Re: [PATCH v7 1/2] drm/i915/selftests: Prevent userspace mapping invalidation

[Andi Shyti <andi.shyti@linux.intel.com>]

Hi Krzysztof,

On Tue, Apr 21, 2026 at 06:17:15AM +0000, Krzysztof Karas wrote:
> Migration testing in i915 assumes current task's address space
> to allocate new userspace mapping and uses it without
> registering real user for that address space in mm_struct.
> On single NUMA node setups PCI probe executes in the same
> context as userspace process calling the test (i915_selftest
> from IGT), but when multiple nodes are available, the PCI code
> puts probe into a kernel workqueue. This switches execution to
> a kworker, which does not have its own address space in
> userspace and must borrow such memory from another process, so
> "current->active_mm" is unknown at the start of the test.
> 
> It was observed that mm->mm_users would occasionally be 0
> or drop to 0 during the test due to short delay between
> scheduling and executing work in forked process, which reaped
> userspace mappings, further leading to failures upon reading
> from userland memory.
> 
> Prevent this by adding a PID parameter to a trusted task, so its
> mm struct may be used if needed.
> 
> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/14204
> Fixes: 34b1c1c71d37 ("i915/selftest/igt_mmap: let mmap tests run in kthread")
> Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>

Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>

...

> +	/**
> +	 * If we are running in a kthread on a multi NUMA system and the user passed
> +	 * a valid PID of a userspace task, then we may borrow its address space
> +	 * to prepare a safe environment for the mmap selftests.
> +	 */

Sebastian is right here, the "**" is more for the documentation,
it can be removed before pushing.

Andi

> +	if (!current->mm && u_pid_nr) {
> +		mm = get_mm(u_pid_nr);
> +		if (mm) {

...