fs/jbd2/recovery.c - kernel/msm-4.9 - Gitiles

 /*
  * linux/fs/jbd2/recovery.c
  *
  * Written by Stephen C. Tweedie <sct@redhat.com>, 1999
  *
  * Copyright 1999-2000 Red Hat Software --- 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.
  *
  * Journal recovery routines for the generic filesystem journaling code;
  * part of the ext2fs journaling system.
  */

 #ifndef __KERNEL__
 #include "jfs_user.h"
 #else
 #include <linux/time.h>
 #include <linux/fs.h>
 #include <linux/jbd2.h>
 #include <linux/errno.h>
 #include <linux/slab.h>
 #endif

 /*
  * Maintain information about the progress of the recovery job, so that
  * the different passes can carry information between them.
  */
 struct recovery_info
 {
 	tid_t		start_transaction;
 	tid_t		end_transaction;

 	int		nr_replays;
 	int		nr_revokes;
 	int		nr_revoke_hits;
 };

 enum passtype {PASS_SCAN, PASS_REVOKE, PASS_REPLAY};
 static int do_one_pass(journal_t *journal,
 				struct recovery_info *info, enum passtype pass);
 static int scan_revoke_records(journal_t *, struct buffer_head *,
 				tid_t, struct recovery_info *);

 #ifdef __KERNEL__

 /* Release readahead buffers after use */
 static void journal_brelse_array(struct buffer_head *b[], int n)
 {
 	while (--n >= 0)
 		brelse (b[n]);
 }


 /*
  * When reading from the journal, we are going through the block device
  * layer directly and so there is no readahead being done for us.  We
  * need to implement any readahead ourselves if we want it to happen at
  * all.  Recovery is basically one long sequential read, so make sure we
  * do the IO in reasonably large chunks.
  *
  * This is not so critical that we need to be enormously clever about
  * the readahead size, though.  128K is a purely arbitrary, good-enough
  * fixed value.
  */

 #define MAXBUF 8
 static int do_readahead(journal_t *journal, unsigned int start)
 {
 	int err;
 	unsigned int max, nbufs, next;
 	unsigned long long blocknr;
 	struct buffer_head *bh;

 	struct buffer_head * bufs[MAXBUF];

 	/* Do up to 128K of readahead */
 	max = start + (128 * 1024 / journal->j_blocksize);
 	if (max > journal->j_maxlen)
 		max = journal->j_maxlen;

 	/* Do the readahead itself.  We'll submit MAXBUF buffer_heads at
 	 * a time to the block device IO layer. */

 	nbufs = 0;

 	for (next = start; next < max; next++) {
 		err = jbd2_journal_bmap(journal, next, &blocknr);

 		if (err) {
 			printk (KERN_ERR "JBD: bad block at offset %u\n",
 				next);
 			goto failed;
 		}

 		bh = __getblk(journal->j_dev, blocknr, journal->j_blocksize);
 		if (!bh) {
 			err = -ENOMEM;
 			goto failed;
 		}

 		if (!buffer_uptodate(bh) && !buffer_locked(bh)) {
 			bufs[nbufs++] = bh;
 			if (nbufs == MAXBUF) {
 				ll_rw_block(READ, nbufs, bufs);
 				journal_brelse_array(bufs, nbufs);
 				nbufs = 0;
 			}
 		} else
 			brelse(bh);
 	}

 	if (nbufs)
 		ll_rw_block(READ, nbufs, bufs);
 	err = 0;

 failed:
 	if (nbufs)
 		journal_brelse_array(bufs, nbufs);
 	return err;
 }

 #endif /* __KERNEL__ */


 /*
  * Read a block from the journal
  */

 static int jread(struct buffer_head **bhp, journal_t *journal,
 		 unsigned int offset)
 {
 	int err;
 	unsigned long long blocknr;
 	struct buffer_head *bh;

 	*bhp = NULL;

 	if (offset >= journal->j_maxlen) {
 		printk(KERN_ERR "JBD: corrupted journal superblock\n");
 		return -EIO;
 	}

 	err = jbd2_journal_bmap(journal, offset, &blocknr);

 	if (err) {
 		printk (KERN_ERR "JBD: bad block at offset %u\n",
 			offset);
 		return err;
 	}

 	bh = __getblk(journal->j_dev, blocknr, journal->j_blocksize);
 	if (!bh)
 		return -ENOMEM;

 	if (!buffer_uptodate(bh)) {
 		/* If this is a brand new buffer, start readahead.
                    Otherwise, we assume we are already reading it.  */
 		if (!buffer_req(bh))
 			do_readahead(journal, offset);
 		wait_on_buffer(bh);
 	}

 	if (!buffer_uptodate(bh)) {
 		printk (KERN_ERR "JBD: Failed to read block at offset %u\n",
 			offset);
 		brelse(bh);
 		return -EIO;
 	}

