Re: [PATCH v6 1/3] selftests: cgroup: Add dmem_selftest helper module

[R Nageswara Sastry <rnsastry@linux.ibm.com>]

On 31.08.2026 6:55 PM, Albert Esteve wrote:
> Currently, dmem charging is driver-driven through direct
> calls to dmem_cgroup_try_charge() (for example TTM).
> That does not give cgroup selftests a generic way to
> trigger alloc/free paths from userspace in a chosen
> cgroup, so coverage of nested dmem.max still
> needs a synthetic region.
>
> Add tools/testing/selftests/cgroup/test_modules/dmem_selftest.c
> as an OOT helper, built like other test_modules (no extra
> Kconfig needed). Tests can insmod the .ko module to register a
> "dmem_selftest" region.
>
> Module parameters drive a single outstanding allocation against
> the calling task's cgroup:
>
>    /sys/module/dmem_selftest/parameters/alloc
>    /sys/module/dmem_selftest/parameters/free
>
> Writing a size to alloc triggers dmem_cgroup_try_charge() for
> the calling task's cgroup. Writing to free with no argument
> releases the outstanding charge via dmem_cgroup_uncharge().
> Only a single outstanding charge is supported.
>
> This provides a deterministic, driver-independent mechanism
> for exercising dmem accounting paths in selftests.
>
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
Tested-by: R Nageswara Sastry <rnsastry@linux.ibm.com>

System: ppc64le LPAR (IBM POWER), Linux 7.3-rc2

# insmod test_modules/dmem_selftest.ko
# ./test_dmem
TAP version 13
1..2
ok 1 test_dmem_max
ok 2 test_dmem_alloc_byte_granularity
# Totals: pass:2 fail:0 xfail:0 xpass:0 skip:0 error:0

