[PATCH v2 0/3] fstests: test that RWF_DONTCACHE I/O drops its folios

[PATCH v2 0/3] fstests: test that RWF_DONTCACHE I/O drops its folios

[Tal Zussman]

Add a test for RWF_DONTCACHE, as suggested by Christoph. [1]

generic/810 writes and reads a 1M file with and without RWF_DONTCACHE
and checks with mincore how much of it is still cached after each step.
On ext4 without the fix, the dontcache overwrite leaves the whole file
cached.

The first two patches make _require_xfs_io_command able to probe
"pwrite -U" and "pread -U".

[1] https://lore.kernel.org/linux-ext4/20261012-ext4-dontcache-complete-in-task-v1-1-6ac801557041@columbia.edu/

---
Changes in v2:
- generic/810: fold the writeback wait into report_cached() with an
  optional delay, and give the plain cases a short delay, per Brian
  Foster.
- Link to v1: https://patch.msgid.link/20261015-dontcache-test-v1-0-6aa7583973a9@columbia.edu

---
Tal Zussman (3):
      common/rc: probe pread options with a real read
      common/rc: pass -V 1 when probing pwrite and pread -U
      generic/810: check that RWF_DONTCACHE I/O drops its folios

 common/rc             | 14 ++++++++++
 tests/generic/810     | 77 +++++++++++++++++++++++++++++++++++++++++++++++++++
 tests/generic/810.out | 11 ++++++++
 3 files changed, 102 insertions(+)
---
base-commit: d07dca72d2455f4ba8c660ad4c008541ccccb707
change-id: 20261015-dontcache-test-be6094aa3f20

Best regards,
--  
Tal Zussman <tz2294@columbia.edu>

[PATCH v2 1/3] common/rc: probe pread options with a real read

[Tal Zussman]

_require_xfs_io_command has no pread case, so a pread option falls back
to a grep of the help text, and that only matches options that take an
argument. A bare -U is reported as unsupported even when xfs_io has it.
Add a pread case that does a real read with the option, like the pwrite
case does.

Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 common/rc | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/common/rc b/common/rc
index 1eccd60c..945c9e42 100644
--- a/common/rc
+++ b/common/rc
@@ -3120,6 +3120,11 @@ _require_xfs_io_command()
 		        "pwrite $pwrite_opts $param 0 $write_size" $testfile 2>&1`
 		param_checked="$pwrite_opts $param"
 		;;
+	"pread")
+		testio=`$XFS_IO_PROG -f -c "pwrite 0 4k" \
+			-c "pread $param 0 4k" $testfile 2>&1`
+		param_checked="$param"
+		;;
 	"scrub"|"repair")
 		test -z "$param" && param="probe"
 		testio=`$XFS_IO_PROG -x -c "$command $param" $TEST_DIR 2>&1`

-- 
2.39.5

[PATCH v2 2/3] common/rc: pass -V 1 when probing pwrite and pread -U

[Tal Zussman]

xfs_io refuses pwritev2() and preadv2() flags unless -V is given, so
probing -U without it always fails. Pass -V 1 for -U in both the pwrite
and pread cases.

Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 common/rc | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/common/rc b/common/rc
index 945c9e42..a0239034 100644
--- a/common/rc
+++ b/common/rc
@@ -3105,6 +3105,10 @@ _require_xfs_io_command()
 			opts+=" -d"
 			pwrite_opts+="-V 1 -b $write_size"
 		fi
+		# -U (RWF_DONTCACHE) only works with vectored I/O writes
+		if [ "$param" == "-U" ]; then
+			pwrite_opts+="-V 1 -b $write_size"
+		fi
 		if [ "$param" == "-A" ]; then
 			opts+=" -d"
 			# try to write the minimum supported atomic write size
@@ -3121,9 +3125,14 @@ _require_xfs_io_command()
 		param_checked="$pwrite_opts $param"
 		;;
 	"pread")
+		# -U (RWF_DONTCACHE) only works with vectored I/O reads
+		local pread_opts=" "
+		if [ "$param" == "-U" ]; then
+			pread_opts+="-V 1 -b 4k"
+		fi
 		testio=`$XFS_IO_PROG -f -c "pwrite 0 4k" \
-			-c "pread $param 0 4k" $testfile 2>&1`
-		param_checked="$param"
+			-c "pread $pread_opts $param 0 4k" $testfile 2>&1`
+		param_checked="$pread_opts $param"
 		;;
 	"scrub"|"repair")
 		test -z "$param" && param="probe"

-- 
2.39.5

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

[Tal Zussman]

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>
---
 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

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

[Brian Foster]

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
> 
> 

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

[Tal Zussman]

On 11/9/26 9:10 AM, Brian Foster wrote:
> 
> I like it. Thanks for the tweaks!
> 
> Reviewed-by: Brian Foster <bfoster@redhat.com>
> 
Thanks, and thanks for the review! The end result looks cleaner.

Re: [PATCH v2 2/3] common/rc: pass -V 1 when probing pwrite and pread -U

[Christoph Hellwig]

On Sat, Nov 07, 2026 at 09:19:40PM -0500, Tal Zussman wrote:
> xfs_io refuses pwritev2() and preadv2() flags unless -V is given, so
> probing -U without it always fails. Pass -V 1 for -U in both the pwrite
> and pread cases.

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

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

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>