drm/amdgpu: Remap all page faults to per process dummy page.

On device removal reroute all CPU mappings to dummy page
per drm_file instance or imported GEM object.

v4:
Update for modified ttm_bo_vm_dummy_page
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210512142648.666476-7-andrey.grodzovsky@amd.com

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c

@@ -48,6 +48,7 @@
 #include <drm/ttm/ttm_placement.h>
 
 #include <drm/amdgpu_drm.h>
+#include <drm/drm_drv.h>
 
 #include "amdgpu.h"
 #include "amdgpu_object.h"
@@ -1905,18 +1906,28 @@ void amdgpu_ttm_set_buffer_funcs_status(struct amdgpu_device *adev, bool enable)
 static vm_fault_t amdgpu_ttm_fault(struct vm_fault *vmf)
 {
 	struct ttm_buffer_object *bo = vmf->vma->vm_private_data;
+	struct drm_device *ddev = bo->base.dev;
 	vm_fault_t ret;
+	int idx;
 
 	ret = ttm_bo_vm_reserve(bo, vmf);
 	if (ret)
 		return ret;
 
+	if (drm_dev_enter(ddev, &idx)) {
 		ret = amdgpu_bo_fault_reserve_notify(bo);
-	if (ret)
+		if (ret) {
+			drm_dev_exit(idx);
 			goto unlock;
+		}
 
 		 ret = ttm_bo_vm_fault_reserved(vmf, vmf->vma->vm_page_prot,
 						TTM_BO_VM_NUM_PREFAULT, 1);
+
+		 drm_dev_exit(idx);
+	} else {
+		ret = ttm_bo_vm_dummy_page(vmf, vmf->vma->vm_page_prot);
+	}
 	if (ret == VM_FAULT_RETRY && !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT))
 		return ret;