Re: [PATCH 08/11] drm/i915: Support for pread/pwrite from/to non shmem backed objects

[Chris Wilson <chris@chris-wilson.co.uk>]

On Thu, Jan 14, 2016 at 11:46:44AM +0530, ankitprasad.r.sharma@intel.com wrote:
> From: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
> 
> This patch adds support for extending the pread/pwrite functionality
> for objects not backed by shmem. The access will be made through
> gtt interface. This will cover objects backed by stolen memory as well
> as other non-shmem backed objects.
> 
> v2: Drop locks around slow_user_access, prefault the pages before
> access (Chris)
> 
> v3: Rebased to the latest drm-intel-nightly (Ankit)
> 
> v4: Moved page base & offset calculations outside the copy loop,
> corrected data types for size and offset variables, corrected if-else
> braces format (Tvrtko/kerneldocs)
> 
> v5: Enabled pread/pwrite for all non-shmem backed objects including
> without tiling restrictions (Ankit)
> 
> v6: Using pwrite_fast for non-shmem backed objects as well (Chris)
> 
> v7: Updated commit message, Renamed i915_gem_gtt_read to i915_gem_gtt_copy,
> added pwrite slow path for non-shmem backed objects (Chris/Tvrtko)
> 
> v8: Updated v7 commit message, mutex unlock around pwrite slow path for
> non-shmem backed objects (Tvrtko)
> 
> v9: Corrected check during pread_ioctl, to avoid shmem_pread being
> called for non-shmem backed objects (Tvrtko)
> 
> v10: Moved the write_domain check to needs_clflush and tiling mode check
> to pwrite_fast (Chris)
> 
> v11: Use pwrite_fast fallback for all objects (shmem and non-shmem backed),
> call fast_user_write regardless of pagefault in previous iteration
> 
> Testcase: igt/gem_stolen

Presumably igt/gem_pread and igt/gem_pwrite didn't break?

We could with the help of say vgem create a few 1+GiB objects that have
no obj->base.filp and so stress the pinning code better. (Or even just
stolen on some machines would be enough to create an unmappable object.)

> Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 155 +++++++++++++++++++++++++++++++++-------
>  1 file changed, 129 insertions(+), 26 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 7642b1b..ab1d043 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -55,6 +55,9 @@ static bool cpu_cache_is_coherent(struct drm_device *dev,
>  
>  static bool cpu_write_needs_clflush(struct drm_i915_gem_object *obj)
>  {
> +	if (obj->base.write_domain == I915_GEM_DOMAIN_CPU)
> +		return false;
> +
>  	if (!cpu_cache_is_coherent(obj->base.dev, obj->cache_level))
>  		return true;
>  
> @@ -646,6 +649,99 @@ shmem_pread_slow(struct page *page, int shmem_page_offset, int page_length,
>  	return ret ? - EFAULT : 0;
>  }
>  
> +static inline uint64_t
> +slow_user_access(struct io_mapping *mapping,
> +		 uint64_t page_base, int page_offset,
> +		 char __user *user_data,
> +		 int length, bool pwrite)

static inline bool
> +{
> +	void __iomem *vaddr_inatomic;

inatomic? We are not!
void __iomem *ioaddr;
void *vaddr;

> +	void *vaddr;
> +	uint64_t unwritten;
> +
> +	vaddr_inatomic = io_mapping_map_wc(mapping, page_base);
> +	/* We can use the cpu mem copy function because this is X86. */
> +	vaddr = (void __force *)vaddr_inatomic + page_offset;
> +	if (pwrite)
> +		unwritten = __copy_from_user(vaddr, user_data, length);
> +	else
> +		unwritten = __copy_to_user(user_data, vaddr, length);

