Re: [PATCH 20/45] xfs: only CIL pushes require a start record

Darrick J. Wong
2021-03-09T00:08:16Z

On Fri, Mar 05, 2021 at 04:11:18PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> So move the one-off start record writing in xlog_write() out into
> the static header that the CIL push builds to write into the log
> initially. This simplifes the xlog_write() logic a lot.
>
> pahole on x86-64 confirms that the xlog_cil_trans_hdr is correctly
> 32 bit aligned and packed for copying the log op and transaction
> headers directly into the log as a single log region copy.
>
> struct xlog_cil_trans_hdr {
> struct xlog_op_header oph[2]; /* 0 24 */
> struct xfs_trans_header thdr; /* 24 16 */
> struct xfs_log_iovec lhdr; /* 40 16 */
>
> /* size: 56, cachelines: 1, members: 3 */
> /* last cacheline: 56 bytes */
> };
>
> A wart is needed to handle the fact that length of the region the
> opheader points to doesn't include the opheader length. hence if
> we embed the opheader, we have to substract the opheader length from
> the length written into the opheader by the generic copying code.
> This will eventually go away when everything is converted to
> embedded opheaders.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

Looks ... ugly, but looking forward a few patches you're clearly getting
ready to refactor a bunch of grody 4-indent code so...

