal->j_flags & JFS_ACK_ERR))) {
            spin_unlock(&journal->j_state_lock);
            ret = -EROFS; 
            goto out;
      }

      /* Wait on the journal's transaction barrier if necessary */
      if (journal->j_barrier_count) {
            spin_unlock(&journal->j_state_lock);
            wait_event(journal->j_wait_transaction_locked,
                        journal->j_barrier_count == 0);
            goto repeat;
      }

      if (!journal->j_running_transaction) {
            if (!new_transaction) {
                  spin_unlock(&journal->j_state_lock);
                  goto alloc_transaction;
            }
            get_transaction(journal, new_transaction);
            new_transaction = NULL;
      }

      transaction = journal->j_running_transaction;

      /*
       * If the current transaction is locked down for commit, wait for the
       * lock to be released.
       */
      if (transaction->t_state == T_LOCKED) {
            DEFINE_WAIT(wait);

            prepare_to_wait(&journal->j_wait_transaction_locked,
                              &wait, TASK_UNINTERRUPTIBLE);
            spin_unlock(&journal->j_state_lock);
            schedule();
            finish_wait(&journal->j_wait_transaction_locked, &wait);
            goto repeat;
      }

      /*
       * If there is not enough space left in the log to write all potential
       * buffers requested by this operation, we need to stall pending a log
       * checkpoint to free some more log space.
       */
      spin_lock(&transaction->t_handle_lock);
      needed = transaction->t_outstanding_credits + nblocks;

      if (needed > journal->j_max_transaction_buffers) {
            /*
             * If the current transaction is already too large, then start
             * to commit it: we can then go back and attach this handle to
             * a new transaction.
             */
            DEFINE_WAIT(wait);

            jbd_debug(2, "Handle %p starting new commit...\n", handle);
            spin_unlock(&transaction->t_handle_lock);
            prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
                        TASK_UNINTERRUPTIBLE);
            __log_start_commit(journal, transaction->t_tid);
            spin_unlock(&journal->j_state_lock);
            schedule();
            finish_wait(&journal->j_wait_transaction_locked, &wait);
            goto repeat;
      }

      /* 
       * The commit code assumes that it can get enough log space
       * without forcing a checkpoint.  This is *critical* for
       * correctness: a checkpoint of a buffer which is also
       * associated with a committing transaction creates a deadlock,
       * so commit simply cannot force through checkpoints.
       *
       * We must therefore ensure the necessary space in the journal
       * *before* starting to dirty potentially checkpointed buffers
       * in the new transaction. 
       *
       * The worst part is, any transaction currently committing can
       * reduce the free space arbitrarily.  Be careful to account for
       * those buffers when checkpointing.
       */

      /*
       * @@@ AKPM: This seems rather over-defensive.  We're giving commit
       * a _lot_ of headroom: 1/4 of the journal plus the size of
       * the committing transaction.  Really, we only need to give it
       * committing_transaction->t_outstanding_credits plus "enough" for
       * the log control blocks.
       * Also, this test is inconsitent with the matching one in
       * journal_extend().
       */
      if (__log_space_left(journal) < jbd_space_needed(journal)) {
            jbd_debug(2, "Handle %p waiting for checkpoint...\n", handle);
            spin_unlock(&transaction->t_handle_lock);
            __log_wait_for_space(journal);
            goto repeat_locked;
      }

      /* OK, account for the buffers that this operation expects to
       * use and add the handle to the running transaction. */

      handle->h_transaction = transaction;
      transaction->t_outstanding_credits += nblocks;
      transaction->t_updates++;
      transaction->t_handle_count++;
      jbd_debug(4, "Handle %p given %d credits (total %d, free %d)\n",
              handle, nblocks, transaction->t_outstanding_credits,
              __log_space_left(journal));
      spin_unlock(&transaction->t_handle_lock);
      spin_unlock(&journal->j_state_lock);
out:
      if (unlikely(new_transaction))            /* It's usually NULL */
            kfree(new_transaction);
      return ret;
}

/* Allocate a new handle.  This should probably be in a slab... */
static handle_t *new_handle(int nblocks)
{
      handle_t *handle = jbd_alloc_handle(GFP_NOFS);
      if (!handle)
            return NULL;
      memset(handle, 0, sizeof(*handle));
      handle->h_buffer_credits = nblocks;
      handle->h_ref = 1;

      return handle;
}

/**
 * handle_t *journal_start() - Obtain a new handle.  
 * @journal: Journal to start transaction on.
 * @nblocks: number of block buffer we might modify
 *
 * We make sure that the transaction can guarantee at least nblocks of
 * modified buffers in the log.  We block until the log can guarantee
 * that much space.  
 *
 * This function is visible to journal users (like ext3fs), so is not
 * called with the journal already locked.
 *
 * Return a pointer to a newly allocated handle, or NULL on failure
 */
handle_t *journal_start(journal_t *journal, int nblocks)
{
      handle_t *handle = journal_current_handle();
      int err;

      if (!journal)
            return ERR_PTR(-EROFS);

      if (handle) {
            J_ASSERT(handle->h_transaction->t_journal == journal);
            handle->h_ref++;
            return handle;
      }

      handle = new_handle(nblocks);
      if (!handle)
            return ERR_PTR(-ENOMEM);

      current->journal_info = handle;

      err = start_this_handle(journal, handle);
      if (err < 0) {
            jbd_free_handle(handle);
            current->journal_info = NULL;
            handle = ERR_PTR(err);
      }
      return handle;
}

/**
 * int journal_extend() - extend buffer credits.
 * @handle:  handle to 'extend'
 * @nblocks: nr blocks to try to extend by.
 * 
 * Some transactions, such as large extends and truncates, can be done
 * atomically all at once or in several stages.  The operation requests
 * a credit for a number of buffer modications in advance, but can
 * extend its credit if it needs more.  
 *
 * journal_extend tries to give the running handle more buffer credits.
 * It does not guarantee that allocation - this is a best-effort only.
 * The calling process MUST be able to deal cleanly with a failure to
 * extend here.
 *
 * Return 0 on success, non-zero on failure.
 *
 * return code < 0 implies an error
 * return code > 0 implies normal transaction-full status.
 */
int journal_extend(handle_t *handle, int nblocks)
{
      transaction_t *transaction = handle->h_transaction;
      journal_t *journal = transaction->t_journal;
      int result;
      int wanted;

      result = -EIO;
      if (is_handle_aborted(handle))
            goto out;

      result = 1;

      spin_lock(&journal->j_state_lock);

      /* Don't extend a locked-down transaction! */
      if (handle->h_transaction->t_state != T_RUNNING) {
            jbd_debug(3, "denied handle %p %d blocks: "
                    "transaction not running\n", handle, nblocks);
            goto error_out;
      }

      spin_lock(&transaction->t_handle_lock);
      wanted = transaction->t_outstanding_credits + nblocks;

      if (wanted > journal->j_max_transaction_buffers) {
            jbd_debug(3, "denied handle %p %d blocks: "
                    "transaction too large\n", handle, nblocks);
            goto unlock;
      }

      if (wanted > __log_space_left(journal)) {
            jbd_debug(3, "denied handle %p %d blocks: "
                    "insufficient log space\n", handle, nblocks);
            goto unlock;
      }

      handle->h_buffer_credits += nblocks;
      transaction->t_outstanding_credits += nblocks;
      result = 0;

      jbd_debug(3, "extended handle %p by %d\n", handle, nblocks);
unlock:
      spin_unlock(&transaction->t_handle_lock);
error_out:
      spin_unlock(&journal->j_state_lock);
out:
      return result;
}


/**
 * int journal_restart() - restart a handle .
 * @handle:  handle to restart
 * @nblocks: nr credits requested
 * 
 * Restart a handle for a multi-transaction filesystem
 * operation.
 *
 * If the journal_extend() call above fails to grant new buffer credits
 * to a running handle, a call to journal_restart will commit the
 * handle's transaction so far and reattach the handle to a new
 * transaction capabable of guaranteeing the requested number of
 * credits.
 */

int journal_restart(handle_t *handle, int nblocks)
{
      transaction_t *transaction = handle->h_transaction;
      journal_t *journal = transaction->t_journal;
      int ret;

