Re: [PATCH v6 10/10] selftests/dax: add dax/kmem hotplug sysfs regression test

[Richard Cheng <icheng@nvidia.com>]

On Tue, Jun 30, 2026 at 05:18:42PM +0800, Gregory Price wrote:
> Add a kselftest for the dax/kmem whole-device "state" sysfs attribute
> (/sys/bus/dax/devices/daxX.Y/state), which transitions a kmem-backed
> dax device between "unplugged", "online" and "online_movable".
> 
> The kselftest also includes a test to demonstrate the force-unbind
> does not deadlock - but this is a destructive test.  The dax device
> can never be rebound after doing this.
> 
> Provisioning a devdax device and binding it to kmem needs daxctl/ndctl
> out of scope for an in-tree selftest, so the test discovers an already
> kmem-bound dax device and SKIPs when none are present or the memory
> cannot be freed to reach a known baseline.
> 
> When a device is available it validates the interface contract:
>   - online / online_movable actually add memory (MemTotal grows),
>   - online is idempotent,
>   - switching between online types without unplug is rejected,
>   - unplug removes memory and the reported state is "unplugged"
>   - invalid input is rejected.
> 
> One specific regression test:
>     online -> unplug -> online_movable -> unplug
> 
> Re-online must re-reserve per-range resources so subsequent unplug
> actually offlines and removes instead of silently reporting success
> while the memory stays online.
> 
> Signed-off-by: Gregory Price <gourry@gourry.net>
> ---
>  tools/testing/selftests/Makefile              |   1 +
>  tools/testing/selftests/dax/Makefile          |   6 +
>  tools/testing/selftests/dax/config            |   4 +
>  .../testing/selftests/dax/dax-kmem-hotplug.sh | 190 ++++++++++++++++++
>  tools/testing/selftests/dax/settings          |   1 +
>  5 files changed, 202 insertions(+)
>  create mode 100644 tools/testing/selftests/dax/Makefile
>  create mode 100644 tools/testing/selftests/dax/config
>  create mode 100755 tools/testing/selftests/dax/dax-kmem-hotplug.sh
>  create mode 100644 tools/testing/selftests/dax/settings
> 
> diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
> index 6e59b8f63e41..8c2b4f97619c 100644
> --- a/tools/testing/selftests/Makefile
> +++ b/tools/testing/selftests/Makefile
> @@ -14,6 +14,7 @@ TARGETS += core
>  TARGETS += cpufreq
>  TARGETS += cpu-hotplug
>  TARGETS += damon
> +TARGETS += dax
>  TARGETS += devices/error_logs
>  TARGETS += devices/probe
>  TARGETS += dmabuf-heaps
> diff --git a/tools/testing/selftests/dax/Makefile b/tools/testing/selftests/dax/Makefile
> new file mode 100644
> index 000000000000..25a4f3d73a5b
> --- /dev/null
> +++ b/tools/testing/selftests/dax/Makefile
> @@ -0,0 +1,6 @@
> +# SPDX-License-Identifier: GPL-2.0
> +all:
> +
> +TEST_PROGS := dax-kmem-hotplug.sh
> +
> +include ../lib.mk
> diff --git a/tools/testing/selftests/dax/config b/tools/testing/selftests/dax/config
> new file mode 100644
> index 000000000000..4c9aaeb6ceb4
> --- /dev/null
> +++ b/tools/testing/selftests/dax/config
> @@ -0,0 +1,4 @@
> +CONFIG_DEV_DAX=m
> +CONFIG_DEV_DAX_KMEM=m
> +CONFIG_MEMORY_HOTPLUG=y
> +CONFIG_MEMORY_HOTREMOVE=y
> diff --git a/tools/testing/selftests/dax/dax-kmem-hotplug.sh b/tools/testing/selftests/dax/dax-kmem-hotplug.sh
> new file mode 100755
> index 000000000000..c8bbaf6178ed
> --- /dev/null
> +++ b/tools/testing/selftests/dax/dax-kmem-hotplug.sh
> @@ -0,0 +1,190 @@
> +#!/bin/bash
> +# SPDX-License-Identifier: GPL-2.0
> +#
> +# Exercise the dax/kmem "state" sysfs attribute:
> +#   /sys/bus/dax/devices/daxX.Y/state  ->  unplugged | online | online_kernel | online_movable
> +#
> +# The test needs a dax device already bound to the kmem driver.
> +# If no suitable device is found the tests SKIP.
> +#
> +# A dax device can be provisioned with the memmap= boot param, e.g.:
> +#   memmap=2G!4G
> +#
> +# then, in the booted system:
> +#
> +#   ndctl create-namespace -m devdax -e namespace0.0 -f
> +#   daxctl reconfigure-device -N -m system-ram dax0.0   # bind kmem
> +#   ./dax-kmem-hotplug.sh
> +
> +# shellcheck disable=SC1091
> +DIR="$(dirname "$(readlink -f "$0")")"
> +. "$DIR"/../kselftest/ktap_helpers.sh
> +
> +DAX_BASE=/sys/bus/dax/devices
> +
> +memtotal_kb() { awk '/^MemTotal:/ {print $2}' /proc/meminfo; }
> +get_state() { cat "$HP" 2>/dev/null; }
> +# set_state STATE -- write a state to the state attribute; returns the
> +# write's exit status (0 = accepted by the kernel)
> +set_state() { echo "$1" > "$HP" 2>/dev/null; }
> +
> +find_kmem_dax() {
> +	local d drv
> +	for d in "$DAX_BASE"/dax*; do
> +		[ -e "$d/state" ] || continue
> +		drv=$(readlink "$d/driver" 2>/dev/null)
> +		[ "$(basename "${drv:-}")" = kmem ] || continue
> +		basename "$d"
> +		return 0
> +	done
> +	return 1
> +}