 	*bhp = bh;
 	return 0;
 }


 /*
  * Count the number of in-use tags in a journal descriptor block.
  */

 static int count_tags(journal_t *journal, struct buffer_head *bh)
 {
 	char *			tagp;
 	journal_block_tag_t *	tag;
 	int			nr = 0, size = journal->j_blocksize;
 	int			tag_bytes = journal_tag_bytes(journal);

 	tagp = &bh->b_data[sizeof(journal_header_t)];

 	while ((tagp - bh->b_data + tag_bytes) <= size) {
 		tag = (journal_block_tag_t *) tagp;

 		nr++;
 		tagp += tag_bytes;
 		if (!(tag->t_flags & cpu_to_be32(JBD2_FLAG_SAME_UUID)))
 			tagp += 16;

 		if (tag->t_flags & cpu_to_be32(JBD2_FLAG_LAST_TAG))
 			break;
 	}

 	return nr;
 }


 /* Make sure we wrap around the log correctly! */
 #define wrap(journal, var)						\
 do {									\
 	if (var >= (journal)->j_last)					\
 		var -= ((journal)->j_last - (journal)->j_first);	\
 } while (0)

 /**
  * jbd2_journal_recover - recovers a on-disk journal
  * @journal: the journal to recover
  *
  * The primary function for recovering the log contents when mounting a
  * journaled device.
  *
  * Recovery is done in three passes.  In the first pass, we look for the
  * end of the log.  In the second, we assemble the list of revoke
  * blocks.  In the third and final pass, we replay any un-revoked blocks
  * in the log.
  */
 int jbd2_journal_recover(journal_t *journal)
 {
 	int			err;
 	journal_superblock_t *	sb;

 	struct recovery_info	info;

 	memset(&info, 0, sizeof(info));
 	sb = journal->j_superblock;

 	/*
 	 * The journal superblock's s_start field (the current log head)
 	 * is always zero if, and only if, the journal was cleanly
 	 * unmounted.
 	 */

 	if (!sb->s_start) {
 		jbd_debug(1, "No recovery required, last transaction %d\n",
 			  be32_to_cpu(sb->s_sequence));
 		journal->j_transaction_sequence = be32_to_cpu(sb->s_sequence) + 1;
 		return 0;
 	}

 	err = do_one_pass(journal, &info, PASS_SCAN);
 	if (!err)
 		err = do_one_pass(journal, &info, PASS_REVOKE);
 	if (!err)
 		err = do_one_pass(journal, &info, PASS_REPLAY);

 	jbd_debug(1, "JBD: recovery, exit status %d, "
 		  "recovered transactions %u to %u\n",
 		  err, info.start_transaction, info.end_transaction);
 	jbd_debug(1, "JBD: Replayed %d and revoked %d/%d blocks\n",
 		  info.nr_replays, info.nr_revoke_hits, info.nr_revokes);

 	/* Restart the log at the next transaction ID, thus invalidating
 	 * any existing commit records in the log. */
 	journal->j_transaction_sequence = ++info.end_transaction;

 	jbd2_journal_clear_revoke(journal);
 	sync_blockdev(journal->j_fs_dev);
 	return err;
 }

 /**
  * jbd2_journal_skip_recovery - Start journal and wipe exiting records
  * @journal: journal to startup
  *
  * Locate any valid recovery information from the journal and set up the
  * journal structures in memory to ignore it (presumably because the
  * caller has evidence that it is out of date).
  * This function does'nt appear to be exorted..
  *
  * We perform one pass over the journal to allow us to tell the user how
  * much recovery information is being erased, and to let us initialise
  * the journal transaction sequence numbers to the next unused ID.
  */
 int jbd2_journal_skip_recovery(journal_t *journal)
 {
 	int			err;
 	journal_superblock_t *	sb;

 	struct recovery_info	info;

 	memset (&info, 0, sizeof(info));
 	sb = journal->j_superblock;

 	err = do_one_pass(journal, &info, PASS_SCAN);

 	if (err) {
 		printk(KERN_ERR "JBD: error %d scanning journal\n", err);
 		++journal->j_transaction_sequence;
 	} else {
 #ifdef CONFIG_JBD2_DEBUG
 		int dropped = info.end_transaction - be32_to_cpu(sb->s_sequence);
 #endif
 		jbd_debug(1,
 			  "JBD: ignoring %d transaction%s from the journal.\n",
 			  dropped, (dropped == 1) ? "" : "s");
 		journal->j_transaction_sequence = ++info.end_transaction;
 	}