      /* If we've had an abort of any type, don't even think about
       * actually doing the restart! */
      if (is_handle_aborted(handle))
            return 0;

      /*
       * First unlink the handle from its current transaction, and start the
       * commit on that.
       */
      J_ASSERT(transaction->t_updates > 0);
      J_ASSERT(journal_current_handle() == handle);

      spin_lock(&journal->j_state_lock);
      spin_lock(&transaction->t_handle_lock);
      transaction->t_outstanding_credits -= handle->h_buffer_credits;
      transaction->t_updates--;

      if (!transaction->t_updates)
            wake_up(&journal->j_wait_updates);
      spin_unlock(&transaction->t_handle_lock);

      jbd_debug(2, "restarting handle %p\n", handle);
      __log_start_commit(journal, transaction->t_tid);
      spin_unlock(&journal->j_state_lock);

      handle->h_buffer_credits = nblocks;
      ret = start_this_handle(journal, handle);
      return ret;
}


/**
 * void journal_lock_updates () - establish a transaction barrier.
 * @journal:  Journal to establish a barrier on.
 *
 * This locks out any further updates from being started, and blocks
 * until all existing updates have completed, returning only once the
 * journal is in a quiescent state with no updates running.
 *
 * The journal lock should not be held on entry.
 */
void journal_lock_updates(journal_t *journal)
{
      DEFINE_WAIT(wait);

      spin_lock(&journal->j_state_lock);
      ++journal->j_barrier_count;

      /* Wait until there are no running updates */
      while (1) {
            transaction_t *transaction = journal->j_running_transaction;

            if (!transaction)
                  break;

            spin_lock(&transaction->t_handle_lock);
            if (!transaction->t_updates) {
                  spin_unlock(&transaction->t_handle_lock);
                  break;
            }
            prepare_to_wait(&journal->j_wait_updates, &wait,
                        TASK_UNINTERRUPTIBLE);
            spin_unlock(&transaction->t_handle_lock);
            spin_unlock(&journal->j_state_lock);
            schedule();
            finish_wait(&journal->j_wait_updates, &wait);
            spin_lock(&journal->j_state_lock);
      }
      spin_unlock(&journal->j_state_lock);

      /*
       * We have now established a barrier against other normal updates, but
       * we also need to barrier against other journal_lock_updates() calls
       * to make sure that we serialise special journal-locked operations
       * too.
       */
      mutex_lock(&journal->j_barrier);
}

/**
 * void journal_unlock_updates (journal_t* journal) - release barrier
 * @journal:  Journal to release the barrier on.
 * 
 * Release a transaction barrier obtained with journal_lock_updates().
 *
 * Should be called without the journal lock held.
 */
void journal_unlock_updates (journal_t *journal)
{
      J_ASSERT(journal->j_barrier_count != 0);

      mutex_unlock(&journal->j_barrier);
      spin_lock(&journal->j_state_lock);
      --journal->j_barrier_count;
      spin_unlock(&journal->j_state_lock);
      wake_up(&journal->j_wait_transaction_locked);
}

/*
 * Report any unexpected dirty buffers which turn up.  Normally those
 * indicate an error, but they can occur if the user is running (say)
 * tune2fs to modify the live filesystem, so we need the option of
 * continuing as gracefully as possible.  #
 *
 * The caller should already hold the journal lock and
 * j_list_lock spinlock: most callers will need those anyway
 * in order to probe the buffer's journaling state safely.
 */
static void jbd_unexpected_dirty_buffer(struct journal_head *jh)
{
      int jlist;

      /* If this buffer is one which might reasonably be dirty
       * --- ie. data, or not part of this journal --- then
       * we're OK to leave it alone, but otherwise we need to
       * move the dirty bit to the journal's own internal
       * JBDDirty bit. */
      jlist = jh->b_jlist;

      if (jlist == BJ_Metadata || jlist == BJ_Reserved ||
          jlist == BJ_Shadow || jlist == BJ_Forget) {
            struct buffer_head *bh = jh2bh(jh);

            if (test_clear_buffer_dirty(bh))
                  set_buffer_jbddirty(bh);
      }
}

/*
 * If the buffer is already part of the current transaction, then there
 * is nothing we need to do.  If it is already part of a prior
 * transaction which we are still committing to disk, then we need to
 * make sure that we do not overwrite the old copy: we do copy-out to
 * preserve the copy going to disk.  We also account the buffer against
 * the handle's metadata buffer credits (unless the buffer is already
 * part of the transaction, that is).
 *
 */
static int
do_get_write_access(handle_t *handle, struct journal_head *jh,
                  int force_copy)
{
      struct buffer_head *bh;
      transaction_t *transaction;
      journal_t *journal;
      int error;
      char *frozen_buffer = NULL;
      int need_copy = 0;

      if (is_handle_aborted(handle))
            return -EROFS;

      transaction = handle->h_transaction;
      journal = transaction->t_journal;

      jbd_debug(5, "buffer_head %p, force_copy %d\n", jh, force_copy);

      JBUFFER_TRACE(jh, "entry");
repeat:
      bh = jh2bh(jh);

      /* @@@ Need to check for errors here at some point. */

      lock_buffer(bh);
      jbd_lock_bh_state(bh);

      /* We now hold the buffer lock so it is safe to query the buffer
       * state.  Is the buffer dirty? 
       * 
       * If so, there are two possibilities.  The buffer may be
       * non-journaled, and undergoing a quite legitimate writeback.
       * Otherwise, it is journaled, and we don't expect dirty buffers
       * in that state (the buffers should be marked JBD_Dirty
       * instead.)  So either the IO is being done under our own
       * control and this is a bug, or it's a third party IO such as
       * dump(8) (which may leave the buffer scheduled for read ---
       * ie. locked but not dirty) or tune2fs (which may actually have
       * the buffer dirtied, ugh.)  */

      if (buffer_dirty(bh)) {
            /*
             * First question: is this buffer already part of the current
             * transaction or the existing committing transaction?
             */
            if (jh->b_transaction) {
                  J_ASSERT_JH(jh,
                        jh->b_transaction == transaction || 
                        jh->b_transaction ==
                              journal->j_committing_transaction);
                  if (jh->b_next_transaction)
                        J_ASSERT_JH(jh, jh->b_next_transaction ==
                                          transaction);
            }
            /*
             * In any case we need to clean the dirty flag and we must
             * do it under the buffer lock to be sure we don't race
             * with running write-out.
             */
            JBUFFER_TRACE(jh, "Unexpected dirty buffer");
            jbd_unexpected_dirty_buffer(jh);
      }

      unlock_buffer(bh);

      error = -EROFS;
      if (is_handle_aborted(handle)) {
            jbd_unlock_bh_state(bh);
            goto out;
      }
      error = 0;

      /*
       * The buffer is already part of this transaction if b_transaction or
       * b_next_transaction points to it
       */
      if (jh->b_transaction == transaction ||
          jh->b_next_transaction == transaction)
            goto done;

      /*
       * If there is already a copy-out version of this buffer, then we don't
       * need to make another one
       */
      if (jh->b_frozen_data) {
            JBUFFER_TRACE(jh, "has frozen data");
            J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
            jh->b_next_transaction = transaction;
            goto done;
      }

      /* Is there data here we need to preserve? */

      if (jh->b_transaction && jh->b_transaction != transaction) {
            JBUFFER_TRACE(jh, "owned by older transaction");
            J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
            J_ASSERT_JH(jh, jh->b_transaction ==
                              journal->j_committing_transaction);

            /* There is one case we have to be very careful about.
             * If the committing transaction is currently writing
             * this buffer out to disk and has NOT made a copy-out,
             * then we cannot modify the buffer contents at all
             * right now.  The essence of copy-out is that it is the
             * extra copy, not the primary copy, which gets
             * journaled.  If the primary copy is already going to
             * disk then we cannot do copy-out here. */

            if (jh->b_jlist == BJ_Shadow) {
                  DEFINE_WAIT_BIT(wait, &bh->b_state, BH_Unshadow);
                  wait_queue_head_t *wqh;

                  wqh = bit_waitqueue(&bh->b_state, BH_Unshadow);

