[release-branch.go1.12] runtime: make mTreap.find actually find the best fit

This change modifies the implementation of mTreap.find to find the
best-fit span with the lowest possible base address.

Fixes #31677.

Change-Id: Ib4bda0f85d7d0590326f939a243a6e4665f37d3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/173479
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
(cherry picked from commit 8c05d676)
Reviewed-on: https://go-review.googlesource.com/c/go/+/173939Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mgclarge.go

@@ -301,24 +301,30 @@ func (root *mTreap) removeNode(t *treapNode) {
 // find searches for, finds, and returns the treap node containing the
 // smallest span that can hold npages. If no span has at least npages
 // it returns nil.
-// This is slightly more complicated than a simple binary tree search
-// since if an exact match is not found the next larger node is
-// returned.
+// This is a simple binary tree search that tracks the best-fit node found
+// so far. The best-fit node is guaranteed to be on the path to a
+// (maybe non-existent) lowest-base exact match.
 func (root *mTreap) find(npages uintptr) *treapNode {
+	var best *treapNode
 	t := root.treap
 	for t != nil {
 		if t.spanKey == nil {
 			throw("treap node with nil spanKey found")
 		}
-		if t.npagesKey < npages {
-			t = t.right
-		} else if t.left != nil && t.left.npagesKey >= npages {
+		// If we found an exact match, try to go left anyway. There could be
+		// a span there with a lower base address.
+		//
+		// Don't bother checking nil-ness of left and right here; even if t
+		// becomes nil, we already know the other path had nothing better for
+		// us anyway.
+		if t.npagesKey >= npages {
+			best = t
 			t = t.left
 		} else {
-			return t
+			t = t.right
 		}
 	}
-	return nil
+	return best
 }
 
 // removeSpan searches for, finds, deletes span along with

src/runtime/treap_test.go

@@ -58,11 +58,7 @@ func TestTreap(t *testing.T) {
 		}
 		tr.RemoveSpan(spans[0])
 	})
-	t.Run("Find", func(t *testing.T) {
-		// Note that Find doesn't actually find the best-fit
-		// element, so just make sure it always returns an element
-		// that is at least large enough to satisfy the request.
-		//
+	t.Run("FindBestFit", func(t *testing.T) {
 		// Run this 10 times, recreating the treap each time.
 		// Because of the non-deterministic structure of a treap,
 		// we'll be able to test different structures this way.
@@ -72,8 +68,10 @@ func TestTreap(t *testing.T) {
 				tr.Insert(s)
 			}
 			i := tr.Find(5)
-			if i.Span().Pages() < 5 {
-				t.Fatalf("expected span of size at least 5, got size %d", i.Span().Pages())
+			if i.Span().Pages() != 5 {
+				t.Fatalf("expected span of size 5, got span of size %d", i.Span().Pages())
+			} else if i.Span().Base() != 0xc0040000 {
+				t.Fatalf("expected span to have the lowest base address, instead got base %x", i.Span().Base())
 			}
 			for _, s := range spans {
 				tr.RemoveSpan(s)