Re: [PATCH v2 3/3] generic/810: check that RWF_DONTCACHE I/O drops its folios

[Brian Foster <bfoster@redhat.com>]

On Sat, Nov 07, 2026 at 09:19:41PM -0500, Tal Zussman wrote:
> A dontcache write drops its folios from the page cache once writeback
> completes, and a dontcache read drops them once the read completes.
> 
> Test this by writing and reading a 1M file with and without
> RWF_DONTCACHE and checking with mincore how much of it is cached
> afterwards: all of it for plain I/O, none of it for dontcache I/O.
> 
> The overwrite is a separate case because the drop is skipped when
> writeback completes in interrupt context, which is what happens on ext4
> without commit "ext4: complete dropbehind writeback in task context"
> [1].
> 
> Link: https://lore.kernel.org/linux-ext4/20261012-ext4-dontcache-complete-in-task-v1-1-6ac801557041@columbia.edu/ [1]
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>
> ---

I like it. Thanks for the tweaks!

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  tests/generic/810     | 77 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  tests/generic/810.out | 11 ++++++++
>  2 files changed, 88 insertions(+)
> 
> diff --git a/tests/generic/810 b/tests/generic/810
> new file mode 100755
> index 00000000..1866848f
> --- /dev/null
> +++ b/tests/generic/810
> @@ -0,0 +1,77 @@
> +#! /bin/bash
> +# SPDX-License-Identifier: GPL-2.0
> +# Copyright (c) 2026 Tal Zussman.  All Rights Reserved.
> +#
> +# FS QA Test No. 810
> +#
> +# Check that RWF_DONTCACHE I/O drops its folios from the page cache: a
> +# dontcache write once its writeback completes, and a dontcache read once the
> +# read completes. Plain buffered I/O on the same file has to leave them cached.
> +#
> +. ./common/preamble
> +_begin_fstest auto quick rw
> +
> +_require_scratch
> +_require_xfs_io_command "pwrite" "-U"
> +_require_xfs_io_command "pread" "-U"
> +_require_xfs_io_command "mincore"
> +_exclude_scratch_mount_option "dax"
> +
> +_scratch_mkfs > $seqres.full 2>&1
> +_scratch_mount
> +
> +size=$((1024 * 1024))
> +file=$SCRATCH_MNT/file
> +pagesize=$(_get_page_size)
> +
> +# Count the bytes of the file that are in the page cache.
> +cached_bytes()
> +{
> +	local pages
> +
> +	pages=$($XFS_IO_PROG -c "mmap -r 0 $size" -c "mincore" $file | \
> +		awk '{ n += $4 } END { print n + 0 }')
> +	echo $((pages * pagesize))
> +}
> +
> +# Report how many bytes are cached, after waiting up to $1 seconds for the
> +# cache to empty. Dontcache writeback can drop folios just after fsync returns.
> +report_cached()
> +{
> +	local delay=${1:-0}
> +	local i
> +
> +	for ((i = 0; i < delay * 10; i++)); do
> +		[ "$(cached_bytes)" -eq 0 ] && break
> +		sleep 0.1
> +	done
> +	echo "$(cached_bytes) of $size bytes cached"
> +}
> +
> +echo "plain write"
> +$XFS_IO_PROG -f -c "pwrite -q 0 $size" -c fsync $file
> +report_cached 1
> +
> +echo "dontcache write to a new file"
> +rm -f $file
> +$XFS_IO_PROG -f -c "pwrite -q -U -V 1 -b $size 0 $size" -c fsync $file
> +report_cached 5
> +
> +# Start each of the remaining cases from an empty page cache.
> +echo "dontcache overwrite"
> +_scratch_cycle_mount
> +$XFS_IO_PROG -c "pwrite -q -U -V 1 -b $size 0 $size" -c fsync $file
> +report_cached 5
> +
> +echo "dontcache read"
> +_scratch_cycle_mount
> +$XFS_IO_PROG -c "pread -q -U -V 1 -b $size 0 $size" $file
> +report_cached 5
> +
> +echo "plain read"
> +_scratch_cycle_mount
> +$XFS_IO_PROG -c "pread -q 0 $size" $file
> +report_cached 1
> +
> +_scratch_unmount
> +_exit 0
> diff --git a/tests/generic/810.out b/tests/generic/810.out
> new file mode 100644
> index 00000000..cb6c3f2a
> --- /dev/null
> +++ b/tests/generic/810.out
> @@ -0,0 +1,11 @@
> +QA output created by 810
> +plain write
> +1048576 of 1048576 bytes cached
> +dontcache write to a new file
> +0 of 1048576 bytes cached
> +dontcache overwrite
> +0 of 1048576 bytes cached
> +dontcache read
> +0 of 1048576 bytes cached
> +plain read
> +1048576 of 1048576 bytes cached
> 
> -- 
> 2.39.5
> 
>