Patchwork [12/29] jbd2: Transaction reservation support

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Submitter Jan Kara
Date April 8, 2013, 9:32 p.m.
Message ID <1365456754-29373-13-git-send-email-jack@suse.cz>
Download mbox | patch
Permalink /patch/234897/
State Superseded
Headers show

Comments

Jan Kara - April 8, 2013, 9:32 p.m.
In some cases we cannot start a transaction because of locking constraints and
passing started transaction into those places is not handy either because we
could block transaction commit for too long. Transaction reservation is
designed to solve these issues. It reserves a handle with given number of
credits in the journal and the handle can be later attached to the running
transaction without blocking on commit or checkpointing. Reserved handles do
not block transaction commit in any way, they only reduce maximum size of the
running transaction (because we have to always be prepared to accomodate
request for attaching reserved handle).

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/jbd2/commit.c      |    6 +
 fs/jbd2/journal.c     |    2 +
 fs/jbd2/transaction.c |  289 +++++++++++++++++++++++++++++++++++++------------
 include/linux/jbd2.h  |   21 ++++-
 4 files changed, 245 insertions(+), 73 deletions(-)
Zheng Liu - May 5, 2013, 9:39 a.m.
On Mon, Apr 08, 2013 at 11:32:17PM +0200, Jan Kara wrote:
> In some cases we cannot start a transaction because of locking constraints and
> passing started transaction into those places is not handy either because we
> could block transaction commit for too long. Transaction reservation is
> designed to solve these issues. It reserves a handle with given number of
> credits in the journal and the handle can be later attached to the running
> transaction without blocking on commit or checkpointing. Reserved handles do
> not block transaction commit in any way, they only reduce maximum size of the
> running transaction (because we have to always be prepared to accomodate
> request for attaching reserved handle).
> 
> Signed-off-by: Jan Kara <jack@suse.cz>

Some minor nits below.  Otherwise the patch looks good to me.
Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>

> ---
>  fs/jbd2/commit.c      |    6 +
>  fs/jbd2/journal.c     |    2 +
>  fs/jbd2/transaction.c |  289 +++++++++++++++++++++++++++++++++++++------------
>  include/linux/jbd2.h  |   21 ++++-
>  4 files changed, 245 insertions(+), 73 deletions(-)
> 
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index 4863f5b..59c572e 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -522,6 +522,12 @@ void jbd2_journal_commit_transaction(journal_t *journal)
>  	 */
>  	jbd2_journal_switch_revoke_table(journal);
>  
> +	/*
> +	 * Reserved credits cannot be claimed anymore, free them
> +	 */
> +	atomic_sub(atomic_read(&journal->j_reserved_credits),
> +		   &commit_transaction->t_outstanding_credits);
> +
>  	trace_jbd2_commit_flushing(journal, commit_transaction);
>  	stats.run.rs_flushing = jiffies;
>  	stats.run.rs_locked = jbd2_time_diff(stats.run.rs_locked,
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index 63e2929..04c52ac 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -1001,6 +1001,7 @@ static journal_t * journal_init_common (void)
>  	init_waitqueue_head(&journal->j_wait_done_commit);
>  	init_waitqueue_head(&journal->j_wait_commit);
>  	init_waitqueue_head(&journal->j_wait_updates);
> +	init_waitqueue_head(&journal->j_wait_reserved);
>  	mutex_init(&journal->j_barrier);
>  	mutex_init(&journal->j_checkpoint_mutex);
>  	spin_lock_init(&journal->j_revoke_lock);
> @@ -1010,6 +1011,7 @@ static journal_t * journal_init_common (void)
>  	journal->j_commit_interval = (HZ * JBD2_DEFAULT_MAX_COMMIT_AGE);
>  	journal->j_min_batch_time = 0;
>  	journal->j_max_batch_time = 15000; /* 15ms */
> +	atomic_set(&journal->j_reserved_credits, 0);
>  
>  	/* The journal is marked for error until we succeed with recovery! */
>  	journal->j_flags = JBD2_ABORT;
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index 9639e47..036c01c 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -89,7 +89,8 @@ jbd2_get_transaction(journal_t *journal, transaction_t *transaction)
>  	transaction->t_expires = jiffies + journal->j_commit_interval;
>  	spin_lock_init(&transaction->t_handle_lock);
>  	atomic_set(&transaction->t_updates, 0);
> -	atomic_set(&transaction->t_outstanding_credits, 0);
> +	atomic_set(&transaction->t_outstanding_credits,
> +		   atomic_read(&journal->j_reserved_credits));
>  	atomic_set(&transaction->t_handle_count, 0);
>  	INIT_LIST_HEAD(&transaction->t_inode_list);
>  	INIT_LIST_HEAD(&transaction->t_private_list);
> @@ -141,6 +142,91 @@ static inline void update_t_max_wait(transaction_t *transaction,
>  }
>  
>  /*
> + * Wait until running transaction passes T_LOCKED state. Also starts the commit
> + * if needed. The function expects running transaction to exist and releases
> + * j_state_lock.
> + */
> +static void wait_transaction_locked(journal_t *journal)
> +	__releases(journal->j_state_lock)
> +{
> +	DEFINE_WAIT(wait);
> +	int need_to_start;
> +	tid_t tid = journal->j_running_transaction->t_tid;
> +
> +	prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
> +			TASK_UNINTERRUPTIBLE);
> +	need_to_start = !tid_geq(journal->j_commit_request, tid);
> +	read_unlock(&journal->j_state_lock);
> +	if (need_to_start)
> +		jbd2_log_start_commit(journal, tid);
> +	schedule();
> +	finish_wait(&journal->j_wait_transaction_locked, &wait);
> +}
> +
> +/*
> + * Wait until we can add credits for handle to the running transaction.  Called
> + * with j_state_lock held for reading. Returns 0 if handle joined the running
> + * transaction. Returns 1 if we had to wait, j_state_lock is dropped, and
> + * caller must retry.
> + */
> +static int add_transaction_credits(journal_t *journal, handle_t *handle)
> +{
> +	transaction_t *t = journal->j_running_transaction;
> +	int nblocks = handle->h_buffer_credits;
> +	int needed;
> +
> +	/*
> +	 * If the current transaction is locked down for commit, wait
> +	 * for the lock to be released.
> +	 */
> +	if (t->t_state == T_LOCKED) {
> +		wait_transaction_locked(journal);
> +		return 1;
> +	}
> +
> +	/*
> +	 * 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.
> +	 */
> +	needed = atomic_add_return(nblocks, &t->t_outstanding_credits);
> +	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.
> +		 */
> +		jbd_debug(2, "Handle %p starting new commit...\n", handle);
> +		atomic_sub(nblocks, &t->t_outstanding_credits);
> +		wait_transaction_locked(journal);
> +		return 1;
> +	}
> +
> +	/*
> +	 * 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.
> +	 */
> +	if (jbd2_log_space_left(journal) < jbd2_space_needed(journal)) {
> +		jbd_debug(2, "Handle %p waiting for checkpoint...\n", handle);
> +		atomic_sub(nblocks, &t->t_outstanding_credits);
> +		read_unlock(&journal->j_state_lock);
> +		write_lock(&journal->j_state_lock);
> +		if (jbd2_log_space_left(journal) < jbd2_space_needed(journal))
> +			__jbd2_log_wait_for_space(journal);
> +		write_unlock(&journal->j_state_lock);
> +		return 1;
> +	}
> +	return 0;
> +}
> +
> +/*
>   * 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
> @@ -151,12 +237,14 @@ static int start_this_handle(journal_t *journal, handle_t *handle,
>  			     gfp_t gfp_mask)
>  {
>  	transaction_t	*transaction, *new_transaction = NULL;
> -	tid_t		tid;
> -	int		needed, need_to_start;
>  	int		nblocks = handle->h_buffer_credits;
>  	unsigned long ts = jiffies;
>  
> -	if (nblocks > journal->j_max_transaction_buffers) {
> +	/*
> +	 * 1/2 of transaction can be reserved so we can practically handle
> +	 * only 1/2 of maximum transaction size per operation
> +	 */

