xfs: add a helper to figure out the space log reservation per item

Add a new helper xfs_calc_buf_res() to calcuate out the transaction space
reservations per item.  xfs_buf_log_overhead() is used to figure out the
extra space for struct xfs_buf_log_format that gets written into the log
for every buffer as well as a log opheader, i.e. struct xlog_op_header.
Signed-off-by: Jie Liu <jeff.liu@oracle.com>
CC: Dave Chinner <david@fromorbit.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_trans.c

@@ -37,14 +37,45 @@
 #include "xfs_extent_busy.h"
 #include "xfs_bmap.h"
 #include "xfs_quota.h"
+#include "xfs_qm.h"
 #include "xfs_trans_priv.h"
 #include "xfs_trans_space.h"
 #include "xfs_inode_item.h"
+#include "xfs_log_priv.h"
+#include "xfs_buf_item.h"
 #include "xfs_trace.h"
 
 kmem_zone_t	*xfs_trans_zone;
 kmem_zone_t	*xfs_log_item_desc_zone;
 
+/*
+ * A buffer has a format structure overhead in the log in addition
+ * to the data, so we need to take this into account when reserving
+ * space in a transaction for a buffer.  Round the space required up
+ * to a multiple of 128 bytes so that we don't change the historical
+ * reservation that has been used for this overhead.
+ */
+STATIC uint
+xfs_buf_log_overhead(void)
+{
+	return round_up(sizeof(struct xlog_op_header) +
+			sizeof(struct xfs_buf_log_format), 128);
+}
+
+/*
+ * Calculate out transaction log reservation per item in bytes.
+ *
+ * The nbufs argument is used to indicate the number of items that
+ * will be changed in a transaction.  size is used to tell how many
+ * bytes should be reserved per item.
+ */
+STATIC uint
+xfs_calc_buf_res(
+	uint		nbufs,
+	uint		size)
+{
+	return nbufs * (size + xfs_buf_log_overhead());
+}
 
 /*
  * Various log reservation values.