drm/mm: Reject over-sized allocation requests early

As we keep an rbtree of available holes sorted by their size, we can
very easily determine if there is any hole large enough that might
satisfy the allocation request. This helps when dealing with a highly
fragmented address space and a request for a search by address.

To cache the largest size, we convert into the cached rbtree variant
which tracks the leftmost node for us. However, currently we sorted into
ascending size order so the leftmost node is the smallest, and so to
make it the largest hole we need to invert our sorting.

Signed-off-by: Chris Wilson <[e-mail]>
Cc: Joonas Lahtinen <[e-mail]>
Reviewed-by: Joonas Lahtinen <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_mm.c

···
 #define HOLE_SIZE(NODE) ((NODE)->hole_size)
 #define HOLE_ADDR(NODE) (__drm_mm_hole_node_start(NODE))
 
+static u64 rb_to_hole_size(struct rb_node *rb)
+{
+	return rb_entry(rb, struct drm_mm_node, rb_hole_size)->hole_size;
+}
+
+static void insert_hole_size(struct rb_root_cached *root,
+			     struct drm_mm_node *node)
+{
+	struct rb_node **link = &root->rb_root.rb_node, *rb = NULL;
+	u64 x = node->hole_size;
+	bool first = true;
+
+	while (*link) {
+		rb = *link;
+		if (x > rb_to_hole_size(rb)) {
+			link = &rb->rb_left;
+		} else {
+			link = &rb->rb_right;
+			first = false;
+		}
+	}
+
+	rb_link_node(&node->rb_hole_size, rb, link);
+	rb_insert_color_cached(&node->rb_hole_size, root, first);
+}
+
 static void add_hole(struct drm_mm_node *node)
 {
 	struct drm_mm *mm = node->mm;
···
 		__drm_mm_hole_node_end(node) - __drm_mm_hole_node_start(node);
 	DRM_MM_BUG_ON(!drm_mm_hole_follows(node));
 
-	RB_INSERT(mm->holes_size, rb_hole_size, HOLE_SIZE);
+	insert_hole_size(&mm->holes_size, node);
 	RB_INSERT(mm->holes_addr, rb_hole_addr, HOLE_ADDR);
 
 	list_add(&node->hole_stack, &mm->hole_stack);
···
 	DRM_MM_BUG_ON(!drm_mm_hole_follows(node));
 
 	list_del(&node->hole_stack);
-	rb_erase(&node->rb_hole_size, &node->mm->holes_size);
+	rb_erase_cached(&node->rb_hole_size, &node->mm->holes_size);
 	rb_erase(&node->rb_hole_addr, &node->mm->holes_addr);
 	node->hole_size = 0;
 
···
 
 static struct drm_mm_node *best_hole(struct drm_mm *mm, u64 size)
 {
-	struct rb_node *best = NULL;
-	struct rb_node **link = &mm->holes_size.rb_node;
+	struct rb_node *rb = mm->holes_size.rb_root.rb_node;
+	struct drm_mm_node *best = NULL;
 
-	while (*link) {
-		struct rb_node *rb = *link;
+	do {
+		struct drm_mm_node *node =
+			rb_entry(rb, struct drm_mm_node, rb_hole_size);
 
-		if (size <= rb_hole_size(rb)) {
-			link = &rb->rb_left;
-			best = rb;
+		if (size <= node->hole_size) {
+			best = node;
+			rb = rb->rb_right;
 		} else {
-			link = &rb->rb_right;
+			rb = rb->rb_left;
 		}
-	}
+	} while (rb);
 
-	return rb_hole_size_to_node(best);
+	return best;
 }
 
 static struct drm_mm_node *find_hole(struct drm_mm *mm, u64 addr)
 {
+	struct rb_node *rb = mm->holes_addr.rb_node;
 	struct drm_mm_node *node = NULL;
-	struct rb_node **link = &mm->holes_addr.rb_node;
 
-	while (*link) {
+	while (rb) {
 		u64 hole_start;
 
-		node = rb_hole_addr_to_node(*link);
+		node = rb_hole_addr_to_node(rb);
 		hole_start = __drm_mm_hole_node_start(node);
 
 		if (addr < hole_start)
-			link = &node->rb_hole_addr.rb_left;
+			rb = node->rb_hole_addr.rb_left;
 		else if (addr > hole_start + node->hole_size)
-			link = &node->rb_hole_addr.rb_right;
+			rb = node->rb_hole_addr.rb_right;
 		else
 			break;
 	}
···
 	   u64 start, u64 end, u64 size,
 	   enum drm_mm_insert_mode mode)
 {
-	if (RB_EMPTY_ROOT(&mm->holes_size))
-		return NULL;
-
 	switch (mode) {
 	default:
 	case DRM_MM_INSERT_BEST:
···
 	switch (mode) {
 	default:
 	case DRM_MM_INSERT_BEST:
-		return rb_hole_size_to_node(rb_next(&node->rb_hole_size));
+		return rb_hole_size_to_node(rb_prev(&node->rb_hole_size));
 
 	case DRM_MM_INSERT_LOW:
 		return rb_hole_addr_to_node(rb_next(&node->rb_hole_addr));
···
 }
 EXPORT_SYMBOL(drm_mm_reserve_node);
 
+static u64 rb_to_hole_size_or_zero(struct rb_node *rb)
+{
+	return rb ? rb_to_hole_size(rb) : 0;
+}
+
 /**
  * drm_mm_insert_node_in_range - ranged search for space and insert @node
  * @mm: drm_mm to allocate from
···
 	DRM_MM_BUG_ON(range_start >= range_end);
 
 	if (unlikely(size == 0 || range_end - range_start < size))
+		return -ENOSPC;
+
+	if (rb_to_hole_size_or_zero(rb_first_cached(&mm->holes_size)) < size)
 		return -ENOSPC;
 
 	if (alignment <= 1)
···
 
 	if (drm_mm_hole_follows(old)) {
 		list_replace(&old->hole_stack, &new->hole_stack);
-		rb_replace_node(&old->rb_hole_size,
-				&new->rb_hole_size,
-				&mm->holes_size);
+		rb_replace_node_cached(&old->rb_hole_size,
+				       &new->rb_hole_size,
+				       &mm->holes_size);
 		rb_replace_node(&old->rb_hole_addr,
 				&new->rb_hole_addr,
 				&mm->holes_addr);
···
 
 	INIT_LIST_HEAD(&mm->hole_stack);
 	mm->interval_tree = RB_ROOT_CACHED;
-	mm->holes_size = RB_ROOT;
+	mm->holes_size = RB_ROOT_CACHED;
 	mm->holes_addr = RB_ROOT;
 
 	/* Clever trick to avoid a special case in the free hole tracking. */

include/drm/drm_mm.h

···
 	struct drm_mm_node head_node;
 	/* Keep an interval_tree for fast lookup of drm_mm_nodes by address. */
 	struct rb_root_cached interval_tree;
-	struct rb_root holes_size;
+	struct rb_root_cached holes_size;
 	struct rb_root holes_addr;
 
 	unsigned long scan_active;