Re: [RFC v2 7/9] drm/xe/vm: Prevent binding of purged buffer objects

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

On Mon, Dec 01, 2025 at 11:20:17AM +0530, Arvind Yadav wrote:
> Add validation in xe_vm_bind_ioctl_validate_bo() to reject MAP and
> PREFETCH operations on purged buffer objects with -EINVAL.
> 
> Problem:
> When a BO is purged (XE_MADV_PURGEABLE_PURGED state), its backing pages
> have been freed by the kernel. Without this check, VM_BIND operations
> would proceed:
> 
>  1. DRM_XE_VM_BIND_OP_MAP: Attempts to create GPU mappings to freed memory
>     - xe_vma_ops_alloc() creates VMA pointing to invalid BO
>     - Page tables populated with stale/invalid addresses
>     - GPU access leads to undefined behavior or hangs
> 
>  2. DRM_XE_VM_BIND_OP_PREFETCH: Attempts to migrate non-existent pages
>     - Triggers BO validation in TTM
>     - ttm_bo_validate() fails or crashes (no backing store)
>     - Wasted work for permanently invalid BO
> 
> With this check:
>   - MAP/PREFETCH immediately fail with -EINVAL at ioctl boundary
>   - Clear error message at syscall (better UX than deferred GPU hang)
>   - Prevents creation of invalid GPU page table entries
> 
> v2:
>   - Clarify that purged BOs are permanently invalid (i915 semantics)
>   - Remove incorrect claim about madvise(WILLNEED) restoring purged BOs
> 
> 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_vm.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
> index d03e69524369..cc946bff9607 100644
> --- a/drivers/gpu/drm/xe/xe_vm.c
> +++ b/drivers/gpu/drm/xe/xe_vm.c
> @@ -3482,6 +3482,13 @@ static int xe_vm_bind_ioctl_validate_bo(struct xe_device *xe, struct xe_bo *bo,
>  		return -EINVAL;
>  	}
>  
> +	/* Purged BOs are permanently invalid; reject new MAP/PREFETCH. */
> +	if (XE_IOCTL_DBG(xe,
> +			 xe_bo_is_purged(bo) &&
> +			 (op == DRM_XE_VM_BIND_OP_MAP ||
> +			  op == DRM_XE_VM_BIND_OP_PREFETCH)))
> +		return -EINVAL;

I'd move this check to later in the bind pipeline once we have BO's
dma-resv lock to make this race free. I think vma_lock_and_validate is
likely the correct function after drm_exec_lock_obj but before
xe_bo_validate. Or you could just make xe_bo_validate fail if the object
is purged.

Matt 

> +
>  	/*
>  	 * Some platforms require 64k VM_BIND alignment,
>  	 * specifically those with XE_VRAM_FLAGS_NEED64K.
> -- 
> 2.43.0
>