drm/i915/gem: Store mmap_offsets in an rbtree rather than a plain list

Currently we create a new mmap_offset for every call to
mmap_offset_ioctl. This exposes ourselves to an abusive client that may
simply create new mmap_offsets ad infinitum, which will exhaust physical
memory and the virtual address space. In addition to the exhaustion, a
very long linear list of mmap_offsets causes other clients using the
object to incur long list walks -- these long lists can also be
generated by simply having many clients generate their own mmap_offset.

However, we can simply use the drm_vma_node itself to manage the file
association (allow/revoke) dropping our need to keep an mmo per-file.
Then if we keep a small rbtree of per-type mmap_offsets, we can lookup
duplicate requests quickly.

Fixes: cc662126 ("drm/i915: Introduce DRM_I915_GEM_MMAP_OFFSET")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200120104924.4000706-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_mman.c

@@ -455,10 +455,11 @@ static void i915_gem_object_release_mmap_gtt(struct drm_i915_gem_object *obj)
 
 void i915_gem_object_release_mmap_offset(struct drm_i915_gem_object *obj)
 {
-	struct i915_mmap_offset *mmo;
+	struct i915_mmap_offset *mmo, *mn;
 
 	spin_lock(&obj->mmo.lock);
-	list_for_each_entry(mmo, &obj->mmo.offsets, offset) {
+	rbtree_postorder_for_each_entry_safe(mmo, mn,
+					     &obj->mmo.offsets, offset) {
 		/*
 		 * vma_node_unmap for GTT mmaps handled already in
 		 * __i915_gem_object_release_mmap_gtt
@@ -487,6 +488,67 @@ void i915_gem_object_release_mmap(struct drm_i915_gem_object *obj)
 	i915_gem_object_release_mmap_offset(obj);
 }
 
+static struct i915_mmap_offset *
+lookup_mmo(struct drm_i915_gem_object *obj,
+	   enum i915_mmap_type mmap_type)
+{
+	struct rb_node *rb;
+
+	spin_lock(&obj->mmo.lock);
+	rb = obj->mmo.offsets.rb_node;
+	while (rb) {
+		struct i915_mmap_offset *mmo =
+			rb_entry(rb, typeof(*mmo), offset);
+
+		if (mmo->mmap_type == mmap_type) {
+			spin_unlock(&obj->mmo.lock);
+			return mmo;
+		}
+
+		if (mmo->mmap_type < mmap_type)
+			rb = rb->rb_right;
+		else
+			rb = rb->rb_left;
+	}
+	spin_unlock(&obj->mmo.lock);
+
+	return NULL;
+}
+
+static struct i915_mmap_offset *
+insert_mmo(struct drm_i915_gem_object *obj, struct i915_mmap_offset *mmo)
+{
+	struct rb_node *rb, **p;
+
+	spin_lock(&obj->mmo.lock);
+	rb = NULL;
+	p = &obj->mmo.offsets.rb_node;
+	while (*p) {
+		struct i915_mmap_offset *pos;
+
+		rb = *p;
+		pos = rb_entry(rb, typeof(*pos), offset);
+
+		if (pos->mmap_type == mmo->mmap_type) {
+			spin_unlock(&obj->mmo.lock);
+			drm_vma_offset_remove(obj->base.dev->vma_offset_manager,
+					      &mmo->vma_node);
+			kfree(mmo);
+			return pos;
+		}
+
+		if (pos->mmap_type < mmo->mmap_type)
+			p = &rb->rb_right;
+		else
+			p = &rb->rb_left;
+	}
+	rb_link_node(&mmo->offset, rb, p);
+	rb_insert_color(&mmo->offset, &obj->mmo.offsets);
+	spin_unlock(&obj->mmo.lock);
+
+	return mmo;
+}
+
 static struct i915_mmap_offset *
 mmap_offset_attach(struct drm_i915_gem_object *obj,
 		   enum i915_mmap_type mmap_type,
@@ -496,20 +558,22 @@ mmap_offset_attach(struct drm_i915_gem_object *obj,
 	struct i915_mmap_offset *mmo;
 	int err;
 
+	mmo = lookup_mmo(obj, mmap_type);
+	if (mmo)
+		goto out;
+
 	mmo = kmalloc(sizeof(*mmo), GFP_KERNEL);
 	if (!mmo)
 		return ERR_PTR(-ENOMEM);
 
 	mmo->obj = obj;
-	mmo->dev = obj->base.dev;
-	mmo->file = file;
 	mmo->mmap_type = mmap_type;
 	drm_vma_node_reset(&mmo->vma_node);
 
-	err = drm_vma_offset_add(mmo->dev->vma_offset_manager, &mmo->vma_node,
-				 obj->base.size / PAGE_SIZE);
+	err = drm_vma_offset_add(obj->base.dev->vma_offset_manager,
+				 &mmo->vma_node, obj->base.size / PAGE_SIZE);
 	if (likely(!err))
-		goto out;
+		goto insert;
 
