Re: [PATCH 13/45] xfs: xfs_log_force_lsn isn't passed a LSN

From: "Darrick J. Wong" <djwong@kernel.org>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-xfs@vger.kernel.org
Subject: Re: [PATCH 13/45] xfs: xfs_log_force_lsn isn't passed a LSN
Date: Mon, 8 Mar 2021 14:53:23 -0800	[thread overview]
Message-ID: <20210308225323.GC3419940@magnolia> (raw)
In-Reply-To: <20210305051143.182133-14-david@fromorbit.com>

On Fri, Mar 05, 2021 at 04:11:11PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> In doing an investigation into AIL push stalls, I was looking at the
> log force code to see if an async CIL push could be done instead.
> This lead me to xfs_log_force_lsn() and looking at how it works.
> 
> xfs_log_force_lsn() is only called from inode synchronisation
> contexts such as fsync(), and it takes the ip->i_itemp->ili_last_lsn
> value as the LSN to sync the log to. This gets passed to
> xlog_cil_force_lsn() via xfs_log_force_lsn() to flush the CIL to the
> journal, and then used by xfs_log_force_lsn() to flush the iclogs to
> the journal.
> 
> The problem with is that ip->i_itemp->ili_last_lsn does not store a
> log sequence number. What it stores is passed to it from the
> ->iop_committing method, which is called by xfs_log_commit_cil().
> The value this passes to the iop_committing method is the CIL
> context sequence number that the item was committed to.
> 
> As it turns out, xlog_cil_force_lsn() converts the sequence to an
> actual commit LSN for the related context and returns that to
> xfs_log_force_lsn(). xfs_log_force_lsn() overwrites it's "lsn"
> variable that contained a sequence with an actual LSN and then uses
> that to sync the iclogs.
> 
> This caused me some confusion for a while, even though I originally
> wrote all this code a decade ago. ->iop_committing is only used by
> a couple of log item types, and only inode items use the sequence
> number it is passed.
> 
> Let's clean up the API, CIL structures and inode log item to call it
> a sequence number, and make it clear that the high level code is
> using CIL sequence numbers and not on-disk LSNs for integrity
> synchronisation purposes.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/libxfs/xfs_types.h |  1 +
>  fs/xfs/xfs_buf_item.c     |  2 +-
>  fs/xfs/xfs_dquot_item.c   |  2 +-
>  fs/xfs/xfs_file.c         | 14 +++++++-------
>  fs/xfs/xfs_inode.c        | 10 +++++-----
>  fs/xfs/xfs_inode_item.c   |  4 ++--
>  fs/xfs/xfs_inode_item.h   |  2 +-
>  fs/xfs/xfs_log.c          | 27 ++++++++++++++-------------
>  fs/xfs/xfs_log.h          |  4 +---
>  fs/xfs/xfs_log_cil.c      | 22 +++++++++-------------
>  fs/xfs/xfs_log_priv.h     | 15 +++++++--------
>  fs/xfs/xfs_trans.c        |  6 +++---
>  fs/xfs/xfs_trans.h        |  4 ++--
>  13 files changed, 54 insertions(+), 59 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_types.h b/fs/xfs/libxfs/xfs_types.h
> index 064bd6e8c922..0870ef6f933d 100644
> --- a/fs/xfs/libxfs/xfs_types.h
> +++ b/fs/xfs/libxfs/xfs_types.h
> @@ -21,6 +21,7 @@ typedef int32_t		xfs_suminfo_t;	/* type of bitmap summary info */
>  typedef uint32_t	xfs_rtword_t;	/* word type for bitmap manipulations */
>  
>  typedef int64_t		xfs_lsn_t;	/* log sequence number */
> +typedef int64_t		xfs_csn_t;	/* CIL sequence number */

I'm unfamiliar with the internal format of CIL sequence numbers.  Do
they have the same cycle:offset segmented structure as LSNs do?  Or are
they a simple linear integer that increases as we checkpoint committed
items?

Looking through the current code, I see a couple of places where we
initialize them to 1, and I also see that when we create a new cil
context we set its sequence to one more than the context that it will
replace.

I also see a bunch of comparisons of cil context sequence numbers that
use standard integer operators, but then I also see one instance of:

	if (XFS_LSN_CMP(lip->li_seq, ctx->sequence) != 0)
		return false;
	return true

in xfs_log_item_in_current_chkpt.  AFAICT this could be replaced with a
simple:

	return lip->li_seq == ctx->sequence;

But the fact that we're using LSN_CMP in /one/ place sticks out like a
sore thumb to me, and now I'm confused.  Is my understanding incorrect,
or is this operator use incorrect?

