patch-2.4.15 linux/fs/jbd/journal.c

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diff -u --recursive --new-file v2.4.14/linux/fs/jbd/journal.c linux/fs/jbd/journal.c
@@ -0,0 +1,1813 @@
+/*
+ * linux/fs/journal.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 journal-writing code; part of the ext2fs
+ * journaling system.
+ *
+ * This file manages journals: areas of disk reserved for logging
+ * transactional updates.  This includes the kernel journaling thread
+ * which is responsible for scheduling updates to the log.
+ *
+ * We do not actually manage the physical storage of the journal in this
+ * file: that is left to a per-journal policy function, which allows us
+ * to store the journal within a filesystem-specified area for ext2
+ * journaling (ext2 can use a reserved inode for storing the log).
+ */
+
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/fs.h>
+#include <linux/jbd.h>
+#include <linux/errno.h>
+#include <linux/slab.h>
+#include <linux/locks.h>
+#include <linux/smp_lock.h>
+#include <linux/sched.h>
+#include <linux/init.h>
+#include <linux/mm.h>
+#include <linux/slab.h>
+#include <asm/uaccess.h>
+#include <linux/proc_fs.h>
+
+EXPORT_SYMBOL(journal_start);
+EXPORT_SYMBOL(journal_try_start);
+EXPORT_SYMBOL(journal_restart);
+EXPORT_SYMBOL(journal_extend);
+EXPORT_SYMBOL(journal_stop);
+EXPORT_SYMBOL(journal_lock_updates);
+EXPORT_SYMBOL(journal_unlock_updates);
+EXPORT_SYMBOL(journal_get_write_access);
+EXPORT_SYMBOL(journal_get_create_access);
+EXPORT_SYMBOL(journal_get_undo_access);
+EXPORT_SYMBOL(journal_dirty_data);
+EXPORT_SYMBOL(journal_dirty_metadata);
+#if 0
+EXPORT_SYMBOL(journal_release_buffer);
+#endif
+EXPORT_SYMBOL(journal_forget);
+#if 0
+EXPORT_SYMBOL(journal_sync_buffer);
+#endif
+EXPORT_SYMBOL(journal_flush);
+EXPORT_SYMBOL(journal_revoke);
+
+EXPORT_SYMBOL(journal_init_dev);
+EXPORT_SYMBOL(journal_init_inode);
+EXPORT_SYMBOL(journal_update_format);
+EXPORT_SYMBOL(journal_check_used_features);
+EXPORT_SYMBOL(journal_check_available_features);
+EXPORT_SYMBOL(journal_set_features);
+EXPORT_SYMBOL(journal_create);
+EXPORT_SYMBOL(journal_load);
+EXPORT_SYMBOL(journal_destroy);
+EXPORT_SYMBOL(journal_recover);
+EXPORT_SYMBOL(journal_update_superblock);
+EXPORT_SYMBOL(__journal_abort);
+EXPORT_SYMBOL(journal_abort);
+EXPORT_SYMBOL(journal_errno);
+EXPORT_SYMBOL(journal_ack_err);
+EXPORT_SYMBOL(journal_clear_err);
+EXPORT_SYMBOL(log_wait_commit);
+EXPORT_SYMBOL(log_start_commit);
+EXPORT_SYMBOL(journal_wipe);
+EXPORT_SYMBOL(journal_blocks_per_page);
+EXPORT_SYMBOL(journal_flushpage);
+EXPORT_SYMBOL(journal_try_to_free_buffers);
+EXPORT_SYMBOL(journal_bmap);
+EXPORT_SYMBOL(journal_force_commit);
+
+static int journal_convert_superblock_v1(journal_t *, journal_superblock_t *);
+
+/*
+ * journal_datalist_lock is used to protect data buffers:
+ *
+ *	bh->b_transaction
+ *	bh->b_tprev
+ *	bh->b_tnext
+ *
+ * journal_free_buffer() is called from journal_try_to_free_buffer(), and is
+ * async wrt everything else.
+ *
+ * It is also used for checkpoint data, also to protect against
+ * journal_try_to_free_buffer():
+ *
+ *	bh->b_cp_transaction
+ *	bh->b_cpnext
+ *	bh->b_cpprev
+ *	transaction->t_checkpoint_list
+ *	transaction->t_cpnext
+ *	transaction->t_cpprev
+ *	journal->j_checkpoint_transactions
+ *
+ * It is global at this time rather than per-journal because it's
+ * impossible for __journal_free_buffer to go from a buffer_head
+ * back to a journal_t unracily (well, not true.  Fix later)
+ *
+ *
+ * The `datalist' and `checkpoint list' functions are quite
+ * separate and we could use two spinlocks here.
+ *
+ * lru_list_lock nests inside journal_datalist_lock.
+ */
+spinlock_t journal_datalist_lock = SPIN_LOCK_UNLOCKED;
+
+/*
+ * jh_splice_lock needs explantion.
+ *
+ * In a number of places we want to do things like:
+ *
+ *	if (buffer_jbd(bh) && bh2jh(bh)->foo)
+ *
+ * This is racy on SMP, because another CPU could remove the journal_head
+ * in the middle of this expression.  We need locking.
+ *
+ * But we can greatly optimise the locking cost by testing BH_JBD
+ * outside the lock.  So, effectively:
+ *
+ *	ret = 0;
+ *	if (buffer_jbd(bh)) {
+ *		spin_lock(&jh_splice_lock);
+ *		if (buffer_jbd(bh)) {	 (* Still there? *)
+ *			ret = bh2jh(bh)->foo;
+ *		}
+ *		spin_unlock(&jh_splice_lock);
+ *	}
+ *	return ret;
+ *
+ * Now, that protects us from races where another CPU can remove the
+ * journal_head.  But it doesn't defend us from the situation where another
+ * CPU can *add* a journal_head.  This is a correctness issue.  But it's not
+ * a problem because a) the calling code was *already* racy and b) it often
+ * can't happen at the call site and c) the places where we add journal_heads
+ * tend to be under external locking.
+ */
+spinlock_t jh_splice_lock = SPIN_LOCK_UNLOCKED;
+
+/*
+ * List of all journals in the system.  Protected by the BKL.
+ */
+static LIST_HEAD(all_journals);
+
+/*
+ * Helper function used to manage commit timeouts
+ */
+
+static void commit_timeout(unsigned long __data)
+{
+	struct task_struct * p = (struct task_struct *) __data;
+
+	wake_up_process(p);
+}
+
+/* Static check for data structure consistency.  There's no code
+ * invoked --- we'll just get a linker failure if things aren't right.
+ */
+void __journal_internal_check(void)
+{
+	extern void journal_bad_superblock_size(void);
+	if (sizeof(struct journal_superblock_s) != 1024)
+		journal_bad_superblock_size();
+}
+
+/*
+ * kjournald: The main thread function used to manage a logging device
+ * journal.
+ *
+ * This kernel thread is responsible for two things:
+ *
+ * 1) COMMIT:  Every so often we need to commit the current state of the
+ *    filesystem to disk.  The journal thread is responsible for writing
+ *    all of the metadata buffers to disk.
+ *
+ * 2) CHECKPOINT: We cannot reuse a used section of the log file until all
+ *    of the data in that part of the log has been rewritten elsewhere on
+ *    the disk.  Flushing these old buffers to reclaim space in the log is
+ *    known as checkpointing, and this thread is responsible for that job.
