Re: [PATCH] generic/211: verify if the filesystem being tested supports in place writes

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

On Wed, Jul 23, 2025 at 01:04:06PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> The test currently assumes the filesystem can do in place writes (no
> Copy-On-Write, no allocation of new extents) when overwriting a file.
> While that is the case for most filesystems in most configurations, there
> are exceptions such as zoned xfs where overwriting results in allocating
> new extents for the new data.
> 
> So make the test check that in place writes are supported and skip the
> test if they are not supported.
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

This is a smarter sollution than the fugly hack that I came up with:

# Don't run on mounted filesystems that only support out of place writes
if [ "$FSTYP" = "xfs" ]; then
	_require_no_xfs_always_cow
	_require_xfs_scratch_non_zoned
fi

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> ---
>  common/rc         | 59 +++++++++++++++++++++++++++++++++++++++++++++++
>  tests/generic/211 |  1 +
>  2 files changed, 60 insertions(+)
> 
> diff --git a/common/rc b/common/rc
> index 96578d15..52aade10 100644
> --- a/common/rc
> +++ b/common/rc
> @@ -5873,6 +5873,65 @@ _require_program() {
>  	_have_program "$1" || _notrun "$tag required"
>  }
>  
> +# Test that a filesystem can do writes to a file in place (without allocating
> +# new extents, without Copy-On-Write semantics).
> +_require_inplace_writes()
> +{
> +	_require_xfs_io_command "fiemap"
> +
> +	local target=$1
> +	local test_file="${target}/test_inplace_writes"
> +	local fiemap_before
> +	local fiemap_after
> +
> +	if [ -z "$target" ]; then
> +		_fail "Usage: _require_inplace_writes <filesystem path>"
> +	fi
> +
> +	rm -f "$test_file"
> +	touch "$test_file"
> +
> +	# Set the file to NOCOW mode on btrfs, which must be done while the file
> +	# is empty, otherwise it fails.
> +	if [ "$FSTYP" == "btrfs" ]; then
> +		_require_chattr C
> +		$CHATTR_PROG +C "$test_file"
> +	fi
> +
> +	$XFS_IO_PROG -f -c "pwrite 0 128K" -c "fsync" "$test_file" &> /dev/null
> +	if [ $? -ne 0 ]; then
> +		rm -f "$test_file"
> +		_fail "_require_inplace_writes failed to write to test file"
> +	fi
> +
> +	# Grab fiemap output before overwriting.
> +	fiemap_before=$($XFS_IO_PROG -c "fiemap" "$test_file")
> +	if [ $? -ne 0 ]; then
> +		rm -f "$test_file"
> +		_fail "_require_inplace_writes first fiemap call failed"
> +	fi
> +
> +	$XFS_IO_PROG -c "pwrite 0 128K" -c "fsync" "$test_file" &> /dev/null
> +	if [ $? -ne 0 ]; then
> +		rm -f "$test_file"
> +		_fail "_require_inplace_writes failed to overwrite test file"
> +	fi
> +
> +	fiemap_after=$($XFS_IO_PROG -c "fiemap" "$test_file")
> +	if [ $? -ne 0 ]; then
> +		rm -f "$test_file"
> +		_fail "_require_inplace_writes second fiemap call failed"
> +	fi
> +
> +	rm -f "$test_file"
> +
> +	# If the filesystem supports inplace writes, then the extent mapping is
> +	# the same before and after overwriting.
> +	if [ "${fiemap_after}" != "${fiemap_before}" ]; then
> +		_notrun "inplace writes not supported"
> +	fi
> +}
> +
>  ################################################################################
>  # make sure this script returns success
>  /bin/true
> diff --git a/tests/generic/211 b/tests/generic/211
> index e87d1e01..ed426db2 100755
> --- a/tests/generic/211
> +++ b/tests/generic/211
> @@ -27,6 +27,7 @@ fs_size=$(_small_fs_size_mb 512)
>  _try_scratch_mkfs_sized $((fs_size * 1024 * 1024)) >>$seqres.full 2>&1 || \
>  	_fail "mkfs failed"
>  _scratch_mount
> +_require_inplace_writes "${SCRATCH_MNT}"
>  
>  touch $SCRATCH_MNT/foobar
>  
> -- 
> 2.47.2
> 
>