Sorry, but I don't understand here why we only reserve 1/2 of maximum
transaction size.

> +	if (nblocks > journal->j_max_transaction_buffers / 2) {
>  		printk(KERN_ERR "JBD2: %s wants too many credits (%d > %d)\n",
>  		       current->comm, nblocks,
>  		       journal->j_max_transaction_buffers);
> @@ -223,75 +311,18 @@ repeat:
>  
>  	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);
> -		read_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.
> -	 */
> -	needed = atomic_add_return(nblocks,
> -				   &transaction->t_outstanding_credits);
> -
> -	if (needed > journal->j_max_transaction_buffers) {
> +	if (!handle->h_reserved) {

Maybe we need to add a comment here because we release j_state_lock in
add_transaction_credits.

Regards,
                                                - Zheng

> +		if (add_transaction_credits(journal, handle))
> +			goto repeat;
> +	} else {
>  		/*
> -		 * 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.
> +		 * We have handle reserved so we are allowed to join T_LOCKED
> +		 * transaction and we don't have to check for transaction size
> +		 * and journal space.
>  		 */
> -		DEFINE_WAIT(wait);
> -
> -		jbd_debug(2, "Handle %p starting new commit...\n", handle);
> -		atomic_sub(nblocks, &transaction->t_outstanding_credits);
> -		prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
> -				TASK_UNINTERRUPTIBLE);
> -		tid = transaction->t_tid;
> -		need_to_start = !tid_geq(journal->j_commit_request, tid);
> -		read_unlock(&journal->j_state_lock);
> -		if (need_to_start)
> -			jbd2_log_start_commit(journal, tid);
> -		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.
> -	 */
> -	if (jbd2_log_space_left(journal) < jbd2_space_needed(journal)) {
> -		jbd_debug(2, "Handle %p waiting for checkpoint...\n", handle);
> -		atomic_sub(nblocks, &transaction->t_outstanding_credits);
> -		read_unlock(&journal->j_state_lock);
> -		write_lock(&journal->j_state_lock);
> -		if (jbd2_log_space_left(journal) < jbd2_space_needed(journal))
> -			__jbd2_log_wait_for_space(journal);
> -		write_unlock(&journal->j_state_lock);
> -		goto repeat;
> +		atomic_sub(nblocks, &journal->j_reserved_credits);
> +		wake_up(&journal->j_wait_reserved);
> +		handle->h_reserved = 0;
>  	}
>  
>  	/* OK, account for the buffers that this operation expects to
> @@ -390,6 +421,122 @@ handle_t *jbd2_journal_start(journal_t *journal, int nblocks)
>  }
>  EXPORT_SYMBOL(jbd2_journal_start);
>  
> +/**
> + * handle_t *jbd2_journal_reserve(journal_t *journal, int nblocks)
> + * @journal: journal to reserve transaction on.
> + * @nblocks: number of blocks we might modify
> + *
> + * This function reserves transaction with @nblocks blocks in @journal.  The
> + * function waits for enough journal space to be available and possibly also
> + * for some reservations to be converted to real transactions if there are too
> + * many of them. Note that this means that calling this function while having
> + * another transaction started or reserved can cause deadlock. The returned
> + * handle cannot be used for anything until it is started using
> + * jbd2_journal_start_reserved().
> + */
> +handle_t *jbd2_journal_reserve(journal_t *journal, int nblocks,
> +			       unsigned int type, unsigned int line_no)
> +{
> +	handle_t *handle;
> +	unsigned long wanted;
> +
> +	handle = new_handle(nblocks);
> +	if (!handle)
> +		return ERR_PTR(-ENOMEM);
> +	handle->h_journal = journal;
> +	handle->h_reserved = 1;
> +	handle->h_type = type;
> +	handle->h_line_no = line_no;
> +
> +repeat:
> +	/*
> +	 * We need j_state_lock early to avoid transaction creation to race
> +	 * with us and using elevated j_reserved_credits.
> +	 */
> +	read_lock(&journal->j_state_lock);
> +	wanted = atomic_add_return(nblocks, &journal->j_reserved_credits);
> +	/* We allow at most half of a transaction to be reserved */
> +	if (wanted > journal->j_max_transaction_buffers / 2) {
> +		atomic_sub(nblocks, &journal->j_reserved_credits);
> +		read_unlock(&journal->j_state_lock);
> +		wait_event(journal->j_wait_reserved,
> +			   atomic_read(&journal->j_reserved_credits) + nblocks
> +			   <= journal->j_max_transaction_buffers / 2);
> +		goto repeat;
> +	}
> +	if (journal->j_running_transaction) {
> +		transaction_t *t = journal->j_running_transaction;
> +
> +		wanted = atomic_add_return(nblocks,
> +					   &t->t_outstanding_credits);
> +		if (wanted > journal->j_max_transaction_buffers) {
> +			atomic_sub(nblocks, &t->t_outstanding_credits);
> +			atomic_sub(nblocks, &journal->j_reserved_credits);
> +			wait_transaction_locked(journal);
> +			goto repeat;
> +		}
> +	}
> +	read_unlock(&journal->j_state_lock);
> +
> +	return handle;
> +}
> +EXPORT_SYMBOL(jbd2_journal_reserve);
> +
> +void jbd2_journal_free_reserved(handle_t *handle)
> +{
> +	journal_t *journal = handle->h_journal;
> +
> +	atomic_sub(handle->h_buffer_credits, &journal->j_reserved_credits);
> +	wake_up(&journal->j_wait_reserved);
> +	jbd2_free_handle(handle);
> +}
> +EXPORT_SYMBOL(jbd2_journal_free_reserved);
> +
> +/**
> + * int jbd2_journal_start_reserved(handle_t *handle) - start reserved handle
> + * @handle: handle to start
> + *
> + * Start handle that has been previously reserved with jbd2_journal_reserve().
> + * This attaches @handle to the running transaction (or creates one if there's
> + * not transaction running). Unlike jbd2_journal_start() this function cannot
> + * block on journal commit, checkpointing, or similar stuff. It can block on
> + * memory allocation or frozen journal though.
> + *
> + * Return 0 on success, non-zero on error - handle is freed in that case.
> + */
> +int jbd2_journal_start_reserved(handle_t *handle)
> +{
> +	journal_t *journal = handle->h_journal;
> +	int ret = -EIO;
> +
> +	if (WARN_ON(!handle->h_reserved)) {
> +		/* Someone passed in normal handle? Just stop it. */
> +		jbd2_journal_stop(handle);
> +		return ret;
> +	}
> +	/*
> +	 * Usefulness of mixing of reserved and unreserved handles is
> +	 * questionable. So far nobody seems to need it so just error out.
> +	 */
> +	if (WARN_ON(current->journal_info)) {
> +		jbd2_journal_free_reserved(handle);
> +		return ret;
> +	}
> +
> +	handle->h_journal = NULL;
> +	current->journal_info = handle;
> +	/*
> +	 * GFP_NOFS is here because callers are likely from writeback or