> +
> +	io_mapping_unmap(vaddr_inatomic);
> +	return unwritten;
> +}
> +
> +static int
> +i915_gem_gtt_copy(struct drm_device *dev,
> +		   struct drm_i915_gem_object *obj, uint64_t size,
> +		   uint64_t data_offset, uint64_t data_ptr)
> +{
> +	struct drm_i915_private *dev_priv = dev->dev_private;
> +	char __user *user_data;
> +	uint64_t remain;
> +	uint64_t offset, page_base;
> +	int page_offset, page_length, ret = 0;
> +
> +	ret = i915_gem_obj_ggtt_pin(obj, 0, PIN_MAPPABLE);
> +	if (ret)
> +		goto out;
> +
> +	ret = i915_gem_object_set_to_gtt_domain(obj, false);
> +	if (ret)
> +		goto out_unpin;
> +
> +	ret = i915_gem_object_put_fence(obj);
> +	if (ret)
> +		goto out_unpin;
> +
> +	user_data = to_user_ptr(data_ptr);
> +	remain = size;
> +	offset = i915_gem_obj_ggtt_offset(obj) + data_offset;
> +
> +	mutex_unlock(&dev->struct_mutex);
> +	if (likely(!i915.prefault_disable))
> +		ret = fault_in_multipages_writeable(user_data, remain);
> +
> +	/*
> +	 * page_offset = offset within page
> +	 * page_base = page offset within aperture
> +	 */
> +	page_offset = offset_in_page(offset);
> +	page_base = offset & PAGE_MASK;

Where's the page-by-page copy? (Think about objects larger than the
aperture, or trying to operate in parallel with many - pinning aperture
space is bad practice.)

> +	while (remain > 0) {
> +		/* page_length = bytes to copy for this page */
> +		page_length = remain;
> +		if ((page_offset + remain) > PAGE_SIZE)
> +			page_length = PAGE_SIZE - page_offset;
> +
> +		/* This is a slow read/write as it tries to read from
> +		 * and write to user memory which may result into page
> +		 * faults

...and so we cannot perform this under the struct_mutex.

> +		 */
> +		ret = slow_user_access(dev_priv->gtt.mappable, page_base,
> +				       page_offset, user_data,
> +				       page_length, false);
> +
> +		if (ret) {

Fortuituously we don't copy more than a page, so this truncation error
should never hit. Still sloppy though.

> +			ret = -EFAULT;
> +			break;
> +		}
> +
> +		remain -= page_length;
> +		user_data += page_length;
> +		page_base += page_length;
> +		page_offset = 0;
> +	}
> +
> +	mutex_lock(&dev->struct_mutex);

This was the unlocked path, reset the domain. We could do some error
detection, but it is a user error so they get to keep the broken
pieces. (We should be consistent, always restore the domain after the
unlock/lock, or never. I'm actually thinking never, user error and all.)

> +
> +out_unpin:
> +	i915_gem_object_ggtt_unpin(obj);
> +out:
> +	return ret;
> +}
> +
>  static int
>  i915_gem_shmem_pread(struct drm_device *dev,
>  		     struct drm_i915_gem_object *obj,
> @@ -769,17 +865,14 @@ i915_gem_pread_ioctl(struct drm_device *dev, void *data,
>  		goto out;
>  	}
>  
> -	/* prime objects have no backing filp to GEM pread/pwrite
> -	 * pages from.
> -	 */
> -	if (!obj->base.filp) {
> -		ret = -EINVAL;
> -		goto out;
> -	}
> -
>  	trace_i915_gem_object_pread(obj, args->offset, args->size);
>  
> -	ret = i915_gem_shmem_pread(dev, obj, args, file);
> +	/* pread for non shmem backed objects */
> +	if (!obj->base.filp && obj->tiling_mode == I915_TILING_NONE)
> +		ret = i915_gem_gtt_copy(dev, obj, args->size,
> +					args->offset, args->data_ptr);
> +	else if (obj->base.filp)
> +		ret = i915_gem_shmem_pread(dev, obj, args, file);

I think this is much more readable as

ret = i915_gem_shmem_pread(##args);
if (ret == -ENODEV) /* or -EFAULT */
	ret = i915_gem_ggtt_pread(##args);

i.e. push the condition checking down and we know that a read of the
backing storage + clflush is the preferred option and should *always* be
tried first.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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