1//go:build unix
 2
 3package alloc
 4
 5import (
 6	"math"
 7
 8	"github.com/tetratelabs/wazero/experimental"
 9	"golang.org/x/sys/unix"
10)
11
12func NewMemory(_, max uint64) experimental.LinearMemory {
13	// Round up to the page size.
14	rnd := uint64(unix.Getpagesize() - 1)
15	max = (max + rnd) &^ rnd
16
17	if max > math.MaxInt {
18		// This ensures int(max) overflows to a negative value,
19		// and unix.Mmap returns EINVAL.
20		max = math.MaxUint64
21	}
22
23	// Reserve max bytes of address space, to ensure we won't need to move it.
24	// A protected, private, anonymous mapping should not commit memory.
25	b, err := unix.Mmap(-1, 0, int(max), unix.PROT_NONE, unix.MAP_PRIVATE|unix.MAP_ANON)
26	if err != nil {
27		panic(err)
28	}
29	return &mmappedMemory{buf: b[:0]}
30}
31
32// The slice covers the entire mmapped memory:
33//   - len(buf) is the already committed memory,
34//   - cap(buf) is the reserved address space.
35type mmappedMemory struct {
36	buf []byte
37}
38
39func (m *mmappedMemory) Reallocate(size uint64) []byte {
40	com := uint64(len(m.buf))
41	res := uint64(cap(m.buf))
42	if com < size && size <= res {
43		// Round up to the page size.
44		rnd := uint64(unix.Getpagesize() - 1)
45		new := (size + rnd) &^ rnd
46
47		// Commit additional memory up to new bytes.
48		err := unix.Mprotect(m.buf[com:new], unix.PROT_READ|unix.PROT_WRITE)
49		if err != nil {
50			return nil
51		}
52
53		// Update committed memory.
54		m.buf = m.buf[:new]
55	}
56	// Limit returned capacity because bytes beyond
57	// len(m.buf) have not yet been committed.
58	return m.buf[:size:len(m.buf)]
59}
60
61func (m *mmappedMemory) Free() {
62	err := unix.Munmap(m.buf[:cap(m.buf)])
63	if err != nil {
64		panic(err)
65	}
66	m.buf = nil
67}