> +	 * similarly constrained call sites
> +	 */
> +	ret = start_this_handle(journal, handle, GFP_NOFS);
> +	if (ret < 0) {
> +		current->journal_info = NULL;
> +		jbd2_journal_free_reserved(handle);
> +	}
> +	return ret;
> +}
> +EXPORT_SYMBOL(jbd2_journal_start_reserved);
>  
>  /**
>   * int jbd2_journal_extend() - extend buffer credits.
> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> index ad4b3bb..b3c1283 100644
> --- a/include/linux/jbd2.h
> +++ b/include/linux/jbd2.h
> @@ -410,8 +410,12 @@ struct jbd2_revoke_table_s;
>  
>  struct jbd2_journal_handle
>  {
> -	/* Which compound transaction is this update a part of? */
> -	transaction_t		*h_transaction;
> +	union {
> +		/* Which compound transaction is this update a part of? */
> +		transaction_t	*h_transaction;
> +		/* Which journal handle belongs to - used iff h_reserved set */
> +		journal_t	*h_journal;
> +	};
>  
>  	/* Number of remaining buffers we are allowed to dirty: */
>  	int			h_buffer_credits;
> @@ -426,6 +430,7 @@ struct jbd2_journal_handle
>  	/* Flags [no locking] */
>  	unsigned int	h_sync:		1;	/* sync-on-close */
>  	unsigned int	h_jdata:	1;	/* force data journaling */
> +	unsigned int	h_reserved:	1;	/* handle with reserved credits */
>  	unsigned int	h_aborted:	1;	/* fatal error on handle */
>  	unsigned int	h_type:		8;	/* for handle statistics */
>  	unsigned int	h_line_no:	16;	/* for handle statistics */
> @@ -689,6 +694,7 @@ jbd2_time_diff(unsigned long start, unsigned long end)
>   * @j_wait_done_commit: Wait queue for waiting for commit to complete
>   * @j_wait_commit: Wait queue to trigger commit
>   * @j_wait_updates: Wait queue to wait for updates to complete
> + * @j_wait_reserved: Wait queue to wait for reserved buffer credits to drop
>   * @j_checkpoint_mutex: Mutex for locking against concurrent checkpoints
>   * @j_head: Journal head - identifies the first unused block in the journal
>   * @j_tail: Journal tail - identifies the oldest still-used block in the
> @@ -702,6 +708,7 @@ jbd2_time_diff(unsigned long start, unsigned long end)
>   *     journal
>   * @j_fs_dev: Device which holds the client fs.  For internal journal this will
>   *     be equal to j_dev
> + * @j_reserved_credits: Number of buffers reserved from the running transaction
>   * @j_maxlen: Total maximum capacity of the journal region on disk.
>   * @j_list_lock: Protects the buffer lists and internal buffer state.
>   * @j_inode: Optional inode where we store the journal.  If present, all journal
> @@ -800,6 +807,9 @@ struct journal_s
>  	/* Wait queue to wait for updates to complete */
>  	wait_queue_head_t	j_wait_updates;
>  
> +	/* Wait queue to wait for reserved buffer credits to drop */
> +	wait_queue_head_t	j_wait_reserved;
> +
>  	/* Semaphore for locking against concurrent checkpoints */
>  	struct mutex		j_checkpoint_mutex;
>  
> @@ -854,6 +864,9 @@ struct journal_s
>  	/* Total maximum capacity of the journal region on disk. */
>  	unsigned int		j_maxlen;
>  
> +	/* Number of buffers reserved from the running transaction */
> +	atomic_t		j_reserved_credits;
> +
>  	/*
>  	 * Protects the buffer lists and internal buffer state.
>  	 */
> @@ -1094,6 +1107,10 @@ extern handle_t *jbd2__journal_start(journal_t *, int nblocks, gfp_t gfp_mask,
>  				     unsigned int type, unsigned int line_no);
>  extern int	 jbd2_journal_restart(handle_t *, int nblocks);
>  extern int	 jbd2__journal_restart(handle_t *, int nblocks, gfp_t gfp_mask);
> +extern handle_t *jbd2_journal_reserve(journal_t *, int nblocks,
> +				      unsigned int type, unsigned int line_no);
> +extern int	 jbd2_journal_start_reserved(handle_t *handle);
> +extern void	 jbd2_journal_free_reserved(handle_t *handle);
>  extern int	 jbd2_journal_extend (handle_t *, int nblocks);
>  extern int	 jbd2_journal_get_write_access(handle_t *, struct buffer_head *);
>  extern int	 jbd2_journal_get_create_access (handle_t *, struct buffer_head *);
> -- 
> 1.7.1
> 
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Jan Kara - May 6, 2013, 12:49 p.m.
On Sun 05-05-13 17:39:39, Zheng Liu wrote:
> On Mon, Apr 08, 2013 at 11:32:17PM +0200, Jan Kara wrote:
> > In some cases we cannot start a transaction because of locking constraints and
> > passing started transaction into those places is not handy either because we
> > could block transaction commit for too long. Transaction reservation is
> > designed to solve these issues. It reserves a handle with given number of
> > credits in the journal and the handle can be later attached to the running
> > transaction without blocking on commit or checkpointing. Reserved handles do
> > not block transaction commit in any way, they only reduce maximum size of the
> > running transaction (because we have to always be prepared to accomodate
> > request for attaching reserved handle).
> > 
> > Signed-off-by: Jan Kara <jack@suse.cz>
> 
> Some minor nits below.  Otherwise the patch looks good to me.
> Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>
...
> 
> > +/*
> >   * 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
> > @@ -151,12 +237,14 @@ static int start_this_handle(journal_t *journal, handle_t *handle,
> >  			     gfp_t gfp_mask)
> >  {
> >  	transaction_t	*transaction, *new_transaction = NULL;
> > -	tid_t		tid;
> > -	int		needed, need_to_start;
> >  	int		nblocks = handle->h_buffer_credits;
> >  	unsigned long ts = jiffies;
> >  
> > -	if (nblocks > journal->j_max_transaction_buffers) {
> > +	/*
> > +	 * 1/2 of transaction can be reserved so we can practically handle
> > +	 * only 1/2 of maximum transaction size per operation
> > +	 */
> 
> Sorry, but I don't understand here why we only reserve 1/2 of maximum
> transaction size.
  Well, we allow 1/2 of maximum transaction size to be allocated in already
reserved handles. So if someone submitted a request for a handle with
more than 1/2 of maximum transaction size, then we might have to wait for
reserved handles to be freed. That would be a slight complication in the
code and it would also possibly introduce livelocking issues - after a
reserved transaction is freed, someone can reserve a new one before the
large handle creation request is satisfied. Again this can be solved but
the complications simply doesn't seem to be worth it.

