Re: [RFC 3/5] drm/i915: Add support for CPU mapping to DRM_IOCTL_I915_GEM_MMAP_GTT

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

On 26/01/16 16:59, Chris Wilson wrote:
> On Tue, Jan 26, 2016 at 04:23:28PM +0000, Tvrtko Ursulin wrote:
>>
>> On 26/01/16 15:10, Chris Wilson wrote:
>>> On Tue, Jan 26, 2016 at 02:53:31PM +0000, Tvrtko Ursulin wrote:
>>>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>
>>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>> ---
>>>>   drivers/gpu/drm/i915/i915_gem.c | 96 ++++++++++++++++++++++++++++++++++++++---
>>>>   include/uapi/drm/i915_drm.h     |  3 ++
>>>>   2 files changed, 93 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
>>>> index dacf6a0013c5..039d55a49fc6 100644
>>>> --- a/drivers/gpu/drm/i915/i915_gem.c
>>>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>>>> @@ -1954,6 +1954,60 @@ out:
>>>>   	return i915_gem_ret_to_vm_ret(dev_priv, ret);
>>>>   }
>>>>
>>>> +static int
>>>> +i915_gem_cpu_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
>>>> +{
>>>> +	struct drm_i915_gem_object *obj = to_intel_bo(vma->vm_private_data);
>>>> +	struct drm_device *dev = obj->base.dev;
>>>> +	struct drm_i915_private *dev_priv = dev->dev_private;
>>>> +	bool write = !!(vmf->flags & FAULT_FLAG_WRITE);
>>>> +	pgoff_t page_offset;
>>>> +	struct page *page;
>>>> +	int ret;
>>>> +
>>>> +	/* We don't use vmf->pgoff since that has the fake offset */
>>>> +	page_offset = ((unsigned long)vmf->virtual_address - vma->vm_start) >>
>>>> +			PAGE_SHIFT;
>>>> +
>>>> +	trace_i915_gem_object_fault(obj, page_offset, true, write);
>>>> +
>>>> +	intel_runtime_pm_get(dev_priv);
>>>> +
>>>> +	ret = i915_mutex_lock_interruptible(dev);
>>>> +	if (ret)
>>>> +		goto out;
>>>> +
>>>> +	ret = i915_gem_object_set_to_cpu_domain(obj, write);
>>>> +	if (ret)
>>>> +		goto out_unlock;
>>>
>>> That was a mistake in the GTT gem_fault(). If you do this, we also want
>>> the nonblocking wait for obvious reasons.
>>
>> You suggest leaving it for userspace?
>
> It is userspace's responsibility. Page faults are random and do not
> occur around every pointer acceess - userspace has to mark the domain
> changes on its boundaries (and coordinate amongst its peers).
>
>> And how would a non-blocking wait work?
>
> Before set-to-cpu-domain, we do a wait_rendering_nonblocking which drops
> and then reacquires the mutex. (That allows for multiple waiters which
> tends to be the lowest hanging fruit with struct_mutex contention.) Then
> set-to-cpu domain does a blocking wait to ensure nothing snuck in.
>
> But I don't think we want this. And we can then reduce the
> i915_mutex_lock_interruptible() to a plain mutex_lock_interruptible() as
> we are not touching the GPU.
>
>>>> +	ret = i915_gem_object_get_pages(obj);
>>>> +	if (ret)
>>>> +		goto out_unlock;
>>>> +
>>>> +	page = i915_gem_object_get_page(obj, page_offset);
>>>> +	if (!page) {
>>>> +		ret = -ERANGE;
>
> ret = -EFAULT;
>
> though it would definitely be a stack bug.
>
>>>> +		goto out_unlock;
>>>> +	}
>>>> +
>>>> +	mutex_unlock(&dev->struct_mutex);
>>>> +
>>>> +	ret = vm_insert_pfn(vma, (unsigned long)vmf->virtual_address,
>>>> +			    page_to_pfn(page));
>>>
>>> We don't have a page ref at this point, so this obj+page could be
>>> freed (via the shrinker at least) before we insert it.
>>
>> Oh yeah, need to pin the pages..
>
> But only whilst inserting. Once inserted they need to be evicted, and I
> was wondering whether we should do the zap on put_pages(). If we don't,
> it means that the shrinker is neutered.

Ah I forgot this in v2. So we would need something like 
obj->fault_mappable so that i915_gem_release_mmap runs, yes?

>>> I would also be more interested in having a version that faulted the
>>> entire object at once - though maybe we will see more random access in
>>> future.
>>
>> Yeah I did not want to concern myself with more code since this was
>> a proof of concept anyway.
>
> No worries, the transformation is simple with a certain remap function.
>
>>> It looks fairly sane. I wanted this just a short while ago, but figured
>>> out a way of using regular mmap() to give me the inheritance instead.
>>
>> So would it be useful to cleanup and finish this work or not?
>
> I agree that it closes a big hole in the API - the ability to CPU mmap
> non-shmemfs object (i.e. userptr, dmabuf). With a bit of polish we
> should be able to offer something to take advantage of the existing GEM
> infrastructure better than a regular CPU mmapping - though off the top
> of my head, I don't have anything that is ratelimited by CPU pagefaults.
>
> Another thing I realised was that this severely limits the mmap space on
> 32-bit systems, as the vma manager is unsigned long. The CPU mmaping was
> a way around some of the restrictions. That would seem fairly easy to
> lift (and I hope without consequence).

I did not manage to figure out what here limits the space on 32-bit systems?

Regards,

Tvrtko
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