FAILED: patch "[PATCH] drm/xe/vm: nuke PTs only after unlinking contested VMAs" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 24a22fb3c731474b68e986af6804db450fb88617
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026092919-qualm-steadily-7858@gregkh' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 24a22fb3c731474b68e986af6804db450fb88617 Mon Sep 17 00:00:00 2001
From: Matthew Auld <[email protected]>
Date: Fri, 18 Sep 2026 14:10:35 +0100
Subject: [PATCH] drm/xe/vm: nuke PTs only after unlinking contested VMAs
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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.

index 23952ad8951e..ef20e205a734 100644
@@ -1947,21 +1947,13 @@ 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) {

@@ -1969,6 +1961,11 @@ 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);

From: Matthew Auld <[email protected]>

[ Upstream commit 24a22fb3c731474b68e986af6804db450fb88617 ]

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.

---
 drivers/gpu/drm/xe/xe_vm.c | 29 +++++++++++++----------------
 1 file changed, 13 insertions(+), 16 deletions(-)

index deb4d0314db3a..fa0d7c6414022 100644
@@ -1678,13 +1678,21 @@ void xe_vm_close_and_put(struct xe_vm *vm)
 		vma->gpuva.flags |= XE_VMA_DESTROYED;
 	}
 
+	xe_vm_unlock(vm);
+
 	/*
-	 * 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.
+	 * 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) {
+		list_del_init(&vma->combined_links.destroy);
+		xe_vma_destroy_unlocked(vma);
+	}
+
+	xe_vm_lock(vm, false);
 	xe_vm_free_scratch(vm);
 
 	for_each_tile(tile, xe, id) {

@@ -1695,17 +1703,6 @@ void xe_vm_close_and_put(struct xe_vm *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.
-	 */
-	list_for_each_entry_safe(vma, next_vma, &contested,
-				 combined_links.destroy) {
-		list_del_init(&vma->combined_links.destroy);
-		xe_vma_destroy_unlocked(vma);
-	}
-
 	up_write(&vm->lock);
 
 	down_write(&xe->usm.lock);

2.53.0