[PATCH v2] generic: test that preallocated blocks still read zeroes after I/O errors

[PATCH v2] generic: test that preallocated blocks still read zeroes after I/O errors

[Daejun Park via B4 Relay]

From: Daejun Park <daejun7.park@samsung.com>

Write one block of a preallocated range while I/O to the rest of the
range fails, and check that the rest still reads back as zeroes rather
than the old contents of the disk.

ext4 got this wrong. When a write converts part of a small unwritten
extent, ext4 may zero out the rest of the extent and convert all of it
to written. If that zeroout fails, ext4 falls back to splitting the
extent, but it cached the blocks it failed to zero out as written in the
extent status tree, so reads of those blocks returned the old contents
of the disk.

On ext4 the test mounts with dioread_lock and nodelalloc and sets
extent_max_zeroout_kb to get into that path: with the default
dioread_nolock, the extent stays unwritten until the I/O completes and
is converted then, without this zeroout. generic/250 and generic/252 do
not hit it either, as they write with direct I/O while dm-error is
loaded, which also leaves the extent unwritten. btrfs is excluded, as
fiemap does not report device offsets there.

The ext4 fix is "ext4: don't cache unzeroed blocks as written after a
failed zeroout".

Link: https://lore.kernel.org/r/20261008-ext4-fc-zeroout-v2-1-55cab1e24fa3@samsung.com
Suggested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Signed-off-by: Daejun Park <daejun7.park@samsung.com>
---
The ext4 fix is 1/2 of the series in the Link: and is not merged yet,
hence xxxxxxxxxxxx in _fixed_by_fs_commit. The test number may need to
change.

Tested in QEMU. On ext4 dev 9091c97be340 without the fix, blocks 1-7
read back as 0x53. With the fix, alone or with the rest of the series,
the test passes with 1k and 4k blocks. It passes on xfs with 1k and 4k
blocks.
---
Changes in v2:
- Make it a generic test (Christoph). The ext4 mount options and
  extent_max_zeroout_kb are set only on ext4. btrfs is excluded, as
  fiemap does not report device offsets there, and xfs gets the zoned
  and realtime handling of generic/791.
- Link to v1: https://lore.kernel.org/r/20261008-ext4-zeroout-eio-test-v1-1-9bb66deaf646@samsung.com
---
 tests/generic/806     | 106 ++++++++++++++++++++++++++++++++++++++++++++++++++
 tests/generic/806.out |   5 +++
 2 files changed, 111 insertions(+)

diff --git a/tests/generic/806 b/tests/generic/806
new file mode 100755
index 0000000..8991b6e
--- /dev/null
+++ b/tests/generic/806
@@ -0,0 +1,106 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 Samsung Electronics Co., Ltd. All Rights Reserved.
+#
+# FS QA Test No. 806
+#
+# Write one block of a preallocated range while I/O to the rest of the range
+# fails, then check that the rest still reads back as zeroes, not as the old
+# contents of the disk.
+#
+# ext4 zeroes out the rest of a small unwritten extent when a write converts
+# part of it. When that zeroout failed, it fell back to splitting the extent,
+# but it cached the blocks it failed to zero out as written in the extent
+# status tree, so reading them returned the old contents of the disk.
+#
+. ./common/preamble
+_begin_fstest auto quick rw prealloc eio
+
+_cleanup()
+{
+	cd /
+	rm -f $tmp.*
+	_dmerror_cleanup
+}
+
+. ./common/dmerror
+
+# The test writes to the device at the offsets fiemap reports, which are not
+# device offsets on btrfs.
+_exclude_fs btrfs
+
+_require_scratch
+_require_dm_target error
+_require_xfs_io_command "falloc"
+_require_xfs_io_command "fiemap"
+test $FSTYP = ext4 && _require_scratch_ext4_feature_enabled "extent"
+
+_fixed_by_fs_commit ext4 xxxxxxxxxxxx \
+	"ext4: don't cache unzeroed blocks as written after a failed zeroout"
+
+_scratch_mkfs >> $seqres.full 2>&1
+
+# dm-error table sizes need zone alignment on zoned devices, and the check
+# needs a mounted file system.
+_scratch_mount
+test $FSTYP = xfs && _require_xfs_scratch_non_zoned
+_scratch_unmount
+
+# ext4 does the zeroout when a write converts the extent itself. With the
+# default dioread_nolock, the extent stays unwritten until the I/O completes,
+# so mount with dioread_lock, and with nodelalloc to convert the extent at
+# write time.
+mount_opts=
+test $FSTYP = ext4 && mount_opts="-o nodelalloc,dioread_lock"
+
+do_mount()
+{
+	_dmerror_mount $mount_opts >> $seqres.full 2>&1 || _fail "mount failed"
+	test $FSTYP = xfs && _xfs_force_bdev data $SCRATCH_MNT
+}
+
+_dmerror_init
+do_mount
+
+nblks=8
+blksz=$(_get_file_block_size $SCRATCH_MNT)
+sects_per_blk=$((blksz / 512))
+file=$SCRATCH_MNT/file
+
+$XFS_IO_PROG -f -c "falloc 0 $((nblks * blksz))" -c fsync $file >> $seqres.full
+extent="$($XFS_IO_PROG -c "fiemap -v" $file | grep "^[[:space:]]*0:")"
+echo "$extent" >> $seqres.full
+phys=$(echo "$extent" | $AWK_PROG '{print $3}' | sed -e 's/\.\..*//')
+len=$(echo "$extent" | $AWK_PROG '{print $4}')
+flags=$(echo "$extent" | $AWK_PROG '{print $5}')
+flags=${flags:-0}
+test "$len" = $((nblks * sects_per_blk)) -a $((flags & 0x800)) -ne 0 || \
+	_notrun "could not allocate one unwritten extent of $nblks blocks"
+
+# Put something other than zeroes in the preallocated blocks
+_dmerror_unmount
+$XFS_IO_PROG -d \
+	-c "pwrite -S 0x53 -b $blksz $((phys * 512)) $((nblks * blksz))" \
+	$DMERROR_DEV >> $seqres.full
+do_mount
+
+# ext4 zeroes out the rest of the extent if the whole extent fits in
+# extent_max_zeroout_kb, which mount resets.
+test $FSTYP = ext4 && _set_fs_sysfs_attr $DMERROR_DEV \
+	extent_max_zeroout_kb $((nblks * blksz / 1024))
+
+# Fail I/O to blocks 1 to the end of the range and write block 0
+_dmerror_reset_table
+_dmerror_mark_range_bad $((phys + sects_per_blk)) \
+	$(((nblks - 1) * sects_per_blk))
+_dmerror_load_error_table
+$XFS_IO_PROG -c "pwrite -S 0x41 0 $blksz" -c fsync $file >> $seqres.full
+_dmerror_load_working_table
+
+echo "block 0:"
+od -An -v -tx1 -N $blksz $file | sort -u
+echo "blocks 1-$((nblks - 1)):"
+od -An -v -tx1 -j $blksz $file | sort -u
+
+# success, all done
+_exit 0
diff --git a/tests/generic/806.out b/tests/generic/806.out
new file mode 100644
index 0000000..84b3a35
--- /dev/null
+++ b/tests/generic/806.out
@@ -0,0 +1,5 @@
+QA output created by 806
+block 0:
+ 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41
+blocks 1-7:
+ 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

