Re: [PATCH v9 2/3] drm: Add drm_user_fence helper

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Build failure on NOMMU architectures due to unconditional use of mmput_async()
--

commit dc8e122093c744c8bd5d72581571fdb0ccb6856c
Author: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>

drm: Add drm_user_fence helper

Introduce a common DRM user fence helper providing the kref-managed,
MM-borrowing dma-fence-callback-to-workqueue pattern used by drivers
that must access userspace memory from a kthread context when a GPU
fence signals.

> diff --git a/drivers/gpu/drm/drm_user_fence.c b/drivers/gpu/drm/drm_user_fence.c
> new file mode 100644
> index 0000000000000..f7882208a8b73
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_user_fence.c
[ ... ]
> +static void drm_user_fence_do_work(struct drm_work_fence *wfence)
> +{
> +	struct drm_user_fence *ufence =
> +		container_of(wfence, struct drm_user_fence, base);
> +	struct mm_struct *mm = NULL;
> +
> +	if (mmget_not_zero(ufence->mm)) {
> +		mm = ufence->mm;
> +		kthread_use_mm(mm);
> +	}
> +
> +	ufence->ops->writeback(ufence, !!mm);
> +
> +	if (mm) {
> +		kthread_unuse_mm(mm);
> +		mmput_async(mm);	/* requires CONFIG_MMU — GPU requires MMU */

[Severity: Low]
Does this cause a build failure on architectures where CONFIG_MMU is disabled?

Although the comment notes that GPUs require an MMU, the DRM core and this
file are compiled unconditionally as part of CONFIG_DRM. Calling mmput_async()
here without an #ifdef CONFIG_MMU guard or similar protection could result in 
an implicit function declaration or linker error on NOMMU builds.

> +	}
> +}
> +

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260909044454.399340-1-srinivasan.shanmugam@amd.com?part=2