drm/i915: Tidy generation of the GTT mmap offset

If we make the observation that mmap-offsets are only released when we
free an object, we can then deduce that the shrinker only creates free
space in the mmap arena indirectly by flushing the request list and
freeing expired objects. If we combine this with the lockless
vma-manager and lockless idling, we can avoid taking our big struct_mutex
until we need to actually free the requests.

One side-effect is that we defer the madvise checking until we need the
pages (i.e. the fault handler). This brings us into line with the other
delayed checks (and madvise in general).

v2: s/ret/err/ and use if (!err) rather than if (ret == 0)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470388464-28458-9-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -1898,36 +1898,28 @@ u64 i915_gem_get_ggtt_alignment(struct drm_i915_private *dev_priv, u64 size,
 static int i915_gem_object_create_mmap_offset(struct drm_i915_gem_object *obj)
 {
 	struct drm_i915_private *dev_priv = to_i915(obj->base.dev);
-	int ret;
-
-	dev_priv->mm.shrinker_no_lock_stealing = true;
+	int err;
 
-	ret = drm_gem_create_mmap_offset(&obj->base);
-	if (ret != -ENOSPC)
-		goto out;
+	err = drm_gem_create_mmap_offset(&obj->base);
+	if (!err)
+		return 0;
 
-	/* Badly fragmented mmap space? The only way we can recover
-	 * space is by destroying unwanted objects. We can't randomly release
-	 * mmap_offsets as userspace expects them to be persistent for the
-	 * lifetime of the objects. The closest we can is to release the
-	 * offsets on purgeable objects by truncating it and marking it purged,
-	 * which prevents userspace from ever using that object again.
+	/* We can idle the GPU locklessly to flush stale objects, but in order
+	 * to claim that space for ourselves, we need to take the big
+	 * struct_mutex to free the requests+objects and allocate our slot.
 	 */
-	i915_gem_shrink(dev_priv,
-			obj->base.size >> PAGE_SHIFT,
-			I915_SHRINK_BOUND |
-			I915_SHRINK_UNBOUND |
-			I915_SHRINK_PURGEABLE);
-	ret = drm_gem_create_mmap_offset(&obj->base);
-	if (ret != -ENOSPC)
-		goto out;
+	err = i915_gem_wait_for_idle(dev_priv, true);
+	if (err)
+		return err;
 
-	i915_gem_shrink_all(dev_priv);
-	ret = drm_gem_create_mmap_offset(&obj->base);
-out:
-	dev_priv->mm.shrinker_no_lock_stealing = false;
+	err = i915_mutex_lock_interruptible(&dev_priv->drm);
+	if (!err) {
+		i915_gem_retire_requests(dev_priv);
+		err = drm_gem_create_mmap_offset(&obj->base);
+		mutex_unlock(&dev_priv->drm.struct_mutex);
+	}
 
-	return ret;
+	return err;
 }
 
 static void i915_gem_object_free_mmap_offset(struct drm_i915_gem_object *obj)
@@ -1944,32 +1936,15 @@ i915_gem_mmap_gtt(struct drm_file *file,
 	struct drm_i915_gem_object *obj;
 	int ret;
 
-	ret = i915_mutex_lock_interruptible(dev);
-	if (ret)
-		return ret;
-
 	obj = i915_gem_object_lookup(file, handle);
-	if (!obj) {
-		ret = -ENOENT;
-		goto unlock;
-	}
-
-	if (obj->madv != I915_MADV_WILLNEED) {
-		DRM_DEBUG("Attempting to mmap a purgeable buffer\n");
-		ret = -EFAULT;
-		goto out;
-	}
+	if (!obj)
+		return -ENOENT;
 
 	ret = i915_gem_object_create_mmap_offset(obj);
-	if (ret)
-		goto out;
-
+	if (ret == 0)
 		*offset = drm_vma_node_offset_addr(&obj->base.vma_node);
 
-out:
-	i915_gem_object_put(obj);
-unlock:
-	mutex_unlock(&dev->struct_mutex);
+	i915_gem_object_put_unlocked(obj);
 	return ret;
 }