[release-branch.go1.15] runtime: fix ReadMemStatsSlow's and CheckScavengedBits' chunk iteration

Both ReadMemStatsSlow and CheckScavengedBits iterate over the page
allocator's chunks but don't actually check if they exist. During the
development process the chunks index became sparse, so now this was a
possibility. If the runtime tests' heap is sparse we might end up
segfaulting in either one of these functions, though this will generally
be very rare.

The pattern here to return nil for a nonexistent chunk is also useful
elsewhere, so this change introduces tryChunkOf which won't throw, but
might return nil. It also updates the documentation of chunkOf.

For #41296.
Fixes #41317.

Change-Id: Id5ae0ca3234480de1724fdf2e3677eeedcf76fa0
Reviewed-on: https://go-review.googlesource.com/c/go/+/253777
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 34835df048)
Reviewed-on: https://go-review.googlesource.com/c/go/+/253917

src/runtime/export_test.go

@@ -360,7 +360,11 @@ func ReadMemStatsSlow() (base, slow MemStats) {
 		}
 
 		for i := mheap_.pages.start; i < mheap_.pages.end; i++ {
-			pg := mheap_.pages.chunkOf(i).scavenged.popcntRange(0, pallocChunkPages)
+			chunk := mheap_.pages.tryChunkOf(i)
+			if chunk == nil {
+				continue
+			}
+			pg := chunk.scavenged.popcntRange(0, pallocChunkPages)
 			slow.HeapReleased += uint64(pg) * pageSize
 		}
 		for _, p := range allp {
@@ -758,11 +762,7 @@ func (p *PageAlloc) InUse() []AddrRange {
 // Returns nil if the PallocData's L2 is missing.
 func (p *PageAlloc) PallocData(i ChunkIdx) *PallocData {
 	ci := chunkIdx(i)
-	l2 := (*pageAlloc)(p).chunks[ci.l1()]
+	return (*PallocData)((*pageAlloc)(p).tryChunkOf(ci))
-	if l2 == nil {
-		return nil
-	}
-	return (*PallocData)(&l2[ci.l2()])
 }
 
 // AddrRange represents a range over addresses.
@@ -902,7 +902,10 @@ func CheckScavengedBitsCleared(mismatches []BitsMismatch) (n int, ok bool) {
 		lock(&mheap_.lock)
 	chunkLoop:
 		for i := mheap_.pages.start; i < mheap_.pages.end; i++ {
-			chunk := mheap_.pages.chunkOf(i)
+			chunk := mheap_.pages.tryChunkOf(i)
+			if chunk == nil {
+				continue
+			}
 			for j := 0; j < pallocChunkPages/64; j++ {
 				// Run over each 64-bit bitmap section and ensure
 				// scavenged is being cleared properly on allocation.

src/runtime/mpagealloc.go

@@ -326,7 +326,20 @@ func (s *pageAlloc) init(mheapLock *mutex, sysStat *uint64) {
 	s.scav.scavLWM = maxSearchAddr
 }
 
+// tryChunkOf returns the bitmap data for the given chunk.
+//
+// Returns nil if the chunk data has not been mapped.
+func (s *pageAlloc) tryChunkOf(ci chunkIdx) *pallocData {
+	l2 := s.chunks[ci.l1()]
+	if l2 == nil {
+		return nil
+	}
+	return &l2[ci.l2()]
+}
+
 // chunkOf returns the chunk at the given chunk index.
+//
+// The chunk index must be valid or this method may throw.
 func (s *pageAlloc) chunkOf(ci chunkIdx) *pallocData {
 	return &s.chunks[ci.l1()][ci.l2()]
 }