> > +	if (nblocks > journal->j_max_transaction_buffers / 2) {
> >  		printk(KERN_ERR "JBD2: %s wants too many credits (%d > %d)\n",
> >  		       current->comm, nblocks,
> >  		       journal->j_max_transaction_buffers);
> > @@ -223,75 +311,18 @@ repeat:
> >  
> >  	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);
> > -		read_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.
> > -	 */
> > -	needed = atomic_add_return(nblocks,
> > -				   &transaction->t_outstanding_credits);
> > -
> > -	if (needed > journal->j_max_transaction_buffers) {
> > +	if (!handle->h_reserved) {
> 
> Maybe we need to add a comment here because we release j_state_lock in
> add_transaction_credits.
  OK, I've added a comment regarding that. Thanks for the review!

								Honza

> > +		if (add_transaction_credits(journal, handle))
> > +			goto repeat;
> > +	} else {
> >  		/*
> > -		 * 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.
> > +		 * We have handle reserved so we are allowed to join T_LOCKED
> > +		 * transaction and we don't have to check for transaction size
> > +		 * and journal space.
> >  		 */
> > -		DEFINE_WAIT(wait);
> > -
> > -		jbd_debug(2, "Handle %p starting new commit...\n", handle);
> > -		atomic_sub(nblocks, &transaction->t_outstanding_credits);
> > -		prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
> > -				TASK_UNINTERRUPTIBLE);
> > -		tid = transaction->t_tid;
> > -		need_to_start = !tid_geq(journal->j_commit_request, tid);
> > -		read_unlock(&journal->j_state_lock);
> > -		if (need_to_start)
> > -			jbd2_log_start_commit(journal, tid);
> > -		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.
> > -	 */
> > -	if (jbd2_log_space_left(journal) < jbd2_space_needed(journal)) {
> > -		jbd_debug(2, "Handle %p waiting for checkpoint...\n", handle);
> > -		atomic_sub(nblocks, &transaction->t_outstanding_credits);
> > -		read_unlock(&journal->j_state_lock);
> > -		write_lock(&journal->j_state_lock);
> > -		if (jbd2_log_space_left(journal) < jbd2_space_needed(journal))
> > -			__jbd2_log_wait_for_space(journal);
> > -		write_unlock(&journal->j_state_lock);
> > -		goto repeat;
> > +		atomic_sub(nblocks, &journal->j_reserved_credits);
> > +		wake_up(&journal->j_wait_reserved);
> > +		handle->h_reserved = 0;
> >  	}
> >  
> >  	/* OK, account for the buffers that this operation expects to
> > @@ -390,6 +421,122 @@ handle_t *jbd2_journal_start(journal_t *journal, int nblocks)
> >  }
> >  EXPORT_SYMBOL(jbd2_journal_start);
> >  
> > +/**
> > + * handle_t *jbd2_journal_reserve(journal_t *journal, int nblocks)
> > + * @journal: journal to reserve transaction on.
> > + * @nblocks: number of blocks we might modify
> > + *
> > + * This function reserves transaction with @nblocks blocks in @journal.  The
> > + * function waits for enough journal space to be available and possibly also
> > + * for some reservations to be converted to real transactions if there are too
> > + * many of them. Note that this means that calling this function while having
> > + * another transaction started or reserved can cause deadlock. The returned
> > + * handle cannot be used for anything until it is started using
> > + * jbd2_journal_start_reserved().
> > + */
> > +handle_t *jbd2_journal_reserve(journal_t *journal, int nblocks,
> > +			       unsigned int type, unsigned int line_no)
> > +{
> > +	handle_t *handle;
> > +	unsigned long wanted;
> > +
> > +	handle = new_handle(nblocks);
> > +	if (!handle)
> > +		return ERR_PTR(-ENOMEM);
> > +	handle->h_journal = journal;
> > +	handle->h_reserved = 1;
> > +	handle->h_type = type;
> > +	handle->h_line_no = line_no;
> > +
> > +repeat:
> > +	/*
> > +	 * We need j_state_lock early to avoid transaction creation to race
> > +	 * with us and using elevated j_reserved_credits.
> > +	 */
> > +	read_lock(&journal->j_state_lock);
> > +	wanted = atomic_add_return(nblocks, &journal->j_reserved_credits);
> > +	/* We allow at most half of a transaction to be reserved */
> > +	if (wanted > journal->j_max_transaction_buffers / 2) {
> > +		atomic_sub(nblocks, &journal->j_reserved_credits);
> > +		read_unlock(&journal->j_state_lock);
> > +		wait_event(journal->j_wait_reserved,
> > +			   atomic_read(&journal->j_reserved_credits) + nblocks
> > +			   <= journal->j_max_transaction_buffers / 2);
> > +		goto repeat;
> > +	}
> > +	if (journal->j_running_transaction) {
> > +		transaction_t *t = journal->j_running_transaction;
> > +
> > +		wanted = atomic_add_return(nblocks,
> > +					   &t->t_outstanding_credits);
> > +		if (wanted > journal->j_max_transaction_buffers) {
> > +			atomic_sub(nblocks, &t->t_outstanding_credits);
> > +			atomic_sub(nblocks, &journal->j_reserved_credits);
> > +			wait_transaction_locked(journal);
> > +			goto repeat;
> > +		}
> > +	}
> > +	read_unlock(&journal->j_state_lock);
> > +
> > +	return handle;
> > +}
> > +EXPORT_SYMBOL(jbd2_journal_reserve);
> > +
> > +void jbd2_journal_free_reserved(handle_t *handle)
> > +{
> > +	journal_t *journal = handle->h_journal;
> > +
> > +	atomic_sub(handle->h_buffer_credits, &journal->j_reserved_credits);
> > +	wake_up(&journal->j_wait_reserved);
> > +	jbd2_free_handle(handle);
> > +}
> > +EXPORT_SYMBOL(jbd2_journal_free_reserved);
> > +
> > +/**
> > + * int jbd2_journal_start_reserved(handle_t *handle) - start reserved handle
> > + * @handle: handle to start
> > + *
> > + * Start handle that has been previously reserved with jbd2_journal_reserve().
> > + * This attaches @handle to the running transaction (or creates one if there's
> > + * not transaction running). Unlike jbd2_journal_start() this function cannot
> > + * block on journal commit, checkpointing, or similar stuff. It can block on
> > + * memory allocation or frozen journal though.
> > + *
> > + * Return 0 on success, non-zero on error - handle is freed in that case.
> > + */
> > +int jbd2_journal_start_reserved(handle_t *handle)
> > +{
> > +	journal_t *journal = handle->h_journal;
> > +	int ret = -EIO;
> > +
> > +	if (WARN_ON(!handle->h_reserved)) {
> > +		/* Someone passed in normal handle? Just stop it. */
> > +		jbd2_journal_stop(handle);