 	journal->j_tail = 0;
 	return err;
 }

 static inline unsigned long long read_tag_block(int tag_bytes, journal_block_tag_t *tag)
 {
 	unsigned long long block = be32_to_cpu(tag->t_blocknr);
 	if (tag_bytes > JBD_TAG_SIZE32)
 		block |= (u64)be32_to_cpu(tag->t_blocknr_high) << 32;
 	return block;
 }

 static int do_one_pass(journal_t *journal,
 			struct recovery_info *info, enum passtype pass)
 {
 	unsigned int		first_commit_ID, next_commit_ID;
 	unsigned long		next_log_block;
 	int			err, success = 0;
 	journal_superblock_t *	sb;
 	journal_header_t *	tmp;
 	struct buffer_head *	bh;
 	unsigned int		sequence;
 	int			blocktype;
 	int			tag_bytes = journal_tag_bytes(journal);

 	/* Precompute the maximum metadata descriptors in a descriptor block */
 	int			MAX_BLOCKS_PER_DESC;
 	MAX_BLOCKS_PER_DESC = ((journal->j_blocksize-sizeof(journal_header_t))
 			       / tag_bytes);

 	/*
 	 * First thing is to establish what we expect to find in the log
 	 * (in terms of transaction IDs), and where (in terms of log
 	 * block offsets): query the superblock.
 	 */

 	sb = journal->j_superblock;
 	next_commit_ID = be32_to_cpu(sb->s_sequence);
 	next_log_block = be32_to_cpu(sb->s_start);

 	first_commit_ID = next_commit_ID;
 	if (pass == PASS_SCAN)
 		info->start_transaction = first_commit_ID;

 	jbd_debug(1, "Starting recovery pass %d\n", pass);

 	/*
 	 * Now we walk through the log, transaction by transaction,
 	 * making sure that each transaction has a commit block in the
 	 * expected place.  Each complete transaction gets replayed back
 	 * into the main filesystem.
 	 */

 	while (1) {
 		int			flags;
 		char *			tagp;
 		journal_block_tag_t *	tag;
 		struct buffer_head *	obh;
 		struct buffer_head *	nbh;

 		cond_resched();		/* We're under lock_kernel() */

 		/* If we already know where to stop the log traversal,
 		 * check right now that we haven't gone past the end of
 		 * the log. */

 		if (pass != PASS_SCAN)
 			if (tid_geq(next_commit_ID, info->end_transaction))
 				break;

 		jbd_debug(2, "Scanning for sequence ID %u at %lu/%lu\n",
 			  next_commit_ID, next_log_block, journal->j_last);

 		/* Skip over each chunk of the transaction looking
 		 * either the next descriptor block or the final commit
 		 * record. */

 		jbd_debug(3, "JBD: checking block %ld\n", next_log_block);
 		err = jread(&bh, journal, next_log_block);
 		if (err)
 			goto failed;

 		next_log_block++;
 		wrap(journal, next_log_block);

 		/* What kind of buffer is it?
 		 *
 		 * If it is a descriptor block, check that it has the
 		 * expected sequence number.  Otherwise, we're all done
 		 * here. */

 		tmp = (journal_header_t *)bh->b_data;

 		if (tmp->h_magic != cpu_to_be32(JBD2_MAGIC_NUMBER)) {
 			brelse(bh);
 			break;
 		}

 		blocktype = be32_to_cpu(tmp->h_blocktype);
 		sequence = be32_to_cpu(tmp->h_sequence);
 		jbd_debug(3, "Found magic %d, sequence %d\n",
 			  blocktype, sequence);

 		if (sequence != next_commit_ID) {
 			brelse(bh);
 			break;
 		}

 		/* OK, we have a valid descriptor block which matches
 		 * all of the sequence number checks.  What are we going
 		 * to do with it?  That depends on the pass... */

 		switch(blocktype) {
 		case JBD2_DESCRIPTOR_BLOCK:
 			/* If it is a valid descriptor block, replay it
 			 * in pass REPLAY; otherwise, just skip over the
 			 * blocks it describes. */
 			if (pass != PASS_REPLAY) {
 				next_log_block += count_tags(journal, bh);
 				wrap(journal, next_log_block);
 				brelse(bh);
 				continue;
 			}

