[PATCH] xfs: test that zone_gc_low_space writes start gc for rw fses

Test that writes to the systfs attribute zone_gc_low_space triggers
garbage collection for rw (but not ro) file systems.

Signed-off-by: Hans Holmberg <[email protected]>
---

This test will fail currently, but i've submitted a fix:
https://lore.kernel.org/linux-xfs/[email protected]/

 tests/xfs/999     | 89 +++++++++++++++++++++++++++++++++++++++++++++++
 tests/xfs/999.out |  2 ++
 2 files changed, 91 insertions(+)
 create mode 100755 tests/xfs/999
 create mode 100644 tests/xfs/999.out

diff --git a/tests/xfs/999 b/tests/xfs/999
new file mode 100755
index 000000000000..f3c2f868cb1e
--- /dev/null
+++ b/tests/xfs/999
@@ -0,0 +1,89 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 Western Digital Corporation
+#
+# FS QA Test No. 999
+#
+# Tests that writes to gc_low_space will trigger start of garbage
+# collection for rw (but not ro) file systems
+#
+. ./common/preamble
+_begin_fstest auto rw zone quick
+
+. ./common/zoned
+
+_require_scratch
+_require_odirect
+
+get_scratch_mountstat() {
+	awk -v mount="$SCRATCH_MNT" -v stat="$1" \
+		'$0~"mounted on "mount" with fstype xfs"{f=1} f&&index($0,stat){print $NF;exit}' \
+		/proc/self/mountstats
+}
+
+get_user_available_rt_blocks() {
+	get_scratch_mountstat "user available RT blocks:"
+}
+
+gc_required() {
+	get_scratch_mountstat "RT GC required:"
+}
+
+# figure out if the rt section is internal or not
+if [ -z "$SCRATCH_RTDEV" ]; then
+	zdev=$SCRATCH_DEV
+else
+	zdev=$SCRATCH_RTDEV
+fi
+
+capacity=$(_zone_capacity 0 $zdev)
+_scratch_mkfs_sized $(($capacity * 6)) > $seqres.full 2>&1
+
+# limit max open zones to avoid gc to trigger before we want it to
+export MOUNT_OPTIONS="$MOUNT_OPTIONS -o max_open_zones=2"
+_try_scratch_mount || _notrun "mount option not supported"
+_require_xfs_scratch_zoned
+
+# create three zone-sized files and invalidate 95% of the data
+for i in $(seq 1 3); do
+	filename=$SCRATCH_MNT/data_$i
+	dd if=/dev/zero of=$filename bs=1M oflag=direct\
+		count=$(($capacity / (1024 * 1024))) >> $seqres.full 2>&1
+	truncate -s $(($capacity / 20)) $filename
+done
+
+# remount read-only and set the gc limit to free up unused space
+_scratch_remount ro
+rt_available_pre="$(get_user_available_rt_blocks)"
+_set_fs_sysfs_attr $SCRATCH_DEV "zoned/zonegc_low_space" 100
+
+# reclaiming a zone with 5% used blocks should take way less than 1 second
+sleep 1
+
+# gc should not run for read only file systems
+rt_available_post="$(get_user_available_rt_blocks)"
+[ "$rt_available_post" -gt "$rt_available_pre" ] && _fail "gc should not run while read only"
+
+_set_fs_sysfs_attr $SCRATCH_DEV "zoned/zonegc_low_space" 0
+
+# remount rw and check that gc starts, frees up blocks and stops after
+# the threshold is set to reclaim the unused blocks
+_scratch_remount rw
+
+rt_available_pre="$(get_user_available_rt_blocks)"
+_set_fs_sysfs_attr $SCRATCH_DEV "zoned/zonegc_low_space" 100
+
+i=0
+while [ $(gc_required) -eq "1" ]; do
+	sleep 1
+	let i=$i+1
+	[ $i -gt 3 ] && _fail "gc did not complete within a reasonable time"
+done
+
+rt_available_post="$(get_user_available_rt_blocks)"
+[ "$rt_available_pre" -ge "$rt_available_post" ] && _fail "gc did not free up space"
+
+echo "Silence is golden"
+status=0
+
+exit
diff --git a/tests/xfs/999.out b/tests/xfs/999.out
new file mode 100644
index 000000000000..3b276ca804fc
--- /dev/null
+++ b/tests/xfs/999.out
@@ -0,0 +1,2 @@
+QA output created by 999
+Silence is golden
-- 
2.34.1