fix cache flushes for the RT device

fix cache flushes for the RT device

[Christoph Hellwig]

Hi all,

when tracing workloads, I realized that currently the volatile write
cache on RT devices is only flushed by fsync, but never at all for
workloads that do not use fsync.

Changes since v1:
 - drop the patches that are pure optimizations for a minimal fix
   series
 - improve a commit log

Diffstat:
 xfs_file.c    |   47 ++++++++++++++---------------------------------
 xfs_log.c     |   45 +++++++++++++++++++++++++++++++--------------
 xfs_log_cil.c |    7 ++++---
 3 files changed, 49 insertions(+), 50 deletions(-)

[PATCH 1/3] xfs: also flush the RT device cache in xlog_write_iclog

[Christoph Hellwig]

The cache flush before writing the CIL start record no only needs to
ensure any metadata covered by the overwritten part of the log is on
stable storage, but also that any data pointed to by metadata logged
is on stable storage, as otherwise log recovery could created allocated
blocks that point to stale data.  Fortunately the code already
handles this right for the data device, but it also needs to flush
the RT device for this to work for data on the RT device.

Also update the comments to explicitly mention this case.

This omission goes back to the first days of cache control in XFS.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
---
 fs/xfs/xfs_log.c     | 45 ++++++++++++++++++++++++++++++--------------
 fs/xfs/xfs_log_cil.c |  7 ++++---
 2 files changed, 35 insertions(+), 17 deletions(-)

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 2a34611d81f6..f4f81d893e8c 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -1544,6 +1544,35 @@ xlog_bio_end_io(
 		   &iclog->ic_end_io_work);
 }
 
+/*
+ * When using multiple devices, we also need to flush the data and RT device
+ * caches first to ensure that all metadata writeback covered by the LSN in
+ * this iclog is on stable storage. This is slow, but it *must* complete
+ * before we issue the external log IO.
+ *
+ * If the flush fails, we cannot conclude that past metadata writeback from
+ * the log succeeded.  Repeating the flush is not possible, hence we must
+ * shut down with log IO error to avoid shutdown re-entering this path and
+ * erroring out again.
+ */
+static int
+xlog_flush_data_caches(
+	struct xlog		*log)
+{
+	struct xfs_mount	*mp = log->l_mp;
+
+	if (log->l_targ != mp->m_ddev_targp) {
+		if (blkdev_issue_flush(mp->m_ddev_targp->bt_bdev))
+			return -EIO;
+	}
+	if (mp->m_rtdev_targp && mp->m_rtdev_targp != mp->m_ddev_targp) {
+		if (blkdev_issue_flush(mp->m_rtdev_targp->bt_bdev))
+			return -EIO;
+	}
+
+	return 0;
+}
+
 STATIC void
 xlog_write_iclog(
 	struct xlog		*log,
@@ -1588,21 +1617,9 @@ xlog_write_iclog(
 	iclog->ic_bio.bi_private = iclog;
 
 	if (iclog->ic_flags & XLOG_ICL_NEED_FLUSH) {
-		iclog->ic_bio.bi_opf |= REQ_PREFLUSH;
-		/*
-		 * For external log devices, we also need to flush the data
-		 * device cache first to ensure all metadata writeback covered
-		 * by the LSN in this iclog is on stable storage. This is slow,
-		 * but it *must* complete before we issue the external log IO.
-		 *
-		 * If the flush fails, we cannot conclude that past metadata
-		 * writeback from the log succeeded.  Repeating the flush is
-		 * not possible, hence we must shut down with log IO error to
-		 * avoid shutdown re-entering this path and erroring out again.
-		 */
-		if (log->l_targ != log->l_mp->m_ddev_targp &&
-		    blkdev_issue_flush(log->l_mp->m_ddev_targp->bt_bdev))
+		if (xlog_flush_data_caches(log))
 			goto shutdown;
+		iclog->ic_bio.bi_opf |= REQ_PREFLUSH;
 	}
 	if (iclog->ic_flags & XLOG_ICL_NEED_FUA)
 		iclog->ic_bio.bi_opf |= REQ_FUA;
diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index 166531018ce4..f9e07a32f60f 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -1055,9 +1055,10 @@ xlog_cil_set_ctx_write_state(
 		spin_unlock(&cil->xc_push_lock);
 
