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

Matthew Brost
2026-03-26T01:34:04Z

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
>