Re: [Intel-gfx] [PATCH 3/4] drm/i915/userptr: Probe existence of backing struct pages upon creation

[Matthew Auld <matthew.william.auld@gmail.com>]

On Wed, 21 Jul 2021 at 21:28, Jason Ekstrand <jason@jlekstrand.net> wrote:
>
> On Thu, Jul 15, 2021 at 5:16 AM Matthew Auld <matthew.auld@intel.com> wrote:
> >
> > From: Chris Wilson <chris@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 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 PROPBE flag, so userspace can easily
> >   check if the kernel supports it(Jason).
> > - use mmap_read_{lock, unlock}.
> > - add some kernel-doc.
> >
> > Testcase: igt/gem_userptr_blits/probe
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> > Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> > Cc: Jordan Justen <jordan.l.justen@intel.com>
> > Cc: Kenneth Graunke <kenneth@whitecape.org>
> > Cc: Jason Ekstrand <jason@jlekstrand.net>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Cc: Ramalingam C <ramalingam.c@intel.com>
> > ---
> >  drivers/gpu/drm/i915/gem/i915_gem_userptr.c | 40 ++++++++++++++++++++-
> >  drivers/gpu/drm/i915/i915_getparam.c        |  3 ++
> >  include/uapi/drm/i915_drm.h                 | 18 ++++++++++
> >  3 files changed, 60 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..fd6880328596 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> > @@ -422,6 +422,33 @@ 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) {
> > +               if (vma->vm_start > addr)
>
> Why isn't this > end?  Are we somehow guaranteed that one vma covers
> the entire range?

AFAIK we are just making sure we don't have a hole(note that we also
update addr below), for example the user might have done a partial
munmap. There could be multiple vma's if the kernel was unable to
merge them. If we reach the vm_end >= end, then we know we have a
"valid" range.

>
> > +                       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 +504,8 @@ 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 +532,16 @@ 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..d6d2e1a10d14 100644
> > --- a/drivers/gpu/drm/i915/i915_getparam.c
> > +++ b/drivers/gpu/drm/i915/i915_getparam.c
> > @@ -163,6 +163,9 @@ int i915_getparam_ioctl(struct drm_device *dev, void *data,
> >         case I915_PARAM_PERF_REVISION:
> >                 value = i915_perf_ioctl_version();
> >                 break;
> > +       case I915_PARAM_HAS_USERPTR_PROBE:
> > +               value = true;
> > +               break;
> >         default:
> >                 DRM_DEBUG("Unknown parameter %d\n", param->param);
> >                 return -EINVAL;
> > diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> > index e20eeeca7a1c..2e4112bf4d38 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 {
> > @@ -2178,12 +2181,27 @@ 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.
> > +        *
> > +        * 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.
> > --
> > 2.26.3
> >
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