>  typedef uint32_t	xfs_dablk_t;	/* dir/attr block number (in file) */
>  typedef uint32_t	xfs_dahash_t;	/* dir/attr hash value */
> diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
> index 14d1fefcbf4c..1cb087b320b1 100644
> --- a/fs/xfs/xfs_buf_item.c
> +++ b/fs/xfs/xfs_buf_item.c
> @@ -713,7 +713,7 @@ xfs_buf_item_release(
>  STATIC void
>  xfs_buf_item_committing(
>  	struct xfs_log_item	*lip,
> -	xfs_lsn_t		commit_lsn)
> +	xfs_csn_t		seq)
>  {
>  	return xfs_buf_item_release(lip);
>  }
> diff --git a/fs/xfs/xfs_dquot_item.c b/fs/xfs/xfs_dquot_item.c
> index 8c1fdf37ee8f..8ed47b739b6c 100644
> --- a/fs/xfs/xfs_dquot_item.c
> +++ b/fs/xfs/xfs_dquot_item.c
> @@ -188,7 +188,7 @@ xfs_qm_dquot_logitem_release(
>  STATIC void
>  xfs_qm_dquot_logitem_committing(
>  	struct xfs_log_item	*lip,
> -	xfs_lsn_t		commit_lsn)
> +	xfs_csn_t		seq)
>  {
>  	return xfs_qm_dquot_logitem_release(lip);

Weird, I didn't know that you could use return like this -- both
_release and _committing return void.

