runtime: add physHugePageSize

This change adds the global physHugePageSize which is initialized in
osinit(). physHugePageSize contains the system's transparent huge page
(or superpage) size in bytes.

For #30333.

Change-Id: I2f0198c40729dbbe6e6f2676cef1d57dd107562c
Reviewed-on: https://go-review.googlesource.com/c/go/+/170858
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/malloc.go

@@ -324,6 +324,14 @@ const (
 // mallocinit.
 var physPageSize uintptr
 
+// physHugePageSize is the size in bytes of the OS's default physical huge
+// page size whose allocation is opaque to the application.
+//
+// If set, this must be set by the OS init code (typically in osinit) before
+// mallocinit. However, setting it at all is optional, and leaving the default
+// value is always safe (though potentially less efficient).
+var physHugePageSize uintptr
+
 // OS-defined helpers:
 //
 // sysAlloc obtains a large chunk of zeroed memory from the

src/runtime/os_linux.go

@@ -261,8 +261,31 @@ func sysauxv(auxv []uintptr) int {
 	return i / 2
 }
 
+var sysTHPSizePath = []byte("/sys/kernel/mm/transparent_hugepage/hpage_pmd_size\x00")
+
+func getHugePageSize() uintptr {
+	var numbuf [20]byte
+	fd := open(&sysTHPSizePath[0], 0 /* O_RDONLY */, 0)
+	if fd < 0 {
+		return 0
+	}
+	n := read(fd, noescape(unsafe.Pointer(&numbuf[0])), int32(len(numbuf)))
+	if n <= 0 {
+		closefd(fd)
+		return 0
+	}
+	l := n - 1 // remove trailing newline
+	v, ok := atoi(slicebytetostringtmp(numbuf[:l]))
+	if !ok || v < 0 {
+		v = 0
+	}
+	closefd(fd)
+	return uintptr(v)
+}
+
 func osinit() {
 	ncpu = getproccount()
+	physHugePageSize = getHugePageSize()
 }
 
 var urandom_dev = []byte("/dev/urandom\x00")