Re: [PATCH v5 4/6] drm/syncobj: Avoid temporary allocation in drm_syncobj_timeline_signal_ioctl

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

On 6/11/25 16:00, Tvrtko Ursulin wrote:
> We can avoid one of the two temporary allocations if we read the userspace
> supplied timeline points as we go along.

The problem with that is that calling copy_from_user multiple times is really inefficient.

So that improves performance with few entries and decreases performance with many entries.

Not sure if having many entries is really a valid use case here.

Regards,
Christian.

> 
> The only new complication is to unwind unused fence chains on the error
> path, but even that code was already present in the function.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> Reviewed-by: Maíra Canal <mcanal@igalia.com> #v1
> ---
> v2:
>  * Change back to copy_from_user due 32-bit ARM not implementing 64-bit
>    get_user.
> 
> v3:
> * Fix argument order mixup.
> ---
>  drivers/gpu/drm/drm_syncobj.c | 43 ++++++++++++++---------------------
>  1 file changed, 17 insertions(+), 26 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> index 497009fc66f8..9968d9429d90 100644
> --- a/drivers/gpu/drm/drm_syncobj.c
> +++ b/drivers/gpu/drm/drm_syncobj.c
> @@ -1579,10 +1579,10 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, void *data,
>  				  struct drm_file *file_private)
>  {
>  	struct drm_syncobj_timeline_array *args = data;
> +	uint64_t __user *points = u64_to_user_ptr(args->points);
> +	uint32_t i, j, count = args->count_handles;
>  	struct drm_syncobj **syncobjs;
>  	struct dma_fence_chain **chains;
> -	uint64_t *points;
> -	uint32_t i, j;
>  	int ret;
>  
>  	if (!drm_core_check_feature(dev, DRIVER_SYNCOBJ_TIMELINE))
> @@ -1596,31 +1596,17 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, void *data,
>  
>  	ret = drm_syncobj_array_find(file_private,
>  				     u64_to_user_ptr(args->handles),
> -				     args->count_handles,
> +				     count,
>  				     &syncobjs);
>  	if (ret < 0)
>  		return ret;
>  
> -	points = kmalloc_array(args->count_handles, sizeof(*points),
> -			       GFP_KERNEL);
> -	if (!points) {
> -		ret = -ENOMEM;
> -		goto out;
> -	}
> -	if (!u64_to_user_ptr(args->points)) {
> -		memset(points, 0, args->count_handles * sizeof(uint64_t));
> -	} else if (copy_from_user(points, u64_to_user_ptr(args->points),
> -				  sizeof(uint64_t) * args->count_handles)) {
> -		ret = -EFAULT;
> -		goto err_points;
> -	}
> -
> -	chains = kmalloc_array(args->count_handles, sizeof(void *), GFP_KERNEL);
> +	chains = kmalloc_array(count, sizeof(void *), GFP_KERNEL);
>  	if (!chains) {
>  		ret = -ENOMEM;
> -		goto err_points;
> +		goto out;
>  	}
> -	for (i = 0; i < args->count_handles; i++) {
> +	for (i = 0; i < count; i++) {
>  		chains[i] = dma_fence_chain_alloc();
>  		if (!chains[i]) {
>  			for (j = 0; j < i; j++)
> @@ -1630,19 +1616,24 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, void *data,
>  		}
>  	}
>  
> -	for (i = 0; i < args->count_handles; i++) {
> +	for (i = 0; i < count; i++) {
>  		struct dma_fence *fence = dma_fence_get_stub();
> +		u64 point = 0;
>  
> -		drm_syncobj_add_point(syncobjs[i], chains[i],
> -				      fence, points[i]);
> +		if (points && copy_from_user(&point, points++, sizeof(point))) {
> +			ret =  -EFAULT;
> +			for (j = i; j < count; j++)
> +				dma_fence_chain_free(chains[j]);
> +			goto err_chains;
> +		}
> +
> +		drm_syncobj_add_point(syncobjs[i], chains[i], fence, point);
>  		dma_fence_put(fence);
>  	}
>  err_chains:
>  	kfree(chains);
> -err_points:
> -	kfree(points);
>  out:
> -	drm_syncobj_array_free(syncobjs, args->count_handles);
> +	drm_syncobj_array_free(syncobjs, count);
>  
>  	return ret;
>  }