Re: [PATCH 5/6] xfs: optimize cache flushing for CIL commits on multi-device file systems

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Sep 02, 2026 at 08:49:19AM +0300, Christoph Hellwig wrote:
> While the flush before writing a start record needs to cover data on the
> data and possibly RT devices, the flush before writing the commit record
> iclog only needs to ensure previous iclogs are stable on disk.
> 
> Split out a new XLOG_ICL_NEED_FLUSH_LOG flag out of XLOG_ICL_NEED_FLUSH
> to signal that only the log device needs to be flushed.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_log.c      |  6 ++++--
>  fs/xfs/xfs_log_cil.c  |  9 ++++++---
>  fs/xfs/xfs_log_priv.h | 10 ++++++----
>  3 files changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 8079f2e003db..a5870877baed 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -1613,12 +1613,14 @@ xlog_write_iclog(
>  	if (iclog->ic_flags & XLOG_ICL_NEED_FLUSH) {
>  		if (xlog_flush_data_caches(log))
>  			goto shutdown;
> -		iclog->ic_bio.bi_opf |= REQ_PREFLUSH;
>  	}
> +	if (iclog->ic_flags & (XLOG_ICL_NEED_FLUSH | XLOG_ICL_NEED_FLUSH_LOG))
> +		iclog->ic_bio.bi_opf |= REQ_PREFLUSH;
>  	if (iclog->ic_flags & XLOG_ICL_NEED_FUA)
>  		iclog->ic_bio.bi_opf |= REQ_FUA;
>  
> -	iclog->ic_flags &= ~(XLOG_ICL_NEED_FLUSH | XLOG_ICL_NEED_FUA);
> +	iclog->ic_flags &= ~(XLOG_ICL_NEED_FLUSH | XLOG_ICL_NEED_FLUSH_LOG |
> +			     XLOG_ICL_NEED_FUA);
>  
>  	if (is_vmalloc_addr(iclog->ic_header)) {
>  		if (!bio_add_vmalloc(&iclog->ic_bio, iclog->ic_header, count))
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 3d159b1350df..e05b77fbfc3d 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -1562,10 +1562,13 @@ xlog_cil_push_work(
>  		}
>  
>  		/*
> -		 * We need to issue a pre-flush so that the ordering for this
> -		 * checkpoint is correctly preserved down to stable storage.
> +		 * We need to issue a pre-flush on the device containing the log
> +		 * so that the ordering for this checkpoint is correctly
> +		 * preserved down to stable storage.
> +		 * There is no need for an extra flush on devices that only
> +		 * contain data or non-log metadata.
>  		 */
> -		ctx->commit_iclog->ic_flags |= XLOG_ICL_NEED_FLUSH;
> +		ctx->commit_iclog->ic_flags |= XLOG_ICL_NEED_FLUSH_LOG;

This is the "wait for all previous iclogs" case, so yes, we only need to
flush the log device.

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

>  	}
>  
>  	/*
> diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> index cf1e4ce61a8c..7c72c3b3ffe1 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -71,12 +71,14 @@ enum xlog_iclog_state {
>  /*
>   * In core log flags
>   */
> -#define XLOG_ICL_NEED_FLUSH	(1u << 0)	/* iclog needs REQ_PREFLUSH */
> -#define XLOG_ICL_NEED_FUA	(1u << 1)	/* iclog needs REQ_FUA */
> +#define XLOG_ICL_NEED_FLUSH	(1u << 0)	/* all devs need REQ_PREFLUSH */
> +#define XLOG_ICL_NEED_FLUSH_LOG	(1u << 0)	/* logdev needs REQ_PREFLUSH */
> +#define XLOG_ICL_NEED_FUA	(1u << 2)	/* iclog needs REQ_FUA */
>  
>  #define XLOG_ICL_STRINGS \
> -	{ XLOG_ICL_NEED_FLUSH,	"XLOG_ICL_NEED_FLUSH" }, \
> -	{ XLOG_ICL_NEED_FUA,	"XLOG_ICL_NEED_FUA" }
> +	{ XLOG_ICL_NEED_FLUSH,		"XLOG_ICL_NEED_FLUSH" }, \
> +	{ XLOG_ICL_NEED_FLUSH_LOG,	"XLOG_ICL_NEED_FLUSH_LOG" }, \
> +	{ XLOG_ICL_NEED_FUA,		"XLOG_ICL_NEED_FUA" }
>  
>  
>  /*
> -- 
> 2.53.0
> 
>