Re: [RFC PATCH 4/5] drm/ttm: Document and optimize ttm_bo_pipeline_gutting()

["Christian König" <christian.koenig@amd.com>]

Am 20.05.21 um 17:09 schrieb Thomas Hellström:
> If the bo is idle when calling ttm_bo_pipeline_gutting(), we unnecessarily
> create a ghost object and push it out to delayed destroy.
> Fix this by adding a path for idle, and document the function.
>
> Also avoid having the bo end up in a bad state vulnerable to user-space
> triggered kernel BUGs if the call to ttm_tt_create() fails.
>
> Finally reuse ttm_bo_pipeline_gutting() in ttm_bo_evict().
>
> Cc: Christian König <christian.koenig@amd.com>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>   drivers/gpu/drm/ttm/ttm_bo.c      | 20 +++++-----
>   drivers/gpu/drm/ttm/ttm_bo_util.c | 63 ++++++++++++++++++++++++-------
>   drivers/gpu/drm/ttm/ttm_tt.c      |  5 +++
>   include/drm/ttm/ttm_tt.h          | 10 +++++
>   4 files changed, 75 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
> index ca1b098b6a56..a8fa3375b8aa 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -501,10 +501,15 @@ static int ttm_bo_evict(struct ttm_buffer_object *bo,
>   	bdev->funcs->evict_flags(bo, &placement);
>   
>   	if (!placement.num_placement && !placement.num_busy_placement) {
> -		ttm_bo_wait(bo, false, false);
> +		ret = ttm_bo_wait(bo, true, false);
> +		if (ret)
> +			return ret;
>   
> -		ttm_bo_cleanup_memtype_use(bo);
> -		return ttm_tt_create(bo, false);
> +		/*
> +		 * Since we've already synced, this frees backing store
> +		 * immediately.
> +		 */
> +		return ttm_bo_pipeline_gutting(bo);

Yeah, we tried to avoid pipeline_gutting here because of eviction. But I 
think when you wait before that should work.

>   	}
>   
>   	ret = ttm_bo_mem_space(bo, &placement, &evict_mem, ctx);
> @@ -974,13 +979,8 @@ int ttm_bo_validate(struct ttm_buffer_object *bo,
>   	/*
>   	 * Remove the backing store if no placement is given.
>   	 */
> -	if (!placement->num_placement && !placement->num_busy_placement) {
> -		ret = ttm_bo_pipeline_gutting(bo);
> -		if (ret)
> -			return ret;
> -
> -		return ttm_tt_create(bo, false);
> -	}
> +	if (!placement->num_placement && !placement->num_busy_placement)
> +		return ttm_bo_pipeline_gutting(bo);
>   
>   	/*
>   	 * Check whether we need to move buffer.
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
> index 919ee03f7eb3..1860e2e7563f 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_util.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
> @@ -479,7 +479,8 @@ static void ttm_transfered_destroy(struct ttm_buffer_object *bo)
>    */
>   
>   static int ttm_buffer_object_transfer(struct ttm_buffer_object *bo,
> -				      struct ttm_buffer_object **new_obj)
> +				      struct ttm_buffer_object **new_obj,
> +				      bool realloc_tt)
>   {
>   	struct ttm_transfer_obj *fbo;
>   	int ret;
> @@ -493,6 +494,17 @@ static int ttm_buffer_object_transfer(struct ttm_buffer_object *bo,
>   	ttm_bo_get(bo);
>   	fbo->bo = bo;
>   
> +	if (realloc_tt) {
> +		bo->ttm = NULL;
> +		ret = ttm_tt_create(bo, true);
> +		if (ret) {
> +			bo->ttm = fbo->base.ttm;
> +			kfree(fbo);
> +			ttm_bo_put(bo);
> +			return ret;
> +		}
> +	}
> +

Can't we keep that logic in the caller? I think that would be cleaner.

>   	/**
>   	 * Fix up members that we shouldn't copy directly:
>   	 * TODO: Explicit member copy would probably be better here.
> @@ -763,7 +775,7 @@ static int ttm_bo_move_to_ghost(struct ttm_buffer_object *bo,
>   	dma_fence_put(bo->moving);
>   	bo->moving = dma_fence_get(fence);
>   
> -	ret = ttm_buffer_object_transfer(bo, &ghost_obj);
> +	ret = ttm_buffer_object_transfer(bo, &ghost_obj, false);
>   	if (ret)
>   		return ret;
>   
> @@ -836,26 +848,51 @@ int ttm_bo_move_accel_cleanup(struct ttm_buffer_object *bo,
>   }
>   EXPORT_SYMBOL(ttm_bo_move_accel_cleanup);
>   
> +/**
> + * ttm_bo_pipeline_gutting - purge the contents of a bo
> + * @bo: The buffer object
> + *
> + * Purge the contents of a bo, async if the bo is not idle.
> + * After a successful call, the bo is left unpopulated in
> + * system placement. The function may wait uninterruptible
> + * for idle on OOM.
> + *
> + * Return: 0 if successful, negative error code on failure.
> + */
>   int ttm_bo_pipeline_gutting(struct ttm_buffer_object *bo)
>   {
>   	static const struct ttm_place sys_mem = { .mem_type = TTM_PL_SYSTEM };
>   	struct ttm_buffer_object *ghost;
>   	int ret;
>   
> -	ret = ttm_buffer_object_transfer(bo, &ghost);
> -	if (ret)
> -		return ret;
> +	/* If already idle, no need for ghost object dance. */
> +	ret = ttm_bo_wait(bo, false, true);
> +	if (ret == -EBUSY) {
> +		ret = ttm_buffer_object_transfer(bo, &ghost, true);
> +		if (ret)
> +			return ret;