 			/* A descriptor block: we can now write all of
 			 * the data blocks.  Yay, useful work is finally
 			 * getting done here! */

 			tagp = &bh->b_data[sizeof(journal_header_t)];
 			while ((tagp - bh->b_data + tag_bytes)
 			       <= journal->j_blocksize) {
 				unsigned long io_block;

 				tag = (journal_block_tag_t *) tagp;
 				flags = be32_to_cpu(tag->t_flags);

 				io_block = next_log_block++;
 				wrap(journal, next_log_block);
 				err = jread(&obh, journal, io_block);
 				if (err) {
 					/* Recover what we can, but
 					 * report failure at the end. */
 					success = err;
 					printk (KERN_ERR
 						"JBD: IO error %d recovering "
 						"block %ld in log\n",
 						err, io_block);
 				} else {
 					unsigned long long blocknr;

 					J_ASSERT(obh != NULL);
 					blocknr = read_tag_block(tag_bytes,
 								 tag);

 					/* If the block has been
 					 * revoked, then we're all done
 					 * here. */
 					if (jbd2_journal_test_revoke
 					    (journal, blocknr,
 					     next_commit_ID)) {
 						brelse(obh);
 						++info->nr_revoke_hits;
 						goto skip_write;
 					}

 					/* Find a buffer for the new
 					 * data being restored */
 					nbh = __getblk(journal->j_fs_dev,
 							blocknr,
 							journal->j_blocksize);
 					if (nbh == NULL) {
 						printk(KERN_ERR
 						       "JBD: Out of memory "
 						       "during recovery.\n");
 						err = -ENOMEM;
 						brelse(bh);
 						brelse(obh);
 						goto failed;
 					}

 					lock_buffer(nbh);
 					memcpy(nbh->b_data, obh->b_data,
 							journal->j_blocksize);
 					if (flags & JBD2_FLAG_ESCAPE) {
 						*((__be32 *)bh->b_data) =
 						cpu_to_be32(JBD2_MAGIC_NUMBER);
 					}

 					BUFFER_TRACE(nbh, "marking dirty");
 					set_buffer_uptodate(nbh);
 					mark_buffer_dirty(nbh);
 					BUFFER_TRACE(nbh, "marking uptodate");
 					++info->nr_replays;
 					/* ll_rw_block(WRITE, 1, &nbh); */
 					unlock_buffer(nbh);
 					brelse(obh);
 					brelse(nbh);
 				}

 			skip_write:
 				tagp += tag_bytes;
 				if (!(flags & JBD2_FLAG_SAME_UUID))
 					tagp += 16;

 				if (flags & JBD2_FLAG_LAST_TAG)
 					break;
 			}

 			brelse(bh);
 			continue;

 		case JBD2_COMMIT_BLOCK:
 			/* Found an expected commit block: not much to
 			 * do other than move on to the next sequence
 			 * number. */
 			brelse(bh);
 			next_commit_ID++;
 			continue;

 		case JBD2_REVOKE_BLOCK:
 			/* If we aren't in the REVOKE pass, then we can
 			 * just skip over this block. */
 			if (pass != PASS_REVOKE) {
 				brelse(bh);
 				continue;
 			}

 			err = scan_revoke_records(journal, bh,
 						  next_commit_ID, info);
 			brelse(bh);
 			if (err)
 				goto failed;
 			continue;

 		default:
 			jbd_debug(3, "Unrecognised magic %d, end of scan.\n",
 				  blocktype);
 			brelse(bh);
 			goto done;
 		}
 	}

  done:
 	/*
 	 * We broke out of the log scan loop: either we came to the
 	 * known end of the log or we found an unexpected block in the
 	 * log.  If the latter happened, then we know that the "current"
 	 * transaction marks the end of the valid log.
 	 */

 	if (pass == PASS_SCAN)
 		info->end_transaction = next_commit_ID;
 	else {
 		/* It's really bad news if different passes end up at
 		 * different places (but possible due to IO errors). */
 		if (info->end_transaction != next_commit_ID) {
 			printk (KERN_ERR "JBD: recovery pass %d ended at "
 				"transaction %u, expected %u\n",
 				pass, next_commit_ID, info->end_transaction);
 			if (!success)
 				success = -EIO;
 		}
 	}

 	return success;

  failed:
 	return err;
 }


 /* Scan a revoke record, marking all blocks mentioned as revoked. */

 static int scan_revoke_records(journal_t *journal, struct buffer_head *bh,
 			       tid_t sequence, struct recovery_info *info)
 {
 	jbd2_journal_revoke_header_t *header;
 	int offset, max;
 	int record_len = 4;

 	header = (jbd2_journal_revoke_header_t *) bh->b_data;
 	offset = sizeof(jbd2_journal_revoke_header_t);
 	max = be32_to_cpu(header->r_count);

