Re: [PATCH v6 1/3] xfstests: btrfs: add functions to create dm-error device

[Dave Chinner <david@fromorbit.com>]

On Thu, Aug 20, 2015 at 03:09:05PM +0800, Anand Jain wrote:
> 
>  (thanks for the off-ML emails from the people who helped me
>  to understand).
> 
> Dave,
> 
>  looks like you are suggesting something like..
> 
> -------
> +_dmerror_mount_options()
> +{
> +       _scratch_options mount
> +       echo $SCRATCH_OPTIONS $MOUNT_OPTIONS $SELINUX_MOUNT_OPTIONS \
> +                                       $* $DMERROR_DEV $SCRATCH_MNT
> +}
> +
> +_dmerror_mount()
> +{
> +       _mount -t $FSTYP `_dmerror_mount_options $*`
> +}
> --------

Very close. :)

You've got the structure right, but missed one more thing: the
options are still cut'n'paste from _scratch_mount_options, and so
should be /factored/ into a common function...

_scratch_options is only useful to XFS to set external log/realtime
devices, and we don't want to do that for the dmerror device.
Everthing else is almost the same as _scratch_mount_options(),
however.  So:

# Used for mounting non-scratch devices (e.g. loop, dm constructs)
# with the safe set of scratch mount options (e.g. loop image may be
# hosted on $SCRATCH_DEV, so can't use external scratch devices).
_common_dev_mount_options()
{
	echo $MOUNT_OPTIONS $SELINUX_MOUNT_OPTIONS $*
}

_scratch_mount_options()
{
	_scratch_options mount

	echo `_common_dev_mount_options $*` $SCRATCH_OPTIONS \
					    $SCRATCH_DEV $SCRATCH_MNT

}

_dmerror_mount_options()
{
       echo `_common_dev_mount_options $*` $DMERROR_DEV $SCRATCH_MNT
}

And to demonstrate why factoring like this is useful, this can also
be applied to _mount_flakey(), and we could also replace most of the
open-coded loop device mounts with a generic "mount device" function
like:

_mount_dev()
{
	_mount `_common_dev_mount_options $*`
}

and call it like:

_mount_dev $LOOP_DEV $MNT_POINT

or even

_mount_dev -o loop $MNT_POINT

We can then take it further: if we have a _mount_dev() wrapper we
can factor _common_dev_mount_options() from all the
_*_mount_options() functions. i.e:

_scratch_mount_options()
{
	_scratch_options mount

	echo $SCRATCH_OPTIONS $SCRATCH_DEV $SCRATCH_MNT
}

_scratch_mount()
{
	_mount_dev $* `_scratch_mount_options`
}

The result is we end up with a much cleaner set of mount functions
that are more generic, more extensible and easier to use, as well as
being more maintainable....

>  Sorry that the word 'should mirror _scratch_mount()' confused me.

Ok, I'll try to be more clear in future.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com