                  JBUFFER_TRACE(jh, "on shadow: sleep");
                  jbd_unlock_bh_state(bh);
                  /* commit wakes up all shadow buffers after IO */
                  for ( ; ; ) {
                        prepare_to_wait(wqh, &wait.wait,
                                    TASK_UNINTERRUPTIBLE);
                        if (jh->b_jlist != BJ_Shadow)
                              break;
                        schedule();
                  }
                  finish_wait(wqh, &wait.wait);
                  goto repeat;
            }

            /* Only do the copy if the currently-owning transaction
             * still needs it.  If it is on the Forget list, the
             * committing transaction is past that stage.  The
             * buffer had better remain locked during the kmalloc,
             * but that should be true --- we hold the journal lock
             * still and the buffer is already on the BUF_JOURNAL
             * list so won't be flushed. 
             *
             * Subtle point, though: if this is a get_undo_access,
             * then we will be relying on the frozen_data to contain
             * the new value of the committed_data record after the
             * transaction, so we HAVE to force the frozen_data copy
             * in that case. */

            if (jh->b_jlist != BJ_Forget || force_copy) {
                  JBUFFER_TRACE(jh, "generate frozen data");
                  if (!frozen_buffer) {
                        JBUFFER_TRACE(jh, "allocate memory for buffer");
                        jbd_unlock_bh_state(bh);
                        frozen_buffer =
                              jbd_slab_alloc(jh2bh(jh)->b_size,
                                           GFP_NOFS);
                        if (!frozen_buffer) {
                              printk(KERN_EMERG
                                     "%s: OOM for frozen_buffer\n",
                                     __FUNCTION__);
                              JBUFFER_TRACE(jh, "oom!");
                              error = -ENOMEM;
                              jbd_lock_bh_state(bh);
                              goto done;
                        }
                        goto repeat;
                  }
                  jh->b_frozen_data = frozen_buffer;
                  frozen_buffer = NULL;
                  need_copy = 1;
            }
            jh->b_next_transaction = transaction;
      }


      /*
       * Finally, if the buffer is not journaled right now, we need to make
       * sure it doesn't get written to disk before the caller actually
       * commits the new data
       */
      if (!jh->b_transaction) {
            JBUFFER_TRACE(jh, "no transaction");
            J_ASSERT_JH(jh, !jh->b_next_transaction);
            jh->b_transaction = transaction;
            JBUFFER_TRACE(jh, "file as BJ_Reserved");
            spin_lock(&journal->j_list_lock);
            __journal_file_buffer(jh, transaction, BJ_Reserved);
            spin_unlock(&journal->j_list_lock);
      }

done:
      if (need_copy) {
            struct page *page;
            int offset;
            char *source;

            J_EXPECT_JH(jh, buffer_uptodate(jh2bh(jh)),
                      "Possible IO failure.\n");
            page = jh2bh(jh)->b_page;
            offset = ((unsigned long) jh2bh(jh)->b_data) & ~PAGE_MASK;
            source = kmap_atomic(page, KM_USER0);
            memcpy(jh->b_frozen_data, source+offset, jh2bh(jh)->b_size);
            kunmap_atomic(source, KM_USER0);
      }
      jbd_unlock_bh_state(bh);

      /*
       * If we are about to journal a buffer, then any revoke pending on it is
       * no longer valid
       */
      journal_cancel_revoke(handle, jh);

out:
      if (unlikely(frozen_buffer))  /* It's usually NULL */
            jbd_slab_free(frozen_buffer, bh->b_size);

      JBUFFER_TRACE(jh, "exit");
      return error;
}

/**
 * int journal_get_write_access() - notify intent to modify a buffer for metadata (not data) update.
 * @handle: transaction to add buffer modifications to
 * @bh:     bh to be used for metadata writes
 * @credits: variable that will receive credits for the buffer
 *
 * Returns an error code or 0 on success.
 *
 * In full data journalling mode the buffer may be of type BJ_AsyncData,
 * because we're write()ing a buffer which is also part of a shared mapping.
 */

int journal_get_write_access(handle_t *handle, struct buffer_head *bh)
{
      struct journal_head *jh = journal_add_journal_head(bh);
      int rc;

      /* We do not want to get caught playing with fields which the
       * log thread also manipulates.  Make sure that the buffer
       * completes any outstanding IO before proceeding. */
      rc = do_get_write_access(handle, jh, 0);
      journal_put_journal_head(jh);
      return rc;
}


/*
 * When the user wants to journal a newly created buffer_head
 * (ie. getblk() returned a new buffer and we are going to populate it
 * manually rather than reading off disk), then we need to keep the
 * buffer_head locked until it has been completely filled with new
 * data.  In this case, we should be able to make the assertion that
 * the bh is not already part of an existing transaction.  
 * 
 * The buffer should already be locked by the caller by this point.
 * There is no lock ranking violation: it was a newly created,
 * unlocked buffer beforehand. */

/**
 * int journal_get_create_access () - notify intent to use newly created bh
 * @handle: transaction to new buffer to
 * @bh: new buffer.
 *
 * Call this if you create a new bh.
 */
int journal_get_create_access(handle_t *handle, struct buffer_head *bh) 
{
      transaction_t *transaction = handle->h_transaction;
      journal_t *journal = transaction->t_journal;
      struct journal_head *jh = journal_add_journal_head(bh);
      int err;

      jbd_debug(5, "journal_head %p\n", jh);
      err = -EROFS;
      if (is_handle_aborted(handle))
            goto out;
      err = 0;

      JBUFFER_TRACE(jh, "entry");
      /*
       * The buffer may already belong to this transaction due to pre-zeroing
       * in the filesystem's new_block code.  It may also be on the previous,
       * committing transaction's lists, but it HAS to be in Forget state in
       * that case: the transaction must have deleted the buffer for it to be
       * reused here.
       */
      jbd_lock_bh_state(bh);
      spin_lock(&journal->j_list_lock);
      J_ASSERT_JH(jh, (jh->b_transaction == transaction ||
            jh->b_transaction == NULL ||
            (jh->b_transaction == journal->j_committing_transaction &&
                    jh->b_jlist == BJ_Forget)));

      J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
      J_ASSERT_JH(jh, buffer_locked(jh2bh(jh)));

      if (jh->b_transaction == NULL) {
            jh->b_transaction = transaction;
            JBUFFER_TRACE(jh, "file as BJ_Reserved");
            __journal_file_buffer(jh, transaction, BJ_Reserved);
      } else if (jh->b_transaction == journal->j_committing_transaction) {
            JBUFFER_TRACE(jh, "set next transaction");
            jh->b_next_transaction = transaction;
      }
      spin_unlock(&journal->j_list_lock);
      jbd_unlock_bh_state(bh);

      /*
       * akpm: I added this.  ext3_alloc_branch can pick up new indirect
       * blocks which contain freed but then revoked metadata.  We need
       * to cancel the revoke in case we end up freeing it yet again
       * and the reallocating as data - this would cause a second revoke,
       * which hits an assertion error.
       */
      JBUFFER_TRACE(jh, "cancelling revoke");
      journal_cancel_revoke(handle, jh);
      journal_put_journal_head(jh);
out:
      return err;
}

/**
 * int journal_get_undo_access() -  Notify intent to modify metadata with
 *     non-rewindable consequences
 * @handle: transaction
 * @bh: buffer to undo
 * @credits: store the number of taken credits here (if not NULL)
 *
 * Sometimes there is a need to distinguish between metadata which has
 * been committed to disk and that which has not.  The ext3fs code uses
 * this for freeing and allocating space, we have to make sure that we
 * do not reuse freed space until the deallocation has been committed,
 * since if we overwrote that space we would make the delete
 * un-rewindable in case of a crash.
 * 
 * To deal with that, journal_get_undo_access requests write access to a
 * buffer for parts of non-rewindable operations such as delete
 * operations on the bitmaps.  The journaling code must keep a copy of
 * the buffer's contents prior to the undo_access call until such time
 * as we know that the buffer has definitely been committed to disk.
 * 
 * We never need to know which transaction the committed data is part
 * of, buffers touched here are guaranteed to be dirtied later and so
 * will be committed to a new transaction in due course, at which point
 * we can discard the old committed data pointer.
 *
 * Returns error number or 0 on success.
 */
int journal_get_undo_access(handle_t *handle, struct buffer_head *bh)
{
      int err;
      struct journal_head *jh = journal_add_journal_head(bh);
      char *committed_data = NULL;

