Re: [PATCH 04/16] xfs: embed the xlog_op_header in the commit record

[Chandan Babu R <chandan.babu@oracle.com>]

On 19 Nov 2021 at 04:43, Dave Chinner wrote:
> Remove the final case where xlog_write() has to prepend an opheader
> to a log transaction. Similar to the start record, the commit record
> is just an empty opheader with a XLOG_COMMIT_TRANS type, so we can
> just make this the payload for the region being passed to
> xlog_write() and remove the special handling in xlog_write() for
> the commit record.
>

I have verfified the following,
1. xfs_log_vec and xfs_log_iovec contents have been initialized correctly.
2. Checkpoint transaction's log reservation is accounted for correctly.
3. Commit record's embedded xlog_op_header's oh_len is correctly set to 0
   by xlog_write() since there is no corresponding payload.

Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>

> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_log.c     | 22 ++++++----------------
>  fs/xfs/xfs_log_cil.c | 11 +++++++++--
>  2 files changed, 15 insertions(+), 18 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index e5515d0c85c4..f789acd2f755 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -2281,11 +2281,10 @@ xlog_write_calc_vec_length(
>  
>  	/* Don't account for regions with embedded ophdrs */
>  	if (optype && headers > 0) {
> -		if (optype & XLOG_UNMOUNT_TRANS)
> -			headers--;
> +		headers--;
>  		if (optype & XLOG_START_TRANS) {
> -			ASSERT(headers >= 2);
> -			headers -= 2;
> +			ASSERT(headers >= 1);
> +			headers--;
>  		}
>  	}
>  
> @@ -2486,14 +2485,6 @@ xlog_write(
>  	int			data_cnt = 0;
>  	int			error = 0;
>  
> -	/*
> -	 * 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
> -	 * 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)
> -		ticket->t_curr_res -= sizeof(struct xlog_op_header);
>  	if (ticket->t_curr_res < 0) {
>  		xfs_alert_tag(log->l_mp, XFS_PTAG_LOGRES,
>  		     "ctx ticket reservation ran out. Need to up reservation");
> @@ -2550,14 +2541,13 @@ xlog_write(
>  			/*
>  			 * The XLOG_START_TRANS has embedded ophdrs for the
>  			 * start record and transaction header. They will always
> -			 * be the first two regions in the lv chain.
> +			 * be the first two regions in the lv chain. Commit and
> +			 * unmount records also have embedded ophdrs.
>  			 */
> -			if (optype & XLOG_START_TRANS) {
> +			if (optype) {
>  				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);
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 9a810a2c92e9..2c4fb55d4897 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -810,9 +810,14 @@ xlog_cil_write_commit_record(
>  	struct xfs_cil_ctx	*ctx)
>  {
>  	struct xlog		*log = ctx->cil->xc_log;
> +	struct xlog_op_header	ophdr = {
> +		.oh_clientid = XFS_TRANSACTION,
> +		.oh_tid = cpu_to_be32(ctx->ticket->t_tid),
> +		.oh_flags = XLOG_COMMIT_TRANS,
> +	};
>  	struct xfs_log_iovec	reg = {
> -		.i_addr = NULL,
> -		.i_len = 0,
> +		.i_addr = &ophdr,
> +		.i_len = sizeof(struct xlog_op_header),
>  		.i_type = XLOG_REG_TYPE_COMMIT,
>  	};
>  	struct xfs_log_vec	vec = {
> @@ -828,6 +833,8 @@ xlog_cil_write_commit_record(
>  	if (error)
>  		return error;
>  
> +	/* account for space used by record data */
> +	ctx->ticket->t_curr_res -= reg.i_len;
>  	error = xlog_write(log, ctx, &vec, ctx->ticket, XLOG_COMMIT_TRANS);
>  	if (error)
>  		xfs_force_shutdown(log->l_mp, SHUTDOWN_LOG_IO_ERROR);


-- 
chandan