 	if (JBD2_HAS_INCOMPAT_FEATURE(journal, JBD2_FEATURE_INCOMPAT_64BIT))
 		record_len = 8;

 	while (offset + record_len <= max) {
 		unsigned long long blocknr;
 		int err;

 		if (record_len == 4)
 			blocknr = be32_to_cpu(* ((__be32 *) (bh->b_data+offset)));
 		else
 			blocknr = be64_to_cpu(* ((__be64 *) (bh->b_data+offset)));
 		offset += record_len;
 		err = jbd2_journal_set_revoke(journal, blocknr, sequence);
 		if (err)
 			return err;
 		++info->nr_revokes;
 	}
 	return 0;
 }
	/*
	* linux/fs/jbd2/recovery.c
	*
	* Written by Stephen C. Tweedie <sct@redhat.com>, 1999
	*
	* Copyright 1999-2000 Red Hat Software --- 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.
	*
	* Journal recovery routines for the generic filesystem journaling code;
	* part of the ext2fs journaling system.
	*/

	#ifndef __KERNEL__
	#include "jfs_user.h"
	#else
	#include <linux/time.h>
	#include <linux/fs.h>
	#include <linux/jbd2.h>
	#include <linux/errno.h>
	#include <linux/slab.h>
	#endif

	/*
	* Maintain information about the progress of the recovery job, so that
	* the different passes can carry information between them.
	*/
	struct recovery_info
	{
	tid_t start_transaction;
	tid_t end_transaction;

	int nr_replays;
	int nr_revokes;
	int nr_revoke_hits;
	};

	enum passtype {PASS_SCAN, PASS_REVOKE, PASS_REPLAY};
	static int do_one_pass(journal_t *journal,
	struct recovery_info *info, enum passtype pass);
	static int scan_revoke_records(journal_t , struct buffer_head ,
	tid_t, struct recovery_info *);

	#ifdef __KERNEL__

	/* Release readahead buffers after use */
	static void journal_brelse_array(struct buffer_head *b[], int n)
	{
	while (--n >= 0)
	brelse (b[n]);
	}


	/*
	* When reading from the journal, we are going through the block device
	* layer directly and so there is no readahead being done for us. We
	* need to implement any readahead ourselves if we want it to happen at
	* all. Recovery is basically one long sequential read, so make sure we
	* do the IO in reasonably large chunks.
	*
	* This is not so critical that we need to be enormously clever about
	* the readahead size, though. 128K is a purely arbitrary, good-enough
	* fixed value.
	*/

	#define MAXBUF 8
	static int do_readahead(journal_t *journal, unsigned int start)
	{
	int err;
	unsigned int max, nbufs, next;
	unsigned long long blocknr;
	struct buffer_head *bh;

	struct buffer_head * bufs[MAXBUF];

	/* Do up to 128K of readahead */
	max = start + (128 * 1024 / journal->j_blocksize);
	if (max > journal->j_maxlen)
	max = journal->j_maxlen;

	/* Do the readahead itself. We'll submit MAXBUF buffer_heads at
	* a time to the block device IO layer. */

	nbufs = 0;

	for (next = start; next < max; next++) {
	err = jbd2_journal_bmap(journal, next, &blocknr);

	if (err) {
	printk (KERN_ERR "JBD: bad block at offset %u\n",
	next);
	goto failed;
	}

	bh = __getblk(journal->j_dev, blocknr, journal->j_blocksize);
	if (!bh) {
	err = -ENOMEM;
	goto failed;
	}

	if (!buffer_uptodate(bh) && !buffer_locked(bh)) {
	bufs[nbufs++] = bh;
	if (nbufs == MAXBUF) {
	ll_rw_block(READ, nbufs, bufs);
	journal_brelse_array(bufs, nbufs);
	nbufs = 0;
	}
	} else
	brelse(bh);
	}

	if (nbufs)
	ll_rw_block(READ, nbufs, bufs);
	err = 0;

	failed:
	if (nbufs)
	journal_brelse_array(bufs, nbufs);
	return err;
	}

	#endif /* __KERNEL__ */


	/*
	* Read a block from the journal
	*/

	static int jread(struct buffer_head *bhp, journal_t journal,
	unsigned int offset)
	{
	int err;
	unsigned long long blocknr;
	struct buffer_head *bh;

