runtime: bring back minHeapIdx in scavenge index

The scavenge index currently doesn't guard against overflow, and CL
436395 removed the minHeapIdx optimization that allows the chunk scan to
skip scanning chunks that haven't been mapped for the heap, and are only
available as a consequence of chunks' mapped region being rounded out to
a page on both ends.

Because the 0'th chunk is never mapped, minHeapIdx effectively prevents
overflow, fixing the iOS breakage.

This change also refactors growth and initialization a little bit to
decouple it from pageAlloc a bit and share code across platforms.

Change-Id: If7fc3245aa81cf99451bf8468458da31986a9b0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/486695
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>

1 files changed, 23 insertions, 12 deletions

diff --git a/src/runtime/mpagealloc_32bit.go b/src/runtime/mpagealloc_32bit.go
index 03990e47cf..900146e363 100644
--- a/src/runtime/mpagealloc_32bit.go
+++ b/src/runtime/mpagealloc_32bit.go
@@ -93,18 +93,6 @@ func (p *pageAlloc) sysInit(test bool) {
 
 		reservation = add(reservation, uintptr(entries)*pallocSumBytes)
 	}
-
-	if test {
-		// Set up the scavenge index via sysAlloc so the test can free it later.
-		scavIndexSize := uintptr(len(scavengeIndexArray)) * unsafe.Sizeof(atomicScavChunkData{})
-		p.scav.index.chunks = ((*[(1 << heapAddrBits) / pallocChunkBytes]atomicScavChunkData)(sysAlloc(scavIndexSize, p.sysStat)))[:]
-		p.summaryMappedReady += scavIndexSize
-	} else {
-		// Set up the scavenge index.
-		p.scav.index.chunks = scavengeIndexArray[:]
-	}
-	p.scav.index.min.Store(1) // The 0th chunk is never going to be mapped for the heap.
-	p.scav.index.max.Store(uintptr(len(p.scav.index.chunks)))
 }
 
 // See mpagealloc_64bit.go for details.
@@ -127,3 +115,26 @@ func (p *pageAlloc) sysGrow(base, limit uintptr) {
 		}
 	}
 }
+
+// sysInit initializes the scavengeIndex' chunks array.
+//
+// Returns the amount of memory added to sysStat.
+func (s *scavengeIndex) sysInit(test bool, sysStat *sysMemStat) (mappedReady uintptr) {
+	if test {
+		// Set up the scavenge index via sysAlloc so the test can free it later.
+		scavIndexSize := uintptr(len(scavengeIndexArray)) * unsafe.Sizeof(atomicScavChunkData{})
+		s.chunks = ((*[(1 << heapAddrBits) / pallocChunkBytes]atomicScavChunkData)(sysAlloc(scavIndexSize, sysStat)))[:]
+		mappedReady = scavIndexSize
+	} else {
+		// Set up the scavenge index.
+		s.chunks = scavengeIndexArray[:]
+	}
+	s.min.Store(1) // The 0th chunk is never going to be mapped for the heap.
+	s.max.Store(uintptr(len(s.chunks)))
+	return
+}
+
+// sysGrow is a no-op on 32-bit platforms.
+func (s *scavengeIndex) sysGrow(base, limit uintptr, sysStat *sysMemStat) uintptr {
+	return 0
+}