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

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

On Mon, 26 Jul 2021 at 17:10, Tvrtko Ursulin
<tvrtko.ursulin@linux.intel.com> wrote:
>
>
> On 26/07/2021 16:14, Jason Ekstrand wrote:
> > On Mon, Jul 26, 2021 at 3:31 AM Maarten Lankhorst
> > <maarten.lankhorst@linux.intel.com> wrote:
> >>
> >> Op 23-07-2021 om 13:34 schreef Matthew Auld:
> >>> 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 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@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>
> >>> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> >>> Acked-by: Kenneth Graunke <kenneth@whitecape.org>
> >>> Reviewed-by: Jason Ekstrand <jason@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?
> >
> > I only care that the name matches what it does.  _VALIDATE sounds like
> > it does a full validation of everything such that, if the import
> > succeeds, execbuf will as well.  If we pin the pages at the same time,
> > maybe that's true?  _PROBE, on the other hand, sounds a lot more like
>
> No it is not possible to guarantee backing store remains valid until
> execbuf.
>
> > a one-time best-effort check which may race with other stuff and
> > doesn't guarantee future success.  That's in line with what the
> > current patch does.
> >
> >> We already have i915_gem_object_userptr_validate, no need to dupe it.
> >
> > I have no opinion on this.
>
> I was actually suggesting the same as Maarten here - that we should add
> a "populate" flag. But opinion was that was not desired - please look
> for the older threads to see the reasoning there.

So how should we proceed here? Maarten?

>
> Regards,
>
> Tvrtko
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