	*bhp = NULL;

	if (offset >= journal->j_maxlen) {
	printk(KERN_ERR "JBD: corrupted journal superblock\n");
	return -EIO;
	}

	err = jbd2_journal_bmap(journal, offset, &blocknr);

	if (err) {
	printk (KERN_ERR "JBD: bad block at offset %u\n",
	offset);
	return err;
	}

	bh = __getblk(journal->j_dev, blocknr, journal->j_blocksize);
	if (!bh)
	return -ENOMEM;

	if (!buffer_uptodate(bh)) {
	/* If this is a brand new buffer, start readahead.
	Otherwise, we assume we are already reading it. */
	if (!buffer_req(bh))
	do_readahead(journal, offset);
	wait_on_buffer(bh);
	}

	if (!buffer_uptodate(bh)) {
	printk (KERN_ERR "JBD: Failed to read block at offset %u\n",
	offset);
	brelse(bh);
	return -EIO;
	}

	*bhp = bh;
	return 0;
	}


	/*
	* Count the number of in-use tags in a journal descriptor block.
	*/

	static int count_tags(journal_t journal, struct buffer_head bh)
	{
	char * tagp;
	journal_block_tag_t * tag;
	int nr = 0, size = journal->j_blocksize;
	int tag_bytes = journal_tag_bytes(journal);

	tagp = &bh->b_data[sizeof(journal_header_t)];

	while ((tagp - bh->b_data + tag_bytes) <= size) {
	tag = (journal_block_tag_t *) tagp;

	nr++;
	tagp += tag_bytes;
	if (!(tag->t_flags & cpu_to_be32(JBD2_FLAG_SAME_UUID)))
	tagp += 16;

	if (tag->t_flags & cpu_to_be32(JBD2_FLAG_LAST_TAG))
	break;
	}

	return nr;
	}


	/* Make sure we wrap around the log correctly! */
	#define wrap(journal, var) \
	do { \
	if (var >= (journal)->j_last) \
	var -= ((journal)->j_last - (journal)->j_first); \
	} while (0)

	/**
	* jbd2_journal_recover - recovers a on-disk journal
	* @journal: the journal to recover
	*
	* The primary function for recovering the log contents when mounting a
	* journaled device.
	*
	* Recovery is done in three passes. In the first pass, we look for the
	* end of the log. In the second, we assemble the list of revoke
	* blocks. In the third and final pass, we replay any un-revoked blocks
	* in the log.
	*/
	int jbd2_journal_recover(journal_t *journal)
	{
	int err;
	journal_superblock_t * sb;

	struct recovery_info info;

	memset(&info, 0, sizeof(info));
	sb = journal->j_superblock;

	/*
	* The journal superblock's s_start field (the current log head)
	* is always zero if, and only if, the journal was cleanly
	* unmounted.
	*/

	if (!sb->s_start) {
	jbd_debug(1, "No recovery required, last transaction %d\n",
	be32_to_cpu(sb->s_sequence));
	journal->j_transaction_sequence = be32_to_cpu(sb->s_sequence) + 1;
	return 0;
	}

	err = do_one_pass(journal, &info, PASS_SCAN);
	if (!err)
	err = do_one_pass(journal, &info, PASS_REVOKE);
	if (!err)
	err = do_one_pass(journal, &info, PASS_REPLAY);

	jbd_debug(1, "JBD: recovery, exit status %d, "
	"recovered transactions %u to %u\n",
	err, info.start_transaction, info.end_transaction);
	jbd_debug(1, "JBD: Replayed %d and revoked %d/%d blocks\n",
	info.nr_replays, info.nr_revoke_hits, info.nr_revokes);

	/* Restart the log at the next transaction ID, thus invalidating
	* any existing commit records in the log. */
	journal->j_transaction_sequence = ++info.end_transaction;

	jbd2_journal_clear_revoke(journal);
	sync_blockdev(journal->j_fs_dev);
	return err;
	}

	/**
	* jbd2_journal_skip_recovery - Start journal and wipe exiting records
	* @journal: journal to startup
	*
	* Locate any valid recovery information from the journal and set up the
	* journal structures in memory to ignore it (presumably because the
	* caller has evidence that it is out of date).
	* This function does'nt appear to be exorted..
	*
	* We perform one pass over the journal to allow us to tell the user how
	* much recovery information is being erased, and to let us initialise
	* the journal transaction sequence numbers to the next unused ID.
	*/
	int jbd2_journal_skip_recovery(journal_t *journal)
	{
	int err;
	journal_superblock_t * sb;

	struct recovery_info info;

	memset (&info, 0, sizeof(info));
	sb = journal->j_superblock;

	err = do_one_pass(journal, &info, PASS_SCAN);

