Re: [PATCH v7 08/12] drm/xe/bo: Block mmap of DONTNEED/purged BOs

[Matthew Brost <matthew.brost@intel.com>]

On Mon, Mar 23, 2026 at 03:00:57PM +0530, Arvind Yadav wrote:
> Don't allow new CPU mmaps to BOs marked DONTNEED or PURGED.
> DONTNEED BOs can have their contents discarded at any time, making
> CPU access undefined behavior. PURGED BOs have no backing store and
> are permanently invalid.
> 
> Return -EBUSY for DONTNEED BOs (temporary purgeable state) and
> -EINVAL for purged BOs (permanent, no backing store).
> 
> The mmap offset ioctl now checks the BO's purgeable state before
> allowing userspace to establish a new CPU mapping. This prevents
> the race where userspace gets a valid offset but the BO is purged
> before actual faulting begins.
> 
> Existing mmaps (established before DONTNEED) may still work until
> pages are purged, at which point CPU faults fail with SIGBUS.
> 
> v6:
> - Split DONTNEED → -EBUSY and PURGED → -EINVAL for consistency
>   with the rest of the series (Thomas, Matt)
> 
> v7:
>   - Move purgeable check from xe_gem_mmap_offset_ioctl() into a new
>     xe_gem_object_mmap() callback that wraps drm_gem_ttm_mmap(). (Thomas)
>   - Use an interruptible lock. (Thomas)
> 
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
> Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_bo.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index da18b43650e3..83a1d1ca6cc6 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -2165,10 +2165,32 @@ static const struct vm_operations_struct xe_gem_vm_ops = {
>  	.access = xe_bo_vm_access,
>  };
>  
> +static int xe_gem_object_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
> +{
> +	struct xe_bo *bo = gem_to_xe_bo(obj);
> +	int err = 0;
> +
> +	/*
> +	 * Reject mmap of purgeable BOs. DONTNEED BOs can be purged
> +	 * at any time, making CPU access undefined behavior. Purged BOs have
> +	 * no backing store and are permanently invalid.
> +	 */
> +	xe_bo_lock(bo, true);

You need to check the return of xe_bo_lock if the 2nd argument is true
as it can fail. On failure, kick the error code to the caller.

> +	if (xe_bo_madv_is_dontneed(bo))
> +		err = -EBUSY;
> +	else if (xe_bo_is_purged(bo))
> +		err = -EINVAL;
> +	xe_bo_unlock(bo);
> +	if (err)
> +		return err;
> +
> +	return drm_gem_ttm_mmap(obj, vma);
> +}
> +
>  static const struct drm_gem_object_funcs xe_gem_object_funcs = {
>  	.free = xe_gem_object_free,
>  	.close = xe_gem_object_close,
> -	.mmap = drm_gem_ttm_mmap,
> +	.mmap = xe_gem_object_mmap,
>  	.export = xe_gem_prime_export,
>  	.vm_ops = &xe_gem_vm_ops,
>  };
> @@ -3427,8 +3449,8 @@ int xe_gem_mmap_offset_ioctl(struct drm_device *dev, void *data,
>  
>  	/* The mmap offset was set up at BO allocation time. */
>  	args->offset = drm_vma_node_offset_addr(&gem_obj->vma_node);
> +	drm_gem_object_put(gem_obj);
>  
> -	xe_bo_put(gem_to_xe_bo(gem_obj));

Look unrelated.

Matt

>  	return 0;
>  }
>  
> -- 
> 2.43.0
>