[PATCH 3.2 18/79] xfs: Account log unmount transaction correctly

[Ben Hutchings]

3.2.55-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Dave Chinner <dchinner@xxxxxxxxxx>

commit 3948659e30808fbaa7673bbe89de2ae9769e20a7 upstream.

There have been a few reports of this warning appearing recently:

XFS (dm-4): xlog_space_left: head behind tail
 tail_cycle = 129, tail_bytes = 20163072
 GH   cycle = 129, GH   bytes = 20162880

The common cause appears to be lots of freeze and unfreeze cycles,
and the output from the warnings indicates that we are leaking
around 8 bytes of log space per freeze/unfreeze cycle.

When we freeze the filesystem, we write an unmount record and that
uses xlog_write directly - a special type of transaction,
effectively. What it doesn't do, however, is correctly account for
the log space it uses. The unmount record writes an 8 byte structure
with a special magic number into the log, and the space this
consumes is not accounted for in the log ticket tracking the
operation. Hence we leak 8 bytes every unmount record that is
written.

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Ben Myers <bpm@xxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/xfs/xfs_log.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -653,8 +653,9 @@ xfs_log_unmount_write(xfs_mount_t *mp)
 				.lv_iovecp = &reg,
 			};
 
-			/* remove inited flag */
+			/* remove inited flag, and account for space used */
 			tic->t_flags = 0;
+			tic->t_curr_res -= sizeof(magic);
 			error = xlog_write(log, &vec, tic, &lsn,
 					   NULL, XLOG_UNMOUNT_TRANS);
 			/*

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