> ---
>   tools/testing/selftests/cgroup/Makefile            |   1 +
>   tools/testing/selftests/cgroup/config              |   1 +
>   .../testing/selftests/cgroup/test_modules/Makefile |  17 +++
>   .../selftests/cgroup/test_modules/dmem_selftest.c  | 146 +++++++++++++++++++++
>   4 files changed, 165 insertions(+)
>
> diff --git a/tools/testing/selftests/cgroup/Makefile b/tools/testing/selftests/cgroup/Makefile
> index e01584c2189a..a08661320a07 100644
> --- a/tools/testing/selftests/cgroup/Makefile
> +++ b/tools/testing/selftests/cgroup/Makefile
> @@ -6,6 +6,7 @@ all: ${HELPER_PROGS}
>   TEST_FILES     := with_stress.sh
>   TEST_PROGS     := test_stress.sh test_cpuset_prs.sh test_cpuset_v1_hp.sh
>   TEST_GEN_FILES := wait_inotify
> +TEST_GEN_MODS_DIR := test_modules
>   # Keep the lists lexicographically sorted
>   TEST_GEN_PROGS  = test_core
>   TEST_GEN_PROGS += test_cpu
> diff --git a/tools/testing/selftests/cgroup/config b/tools/testing/selftests/cgroup/config
> index 39f979690dd3..2ee0488c3d65 100644
> --- a/tools/testing/selftests/cgroup/config
> +++ b/tools/testing/selftests/cgroup/config
> @@ -1,5 +1,6 @@
>   CONFIG_CGROUPS=y
>   CONFIG_CGROUP_CPUACCT=y
> +CONFIG_CGROUP_DMEM=y
>   CONFIG_CGROUP_FREEZER=y
>   CONFIG_CGROUP_SCHED=y
>   CONFIG_MEMCG=y
> diff --git a/tools/testing/selftests/cgroup/test_modules/Makefile b/tools/testing/selftests/cgroup/test_modules/Makefile
> new file mode 100644
> index 000000000000..dd8a17313cec
> --- /dev/null
> +++ b/tools/testing/selftests/cgroup/test_modules/Makefile
> @@ -0,0 +1,17 @@
> +# SPDX-License-Identifier: GPL-2.0
> +TESTMODS_DIR := $(realpath $(dir $(abspath $(lastword $(MAKEFILE_LIST)))))
> +KDIR ?= /lib/modules/$(shell uname -r)/build
> +
> +obj-m += dmem_selftest.o
> +
> +# Ensure that KDIR exists, otherwise skip the compilation
> +modules:
> +ifneq ("$(wildcard $(KDIR))", "")
> +	$(Q)$(MAKE) -C $(KDIR) modules KBUILD_EXTMOD=$(TESTMODS_DIR)
> +endif
> +
> +# Ensure that KDIR exists, otherwise skip the clean target
> +clean:
> +ifneq ("$(wildcard $(KDIR))", "")
> +	$(Q)$(MAKE) -C $(KDIR) clean KBUILD_EXTMOD=$(TESTMODS_DIR)
> +endif
> diff --git a/tools/testing/selftests/cgroup/test_modules/dmem_selftest.c b/tools/testing/selftests/cgroup/test_modules/dmem_selftest.c
> new file mode 100644
> index 000000000000..a763a156f13f
> --- /dev/null
> +++ b/tools/testing/selftests/cgroup/test_modules/dmem_selftest.c
> @@ -0,0 +1,146 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Kselftest helper for the dmem cgroup controller.
> + *
> + * Registers a synthetic dmem region so tests can trigger allocations
> + * from the calling task's cgroup via module parameters:
> + *   /sys/module/dmem_selftest/parameters/alloc
> + *   /sys/module/dmem_selftest/parameters/free
> + */
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
> +#include <linux/cgroup_dmem.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/moduleparam.h>
> +#include <linux/mutex.h>
> +#include <linux/panic.h>
> +#include <linux/sysfs.h>
> +
> +#define DM_SELFTEST_REGION_NAME	"dmem_selftest"
> +#define DM_SELFTEST_REGION_SIZE	(256ULL * 1024 * 1024)
> +
> +static struct dmem_cgroup_region *selftest_region;
> +static struct dmem_cgroup_pool_state *alloc_pool;
> +static u64 alloc_size;
> +static DEFINE_MUTEX(alloc_lock);
> +
> +static int param_set_alloc(const char *val, const struct kernel_param *kp)
> +{
> +	struct dmem_cgroup_pool_state *pool = NULL, *limit = NULL;
> +	u64 size;
> +	int ret;
> +
> +	if (!selftest_region)
> +		return -ENODEV;
> +
> +	ret = kstrtou64(val, 0, &size);
> +	if (ret)
> +		return ret;
> +	if (!size || size > DM_SELFTEST_REGION_SIZE)
> +		return -EINVAL;
> +
> +	mutex_lock(&alloc_lock);
> +	if (alloc_pool) {
> +		mutex_unlock(&alloc_lock);
> +		return -EBUSY;
> +	}
> +
> +	ret = dmem_cgroup_try_charge(selftest_region, size, &pool, &limit);
> +	if (ret == -EAGAIN && limit)
> +		dmem_cgroup_pool_state_put(limit);
> +	if (ret) {
> +		mutex_unlock(&alloc_lock);
> +		return ret;
> +	}
> +
> +	alloc_pool = pool;
> +	alloc_size = size;
> +	mutex_unlock(&alloc_lock);
> +	return 0;
> +}
> +
> +static int param_get_alloc(char *buffer, const struct kernel_param *kp)
> +{
> +	u64 size;
> +
> +	mutex_lock(&alloc_lock);
> +	size = alloc_size;
> +	mutex_unlock(&alloc_lock);
> +	return sysfs_emit(buffer, "%llu\n", size);
> +}
> +
> +static const struct kernel_param_ops alloc_ops = {
> +	.set = param_set_alloc,
> +	.get = param_get_alloc,
> +};
> +
> +module_param_cb(alloc, &alloc_ops, NULL, 0644);
> +MODULE_PARM_DESC(alloc, "Allocate (charge) SIZE bytes against the calling task's dmem cgroup");
> +
> +static int param_set_free(const char *val, const struct kernel_param *kp)
> +{
> +	mutex_lock(&alloc_lock);
> +	if (!alloc_pool) {
> +		mutex_unlock(&alloc_lock);
> +		return -EINVAL;
> +	}
> +
> +	dmem_cgroup_uncharge(alloc_pool, alloc_size);
> +	alloc_pool = NULL;
> +	alloc_size = 0;
> +	mutex_unlock(&alloc_lock);
> +	return 0;
> +}
> +
> +static const struct kernel_param_ops free_ops = {
> +	.flags = KERNEL_PARAM_OPS_FL_NOARG,
> +	.set = param_set_free,
> +};
> +
> +module_param_cb(free, &free_ops, NULL, 0200);
> +MODULE_PARM_DESC(free, "Free the outstanding dmem selftest allocation");
> +
> +static int __init dmem_selftest_init(void)
> +{
> +	static const struct dmem_cgroup_init init = {
> +		.size = DM_SELFTEST_REGION_SIZE,
> +	};
> +
> +	selftest_region = dmem_cgroup_register_region(&init, DM_SELFTEST_REGION_NAME);
> +	if (IS_ERR(selftest_region))
> +		return PTR_ERR(selftest_region);
> +	if (!selftest_region)
> +		return -EINVAL;
> +
> +	add_taint(TAINT_TEST, LOCKDEP_STILL_OK);
> +	pr_info("region '%s' registered; parameters alloc/free\n",
> +		DM_SELFTEST_REGION_NAME);
> +	return 0;
> +}
> +
> +static void __exit dmem_selftest_exit(void)
> +{
> +	mutex_lock(&alloc_lock);
> +	if (alloc_pool) {
> +		dmem_cgroup_uncharge(alloc_pool, alloc_size);
> +		alloc_pool = NULL;
> +		alloc_size = 0;
> +	}
> +	mutex_unlock(&alloc_lock);
> +
> +	if (selftest_region) {
> +		dmem_cgroup_unregister_region(selftest_region);
> +		selftest_region = NULL;
> +	}
> +	pr_info("unloaded.\n");
> +}
> +
> +module_init(dmem_selftest_init);
> +module_exit(dmem_selftest_exit);
> +
> +MODULE_AUTHOR("Albert Esteve <aesteve@redhat.com>");
> +MODULE_DESCRIPTION("Kselftest helper for cgroup dmem controller");
> +MODULE_LICENSE("GPL");
> +MODULE_INFO(test, "Y");
>
-- 
Thanks and Regards
R.Nageswara Sastry