	if (err) {
	printk(KERN_ERR "JBD: error %d scanning journal\n", err);
	++journal->j_transaction_sequence;
	} else {
	#ifdef CONFIG_JBD2_DEBUG
	int dropped = info.end_transaction - be32_to_cpu(sb->s_sequence);
	#endif
	jbd_debug(1,
	"JBD: ignoring %d transaction%s from the journal.\n",
	dropped, (dropped == 1) ? "" : "s");
	journal->j_transaction_sequence = ++info.end_transaction;
	}

	journal->j_tail = 0;
	return err;
	}

	static inline unsigned long long read_tag_block(int tag_bytes, journal_block_tag_t *tag)
	{
	unsigned long long block = be32_to_cpu(tag->t_blocknr);
	if (tag_bytes > JBD_TAG_SIZE32)
	block \|= (u64)be32_to_cpu(tag->t_blocknr_high) << 32;
	return block;
	}

	static int do_one_pass(journal_t *journal,
	struct recovery_info *info, enum passtype pass)
	{
	unsigned int first_commit_ID, next_commit_ID;
	unsigned long next_log_block;
	int err, success = 0;
	journal_superblock_t * sb;
	journal_header_t * tmp;
	struct buffer_head * bh;
	unsigned int sequence;
	int blocktype;
	int tag_bytes = journal_tag_bytes(journal);

	/* Precompute the maximum metadata descriptors in a descriptor block */
	int MAX_BLOCKS_PER_DESC;
	MAX_BLOCKS_PER_DESC = ((journal->j_blocksize-sizeof(journal_header_t))
	/ tag_bytes);

	/*
	* First thing is to establish what we expect to find in the log
	* (in terms of transaction IDs), and where (in terms of log
	* block offsets): query the superblock.
	*/

	sb = journal->j_superblock;
	next_commit_ID = be32_to_cpu(sb->s_sequence);
	next_log_block = be32_to_cpu(sb->s_start);

	first_commit_ID = next_commit_ID;
	if (pass == PASS_SCAN)
	info->start_transaction = first_commit_ID;

	jbd_debug(1, "Starting recovery pass %d\n", pass);

	/*
	* Now we walk through the log, transaction by transaction,
	* making sure that each transaction has a commit block in the
	* expected place. Each complete transaction gets replayed back
	* into the main filesystem.
	*/

	while (1) {
	int flags;
	char * tagp;
	journal_block_tag_t * tag;
	struct buffer_head * obh;
	struct buffer_head * nbh;

	cond_resched(); /* We're under lock_kernel() */

	/* If we already know where to stop the log traversal,
	* check right now that we haven't gone past the end of
	* the log. */

	if (pass != PASS_SCAN)
	if (tid_geq(next_commit_ID, info->end_transaction))
	break;

	jbd_debug(2, "Scanning for sequence ID %u at %lu/%lu\n",
	next_commit_ID, next_log_block, journal->j_last);

	/* Skip over each chunk of the transaction looking
	* either the next descriptor block or the final commit
	* record. */

	jbd_debug(3, "JBD: checking block %ld\n", next_log_block);
	err = jread(&bh, journal, next_log_block);
	if (err)
	goto failed;

	next_log_block++;
	wrap(journal, next_log_block);

	/* What kind of buffer is it?
	*
	* If it is a descriptor block, check that it has the
	* expected sequence number. Otherwise, we're all done
	* here. */

	tmp = (journal_header_t *)bh->b_data;

	if (tmp->h_magic != cpu_to_be32(JBD2_MAGIC_NUMBER)) {
	brelse(bh);
	break;
	}

	blocktype = be32_to_cpu(tmp->h_blocktype);
	sequence = be32_to_cpu(tmp->h_sequence);
	jbd_debug(3, "Found magic %d, sequence %d\n",
	blocktype, sequence);

	if (sequence != next_commit_ID) {
	brelse(bh);
	break;
	}

	/* OK, we have a valid descriptor block which matches
	* all of the sequence number checks. What are we going
	* to do with it? That depends on the pass... */

	switch(blocktype) {
	case JBD2_DESCRIPTOR_BLOCK:
	/* If it is a valid descriptor block, replay it
	* in pass REPLAY; otherwise, just skip over the
	* blocks it describes. */
	if (pass != PASS_REPLAY) {
	next_log_block += count_tags(journal, bh);
	wrap(journal, next_log_block);
	brelse(bh);
	continue;
	}

	/* A descriptor block: we can now write all of
	* the data blocks. Yay, useful work is finally
	* getting done here! */

	tagp = &bh->b_data[sizeof(journal_header_t)];
	while ((tagp - bh->b_data + tag_bytes)
	<= journal->j_blocksize) {
	unsigned long io_block;

	tag = (journal_block_tag_t *) tagp;
	flags = be32_to_cpu(tag->t_flags);

	io_block = next_log_block++;
	wrap(journal, next_log_block);
	err = jread(&obh, journal, io_block);
	if (err) {
	/* Recover what we can, but
	* report failure at the end. */
	success = err;
	printk (KERN_ERR
	"JBD: IO error %d recovering "
	"block %ld in log\n",
	err, io_block);
	} else {
	unsigned long long blocknr;