> > +		return ret;
> > +	}
> > +	/*
> > +	 * Usefulness of mixing of reserved and unreserved handles is
> > +	 * questionable. So far nobody seems to need it so just error out.
> > +	 */
> > +	if (WARN_ON(current->journal_info)) {
> > +		jbd2_journal_free_reserved(handle);
> > +		return ret;
> > +	}
> > +
> > +	handle->h_journal = NULL;
> > +	current->journal_info = handle;
> > +	/*
> > +	 * GFP_NOFS is here because callers are likely from writeback or
> > +	 * similarly constrained call sites
> > +	 */
> > +	ret = start_this_handle(journal, handle, GFP_NOFS);
> > +	if (ret < 0) {
> > +		current->journal_info = NULL;
> > +		jbd2_journal_free_reserved(handle);
> > +	}
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL(jbd2_journal_start_reserved);
> >  
> >  /**
> >   * int jbd2_journal_extend() - extend buffer credits.
> > diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> > index ad4b3bb..b3c1283 100644
> > --- a/include/linux/jbd2.h
> > +++ b/include/linux/jbd2.h
> > @@ -410,8 +410,12 @@ struct jbd2_revoke_table_s;
> >  
> >  struct jbd2_journal_handle
> >  {
> > -	/* Which compound transaction is this update a part of? */
> > -	transaction_t		*h_transaction;
> > +	union {
> > +		/* Which compound transaction is this update a part of? */
> > +		transaction_t	*h_transaction;
> > +		/* Which journal handle belongs to - used iff h_reserved set */
> > +		journal_t	*h_journal;
> > +	};
> >  
> >  	/* Number of remaining buffers we are allowed to dirty: */
> >  	int			h_buffer_credits;
> > @@ -426,6 +430,7 @@ struct jbd2_journal_handle
> >  	/* Flags [no locking] */
> >  	unsigned int	h_sync:		1;	/* sync-on-close */
> >  	unsigned int	h_jdata:	1;	/* force data journaling */
> > +	unsigned int	h_reserved:	1;	/* handle with reserved credits */
> >  	unsigned int	h_aborted:	1;	/* fatal error on handle */
> >  	unsigned int	h_type:		8;	/* for handle statistics */
> >  	unsigned int	h_line_no:	16;	/* for handle statistics */
> > @@ -689,6 +694,7 @@ jbd2_time_diff(unsigned long start, unsigned long end)
> >   * @j_wait_done_commit: Wait queue for waiting for commit to complete
> >   * @j_wait_commit: Wait queue to trigger commit
> >   * @j_wait_updates: Wait queue to wait for updates to complete
> > + * @j_wait_reserved: Wait queue to wait for reserved buffer credits to drop
> >   * @j_checkpoint_mutex: Mutex for locking against concurrent checkpoints
> >   * @j_head: Journal head - identifies the first unused block in the journal
> >   * @j_tail: Journal tail - identifies the oldest still-used block in the
> > @@ -702,6 +708,7 @@ jbd2_time_diff(unsigned long start, unsigned long end)
> >   *     journal
> >   * @j_fs_dev: Device which holds the client fs.  For internal journal this will
> >   *     be equal to j_dev
> > + * @j_reserved_credits: Number of buffers reserved from the running transaction
> >   * @j_maxlen: Total maximum capacity of the journal region on disk.
> >   * @j_list_lock: Protects the buffer lists and internal buffer state.
> >   * @j_inode: Optional inode where we store the journal.  If present, all journal
> > @@ -800,6 +807,9 @@ struct journal_s
> >  	/* Wait queue to wait for updates to complete */
> >  	wait_queue_head_t	j_wait_updates;
> >  
> > +	/* Wait queue to wait for reserved buffer credits to drop */
> > +	wait_queue_head_t	j_wait_reserved;
> > +
> >  	/* Semaphore for locking against concurrent checkpoints */
> >  	struct mutex		j_checkpoint_mutex;
> >  
> > @@ -854,6 +864,9 @@ struct journal_s
> >  	/* Total maximum capacity of the journal region on disk. */
> >  	unsigned int		j_maxlen;
> >  
> > +	/* Number of buffers reserved from the running transaction */
> > +	atomic_t		j_reserved_credits;
> > +
> >  	/*
> >  	 * Protects the buffer lists and internal buffer state.
> >  	 */
> > @@ -1094,6 +1107,10 @@ extern handle_t *jbd2__journal_start(journal_t *, int nblocks, gfp_t gfp_mask,
> >  				     unsigned int type, unsigned int line_no);
> >  extern int	 jbd2_journal_restart(handle_t *, int nblocks);
> >  extern int	 jbd2__journal_restart(handle_t *, int nblocks, gfp_t gfp_mask);
> > +extern handle_t *jbd2_journal_reserve(journal_t *, int nblocks,
> > +				      unsigned int type, unsigned int line_no);
> > +extern int	 jbd2_journal_start_reserved(handle_t *handle);
> > +extern void	 jbd2_journal_free_reserved(handle_t *handle);
> >  extern int	 jbd2_journal_extend (handle_t *, int nblocks);
> >  extern int	 jbd2_journal_get_write_access(handle_t *, struct buffer_head *);
> >  extern int	 jbd2_journal_get_create_access (handle_t *, struct buffer_head *);
> > -- 
> > 1.7.1
> > 
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
Zheng Liu - May 7, 2013, 5:22 a.m.
On Mon, May 06, 2013 at 02:49:39PM +0200, Jan Kara wrote:
...
> > > +/*
> > >   * 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
> > > @@ -151,12 +237,14 @@ static int start_this_handle(journal_t *journal, handle_t *handle,
> > >  			     gfp_t gfp_mask)
> > >  {
> > >  	transaction_t	*transaction, *new_transaction = NULL;
> > > -	tid_t		tid;
> > > -	int		needed, need_to_start;
> > >  	int		nblocks = handle->h_buffer_credits;
> > >  	unsigned long ts = jiffies;
> > >  
> > > -	if (nblocks > journal->j_max_transaction_buffers) {
> > > +	/*
> > > +	 * 1/2 of transaction can be reserved so we can practically handle
> > > +	 * only 1/2 of maximum transaction size per operation
> > > +	 */
> > 
> > Sorry, but I don't understand here why we only reserve 1/2 of maximum
> > transaction size.
>   Well, we allow 1/2 of maximum transaction size to be allocated in already
> reserved handles. So if someone submitted a request for a handle with
> more than 1/2 of maximum transaction size, then we might have to wait for
> reserved handles to be freed. That would be a slight complication in the
> code and it would also possibly introduce livelocking issues - after a
> reserved transaction is freed, someone can reserve a new one before the
> large handle creation request is satisfied. Again this can be solved but
> the complications simply doesn't seem to be worth it.