      JBUFFER_TRACE(jh, "entry");

      /*
       * Do this first --- it can drop the journal lock, so we want to
       * make sure that obtaining the committed_data is done
       * atomically wrt. completion of any outstanding commits.
       */
      err = do_get_write_access(handle, jh, 1);
      if (err)
            goto out;

repeat:
      if (!jh->b_committed_data) {
            committed_data = jbd_slab_alloc(jh2bh(jh)->b_size, GFP_NOFS);
            if (!committed_data) {
                  printk(KERN_EMERG "%s: No memory for committed data\n",
                        __FUNCTION__);
                  err = -ENOMEM;
                  goto out;
            }
      }

      jbd_lock_bh_state(bh);
      if (!jh->b_committed_data) {
            /* Copy out the current buffer contents into the
             * preserved, committed copy. */
            JBUFFER_TRACE(jh, "generate b_committed data");
            if (!committed_data) {
                  jbd_unlock_bh_state(bh);
                  goto repeat;
            }

            jh->b_committed_data = committed_data;
            committed_data = NULL;
            memcpy(jh->b_committed_data, bh->b_data, bh->b_size);
      }
      jbd_unlock_bh_state(bh);
out:
      journal_put_journal_head(jh);
      if (unlikely(committed_data))
            jbd_slab_free(committed_data, bh->b_size);
      return err;
}

/** 
 * int journal_dirty_data() -  mark a buffer as containing dirty data which
 *                             needs to be flushed before we can commit the
 *                             current transaction.  
 * @handle: transaction
 * @bh: bufferhead to mark
 * 
 * The buffer is placed on the transaction's data list and is marked as
 * belonging to the transaction.
 *
 * Returns error number or 0 on success.
 *
 * journal_dirty_data() can be called via page_launder->ext3_writepage
 * by kswapd.
 */
int journal_dirty_data(handle_t *handle, struct buffer_head *bh)
{
      journal_t *journal = handle->h_transaction->t_journal;
      int need_brelse = 0;
      struct journal_head *jh;

      if (is_handle_aborted(handle))
            return 0;

      jh = journal_add_journal_head(bh);
      JBUFFER_TRACE(jh, "entry");

      /*
       * The buffer could *already* be dirty.  Writeout can start
       * at any time.
       */
      jbd_debug(4, "jh: %p, tid:%d\n", jh, handle->h_transaction->t_tid);

      /*
       * What if the buffer is already part of a running transaction?
       * 
       * There are two cases:
       * 1) It is part of the current running transaction.  Refile it,
       *    just in case we have allocated it as metadata, deallocated
       *    it, then reallocated it as data. 
       * 2) It is part of the previous, still-committing transaction.
       *    If all we want to do is to guarantee that the buffer will be
       *    written to disk before this new transaction commits, then
       *    being sure that the *previous* transaction has this same 
       *    property is sufficient for us!  Just leave it on its old
       *    transaction.
       *
       * In case (2), the buffer must not already exist as metadata
       * --- that would violate write ordering (a transaction is free
       * to write its data at any point, even before the previous
       * committing transaction has committed).  The caller must
       * never, ever allow this to happen: there's nothing we can do
       * about it in this layer.
       */
      jbd_lock_bh_state(bh);
      spin_lock(&journal->j_list_lock);
      if (jh->b_transaction) {
            JBUFFER_TRACE(jh, "has transaction");
            if (jh->b_transaction != handle->h_transaction) {
                  JBUFFER_TRACE(jh, "belongs to older transaction");
                  J_ASSERT_JH(jh, jh->b_transaction ==
                              journal->j_committing_transaction);

                  /* @@@ IS THIS TRUE  ? */
                  /*
                   * Not any more.  Scenario: someone does a write()
                   * in data=journal mode.  The buffer's transaction has
                   * moved into commit.  Then someone does another
                   * write() to the file.  We do the frozen data copyout
                   * and set b_next_transaction to point to j_running_t.
                   * And while we're in that state, someone does a
                   * writepage() in an attempt to pageout the same area
                   * of the file via a shared mapping.  At present that
                   * calls journal_dirty_data(), and we get right here.
                   * It may be too late to journal the data.  Simply
                   * falling through to the next test will suffice: the
                   * data will be dirty and wil be checkpointed.  The
                   * ordering comments in the next comment block still
                   * apply.
                   */
                  //J_ASSERT_JH(jh, jh->b_next_transaction == NULL);

                  /*
                   * If we're journalling data, and this buffer was
                   * subject to a write(), it could be metadata, forget
                   * or shadow against the committing transaction.  Now,
                   * someone has dirtied the same darn page via a mapping
                   * and it is being writepage()'d.
                   * We *could* just steal the page from commit, with some
                   * fancy locking there.  Instead, we just skip it -
                   * don't tie the page's buffers to the new transaction
                   * at all.
                   * Implication: if we crash before the writepage() data
                   * is written into the filesystem, recovery will replay
                   * the write() data.
                   */
                  if (jh->b_jlist != BJ_None &&
                              jh->b_jlist != BJ_SyncData &&
                              jh->b_jlist != BJ_Locked) {
                        JBUFFER_TRACE(jh, "Not stealing");
                        goto no_journal;
                  }

                  /*
                   * This buffer may be undergoing writeout in commit.  We
                   * can't return from here and let the caller dirty it
                   * again because that can cause the write-out loop in
                   * commit to never terminate.
                   */
                  if (buffer_dirty(bh)) {
                        get_bh(bh);
                        spin_unlock(&journal->j_list_lock);
                        jbd_unlock_bh_state(bh);
                        need_brelse = 1;
                        sync_dirty_buffer(bh);
                        jbd_lock_bh_state(bh);
                        spin_lock(&journal->j_list_lock);
                        /* The buffer may become locked again at any
                           time if it is redirtied */
                  }

                  /* journal_clean_data_list() may have got there first */
                  if (jh->b_transaction != NULL) {
                        JBUFFER_TRACE(jh, "unfile from commit");
                        __journal_temp_unlink_buffer(jh);
                        /* It still points to the committing
                         * transaction; move it to this one so
                         * that the refile assert checks are
                         * happy. */
                        jh->b_transaction = handle->h_transaction;
                  }
                  /* The buffer will be refiled below */

            }
            /*
             * Special case --- the buffer might actually have been
             * allocated and then immediately deallocated in the previous,
             * committing transaction, so might still be left on that
             * transaction's metadata lists.
             */
            if (jh->b_jlist != BJ_SyncData && jh->b_jlist != BJ_Locked) {
                  JBUFFER_TRACE(jh, "not on correct data list: unfile");
                  J_ASSERT_JH(jh, jh->b_jlist != BJ_Shadow);
                  __journal_temp_unlink_buffer(jh);
                  jh->b_transaction = handle->h_transaction;
                  JBUFFER_TRACE(jh, "file as data");
                  __journal_file_buffer(jh, handle->h_transaction,
                                    BJ_SyncData);
            }
      } else {
            JBUFFER_TRACE(jh, "not on a transaction");
            __journal_file_buffer(jh, handle->h_transaction, BJ_SyncData);
      }
no_journal:
      spin_unlock(&journal->j_list_lock);
      jbd_unlock_bh_state(bh);
      if (need_brelse) {
            BUFFER_TRACE(bh, "brelse");
            __brelse(bh);
      }
      JBUFFER_TRACE(jh, "exit");
      journal_put_journal_head(jh);
      return 0;
}

