Re: [PATCH 1/2] drm/i915: Document our internal limit on object size

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 14/10/2016 16:18, Chris Wilson wrote:
> In many places, we try to count pages using a 32 bit integer. That
> implies if we are asked to create an object larger than 43bits, we will
> subtly crash much later. Catch this on the boundary, and add a warning
> to remind ourselves later on our exabyte systems.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_drv.h |  2 +-
>   drivers/gpu/drm/i915/i915_gem.c | 14 ++++++++++++--
>   2 files changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index fe875b27a6bf..43eb1a72f19e 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -3107,7 +3107,7 @@ void i915_gem_object_free(struct drm_i915_gem_object *obj);
>   void i915_gem_object_init(struct drm_i915_gem_object *obj,
>   			 const struct drm_i915_gem_object_ops *ops);
>   struct drm_i915_gem_object *i915_gem_object_create(struct drm_device *dev,
> -						  size_t size);
> +						   u64 size);
>   struct drm_i915_gem_object *i915_gem_object_create_from_data(
>   		struct drm_device *dev, const void *data, size_t size);
>   void i915_gem_close_object(struct drm_gem_object *gem, struct drm_file *file);
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index fe92e28ea0a8..0d1dc04302ec 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -4131,14 +4131,24 @@ static const struct drm_i915_gem_object_ops i915_gem_object_ops = {
>   	.put_pages = i915_gem_object_put_pages_gtt,
>   };
>   
> -struct drm_i915_gem_object *i915_gem_object_create(struct drm_device *dev,
> -						  size_t size)
> +struct drm_i915_gem_object *
> +i915_gem_object_create(struct drm_device *dev, u64 size)
>   {
>   	struct drm_i915_gem_object *obj;
>   	struct address_space *mapping;
>   	gfp_t mask;
>   	int ret;
>   
> +	/* There is a prevalence of the assumption that we fit the object's
> +	 * page count inside a 32bit variable. Let's document this and catch
> +	 * if we ever need to fix it.
> +	 */
> +	if (WARN_ON(size >> PAGE_SHIFT > INT_MAX))
> +		return ERR_PTR(-E2BIG);
> +
> +	if (sizeof(size_t) < sizeof(u64) && size > INT_MAX)
> +		return ERR_PTR(-E2BIG);
> +

Shouldn't it be UINT_MAX in both cases?

We could try to future proof more maybe like 
sizeof(typeof(obj->base.size)), is typeof can be used like that? 
Something similar for sg API if possible. But then again, it could be 
better future proofing to be hardcoded like you wrote it. Yes I think so.

Regards,

Tvrtko



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