[PATCH stable 3.7] xfs: fix segment in xfs_buf_item_format_segment

[Mark Tinguely]

For Linux 3.7-stable.

Upstream commit: 820a554f2f83d21f7e9e608377c8c86af70917bd

Not every segment in a multi-segment buffer is dirty in a
transaction and they will not be outputted. The assert in
xfs_buf_item_format_segment() that checks for the at least
one chunk of data in the segment to be used is not necessary
true for multi-segmented buffers.

Signed-off-by: Mark Tinguely <tinguely at sgi.com>
Reviewed-by: Dave Chinner <dchinner at redhat.com>
Signed-off-by: Ben Myers <bpm at sgi.com>
---
fs/xfs/xfs_buf_item.c |   20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

Index: b/fs/xfs/xfs_buf_item.c
===================================================================
--- a/fs/xfs/xfs_buf_item.c
+++ b/fs/xfs/xfs_buf_item.c
@@ -287,6 +287,17 @@ xfs_buf_item_format_segment(
 	 */
 	base_size = offsetof(struct xfs_buf_log_format, blf_data_map) +
 			(blfp->blf_map_size * sizeof(blfp->blf_data_map[0]));
+
+	nvecs = 0;
+	first_bit = xfs_next_bit(blfp->blf_data_map, blfp->blf_map_size, 0);
+	if (!(bip->bli_flags & XFS_BLI_STALE) && first_bit == -1) {
+		/*
+		 * If the map is not be dirty in the transaction, mark
+		 * the size as zero and do not advance the vector pointer.
+		 */
+		goto out;
+	}
+
 	vecp->i_addr = blfp;
 	vecp->i_len = base_size;
 	vecp->i_type = XLOG_REG_TYPE_BFORMAT;
@@ -301,15 +312,13 @@ xfs_buf_item_format_segment(
 		 */
 		trace_xfs_buf_item_format_stale(bip);
 		ASSERT(blfp->blf_flags & XFS_BLF_CANCEL);
-		blfp->blf_size = nvecs;
-		return vecp;
+		goto out;
 	}
 
 	/*
 	 * Fill in an iovec for each set of contiguous chunks.
 	 */
-	first_bit = xfs_next_bit(blfp->blf_data_map, blfp->blf_map_size, 0);
-	ASSERT(first_bit != -1);
+
 	last_bit = first_bit;
 	nbits = 1;
 	for (;;) {
@@ -371,7 +380,8 @@ xfs_buf_item_format_segment(
 			nbits++;
 		}
 	}
-	bip->__bli_format.blf_size = nvecs;
+out:
+	blfp->blf_size = nvecs;
 	return vecp;
 }