[PATCH] btrfs/255: add test for quota disable in parallel with balance

[Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>]

Test quota disable during btrfs balance and confirm it does not cause
kernel hang. This is a regression test for the problem reported to
linux-btrfs list [1]. The hang was recreated using the test case and
memory backed null_blk device with 5GB size as the scratch device.

[1] https://lore.kernel.org/linux-btrfs/20220115053012.941761-1-shinichiro.kawasaki@wdc.com/

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
---
 tests/btrfs/255     | 42 ++++++++++++++++++++++++++++++++++++++++++
 tests/btrfs/255.out |  2 ++
 2 files changed, 44 insertions(+)
 create mode 100755 tests/btrfs/255
 create mode 100644 tests/btrfs/255.out

diff --git a/tests/btrfs/255 b/tests/btrfs/255
new file mode 100755
index 00000000..16b682ca
--- /dev/null
+++ b/tests/btrfs/255
@@ -0,0 +1,42 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2022 Western Digital Corporation or its affiliates.
+#
+# FS QA Test No. btrfs/255
+#
+# Confirm that disabling quota during balance does not hang
+#
+. ./common/preamble
+_begin_fstest auto qgroup
+
+# real QA test starts here
+_supported_fs btrfs
+_require_scratch
+
+_scratch_mkfs >> $seqres.full 2>&1
+_scratch_mount
+
+# Fill 40% of the device or 2GB
+fill_percent=40
+max_fillsize=$((2*1024*1024*1024))
+
+devsize=$(($(_get_device_size $SCRATCH_DEV) * 512))
+fillsize=$((devsize * fill_percent / 100))
+((fillsize > max_fillsize)) && fillsize=$max_fillsize
+
+fs=$((4096*1024))
+for ((i=0; i * fs < fillsize; i++)); do
+	dd if=/dev/zero of=$SCRATCH_MNT/file.$i bs=$fs count=1 \
+	   >> $seqres.full 2>&1
+done
+echo 3 > /proc/sys/vm/drop_caches
+
+# Run btrfs balance and quota enable/disable in parallel
+_run_btrfs_balance_start $SCRATCH_MNT >> $seqres.full &
+$BTRFS_UTIL_PROG quota enable $SCRATCH_MNT
+$BTRFS_UTIL_PROG quota disable $SCRATCH_MNT
+wait
+
+echo "Silence is golden"
+status=0
+exit
diff --git a/tests/btrfs/255.out b/tests/btrfs/255.out
new file mode 100644
index 00000000..7eefb828
--- /dev/null
+++ b/tests/btrfs/255.out
@@ -0,0 +1,2 @@
+QA output created by 255
+Silence is golden
-- 
2.33.1