Re: [PATCH v5 1/4] cgroup: Add dmem_selftest module

[Michal Koutný]

On Mon, Jul 06, 2026 at 02:06:40PM +0200, Albert Esteve <aesteve@xxxxxxxxxx> wrote:
> Currently, dmem charging is driver-driven through direct
> calls to dmem_cgroup_try_charge(), so cgroup selftests
> do not have a generic way to trigger charge and uncharge
> paths from userspace.
> 
> This limits any selftest coverage to configuration/readout
> checks unless a specific driver exposing charge hooks is
> present in the test environment.

What about that
drivers/gpu/drm/ttm/tests/ttm_resource_test.c
?

> 
> Add kernel/cgroup/dmem_selftest.c as a helper module
> (CONFIG_DMEM_SELFTEST) that registers a synthetic dmem region
> (dmem_selftest) and exposes debugfs control files:
> /sys/kernel/debug/dmem_selftest/charge
> /sys/kernel/debug/dmem_selftest/uncharge

Nit: I'd think about alloc and free (to denote meaning not how it's
implemented). 

> 
> Writing a size to charge triggers dmem_cgroup_try_charge() for
> the calling task's cgroup (the module calls kstrtou64()).
> Writing to uncharge 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@xxxxxxxxxx>
> ---
>  init/Kconfig                  |  12 +++
>  kernel/cgroup/Makefile        |   1 +
>  kernel/cgroup/dmem_selftest.c | 198 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 211 insertions(+)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index 5230d4879b1c8..6fab65d6b44ea 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1267,6 +1267,18 @@ config CGROUP_DMEM
>  	  As an example, it allows you to restrict VRAM usage for applications
>  	  in the DRM subsystem.
>  
> +config DMEM_SELFTEST
> +	tristate "dmem cgroup selftest helper module"
> +	depends on CGROUP_DMEM && DEBUG_FS
> +	default n
> +	help
> +	  Builds a small loadable module that registers a dmem region named
> +	  "dmem_selftest" and exposes debugfs files under
> +	  /sys/kernel/debug/dmem_selftest/ so kselftests can trigger
> +	  dmem charge/uncharge operations from userspace.
> +
> +	  Say N unless you run dmem selftests or develop the dmem controller.

Could this be made w/out introduciton of a new Kconfig option? (E.g.
what tools/testing/selftests/livepatch/test_modules/Makefile does)

(Maybe reword to insmod if you choose building the test module as OOT.)

> +static int __init dmem_selftest_register(void)

Just an idea to reduce the amount debugfs of boilerplage -- the testing
args could be handled by module_param_cb().

> +static int __init dmem_selftest_init(void)
...
> +	add_taint(TAINT_TEST, LOCKDEP_STILL_OK);

Great!

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