xfs: embed the xlog_op_header in the unmount record

Remove another case where xlog_write() has to prepend an opheader to
a log transaction. The unmount record + ophdr is smaller than the
minimum amount of space guaranteed to be free in an iclog (2 *
sizeof(ophdr)) and so we don't have to care about an unmount record
being split across 2 iclogs.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_log.c

@@ -901,12 +901,22 @@ xlog_write_unmount_record(
 	struct xlog		*log,
 	struct xlog_ticket	*ticket)
 {
-	struct xfs_unmount_log_format ulf = {
-		.magic = XLOG_UNMOUNT_TYPE,
+	struct  {
+		struct xlog_op_header ophdr;
+		struct xfs_unmount_log_format ulf;
+	} unmount_rec = {
+		.ophdr = {
+			.oh_clientid = XFS_LOG,
+			.oh_tid = cpu_to_be32(ticket->t_tid),
+			.oh_flags = XLOG_UNMOUNT_TRANS,
+		},
+		.ulf = {
+			.magic = XLOG_UNMOUNT_TYPE,
+		},
 	};
 	struct xfs_log_iovec reg = {
-		.i_addr = &ulf,
-		.i_len = sizeof(ulf),
+		.i_addr = &unmount_rec,
+		.i_len = sizeof(unmount_rec),
 		.i_type = XLOG_REG_TYPE_UNMOUNT,
 	};
 	struct xfs_log_vec vec = {
@@ -914,8 +924,12 @@ xlog_write_unmount_record(
 		.lv_iovecp = &reg,
 	};
 
+	BUILD_BUG_ON((sizeof(struct xlog_op_header) +
+		      sizeof(struct xfs_unmount_log_format)) !=
+							sizeof(unmount_rec));
+
 	/* account for space used by record data */
-	ticket->t_curr_res -= sizeof(ulf);
+	ticket->t_curr_res -= sizeof(unmount_rec);
 
 	return xlog_write(log, NULL, &vec, ticket, XLOG_UNMOUNT_TRANS);
 }
@@ -2253,6 +2267,8 @@ xlog_write_calc_vec_length(
 
 	/* Don't account for regions with embedded ophdrs */
 	if (optype && headers > 0) {
+		if (optype & XLOG_UNMOUNT_TRANS)
+			headers--;
 		if (optype & XLOG_START_TRANS) {
 			ASSERT(headers >= 2);
 			headers -= 2;
@@ -2458,12 +2474,11 @@ 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 for commit and unmount
-	 * records.
+	 * 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 | XLOG_UNMOUNT_TRANS))
+	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,
@@ -2527,6 +2542,8 @@ xlog_write(
 				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);
@@ -2557,7 +2574,7 @@ xlog_write(
 			/*
 			 * Copy region.
 			 *
-			 * Commit and unmount records just log an opheader, so
+			 * Commit 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.