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

--D

> ---
> fs/xfs/xfs_log.c | 90 ++++++++++++++++++++++----------------------
> fs/xfs/xfs_log_cil.c | 44 ++++++++++++++++++----
> 2 files changed, 81 insertions(+), 53 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index f54d48f4584e..b2f9fb1b4fed 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -2106,9 +2106,9 @@ xlog_print_trans(
> }
>
> /*
> - * Calculate the potential space needed by the log vector. We may need a start
> - * record, and each region gets its own struct xlog_op_header and may need to be
> - * double word aligned.
> + * Calculate the potential space needed by the log vector. If this is a start
> + * transaction, the caller has already accounted for both opheaders in the start
> + * transaction, so we don't need to account for them here.
> */
> static int
> xlog_write_calc_vec_length(
> @@ -2121,9 +2121,6 @@ xlog_write_calc_vec_length(
> int len = 0;
> int i;
>
> - if (optype & XLOG_START_TRANS)
> - headers++;
> -
> for (lv = log_vector; lv; lv = lv->lv_next) {
> /* we don't write ordered log vectors */
> if (lv->lv_buf_len == XFS_LOG_VEC_ORDERED)
> @@ -2139,24 +2136,20 @@ xlog_write_calc_vec_length(
> }
> }
>
> + /* Don't account for regions with embedded ophdrs */
> + if (optype && headers > 0) {
> + if (optype & XLOG_START_TRANS) {
> + ASSERT(headers >= 2);
> + headers -= 2;
> + }
> + }
> +
> ticket->t_res_num_ophdrs += headers;
> len += headers * sizeof(struct xlog_op_header);
>
> return len;
> }
>
> -static void
> -xlog_write_start_rec(
> - struct xlog_op_header *ophdr,
> - struct xlog_ticket *ticket)
> -{
> - ophdr->oh_tid = cpu_to_be32(ticket->t_tid);
> - ophdr->oh_clientid = ticket->t_clientid;
> - ophdr->oh_len = 0;
> - ophdr->oh_flags = XLOG_START_TRANS;
> - ophdr->oh_res2 = 0;
> -}
> -
> static xlog_op_header_t *
> xlog_write_setup_ophdr(
> struct xlog *log,
> @@ -2361,9 +2354,11 @@ 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.
> + * to account for an extra xlog_op_header here for commit and unmount
> + * records.
> */
> - ticket->t_curr_res -= sizeof(struct xlog_op_header);
> + if (optype & (XLOG_COMMIT_TRANS | XLOG_UNMOUNT_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");
> @@ -2411,7 +2406,7 @@ xlog_write(
> int copy_len;
> int copy_off;
> bool ordered = false;
> - bool wrote_start_rec = false;
> + bool added_ophdr = false;
>
> /* ordered log vectors have no regions to write */
> if (lv->lv_buf_len == XFS_LOG_VEC_ORDERED) {
> @@ -2425,25 +2420,24 @@ xlog_write(
> ASSERT((unsigned long)ptr % sizeof(int32_t) == 0);
>
> /*
> - * Before we start formatting log vectors, we need to
> - * write a start record. Only do this for the first
> - * iclog we write to.
> + * 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.
> */
> if (optype & XLOG_START_TRANS) {
> - xlog_write_start_rec(ptr, ticket);
> - xlog_write_adv_cnt(&ptr, &len, &log_offset,
> - sizeof(struct xlog_op_header));
> - optype &= ~XLOG_START_TRANS;
> - wrote_start_rec = true;
> - }
> -
> - ophdr = xlog_write_setup_ophdr(log, ptr, ticket, optype);
> - if (!ophdr)
> - return -EIO;
> + ophdr = reg->i_addr;
> + if (index)
> + optype &= ~XLOG_START_TRANS;
> + } else {
> + ophdr = xlog_write_setup_ophdr(log, ptr,
> + ticket, optype);
> + if (!ophdr)
> + return -EIO;
>
> - xlog_write_adv_cnt(&ptr, &len, &log_offset,
> + xlog_write_adv_cnt(&ptr, &len, &log_offset,
> sizeof(struct xlog_op_header));
> -
> + added_ophdr = true;
> + }
> len += xlog_write_setup_copy(ticket, ophdr,
> iclog->ic_size-log_offset,
> reg->i_len,
> @@ -2452,13 +2446,22 @@ xlog_write(
> &partial_copy_len);
> xlog_verify_dest_ptr(log, ptr);
>
> +
> + /*
> + * Wart: need to update length in embedded ophdr not
> + * to include it's own length.
> + */
> + if (!added_ophdr) {
> + ophdr->oh_len = cpu_to_be32(copy_len -
> + sizeof(struct xlog_op_header));
> + }
> /*
> * Copy region.
> *
> - * Unmount records just log an opheader, so 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.
> + * Commit and unmount 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.
> */
> ASSERT(copy_len >= 0);
> if (copy_len > 0) {
> @@ -2466,12 +2469,9 @@ xlog_write(
> xlog_write_adv_cnt(&ptr, &len, &log_offset,
> copy_len);
> }
> - copy_len += sizeof(struct xlog_op_header);
> - record_cnt++;
> - if (wrote_start_rec) {
> + if (added_ophdr)
> copy_len += sizeof(struct xlog_op_header);
> - record_cnt++;
> - }
> + record_cnt++;
> data_cnt += contwr ? copy_len : 0;
>
> error = xlog_write_copy_finish(log, iclog, optype,
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index b515002e7959..e9da074ecd69 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -652,14 +652,22 @@ xlog_cil_process_committed(
> }
>
> struct xlog_cil_trans_hdr {
> + struct xlog_op_header oph[2];
> struct xfs_trans_header thdr;
> - struct xfs_log_iovec lhdr;
> + struct xfs_log_iovec lhdr[2];
> };
>
> /*
> * Build a checkpoint transaction header to begin the journal transaction. We
> * need to account for the space used by the transaction header here as it is
> * not accounted for in xlog_write().
> + *
> + * This is the only place we write a transaction header, so we also build the
> + * log opheaders that indicate the start of a log transaction and wrap the
> + * transaction header. We keep the start record in it's own log vector rather
> + * than compacting them into a single region as this ends up making the logic
> + * in xlog_write() for handling empty opheaders for start, commit and unmount
> + * records much simpler.
> */
> static void
> xlog_cil_build_trans_hdr(
> @@ -669,20 +677,40 @@ xlog_cil_build_trans_hdr(
> int num_iovecs)
> {
> struct xlog_ticket *tic = ctx->ticket;
> + uint32_t tid = cpu_to_be32(tic->t_tid);
>
> memset(hdr, 0, sizeof(*hdr));
>
> + /* Log start record */
> + hdr->oph[0].oh_tid = tid;
> + hdr->oph[0].oh_clientid = XFS_TRANSACTION;
> + hdr->oph[0].oh_flags = XLOG_START_TRANS;
> +
> + /* log iovec region pointer */
> + hdr->lhdr[0].i_addr = &hdr->oph[0];
> + hdr->lhdr[0].i_len = sizeof(struct xlog_op_header);
> + hdr->lhdr[0].i_type = XLOG_REG_TYPE_LRHEADER;
> +
> + /* log opheader */
> + hdr->oph[1].oh_tid = tid;
> + hdr->oph[1].oh_clientid = XFS_TRANSACTION;
> +
> + /* transaction header */
> hdr->thdr.th_magic = XFS_TRANS_HEADER_MAGIC;
> hdr->thdr.th_type = XFS_TRANS_CHECKPOINT;
> - hdr->thdr.th_tid = tic->t_tid;
> + hdr->thdr.th_tid = tid;
> hdr->thdr.th_num_items = num_iovecs;
> - hdr->lhdr.i_addr = &hdr->thdr;
> - hdr->lhdr.i_len = sizeof(xfs_trans_header_t);
> - hdr->lhdr.i_type = XLOG_REG_TYPE_TRANSHDR;
> - tic->t_curr_res -= hdr->lhdr.i_len + sizeof(xlog_op_header_t);
>
> - lvhdr->lv_niovecs = 1;
> - lvhdr->lv_iovecp = &hdr->lhdr;
> + /* log iovec region pointer */
> + hdr->lhdr[1].i_addr = &hdr->oph[1];
> + hdr->lhdr[1].i_len = sizeof(struct xlog_op_header) +
> + sizeof(struct xfs_trans_header);
> + hdr->lhdr[1].i_type = XLOG_REG_TYPE_TRANSHDR;
> +
> + tic->t_curr_res -= hdr->lhdr[0].i_len + hdr->lhdr[1].i_len;
> +
> + lvhdr->lv_niovecs = 2;
> + lvhdr->lv_iovecp = &hdr->lhdr[0];
> lvhdr->lv_next = ctx->lv_chain;
> }
>
> --
> 2.28.0
>