	J_ASSERT(obh != NULL);
	blocknr = read_tag_block(tag_bytes,
	tag);

	/* If the block has been
	* revoked, then we're all done
	* here. */
	if (jbd2_journal_test_revoke
	(journal, blocknr,
	next_commit_ID)) {
	brelse(obh);
	++info->nr_revoke_hits;
	goto skip_write;
	}

	/* Find a buffer for the new
	* data being restored */
	nbh = __getblk(journal->j_fs_dev,
	blocknr,
	journal->j_blocksize);
	if (nbh == NULL) {
	printk(KERN_ERR
	"JBD: Out of memory "
	"during recovery.\n");
	err = -ENOMEM;
	brelse(bh);
	brelse(obh);
	goto failed;
	}

	lock_buffer(nbh);
	memcpy(nbh->b_data, obh->b_data,
	journal->j_blocksize);
	if (flags & JBD2_FLAG_ESCAPE) {
	((__be32 )bh->b_data) =
	cpu_to_be32(JBD2_MAGIC_NUMBER);
	}

	BUFFER_TRACE(nbh, "marking dirty");
	set_buffer_uptodate(nbh);
	mark_buffer_dirty(nbh);
	BUFFER_TRACE(nbh, "marking uptodate");
	++info->nr_replays;
	/* ll_rw_block(WRITE, 1, &nbh); */
	unlock_buffer(nbh);
	brelse(obh);
	brelse(nbh);
	}

	skip_write:
	tagp += tag_bytes;
	if (!(flags & JBD2_FLAG_SAME_UUID))
	tagp += 16;

	if (flags & JBD2_FLAG_LAST_TAG)
	break;
	}

	brelse(bh);
	continue;

	case JBD2_COMMIT_BLOCK:
	/* Found an expected commit block: not much to
	* do other than move on to the next sequence
	* number. */
	brelse(bh);
	next_commit_ID++;
	continue;

	case JBD2_REVOKE_BLOCK:
	/* If we aren't in the REVOKE pass, then we can
	* just skip over this block. */
	if (pass != PASS_REVOKE) {
	brelse(bh);
	continue;
	}

	err = scan_revoke_records(journal, bh,
	next_commit_ID, info);
	brelse(bh);
	if (err)
	goto failed;
	continue;

	default:
	jbd_debug(3, "Unrecognised magic %d, end of scan.\n",
	blocktype);
	brelse(bh);
	goto done;
	}
	}

	done:
	/*
	* We broke out of the log scan loop: either we came to the
	* known end of the log or we found an unexpected block in the
	* log. If the latter happened, then we know that the "current"
	* transaction marks the end of the valid log.
	*/

	if (pass == PASS_SCAN)
	info->end_transaction = next_commit_ID;
	else {
	/* It's really bad news if different passes end up at
	* different places (but possible due to IO errors). */
	if (info->end_transaction != next_commit_ID) {
	printk (KERN_ERR "JBD: recovery pass %d ended at "
	"transaction %u, expected %u\n",
	pass, next_commit_ID, info->end_transaction);
	if (!success)
	success = -EIO;
	}
	}

	return success;

	failed:
	return err;
	}


	/* Scan a revoke record, marking all blocks mentioned as revoked. */

	static int scan_revoke_records(journal_t journal, struct buffer_head bh,
	tid_t sequence, struct recovery_info *info)
	{
	jbd2_journal_revoke_header_t *header;
	int offset, max;
	int record_len = 4;

	header = (jbd2_journal_revoke_header_t *) bh->b_data;
	offset = sizeof(jbd2_journal_revoke_header_t);
	max = be32_to_cpu(header->r_count);

	if (JBD2_HAS_INCOMPAT_FEATURE(journal, JBD2_FEATURE_INCOMPAT_64BIT))
	record_len = 8;

	while (offset + record_len <= max) {
	unsigned long long blocknr;
	int err;

	if (record_len == 4)
	blocknr = be32_to_cpu(* ((__be32 *) (bh->b_data+offset)));
	else
	blocknr = be64_to_cpu(* ((__be64 *) (bh->b_data+offset)));
	offset += record_len;
	err = jbd2_journal_set_revoke(journal, blocknr, sequence);
	if (err)
	return err;
	++info->nr_revokes;
	}
	return 0;
	}