/** 
 * int journal_dirty_metadata() -  mark a buffer as containing dirty metadata
 * @handle: transaction to add buffer to.
 * @bh: buffer to mark 
 * 
 * mark dirty metadata which needs to be journaled as part of the current
 * transaction.
 *
 * The buffer is placed on the transaction's metadata list and is marked
 * as belonging to the transaction.  
 *
 * Returns error number or 0 on success.  
 *
 * Special care needs to be taken if the buffer already belongs to the
 * current committing transaction (in which case we should have frozen
 * data present for that commit).  In that case, we don't relink the
 * buffer: that only gets done when the old transaction finally
 * completes its commit.
 */
int journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
{
      transaction_t *transaction = handle->h_transaction;
      journal_t *journal = transaction->t_journal;
      struct journal_head *jh = bh2jh(bh);

      jbd_debug(5, "journal_head %p\n", jh);
      JBUFFER_TRACE(jh, "entry");
      if (is_handle_aborted(handle))
            goto out;

      jbd_lock_bh_state(bh);

      if (jh->b_modified == 0) {
            /*
             * This buffer's got modified and becoming part
             * of the transaction. This needs to be done
             * once a transaction -bzzz
             */
            jh->b_modified = 1;
            J_ASSERT_JH(jh, handle->h_buffer_credits > 0);
            handle->h_buffer_credits--;
      }

      /*
       * fastpath, to avoid expensive locking.  If this buffer is already
       * on the running transaction's metadata list there is nothing to do.
       * Nobody can take it off again because there is a handle open.
       * I _think_ we're OK here with SMP barriers - a mistaken decision will
       * result in this test being false, so we go in and take the locks.
       */
      if (jh->b_transaction == transaction && jh->b_jlist == BJ_Metadata) {
            JBUFFER_TRACE(jh, "fastpath");
            J_ASSERT_JH(jh, jh->b_transaction ==
                              journal->j_running_transaction);
            goto out_unlock_bh;
      }

      set_buffer_jbddirty(bh);

      /* 
       * Metadata already on the current transaction list doesn't
       * need to be filed.  Metadata on another transaction's list must
       * be committing, and will be refiled once the commit completes:
       * leave it alone for now. 
       */
      if (jh->b_transaction != transaction) {
            JBUFFER_TRACE(jh, "already on other transaction");
            J_ASSERT_JH(jh, jh->b_transaction ==
                              journal->j_committing_transaction);
            J_ASSERT_JH(jh, jh->b_next_transaction == transaction);
            /* And this case is illegal: we can't reuse another
             * transaction's data buffer, ever. */
            goto out_unlock_bh;
      }

      /* That test should have eliminated the following case: */
      J_ASSERT_JH(jh, jh->b_frozen_data == 0);

      JBUFFER_TRACE(jh, "file as BJ_Metadata");
      spin_lock(&journal->j_list_lock);
      __journal_file_buffer(jh, handle->h_transaction, BJ_Metadata);
      spin_unlock(&journal->j_list_lock);
out_unlock_bh:
      jbd_unlock_bh_state(bh);
out:
      JBUFFER_TRACE(jh, "exit");
      return 0;
}

/* 
 * journal_release_buffer: undo a get_write_access without any buffer
 * updates, if the update decided in the end that it didn't need access.
 *
 */
void
journal_release_buffer(handle_t *handle, struct buffer_head *bh)
{
      BUFFER_TRACE(bh, "entry");
}

/** 
 * void journal_forget() - bforget() for potentially-journaled buffers.
 * @handle: transaction handle
 * @bh:     bh to 'forget'
 *
 * We can only do the bforget if there are no commits pending against the
 * buffer.  If the buffer is dirty in the current running transaction we
 * can safely unlink it. 
 *
 * bh may not be a journalled buffer at all - it may be a non-JBD
 * buffer which came off the hashtable.  Check for this.
 *
 * Decrements bh->b_count by one.
 * 
 * Allow this call even if the handle has aborted --- it may be part of
 * the caller's cleanup after an abort.
 */
int journal_forget (handle_t *handle, struct buffer_head *bh)
{
      transaction_t *transaction = handle->h_transaction;
      journal_t *journal = transaction->t_journal;
      struct journal_head *jh;
      int drop_reserve = 0;
      int err = 0;

      BUFFER_TRACE(bh, "entry");

      jbd_lock_bh_state(bh);
      spin_lock(&journal->j_list_lock);

      if (!buffer_jbd(bh))
            goto not_jbd;
      jh = bh2jh(bh);

      /* Critical error: attempting to delete a bitmap buffer, maybe?
       * Don't do any jbd operations, and return an error. */
      if (!J_EXPECT_JH(jh, !jh->b_committed_data,
                   "inconsistent data on disk")) {
            err = -EIO;
            goto not_jbd;
      }

      /*
       * The buffer's going from the transaction, we must drop
       * all references -bzzz
       */
      jh->b_modified = 0;

      if (jh->b_transaction == handle->h_transaction) {
            J_ASSERT_JH(jh, !jh->b_frozen_data);

            /* If we are forgetting a buffer which is already part
             * of this transaction, then we can just drop it from
             * the transaction immediately. */
            clear_buffer_dirty(bh);
            clear_buffer_jbddirty(bh);

            JBUFFER_TRACE(jh, "belongs to current transaction: unfile");

            drop_reserve = 1;

            /* 
             * We are no longer going to journal this buffer.
             * However, the commit of this transaction is still
             * important to the buffer: the delete that we are now
             * processing might obsolete an old log entry, so by
             * committing, we can satisfy the buffer's checkpoint.
             *
             * So, if we have a checkpoint on the buffer, we should
             * now refile the buffer on our BJ_Forget list so that
             * we know to remove the checkpoint after we commit. 
             */

            if (jh->b_cp_transaction) {
                  __journal_temp_unlink_buffer(jh);
                  __journal_file_buffer(jh, transaction, BJ_Forget);
            } else {
                  __journal_unfile_buffer(jh);
                  journal_remove_journal_head(bh);
                  __brelse(bh);
                  if (!buffer_jbd(bh)) {
                        spin_unlock(&journal->j_list_lock);
                        jbd_unlock_bh_state(bh);
                        __bforget(bh);
                        goto drop;
                  }
            }
      } else if (jh->b_transaction) {
            J_ASSERT_JH(jh, (jh->b_transaction == 
                         journal->j_committing_transaction));
            /* However, if the buffer is still owned by a prior
             * (committing) transaction, we can't drop it yet... */
            JBUFFER_TRACE(jh, "belongs to older transaction");
            /* ... but we CAN drop it from the new transaction if we
             * have also modified it since the original commit. */

            if (jh->b_next_transaction) {
                  J_ASSERT(jh->b_next_transaction == transaction);
                  jh->b_next_transaction = NULL;
                  drop_reserve = 1;
            }
      }

not_jbd:
      spin_unlock(&journal->j_list_lock);
      jbd_unlock_bh_state(bh);
      __brelse(bh);
drop:
      if (drop_reserve) {
            /* no need to reserve log space for this block -bzzz */
            handle->h_buffer_credits++;
      }
      return err;
}

/**
 * int journal_stop() - complete a transaction
 * @handle: tranaction to complete.
 * 
 * All done for a particular handle.
 *
 * There is not much action needed here.  We just return any remaining
 * buffer credits to the transaction and remove the handle.  The only
 * complication is that we need to start a commit operation if the
 * filesystem is marked for synchronous update.
 *
 * journal_stop itself will not usually return an error, but it may
 * do so in unusual circumstances.  In particular, expect it to 
 * return -EIO if a journal_abort has been executed since the
 * transaction began.
 */
int journal_stop(handle_t *handle)
{
      transaction_t *transaction = handle->h_transaction;
      journal_t *journal = transaction->t_journal;
      int old_handle_count, err;
      pid_t pid;

      J_ASSERT(transaction->t_updates > 0);
      J_ASSERT(journal_current_handle() == handle);

      if (is_handle_aborted(handle))
            err = -EIO;
      else
            err = 0;

      if (--handle->h_ref > 0) {
            jbd_debug(4, "h_ref %d -> %d\n", handle->h_ref + 1,
                    handle->h_ref);
            return err;
      }

      jbd_debug(4, "Handle %p going down\n", handle);

