Re: [PATCH] drm/xe/vm: nuke PTs only after unlinking contested VMAs

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Fri, 2026-09-18 at 14:10 +0100, Matthew Auld wrote:
> In xe_vm_close_and_put(), external-BO VMAs are queued on the
> contested
> list for deferred destruction via xe_vma_destroy_unlocked(). However,
> xe_vm_pt_destroy() was previously invoked before processing contested
> VMAs, destroying vm->pt_root while those VMAs were still linked to
> their
> respective buffer objects (vm_bo->list.gpuva).
> 
> If a concurrent thread evicts one of those shared buffer objects,
> xe_bo_trigger_rebind() holding only bo->resv walks the BO's VMAs and,
> in
> fault mode, calls xe_vm_invalidate_vma() -> xe_pt_zap_ptes(). Because
> vm->pt_root[tile->id] is already NULL, dereferencing pt->level causes
> a
> NULL ptr deref.
> 
> Fix this by deferring xe_vm_free_scratch() and xe_vm_pt_destroy()
> until
> after all contested VMAs have been unlinked and destroyed.
> 
> User is reporting hitting a NULL ptr deref in xe_pt_zap_ptes(), which
> could be explained by this race.
> 
> Assisted-by: LLM
> Fixes: b06d47be7c83 ("drm/xe: Port Xe to GPUVA")
> Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/9290
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: <stable@vger.kernel.org> # v6.12+

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>


> ---
>  drivers/gpu/drm/xe/xe_vm.c | 21 +++++++++------------
>  1 file changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
> index 17dc4debe7c1..390da884c727 100644
> --- a/drivers/gpu/drm/xe/xe_vm.c
> +++ b/drivers/gpu/drm/xe/xe_vm.c
> @@ -1979,21 +1979,13 @@ void xe_vm_close_and_put(struct xe_vm *vm)
>  		vma->gpuva.flags |= XE_VMA_DESTROYED;
>  	}
>  
> -	/*
> -	 * All vm operations will add shared fences to resv.
> -	 * The only exception is eviction for a shared object,
> -	 * but even so, the unbind when evicted would still
> -	 * install a fence to resv. Hence it's safe to
> -	 * destroy the pagetables immediately.
> -	 */
> -	xe_vm_free_scratch(vm);
> -	xe_vm_pt_destroy(vm);
>  	xe_vm_unlock(vm);
>  
>  	/*
> -	 * VM is now dead, cannot re-add nodes to vm->vmas if it's
> NULL
> -	 * Since we hold a refcount to the bo, we can remove and
> free
> -	 * the members safely without locking.
> +	 * Unlink and destroy all contested external-BO VMAs before
> destroying
> +	 * the page tables. Otherwise, concurrent eviction holding
> only bo->resv
> +	 * can walk the BO's VMAs and attempt to invalidate/zap page
> tables that
> +	 * have already been freed.
>  	 */
>  	list_for_each_entry_safe(vma, next_vma, &contested,
>  				 combined_links.destroy) {
> @@ -2001,6 +1993,11 @@ void xe_vm_close_and_put(struct xe_vm *vm)
>  		xe_vma_destroy_unlocked(vma);
>  	}
>  
> +	xe_vm_lock(vm, false);
> +	xe_vm_free_scratch(vm);
> +	xe_vm_pt_destroy(vm);
> +	xe_vm_unlock(vm);
> +
>  	xe_svm_fini(vm);
>  
>  	up_write(&vm->lock);