+ */
+
+journal_t *current_journal;		// AKPM: debug
+
+int kjournald(void *arg)
+{
+	journal_t *journal = (journal_t *) arg;
+	transaction_t *transaction;
+	struct timer_list timer;
+
+	current_journal = journal;
+
+	lock_kernel();
+	daemonize();
+	spin_lock_irq(&current->sigmask_lock);
+	sigfillset(&current->blocked);
+	recalc_sigpending(current);
+	spin_unlock_irq(&current->sigmask_lock);
+
+	sprintf(current->comm, "kjournald");
+
+	/* Set up an interval timer which can be used to trigger a
+           commit wakeup after the commit interval expires */
+	init_timer(&timer);
+	timer.data = (unsigned long) current;
+	timer.function = commit_timeout;
+	journal->j_commit_timer = &timer;
+
+	/* Record that the journal thread is running */
+	journal->j_task = current;
+	wake_up(&journal->j_wait_done_commit);
+
+	printk(KERN_INFO "kjournald starting.  Commit interval %ld seconds\n",
+			journal->j_commit_interval / HZ);
+	list_add(&journal->j_all_journals, &all_journals);
+
+	/* And now, wait forever for commit wakeup events. */
+	while (1) {
+		if (journal->j_flags & JFS_UNMOUNT)
+			break;
+
+		jbd_debug(1, "commit_sequence=%d, commit_request=%d\n",
+			journal->j_commit_sequence, journal->j_commit_request);
+
+		if (journal->j_commit_sequence != journal->j_commit_request) {
+			jbd_debug(1, "OK, requests differ\n");
+			if (journal->j_commit_timer_active) {
+				journal->j_commit_timer_active = 0;
+				del_timer(journal->j_commit_timer);
+			}
+
+			journal_commit_transaction(journal);
+			continue;
+		}
+
+		wake_up(&journal->j_wait_done_commit);
+		interruptible_sleep_on(&journal->j_wait_commit);
+
+		jbd_debug(1, "kjournald wakes\n");
+
+		/* Were we woken up by a commit wakeup event? */
+		if ((transaction = journal->j_running_transaction) != NULL &&
+		    time_after_eq(jiffies, transaction->t_expires)) {
+			journal->j_commit_request = transaction->t_tid;
+			jbd_debug(1, "woke because of timeout\n");
+		}
+	}
+
+	if (journal->j_commit_timer_active) {
+		journal->j_commit_timer_active = 0;
+		del_timer_sync(journal->j_commit_timer);
+	}
+
+	list_del(&journal->j_all_journals);
+
+	journal->j_task = NULL;
+	wake_up(&journal->j_wait_done_commit);
+	jbd_debug(1, "Journal thread exiting.\n");
+	return 0;
+}
+
+static void journal_start_thread(journal_t *journal)
+{
+	kernel_thread(kjournald, (void *) journal,
+		      CLONE_VM | CLONE_FS | CLONE_FILES);
+	while (!journal->j_task)
+		sleep_on(&journal->j_wait_done_commit);
+}
+
+static void journal_kill_thread(journal_t *journal)
+{
+	journal->j_flags |= JFS_UNMOUNT;
+
+	while (journal->j_task) {
+		wake_up(&journal->j_wait_commit);
+		sleep_on(&journal->j_wait_done_commit);
+	}
+}
+
+#if 0
+
+This is no longer needed - we do it in commit quite efficiently.
+Note that if this function is resurrected, the loop needs to
+be reorganised into the next_jh/last_jh algorithm.
+
+/*
+ * journal_clean_data_list: cleanup after data IO.
+ *
+ * Once the IO system has finished writing the buffers on the transaction's
+ * data list, we can remove those buffers from the list.  This function
+ * scans the list for such buffers and removes them cleanly.
+ *
+ * We assume that the journal is already locked.
+ * We are called with journal_datalist_lock held.
+ *
+ * AKPM: This function looks inefficient.  Approximately O(n^2)
+ * for potentially thousands of buffers.  It no longer shows on profiles
+ * because these buffers are mainly dropped in journal_commit_transaction().
+ */
+
+void __journal_clean_data_list(transaction_t *transaction)
+{
+	struct journal_head *jh, *next;
+
+	assert_spin_locked(&journal_datalist_lock);
+
+restart:
+	jh = transaction->t_sync_datalist;
+	if (!jh)
+		goto out;
+	do {
+		next = jh->b_tnext;
+		if (!buffer_locked(jh2bh(jh)) && !buffer_dirty(jh2bh(jh))) {
+			struct buffer_head *bh = jh2bh(jh);
+			BUFFER_TRACE(bh, "data writeout complete: unfile");
+			__journal_unfile_buffer(jh);
+			jh->b_transaction = NULL;
+			__journal_remove_journal_head(bh);
+			refile_buffer(bh);
+			__brelse(bh);
+			goto restart;
+		}
+		jh = next;
+	} while (transaction->t_sync_datalist &&
+			jh != transaction->t_sync_datalist);
+out:
+	return;
+}
+#endif
+
+/*
+ * journal_write_metadata_buffer: write a metadata buffer to the journal.
+ *
+ * Writes a metadata buffer to a given disk block.  The actual IO is not
+ * performed but a new buffer_head is constructed which labels the data
+ * to be written with the correct destination disk block.
+ *
+ * Any magic-number escaping which needs to be done will cause a
+ * copy-out here.  If the buffer happens to start with the
+ * JFS_MAGIC_NUMBER, then we can't write it to the log directly: the
+ * magic number is only written to the log for descripter blocks.  In
+ * this case, we copy the data and replace the first word with 0, and we
+ * return a result code which indicates that this buffer needs to be
+ * marked as an escaped buffer in the corresponding log descriptor
+ * block.  The missing word can then be restored when the block is read
+ * during recovery.
+ *
+ * If the source buffer has already been modified by a new transaction
+ * since we took the last commit snapshot, we use the frozen copy of
+ * that data for IO.  If we end up using the existing buffer_head's data
+ * for the write, then we *have* to lock the buffer to prevent anyone
+ * else from using and possibly modifying it while the IO is in
+ * progress.
+ *
+ * The function returns a pointer to the buffer_heads to be used for IO.
+ *
+ * We assume that the journal has already been locked in this function.
+ *
+ * Return value:
+ *  <0: Error
+ * >=0: Finished OK
+ *
+ * On success:
+ * Bit 0 set == escape performed on the data
+ * Bit 1 set == buffer copy-out performed (kfree the data after IO)
+ */
+
+static inline unsigned long virt_to_offset(void *p) 
+{return ((unsigned long) p) & ~PAGE_MASK;}
+					       
+int journal_write_metadata_buffer(transaction_t *transaction,
+				  struct journal_head  *jh_in,
+				  struct journal_head **jh_out,
+				  int blocknr)
+{
+	int need_copy_out = 0;
+	int done_copy_out = 0;
+	int do_escape = 0;
+	char *mapped_data;
+	struct buffer_head *new_bh;
+	struct journal_head * new_jh;
+	struct page *new_page;
+	unsigned int new_offset;
+
+	/*
+	 * The buffer really shouldn't be locked: only the current committing
+	 * transaction is allowed to write it, so nobody else is allowed
+	 * to do any IO.
+	 *
+	 * akpm: except if we're journalling data, and write() output is
+	 * also part of a shared mapping, and another thread has
+	 * decided to launch a writepage() against this buffer.
+	 */
+	J_ASSERT_JH(jh_in, buffer_jdirty(jh2bh(jh_in)));
+
+	/*
+	 * If a new transaction has already done a buffer copy-out, then
+	 * we use that version of the data for the commit.
+	 */
+
+	if (jh_in->b_frozen_data) {
+		done_copy_out = 1;
+		new_page = virt_to_page(jh_in->b_frozen_data);
+		new_offset = virt_to_offset(jh_in->b_frozen_data);
+	} else {
+		new_page = jh2bh(jh_in)->b_page;
+		new_offset = virt_to_offset(jh2bh(jh_in)->b_data);
+	}
+
+	mapped_data = ((char *) kmap(new_page)) + new_offset;
+
+	/*
+	 * Check for escaping
+	 */
+	if (* ((unsigned int *) mapped_data) == htonl(JFS_MAGIC_NUMBER)) {
+		need_copy_out = 1;
+		do_escape = 1;
+	}
+
+	/*
+	 * Do we need to do a data copy?
