Re: [PATCH v2 2/8] drm/ttm: Remap all page faults to per process dummy page.

[Daniel Vetter <daniel@ffwll.ch>]

On Sun, Jun 21, 2020 at 02:03:02AM -0400, Andrey Grodzovsky wrote:
> On device removal reroute all CPU mappings to dummy page per drm_file
> instance or imported GEM object.
> 
> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
> ---
>  drivers/gpu/drm/ttm/ttm_bo_vm.c | 65 ++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 57 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> index 389128b..2f8bf5e 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -35,6 +35,8 @@
>  #include <drm/ttm/ttm_bo_driver.h>
>  #include <drm/ttm/ttm_placement.h>
>  #include <drm/drm_vma_manager.h>
> +#include <drm/drm_drv.h>
> +#include <drm/drm_file.h>
>  #include <linux/mm.h>
>  #include <linux/pfn_t.h>
>  #include <linux/rbtree.h>
> @@ -328,19 +330,66 @@ vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf)

Hm I think diff and code flow look a bit bad now. What about renaming the
current function to __ttm_bo_vm_fault and then having something like the
below:

ttm_bo_vm_fault(args) {

	if (drm_dev_enter()) {
		__ttm_bo_vm_fault(args);
		drm_dev_exit();
	} else  {
		drm_gem_insert_dummy_pfn();
	}
}

I think drm_gem_insert_dummy_pfn(); should be portable across drivers, so
another nice point to try to unifiy drivers as much as possible.
-Daniel

>  	pgprot_t prot;
>  	struct ttm_buffer_object *bo = vma->vm_private_data;
>  	vm_fault_t ret;
> +	int idx;
> +	struct drm_device *ddev = bo->base.dev;
>  
> -	ret = ttm_bo_vm_reserve(bo, vmf);
> -	if (ret)
> -		return ret;
> +	if (drm_dev_enter(ddev, &idx)) {
> +		ret = ttm_bo_vm_reserve(bo, vmf);
> +		if (ret)
> +			goto exit;
> +
> +		prot = vma->vm_page_prot;
>  
> -	prot = vma->vm_page_prot;
> -	ret = ttm_bo_vm_fault_reserved(vmf, prot, TTM_BO_VM_NUM_PREFAULT);
> -	if (ret == VM_FAULT_RETRY && !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT))
> +		ret = ttm_bo_vm_fault_reserved(vmf, prot, TTM_BO_VM_NUM_PREFAULT);
> +		if (ret == VM_FAULT_RETRY && !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT))
> +			goto exit;
> +
> +		dma_resv_unlock(bo->base.resv);
> +
> +exit:
> +		drm_dev_exit(idx);
>  		return ret;
> +	} else {
>  
> -	dma_resv_unlock(bo->base.resv);
> +		struct drm_file *file = NULL;
> +		struct page *dummy_page = NULL;
> +		int handle;
>  
> -	return ret;
> +		/* We are faulting on imported BO from dma_buf */
> +		if (bo->base.dma_buf && bo->base.import_attach) {
> +			dummy_page = bo->base.dummy_page;
> +		/* We are faulting on non imported BO, find drm_file owning the BO*/

Uh, we can't fish that out of the vma->vm_file pointer somehow? Or is that
one all wrong? Doing this kind of list walk looks pretty horrible.

If the vma doesn't have the right pointer I guess next option is that we
store the drm_file page in gem_bo->dummy_page, and replace it on first
export. But that's going to be tricky to track ...

> +		} else {
> +			struct drm_gem_object *gobj;
> +
> +			mutex_lock(&ddev->filelist_mutex);
> +			list_for_each_entry(file, &ddev->filelist, lhead) {
> +				spin_lock(&file->table_lock);
> +				idr_for_each_entry(&file->object_idr, gobj, handle) {
> +					if (gobj == &bo->base) {
> +						dummy_page = file->dummy_page;
> +						break;
> +					}
> +				}
> +				spin_unlock(&file->table_lock);
> +			}
> +			mutex_unlock(&ddev->filelist_mutex);
> +		}
> +
> +		if (dummy_page) {
> +			/*
> +			 * Let do_fault complete the PTE install e.t.c using vmf->page
> +			 *
> +			 * TODO - should i call free_page somewhere ?

Nah, instead don't call get_page. The page will be around as long as
there's a reference for the drm_file or gem_bo, which is longer than any
mmap. Otherwise yes this would like really badly.

> +			 */
> +			get_page(dummy_page);
> +			vmf->page = dummy_page;
> +			return 0;
> +		} else {
> +			return VM_FAULT_SIGSEGV;

Hm that would be a kernel bug, wouldn't it? WARN_ON() required here imo.
-Daniel

> +		}
> +	}
>  }
>  EXPORT_SYMBOL(ttm_bo_vm_fault);
>  
> -- 
> 2.7.4
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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