[PATCH] drm/i915/userptr: Probe existence of backing struct pages upon creation

[Maarten Lankhorst]

Op 23-07-2021 om 13:34 schreef Matthew Auld:
> From: Chris Wilson <chris at chris-wilson.co.uk>
>
> Jason Ekstrand requested a more efficient method than userptr+set-domain
> to determine if the userptr object was backed by a complete set of pages
> upon creation. To be more efficient than simply populating the userptr
> using get_user_pages() (as done by the call to set-domain or execbuf),
> we can walk the tree of vm_area_struct and check for gaps or vma not
> backed by struct page (VM_PFNMAP). The question is how to handle
> VM_MIXEDMAP which may be either struct page or pfn backed...
>
> With discrete we are going to drop support for set_domain(), so offering
> a way to probe the pages, without having to resort to dummy batches has
> been requested.
>
> v2:
> - add new query param for the PROBE flag, so userspace can easily
>   check if the kernel supports it(Jason).
> - use mmap_read_{lock, unlock}.
> - add some kernel-doc.
> v3:
> - In the docs also mention that PROBE doesn't guarantee that the pages
>   will remain valid by the time they are actually used(Tvrtko).
> - Add a small comment for the hole finding logic(Jason).
> - Move the param next to all the other params which just return true.
>
> Testcase: igt/gem_userptr_blits/probe
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> Cc: Jordan Justen <jordan.l.justen at intel.com>
> Cc: Kenneth Graunke <kenneth at whitecape.org>
> Cc: Jason Ekstrand <jason at jlekstrand.net>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Ramalingam C <ramalingam.c at intel.com>
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Acked-by: Kenneth Graunke <kenneth at whitecape.org>
> Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_userptr.c | 41 ++++++++++++++++++++-
>  drivers/gpu/drm/i915/i915_getparam.c        |  1 +
>  include/uapi/drm/i915_drm.h                 | 20 ++++++++++
>  3 files changed, 61 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> index 56edfeff8c02..468a7a617fbf 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> @@ -422,6 +422,34 @@ static const struct drm_i915_gem_object_ops i915_gem_userptr_ops = {
>  
>  #endif
>  
> +static int
> +probe_range(struct mm_struct *mm, unsigned long addr, unsigned long len)
> +{
> +	const unsigned long end = addr + len;
> +	struct vm_area_struct *vma;
> +	int ret = -EFAULT;
> +
> +	mmap_read_lock(mm);
> +	for (vma = find_vma(mm, addr); vma; vma = vma->vm_next) {
> +		/* Check for holes, note that we also update the addr below */
> +		if (vma->vm_start > addr)
> +			break;
> +
> +		if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
> +			break;
> +
> +		if (vma->vm_end >= end) {
> +			ret = 0;
> +			break;
> +		}
> +
> +		addr = vma->vm_end;
> +	}
> +	mmap_read_unlock(mm);
> +
> +	return ret;
> +}
> +
>  /*
>   * Creates a new mm object that wraps some normal memory from the process
>   * context - user memory.
> @@ -477,7 +505,8 @@ i915_gem_userptr_ioctl(struct drm_device *dev,
>  	}
>  
>  	if (args->flags & ~(I915_USERPTR_READ_ONLY |
> -			    I915_USERPTR_UNSYNCHRONIZED))
> +			    I915_USERPTR_UNSYNCHRONIZED |
> +			    I915_USERPTR_PROBE))
>  		return -EINVAL;
>  
>  	if (i915_gem_object_size_2big(args->user_size))
> @@ -504,6 +533,16 @@ i915_gem_userptr_ioctl(struct drm_device *dev,
>  			return -ENODEV;
>  	}
>  
> +	if (args->flags & I915_USERPTR_PROBE) {
> +		/*
> +		 * Check that the range pointed to represents real struct
> +		 * pages and not iomappings (at this moment in time!)
> +		 */
> +		ret = probe_range(current->mm, args->user_ptr, args->user_size);
> +		if (ret)
> +			return ret;
> +	}
> +
>  #ifdef CONFIG_MMU_NOTIFIER
>  	obj = i915_gem_object_alloc();
>  	if (obj == NULL)
> diff --git a/drivers/gpu/drm/i915/i915_getparam.c b/drivers/gpu/drm/i915/i915_getparam.c
> index 24e18219eb50..bbb7cac43eb4 100644
> --- a/drivers/gpu/drm/i915/i915_getparam.c
> +++ b/drivers/gpu/drm/i915/i915_getparam.c
> @@ -134,6 +134,7 @@ int i915_getparam_ioctl(struct drm_device *dev, void *data,
>  	case I915_PARAM_HAS_EXEC_FENCE_ARRAY:
>  	case I915_PARAM_HAS_EXEC_SUBMIT_FENCE:
>  	case I915_PARAM_HAS_EXEC_TIMELINE_FENCES:
> +	case I915_PARAM_HAS_USERPTR_PROBE:
>  		/* For the time being all of these are always true;
>  		 * if some supported hardware does not have one of these
>  		 * features this value needs to be provided from
> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> index 975087553ea0..0d290535a6e5 100644
> --- a/include/uapi/drm/i915_drm.h
> +++ b/include/uapi/drm/i915_drm.h
> @@ -674,6 +674,9 @@ typedef struct drm_i915_irq_wait {
>   */
>  #define I915_PARAM_HAS_EXEC_TIMELINE_FENCES 55
>  
> +/* Query if the kernel supports the I915_USERPTR_PROBE flag. */
> +#define I915_PARAM_HAS_USERPTR_PROBE 56
> +
>  /* Must be kept compact -- no holes and well documented */
>  
>  typedef struct drm_i915_getparam {
> @@ -2222,12 +2225,29 @@ struct drm_i915_gem_userptr {
>  	 * through the GTT. If the HW can't support readonly access, an error is
>  	 * returned.
>  	 *
> +	 * I915_USERPTR_PROBE:
> +	 *
> +	 * Probe the provided @user_ptr range and validate that the @user_ptr is
> +	 * indeed pointing to normal memory and that the range is also valid.
> +	 * For example if some garbage address is given to the kernel, then this
> +	 * should complain.
> +	 *
> +	 * Returns -EFAULT if the probe failed.
> +	 *
> +	 * Note that this doesn't populate the backing pages, and also doesn't
> +	 * guarantee that the object will remain valid when the object is
> +	 * eventually used.
> +	 *
> +	 * The kernel supports this feature if I915_PARAM_HAS_USERPTR_PROBE
> +	 * returns a non-zero value.
> +	 *
>  	 * I915_USERPTR_UNSYNCHRONIZED:
>  	 *
>  	 * NOT USED. Setting this flag will result in an error.
>  	 */
>  	__u32 flags;
>  #define I915_USERPTR_READ_ONLY 0x1
> +#define I915_USERPTR_PROBE 0x2
>  #define I915_USERPTR_UNSYNCHRONIZED 0x80000000
>  	/**
>  	 * @handle: Returned handle for the object.

Could we use _VALIDATE instead of probe? Or at least pin the pages as well, so we don't have to do it later?

We already have i915_gem_object_userptr_validate, no need to dupe it.