+	 */
+
+	if (need_copy_out && !done_copy_out) {
+		char *tmp;
+		tmp = jbd_rep_kmalloc(jh2bh(jh_in)->b_size, GFP_NOFS);
+
+		jh_in->b_frozen_data = tmp;
+		memcpy (tmp, mapped_data, jh2bh(jh_in)->b_size);
+		
+		/* If we get to this path, we'll always need the new
+		   address kmapped so that we can clear the escaped
+		   magic number below. */
+		kunmap(new_page);
+		new_page = virt_to_page(tmp);
+		new_offset = virt_to_offset(tmp);
+		mapped_data = ((char *) kmap(new_page)) + new_offset;
+		
+		done_copy_out = 1;
+	}
+
+	/*
+	 * Right, time to make up the new buffer_head.
+	 */
+	do {
+		new_bh = get_unused_buffer_head(0);
+		if (!new_bh) {
+			printk (KERN_NOTICE __FUNCTION__
+				": ENOMEM at get_unused_buffer_head, "
+				"trying again.\n");
+			current->policy |= SCHED_YIELD;
+			schedule();
+		}
+	} while (!new_bh);
+	/* keep subsequent assertions sane */
+	new_bh->b_prev_free = 0;
+	new_bh->b_next_free = 0;
+	new_bh->b_state = 0;
+	init_buffer(new_bh, NULL, NULL);
+	atomic_set(&new_bh->b_count, 1);
+	new_jh = journal_add_journal_head(new_bh);
+
+	set_bh_page(new_bh, new_page, new_offset);
+
+	new_jh->b_transaction = NULL;
+	new_bh->b_size = jh2bh(jh_in)->b_size;
+	new_bh->b_dev = transaction->t_journal->j_dev;
+	new_bh->b_blocknr = blocknr;
+	new_bh->b_state |= (1 << BH_Mapped) | (1 << BH_Dirty);
+
+	*jh_out = new_jh;
+
+	/*
+	 * Did we need to do an escaping?  Now we've done all the
+	 * copying, we can finally do so.
+	 */
+
+	if (do_escape)
+		* ((unsigned int *) mapped_data) = 0;
+	kunmap(new_page);
+	
+	/*
+	 * The to-be-written buffer needs to get moved to the io queue,
+	 * and the original buffer whose contents we are shadowing or
+	 * copying is moved to the transaction's shadow queue.
+	 */
+	JBUFFER_TRACE(jh_in, "file as BJ_Shadow");
+	journal_file_buffer(jh_in, transaction, BJ_Shadow);
+	JBUFFER_TRACE(new_jh, "file as BJ_IO");
+	journal_file_buffer(new_jh, transaction, BJ_IO);
+
+	return do_escape | (done_copy_out << 1);
+}
+
+/*
+ * Allocation code for the journal file.  Manage the space left in the
+ * journal, so that we can begin checkpointing when appropriate.
+ */
+
+/*
+ * log_space_left: Return the number of free blocks left in the journal.
+ *
+ * Called with the journal already locked.
+ */
+
+int log_space_left (journal_t *journal)
+{
+	int left = journal->j_free;
+
+	/* Be pessimistic here about the number of those free blocks
+	 * which might be required for log descriptor control blocks. */
+
+#define MIN_LOG_RESERVED_BLOCKS 32 /* Allow for rounding errors */
+
+	left -= MIN_LOG_RESERVED_BLOCKS;
+
+	if (left <= 0)
+		return 0;
+	left -= (left >> 3);
+	return left;
+}
+
+/*
+ * This function must be non-allocating for PF_MEMALLOC tasks
+ */
+tid_t log_start_commit (journal_t *journal, transaction_t *transaction)
+{
+	tid_t target = journal->j_commit_request;
+
+	lock_kernel(); /* Protect journal->j_running_transaction */
+	
+	/*
+	 * A NULL transaction asks us to commit the currently running
+	 * transaction, if there is one.  
+	 */
+	if (transaction)
+		target = transaction->t_tid;
+	else {
+		transaction = journal->j_running_transaction;
+		if (!transaction)
+			goto out;
+		target = transaction->t_tid;
+	}
+		
+	/*
+	 * Are we already doing a recent enough commit?
+	 */
+	if (tid_geq(journal->j_commit_request, target))
+		goto out;
+
+	/*
+	 * We want a new commit: OK, mark the request and wakup the
+	 * commit thread.  We do _not_ do the commit ourselves.
+	 */
+
+	journal->j_commit_request = target;
+	jbd_debug(1, "JBD: requesting commit %d/%d\n",
+		  journal->j_commit_request,
+		  journal->j_commit_sequence);
+	wake_up(&journal->j_wait_commit);
+
+out:
+	unlock_kernel();
+	return target;
+}
+
+/*
+ * Wait for a specified commit to complete.
+ * The caller may not hold the journal lock.
+ */
+void log_wait_commit (journal_t *journal, tid_t tid)
+{
+	lock_kernel();
+#ifdef CONFIG_JBD_DEBUG
+	lock_journal(journal);
+	if (!tid_geq(journal->j_commit_request, tid)) {
+		printk(KERN_EMERG __FUNCTION__
+			": error: j_commit_request=%d, tid=%d\n",
+			journal->j_commit_request, tid);
+	}
+	unlock_journal(journal);
+#endif
+	while (tid_gt(tid, journal->j_commit_sequence)) {
+		jbd_debug(1, "JBD: want %d, j_commit_sequence=%d\n",
+				  tid, journal->j_commit_sequence);
+		wake_up(&journal->j_wait_commit);
+		sleep_on(&journal->j_wait_done_commit);
+	}
+	unlock_kernel();
+}
+
+/*
+ * Log buffer allocation routines:
+ */
+
+unsigned long journal_next_log_block(journal_t *journal)
+{
+	unsigned long blocknr;
+
+	J_ASSERT(journal->j_free > 1);
+
+	blocknr = journal->j_head;
+	journal->j_head++;
+	journal->j_free--;
+	if (journal->j_head == journal->j_last)
+		journal->j_head = journal->j_first;
+	return journal_bmap(journal, blocknr);
+}
+
+/*
+ * Conversion of logical to physical block numbers for the journal
+ *
+ * On external journals the journal blocks are identity-mapped, so
+ * this is a no-op.  If needed, we can use j_blk_offset - everything is
+ * ready.
+ */
+unsigned long journal_bmap(journal_t *journal, unsigned long blocknr)
+{
+	unsigned long ret;
+
+	if (journal->j_inode) {
+		ret = bmap(journal->j_inode, blocknr);
+		J_ASSERT(ret != 0);
+	} else {
+		ret = blocknr;	 /* +journal->j_blk_offset */
+	}
+	return ret;
+}
+
+/*
+ * We play buffer_head aliasing tricks to write data/metadata blocks to
+ * the journal without copying their contents, but for journal
+ * descriptor blocks we do need to generate bona fide buffers.
