[PATCH v2] generic/798: chain xfs_io commands into a single invocation

[Avinesh Kumar <avinesh.kumar@suse.com>]

From: Avinesh Kumar <avinesh.kumar@suse.com>

The write, cachestat, fsync, and cachestat calls were each run as
separate xfs_io invocations, closing and reopening the file in
between. Some filesystems flush dirty data back on close(), clearing
the dirty bit before the first cachestat ever ran, causing test failure

- output mismatch (see /opt/xfstests/results//generic/798.out.bad)
    --- tests/generic/798.out	2026-08-20 05:25:39.000000000 -0400
    +++ /opt/xfstests/results//generic/798.out.bad	2026-08-21 12:30:39.831381108 -0400
    @@ -1,16 +1,16 @@
     QA output created by 798
     === Test with 1 pages ===
    -Cached: 1, Dirty: 1, Writeback: 0, Evicted: 0, Recently Evicted: 0
    +Cached: 1, Dirty: 0, Writeback: 0, Evicted: 0, Recently Evicted: 0
     Cached: 1, Dirty: 0, Writeback: 0, Evicted: 0, Recently Evicted: 0
     === Test with 2 pages ===
    -Cached: 2, Dirty: 2, Writeback: 0, Evicted: 0, Recently Evicted: 0
    ...

Run the whole sequence on one open file descriptor instead by chaining
all xfs_io commands in single invocation.

Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Avinesh Kumar <avinesh.kumar@suse.com>
---
 tests/generic/798 | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/tests/generic/798 b/tests/generic/798
index 0fe37646..171878ee 100755
--- a/tests/generic/798
+++ b/tests/generic/798
@@ -27,12 +27,22 @@ for num_page in 1 2 4 8 16; do
 	echo "=== Test with $num_page pages ==="
 
 	rm -f $SCRATCH_MNT/foobar
-	$XFS_IO_PROG -f -c "pwrite -b $pagesize 0 $size" $SCRATCH_MNT/foobar > /dev/null
-	# Basic cached number reporting
-	$XFS_IO_PROG -c "cachestat 0 $size" $SCRATCH_MNT/foobar
 
-	# Test dirty page number reporting after a fsync.
+	# Basic cached number reporting and test dirty page number
+	# reporting after a fsync.
+	#
+	# Do the whole write+cachestat+fsync+cachestat sequence in a
+	# single xfs_io invocation. Some filesystems (e.g. NFS) flush
+	# dirty data back to the server as part of their own close()-time
+	# cache consistency, which would clear the dirty bits before the
+	# second xfs_io process ever got to run cachestat.
 	$XFS_IO_PROG -c "fsync" -c "cachestat 0 $size" $SCRATCH_MNT/foobar
+		$XFS_IO_PROG -f \
+		-c "pwrite -q -b $pagesize 0 $size" \
+		-c "cachestat 0 $size" \
+		-c "fsync" \
+		-c "cachestat 0 $size" \
+		$SCRATCH_MNT/foobar
 done
 
 _scratch_unmount
-- 
2.55.0