Re: [PATCH v6 08/12] drm/xe/bo: Block mmap of DONTNEED/purged BOs

Thomas Hellström
2026-03-10T10:17:22Z

On Tue, 2026-03-03 at 20:50 +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)
>
> 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 | 28 +++++++++++++++++++++++++++-
>  1 file changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index d05a73756905..3a4965bdadf2 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -3396,6 +3396,8 @@ int xe_gem_mmap_offset_ioctl(struct drm_device

This needs to be done in the mmap() callback. It should be OK to wrap
the existing callback there.

> *dev, void *data,
>   struct xe_device *xe = to_xe_device(dev);
>   struct drm_xe_gem_mmap_offset *args = data;
>   struct drm_gem_object *gem_obj;
> + struct xe_bo *bo;
> + int err;
>
>   if (XE_IOCTL_DBG(xe, args->extensions) ||
>       XE_IOCTL_DBG(xe, args->reserved[0] || args-
> >reserved[1]))
> @@ -3425,11 +3427,35 @@ int xe_gem_mmap_offset_ioctl(struct
> drm_device *dev, void *data,
>   if (XE_IOCTL_DBG(xe, !gem_obj))
>   return -ENOENT;
>
> + bo = gem_to_xe_bo(gem_obj);
> +
> + /*
> + * Reject new mmap to 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, false);

sleeping locks need to be interruptible whenever possible.

Thanks,
Thomas

> + if (xe_bo_madv_is_dontneed(bo)) {
> + err = -EBUSY;
> + goto out_unlock;
> + }
> +
> + if (xe_bo_is_purged(bo)) {
> + err = -EINVAL;
> + goto out_unlock;
> + }
> + xe_bo_unlock(bo);
> +
>   /* The mmap offset was set up at BO allocation time. */
>   args->offset = drm_vma_node_offset_addr(&gem_obj->vma_node);
> + xe_bo_put(bo);
>
> - xe_bo_put(gem_to_xe_bo(gem_obj));
>   return 0;
> +
> +out_unlock:
> + xe_bo_unlock(bo);
> + xe_bo_put(bo);
> + return err;
>  }
>
>  /**