      /*
       * Implement synchronous transaction batching.  If the handle
       * was synchronous, don't force a commit immediately.  Let's
       * yield and let another thread piggyback onto this transaction.
       * Keep doing that while new threads continue to arrive.
       * It doesn't cost much - we're about to run a commit and sleep
       * on IO anyway.  Speeds up many-threaded, many-dir operations
       * by 30x or more...
       *
       * But don't do this if this process was the most recent one to
       * perform a synchronous write.  We do this to detect the case where a
       * single process is doing a stream of sync writes.  No point in waiting
       * for joiners in that case.
       */
      pid = current->pid;
      if (handle->h_sync && journal->j_last_sync_writer != pid) {
            journal->j_last_sync_writer = pid;
            do {
                  old_handle_count = transaction->t_handle_count;
                  schedule_timeout_uninterruptible(1);
            } while (old_handle_count != transaction->t_handle_count);
      }

      current->journal_info = NULL;
      spin_lock(&journal->j_state_lock);
      spin_lock(&transaction->t_handle_lock);
      transaction->t_outstanding_credits -= handle->h_buffer_credits;
      transaction->t_updates--;
      if (!transaction->t_updates) {
            wake_up(&journal->j_wait_updates);
            if (journal->j_barrier_count)
                  wake_up(&journal->j_wait_transaction_locked);
      }

      /*
       * If the handle is marked SYNC, we need to set another commit
       * going!  We also want to force a commit if the current
       * transaction is occupying too much of the log, or if the
       * transaction is too old now.
       */
      if (handle->h_sync ||
                  transaction->t_outstanding_credits >
                        journal->j_max_transaction_buffers ||
                  time_after_eq(jiffies, transaction->t_expires)) {
            /* Do this even for aborted journals: an abort still
             * completes the commit thread, it just doesn't write
             * anything to disk. */
            tid_t tid = transaction->t_tid;

            spin_unlock(&transaction->t_handle_lock);
            jbd_debug(2, "transaction too old, requesting commit for "
                              "handle %p\n", handle);
            /* This is non-blocking */
            __log_start_commit(journal, transaction->t_tid);
            spin_unlock(&journal->j_state_lock);

            /*
             * Special case: JFS_SYNC synchronous updates require us
             * to wait for the commit to complete.  
             */
            if (handle->h_sync && !(current->flags & PF_MEMALLOC))
                  err = log_wait_commit(journal, tid);
      } else {
            spin_unlock(&transaction->t_handle_lock);
            spin_unlock(&journal->j_state_lock);
      }

      jbd_free_handle(handle);
      return err;
}

/**int journal_force_commit() - force any uncommitted transactions
 * @journal: journal to force
 *
 * For synchronous operations: force any uncommitted transactions
 * to disk.  May seem kludgy, but it reuses all the handle batching
 * code in a very simple manner.
 */
int journal_force_commit(journal_t *journal)
{
      handle_t *handle;
      int ret;

      handle = journal_start(journal, 1);
      if (IS_ERR(handle)) {
            ret = PTR_ERR(handle);
      } else {
            handle->h_sync = 1;
            ret = journal_stop(handle);
      }
      return ret;
}

/*
 *
 * List management code snippets: various functions for manipulating the
 * transaction buffer lists.
 *
 */

/*
 * Append a buffer to a transaction list, given the transaction's list head
 * pointer.
 *
 * j_list_lock is held.
 *
 * jbd_lock_bh_state(jh2bh(jh)) is held.
 */

static inline void 
__blist_add_buffer(struct journal_head **list, struct journal_head *jh)
{
      if (!*list) {
            jh->b_tnext = jh->b_tprev = jh;
            *list = jh;
      } else {
            /* Insert at the tail of the list to preserve order */
            struct journal_head *first = *list, *last = first->b_tprev;
            jh->b_tprev = last;
            jh->b_tnext = first;
            last->b_tnext = first->b_tprev = jh;
      }
}

/* 
 * Remove a buffer from a transaction list, given the transaction's list
 * head pointer.
 *
 * Called with j_list_lock held, and the journal may not be locked.
 *
 * jbd_lock_bh_state(jh2bh(jh)) is held.
 */

static inline void
__blist_del_buffer(struct journal_head **list, struct journal_head *jh)
{
      if (*list == jh) {
            *list = jh->b_tnext;
            if (*list == jh)
                  *list = NULL;
      }
      jh->b_tprev->b_tnext = jh->b_tnext;
      jh->b_tnext->b_tprev = jh->b_tprev;
}

/* 
 * Remove a buffer from the appropriate transaction list.
 *
 * Note that this function can *change* the value of
 * bh->b_transaction->t_sync_datalist, t_buffers, t_forget,
 * t_iobuf_list, t_shadow_list, t_log_list or t_reserved_list.  If the caller
 * is holding onto a copy of one of thee pointers, it could go bad.
 * Generally the caller needs to re-read the pointer from the transaction_t.
 *
 * Called under j_list_lock.  The journal may not be locked.
 */
void __journal_temp_unlink_buffer(struct journal_head *jh)
{
      struct journal_head **list = NULL;
      transaction_t *transaction;
      struct buffer_head *bh = jh2bh(jh);

      J_ASSERT_JH(jh, jbd_is_locked_bh_state(bh));
      transaction = jh->b_transaction;
      if (transaction)
            assert_spin_locked(&transaction->t_journal->j_list_lock);

      J_ASSERT_JH(jh, jh->b_jlist < BJ_Types);
      if (jh->b_jlist != BJ_None)
            J_ASSERT_JH(jh, transaction != 0);

      switch (jh->b_jlist) {
      case BJ_None:
            return;
      case BJ_SyncData:
            list = &transaction->t_sync_datalist;
            break;
      case BJ_Metadata:
            transaction->t_nr_buffers--;
            J_ASSERT_JH(jh, transaction->t_nr_buffers >= 0);
            list = &transaction->t_buffers;
            break;
      case BJ_Forget:
            list = &transaction->t_forget;
            break;
      case BJ_IO:
            list = &transaction->t_iobuf_list;
            break;
      case BJ_Shadow:
            list = &transaction->t_shadow_list;
            break;
      case BJ_LogCtl:
            list = &transaction->t_log_list;
            break;
      case BJ_Reserved:
            list = &transaction->t_reserved_list;
            break;
      case BJ_Locked:
            list = &transaction->t_locked_list;
            break;
      }

      __blist_del_buffer(list, jh);
      jh->b_jlist = BJ_None;
      if (test_clear_buffer_jbddirty(bh))
            mark_buffer_dirty(bh);  /* Expose it to the VM */
}

void __journal_unfile_buffer(struct journal_head *jh)
{
      __journal_temp_unlink_buffer(jh);
      jh->b_transaction = NULL;
}

void journal_unfile_buffer(journal_t *journal, struct journal_head *jh)
{
      jbd_lock_bh_state(jh2bh(jh));
      spin_lock(&journal->j_list_lock);
      __journal_unfile_buffer(jh);
      spin_unlock(&journal->j_list_lock);
      jbd_unlock_bh_state(jh2bh(jh));
}

/*
 * Called from journal_try_to_free_buffers().
 *
 * Called under jbd_lock_bh_state(bh)
 */
static void
__journal_try_to_free_buffer(journal_t *journal, struct buffer_head *bh)
{
      struct journal_head *jh;

      jh = bh2jh(bh);

      if (buffer_locked(bh) || buffer_dirty(bh))
            goto out;

      if (jh->b_next_transaction != 0)
            goto out;

      spin_lock(&journal->j_list_lock);
      if (jh->b_transaction != 0 && jh->b_cp_transaction == 0) {
            if (jh->b_jlist == BJ_SyncData || jh->b_jlist == BJ_Locked) {
                  /* A written-back ordered data buffer */
                  JBUFFER_TRACE(jh, "release data");
                  __journal_unfile_buffer(jh);
                  journal_remove_journal_head(bh);
                  __brelse(bh);
            }
      } else if (jh->b_cp_transaction != 0 && jh->b_transaction == 0) {
            /* written-back checkpointed metadata buffer */
            if (jh->b_jlist == BJ_None) {
                  JBUFFER_TRACE(jh, "remove from checkpoint list");
                  __journal_remove_checkpoint(jh);
                  journal_remove_journal_head(bh);
                  __brelse(bh);
            }
      }
      spin_unlock(&journal->j_list_lock);
out:
      return;
}