+ */
+
+struct journal_head * journal_get_descriptor_buffer(journal_t *journal)
+{
+	struct buffer_head *bh;
+	unsigned long blocknr = journal_next_log_block(journal);
+
+	bh = getblk(journal->j_dev, blocknr, journal->j_blocksize);
+	bh->b_state |= (1 << BH_Dirty);
+	BUFFER_TRACE(bh, "return this buffer");
+	return journal_add_journal_head(bh);
+}
+
+/*
+ * Management for journal control blocks: functions to create and
+ * destroy journal_t structures, and to initialise and read existing
+ * journal blocks from disk.  */
+
+/* First: create and setup a journal_t object in memory.  We initialise
+ * very few fields yet: that has to wait until we have created the
+ * journal structures from from scratch, or loaded them from disk. */
+
+static journal_t * journal_init_common (void)
+{
+	journal_t *journal;
+	int err;
+
+	MOD_INC_USE_COUNT;
+
+	journal = jbd_kmalloc(sizeof(*journal), GFP_KERNEL);
+	if (!journal)
+		goto fail;
+	memset(journal, 0, sizeof(*journal));
+
+	init_waitqueue_head(&journal->j_wait_transaction_locked);
+	init_waitqueue_head(&journal->j_wait_logspace);
+	init_waitqueue_head(&journal->j_wait_done_commit);
+	init_waitqueue_head(&journal->j_wait_checkpoint);
+	init_waitqueue_head(&journal->j_wait_commit);
+	init_waitqueue_head(&journal->j_wait_updates);
+	init_MUTEX(&journal->j_barrier);
+	init_MUTEX(&journal->j_checkpoint_sem);
+	init_MUTEX(&journal->j_sem);
+
+	journal->j_commit_interval = (HZ * 5);
+
+	/* The journal is marked for error until we succeed with recovery! */
+	journal->j_flags = JFS_ABORT;
+
+	/* Set up a default-sized revoke table for the new mount. */
+	err = journal_init_revoke(journal, JOURNAL_REVOKE_DEFAULT_HASH);
+	if (err) {
+		kfree(journal);
+		goto fail;
+	}
+	return journal;
+fail:
+	MOD_DEC_USE_COUNT;
+	return NULL;
+}
+
+/* journal_init_dev and journal_init_inode:
+ *
+ * Create a journal structure assigned some fixed set of disk blocks to
+ * the journal.  We don't actually touch those disk blocks yet, but we
+ * need to set up all of the mapping information to tell the journaling
+ * system where the journal blocks are.
+ *
+ * journal_init_dev creates a journal which maps a fixed contiguous
+ * range of blocks on an arbitrary block device.
+ *
+ * journal_init_inode creates a journal which maps an on-disk inode as
+ * the journal.  The inode must exist already, must support bmap() and
+ * must have all data blocks preallocated.
+ */
+
+journal_t * journal_init_dev(kdev_t dev, kdev_t fs_dev,
+			int start, int len, int blocksize)
+{
+	journal_t *journal = journal_init_common();
+	struct buffer_head *bh;
+
+	if (!journal)
+		return NULL;
+
+	journal->j_dev = dev;
+	journal->j_fs_dev = fs_dev;
+	journal->j_blk_offset = start;
+	journal->j_maxlen = len;
+	journal->j_blocksize = blocksize;
+
+	bh = getblk(journal->j_dev, start, journal->j_blocksize);
+	J_ASSERT(bh != NULL);
+	journal->j_sb_buffer = bh;
+	journal->j_superblock = (journal_superblock_t *)bh->b_data;
+
+	return journal;
+}
+
+journal_t * journal_init_inode (struct inode *inode)
+{
+	struct buffer_head *bh;
+	journal_t *journal = journal_init_common();
+	int blocknr;
+
+	if (!journal)
+		return NULL;
+
+	journal->j_dev = inode->i_dev;
+	journal->j_fs_dev = inode->i_dev;
+	journal->j_inode = inode;
+	jbd_debug(1,
+		  "journal %p: inode %s/%ld, size %Ld, bits %d, blksize %ld\n",
+		  journal, bdevname(inode->i_dev), inode->i_ino, inode->i_size,
+		  inode->i_sb->s_blocksize_bits, inode->i_sb->s_blocksize);
+
+	journal->j_maxlen = inode->i_size >> inode->i_sb->s_blocksize_bits;
+	journal->j_blocksize = inode->i_sb->s_blocksize;
+
+	blocknr = journal_bmap(journal, 0);
+	bh = getblk(journal->j_dev, blocknr, journal->j_blocksize);
+	J_ASSERT(bh != NULL);
+	journal->j_sb_buffer = bh;
+	journal->j_superblock = (journal_superblock_t *)bh->b_data;
+
+	return journal;
+}
+
+/*
+ * Given a journal_t structure, initialise the various fields for
+ * startup of a new journaling session.  We use this both when creating
+ * a journal, and after recovering an old journal to reset it for
+ * subsequent use.
+ */
+
+static int journal_reset (journal_t *journal)
+{
+	journal_superblock_t *sb = journal->j_superblock;
+	unsigned int first, last;
+
+	first = ntohl(sb->s_first);
+	last = ntohl(sb->s_maxlen);
+
+	journal->j_first = first;
+	journal->j_last = last;
+
+	journal->j_head = first;
+	journal->j_tail = first;
+	journal->j_free = last - first;
+
+	journal->j_tail_sequence = journal->j_transaction_sequence;
+	journal->j_commit_sequence = journal->j_transaction_sequence - 1;
+	journal->j_commit_request = journal->j_commit_sequence;
+
+	journal->j_max_transaction_buffers = journal->j_maxlen / 4;
+
+	/* Add the dynamic fields and write it to disk. */
+	journal_update_superblock(journal, 1);
+
+	lock_journal(journal);
+	journal_start_thread(journal);
+	unlock_journal(journal);
+
+	return 0;
+}
+
+/*
+ * Given a journal_t structure which tells us which disk blocks we can
+ * use, create a new journal superblock and initialise all of the
+ * journal fields from scratch.  */
+
+int journal_create (journal_t *journal)
+{
+	int blocknr;
+	struct buffer_head *bh;
+	journal_superblock_t *sb;
+	int i;
+
+	if (journal->j_maxlen < JFS_MIN_JOURNAL_BLOCKS) {
+		printk (KERN_ERR "Journal length (%d blocks) too short.\n",
+			journal->j_maxlen);
+		return -EINVAL;
+	}
+
+	if (journal->j_inode == NULL) {
+		/*
+		 * We don't know what block to start at!
+		 */
+		printk(KERN_EMERG __FUNCTION__
+			": creation of journal on external device!\n");
+		BUG();
+	}
+
+	/* Zero out the entire journal on disk.  We cannot afford to
+	   have any blocks on disk beginning with JFS_MAGIC_NUMBER. */
+	jbd_debug(1, "JBD: Zeroing out journal blocks...\n");
+	for (i = 0; i < journal->j_maxlen; i++) {
+		blocknr = journal_bmap(journal, i);
+		bh = getblk(journal->j_dev, blocknr, journal->j_blocksize);
+		wait_on_buffer(bh);
+		memset (bh->b_data, 0, journal->j_blocksize);
+		BUFFER_TRACE(bh, "marking dirty");
+		mark_buffer_dirty(bh);
+		BUFFER_TRACE(bh, "marking uptodate");
+		mark_buffer_uptodate(bh, 1);
+		__brelse(bh);
+	}
+	sync_dev(journal->j_dev);
+	jbd_debug(1, "JBD: journal cleared.\n");
+
+	/* OK, fill in the initial static fields in the new superblock */
+	sb = journal->j_superblock;
+
+	sb->s_header.h_magic	 = htonl(JFS_MAGIC_NUMBER);
+	sb->s_header.h_blocktype = htonl(JFS_SUPERBLOCK_V2);
+
+	sb->s_blocksize	= htonl(journal->j_blocksize);
+	sb->s_maxlen	= htonl(journal->j_maxlen);
+	sb->s_first	= htonl(1);
+
+	journal->j_transaction_sequence = 1;
+
+	journal->j_flags &= ~JFS_ABORT;
+	journal->j_format_version = 2;
+
+	return journal_reset(journal);
+}
+
+/*
+ * Update a journal's dynamic superblock fields and write it to disk,
+ * optionally waiting for the IO to complete.
