xfs: only CIL pushes require a start record

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[2];              /*    40    32 */

        /* size: 72, cachelines: 2, members: 3 */
        /* last cacheline: 8 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>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_log.c

@@ -2221,9 +2221,9 @@ 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(
@@ -2236,9 +2236,6 @@ 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)
@@ -2254,24 +2251,20 @@ 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,
@@ -2467,8 +2460,10 @@ 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.
 	 */
+	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,
@@ -2510,7 +2505,7 @@ 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) {
@@ -2524,25 +2519,24 @@ 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));
+				ophdr = reg->i_addr;
+				if (index)
 					optype &= ~XLOG_START_TRANS;
-				wrote_start_rec = true;
-			}
-
-			ophdr = xlog_write_setup_ophdr(log, ptr, ticket, optype);
+			} else {
+				ophdr = xlog_write_setup_ophdr(log, ptr,
+							ticket, optype);
 				if (!ophdr)
 					return -EIO;
 
 				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,
@@ -2551,13 +2545,22 @@ 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) {
@@ -2565,12 +2568,9 @@ xlog_write(
 				xlog_write_adv_cnt(&ptr, &len, &log_offset,
 						   copy_len);
 			}
+			if (added_ophdr)
 				copy_len += sizeof(struct xlog_op_header);
 			record_cnt++;
-			if (wrote_start_rec) {
-				copy_len += sizeof(struct xlog_op_header);
-				record_cnt++;
-			}
 			data_cnt += contwr ? copy_len : 0;
 
 			error = xlog_write_copy_finish(log, iclog, optype,

fs/xfs/xfs_log_cil.c

@@ -869,14 +869,22 @@ xlog_cil_write_commit_record(
 }
 
 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(
@@ -886,20 +894,41 @@ xlog_cil_build_trans_hdr(
 	int			num_iovecs)
 {
 	struct xlog_ticket	*tic = ctx->ticket;
+	__be32			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;
+	hdr->oph[1].oh_len = cpu_to_be32(sizeof(struct xfs_trans_header));
+
+	/* transaction header in host byte order format */
 	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_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(struct xlog_op_header);
 
-	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;
 }