 		/*
-		 * Make sure the metadata we are about to overwrite in the log
-		 * has been flushed to stable storage before this iclog is
-		 * issued.
+		 * Flush the write cache before writing the start record so that
+		 * the metadata we are about to overwrite in the log and the
+		 * data that new allocations in this context refer to are
+		 * persisted to stable storage before this iclog is written.
 		 */
 		spin_lock(&cil->xc_log->l_icloglock);
 		iclog->ic_flags |= XLOG_ICL_NEED_FLUSH;
-- 
2.53.0

[PATCH 2/3] xfs: don't continue on error in xfs_fsync

[Christoph Hellwig]

As soon as we get an error from cache flushing or log forcing, there
is no point in continuing as the data integrity is already impacted.
Return the error instead of continuing to do more work.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
---
 fs/xfs/xfs_file.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 426a67b813a7..0d31fea67a2c 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -130,8 +130,8 @@ xfs_file_fsync(
 {
 	struct xfs_inode	*ip = XFS_I(file->f_mapping->host);
 	struct xfs_mount	*mp = ip->i_mount;
-	int			error, err2;
 	int			log_flushed = 0;
+	int			error;
 
 	trace_xfs_file_fsync(ip);
 
@@ -154,15 +154,17 @@ xfs_file_fsync(
 		error = blkdev_issue_flush(mp->m_rtdev_targp->bt_bdev);
 	else if (mp->m_logdev_targp != mp->m_ddev_targp)
 		error = blkdev_issue_flush(mp->m_ddev_targp->bt_bdev);
+	if (error)
+		return error;
 
 	/*
 	 * If the inode has a inode log item attached, it may need the journal
 	 * flushed to persist any changes the log item might be tracking.
 	 */
 	if (ip->i_itemp) {
-		err2 = xfs_fsync_flush_log(ip, datasync, &log_flushed);
-		if (err2 && !error)
-			error = err2;
+		error = xfs_fsync_flush_log(ip, datasync, &log_flushed);
+		if (error)
+			return error;
 	}
 
 	/*
@@ -178,14 +180,11 @@ xfs_file_fsync(
 	if (!log_flushed) {
 		struct xfs_buftarg *file_targp = xfs_inode_buftarg(ip);
 
-		if (mp->m_logdev_targp == file_targp) {
-			err2 = blkdev_issue_flush(file_targp->bt_bdev);
-			if (err2 && !error)
-				error = err2;
-		}
+		if (mp->m_logdev_targp == file_targp)
+			return blkdev_issue_flush(file_targp->bt_bdev);
 	}
 
-	return error;
+	return 0;
 }
 
 static int
-- 
2.53.0

[PATCH 3/3] xfs: avoid extra cache flushes for multi-device file systems in xfs_fsync

[Christoph Hellwig]

When xlog_force_lsn sets log_flushed, it has just called xlog_force_iclog
through xlog_force_and_check_iclog, which sets XLOG_ICL_NEED_FLUSH before
writing out the head iclog.  This means that we already flushed the log,
data, and (with the recent fix) RT devices before writing out the iclog
start record and no extra cache flushed is required.

This optimizes the external log case, and fixes a performance regression
due to double RT dev flushes with "xfs: also flush the RT device cache in
xlog_write_iclog".

The explicit flush of the data that the device resides on when no iclog
was written out is still required.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
---
 fs/xfs/xfs_file.c | 36 +++++++++---------------------------
 1 file changed, 9 insertions(+), 27 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 0d31fea67a2c..d8202da15aca 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -129,7 +129,6 @@ xfs_file_fsync(
 	int			datasync)
 {
 	struct xfs_inode	*ip = XFS_I(file->f_mapping->host);
-	struct xfs_mount	*mp = ip->i_mount;
 	int			log_flushed = 0;
 	int			error;
 
@@ -139,27 +138,17 @@ xfs_file_fsync(
 	if (error)
 		return error;
 
-	if (xfs_is_shutdown(mp))
+	if (xfs_is_shutdown(ip->i_mount))
 		return -EIO;
 
 	xfs_iflags_clear(ip, XFS_ITRUNCATED);
 