It picks the first kmem-bound dax device and runs online/offline cycles on it.
If the selected device is a real device with production usage, offlining movable
memory will migrate all in-use pages.

Could the destructive parts be gated behind an opt-in, e.g. an environment var ?
Or find the testable backend device instead of just picking the first one?

Best regards,
Richard Cheng.

> +
> +ktap_print_header
> +
> +if [ "$UID" != 0 ]; then
> +	ktap_skip_all "must be run as root"
> +	exit "$KSFT_SKIP"
> +fi
> +
> +DAX=$(find_kmem_dax)
> +if [ -z "$DAX" ]; then
> +	ktap_skip_all "no kmem-bound dax device with a state attribute"
> +	exit "$KSFT_SKIP"
> +fi
> +HP=$DAX_BASE/$DAX/state
> +ORIG=$(get_state)
> +
> +# A failure to reach the baseline is environmental (memory in use), not an
> +# interface failure, so skip rather than fail.
> +set_state unplugged; rc=$?
> +if [ "$rc" != 0 ] || [ "$(get_state)" != unplugged ]; then
> +	ktap_skip_all "$DAX: cannot reach 'unplugged' baseline (memory in use?)"
> +	[ -n "$ORIG" ] && set_state "$ORIG"
> +	exit "$KSFT_SKIP"
> +fi
> +mt_unplugged=$(memtotal_kb)
> +
> +DRV=/sys/bus/dax/drivers/kmem
> +AOB=/sys/devices/system/memory/auto_online_blocks
> +
> +ktap_print_msg "using $DAX (initial state was: $ORIG)"
> +ktap_set_plan 8
> +
> +# A public (N_MEMORY) kmem node onlined into a kernel zone (online/online_kernel)
> +# collects unmovable allocations and can then never be offlined, which would
> +# wedge the device for the rest of this test.  So this test only ever
> +# successfully onlines online_movable, the one mode that is reliably unpluggable.
> +
> +set_state online_movable; rc=$?
> +mt_online=$(memtotal_kb)
> +if [ "$rc" = 0 ] && [ "$(get_state)" = online_movable ] && [ "$mt_online" -gt "$mt_unplugged" ]; then
> +	ktap_test_pass "online_movable: state=online_movable, MemTotal $mt_unplugged -> $mt_online kB"
> +else
> +	ktap_test_fail "online_movable: rc=$rc state=$(get_state) MemTotal $mt_unplugged -> $mt_online"
> +fi
> +
> +set_state online_movable; rc=$?
> +if [ "$rc" = 0 ] && [ "$(get_state)" = online_movable ]; then
> +	ktap_test_pass "online_movable idempotent"
> +else
> +	ktap_test_fail "online_movable idempotent: rc=$rc state=$(get_state)"
> +fi
> +
> +# A different online type is rejected without an intervening unplug.  The write
> +# is refused before any hotplug, so this never actually onlines a kernel zone.
> +set_state online_kernel; rc=$?
> +if [ "$rc" != 0 ] && [ "$(get_state)" = online_movable ]; then
> +	ktap_test_pass "reject online_kernel without intervening unplug (no kernel-zone online)"
> +else
> +	ktap_test_fail "online_movable->online_kernel not rejected: rc=$rc state=$(get_state)"
> +fi
> +
> +set_state unplugged; rc=$?
> +mt=$(memtotal_kb)
> +if [ "$rc" = 0 ] && [ "$(get_state)" = unplugged ] && [ "$mt" -lt "$mt_online" ]; then
> +	ktap_test_pass "unplug from online_movable: MemTotal $mt_online -> $mt kB"
> +else
> +	ktap_test_fail "unplug from online_movable: rc=$rc state=$(get_state) MemTotal $mt_online -> $mt"
> +fi
> +
> +before=$(get_state)
> +set_state bogus_state; rc=$?
> +if [ "$rc" != 0 ] && [ "$(get_state)" = "$before" ]; then
> +	ktap_test_pass "reject invalid state string"
> +else
> +	ktap_test_fail "invalid state not rejected: rc=$rc state=$(get_state)"
> +fi
> +
> +# The online_movable -> unplug cycle once regressed: a re-online failed to
> +# re-reserve the per-range resources, so a later unplug reported success while
