Re: [PATCH v7 01/11] fstests: add _loop_image_create_clone() helper

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Jun 17, 2026 at 07:20:28PM +0800, Anand Jain wrote:
> Introduce _loop_image_create_clone() and _loop_image_destroy() to mkfs an
> image file and clone it to another image file, and attach a loop device to
> them. And its destroy part.
> 
> Signed-off-by: Anand Jain <asj@kernel.org>
> ---
>  common/rc | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 63 insertions(+)
> 
> diff --git a/common/rc b/common/rc
> index 79189e7e6e94..d7e3e0bdfb1e 100644
> --- a/common/rc
> +++ b/common/rc
> @@ -1520,6 +1520,69 @@ _scratch_resvblks()
>  	esac
>  }
>  
> +# Create a small loop image, run an optional tuning function ($2) on it,
> +# clone it, and attach both to loop devices, returned in ($1).
> +# Args:
> +#   $1: Nameref to return the array of allocated loop devices [base, clone].
> +#   $2: Optional callback function to tune the base filesystem before cloning.
> +_loop_image_create_clone()
> +{
> +	local -n _ret=$1
> +	local pre_clone_tune_func="$2"
> +	local img_file=$TEST_DIR/${seq}.img
> +	local img_file_clone=$TEST_DIR/${seq}_clone.img
> +	local size=$(_small_fs_size_mb 128) # Smallest possible
> +	local loop_devs
> +
> +	# Since we copy the block device image, we keep its size small.
> +	_require_fs_space $TEST_DIR $((size * 1024))
> +
> +	_create_file_sized $((size * 1024 * 1024)) $img_file ||
> +				_fail "Failed: Create $img_file $size"
> +
> +	loop_devs=$(_create_loop_device $img_file)
> +	_ret=($loop_devs)
> +
> +	case $FSTYP in
> +	xfs)
> +		_mkfs_dev "-s size=4096" ${loop_devs[0]}

Not sure why you pass two separate cli arguments as a quoted string, but
my guess is it "doesn't matter" because _try_mkfs_dev uses $* unquoted,
which separates them again.

I HATE BASH.

> +		;;
> +	btrfs)
> +		_mkfs_dev ${loop_devs[0]}

And while I'm whining:    ^^^^^^^^^^^^^^^ actually should be quoted.
Not that fstests is at all good at getting this right.

> +		;;
> +	*)
> +		_mkfs_dev ${loop_devs[0]}
> +		;;
> +	esac
> +
> +	# Only execute if the function argument is not empty
> +	if [ -n "$pre_clone_tune_func" ]; then
> +		$pre_clone_tune_func ${loop_devs[0]}
> +	fi
> +
> +	sync ${loop_devs[0]}
> +	cp $img_file $img_file_clone

What if cp doesn't create a reflink copy?  Can we fill up the $TEST_DIR
despite having checked it for sufficient free space?  Especially on
filesystems that don't support sparse holes?

> +
> +	loop_devs="$loop_devs $(_create_loop_device $img_file_clone)"
> +
> +	_ret=($loop_devs)

Hmm.  Should this function return nonzero if any part of the clone
creation fails?  Or are callers expected to notice that _ret only has
one element?

--D

> +}
> +
> +# Teardown loop devices and delete their underlying backing image files.
> +# Accepts a list of loop device paths (e.g., /dev/loop0 /dev/loop1).
> +_loop_image_destroy()
> +{
> +	for d in "$@"; do
> +		# Retrieve the path of the backing file
> +		local f=$(losetup --noheadings --output BACK-FILE $d)
> +
> +		# Detach the loop device from the backing file
> +		_destroy_loop_device "$d"
> +
> +		# Clean up the backing disk image file
> +		[ -n "$f" ] && rm -f "$f"
> +	done
> +}
>  
>  # Repair scratch filesystem.  Returns 0 if the FS is good to go (either no
>  # errors found or errors were fixed) and nonzero otherwise; also spits out
> -- 
> 2.43.0
> 
>