runtime: notify OS more aggressively about freed pages

This causes the number of dirty pages reported by
vmmap to decrease after a scavenge.

I don't know why this works. Apparently neither do the folks
on StackOverflow who suggested it:

https://stackoverflow.com/questions/7718964/

For more history and a discussion of this StackOverflow post,
see golang#29844.

The key observation is that MADV_FREE_REUSABLE allows the OS
to decide that pages may be considered not-dirty,
but the OS is under no immediate obligation to do so.
And apparently iOS does not do so promptly,
and thus the accounting is not updated promptly.

Change-Id: Iec09510c5cc57c24c1f190e5f658096b12b7d691

src/runtime/mem_darwin.go

@@ -24,6 +24,8 @@ func sysUnused(v unsafe.Pointer, n uintptr) {
 	// MADV_FREE_REUSABLE is like MADV_FREE except it also propagates
 	// accounting information about the process to task_info.
 	madvise(v, n, _MADV_FREE_REUSABLE)
+	mprotect(v, n, _PROT_NONE)
+	mprotect(v, n, _PROT_READ|_PROT_WRITE)
 }
 
 func sysUsed(v unsafe.Pointer, n uintptr) {

src/runtime/sys_darwin.go

@@ -185,6 +185,13 @@ func madvise(addr unsafe.Pointer, n uintptr, flags int32) {
 }
 func madvise_trampoline()
 
+//go:nosplit
+//go:cgo_unsafe_args
+func mprotect(addr unsafe.Pointer, n uintptr, flags int32) {
+	libcCall(unsafe.Pointer(abi.FuncPCABI0(mprotect_trampoline)), unsafe.Pointer(&addr))
+}
+func mprotect_trampoline()
+
 //go:nosplit
 //go:cgo_unsafe_args
 func mlock(addr unsafe.Pointer, n uintptr) {

src/runtime/sys_darwin_arm64.s

@@ -120,6 +120,13 @@ TEXT runtime·madvise_trampoline(SB),NOSPLIT,$0
 	BL	libc_madvise(SB)
 	RET
 
+TEXT runtime·mprotect_trampoline(SB),NOSPLIT,$0
+	MOVD	8(R0), R1	// arg 2 len
+	MOVW	16(R0), R2	// arg 3 advice
+	MOVD	0(R0), R0	// arg 1 addr
+	BL	libc_mprotect(SB)
+	RET
+
 TEXT runtime·mlock_trampoline(SB),NOSPLIT,$0
 	MOVD	8(R0), R1	// arg 2 len
 	MOVD	0(R0), R0	// arg 1 addr