Re: [PATCH 21/45] xfs: embed the xlog_op_header in the unmount record

[Dave Chinner <david@fromorbit.com>]

On Mon, Mar 08, 2021 at 04:15:23PM -0800, Darrick J. Wong wrote:
> On Fri, Mar 05, 2021 at 04:11:19PM +1100, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > 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>
> > ---
> >  fs/xfs/xfs_log.c | 35 ++++++++++++++++++++++++-----------
> >  1 file changed, 24 insertions(+), 11 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> > index b2f9fb1b4fed..94711b9ff007 100644
> > --- a/fs/xfs/xfs_log.c
> > +++ b/fs/xfs/xfs_log.c
> > @@ -798,12 +798,22 @@ 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 = {
> 
> I wonder, should we have a BUILD_BUG_ON to confirm sizeof(umount_rec)
> just in case some weird architecture injects padding between these two?
> Prior to this code we formatted the op header and unmount record in
> separate incore buffers and wrote them to disk with no gap, right?

Yup. Easy enough to add.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com