/** 
 * int journal_try_to_free_buffers() - try to free page buffers.
 * @journal: journal for operation
 * @page: to try and free
 * @unused_gfp_mask: unused
 *
 * 
 * For all the buffers on this page,
 * if they are fully written out ordered data, move them onto BUF_CLEAN
 * so try_to_free_buffers() can reap them.
 * 
 * This function returns non-zero if we wish try_to_free_buffers()
 * to be called. We do this if the page is releasable by try_to_free_buffers().
 * We also do it if the page has locked or dirty buffers and the caller wants
 * us to perform sync or async writeout.
 *
 * This complicates JBD locking somewhat.  We aren't protected by the
 * BKL here.  We wish to remove the buffer from its committing or
 * running transaction's ->t_datalist via __journal_unfile_buffer.
 *
 * This may *change* the value of transaction_t->t_datalist, so anyone
 * who looks at t_datalist needs to lock against this function.
 *
 * Even worse, someone may be doing a journal_dirty_data on this
 * buffer.  So we need to lock against that.  journal_dirty_data()
 * will come out of the lock with the buffer dirty, which makes it
 * ineligible for release here.
 *
 * Who else is affected by this?  hmm...  Really the only contender
 * is do_get_write_access() - it could be looking at the buffer while
 * journal_try_to_free_buffer() is changing its state.  But that
 * cannot happen because we never reallocate freed data as metadata
 * while the data is part of a transaction.  Yes?
 */
int journal_try_to_free_buffers(journal_t *journal, 
                        struct page *page, gfp_t unused_gfp_mask)
{
      struct buffer_head *head;
      struct buffer_head *bh;
      int ret = 0;

      J_ASSERT(PageLocked(page));

      head = page_buffers(page);
      bh = head;
      do {
            struct journal_head *jh;

            /*
             * We take our own ref against the journal_head here to avoid
             * having to add tons of locking around each instance of
             * journal_remove_journal_head() and journal_put_journal_head().
             */
            jh = journal_grab_journal_head(bh);
            if (!jh)
                  continue;

            jbd_lock_bh_state(bh);
            __journal_try_to_free_buffer(journal, bh);
            journal_put_journal_head(jh);
            jbd_unlock_bh_state(bh);
            if (buffer_jbd(bh))
                  goto busy;
      } while ((bh = bh->b_this_page) != head);
      ret = try_to_free_buffers(page);
busy:
      return ret;
}

/*
 * This buffer is no longer needed.  If it is on an older transaction's
 * checkpoint list we need to record it on this transaction's forget list
 * to pin this buffer (and hence its checkpointing transaction) down until
 * this transaction commits.  If the buffer isn't on a checkpoint list, we
 * release it.
 * Returns non-zero if JBD no longer has an interest in the buffer.
 *
 * Called under j_list_lock.
 *
 * Called under jbd_lock_bh_state(bh).
 */
static int __dispose_buffer(struct journal_head *jh, transaction_t *transaction)
{
      int may_free = 1;
      struct buffer_head *bh = jh2bh(jh);

      __journal_unfile_buffer(jh);

      if (jh->b_cp_transaction) {
            JBUFFER_TRACE(jh, "on running+cp transaction");
            __journal_file_buffer(jh, transaction, BJ_Forget);
            clear_buffer_jbddirty(bh);
            may_free = 0;
      } else {
            JBUFFER_TRACE(jh, "on running transaction");
            journal_remove_journal_head(bh);
            __brelse(bh);
      }
      return may_free;
}

/*
 * journal_invalidatepage 
 *
 * This code is tricky.  It has a number of cases to deal with.
 *
 * There are two invariants which this code relies on:
 *
 * i_size must be updated on disk before we start calling invalidatepage on the
 * data.
 * 
 *  This is done in ext3 by defining an ext3_setattr method which
 *  updates i_size before truncate gets going.  By maintaining this
 *  invariant, we can be sure that it is safe to throw away any buffers
 *  attached to the current transaction: once the transaction commits,
 *  we know that the data will not be needed.
 * 
 *  Note however that we can *not* throw away data belonging to the
 *  previous, committing transaction!  
 *
 * Any disk blocks which *are* part of the previous, committing
 * transaction (and which therefore cannot be discarded immediately) are
 * not going to be reused in the new running transaction
 *
 *  The bitmap committed_data images guarantee this: any block which is
 *  allocated in one transaction and removed in the next will be marked
 *  as in-use in the committed_data bitmap, so cannot be reused until
 *  the next transaction to delete the block commits.  This means that
 *  leaving committing buffers dirty is quite safe: the disk blocks
 *  cannot be reallocated to a different file and so buffer aliasing is
 *  not possible.
 *
 *
 * The above applies mainly to ordered data mode.  In writeback mode we
 * don't make guarantees about the order in which data hits disk --- in
 * particular we don't guarantee that new dirty data is flushed before
 * transaction commit --- so it is always safe just to discard data
 * immediately in that mode.  --sct 
 */

/*
 * The journal_unmap_buffer helper function returns zero if the buffer
 * concerned remains pinned as an anonymous buffer belonging to an older
 * transaction.
 *
 * We're outside-transaction here.  Either or both of j_running_transaction
 * and j_committing_transaction may be NULL.
 */
static int journal_unmap_buffer(journal_t *journal, struct buffer_head *bh)
{
      transaction_t *transaction;
      struct journal_head *jh;
      int may_free = 1;
      int ret;

      BUFFER_TRACE(bh, "entry");

      /*
       * It is safe to proceed here without the j_list_lock because the
       * buffers cannot be stolen by try_to_free_buffers as long as we are
       * holding the page lock. --sct
       */

      if (!buffer_jbd(bh))
            goto zap_buffer_unlocked;

      spin_lock(&journal->j_state_lock);
      jbd_lock_bh_state(bh);
      spin_lock(&journal->j_list_lock);

      jh = journal_grab_journal_head(bh);
      if (!jh)
            goto zap_buffer_no_jh;

      transaction = jh->b_transaction;
      if (transaction == NULL) {
            /* First case: not on any transaction.  If it
             * has no checkpoint link, then we can zap it:
             * it's a writeback-mode buffer so we don't care
             * if it hits disk safely. */
            if (!jh->b_cp_transaction) {
                  JBUFFER_TRACE(jh, "not on any transaction: zap");
                  goto zap_buffer;
            }

            if (!buffer_dirty(bh)) {
                  /* bdflush has written it.  We can drop it now */
                  goto zap_buffer;
            }

            /* OK, it must be in the journal but still not
             * written fully to disk: it's metadata or
             * journaled data... */

            if (journal->j_running_transaction) {
                  /* ... and once the current transaction has
                   * committed, the buffer won't be needed any
                   * longer. */
                  JBUFFER_TRACE(jh, "checkpointed: add to BJ_Forget");
                  ret = __dispose_buffer(jh,
                              journal->j_running_transaction);
                  journal_put_journal_head(jh);
                  spin_unlock(&journal->j_list_lock);
                  jbd_unlock_bh_state(bh);
                  spin_unlock(&journal->j_state_lock);
                  return ret;
            } else {
                  /* There is no currently-running transaction. So the
                   * orphan record which we wrote for this file must have
                   * passed into commit.  We must attach this buffer to
                   * the committing transaction, if it exists. */
                  if (journal->j_committing_transaction) {
                        JBUFFER_TRACE(jh, "give to committing trans");
                        ret = __dispose_buffer(jh,
                              journal->j_committing_transaction);
                        journal_put_journal_head(jh);
                        spin_unlock(&journal->j_list_lock);
                        jbd_unlock_bh_state(bh);
                        spin_unlock(&journal->j_state_lock);
                        return ret;
                  } else {
                        /* The orphan record's transaction has
                         * committed.  We can cleanse this buffer */
                        clear_buffer_jbddirty(bh);
                        goto zap_buffer;
                  }
            }
      } else if (transaction == journal->j_committing_transaction) {
            if (jh->b_jlist == BJ_Locked) {
                  /*
                   * The buffer is on the committing transaction's locked
                   * list.  We have the buffer locked, so I/O has
                   * completed.  So we can nail the buffer now.
                   */
                  may_free = __dispose_buffer(jh, transaction);
                  goto zap_buffer;
            }
            /*
             * If it is committing, we simply cannot touch it.  We
             * can remove it's next_transaction pointer from the
             * running transaction if that is set, but nothing
             * else. */
            JBUFFER_TRACE(jh, "on committing transaction");
            set_buffer_freed(bh);
            if (jh->b_next_transaction) {
                  J_ASSERT(jh->b_next_transaction ==
                              journal->j_running_transaction);
                  jh->b_next_transaction = NULL;
            }
            journal_put_journal_head(jh);
            spin_unlock(&journal->j_list_lock);
            jbd_unlock_bh_state(bh);
            spin_unlock(&journal->j_state_lock);
            return 0;
      } else {
            /* Good, the buffer belongs to the running transaction.
             * We are writing our own transaction's data, not any
             * previous one's, so it is safe to throw it away
             * (remember that we expect the filesystem to have set
             * i_size already for this truncate so recovery will not
             * expose the disk blocks we are discarding here.) */
            J_ASSERT_JH(jh, transaction == journal->j_running_transaction);
            may_free = __dispose_buffer(jh, transaction);
      }

