[Intel-gfx] [PATCH 2/3] drm/i915: Infrastructure for supporting different GGTT views per object

[Daniel Vetter]

On Thu, Dec 04, 2014 at 10:26:14AM +0000, Chris Wilson wrote:
> On Thu, Dec 04, 2014 at 10:19:09AM +0000, Tvrtko Ursulin wrote:
> > 
> > On 12/04/2014 09:53 AM, Chris Wilson wrote:
> > >On Wed, Dec 03, 2014 at 02:59:25PM +0000, Tvrtko Ursulin wrote:
> > >>+void i915_vma_bind(struct i915_vma *vma, enum i915_cache_level cache_level,
> > >>+		   u32 flags)
> > >>+{
> > >>+	struct sg_table *pages = i915_ggtt_view_pages(vma);
> > >>+
> > >>+	if (pages && !IS_ERR(pages)) {
> > >>+		vma->bind_vma(vma, pages, cache_level, flags);
> > >>+
> > >>+		if (vma->ggtt_view.type != I915_GGTT_VIEW_NORMAL) {
> > >>+			sg_free_table(pages);
> > >>+			kfree(pages);
> > >>+		}
> > >>+	}
> > >>+}
> > >
> > >Stop. Even if the failure path is impossible with the present
> > >implementation, here you are masking the error only to go and pretend
> > >the binding succeeded.
> > >
> > >Don't be lazy, this is a very nasty bug that should be hit during igt -
> > >or else you are not testing well enough.
> > 
> > Fair comment, even if a bit too assuming. I actually had this as
> > TODO but somehow lost it.
> > 
> > I don't have any ideas on how to provoke this to fail from an IGT?
> > Even with future implementations it boils down to a couple of small
> > allocations which would have to fail reliably.
> 
> We have quite a few thrash tests now that are fairly good at getting
> even the small allocations to fail.
> 
> What we don't have is a single-fd, multi-ctx thrash test (well except
> for some GL tests...)

But none of these tests result in permanent memory failures (only the
occasional ioctl restart when waiting for gpu rendering). And sg table
alloc only recurses through the shrinker so that can't happen. So I think
we just have to get by with review.

We did have issues with sg table allocations in stress tests though,
before we've added the recursive shrinker locking, hence sg table alloc
can indeed go south.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch