[PATCH 6/8] xfs: Account log unmount transaction correctly

[Ben Myers]

On Thu, Mar 22, 2012 at 04:15:11PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner at redhat.com>
> 
> 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 acocunt 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 at redhat.com>

Looks good.
Reviewed-by: Ben Myers <bpm at sgi.com>