zap_buffer:
      journal_put_journal_head(jh);
zap_buffer_no_jh:
      spin_unlock(&journal->j_list_lock);
      jbd_unlock_bh_state(bh);
      spin_unlock(&journal->j_state_lock);
zap_buffer_unlocked:
      clear_buffer_dirty(bh);
      J_ASSERT_BH(bh, !buffer_jbddirty(bh));
      clear_buffer_mapped(bh);
      clear_buffer_req(bh);
      clear_buffer_new(bh);
      bh->b_bdev = NULL;
      return may_free;
}

/** 
 * void journal_invalidatepage()
 * @journal: journal to use for flush... 
 * @page:    page to flush
 * @offset:  length of page to invalidate.
 *
 * Reap page buffers containing data after offset in page.
 *
 */
void journal_invalidatepage(journal_t *journal,
                  struct page *page, 
                  unsigned long offset)
{
      struct buffer_head *head, *bh, *next;
      unsigned int curr_off = 0;
      int may_free = 1;

      if (!PageLocked(page))
            BUG();
      if (!page_has_buffers(page))
            return;

      /* We will potentially be playing with lists other than just the
       * data lists (especially for journaled data mode), so be
       * cautious in our locking. */

      head = bh = page_buffers(page);
      do {
            unsigned int next_off = curr_off + bh->b_size;
            next = bh->b_this_page;

            if (offset <= curr_off) {
                  /* This block is wholly outside the truncation point */
                  lock_buffer(bh);
                  may_free &= journal_unmap_buffer(journal, bh);
                  unlock_buffer(bh);
            }
            curr_off = next_off;
            bh = next;

      } while (bh != head);

      if (!offset) {
            if (may_free && try_to_free_buffers(page))
                  J_ASSERT(!page_has_buffers(page));
      }
}

/* 
 * File a buffer on the given transaction list. 
 */
void __journal_file_buffer(struct journal_head *jh,
                  transaction_t *transaction, int jlist)
{
      struct journal_head **list = NULL;
      int was_dirty = 0;
      struct buffer_head *bh = jh2bh(jh);

      J_ASSERT_JH(jh, jbd_is_locked_bh_state(bh));
      assert_spin_locked(&transaction->t_journal->j_list_lock);

      J_ASSERT_JH(jh, jh->b_jlist < BJ_Types);
      J_ASSERT_JH(jh, jh->b_transaction == transaction ||
                        jh->b_transaction == 0);

      if (jh->b_transaction && jh->b_jlist == jlist)
            return;

      /* The following list of buffer states needs to be consistent
       * with __jbd_unexpected_dirty_buffer()'s handling of dirty
       * state. */

      if (jlist == BJ_Metadata || jlist == BJ_Reserved || 
          jlist == BJ_Shadow || jlist == BJ_Forget) {
            if (test_clear_buffer_dirty(bh) ||
                test_clear_buffer_jbddirty(bh))
                  was_dirty = 1;
      }

      if (jh->b_transaction)
            __journal_temp_unlink_buffer(jh);
      jh->b_transaction = transaction;

      switch (jlist) {
      case BJ_None:
            J_ASSERT_JH(jh, !jh->b_committed_data);
            J_ASSERT_JH(jh, !jh->b_frozen_data);
            return;
      case BJ_SyncData:
            list = &transaction->t_sync_datalist;
            break;
      case BJ_Metadata:
            transaction->t_nr_buffers++;
            list = &transaction->t_buffers;
            break;
      case BJ_Forget:
            list = &transaction->t_forget;
            break;
      case BJ_IO:
            list = &transaction->t_iobuf_list;
            break;
      case BJ_Shadow:
            list = &transaction->t_shadow_list;
            break;
      case BJ_LogCtl:
            list = &transaction->t_log_list;
            break;
      case BJ_Reserved:
            list = &transaction->t_reserved_list;
            break;
      case BJ_Locked:
            list =  &transaction->t_locked_list;
            break;
      }

      __blist_add_buffer(list, jh);
      jh->b_jlist = jlist;

      if (was_dirty)
            set_buffer_jbddirty(bh);
}

void journal_file_buffer(struct journal_head *jh,
                        transaction_t *transaction, int jlist)
{
      jbd_lock_bh_state(jh2bh(jh));
      spin_lock(&transaction->t_journal->j_list_lock);
      __journal_file_buffer(jh, transaction, jlist);
      spin_unlock(&transaction->t_journal->j_list_lock);
      jbd_unlock_bh_state(jh2bh(jh));
}

/* 
 * Remove a buffer from its current buffer list in preparation for
 * dropping it from its current transaction entirely.  If the buffer has
 * already started to be used by a subsequent transaction, refile the
 * buffer on that transaction's metadata list.
 *
 * Called under journal->j_list_lock
 *
 * Called under jbd_lock_bh_state(jh2bh(jh))
 */
void __journal_refile_buffer(struct journal_head *jh)
{
      int was_dirty;
      struct buffer_head *bh = jh2bh(jh);

      J_ASSERT_JH(jh, jbd_is_locked_bh_state(bh));
      if (jh->b_transaction)
            assert_spin_locked(&jh->b_transaction->t_journal->j_list_lock);

      /* If the buffer is now unused, just drop it. */
      if (jh->b_next_transaction == NULL) {
            __journal_unfile_buffer(jh);
            return;
      }

      /*
       * It has been modified by a later transaction: add it to the new
       * transaction's metadata list.
       */

      was_dirty = test_clear_buffer_jbddirty(bh);
      __journal_temp_unlink_buffer(jh);
      jh->b_transaction = jh->b_next_transaction;
      jh->b_next_transaction = NULL;
      __journal_file_buffer(jh, jh->b_transaction,
                        was_dirty ? BJ_Metadata : BJ_Reserved);
      J_ASSERT_JH(jh, jh->b_transaction->t_state == T_RUNNING);

      if (was_dirty)
            set_buffer_jbddirty(bh);
}

/*
 * For the unlocked version of this call, also make sure that any
 * hanging journal_head is cleaned up if necessary.
 *
 * __journal_refile_buffer is usually called as part of a single locked
 * operation on a buffer_head, in which the caller is probably going to
 * be hooking the journal_head onto other lists.  In that case it is up
 * to the caller to remove the journal_head if necessary.  For the
 * unlocked journal_refile_buffer call, the caller isn't going to be
 * doing anything else to the buffer so we need to do the cleanup
 * ourselves to avoid a jh leak. 
 *
 * *** The journal_head may be freed by this call! ***
 */
void journal_refile_buffer(journal_t *journal, struct journal_head *jh)
{
      struct buffer_head *bh = jh2bh(jh);

      jbd_lock_bh_state(bh);
      spin_lock(&journal->j_list_lock);

      __journal_refile_buffer(jh);
      jbd_unlock_bh_state(bh);
      journal_remove_journal_head(bh);

      spin_unlock(&journal->j_list_lock);
      __brelse(bh);
}

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