Re: [PATCH] generic/563: use a loop device to avoid partition incompatibility

[Zorro Lang <zlang@redhat.com>]

On Thu, Dec 10, 2020 at 11:14:26AM -0500, Brian Foster wrote:
> cgroup writeback accounting does not track partition level
> statistics. Instead, I/O is accounted against the parent device. As
> a result, the test fails if the scratch device happens to be a
> device partition. Since parent level stats are potentially polluted
> by factors external to the test, wrap the scratch device in a
> loopback device to guarantee the test always runs on a top-level
> block device.
> 
> Reported-by: Boyang Xue <bxue@redhat.com>
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  tests/generic/563 | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/tests/generic/563 b/tests/generic/563
> index 51deaa2f..9292dece 100755
> --- a/tests/generic/563
> +++ b/tests/generic/563
> @@ -2,7 +2,7 @@
>  # SPDX-License-Identifier: GPL-2.0
>  # Copyright (c) 2019 Red Hat, Inc.  All Rights Reserved.
>  #
> -# FS QA Test No. 011
> +# FS QA Test No. 563
>  #
>  # This test verifies that cgroup aware writeback properly accounts I/Os in
>  # various scenarios. We perform reads/writes from different combinations of
> @@ -26,6 +26,8 @@ _cleanup()
>  
>  	echo $$ > $cgdir/cgroup.procs
>  	rmdir $cgdir/$seq-cg* > /dev/null 2>&1
> +	umount $SCRATCH_MNT > /dev/null 2>&1
> +	_destroy_loop_device $LOOP_DEV > /dev/null 2>&1
>  }
>  
>  # get standard environment, filters and checks
> @@ -42,14 +44,12 @@ rm -f $seqres.full
>  _supported_fs generic
>  _require_scratch
>  _require_cgroup2 io
> +_require_loop
>  
>  # cgroup v2 writeback is only support on block devices so far
>  _require_block_device $SCRATCH_DEV
>  
> -smajor=$((0x`stat -L -c %t $SCRATCH_DEV`))
> -sminor=$((0x`stat -L -c %T $SCRATCH_DEV`))
>  cgdir=$CGROUP2_PATH
> -
>  iosize=$((1024 * 1024 * 8))
>  
>  # Check cgroup read/write charges against expected values. Allow for some
> @@ -89,12 +89,19 @@ reset()
>  	rmdir $cgdir/$seq-cg* > /dev/null 2>&1
>  	$XFS_IO_PROG -fc "pwrite 0 $iosize" $SCRATCH_MNT/file \
>  		>> $seqres.full 2>&1
> -	_scratch_cycle_mount || _fail "mount failed"
> +	umount $SCRATCH_MNT || _fail "umount failed"
> +	_mount $LOOP_DEV $SCRATCH_MNT || _fail "mount failed"
>  	stat $SCRATCH_MNT/file > /dev/null
>  }
>  
> -_scratch_mkfs >> $seqres.full 2>&1
> -_scratch_mount
> +# cgroup I/O accounting doesn't work on partitions. Use a loop device to rule
> +# that out.
> +LOOP_DEV=$(_create_loop_device $SCRATCH_DEV)

I recommend using a file to create loop device. If you'd like to use SCRATCH_DEV
to create loop device directly, you'd better to change the "_require_scratch"
to "_require_scratch_nocheck". Or I think it might be failed, e.g. if SCRATCH_DEV
is a 4k sector size device.

Others look good to me.

Thanks,
Zorro

> +smajor=$((0x`stat -L -c %t $LOOP_DEV`))
> +sminor=$((0x`stat -L -c %T $LOOP_DEV`))
> +
> +_mkfs_dev $LOOP_DEV >> $seqres.full 2>&1
> +_mount $LOOP_DEV $SCRATCH_MNT || _fail "mount failed"
>  
>  echo "+io" > $cgdir/cgroup.subtree_control || _fail "subtree control"
>  
> -- 
> 2.26.2
>