--D

>  }
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 24c7f45fc4eb..ac3120dfe477 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -119,8 +119,8 @@ xfs_dir_fsync(
>  	return xfs_log_force_inode(ip);
>  }
>  
> -static xfs_lsn_t
> -xfs_fsync_lsn(
> +static xfs_csn_t
> +xfs_fsync_seq(
>  	struct xfs_inode	*ip,
>  	bool			datasync)
>  {
> @@ -128,7 +128,7 @@ xfs_fsync_lsn(
>  		return 0;
>  	if (datasync && !(ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP))
>  		return 0;
> -	return ip->i_itemp->ili_last_lsn;
> +	return ip->i_itemp->ili_commit_seq;
>  }
>  
>  /*
> @@ -151,12 +151,12 @@ xfs_fsync_flush_log(
>  	int			*log_flushed)
>  {
>  	int			error = 0;
> -	xfs_lsn_t		lsn;
> +	xfs_csn_t		seq;
>  
>  	xfs_ilock(ip, XFS_ILOCK_SHARED);
> -	lsn = xfs_fsync_lsn(ip, datasync);
> -	if (lsn) {
> -		error = xfs_log_force_lsn(ip->i_mount, lsn, XFS_LOG_SYNC,
> +	seq = xfs_fsync_seq(ip, datasync);
> +	if (seq) {
> +		error = xfs_log_force_seq(ip->i_mount, seq, XFS_LOG_SYNC,
>  					  log_flushed);
>  
>  		spin_lock(&ip->i_itemp->ili_lock);
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index bed2beb169e4..1c2ef1f1859a 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -2644,7 +2644,7 @@ xfs_iunpin(
>  	trace_xfs_inode_unpin_nowait(ip, _RET_IP_);
>  
>  	/* Give the log a push to start the unpinning I/O */
> -	xfs_log_force_lsn(ip->i_mount, ip->i_itemp->ili_last_lsn, 0, NULL);
> +	xfs_log_force_seq(ip->i_mount, ip->i_itemp->ili_commit_seq, 0, NULL);
>  
>  }
>  
> @@ -3652,16 +3652,16 @@ int
>  xfs_log_force_inode(
>  	struct xfs_inode	*ip)
>  {
> -	xfs_lsn_t		lsn = 0;
> +	xfs_csn_t		seq = 0;
>  
>  	xfs_ilock(ip, XFS_ILOCK_SHARED);
>  	if (xfs_ipincount(ip))
> -		lsn = ip->i_itemp->ili_last_lsn;
> +		seq = ip->i_itemp->ili_commit_seq;
>  	xfs_iunlock(ip, XFS_ILOCK_SHARED);
>  
> -	if (!lsn)
> +	if (!seq)
>  		return 0;
> -	return xfs_log_force_lsn(ip->i_mount, lsn, XFS_LOG_SYNC, NULL);
> +	return xfs_log_force_seq(ip->i_mount, seq, XFS_LOG_SYNC, NULL);
>  }
>  
>  /*
> diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
> index 6ff91e5bf3cd..3aba4559469f 100644
> --- a/fs/xfs/xfs_inode_item.c
> +++ b/fs/xfs/xfs_inode_item.c
> @@ -617,9 +617,9 @@ xfs_inode_item_committed(
>  STATIC void
>  xfs_inode_item_committing(
>  	struct xfs_log_item	*lip,
> -	xfs_lsn_t		commit_lsn)
> +	xfs_csn_t		seq)
>  {
> -	INODE_ITEM(lip)->ili_last_lsn = commit_lsn;
> +	INODE_ITEM(lip)->ili_commit_seq = seq;
>  	return xfs_inode_item_release(lip);
>  }
>  
> diff --git a/fs/xfs/xfs_inode_item.h b/fs/xfs/xfs_inode_item.h
> index 4b926e32831c..403b45ab9aa2 100644
> --- a/fs/xfs/xfs_inode_item.h
> +++ b/fs/xfs/xfs_inode_item.h
> @@ -33,7 +33,7 @@ struct xfs_inode_log_item {
>  	unsigned int		ili_fields;	   /* fields to be logged */
>  	unsigned int		ili_fsync_fields;  /* logged since last fsync */
>  	xfs_lsn_t		ili_flush_lsn;	   /* lsn at last flush */
> -	xfs_lsn_t		ili_last_lsn;	   /* lsn at last transaction */
> +	xfs_csn_t		ili_commit_seq;	   /* last transaction commit */
>  };
>  
>  static inline int xfs_inode_clean(struct xfs_inode *ip)
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index ed44d67d7099..145db0f88060 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -3273,14 +3273,13 @@ xfs_log_force(
>  }
>  
>  static int
> -__xfs_log_force_lsn(
> -	struct xfs_mount	*mp,
> +xlog_force_lsn(
> +	struct xlog		*log,
>  	xfs_lsn_t		lsn,
>  	uint			flags,
>  	int			*log_flushed,
>  	bool			already_slept)
>  {
> -	struct xlog		*log = mp->m_log;
>  	struct xlog_in_core	*iclog;
>  
>  	spin_lock(&log->l_icloglock);
> @@ -3313,8 +3312,6 @@ __xfs_log_force_lsn(
>  		if (!already_slept &&
>  		    (iclog->ic_prev->ic_state == XLOG_STATE_WANT_SYNC ||
>  		     iclog->ic_prev->ic_state == XLOG_STATE_SYNCING)) {
> -			XFS_STATS_INC(mp, xs_log_force_sleep);
> -
>  			xlog_wait(&iclog->ic_prev->ic_write_wait,
>  					&log->l_icloglock);
>  			return -EAGAIN;
