[Intel-gfx] [PATCH v3] drm/i915/userptr: Probe vma range before gup

[Chris Wilson]

Quoting Chris Wilson (2017-12-15 10:09:02)
> We want to exclude any GGTT objects from being present on our internal
> lists to avoid the deadlock we may run into with our requirement for
> struct_mutex during invalidate. However, if the gup_fast fails, we put
> the userptr onto the workqueue and mark it as active, so that we
> remember to serialise the worker upon mmu_invalidate.
> 
> v2: Hold mmap_sem to prevent modifications to the mm while we probe and
> add ourselves to the interval-tree for notificiation.
> v3: Rely on mmap_sem for a simpler patch.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104209
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Michał Winiarski <michal.winiarski at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem_userptr.c | 42 +++++++++++++++++++++++++++++++--
>  1 file changed, 40 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> index 382a77a1097e..f879c76e46ee 100644
> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> @@ -598,6 +598,37 @@ __i915_gem_userptr_get_pages_schedule(struct drm_i915_gem_object *obj)
>         return ERR_PTR(-EAGAIN);
>  }
>  
> +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;
> +
> +       for (vma = find_vma(mm, addr); vma; vma = vma->vm_next) {
> +               if (vma->vm_start > addr)
> +                       break;
> +
> +               /*
> +                * Exclude any VMA that is backed only by struct_page, i.e.
> +                * IO regions that include our own GGTT mmaps. We cannot handle
> +                * such ranges, as we may encounter deadlocks around our
> +                * struct_mutex on mmu_invalidate_range.
> +                */
> +               if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
> +                       break;
> +
> +               if (vma->vm_end >= end) {
> +                       ret = 0;
> +                       break;
> +               }
> +
> +               addr = vma->vm_end;
> +       }
> +
> +       return ret;
> +}
> +
>  static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
>  {
>         const int num_pages = obj->base.size >> PAGE_SHIFT;
> @@ -632,9 +663,15 @@ static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
>                         return -EAGAIN;
>         }
>  
> -       pvec = NULL;
> -       pinned = 0;
> +       /* Quickly exclude any invalid VMA */
> +       down_read(&mm->mmap_sem);

Regardless, mmap_sem is not allowed here, as we may be underneath
struct_mutex and a mmap_sem from a pagefault.

Please look the other way as I investigate nesting...
-Chris