xfs: l_last_sync_lsn is really AIL state

The current implementation of xlog_assign_tail_lsn() assumes that
when the AIL is empty, the log tail matches the LSN of the last
written commit record. This is recorded in xlog_state_set_callback()
as log->l_last_sync_lsn when the iclog state changes to
XLOG_STATE_CALLBACK. This change is then immediately followed by
running the callbacks on the iclog which then insert the log items
into the AIL at the "commit lsn" of that checkpoint.

The AIL tracks log items via the start record LSN of the checkpoint,
not the commit record LSN. This is because we can pipeline multiple
checkpoints, and so the start record of checkpoint N+1 can be
written before the commit record of checkpoint N. i.e:

     start N			commit N
	+-------------+------------+----------------+
		  start N+1			commit N+1

The tail of the log cannot be moved to the LSN of commit N when all
the items of that checkpoint are written back, because then the
start record for N+1 is no longer in the active portion of the log
and recovery will fail/corrupt the filesystem.

Hence when all the log items in checkpoint N are written back, the
tail of the log most now only move as far forwards as the start LSN
of checkpoint N+1.

Hence we cannot use the maximum start record LSN the AIL sees as a
replacement the pointer to the current head of the on-disk log
records. However, we currently only use the l_last_sync_lsn when the
AIL is empty - when there is no start LSN remaining, the tail of the
log moves to the LSN of the last commit record as this is where
recovery needs to start searching for recoverable records. THe next
checkpoint will have a start record LSN that is higher than
l_last_sync_lsn, and so everything still works correctly when new
checkpoints are written to an otherwise empty log.

l_last_sync_lsn is an atomic variable because it is currently
updated when an iclog with callbacks attached moves to the CALLBACK
state. While we hold the icloglock at this point, we don't hold the
AIL lock. When we assign the log tail, we hold the AIL lock, not the
icloglock because we have to look up the AIL. Hence it is an atomic
variable so it's not bound to a specific lock context.

However, the iclog callbacks are only used for CIL checkpoints. We
don't use callbacks with unmount record writes, so the
l_last_sync_lsn variable only gets updated when we are processing
CIL checkpoint callbacks. And those callbacks run under AIL lock
contexts, not icloglock context. The CIL checkpoint already knows
what the LSN of the iclog the commit record was written to (obtained
when written into the iclog before submission) and so we can update
the l_last_sync_lsn under the AIL lock in this callback. No other
iclog callbacks will run until the currently executing one
completes, and hence we can update the l_last_sync_lsn under the AIL
lock safely.

This means l_last_sync_lsn can move to the AIL as the "ail_head_lsn"
and it can be used to replace the atomic l_last_sync_lsn in the
iclog code. This makes tracking the log tail belong entirely to the
AIL, rather than being smeared across log, iclog and AIL state and
locking.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

1 files changed, 9 insertions, 72 deletions

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index ae22f361627f..1977afecd385 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -1231,47 +1231,6 @@ xfs_log_cover(
 }
 
 /*
- * We may be holding the log iclog lock upon entering this routine.
- */
-xfs_lsn_t
-xlog_assign_tail_lsn_locked(
-	struct xfs_mount	*mp)
-{
-	struct xlog		*log = mp->m_log;
-	struct xfs_log_item	*lip;
-	xfs_lsn_t		tail_lsn;
-
-	assert_spin_locked(&mp->m_ail->ail_lock);
-
-	/*
-	 * To make sure we always have a valid LSN for the log tail we keep
-	 * track of the last LSN which was committed in log->l_last_sync_lsn,
-	 * and use that when the AIL was empty.
-	 */
-	lip = xfs_ail_min(mp->m_ail);
-	if (lip)
-		tail_lsn = lip->li_lsn;
-	else
-		tail_lsn = atomic64_read(&log->l_last_sync_lsn);
-	trace_xfs_log_assign_tail_lsn(log, tail_lsn);
-	atomic64_set(&log->l_tail_lsn, tail_lsn);
-	return tail_lsn;
-}
-
-xfs_lsn_t
-xlog_assign_tail_lsn(
-	struct xfs_mount	*mp)
-{
-	xfs_lsn_t		tail_lsn;
-
-	spin_lock(&mp->m_ail->ail_lock);
-	tail_lsn = xlog_assign_tail_lsn_locked(mp);
-	spin_unlock(&mp->m_ail->ail_lock);
-
-	return tail_lsn;
-}
-
-/*
  * Return the space in the log between the tail and the head.  The head
  * is passed in the cycle/bytes formal parms.  In the special case where
  * the reserve head has wrapped passed the tail, this calculation is no
@@ -1501,7 +1460,6 @@ xlog_alloc_log(
 	log->l_prev_block  = -1;
 	/* log->l_tail_lsn = 0x100000000LL; cycle = 1; current block = 0 */
 	xlog_assign_atomic_lsn(&log->l_tail_lsn, 1, 0);
-	xlog_assign_atomic_lsn(&log->l_last_sync_lsn, 1, 0);
 	log->l_curr_cycle  = 1;	    /* 0 is bad since this is initial value */
 
 	if (xfs_has_logv2(mp) && mp->m_sb.sb_logsunit > 1)
@@ -2549,44 +2507,23 @@ xlog_get_lowest_lsn(
 	return lowest_lsn;
 }
 
-/*
- * Completion of a iclog IO does not imply that a transaction has completed, as
- * transactions can be large enough to span many iclogs. We cannot change the
- * tail of the log half way through a transaction as this may be the only
- * transaction in the log and moving the tail to point to the middle of it
- * will prevent recovery from finding the start of the transaction. Hence we
- * should only update the last_sync_lsn if this iclog contains transaction
- * completion callbacks on it.
- *
- * We have to do this before we drop the icloglock to ensure we are the only one
- * that can update it.
- *
- * If we are moving the last_sync_lsn forwards, we also need to ensure we kick
- * the reservation grant head pushing. This is due to the fact that the push
- * target is bound by the current last_sync_lsn value. Hence if we have a large
- * amount of log space bound up in this committing transaction then the
- * last_sync_lsn value may be the limiting factor preventing tail pushing from
- * freeing space in the log. Hence once we've updated the last_sync_lsn we
- * should push the AIL to ensure the push target (and hence the grant head) is
- * no longer bound by the old log head location and can move forwards and make
- * progress again.
- */
 static void
 xlog_state_set_callback(
 	struct xlog		*log,
 	struct xlog_in_core	*iclog,
 	xfs_lsn_t		header_lsn)
 {
+	/*
+	 * If there are no callbacks on this iclog, we can mark it clean
+	 * immediately and return. Otherwise we need to run the
+	 * callbacks.
+	 */
+	if (list_empty(&iclog->ic_callbacks)) {
+		xlog_state_clean_iclog(log, iclog);
+		return;
+	}
 	trace_xlog_iclog_callback(iclog, _RET_IP_);
 	iclog->ic_state = XLOG_STATE_CALLBACK;
-
-	ASSERT(XFS_LSN_CMP(atomic64_read(&log->l_last_sync_lsn),
-			   header_lsn) <= 0);
-
-	if (list_empty_careful(&iclog->ic_callbacks))
-		return;
-
-	atomic64_set(&log->l_last_sync_lsn, header_lsn);
 }
 
 /*