[PATCH 1/1] drm/amdgpu: workaround failed COW checks for Thunk VMAs

[Christian König]

Am 15.07.21 um 21:05 schrieb Felix Kuehling:
> KFD Thunk maps invisible VRAM BOs with PROT_NONE, MAP_PRIVATE.
> is_cow_mapping returns true for these mappings, which causes mmap to fail
> in ttm_bo_mmap_obj.
>
> As a workaround, clear VM_MAYWRITE for PROT_NONE-COW mappings. This
> should prevent the mapping from ever becoming writable and makes
> is_cow_mapping(vm_flags) false.
>
> Fixes: f91142c62161 ("drm/ttm: nuke VM_MIXEDMAP on BO mappings v3")
> Suggested-by: Daniel Vetter <daniel.vetter at intel.com>
> Tested-by: Felix Kuehling <Felix.Kuehling at amd.com>
> Signed-off-by: Felix Kuehling <Felix.Kuehling at amd.com>

I've reviewed and pushed the patch to drm-misc-fixes.

Regards,
Christian.

> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> index b3404c43a911..9f952b7fc197 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> @@ -255,6 +255,15 @@ static int amdgpu_gem_object_mmap(struct drm_gem_object *obj, struct vm_area_str
>   	if (bo->flags & AMDGPU_GEM_CREATE_NO_CPU_ACCESS)
>   		return -EPERM;
>   
> +	/* Workaround for Thunk bug creating PROT_NONE,MAP_PRIVATE mappings
> +	 * for debugger access to invisible VRAM. Should have used MAP_SHARED
> +	 * instead. Clearing VM_MAYWRITE prevents the mapping from ever
> +	 * becoming writable and makes is_cow_mapping(vm_flags) false.
> +	 */
> +	if (is_cow_mapping(vma->vm_flags) &&
> +	    !(vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC)))
> +		vma->vm_flags &= ~VM_MAYWRITE;
> +
>   	return drm_gem_ttm_mmap(obj, vma);
>   }
>