Re: [PATCH 01/11] xfs: don't try to write a start record into every iclog

Brian Foster
2020-03-05T18:05:38Z

On Wed, Mar 04, 2020 at 06:53:51PM +1100, Dave Chinner 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() we only ever need to write a start record once per

^ "so we only ever ..." ?

> 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>
> ---
> fs/xfs/xfs_log.c | 68 +++++++++++++++++++++++++-----------------------
> 1 file changed, 35 insertions(+), 33 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index f6006d94a581..5b0568a86c07 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
...
> @@ -2404,25 +2391,29 @@ xlog_write(
> int record_cnt = 0;
> int data_cnt = 0;
> int error = 0;
> + int start_rec_size = sizeof(struct xlog_op_header);
>
> *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)
> + if (ticket->t_flags & XLOG_TIC_INITED) {
> ticket->t_curr_res -= sizeof(xlog_op_header_t);
> + 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))
> + if (flags & (XLOG_COMMIT_TRANS | XLOG_UNMOUNT_TRANS)) {
> ticket->t_curr_res -= sizeof(xlog_op_header_t);
> + start_rec_size = 0;
> + }

This is more of a flaw in the existing code than this patch, but please
update the comment to document why we reset start_rec_size here since
it's not obvious from the context of the function. Otherwise looks Ok to
me modulo Christoph's comments (in particular the data_cnt bit). My one
nit is that I think start_rec might be a better name than start_rec_size
given how it's used, but I don't care that much.

Brian

>
> if (ticket->t_curr_res < 0) {
> xfs_alert_tag(log->l_mp, XFS_PTAG_LOGRES,
> @@ -2431,6 +2422,8 @@ xlog_write(
> xfs_force_shutdown(log->l_mp, SHUTDOWN_LOG_IO_ERROR);
> }
>
> + len = xlog_write_calc_vec_length(ticket, log_vector, start_rec_size);
> +
> index = 0;
> lv = log_vector;
> vecp = lv->lv_iovecp;
> @@ -2446,9 +2439,20 @@ xlog_write(
> ASSERT(log_offset <= iclog->ic_size - 1);
> ptr = iclog->ic_datap + log_offset;
>
> - /* start_lsn is the first lsn written to. That's all we need. */
> + /*
> + * Before we start formatting log vectors, we need to write a
> + * start record and record the lsn of the iclog that we write
> + * the start record into. Only do this for the first iclog we
> + * write to.
> + */
> if (!*start_lsn)
> *start_lsn = be64_to_cpu(iclog->ic_header.h_lsn);
> + if (start_rec_size) {
> + xlog_write_start_rec(ptr, ticket);
> + xlog_write_adv_cnt(&ptr, &len, &log_offset,
> + start_rec_size);
> + record_cnt++;
> + }
>
> /*
> * This loop writes out as many regions as can fit in the amount
> @@ -2457,7 +2461,6 @@ 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;
> @@ -2473,13 +2476,6 @@ 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++;
> - xlog_write_adv_cnt(&ptr, &len, &log_offset,
> - start_rec_copy);
> - }
> -
> ophdr = xlog_write_setup_ophdr(log, ptr, ticket, flags);
> if (!ophdr)
> return -EIO;
> @@ -2509,10 +2505,15 @@ 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(xlog_op_header_t);
> record_cnt++;
> data_cnt += contwr ? copy_len : 0;
>
> + if (start_rec_size) {
> + copy_len += start_rec_size;
> + start_rec_size = 0;
> + }
> +
> error = xlog_write_copy_finish(log, iclog, flags,
> &record_cnt, &data_cnt,
> &partial_copy,
> @@ -2550,6 +2551,7 @@ xlog_write(
> break;
> }
> }
> + start_rec_size = 0;
> }
>
> ASSERT(len == 0);
> --
> 2.24.0.rc0
>