 	/*
-	 * If we have an RT and/or log subvolume we need to make sure to flush
-	 * the write cache the device used for file data first.  This is to
-	 * ensure newly written file data make it to disk before logging the new
-	 * inode size in case of an extending write.
-	 */
-	if (XFS_IS_REALTIME_INODE(ip) && mp->m_rtdev_targp != mp->m_ddev_targp)
-		error = blkdev_issue_flush(mp->m_rtdev_targp->bt_bdev);
-	else if (mp->m_logdev_targp != mp->m_ddev_targp)
-		error = blkdev_issue_flush(mp->m_ddev_targp->bt_bdev);
-	if (error)
-		return error;
-
-	/*
-	 * If the inode has a inode log item attached, it may need the journal
-	 * flushed to persist any changes the log item might be tracking.
+	 * If the inode has a log item attached, we must force the log up to the
+	 * last LSN in which the inode was modified to ensure all metadata is
+	 * persisted.  The log force will flush the caches for all devices
+	 * before writing the log records unless it is a no-op because there are
+	 * no modifications to this inode that need to be pushed out.
 	 */
 	if (ip->i_itemp) {
 		error = xfs_fsync_flush_log(ip, datasync, &log_flushed);
@@ -173,17 +162,10 @@ xfs_file_fsync(
 	 * when no metadata needed to be committed.
 	 *
 	 * Use the inode's actual file data target rather than assuming the
-	 * main data device. Realtime inodes with a separate realtime device
-	 * are flushed before the log force, so this fallback only applies
-	 * when the file data target is the same as the log target.
+	 * main data device.
 	 */
-	if (!log_flushed) {
-		struct xfs_buftarg *file_targp = xfs_inode_buftarg(ip);
-
-		if (mp->m_logdev_targp == file_targp)
-			return blkdev_issue_flush(file_targp->bt_bdev);
-	}
-
+	if (!log_flushed)
+		return blkdev_issue_flush(xfs_inode_buftarg(ip)->bt_bdev);
 	return 0;
 }
 
-- 
2.53.0

Re: [PATCH 1/3] xfs: also flush the RT device cache in xlog_write_iclog

[Carlos Maiolino]

On Mon, Sep 07, 2026 at 10:33:07AM +0300, Christoph Hellwig wrote:
> The cache flush before writing the CIL start record no only needs to
> ensure any metadata covered by the overwritten part of the log is on
> stable storage, but also that any data pointed to by metadata logged
> is on stable storage, as otherwise log recovery could created allocated
> blocks that point to stale data.  Fortunately the code already
> handles this right for the data device, but it also needs to flush
> the RT device for this to work for data on the RT device.
> 
> Also update the comments to explicitly mention this case.
> 
> This omission goes back to the first days of cache control in XFS.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
> ---
>  fs/xfs/xfs_log.c     | 45 ++++++++++++++++++++++++++++++--------------
>  fs/xfs/xfs_log_cil.c |  7 ++++---
>  2 files changed, 35 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 2a34611d81f6..f4f81d893e8c 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -1544,6 +1544,35 @@ xlog_bio_end_io(
>  		   &iclog->ic_end_io_work);
>  }
>  
> +/*
> + * When using multiple devices, we also need to flush the data and RT device
> + * caches first to ensure that all metadata writeback covered by the LSN in
> + * this iclog is on stable storage. This is slow, but it *must* complete
> + * before we issue the external log IO.
> + *
> + * If the flush fails, we cannot conclude that past metadata writeback from
> + * the log succeeded.  Repeating the flush is not possible, hence we must
> + * shut down with log IO error to avoid shutdown re-entering this path and
> + * erroring out again.
> + */
> +static int
> +xlog_flush_data_caches(
> +	struct xlog		*log)
> +{
> +	struct xfs_mount	*mp = log->l_mp;
> +
> +	if (log->l_targ != mp->m_ddev_targp) {
> +		if (blkdev_issue_flush(mp->m_ddev_targp->bt_bdev))
> +			return -EIO;
> +	}
> +	if (mp->m_rtdev_targp && mp->m_rtdev_targp != mp->m_ddev_targp) {
> +		if (blkdev_issue_flush(mp->m_rtdev_targp->bt_bdev))
> +			return -EIO;
> +	}
> +
> +	return 0;
> +}
> +
>  STATIC void
>  xlog_write_iclog(
>  	struct xlog		*log,
> @@ -1588,21 +1617,9 @@ xlog_write_iclog(
>  	iclog->ic_bio.bi_private = iclog;
>  
>  	if (iclog->ic_flags & XLOG_ICL_NEED_FLUSH) {
> -		iclog->ic_bio.bi_opf |= REQ_PREFLUSH;
> -		/*
> -		 * For external log devices, we also need to flush the data
> -		 * device cache first to ensure all metadata writeback covered
> -		 * by the LSN in this iclog is on stable storage. This is slow,
> -		 * but it *must* complete before we issue the external log IO.
> -		 *
> -		 * If the flush fails, we cannot conclude that past metadata
> -		 * writeback from the log succeeded.  Repeating the flush is
> -		 * not possible, hence we must shut down with log IO error to
> -		 * avoid shutdown re-entering this path and erroring out again.
> -		 */
> -		if (log->l_targ != log->l_mp->m_ddev_targp &&
> -		    blkdev_issue_flush(log->l_mp->m_ddev_targp->bt_bdev))
> +		if (xlog_flush_data_caches(log))
>  			goto shutdown;
> +		iclog->ic_bio.bi_opf |= REQ_PREFLUSH;
>  	}
>  	if (iclog->ic_flags & XLOG_ICL_NEED_FUA)
>  		iclog->ic_bio.bi_opf |= REQ_FUA;
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 166531018ce4..f9e07a32f60f 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -1055,9 +1055,10 @@ xlog_cil_set_ctx_write_state(
>  		spin_unlock(&cil->xc_push_lock);
>  
>  		/*
> -		 * Make sure the metadata we are about to overwrite in the log
> -		 * has been flushed to stable storage before this iclog is
> -		 * issued.
> +		 * Flush the write cache before writing the start record so that
> +		 * the metadata we are about to overwrite in the log and the
> +		 * data that new allocations in this context refer to are
> +		 * persisted to stable storage before this iclog is written.
>  		 */
>  		spin_lock(&cil->xc_log->l_icloglock);
>  		iclog->ic_flags |= XLOG_ICL_NEED_FLUSH;
> -- 
> 2.53.0
> 
> 