> +# leaving the memory online.  Assert each iteration really adds and frees memory.
> +set_state unplugged
> +cycle_ok=1; fail_i=0; on=0; off=0
> +for i in 1 2 3; do
> +	if ! set_state online_movable; then cycle_ok=0; fail_i=$i; break; fi
> +	on=$(memtotal_kb)
> +	if ! set_state unplugged; then cycle_ok=0; fail_i=$i; break; fi
> +	off=$(memtotal_kb)
> +	if [ "$on" -le "$mt_unplugged" ] || [ "$off" -ge "$on" ]; then
> +		cycle_ok=0; fail_i=$i; break
> +	fi
> +done
> +if [ "$cycle_ok" = 1 ]; then
> +	ktap_test_pass "online_movable/unplug cycle re-acquires resources (3x: added and freed each time)"
> +else
> +	ktap_test_fail "online_movable/unplug cycle regressed at iteration $fail_i (on=$on off=$off baseline=$mt_unplugged)"
> +fi
> +
> +# change system default online policy while the device is unbound, and show
> +# the new system default policy is utilized across bindings.
> +set_state unplugged
> +if [ -w "$AOB" ] && [ -w "$DRV/unbind" ] && [ -w "$DRV/bind" ]; then
> +	orig_aob=$(cat "$AOB")
> +	echo "$DAX" > "$DRV/unbind" 2>/dev/null
> +	echo offline > "$AOB" 2>/dev/null
> +	echo "$DAX" > "$DRV/bind" 2>/dev/null
> +	sleep 1
> +	st=$(get_state)
> +	echo "$orig_aob" > "$AOB" 2>/dev/null		# restore system policy
> +	if [ "$st" = offline ]; then
> +		ktap_test_pass "online policy resolved at bind: auto_online_blocks=offline -> state=offline"
> +	else
> +		ktap_test_fail "bind-time policy not honored: state=$st (expected offline)"
> +	fi
> +	set_state unplugged 2>/dev/null
> +else
> +	ktap_test_skip "auto_online_blocks or driver bind/unbind not writable"
> +fi
> +
> +[ -n "$ORIG" ] && set_state "$ORIG"
> +
> +# DESTRUCTIVE: unbinding the driver while memory is online causes the resources
> +# to leak - but the unbind should not deadlock.  Instead the driver leaks it
> +# with a single "stuck online" warning. This leaves the memory online and the
> +# device unbound until reboot, so it runs last - and only if we can run it,
> +# leaving the restored state above untouched otherwise.  online_movable only:
> +# this test never onlines a public node into a kernel zone.
> +if [ -w "$DRV/unbind" ]; then
> +	set_state unplugged; set_state online_movable
> +fi
> +if [ "$(get_state)" = online_movable ] && [ -w "$DRV/unbind" ]; then
> +	mt_on=$(memtotal_kb)
> +	dmesg -C 2>/dev/null
> +	echo "$DAX" > "$DRV/unbind" 2>/dev/null
> +	mt_after=$(memtotal_kb)
> +	# The leaked "System RAM (kmem)" regions stay in the iomem tree; reading
> +	# their names dereferences res_name, which a buggy unbind already freed.
> +	# Walk /proc/iomem to provoke that use-after-free (caught by KASAN).
> +	cat /proc/iomem > /dev/null 2>&1
> +	splat=$(dmesg 2>/dev/null | grep -ciE "KASAN|BUG:|use-after-free|general protection|Oops|refcount_t")
> +	if [ "$splat" = 0 ] && [ "$mt_after" -ge "$mt_on" ]; then
> +		ktap_test_pass "unbind while online: memory left online, no UAF/oops (MemTotal $mt_on -> $mt_after kB)"
> +	else
> +		ktap_test_fail "unbind while online regressed: splat=$splat MemTotal $mt_on -> $mt_after kB"
> +	fi
> +else
> +	ktap_test_skip "could not online device for unbind-while-online test"
> +fi
> +
> +ktap_finished
> diff --git a/tools/testing/selftests/dax/settings b/tools/testing/selftests/dax/settings
> new file mode 100644
> index 000000000000..ba4d85f74cd6
> --- /dev/null
> +++ b/tools/testing/selftests/dax/settings
> @@ -0,0 +1 @@
> +timeout=90
> -- 
> 2.53.0-Meta
> 
>