[PATCH v2] fstests: generic: add a basic cachestat test case

[Qu Wenruo <wqu@suse.com>]

The test case is inspired by LTP, where there is a regression on 64K
page size systems with btrfs, that after a fsync, cachestat() still
report dirty pages.

The test case itself is pretty simple, fill the file with a buffered write that is
1/2/4/8/16 page sized, call cachestat() to make sure the cached/dirtied
number match the page number.

Then do a fsync(), and make sure the dirty page number reduced to 0
meanwhile cached is still the same.

Link: https://bugzilla.suse.com/show_bug.cgi?id=1270397
Reviewed-by: Anand Jain <asj@kernel.org>
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
Changelog:
v2:
- Reject "sync" and "dax" mount options
  Those mount options write back data synchronously, will screw up the
  buffered write dirtied page reporting.

- Use AWK_PROG everywhere.

- Remove the unnecessary redirection for fsync
---
 tests/generic/798     | 61 +++++++++++++++++++++++++++++++++++++++++++
 tests/generic/798.out |  2 ++
 2 files changed, 63 insertions(+)
 create mode 100755 tests/generic/798
 create mode 100644 tests/generic/798.out

diff --git a/tests/generic/798 b/tests/generic/798
new file mode 100755
index 00000000..8888f6a3
--- /dev/null
+++ b/tests/generic/798
@@ -0,0 +1,61 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 SUSE S.A.  All Rights Reserved.
+#
+# FS QA Test 798
+#
+# Basic tests for cachestat()
+#
+. ./common/preamble
+_begin_fstest auto quick
+
+_require_xfs_io_command "cachestat"
+_require_scratch
+# Any mount option that writes data back synchronously should be rejected.
+# Or it will screw up the dirtied page reporting for buffered writes.
+_exclude_scratch_mount_option "dax"
+_exclude_scratch_mount_option "sync"
+
+pagesize=$(_get_page_size)
+
+for num_page in 1 2 4 8 16; do
+	size=$(($pagesize * $num_page))
+
+	echo "=== Test with $num_page pages ===" >> $seqres.full
+	_scratch_mkfs > /dev/null
+	_scratch_mount
+
+	# Basic cached number reporting
+	$XFS_IO_PROG -f -c "pwrite -b $pagesize 0 $size" \
+		$SCRATCH_MNT/foobar >> $seqres.full
+	$XFS_IO_PROG -c "cachestat 0 $size" $SCRATCH_MNT/foobar > $tmp.output
+	cat $tmp.output >> $seqres.full
+	cached=$(cat $tmp.output | cut -f1 -d, | $AWK_PROG '{print $2}')
+	dirtied=$(cat $tmp.output | cut -f2 -d, | $AWK_PROG '{print $2}')
+
+	if [ "$cached" -ne "$num_page" ]; then
+		_fail "cached not matching the page number"
+	fi
+
+	if [ "$cached" -ne "$dirtied" ]; then
+		_fail "dirtied not matching the page number"
+	fi
+	$XFS_IO_PROG -c "fsync" $SCRATCH_MNT/foobar
+
+	# Test dirty page number reporting after a fsync.
+	$XFS_IO_PROG -c "cachestat 0 $size" $SCRATCH_MNT/foobar > $tmp.output
+	cat $tmp.output >> $seqres.full
+	_scratch_unmount
+	cached=$(cat $tmp.output | cut -f1 -d, | $AWK_PROG '{print $2}')
+	dirtied=$(cat $tmp.output | cut -f2 -d, | $AWK_PROG '{print $2}')
+
+	if [ "$cached" -ne "$num_page" ]; then
+		_fail "cached not matching the page number"
+	fi
+	if [ "$dirtied" -ne 0 ]; then
+		_fail "dirtied pages not zero"
+	fi
+done
+
+echo "Silence is golden"
+_exit 0
diff --git a/tests/generic/798.out b/tests/generic/798.out
new file mode 100644
index 00000000..216d6e93
--- /dev/null
+++ b/tests/generic/798.out
@@ -0,0 +1,2 @@
+QA output created by 798
+Silence is golden
-- 
2.51.2