Re: [PATCH 2/3] xfs: don't continue on error in xfs_fsync

[Carlos Maiolino]

On Mon, Sep 07, 2026 at 10:33:08AM +0300, Christoph Hellwig wrote:
> As soon as we get an error from cache flushing or log forcing, there
> is no point in continuing as the data integrity is already impacted.
> Return the error instead of continuing to do more work.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
> ---

Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>

>  fs/xfs/xfs_file.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 426a67b813a7..0d31fea67a2c 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -130,8 +130,8 @@ xfs_file_fsync(
>  {
>  	struct xfs_inode	*ip = XFS_I(file->f_mapping->host);
>  	struct xfs_mount	*mp = ip->i_mount;
> -	int			error, err2;
>  	int			log_flushed = 0;
> +	int			error;
>  
>  	trace_xfs_file_fsync(ip);
>  
> @@ -154,15 +154,17 @@ xfs_file_fsync(
>  		error = blkdev_issue_flush(mp->m_rtdev_targp->bt_bdev);
>  	else if (mp->m_logdev_targp != mp->m_ddev_targp)
>  		error = blkdev_issue_flush(mp->m_ddev_targp->bt_bdev);
> +	if (error)
> +		return error;
>  
>  	/*
>  	 * If the inode has a inode log item attached, it may need the journal
>  	 * flushed to persist any changes the log item might be tracking.
>  	 */
>  	if (ip->i_itemp) {
> -		err2 = xfs_fsync_flush_log(ip, datasync, &log_flushed);
> -		if (err2 && !error)
> -			error = err2;
> +		error = xfs_fsync_flush_log(ip, datasync, &log_flushed);
> +		if (error)
> +			return error;
>  	}
>  
>  	/*
> @@ -178,14 +180,11 @@ xfs_file_fsync(
>  	if (!log_flushed) {
>  		struct xfs_buftarg *file_targp = xfs_inode_buftarg(ip);
>  
> -		if (mp->m_logdev_targp == file_targp) {
> -			err2 = blkdev_issue_flush(file_targp->bt_bdev);
> -			if (err2 && !error)
> -				error = err2;
> -		}
> +		if (mp->m_logdev_targp == file_targp)
> +			return blkdev_issue_flush(file_targp->bt_bdev);
>  	}
>  
> -	return error;
> +	return 0;
>  }
>  
>  static int
> -- 
> 2.53.0
> 
> 

Re: [PATCH 3/3] xfs: avoid extra cache flushes for multi-device file systems in xfs_fsync

[Carlos Maiolino]

