Re: [PATCH 14/39] xfs: embed the xlog_op_header in the unmount record

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

On Wed, May 19, 2021 at 10:12:52PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> Remove another case where xlog_write() has to prepend an opheader to
> a log transaction. The unmount record + ophdr is smaller than the
> minimum amount of space guaranteed to be free in an iclog (2 *
> sizeof(ophdr)) and so we don't have to care about an unmount record
> being split across 2 iclogs.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>

I love me some BUILD_BUG_ON, so

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

--D

> ---
>  fs/xfs/xfs_log.c | 39 ++++++++++++++++++++++++++++-----------
>  1 file changed, 28 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 981cd6f8f0ff..e7a135ffa66f 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -800,12 +800,22 @@ xlog_write_unmount_record(
>  	struct xlog		*log,
>  	struct xlog_ticket	*ticket)
>  {
> -	struct xfs_unmount_log_format ulf = {
> -		.magic = XLOG_UNMOUNT_TYPE,
> +	struct  {
> +		struct xlog_op_header ophdr;
> +		struct xfs_unmount_log_format ulf;
> +	} unmount_rec = {
> +		.ophdr = {
> +			.oh_clientid = XFS_LOG,
> +			.oh_tid = cpu_to_be32(ticket->t_tid),
> +			.oh_flags = XLOG_UNMOUNT_TRANS,
> +		},
> +		.ulf = {
> +			.magic = XLOG_UNMOUNT_TYPE,
> +		},
>  	};
>  	struct xfs_log_iovec reg = {
> -		.i_addr = &ulf,
> -		.i_len = sizeof(ulf),
> +		.i_addr = &unmount_rec,
> +		.i_len = sizeof(unmount_rec),
>  		.i_type = XLOG_REG_TYPE_UNMOUNT,
>  	};
>  	struct xfs_log_vec vec = {
> @@ -813,8 +823,12 @@ xlog_write_unmount_record(
>  		.lv_iovecp = &reg,
>  	};
>  
> +	BUILD_BUG_ON((sizeof(struct xlog_op_header) +
> +		      sizeof(struct xfs_unmount_log_format)) !=
> +							sizeof(unmount_rec));
> +
>  	/* account for space used by record data */
> -	ticket->t_curr_res -= sizeof(ulf);
> +	ticket->t_curr_res -= sizeof(unmount_rec);
>  
>  	/*
>  	 * For external log devices, we need to flush the data device cache
> @@ -2145,6 +2159,8 @@ xlog_write_calc_vec_length(
>  
>  	/* Don't account for regions with embedded ophdrs */
>  	if (optype && headers > 0) {
> +		if (optype & XLOG_UNMOUNT_TRANS)
> +			headers--;
>  		if (optype & XLOG_START_TRANS) {
>  			ASSERT(headers >= 2);
>  			headers -= 2;
> @@ -2359,12 +2375,11 @@ xlog_write(
>  
>  	/*
>  	 * If this is a commit or unmount transaction, we don't need a start
> -	 * record to be written.  We do, however, have to account for the
> -	 * commit or unmount header that gets written. Hence we always have
> -	 * to account for an extra xlog_op_header here for commit and unmount
> -	 * records.
> +	 * record to be written.  We do, however, have to account for the commit
> +	 * header that gets written. Hence we always have to account for an
> +	 * extra xlog_op_header here for commit records.
>  	 */
> -	if (optype & (XLOG_COMMIT_TRANS | XLOG_UNMOUNT_TRANS))
> +	if (optype & XLOG_COMMIT_TRANS)
>  		ticket->t_curr_res -= sizeof(struct xlog_op_header);
>  	if (ticket->t_curr_res < 0) {
>  		xfs_alert_tag(log->l_mp, XFS_PTAG_LOGRES,
> @@ -2424,6 +2439,8 @@ xlog_write(
>  				ophdr = reg->i_addr;
>  				if (index)
>  					optype &= ~XLOG_START_TRANS;
> +			} else if (optype & XLOG_UNMOUNT_TRANS) {
> +				ophdr = reg->i_addr;
>  			} else {
>  				ophdr = xlog_write_setup_ophdr(log, ptr,
>  							ticket, optype);
> @@ -2454,7 +2471,7 @@ xlog_write(
>  			/*
>  			 * Copy region.
>  			 *
> -			 * Commit and unmount records just log an opheader, so
> +			 * Commit records just log an opheader, so
>  			 * we can have empty payloads with no data region to
>  			 * copy.  Hence we only copy the payload if the vector
>  			 * says it has data to copy.
> -- 
> 2.31.1
>