> @@ -3352,25 +3349,29 @@ __xfs_log_force_lsn(
>   * to disk, that thread will wake up all threads waiting on the queue.
>   */
>  int
> -xfs_log_force_lsn(
> +xfs_log_force_seq(
>  	struct xfs_mount	*mp,
> -	xfs_lsn_t		lsn,
> +	xfs_csn_t		seq,
>  	uint			flags,
>  	int			*log_flushed)
>  {
> +	struct xlog		*log = mp->m_log;
> +	xfs_lsn_t		lsn;
>  	int			ret;
> -	ASSERT(lsn != 0);
> +	ASSERT(seq != 0);
>  
>  	XFS_STATS_INC(mp, xs_log_force);
> -	trace_xfs_log_force(mp, lsn, _RET_IP_);
> +	trace_xfs_log_force(mp, seq, _RET_IP_);
>  
> -	lsn = xlog_cil_force_lsn(mp->m_log, lsn);
> +	lsn = xlog_cil_force_seq(log, seq);
>  	if (lsn == NULLCOMMITLSN)
>  		return 0;
>  
> -	ret = __xfs_log_force_lsn(mp, lsn, flags, log_flushed, false);
> -	if (ret == -EAGAIN)
> -		ret = __xfs_log_force_lsn(mp, lsn, flags, log_flushed, true);
> +	ret = xlog_force_lsn(log, lsn, flags, log_flushed, false);
> +	if (ret == -EAGAIN) {
> +		XFS_STATS_INC(mp, xs_log_force_sleep);
> +		ret = xlog_force_lsn(log, lsn, flags, log_flushed, true);
> +	}
>  	return ret;
>  }
>  
> diff --git a/fs/xfs/xfs_log.h b/fs/xfs/xfs_log.h
> index 044e02cb8921..ba96f4ad9576 100644
> --- a/fs/xfs/xfs_log.h
> +++ b/fs/xfs/xfs_log.h
> @@ -107,7 +106,7 @@ struct xfs_item_ops;
>  struct xfs_trans;
>  
>  int	  xfs_log_force(struct xfs_mount *mp, uint flags);
> -int	  xfs_log_force_lsn(struct xfs_mount *mp, xfs_lsn_t lsn, uint flags,
> +int	  xfs_log_force_seq(struct xfs_mount *mp, xfs_csn_t seq, uint flags,
>  		int *log_forced);
>  int	  xfs_log_mount(struct xfs_mount	*mp,
>  			struct xfs_buftarg	*log_target,
> @@ -132,8 +132,6 @@ bool	xfs_log_writable(struct xfs_mount *mp);
>  struct xlog_ticket *xfs_log_ticket_get(struct xlog_ticket *ticket);
>  void	  xfs_log_ticket_put(struct xlog_ticket *ticket);
>  
> -void	xfs_log_commit_cil(struct xfs_mount *mp, struct xfs_trans *tp,
> -				xfs_lsn_t *commit_lsn, bool regrant);
>  void	xlog_cil_process_committed(struct list_head *list);
>  bool	xfs_log_item_in_current_chkpt(struct xfs_log_item *lip);
>  
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 2f0adc35d8ec..44bb7cc17541 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -794,7 +794,7 @@ xlog_cil_push_work(
>  	 * that higher sequences will wait for us to write out a commit record
>  	 * before they do.
>  	 *
> -	 * xfs_log_force_lsn requires us to mirror the new sequence into the cil
> +	 * xfs_log_force_seq requires us to mirror the new sequence into the cil
>  	 * structure atomically with the addition of this sequence to the
>  	 * committing list. This also ensures that we can do unlocked checks
>  	 * against the current sequence in log forces without risking
> @@ -1058,16 +1058,14 @@ xlog_cil_empty(
>   * allowed again.
>   */
>  void
> -xfs_log_commit_cil(
> -	struct xfs_mount	*mp,
> +xlog_cil_commit(
> +	struct xlog		*log,
>  	struct xfs_trans	*tp,
> -	xfs_lsn_t		*commit_lsn,
> +	xfs_csn_t		*commit_seq,
>  	bool			regrant)
>  {
> -	struct xlog		*log = mp->m_log;
>  	struct xfs_cil		*cil = log->l_cilp;
>  	struct xfs_log_item	*lip, *next;
> -	xfs_lsn_t		xc_commit_lsn;
>  
>  	/*
>  	 * Do all necessary memory allocation before we lock the CIL.
> @@ -1081,10 +1079,6 @@ xfs_log_commit_cil(
>  
>  	xlog_cil_insert_items(log, tp);
>  
> -	xc_commit_lsn = cil->xc_ctx->sequence;
> -	if (commit_lsn)
> -		*commit_lsn = xc_commit_lsn;
> -
>  	if (regrant && !XLOG_FORCED_SHUTDOWN(log))
>  		xfs_log_ticket_regrant(log, tp->t_ticket);
>  	else
> @@ -1107,8 +1101,10 @@ xfs_log_commit_cil(
>  	list_for_each_entry_safe(lip, next, &tp->t_items, li_trans) {
>  		xfs_trans_del_item(lip);
>  		if (lip->li_ops->iop_committing)
> -			lip->li_ops->iop_committing(lip, xc_commit_lsn);
> +			lip->li_ops->iop_committing(lip, cil->xc_ctx->sequence);
>  	}
> +	if (commit_seq)
> +		*commit_seq = cil->xc_ctx->sequence;
>  
>  	/* xlog_cil_push_background() releases cil->xc_ctx_lock */
>  	xlog_cil_push_background(log);
> @@ -1125,9 +1121,9 @@ xfs_log_commit_cil(