+*/
+
+void journal_update_superblock(journal_t *journal, int wait)
+{
+	journal_superblock_t *sb = journal->j_superblock;
+	struct buffer_head *bh = journal->j_sb_buffer;
+
+	jbd_debug(1,"JBD: updating superblock (start %ld, seq %d, errno %d)\n",
+		  journal->j_tail, journal->j_tail_sequence, journal->j_errno);
+
+	sb->s_sequence = htonl(journal->j_tail_sequence);
+	sb->s_start    = htonl(journal->j_tail);
+	sb->s_errno    = htonl(journal->j_errno);
+
+	BUFFER_TRACE(bh, "marking dirty");
+	mark_buffer_dirty(bh);
+	ll_rw_block(WRITE, 1, &bh);
+	if (wait)
+		wait_on_buffer(bh);
+
+	/* If we have just flushed the log (by marking s_start==0), then
+	 * any future commit will have to be careful to update the
+	 * superblock again to re-record the true start of the log. */
+
+	if (sb->s_start)
+		journal->j_flags &= ~JFS_FLUSHED;
+	else
+		journal->j_flags |= JFS_FLUSHED;
+}
+
+
+/*
+ * Read the superblock for a given journal, performing initial
+ * validation of the format.
+ */
+
+static int journal_get_superblock(journal_t *journal)
+{
+	struct buffer_head *bh;
+	journal_superblock_t *sb;
+
+	bh = journal->j_sb_buffer;
+
+	J_ASSERT(bh != NULL);
+	if (!buffer_uptodate(bh)) {
+		ll_rw_block(READ, 1, &bh);
+		wait_on_buffer(bh);
+		if (!buffer_uptodate(bh)) {
+			printk (KERN_ERR
+				"JBD: IO error reading journal superblock\n");
+			return -EIO;
+		}
+	}
+
+	sb = journal->j_superblock;
+
+	if (sb->s_header.h_magic != htonl(JFS_MAGIC_NUMBER) ||
+	    sb->s_blocksize != htonl(journal->j_blocksize)) {
+		printk(KERN_WARNING "JBD: no valid journal superblock found\n");
+		return -EINVAL;
+	}
+
+	switch(ntohl(sb->s_header.h_blocktype)) {
+	case JFS_SUPERBLOCK_V1:
+		journal->j_format_version = 1;
+		break;
+	case JFS_SUPERBLOCK_V2:
+		journal->j_format_version = 2;
+		break;
+	default:
+		printk(KERN_WARNING "JBD: unrecognised superblock format ID\n");
+		return -EINVAL;
+	}
+
+	if (ntohl(sb->s_maxlen) < journal->j_maxlen)
+		journal->j_maxlen = ntohl(sb->s_maxlen);
+	else if (ntohl(sb->s_maxlen) > journal->j_maxlen) {
+		printk (KERN_WARNING "JBD: journal file too short\n");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+/*
+ * Load the on-disk journal superblock and read the key fields into the
+ * journal_t.
+ */
+
+static int load_superblock(journal_t *journal)
+{
+	int err;
+	journal_superblock_t *sb;
+
+	err = journal_get_superblock(journal);
+	if (err)
+		return err;
+
+	sb = journal->j_superblock;
+
+	journal->j_tail_sequence = ntohl(sb->s_sequence);
+	journal->j_tail = ntohl(sb->s_start);
+	journal->j_first = ntohl(sb->s_first);
+	journal->j_last = ntohl(sb->s_maxlen);
+	journal->j_errno = ntohl(sb->s_errno);
+
+	return 0;
+}
+
+
+/*
+ * Given a journal_t structure which tells us which disk blocks contain
+ * a journal, read the journal from disk to initialise the in-memory
+ * structures.
+ */
+
+int journal_load(journal_t *journal)
+{
+	int err;
+
+	err = load_superblock(journal);
+	if (err)
+		return err;
+
+	/* If this is a V2 superblock, then we have to check the
+	 * features flags on it. */
+
+	if (journal->j_format_version >= 2) {
+		journal_superblock_t *sb = journal->j_superblock;
+
+		if ((sb->s_feature_ro_compat &
+		     ~cpu_to_be32(JFS_KNOWN_ROCOMPAT_FEATURES)) ||
+		    (sb->s_feature_incompat &
+		     ~cpu_to_be32(JFS_KNOWN_INCOMPAT_FEATURES))) {
+			printk (KERN_WARNING
+				"JBD: Unrecognised features on journal\n");
+			return -EINVAL;
+		}
+	}
+
+	/* Let the recovery code check whether it needs to recover any
+	 * data from the journal. */
+	if (journal_recover(journal))
+		goto recovery_error;
+
+	/* OK, we've finished with the dynamic journal bits:
+	 * reinitialise the dynamic contents of the superblock in memory
+	 * and reset them on disk. */
+	if (journal_reset(journal))
+		goto recovery_error;
+
+	journal->j_flags &= ~JFS_ABORT;
+	journal->j_flags |= JFS_LOADED;
+	return 0;
+
+recovery_error:
+	printk (KERN_WARNING "JBD: recovery failed\n");
+	return -EIO;
+}
+
+/*
+ * Release a journal_t structure once it is no longer in use by the
+ * journaled object.
+ */
+
+void journal_destroy (journal_t *journal)
+{
+	/* Wait for the commit thread to wake up and die. */
+	journal_kill_thread(journal);
+
+	/* Force a final log commit */
+	if (journal->j_running_transaction)
+		journal_commit_transaction(journal);
+
+	/* Force any old transactions to disk */
+	lock_journal(journal);
+	while (journal->j_checkpoint_transactions != NULL)
+		log_do_checkpoint(journal, 1);
+
+	J_ASSERT(journal->j_running_transaction == NULL);
+	J_ASSERT(journal->j_committing_transaction == NULL);
+	J_ASSERT(journal->j_checkpoint_transactions == NULL);
+
+	/* We can now mark the journal as empty. */
+	journal->j_tail = 0;
+	journal->j_tail_sequence = ++journal->j_transaction_sequence;
+	journal_update_superblock(journal, 1);
+
+	if (journal->j_inode)
+		iput(journal->j_inode);
+	if (journal->j_revoke)
+		journal_destroy_revoke(journal);
+
+	unlock_journal(journal);
+	brelse(journal->j_sb_buffer);
+	kfree(journal);
+	MOD_DEC_USE_COUNT;
+}
+
+
+/* Published API: Check whether the journal uses all of a given set of
+ * features.  Return true (non-zero) if it does. */
+
+int journal_check_used_features (journal_t *journal, unsigned long compat,
+				 unsigned long ro, unsigned long incompat)
+{
+	journal_superblock_t *sb;
+
+	if (!compat && !ro && !incompat)
+		return 1;
+	if (journal->j_format_version == 1)
+		return 0;
+
+	sb = journal->j_superblock;
+
+	if (((be32_to_cpu(sb->s_feature_compat) & compat) == compat) &&
+	    ((be32_to_cpu(sb->s_feature_ro_compat) & ro) == ro) &&
+	    ((be32_to_cpu(sb->s_feature_incompat) & incompat) == incompat))
+		return 1;
+
+	return 0;
+}
+
+/* Published API: Check whether the journaling code supports the use of
+ * all of a given set of features on this journal.  Return true
+ * (non-zero) if it can. */
+
+int journal_check_available_features (journal_t *journal, unsigned long compat,
+				      unsigned long ro, unsigned long incompat)
+{
+	journal_superblock_t *sb;
+
+	if (!compat && !ro && !incompat)
+		return 1;
+
+	sb = journal->j_superblock;
+
+	/* We can support any known requested features iff the
+	 * superblock is in version 2.  Otherwise we fail to support any
+	 * extended sb features. */
+
+	if (journal->j_format_version != 2)
+		return 0;
+
+	if ((compat   & JFS_KNOWN_COMPAT_FEATURES) == compat &&
+	    (ro       & JFS_KNOWN_ROCOMPAT_FEATURES) == ro &&
+	    (incompat & JFS_KNOWN_INCOMPAT_FEATURES) == incompat)
+		return 1;
+
+	return 0;
+}
+
+/* Published API: Mark a given journal feature as present on the
+ * superblock.  Returns true if the requested features could be set. */
+
+int journal_set_features (journal_t *journal, unsigned long compat,
+			  unsigned long ro, unsigned long incompat)
+{
+	journal_superblock_t *sb;
+
+	if (journal_check_used_features(journal, compat, ro, incompat))
+		return 1;
+
+	if (!journal_check_available_features(journal, compat, ro, incompat))
+		return 0;
+
+	jbd_debug(1, "Setting new features 0x%lx/0x%lx/0x%lx\n",
+		  compat, ro, incompat);
+
+	sb = journal->j_superblock;
+
+	sb->s_feature_compat    |= cpu_to_be32(compat);
+	sb->s_feature_ro_compat |= cpu_to_be32(ro);
+	sb->s_feature_incompat  |= cpu_to_be32(incompat);
+
+	return 1;
+}
+
+
+/*
+ * Published API:
+ * Given an initialised but unloaded journal struct, poke about in the
+ * on-disk structure to update it to the most recent supported version.
