xfs: xfs_log_force_lsn isn't passed a LSN

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 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>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

1 files changed, 11 insertions, 19 deletions

diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index 903617e6d054..3c2b1205944d 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -788,7 +788,7 @@ 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
@@ -1057,16 +1057,14 @@ 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.
@@ -1080,10 +1078,6 @@ 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
@@ -1106,8 +1100,10 @@ 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);
@@ -1124,9 +1120,9 @@ 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;
@@ -1222,21 +1218,17 @@ bool
 xfs_log_item_in_current_chkpt(
 	struct xfs_log_item *lip)
 {
-	struct xfs_cil_ctx *ctx;
+	struct xfs_cil_ctx *ctx = lip->li_mountp->m_log->l_cilp->xc_ctx;
 
 	if (list_empty(&lip->li_cil))
 		return false;
 
-	ctx = lip->li_mountp->m_log->l_cilp->xc_ctx;
-
 	/*
 	 * li_seq is written on the first commit of a log item to record the
 	 * first checkpoint it is written to. Hence if it is different to the
 	 * current sequence, we're in a new checkpoint.
 	 */
-	if (XFS_LSN_CMP(lip->li_seq, ctx->sequence) != 0)
-		return false;
-	return true;
+	return lip->li_seq == ctx->sequence;
 }
 
 /*