---
base-commit: 22348afe338c0f6d540c0d7ef0db749eaa51b218
change-id: 20261008-ext4-zeroout-eio-test-ee76bea0a517

Best regards,
-- 
Daejun Park <daejun7.park@samsung.com>

Re: [PATCH v2] generic: test that preallocated blocks still read zeroes after I/O errors

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Medium] tests/generic: missing _set_fs_sysfs_attr definition
- [Low] bash: unquoted $FSTYP in test expression risks syntax error
--

commit 1313e327b3473925cf67777d9fc9f5d95ae058fc
Author: Daejun Park <daejun7.park@samsung.com>

generic: test that preallocated blocks still read zeroes after I/O errors

This commit adds a new fstest to verify that when I/O to a portion of a
preallocated extent fails, the remainder of the extent still reads back
as zeroes. It reproduces an ext4 issue where failed zeroouts left blocks
cached as written, returning stale disk contents.

Link: https://lore.kernel.org/r/20261008-ext4-fc-zeroout-v2-1-55cab1e24fa3@samsung.com

> diff --git a/tests/generic/806 b/tests/generic/806
> new file mode 100755
> index 0000000000000..8991b6e59254a
> --- /dev/null
> +++ b/tests/generic/806

[ ... ]

> +_require_scratch
> +_require_dm_target error
> +_require_xfs_io_command "falloc"
> +_require_xfs_io_command "fiemap"
> +test $FSTYP = ext4 && _require_scratch_ext4_feature_enabled "extent"

[Severity: Low]
Is it possible for the $FSTYP variable to be empty or unset when this
expression in tests/generic/806 is evaluated? If so, this unquoted test
risks an invalid syntax error. Would it be safer to wrap the variable in
double quotes?

[ ... ]

> +do_mount
> +
> +# ext4 zeroes out the rest of the extent if the whole extent fits in
> +# extent_max_zeroout_kb, which mount resets.
> +test $FSTYP = ext4 && _set_fs_sysfs_attr $DMERROR_DEV \
> +	extent_max_zeroout_kb $((nblks * blksz / 1024))

[Severity: Medium]
Does this code call an undefined function? A codebase search indicates that
_set_fs_sysfs_attr is not defined anywhere in the test framework. When this
script executes during an ext4 run, will this cause a 'command not found'
error, leaving the extent_max_zeroout_kb attribute at its default value and
silently failing to configure the test environment?

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261008-ext4-zeroout-eio-test-v2-1-883ac28de4cf@samsung.com?part=1

Re: [PATCH v2] generic: test that preallocated blocks still read zeroes after I/O errors

[Christoph Hellwig]

On Thu, Oct 08, 2026 at 07:18:24PM +0900, Daejun Park via B4 Relay wrote:
> +# dm-error table sizes need zone alignment on zoned devices, and the check
> +# needs a mounted file system.
> +_scratch_mount
> +test $FSTYP = xfs && _require_xfs_scratch_non_zoned

The canonical way to write this would be

_require_non_zoned_device $SCRATCH_RTDEV

Otherwise looks good:


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