When this is a shortcout to avoid work we should rather use the inverse 
notation.

In other words something like that:

if (ret != -EBUSY) {
     ttm_resource_free(bo, &bo->mem);
     ttm_resource_alloc(bo, &sys_mem, &bo->mem);
     ttm_tt_create()...
     return ret;
}

>   
> -	ret = dma_resv_copy_fences(&ghost->base._resv, bo->base.resv);
> -	/* Last resort, wait for the BO to be idle when we are OOM */
> -	if (ret)
> -		ttm_bo_wait(bo, false, false);
> +		ret = dma_resv_copy_fences(&ghost->base._resv, bo->base.resv);
> +		/* Last resort, wait for the BO to be idle when we are OOM */
> +		if (ret)
> +			ttm_bo_wait(bo, false, false);
>   
> -	ttm_resource_alloc(bo, &sys_mem, &bo->mem);
> -	bo->ttm = NULL;
> +		dma_resv_unlock(&ghost->base._resv);
> +		ttm_bo_put(ghost);
> +	} else {
> +		if (!bo->ttm) {
> +			ret = ttm_tt_create(bo, true);
> +			if (ret)
> +				return ret;
> +		} else {
> +			ttm_tt_unpopulate(bo->bdev, bo->ttm);
> +			if (bo->type == ttm_bo_type_device)
> +				ttm_tt_mark_for_clear(bo->ttm);

That's not legal, you can't unpopulate it when the BO is busy.

Instead the TT object must be destroyed with the ghost and a new one 
created.

Christian.

> +		}
> +		ttm_resource_free(bo, &bo->mem);
> +	}
>   
> -	dma_resv_unlock(&ghost->base._resv);
> -	ttm_bo_put(ghost);
> +	ttm_resource_alloc(bo, &sys_mem, &bo->mem);
>   
>   	return 0;
>   }
> diff --git a/drivers/gpu/drm/ttm/ttm_tt.c b/drivers/gpu/drm/ttm/ttm_tt.c
> index 539e0232cb3b..0b1053e93db2 100644
> --- a/drivers/gpu/drm/ttm/ttm_tt.c
> +++ b/drivers/gpu/drm/ttm/ttm_tt.c
> @@ -134,6 +134,11 @@ void ttm_tt_destroy_common(struct ttm_device *bdev, struct ttm_tt *ttm)
>   }
>   EXPORT_SYMBOL(ttm_tt_destroy_common);
>   
> +void ttm_tt_mark_for_clear(struct ttm_tt *ttm)
> +{
> +	ttm->page_flags |= TTM_PAGE_FLAG_ZERO_ALLOC;
> +}
> +
>   void ttm_tt_destroy(struct ttm_device *bdev, struct ttm_tt *ttm)
>   {
>   	bdev->funcs->ttm_tt_destroy(bdev, ttm);
> diff --git a/include/drm/ttm/ttm_tt.h b/include/drm/ttm/ttm_tt.h
> index 134d09ef7766..91552c83ac79 100644
> --- a/include/drm/ttm/ttm_tt.h
> +++ b/include/drm/ttm/ttm_tt.h
> @@ -157,6 +157,16 @@ int ttm_tt_populate(struct ttm_device *bdev, struct ttm_tt *ttm, struct ttm_oper
>    */
>   void ttm_tt_unpopulate(struct ttm_device *bdev, struct ttm_tt *ttm);
>   
> +/**
> + * ttm_tt_mark_for_clear - Mark pages for clearing on populate.
> + *
> + * @ttm: Pointer to the ttm_tt structure
> + *
> + * Marks pages for clearing so that the next time the page vector is
> + * populated, the pages will be cleared.
> + */
> +void ttm_tt_mark_for_clear(struct ttm_tt *ttm);
> +
>   void ttm_tt_mgr_init(unsigned long num_pages, unsigned long num_dma32_pages);
>   
>   #if IS_ENABLED(CONFIG_AGP)