Fair enough.  Thanks for your explanation.

Regards,
                                                - Zheng
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Patch

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 4863f5b..59c572e 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -522,6 +522,12 @@  void jbd2_journal_commit_transaction(journal_t *journal)
 	 */
 	jbd2_journal_switch_revoke_table(journal);
 
+	/*
+	 * Reserved credits cannot be claimed anymore, free them
+	 */
+	atomic_sub(atomic_read(&journal->j_reserved_credits),
+		   &commit_transaction->t_outstanding_credits);
+
 	trace_jbd2_commit_flushing(journal, commit_transaction);
 	stats.run.rs_flushing = jiffies;
 	stats.run.rs_locked = jbd2_time_diff(stats.run.rs_locked,
diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 63e2929..04c52ac 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -1001,6 +1001,7 @@  static journal_t * journal_init_common (void)
 	init_waitqueue_head(&journal->j_wait_done_commit);
 	init_waitqueue_head(&journal->j_wait_commit);
 	init_waitqueue_head(&journal->j_wait_updates);
+	init_waitqueue_head(&journal->j_wait_reserved);
 	mutex_init(&journal->j_barrier);
 	mutex_init(&journal->j_checkpoint_mutex);
 	spin_lock_init(&journal->j_revoke_lock);
@@ -1010,6 +1011,7 @@  static journal_t * journal_init_common (void)
 	journal->j_commit_interval = (HZ * JBD2_DEFAULT_MAX_COMMIT_AGE);
 	journal->j_min_batch_time = 0;
 	journal->j_max_batch_time = 15000; /* 15ms */
+	atomic_set(&journal->j_reserved_credits, 0);
 
 	/* The journal is marked for error until we succeed with recovery! */
 	journal->j_flags = JBD2_ABORT;
diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index 9639e47..036c01c 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -89,7 +89,8 @@  jbd2_get_transaction(journal_t *journal, transaction_t *transaction)
 	transaction->t_expires = jiffies + journal->j_commit_interval;
 	spin_lock_init(&transaction->t_handle_lock);
 	atomic_set(&transaction->t_updates, 0);
-	atomic_set(&transaction->t_outstanding_credits, 0);
+	atomic_set(&transaction->t_outstanding_credits,
+		   atomic_read(&journal->j_reserved_credits));
 	atomic_set(&transaction->t_handle_count, 0);
 	INIT_LIST_HEAD(&transaction->t_inode_list);
 	INIT_LIST_HEAD(&transaction->t_private_list);
@@ -141,6 +142,91 @@  static inline void update_t_max_wait(transaction_t *transaction,
 }
 