+ */
+
+int journal_update_format (journal_t *journal)
+{
+	journal_superblock_t *sb;
+	int err;
+
+	err = journal_get_superblock(journal);
+	if (err)
+		return err;
+
+	sb = journal->j_superblock;
+
+	switch (ntohl(sb->s_header.h_blocktype)) {
+	case JFS_SUPERBLOCK_V2:
+		return 0;
+	case JFS_SUPERBLOCK_V1:
+		return journal_convert_superblock_v1(journal, sb);
+	default:
+		break;
+	}
+	return -EINVAL;
+}
+
+static int journal_convert_superblock_v1(journal_t *journal,
+					 journal_superblock_t *sb)
+{
+	int offset, blocksize;
+	struct buffer_head *bh;
+
+	printk(KERN_WARNING
+		"JBD: Converting superblock from version 1 to 2.\n");
+
+	/* Pre-initialise new fields to zero */
+	offset = ((char *) &(sb->s_feature_compat)) - ((char *) sb);
+	blocksize = ntohl(sb->s_blocksize);
+	memset(&sb->s_feature_compat, 0, blocksize-offset);
+
+	sb->s_nr_users = cpu_to_be32(1);
+	sb->s_header.h_blocktype = cpu_to_be32(JFS_SUPERBLOCK_V2);
+	journal->j_format_version = 2;
+
+	bh = journal->j_sb_buffer;
+	BUFFER_TRACE(bh, "marking dirty");
+	mark_buffer_dirty(bh);
+	ll_rw_block(WRITE, 1, &bh);
+	wait_on_buffer(bh);
+	return 0;
+}
+
+
+/*
+ * Flush all data for a given journal to disk and empty the journal.
+ * Filesystems can use this when remounting readonly to ensure that
+ * recovery does not need to happen on remount.
+ */
+
+int journal_flush (journal_t *journal)
+{
+	int err = 0;
+	transaction_t *transaction = NULL;
+	unsigned long old_tail;
+
+	lock_kernel();
+	
+	/* Force everything buffered to the log... */
+	if (journal->j_running_transaction) {
+		transaction = journal->j_running_transaction;
+		log_start_commit(journal, transaction);
+	} else if (journal->j_committing_transaction)
+		transaction = journal->j_committing_transaction;
+
+	/* Wait for the log commit to complete... */
+	if (transaction)
+		log_wait_commit(journal, transaction->t_tid);
+
+	/* ...and flush everything in the log out to disk. */
+	lock_journal(journal);
+	while (!err && journal->j_checkpoint_transactions != NULL)
+		err = log_do_checkpoint(journal, journal->j_maxlen);
+	cleanup_journal_tail(journal);
+
+	/* Finally, mark the journal as really needing no recovery.
+	 * This sets s_start==0 in the underlying superblock, which is
+	 * the magic code for a fully-recovered superblock.  Any future
+	 * commits of data to the journal will restore the current
+	 * s_start value. */
+	old_tail = journal->j_tail;
+	journal->j_tail = 0;
+	journal_update_superblock(journal, 1);
+	journal->j_tail = old_tail;
+
+	unlock_journal(journal);
+
+	J_ASSERT(!journal->j_running_transaction);
+	J_ASSERT(!journal->j_committing_transaction);
+	J_ASSERT(!journal->j_checkpoint_transactions);
+	J_ASSERT(journal->j_head == journal->j_tail);
+	J_ASSERT(journal->j_tail_sequence == journal->j_transaction_sequence);
+
+	unlock_kernel();
+	
+	return err;
+}
+
+/*
+ * Wipe out all of the contents of a journal, safely.  This will produce
+ * a warning if the journal contains any valid recovery information.
+ * Must be called between journal_init_*() and journal_load().
+ *
+ * If (write) is non-zero, then we wipe out the journal on disk; otherwise
+ * we merely suppress recovery.
+ */
+
+int journal_wipe (journal_t *journal, int write)
+{
+	journal_superblock_t *sb;
+	int err = 0;
+
+	J_ASSERT (!(journal->j_flags & JFS_LOADED));
+
+	err = load_superblock(journal);
+	if (err)
+		return err;
+
+	sb = journal->j_superblock;
+
+	if (!journal->j_tail)
+		goto no_recovery;
+
+	printk (KERN_WARNING "JBD: %s recovery information on journal\n",
+		write ? "Clearing" : "Ignoring");
+
+	err = journal_skip_recovery(journal);
+	if (write)
+		journal_update_superblock(journal, 1);
+
+ no_recovery:
+	return err;
+}
+
+/*
+ * journal_dev_name: format a character string to describe on what
+ * device this journal is present.
+ */
+
+const char * journal_dev_name(journal_t *journal)
+{
+	kdev_t dev;
+
+	if (journal->j_inode)
+		dev = journal->j_inode->i_dev;
+	else
+		dev = journal->j_dev;
+
+	return bdevname(dev);
+}
+
+/*
+ * journal_abort: perform a complete, immediate shutdown of the ENTIRE
+ * journal (not of a single transaction).  This operation cannot be
+ * undone without closing and reopening the journal.
+ *
+ * The journal_abort function is intended to support higher level error
+ * recovery mechanisms such as the ext2/ext3 remount-readonly error
+ * mode.
+ *
+ * Journal abort has very specific semantics.  Any existing dirty,
+ * unjournaled buffers in the main filesystem will still be written to
+ * disk by bdflush, but the journaling mechanism will be suspended
+ * immediately and no further transaction commits will be honoured.
+ *
+ * Any dirty, journaled buffers will be written back to disk without
+ * hitting the journal.  Atomicity cannot be guaranteed on an aborted
+ * filesystem, but we _do_ attempt to leave as much data as possible
+ * behind for fsck to use for cleanup.
+ *
+ * Any attempt to get a new transaction handle on a journal which is in
+ * ABORT state will just result in an -EROFS error return.  A
+ * journal_stop on an existing handle will return -EIO if we have
+ * entered abort state during the update.
+ *
+ * Recursive transactions are not disturbed by journal abort until the
+ * final journal_stop, which will receive the -EIO error.
