Re: [PATCH 6/6] drm/i915: Avoid allocating a vmap arena for a single page

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

On 06/04/16 11:05, Chris Wilson wrote:
> On Wed, Apr 06, 2016 at 10:49:36AM +0100, Tvrtko Ursulin wrote:
>>> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
>>> index 985f067c1f0e..dc8e1b76c896 100644
>>> --- a/drivers/gpu/drm/i915/i915_gem.c
>>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>>> @@ -2233,7 +2233,10 @@ i915_gem_object_put_pages(struct drm_i915_gem_object *obj)
>>>   	list_del(&obj->global_list);
>>>
>>>   	if (obj->vmapping) {
>>> -		vunmap(obj->vmapping);
>>> +		if (obj->base.size == PAGE_SIZE)
>>> +			kunmap(kmap_to_page(obj->vmapping));
>>> +		else
>>> +			vunmap(obj->vmapping);
>>
>> Can't think of a reason why it would be better but there is also
>> is_vmalloc_addr(addr) as used by kvfree.
>
> For consistency with the shrinker (see below).
>
>>>   		obj->vmapping = NULL;
>>>   	}
>>>
>>> @@ -2416,15 +2419,22 @@ void *i915_gem_object_pin_vmap(struct drm_i915_gem_object *obj)
>>>   	i915_gem_object_pin_pages(obj);
>>>
>>>   	if (obj->vmapping == NULL) {
>>> -		struct sg_page_iter sg_iter;
>>>   		struct page **pages;
>>> -		int n;
>>>
>>> -		n = obj->base.size >> PAGE_SHIFT;
>>> -		pages = drm_malloc_gfp(n, sizeof(*pages), GFP_TEMPORARY);
>>> +		pages = NULL;
>>> +		if (obj->base.size == PAGE_SIZE)
>>> +			obj->vmapping = kmap(sg_page(obj->pages->sgl));
>>> +		else
>>> +			pages = drm_malloc_gfp(obj->base.size >> PAGE_SHIFT,
>>> +					       sizeof(*pages),
>>> +					       GFP_TEMPORARY);
>>>   		if (pages != NULL) {
>>> +			struct sg_page_iter sg_iter;
>>> +			int n;
>>> +
>>>   			n = 0;
>>> -			for_each_sg_page(obj->pages->sgl, &sg_iter, obj->pages->nents, 0)
>>> +			for_each_sg_page(obj->pages->sgl, &sg_iter,
>>> +					 obj->pages->nents, 0)
>>>   				pages[n++] = sg_page_iter_page(&sg_iter);
>>>
>>>   			obj->vmapping = vmap(pages, n, 0, PAGE_KERNEL);
>>>
>>
>> Two problems I can spot are:
>>
>> 1. Callers of pin_vmap now don't know which kind of address they are
>> getting. Maybe call it pin_kvmap or something? Just mention in
>> kerneldoc could be enough.
>
> I think just mention, and we can rename this to i915_gem_object_pin_map().
> Hmm. I liked the pin in the name since it ties to to pin_pages (later
> I plan to change that to get_pages and get_vmap/get_map as the pin
> becomes implicit).
>
>> 2. Shrinker will try to kick out kmapped objects because they have
>> obj->vmapping set.
>
> Not caring that much since the vmap_purge is very heavy weight, but we
> can apply is_vmalloc_addr() to the shrinker.
>
> Ok, happy to call this obj->mapping and i915_gem_object_pin_map() ?

Sounds good. (Including the is_vmalloc_addr() smartening in the shrinker.)

Regards,

Tvrtko


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