On Mon, Sep 07, 2026 at 10:33:09AM +0300, Christoph Hellwig wrote:
> When xlog_force_lsn sets log_flushed, it has just called xlog_force_iclog
> through xlog_force_and_check_iclog, which sets XLOG_ICL_NEED_FLUSH before
> writing out the head iclog.  This means that we already flushed the log,
> data, and (with the recent fix) RT devices before writing out the iclog
> start record and no extra cache flushed is required.
> 
> This optimizes the external log case, and fixes a performance regression
> due to double RT dev flushes with "xfs: also flush the RT device cache in
> xlog_write_iclog".
> 
> The explicit flush of the data that the device resides on when no iclog
> was written out is still required.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>

> ---
>  fs/xfs/xfs_file.c | 36 +++++++++---------------------------
>  1 file changed, 9 insertions(+), 27 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 0d31fea67a2c..d8202da15aca 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -129,7 +129,6 @@ xfs_file_fsync(
>  	int			datasync)
>  {
>  	struct xfs_inode	*ip = XFS_I(file->f_mapping->host);
> -	struct xfs_mount	*mp = ip->i_mount;
>  	int			log_flushed = 0;
>  	int			error;
>  
> @@ -139,27 +138,17 @@ xfs_file_fsync(
>  	if (error)
>  		return error;
>  
> -	if (xfs_is_shutdown(mp))
> +	if (xfs_is_shutdown(ip->i_mount))
>  		return -EIO;
>  
>  	xfs_iflags_clear(ip, XFS_ITRUNCATED);
>  
>  	/*
> -	 * If we have an RT and/or log subvolume we need to make sure to flush
> -	 * the write cache the device used for file data first.  This is to
> -	 * ensure newly written file data make it to disk before logging the new
> -	 * inode size in case of an extending write.
> -	 */
> -	if (XFS_IS_REALTIME_INODE(ip) && mp->m_rtdev_targp != mp->m_ddev_targp)
> -		error = blkdev_issue_flush(mp->m_rtdev_targp->bt_bdev);
> -	else if (mp->m_logdev_targp != mp->m_ddev_targp)
> -		error = blkdev_issue_flush(mp->m_ddev_targp->bt_bdev);
> -	if (error)
> -		return error;
> -
> -	/*
> -	 * If the inode has a inode log item attached, it may need the journal
> -	 * flushed to persist any changes the log item might be tracking.
> +	 * If the inode has a log item attached, we must force the log up to the
> +	 * last LSN in which the inode was modified to ensure all metadata is
> +	 * persisted.  The log force will flush the caches for all devices
> +	 * before writing the log records unless it is a no-op because there are
> +	 * no modifications to this inode that need to be pushed out.
>  	 */
>  	if (ip->i_itemp) {
>  		error = xfs_fsync_flush_log(ip, datasync, &log_flushed);
> @@ -173,17 +162,10 @@ xfs_file_fsync(
>  	 * when no metadata needed to be committed.
>  	 *
>  	 * Use the inode's actual file data target rather than assuming the
> -	 * main data device. Realtime inodes with a separate realtime device
> -	 * are flushed before the log force, so this fallback only applies
> -	 * when the file data target is the same as the log target.
> +	 * main data device.
>  	 */
> -	if (!log_flushed) {
> -		struct xfs_buftarg *file_targp = xfs_inode_buftarg(ip);
> -
> -		if (mp->m_logdev_targp == file_targp)
> -			return blkdev_issue_flush(file_targp->bt_bdev);
> -	}
> -
> +	if (!log_flushed)
> +		return blkdev_issue_flush(xfs_inode_buftarg(ip)->bt_bdev);
>  	return 0;
>  }
>  
> -- 
> 2.53.0
> 
> 

Re: fix cache flushes for the RT device

[Carlos Maiolino]

On Mon, 07 Sep 2026 10:33:06 +0300, Christoph Hellwig wrote:
> when tracing workloads, I realized that currently the volatile write
> cache on RT devices is only flushed by fsync, but never at all for
> workloads that do not use fsync.
> 
> Changes since v1:
>  - drop the patches that are pure optimizations for a minimal fix
>    series
>  - improve a commit log
> 
> [...]

Applied to for-next, thanks!

[1/3] xfs: also flush the RT device cache in xlog_write_iclog
      commit: ad0033e2dbd3ecc063dfe613060da5cbab9a4970
[2/3] xfs: don't continue on error in xfs_fsync
      commit: c84455c683eb0b0397b0f5c5f5ce5cd82572a23f
[3/3] xfs: avoid extra cache flushes for multi-device file systems in xfs_fsync
      commit: 761e015e5a54851043c3b5bb7cfb6f539b01a35e

Best regards,
-- 
Carlos Maiolino <cem@kernel.org>