runtime: check whether scavAddr is in inUse on scavengeOne fast path

This change makes it so that we check whether scavAddr is actually
mapped before trying to look at the summary for the fast path, since we
may segfault if that that part of the summary is not mapped in.
Previously this wasn't a problem because we would conservatively map
all memory for the summaries between the lowest mapped heap address and
the highest one.

This change also adds a test for this case.

Change-Id: I2b1d89b5e044dce81745964dfaba829f4becdc57
Reviewed-on: https://go-review.googlesource.com/c/go/+/212637
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 23 insertions, 2 deletions

diff --git a/src/runtime/mgcscavenge_test.go b/src/runtime/mgcscavenge_test.go
index a6eba8d972..58f9e3a80d 100644
--- a/src/runtime/mgcscavenge_test.go
+++ b/src/runtime/mgcscavenge_test.go
@@ -282,12 +282,13 @@ func TestPageAllocScavenge(t *testing.T) {
 	if minPages < 1 {
 		minPages = 1
 	}
-	tests := map[string]struct {
+	type setup struct {
 		beforeAlloc map[ChunkIdx][]BitRange
 		beforeScav  map[ChunkIdx][]BitRange
 		expect      []test
 		afterScav   map[ChunkIdx][]BitRange
-	}{
+	}
+	tests := map[string]setup{
 		"AllFreeUnscavExhaust": {
 			beforeAlloc: map[ChunkIdx][]BitRange{
 				BaseChunkIdx:     {},
@@ -396,6 +397,26 @@ func TestPageAllocScavenge(t *testing.T) {
 			},
 		},
 	}
+	if PageAlloc64Bit != 0 {
+		tests["ScavAllVeryDiscontiguous"] = setup{
+			beforeAlloc: map[ChunkIdx][]BitRange{
+				BaseChunkIdx:          {},
+				BaseChunkIdx + 0x1000: {},
+			},
+			beforeScav: map[ChunkIdx][]BitRange{
+				BaseChunkIdx:          {},
+				BaseChunkIdx + 0x1000: {},
+			},
+			expect: []test{
+				{^uintptr(0), 2 * PallocChunkPages * PageSize},
+				{^uintptr(0), 0},
+			},
+			afterScav: map[ChunkIdx][]BitRange{
+				BaseChunkIdx:          {{0, PallocChunkPages}},
+				BaseChunkIdx + 0x1000: {{0, PallocChunkPages}},
+			},
+		}
+	}
 	for name, v := range tests {
 		v := v
 		runTest := func(t *testing.T, locked bool) {