+ *
+ * Finally, the journal_abort call allows the caller to supply an errno
+ * which will be recored (if possible) in the journal superblock.  This
+ * allows a client to record failure conditions in the middle of a
+ * transaction without having to complete the transaction to record the
+ * failure to disk.  ext3_error, for example, now uses this
+ * functionality.
+ *
+ * Errors which originate from within the journaling layer will NOT
+ * supply an errno; a null errno implies that absolutely no further
+ * writes are done to the journal (unless there are any already in
+ * progress).
+ */
+
+/* Quick version for internal journal use (doesn't lock the journal) */
+void __journal_abort (journal_t *journal)
+{
+	transaction_t *transaction;
+
+	printk (KERN_ERR "Aborting journal on device %s.\n",
+		journal_dev_name(journal));
+
+	journal->j_flags |= JFS_ABORT;
+	transaction = journal->j_running_transaction;
+	if (transaction)
+		log_start_commit(journal, transaction);
+}
+
+/* Full version for external use */
+void journal_abort (journal_t *journal, int errno)
+{
+	lock_journal(journal);
+
+	if (journal->j_flags & JFS_ABORT)
+		goto out;
+
+	if (!journal->j_errno)
+		journal->j_errno = errno;
+
+	__journal_abort(journal);
+
+	if (errno)
+		journal_update_superblock(journal, 1);
+
+ out:
+	unlock_journal(journal);
+}
+
+int journal_errno (journal_t *journal)
+{
+	int err;
+
+	lock_journal(journal);
+	if (journal->j_flags & JFS_ABORT)
+		err = -EROFS;
+	else
+		err = journal->j_errno;
+	unlock_journal(journal);
+	return err;
+}
+
+int journal_clear_err (journal_t *journal)
+{
+	int err = 0;
+
+	lock_journal(journal);
+	if (journal->j_flags & JFS_ABORT)
+		err = -EROFS;
+	else
+		journal->j_errno = 0;
+	unlock_journal(journal);
+	return err;
+}
+
+void journal_ack_err (journal_t *journal)
+{
+	lock_journal(journal);
+	if (journal->j_errno)
+		journal->j_flags |= JFS_ACK_ERR;
+	unlock_journal(journal);
+}
+
+int journal_blocks_per_page(struct inode *inode)
+{
+	return 1 << (PAGE_CACHE_SHIFT - inode->i_sb->s_blocksize_bits);
+}
+
+/*
+ * shrink_journal_memory().
+ * Called when we're under memory pressure.  Free up all the written-back
+ * checkpointed metadata buffers.
+ */
+void shrink_journal_memory(void)
+{
+	struct list_head *list;
+
+	lock_kernel();
+	list_for_each(list, &all_journals) {
+		journal_t *journal =
+			list_entry(list, journal_t, j_all_journals);
+		spin_lock(&journal_datalist_lock);
+		__journal_clean_checkpoint_list(journal);
+		spin_unlock(&journal_datalist_lock);
+	}
+	unlock_kernel();
+}
+
+/*
+ * Simple support for retying memory allocations.  Introduced to help to
+ * debug different VM deadlock avoidance strategies. 
+ */
+/*
+ * Simple support for retying memory allocations.  Introduced to help to
+ * debug different VM deadlock avoidance strategies. 
+ */
+void * __jbd_kmalloc (char *where, size_t size, int flags, int retry)
+{
+	void *p;
+	static unsigned long last_warning;
+	
+	while (1) {
+		p = kmalloc(size, flags);
+		if (p)
+			return p;
+		if (!retry)
+			return NULL;
+		/* Log every retry for debugging.  Also log them to the
+		 * syslog, but do rate-limiting on the non-debugging
+		 * messages. */
+		jbd_debug(1, "ENOMEM in %s, retrying.\n", where);
+
+		if (time_after(jiffies, last_warning + 5*HZ)) {
+			printk(KERN_NOTICE
+			       "ENOMEM in %s, retrying.\n", where);
+			last_warning = jiffies;
+		}
+		
+		current->policy |= SCHED_YIELD;
+		schedule();
+	}
+}
+
+/*
+ * Journal_head storage management
+ */
+static kmem_cache_t *journal_head_cache;
+#ifdef CONFIG_JBD_DEBUG
+static atomic_t nr_journal_heads = ATOMIC_INIT(0);
+#endif
+
+static int journal_init_journal_head_cache(void)
+{
+	int retval;
+
+	J_ASSERT(journal_head_cache == 0);
+	journal_head_cache = kmem_cache_create("journal_head",
+				sizeof(struct journal_head),
+				0,		/* offset */
+				0,		/* flags */
+				NULL,		/* ctor */
+				NULL);		/* dtor */
+	retval = 0;
+	if (journal_head_cache == 0) {
+		retval = -ENOMEM;
+		printk(KERN_EMERG "JBD: no memory for journal_head cache\n");
+	}
+	return retval;
+}
+
+static void journal_destroy_journal_head_cache(void)
+{
+	J_ASSERT(journal_head_cache != NULL);
+	kmem_cache_destroy(journal_head_cache);
+	journal_head_cache = 0;
+}
+
+/*
+ * journal_head splicing and dicing
+ */
+static struct journal_head *journal_alloc_journal_head(void)
+{
+	struct journal_head *ret;
+	static unsigned long last_warning;
+
+#ifdef CONFIG_JBD_DEBUG
+	atomic_inc(&nr_journal_heads);
+#endif
+	ret = kmem_cache_alloc(journal_head_cache, GFP_NOFS);
+	if (ret == 0) {
+		jbd_debug(1, "out of memory for journal_head\n");
+		if (time_after(jiffies, last_warning + 5*HZ)) {
+			printk(KERN_NOTICE "ENOMEM in " __FUNCTION__
+			       ", retrying.\n");
+			last_warning = jiffies;
+		}
+		while (ret == 0) {
+			current->policy |= SCHED_YIELD;
+			schedule();
+			ret = kmem_cache_alloc(journal_head_cache, GFP_NOFS);
+		}
+	}
+	return ret;
+}
+
+static void journal_free_journal_head(struct journal_head *jh)
+{
+#ifdef CONFIG_JBD_DEBUG
+	atomic_dec(&nr_journal_heads);
+	memset(jh, 0x5b, sizeof(*jh));
+#endif
+	kmem_cache_free(journal_head_cache, jh);
+}
+
+/*
+ * A journal_head is attached to a buffer_head whenever JBD has an
+ * interest in the buffer.
+ *
+ * Whenever a buffer has an attached journal_head, its ->b_state:BH_JBD bit
+ * is set.  This bit is tested in core kernel code where we need to take
+ * JBD-specific actions.  Testing the zeroness of ->b_private is not reliable
+ * there.
+ *
+ * When a buffer has its BH_JBD bit set, its ->b_count is elevated by one.
+ *
+ * When a buffer has its BH_JBD bit set it is immune from being released by
+ * core kernel code, mainly via ->b_count.
+ *
+ * A journal_head may be detached from its buffer_head when the journal_head's
+ * b_transaction, b_cp_transaction and b_next_transaction pointers are NULL.
+ * Various places in JBD call journal_remove_journal_head() to indicate that the
+ * journal_head can be dropped if needed.
+ *
+ * Various places in the kernel want to attach a journal_head to a buffer_head
+ * _before_ attaching the journal_head to a transaction.  To protect the
+ * journal_head in this situation, journal_add_journal_head elevates the
+ * journal_head's b_jcount refcount by one.  The caller must call
+ * journal_unlock_journal_head() to undo this.
+ *
+ * So the typical usage would be:
+ *
+ *	(Attach a journal_head if needed.  Increments b_jcount)
+ *	struct journal_head *jh = journal_add_journal_head(bh);
+ *	...