 	/* Attempt to reap some mmap space from dead objects */
 	err = intel_gt_retire_requests_timeout(&i915->gt, MAX_SCHEDULE_TIMEOUT);
@@ -517,19 +581,17 @@ mmap_offset_attach(struct drm_i915_gem_object *obj,
 		goto err;
 
 	i915_gem_drain_freed_objects(i915);
-	err = drm_vma_offset_add(mmo->dev->vma_offset_manager, &mmo->vma_node,
-				 obj->base.size / PAGE_SIZE);
+	err = drm_vma_offset_add(obj->base.dev->vma_offset_manager,
+				 &mmo->vma_node, obj->base.size / PAGE_SIZE);
 	if (err)
 		goto err;
 
+insert:
+	mmo = insert_mmo(obj, mmo);
+	GEM_BUG_ON(lookup_mmo(obj, mmap_type) != mmo);
 out:
 	if (file)
 		drm_vma_node_allow(&mmo->vma_node, file);
-
-	spin_lock(&obj->mmo.lock);
-	list_add(&mmo->offset, &obj->mmo.offsets);
-	spin_unlock(&obj->mmo.lock);
-
 	return mmo;
 
 err:

drivers/gpu/drm/i915/gem/i915_gem_object.c

@@ -63,7 +63,7 @@ void i915_gem_object_init(struct drm_i915_gem_object *obj,
 	INIT_LIST_HEAD(&obj->lut_list);
 
 	spin_lock_init(&obj->mmo.lock);
-	INIT_LIST_HEAD(&obj->mmo.offsets);
+	obj->mmo.offsets = RB_ROOT;
 
 	init_rcu_head(&obj->rcu);
 
@@ -100,8 +100,8 @@ void i915_gem_close_object(struct drm_gem_object *gem, struct drm_file *file)
 {
 	struct drm_i915_gem_object *obj = to_intel_bo(gem);
 	struct drm_i915_file_private *fpriv = file->driver_priv;
+	struct i915_mmap_offset *mmo, *mn;
 	struct i915_lut_handle *lut, *ln;
-	struct i915_mmap_offset *mmo;
 	LIST_HEAD(close);
 
 	i915_gem_object_lock(obj);
@@ -117,14 +117,8 @@ void i915_gem_close_object(struct drm_gem_object *gem, struct drm_file *file)
 	i915_gem_object_unlock(obj);
 
 	spin_lock(&obj->mmo.lock);
-	list_for_each_entry(mmo, &obj->mmo.offsets, offset) {
-		if (mmo->file != file)
-			continue;
-
-		spin_unlock(&obj->mmo.lock);
+	rbtree_postorder_for_each_entry_safe(mmo, mn, &obj->mmo.offsets, offset)
 		drm_vma_node_revoke(&mmo->vma_node, file);
-		spin_lock(&obj->mmo.lock);
-	}
 	spin_unlock(&obj->mmo.lock);
 
 	list_for_each_entry_safe(lut, ln, &close, obj_link) {
@@ -203,12 +197,14 @@ static void __i915_gem_free_objects(struct drm_i915_private *i915,
 
 		i915_gem_object_release_mmap(obj);
 
-		list_for_each_entry_safe(mmo, mn, &obj->mmo.offsets, offset) {
+		rbtree_postorder_for_each_entry_safe(mmo, mn,
+						     &obj->mmo.offsets,
+						     offset) {
 			drm_vma_offset_remove(obj->base.dev->vma_offset_manager,
 					      &mmo->vma_node);
 			kfree(mmo);
 		}
-		INIT_LIST_HEAD(&obj->mmo.offsets);
+		obj->mmo.offsets = RB_ROOT;
 
 		GEM_BUG_ON(atomic_read(&obj->bind_count));
 		GEM_BUG_ON(obj->userfault_count);

drivers/gpu/drm/i915/gem/i915_gem_object_types.h

@@ -71,13 +71,11 @@ enum i915_mmap_type {
 };
 
 struct i915_mmap_offset {
-	struct drm_device *dev;
 	struct drm_vma_offset_node vma_node;
 	struct drm_i915_gem_object *obj;
-	struct drm_file *file;
 	enum i915_mmap_type mmap_type;
 
-	struct list_head offset;
+	struct rb_node offset;
 };
 
 struct drm_i915_gem_object {
@@ -137,7 +135,7 @@ struct drm_i915_gem_object {
 
 	struct {
 		spinlock_t lock; /* Protects access to mmo offsets */
-		struct list_head offsets;
+		struct rb_root offsets;
 	} mmo;
 
 	I915_SELFTEST_DECLARE(struct list_head st_link);