Re: [PATCH 1/8] xfs: don't try to write a start record into every iclog

[Brian Foster <bfoster@redhat.com>]

On Tue, Mar 24, 2020 at 06:44:52PM +0100, Christoph Hellwig wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> The xlog_write() function iterates over iclogs until it completes
> writing all the log vectors passed in. The ticket tracks whether
> a start record has been written or not, so only the first iclog gets
> a start record. We only ever pass single use tickets to
> xlog_write() so we only ever need to write a start record once per
> xlog_write() call.
> 
> Hence we don't need to store whether we should write a start record
> in the ticket as the callers provide all the information we need to
> determine if a start record should be written. For the moment, we
> have to ensure that we clear the XLOG_TIC_INITED appropriately so
> the code in xfs_log_done() still works correctly for committing
> transactions.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> [hch: use an need_start_rec bool]
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_log.c | 63 ++++++++++++++++++++++++------------------------
>  1 file changed, 32 insertions(+), 31 deletions(-)
> 
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 2a90a483c2d6..bf071552094a 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
...
> @@ -2372,25 +2359,29 @@ xlog_write(
>  	int			record_cnt = 0;
>  	int			data_cnt = 0;
>  	int			error = 0;
> +	bool			need_start_rec = true;
>  
>  	*start_lsn = 0;
>  
> -	len = xlog_write_calc_vec_length(ticket, log_vector);
>  
>  	/*
>  	 * Region headers and bytes are already accounted for.
>  	 * We only need to take into account start records and
>  	 * split regions in this function.
>  	 */
> -	if (ticket->t_flags & XLOG_TIC_INITED)
> -		ticket->t_curr_res -= sizeof(xlog_op_header_t);
> +	if (ticket->t_flags & XLOG_TIC_INITED) {
> +		ticket->t_curr_res -= sizeof(struct xlog_op_header);
> +		ticket->t_flags &= ~XLOG_TIC_INITED;
> +	}
>  
>  	/*
>  	 * Commit record headers need to be accounted for. These
>  	 * come in as separate writes so are easy to detect.
>  	 */
> -	if (flags & (XLOG_COMMIT_TRANS | XLOG_UNMOUNT_TRANS))
> -		ticket->t_curr_res -= sizeof(xlog_op_header_t);
> +	if (flags & (XLOG_COMMIT_TRANS | XLOG_UNMOUNT_TRANS)) {
> +		ticket->t_curr_res -= sizeof(struct xlog_op_header);
> +		need_start_rec = false;
> +	}

Hmm.. I was asking for a comment update in v1 for this logic change.
Looking through it again, what happens here if
xfs_log_write_unmount_record() clears the UNMOUNT_TRANS flag for that
summary counter check? That looks like a potential behavior change wrt
to the start record..

Brian

>  
>  	if (ticket->t_curr_res < 0) {
>  		xfs_alert_tag(log->l_mp, XFS_PTAG_LOGRES,
> @@ -2399,6 +2390,8 @@ xlog_write(
>  		xfs_force_shutdown(log->l_mp, SHUTDOWN_LOG_IO_ERROR);
>  	}
>  
> +	len = xlog_write_calc_vec_length(ticket, log_vector, need_start_rec);
> +
>  	index = 0;
>  	lv = log_vector;
>  	vecp = lv->lv_iovecp;
> @@ -2425,7 +2418,6 @@ xlog_write(
>  		while (lv && (!lv->lv_niovecs || index < lv->lv_niovecs)) {
>  			struct xfs_log_iovec	*reg;
>  			struct xlog_op_header	*ophdr;
> -			int			start_rec_copy;
>  			int			copy_len;
>  			int			copy_off;
>  			bool			ordered = false;
> @@ -2441,11 +2433,15 @@ xlog_write(
>  			ASSERT(reg->i_len % sizeof(int32_t) == 0);
>  			ASSERT((unsigned long)ptr % sizeof(int32_t) == 0);
>  
> -			start_rec_copy = xlog_write_start_rec(ptr, ticket);
> -			if (start_rec_copy) {
> -				record_cnt++;
> +			/*
> +			 * Before we start formatting log vectors, we need to
> +			 * write a start record. Only do this for the first
> +			 * iclog we write to.
> +			 */
> +			if (need_start_rec) {
> +				xlog_write_start_rec(ptr, ticket);
>  				xlog_write_adv_cnt(&ptr, &len, &log_offset,
> -						   start_rec_copy);
> +						sizeof(struct xlog_op_header));
>  			}
>  
>  			ophdr = xlog_write_setup_ophdr(log, ptr, ticket, flags);
> @@ -2477,8 +2473,13 @@ xlog_write(
>  				xlog_write_adv_cnt(&ptr, &len, &log_offset,
>  						   copy_len);
>  			}
> -			copy_len += start_rec_copy + sizeof(xlog_op_header_t);
> +			copy_len += sizeof(struct xlog_op_header);
>  			record_cnt++;
> +			if (need_start_rec) {
> +				copy_len += sizeof(struct xlog_op_header);
> +				record_cnt++;
> +				need_start_rec = false;
> +			}
>  			data_cnt += contwr ? copy_len : 0;
>  
>  			error = xlog_write_copy_finish(log, iclog, flags,
> -- 
> 2.25.1
>