 /*
+ * Wait until running transaction passes T_LOCKED state. Also starts the commit
+ * if needed. The function expects running transaction to exist and releases
+ * j_state_lock.
+ */
+static void wait_transaction_locked(journal_t *journal)
+	__releases(journal->j_state_lock)
+{
+	DEFINE_WAIT(wait);
+	int need_to_start;
+	tid_t tid = journal->j_running_transaction->t_tid;
+
+	prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
+			TASK_UNINTERRUPTIBLE);
+	need_to_start = !tid_geq(journal->j_commit_request, tid);
+	read_unlock(&journal->j_state_lock);
+	if (need_to_start)
+		jbd2_log_start_commit(journal, tid);
+	schedule();
+	finish_wait(&journal->j_wait_transaction_locked, &wait);
+}
+
+/*
+ * Wait until we can add credits for handle to the running transaction.  Called
+ * with j_state_lock held for reading. Returns 0 if handle joined the running
+ * transaction. Returns 1 if we had to wait, j_state_lock is dropped, and
+ * caller must retry.
+ */
+static int add_transaction_credits(journal_t *journal, handle_t *handle)
+{
+	transaction_t *t = journal->j_running_transaction;
+	int nblocks = handle->h_buffer_credits;
+	int needed;
+
+	/*
+	 * If the current transaction is locked down for commit, wait
+	 * for the lock to be released.
+	 */
+	if (t->t_state == T_LOCKED) {
+		wait_transaction_locked(journal);
+		return 1;
+	}
+
+	/*
+	 * 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.
+	 */
+	needed = atomic_add_return(nblocks, &t->t_outstanding_credits);
+	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.
+		 */
+		jbd_debug(2, "Handle %p starting new commit...\n", handle);
+		atomic_sub(nblocks, &t->t_outstanding_credits);
+		wait_transaction_locked(journal);
+		return 1;
+	}
+
+	/*
+	 * 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.
+	 */
+	if (jbd2_log_space_left(journal) < jbd2_space_needed(journal)) {
+		jbd_debug(2, "Handle %p waiting for checkpoint...\n", handle);
+		atomic_sub(nblocks, &t->t_outstanding_credits);
+		read_unlock(&journal->j_state_lock);
+		write_lock(&journal->j_state_lock);
+		if (jbd2_log_space_left(journal) < jbd2_space_needed(journal))
+			__jbd2_log_wait_for_space(journal);
+		write_unlock(&journal->j_state_lock);
+		return 1;
+	}
+	return 0;
+}
+
+/*
  * 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
@@ -151,12 +237,14 @@  static int start_this_handle(journal_t *journal, handle_t *handle,
 			     gfp_t gfp_mask)
 {
 	transaction_t	*transaction, *new_transaction = NULL;
-	tid_t		tid;
-	int		needed, need_to_start;
 	int		nblocks = handle->h_buffer_credits;
 	unsigned long ts = jiffies;
 
-	if (nblocks > journal->j_max_transaction_buffers) {
+	/*
+	 * 1/2 of transaction can be reserved so we can practically handle
+	 * only 1/2 of maximum transaction size per operation
+	 */
+	if (nblocks > journal->j_max_transaction_buffers / 2) {
 		printk(KERN_ERR "JBD2: %s wants too many credits (%d > %d)\n",
 		       current->comm, nblocks,
 		       journal->j_max_transaction_buffers);
@@ -223,75 +311,18 @@  repeat:
 
 	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);
-		read_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.
-	 */
-	needed = atomic_add_return(nblocks,
-				   &transaction->t_outstanding_credits);
-
-	if (needed > journal->j_max_transaction_buffers) {
+	if (!handle->h_reserved) {
+		if (add_transaction_credits(journal, handle))
+			goto repeat;
+	} else {
 		/*
-		 * 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.
+		 * We have handle reserved so we are allowed to join T_LOCKED
+		 * transaction and we don't have to check for transaction size
+		 * and journal space.
 		 */
-		DEFINE_WAIT(wait);
-
-		jbd_debug(2, "Handle %p starting new commit...\n", handle);
-		atomic_sub(nblocks, &transaction->t_outstanding_credits);
-		prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
-				TASK_UNINTERRUPTIBLE);
-		tid = transaction->t_tid;
-		need_to_start = !tid_geq(journal->j_commit_request, tid);
-		read_unlock(&journal->j_state_lock);
-		if (need_to_start)
-			jbd2_log_start_commit(journal, tid);
-		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.
-	 */
-	if (jbd2_log_space_left(journal) < jbd2_space_needed(journal)) {
-		jbd_debug(2, "Handle %p waiting for checkpoint...\n", handle);
-		atomic_sub(nblocks, &transaction->t_outstanding_credits);
-		read_unlock(&journal->j_state_lock);
-		write_lock(&journal->j_state_lock);
-		if (jbd2_log_space_left(journal) < jbd2_space_needed(journal))
-			__jbd2_log_wait_for_space(journal);
-		write_unlock(&journal->j_state_lock);
-		goto repeat;
+		atomic_sub(nblocks, &journal->j_reserved_credits);
+		wake_up(&journal->j_wait_reserved);
+		handle->h_reserved = 0;
 	}
 
 	/* OK, account for the buffers that this operation expects to
@@ -390,6 +421,122 @@  handle_t *jbd2_journal_start(journal_t *journal, int nblocks)
 }
 EXPORT_SYMBOL(jbd2_journal_start);
 