+ *	jh->b_transaction = xxx;
+ *	journal_unlock_journal_head(jh);
+ *
+ * Now, the journal_head's b_jcount is zero, but it is safe from being released
+ * because it has a non-zero b_transaction.
+ */
+
+/*
+ * Give a buffer_head a journal_head.
+ *
+ * Doesn't need the journal lock.
+ * May sleep.
+ * Cannot be called with journal_datalist_lock held.
+ */
+struct journal_head *journal_add_journal_head(struct buffer_head *bh)
+{
+	struct journal_head *jh;
+
+	spin_lock(&journal_datalist_lock);
+	if (buffer_jbd(bh)) {
+		jh = bh2jh(bh);
+	} else {
+		J_ASSERT_BH(bh,
+			(atomic_read(&bh->b_count) > 0) ||
+			(bh->b_page && bh->b_page->mapping));
+		spin_unlock(&journal_datalist_lock);
+		jh = journal_alloc_journal_head();
+		memset(jh, 0, sizeof(*jh));
+		spin_lock(&journal_datalist_lock);
+
+		if (buffer_jbd(bh)) {
+			/* Someone did it for us! */
+			J_ASSERT_BH(bh, bh->b_private != NULL);
+			journal_free_journal_head(jh);
+			jh = bh->b_private;
+		} else {
+			/*
+			 * We actually don't need jh_splice_lock when
+			 * adding a journal_head - only on removal.
+			 */
+			spin_lock(&jh_splice_lock);
+			set_bit(BH_JBD, &bh->b_state);
+			bh->b_private = jh;
+			jh->b_bh = bh;
+			atomic_inc(&bh->b_count);
+			spin_unlock(&jh_splice_lock);
+			BUFFER_TRACE(bh, "added journal_head");
+		}
+	}
+	jh->b_jcount++;
+	spin_unlock(&journal_datalist_lock);
+	return bh->b_private;
+}
+
+/*
+ * journal_remove_journal_head(): if the buffer isn't attached to a transaction
+ * and has a zero b_jcount then remove and release its journal_head.   If we did
+ * see that the buffer is not used by any transaction we also "logically"
+ * decrement ->b_count.
+ *
+ * We in fact take an additional increment on ->b_count as a convenience,
+ * because the caller usually wants to do additional things with the bh
+ * after calling here.
+ * The caller of journal_remove_journal_head() *must* run __brelse(bh) at some
+ * time.  Once the caller has run __brelse(), the buffer is eligible for
+ * reaping by try_to_free_buffers().
+ *
+ * Requires journal_datalist_lock.
+ */
+void __journal_remove_journal_head(struct buffer_head *bh)
+{
+	struct journal_head *jh = bh2jh(bh);
+
+	assert_spin_locked(&journal_datalist_lock);
+	J_ASSERT_JH(jh, jh->b_jcount >= 0);
+	atomic_inc(&bh->b_count);
+	if (jh->b_jcount == 0) {
+		if (jh->b_transaction == NULL &&
+				jh->b_next_transaction == NULL &&
+				jh->b_cp_transaction == NULL) {
+			J_ASSERT_BH(bh, buffer_jbd(bh));
+			J_ASSERT_BH(bh, jh2bh(jh) == bh);
+			BUFFER_TRACE(bh, "remove journal_head");
+			spin_lock(&jh_splice_lock);
+			bh->b_private = NULL;
+			jh->b_bh = NULL;	/* debug, really */
+			clear_bit(BH_JBD, &bh->b_state);
+			__brelse(bh);
+			spin_unlock(&jh_splice_lock);
+			journal_free_journal_head(jh);
+		} else {
+			BUFFER_TRACE(bh, "journal_head was locked");
+		}
+	}
+}
+
+void journal_unlock_journal_head(struct journal_head *jh)
+{
+	spin_lock(&journal_datalist_lock);
+	J_ASSERT_JH(jh, jh->b_jcount > 0);
+	--jh->b_jcount;
+	if (!jh->b_jcount && !jh->b_transaction) {
+		struct buffer_head *bh;
+		bh = jh2bh(jh);
+		__journal_remove_journal_head(bh);
+		__brelse(bh);
+	}
+	
+	spin_unlock(&journal_datalist_lock);
+}
+
+void journal_remove_journal_head(struct buffer_head *bh)
+{
+	spin_lock(&journal_datalist_lock);
+	__journal_remove_journal_head(bh);
+	spin_unlock(&journal_datalist_lock);
+}
+
+/*
+ * /proc tunables
+ */
+#if defined(CONFIG_JBD_DEBUG)
+int journal_enable_debug;
+EXPORT_SYMBOL(journal_enable_debug);
+#endif
+
+#if defined(CONFIG_JBD_DEBUG) && defined(CONFIG_PROC_FS)
+
+static struct proc_dir_entry *proc_jbd_debug;
+
+int read_jbd_debug(char *page, char **start, off_t off,
+			  int count, int *eof, void *data)
+{
+	int ret;
+
+	ret = sprintf(page + off, "%d\n", journal_enable_debug);
+	*eof = 1;
+	return ret;
+}
+
+int write_jbd_debug(struct file *file, const char *buffer,
+			   unsigned long count, void *data)
+{
+	char buf[32];
+
+	if (count > ARRAY_SIZE(buf) - 1)
+		count = ARRAY_SIZE(buf) - 1;
+	if (copy_from_user(buf, buffer, count))
+		return -EFAULT;
+	buf[ARRAY_SIZE(buf) - 1] = '\0';
+	journal_enable_debug = simple_strtoul(buf, NULL, 10);
+	return count;
+}
+
+#define JBD_PROC_NAME "sys/fs/jbd-debug"
+
+static void __init create_jbd_proc_entry(void)
+{
+	proc_jbd_debug = create_proc_entry(JBD_PROC_NAME, 0644, NULL);
+	if (proc_jbd_debug) {
+		/* Why is this so hard? */
+		proc_jbd_debug->read_proc = read_jbd_debug;
+		proc_jbd_debug->write_proc = write_jbd_debug;
+	}
+}
+
+static void __exit remove_jbd_proc_entry(void)
+{
+	if (proc_jbd_debug)
+		remove_proc_entry(JBD_PROC_NAME, NULL);
+}
+
+#else
+
+#define create_jbd_proc_entry() do {} while (0)
+#define remove_jbd_proc_entry() do {} while (0)
+
+#endif
+
+/*
+ * Module startup and shutdown
+ */
+
+static int __init journal_init_caches(void)
+{
+	int ret;
+
+	ret = journal_init_revoke_caches();
+	if (ret == 0)
+		ret = journal_init_journal_head_cache();
+	return ret;
+}
+
+static void journal_destroy_caches(void)
+{
+	journal_destroy_revoke_caches();
+	journal_destroy_journal_head_cache();
+}
+
+static int __init journal_init(void)
+{
+	int ret;
+
+	printk(KERN_INFO "Journalled Block Device driver loaded\n");
+	ret = journal_init_caches();
+	if (ret != 0)
+		journal_destroy_caches();
+	create_jbd_proc_entry();
+	return ret;
+}
+
+static void __exit journal_exit(void)
+{
+#ifdef CONFIG_JBD_DEBUG
+	int n = atomic_read(&nr_journal_heads);
+	if (n)
+		printk(KERN_EMERG "JBD: leaked %d journal_heads!\n", n);
+#endif
+	remove_jbd_proc_entry();
+	journal_destroy_caches();
+}
+
+MODULE_LICENSE("GPL");
+module_init(journal_init);
+module_exit(journal_exit);
+
FUNET's LINUX-ADM group, linux-adm@nic.funet.fi
TCL-scripts by Sam Shen (who was at: slshen@lbl.gov)