>   * iclog flush is necessary following this call.
>   */
>  xfs_lsn_t
> -xlog_cil_force_lsn(
> +xlog_cil_force_seq(
>  	struct xlog	*log,
> -	xfs_lsn_t	sequence)
> +	xfs_csn_t	sequence)
>  {
>  	struct xfs_cil		*cil = log->l_cilp;
>  	struct xfs_cil_ctx	*ctx;
> diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> index 0552e96d2b64..31ce2ce21e27 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -234,7 +234,7 @@ struct xfs_cil;
>  
>  struct xfs_cil_ctx {
>  	struct xfs_cil		*cil;
> -	xfs_lsn_t		sequence;	/* chkpt sequence # */
> +	xfs_csn_t		sequence;	/* chkpt sequence # */
>  	xfs_lsn_t		start_lsn;	/* first LSN of chkpt commit */
>  	xfs_lsn_t		commit_lsn;	/* chkpt commit record lsn */
>  	struct xlog_ticket	*ticket;	/* chkpt ticket */
> @@ -272,10 +272,10 @@ struct xfs_cil {
>  	struct xfs_cil_ctx	*xc_ctx;
>  
>  	spinlock_t		xc_push_lock ____cacheline_aligned_in_smp;
> -	xfs_lsn_t		xc_push_seq;
> +	xfs_csn_t		xc_push_seq;
>  	struct list_head	xc_committing;
>  	wait_queue_head_t	xc_commit_wait;
> -	xfs_lsn_t		xc_current_sequence;
> +	xfs_csn_t		xc_current_sequence;
>  	struct work_struct	xc_push_work;
>  	wait_queue_head_t	xc_push_wait;	/* background push throttle */
>  } ____cacheline_aligned_in_smp;
> @@ -552,19 +552,18 @@ int	xlog_cil_init(struct xlog *log);
>  void	xlog_cil_init_post_recovery(struct xlog *log);
>  void	xlog_cil_destroy(struct xlog *log);
>  bool	xlog_cil_empty(struct xlog *log);
> +void	xlog_cil_commit(struct xlog *log, struct xfs_trans *tp,
> +			xfs_csn_t *commit_seq, bool regrant);
>  
>  /*
>   * CIL force routines
>   */
> -xfs_lsn_t
> -xlog_cil_force_lsn(
> -	struct xlog *log,
> -	xfs_lsn_t sequence);
> +xfs_lsn_t xlog_cil_force_seq(struct xlog *log, xfs_csn_t sequence);
>  
>  static inline void
>  xlog_cil_force(struct xlog *log)
>  {
> -	xlog_cil_force_lsn(log, log->l_cilp->xc_current_sequence);
> +	xlog_cil_force_seq(log, log->l_cilp->xc_current_sequence);
>  }
>  
>  /*
> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> index b22a09e9daee..21ac7c048380 100644
> --- a/fs/xfs/xfs_trans.c
> +++ b/fs/xfs/xfs_trans.c
> @@ -851,7 +851,7 @@ __xfs_trans_commit(
>  	bool			regrant)
>  {
>  	struct xfs_mount	*mp = tp->t_mountp;
> -	xfs_lsn_t		commit_lsn = -1;
> +	xfs_csn_t		commit_seq = 0;
>  	int			error = 0;
>  	int			sync = tp->t_flags & XFS_TRANS_SYNC;
>  
> @@ -893,7 +893,7 @@ __xfs_trans_commit(
>  		xfs_trans_apply_sb_deltas(tp);
>  	xfs_trans_apply_dquot_deltas(tp);
>  
> -	xfs_log_commit_cil(mp, tp, &commit_lsn, regrant);
> +	xlog_cil_commit(mp->m_log, tp, &commit_seq, regrant);
>  
>  	xfs_trans_free(tp);
>  
> @@ -902,7 +902,7 @@ __xfs_trans_commit(
>  	 * log out now and wait for it.
>  	 */
>  	if (sync) {
> -		error = xfs_log_force_lsn(mp, commit_lsn, XFS_LOG_SYNC, NULL);
> +		error = xfs_log_force_seq(mp, commit_seq, XFS_LOG_SYNC, NULL);
>  		XFS_STATS_INC(mp, xs_trans_sync);
>  	} else {
>  		XFS_STATS_INC(mp, xs_trans_async);
> diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> index 9dd745cf77c9..6276c7d251e6 100644
> --- a/fs/xfs/xfs_trans.h
> +++ b/fs/xfs/xfs_trans.h
> @@ -43,7 +43,7 @@ struct xfs_log_item {
>  	struct list_head		li_cil;		/* CIL pointers */
>  	struct xfs_log_vec		*li_lv;		/* active log vector */
>  	struct xfs_log_vec		*li_lv_shadow;	/* standby vector */
> -	xfs_lsn_t			li_seq;		/* CIL commit seq */
> +	xfs_csn_t			li_seq;		/* CIL commit seq */
>  };
>  
>  /*
> @@ -69,7 +69,7 @@ struct xfs_item_ops {
>  	void (*iop_pin)(struct xfs_log_item *);
>  	void (*iop_unpin)(struct xfs_log_item *, int remove);
>  	uint (*iop_push)(struct xfs_log_item *, struct list_head *);
> -	void (*iop_committing)(struct xfs_log_item *, xfs_lsn_t commit_lsn);
> +	void (*iop_committing)(struct xfs_log_item *lip, xfs_csn_t seq);
>  	void (*iop_release)(struct xfs_log_item *);
>  	xfs_lsn_t (*iop_committed)(struct xfs_log_item *, xfs_lsn_t);
>  	int (*iop_recover)(struct xfs_log_item *lip,
> -- 
> 2.28.0
> 