+/**
+ * handle_t *jbd2_journal_reserve(journal_t *journal, int nblocks)
+ * @journal: journal to reserve transaction on.
+ * @nblocks: number of blocks we might modify
+ *
+ * This function reserves transaction with @nblocks blocks in @journal.  The
+ * function waits for enough journal space to be available and possibly also
+ * for some reservations to be converted to real transactions if there are too
+ * many of them. Note that this means that calling this function while having
+ * another transaction started or reserved can cause deadlock. The returned
+ * handle cannot be used for anything until it is started using
+ * jbd2_journal_start_reserved().
+ */
+handle_t *jbd2_journal_reserve(journal_t *journal, int nblocks,
+			       unsigned int type, unsigned int line_no)
+{
+	handle_t *handle;
+	unsigned long wanted;
+
+	handle = new_handle(nblocks);
+	if (!handle)
+		return ERR_PTR(-ENOMEM);
+	handle->h_journal = journal;
+	handle->h_reserved = 1;
+	handle->h_type = type;
+	handle->h_line_no = line_no;
+
+repeat:
+	/*
+	 * We need j_state_lock early to avoid transaction creation to race
+	 * with us and using elevated j_reserved_credits.
+	 */
+	read_lock(&journal->j_state_lock);
+	wanted = atomic_add_return(nblocks, &journal->j_reserved_credits);
+	/* We allow at most half of a transaction to be reserved */
+	if (wanted > journal->j_max_transaction_buffers / 2) {
+		atomic_sub(nblocks, &journal->j_reserved_credits);
+		read_unlock(&journal->j_state_lock);
+		wait_event(journal->j_wait_reserved,
+			   atomic_read(&journal->j_reserved_credits) + nblocks
+			   <= journal->j_max_transaction_buffers / 2);
+		goto repeat;
+	}
+	if (journal->j_running_transaction) {
+		transaction_t *t = journal->j_running_transaction;
+
+		wanted = atomic_add_return(nblocks,
+					   &t->t_outstanding_credits);
+		if (wanted > journal->j_max_transaction_buffers) {
+			atomic_sub(nblocks, &t->t_outstanding_credits);
+			atomic_sub(nblocks, &journal->j_reserved_credits);
+			wait_transaction_locked(journal);
+			goto repeat;
+		}
+	}
+	read_unlock(&journal->j_state_lock);
+
+	return handle;
+}
+EXPORT_SYMBOL(jbd2_journal_reserve);
+
+void jbd2_journal_free_reserved(handle_t *handle)
+{
+	journal_t *journal = handle->h_journal;
+
+	atomic_sub(handle->h_buffer_credits, &journal->j_reserved_credits);
+	wake_up(&journal->j_wait_reserved);
+	jbd2_free_handle(handle);
+}
+EXPORT_SYMBOL(jbd2_journal_free_reserved);
+
+/**
+ * int jbd2_journal_start_reserved(handle_t *handle) - start reserved handle
+ * @handle: handle to start
+ *
+ * Start handle that has been previously reserved with jbd2_journal_reserve().
+ * This attaches @handle to the running transaction (or creates one if there's
+ * not transaction running). Unlike jbd2_journal_start() this function cannot
+ * block on journal commit, checkpointing, or similar stuff. It can block on
+ * memory allocation or frozen journal though.
+ *
+ * Return 0 on success, non-zero on error - handle is freed in that case.
+ */
+int jbd2_journal_start_reserved(handle_t *handle)
+{
+	journal_t *journal = handle->h_journal;
+	int ret = -EIO;
+
+	if (WARN_ON(!handle->h_reserved)) {
+		/* Someone passed in normal handle? Just stop it. */
+		jbd2_journal_stop(handle);
+		return ret;
+	}
+	/*
+	 * Usefulness of mixing of reserved and unreserved handles is
+	 * questionable. So far nobody seems to need it so just error out.
+	 */
+	if (WARN_ON(current->journal_info)) {
+		jbd2_journal_free_reserved(handle);
+		return ret;
+	}
+
+	handle->h_journal = NULL;
+	current->journal_info = handle;
+	/*
+	 * GFP_NOFS is here because callers are likely from writeback or
+	 * similarly constrained call sites
+	 */
+	ret = start_this_handle(journal, handle, GFP_NOFS);
+	if (ret < 0) {
+		current->journal_info = NULL;
+		jbd2_journal_free_reserved(handle);
+	}
+	return ret;
+}
+EXPORT_SYMBOL(jbd2_journal_start_reserved);
 
 /**
  * int jbd2_journal_extend() - extend buffer credits.
diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index ad4b3bb..b3c1283 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -410,8 +410,12 @@  struct jbd2_revoke_table_s;
 
 struct jbd2_journal_handle
 {
-	/* Which compound transaction is this update a part of? */
-	transaction_t		*h_transaction;
+	union {
+		/* Which compound transaction is this update a part of? */
+		transaction_t	*h_transaction;
+		/* Which journal handle belongs to - used iff h_reserved set */
+		journal_t	*h_journal;
+	};
 
 	/* Number of remaining buffers we are allowed to dirty: */
 	int			h_buffer_credits;
@@ -426,6 +430,7 @@  struct jbd2_journal_handle
 	/* Flags [no locking] */
 	unsigned int	h_sync:		1;	/* sync-on-close */
 	unsigned int	h_jdata:	1;	/* force data journaling */
+	unsigned int	h_reserved:	1;	/* handle with reserved credits */
 	unsigned int	h_aborted:	1;	/* fatal error on handle */
 	unsigned int	h_type:		8;	/* for handle statistics */
 	unsigned int	h_line_no:	16;	/* for handle statistics */
@@ -689,6 +694,7 @@  jbd2_time_diff(unsigned long start, unsigned long end)
  * @j_wait_done_commit: Wait queue for waiting for commit to complete
  * @j_wait_commit: Wait queue to trigger commit
  * @j_wait_updates: Wait queue to wait for updates to complete
+ * @j_wait_reserved: Wait queue to wait for reserved buffer credits to drop
  * @j_checkpoint_mutex: Mutex for locking against concurrent checkpoints
  * @j_head: Journal head - identifies the first unused block in the journal
  * @j_tail: Journal tail - identifies the oldest still-used block in the
@@ -702,6 +708,7 @@  jbd2_time_diff(unsigned long start, unsigned long end)
  *     journal
  * @j_fs_dev: Device which holds the client fs.  For internal journal this will
  *     be equal to j_dev
+ * @j_reserved_credits: Number of buffers reserved from the running transaction
  * @j_maxlen: Total maximum capacity of the journal region on disk.
  * @j_list_lock: Protects the buffer lists and internal buffer state.
  * @j_inode: Optional inode where we store the journal.  If present, all journal
@@ -800,6 +807,9 @@  struct journal_s
 	/* Wait queue to wait for updates to complete */
 	wait_queue_head_t	j_wait_updates;
 
+	/* Wait queue to wait for reserved buffer credits to drop */
+	wait_queue_head_t	j_wait_reserved;
+
 	/* Semaphore for locking against concurrent checkpoints */
 	struct mutex		j_checkpoint_mutex;
 
@@ -854,6 +864,9 @@  struct journal_s
 	/* Total maximum capacity of the journal region on disk. */
 	unsigned int		j_maxlen;
 
+	/* Number of buffers reserved from the running transaction */
+	atomic_t		j_reserved_credits;
+
 	/*
 	 * Protects the buffer lists and internal buffer state.
 	 */
@@ -1094,6 +1107,10 @@  extern handle_t *jbd2__journal_start(journal_t *, int nblocks, gfp_t gfp_mask,
 				     unsigned int type, unsigned int line_no);
 extern int	 jbd2_journal_restart(handle_t *, int nblocks);
 extern int	 jbd2__journal_restart(handle_t *, int nblocks, gfp_t gfp_mask);
+extern handle_t *jbd2_journal_reserve(journal_t *, int nblocks,
+				      unsigned int type, unsigned int line_no);
+extern int	 jbd2_journal_start_reserved(handle_t *handle);
+extern void	 jbd2_journal_free_reserved(handle_t *handle);
 extern int	 jbd2_journal_extend (handle_t *, int nblocks);
 extern int	 jbd2_journal_get_write_access(handle_t *, struct buffer_head *);
 extern int	 jbd2_journal_get_create_access (handle_t *, struct buffer_head *);