[PATCH] drm/i915/gem: Avoid waiting inside mmu_notifier_invalidate_range

[Chris Wilson]

Since the userptr may be invalidated from inside a reclaim path, we
cannot wait on unbinding the object as we may hold any number of locks,
including the active reference of the object we are trying to
invalidate. E.g.

[<0>] __i915_active_wait+0x70/0xc0 [i915]
[<0>] i915_vma_unbind+0x2f/0x110 [i915]
[<0>] i915_gem_object_unbind+0x17c/0x350 [i915]
[<0>] userptr_mn_invalidate_range_start+0xb6/0x140 [i915]
[<0>] __mmu_notifier_invalidate_range_start+0x105/0x150
[<0>] try_to_unmap_one+0x7fb/0x900
[<0>] rmap_walk_file+0xe4/0x240
[<0>] try_to_unmap+0x85/0xc0
[<0>] shrink_page_list+0x885/0xe80
[<0>] shrink_inactive_list+0x155/0x260
[<0>] shrink_lruvec+0x4ec/0x5f0
[<0>] shrink_node+0x15b/0x410
[<0>] try_to_free_pages+0x169/0x3a0
[<0>] __alloc_pages_slowpath.constprop.0+0x251/0xa00
[<0>] __alloc_pages_nodemask+0x144/0x170
[<0>] new_slab+0x259/0x280
[<0>] ___slab_alloc.isra.0.constprop.0+0x3dd/0x460
[<0>] __slab_alloc.isra.0.constprop.0+0x9/0x10
[<0>] kmem_cache_alloc+0x11a/0x130
[<0>] alloc_pd+0x17/0x60 [i915]
[<0>] __gen8_ppgtt_alloc+0x2d7/0x350 [i915]
[<0>] gen8_ppgtt_alloc+0x35/0x70 [i915]
[<0>] ppgtt_bind_vma+0x29/0x70 [i915]

This situation only occurs when the lru page shrinker tries to sacrifice
one the objects it is trying to allocate for.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1702
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gem/i915_gem_userptr.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
index 8b0708708671..7baa2a094ed5 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
@@ -126,14 +126,12 @@ userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
 		}
 		spin_unlock(&mn->lock);
 
-		ret = i915_gem_object_unbind(obj,
-					     I915_GEM_OBJECT_UNBIND_ACTIVE |
-					     I915_GEM_OBJECT_UNBIND_BARRIER);
+		ret = i915_gem_object_unbind(obj, 0);
 		if (ret == 0)
 			ret = __i915_gem_object_put_pages(obj);
 		i915_gem_object_put(obj);
 		if (ret)
-			return ret;
+			return -EAGAIN;
 
 		spin_lock(&mn->lock);
 
-- 
2.20.1