1// Copyright 2019 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5//go:build unix
6
7package bio
8
9import (
10	"runtime"
11	"sync/atomic"
12	"syscall"
13)
14
15// mmapLimit is the maximum number of mmaped regions to create before
16// falling back to reading into a heap-allocated slice. This exists
17// because some operating systems place a limit on the number of
18// distinct mapped regions per process. As of this writing:
19//
20//	Darwin    unlimited
21//	DragonFly   1000000 (vm.max_proc_mmap)
22//	FreeBSD   unlimited
23//	Linux         65530 (vm.max_map_count) // TODO: query /proc/sys/vm/max_map_count?
24//	NetBSD    unlimited
25//	OpenBSD   unlimited
26var mmapLimit int32 = 1<<31 - 1
27
28func init() {
29	// Linux is the only practically concerning OS.
30	if runtime.GOOS == "linux" {
31		mmapLimit = 30000
32	}
33}
34
35func (r *Reader) sliceOS(length uint64) ([]byte, bool) {
36	// For small slices, don't bother with the overhead of a
37	// mapping, especially since we have no way to unmap it.
38	const threshold = 16 << 10
39	if length < threshold {
40		return nil, false
41	}
42
43	// Have we reached the mmap limit?
44	if atomic.AddInt32(&mmapLimit, -1) < 0 {
45		atomic.AddInt32(&mmapLimit, 1)
46		return nil, false
47	}
48
49	// Page-align the offset.
50	off := r.Offset()
51	align := syscall.Getpagesize()
52	aoff := off &^ int64(align-1)
53
54	data, err := syscall.Mmap(int(r.f.Fd()), aoff, int(length+uint64(off-aoff)), syscall.PROT_READ, syscall.MAP_SHARED|syscall.MAP_FILE)
55	if err != nil {
56		return nil, false
57	}
58
59	data = data[off-aoff:]
60	r.MustSeek(int64(length), 1)
61	return data, true
62}
63