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 // +build darwin dragonfly freebsd linux netbsd openbsd
6 7 package bio
8 9 import (
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
26 var mmapLimit int32 = 1<<31 - 1
27 28 func init() {
29 // Linux is the only practically concerning OS.
30 if runtime.GOOS == "linux" {
31 mmapLimit = 30000
32 }
33 }
34 35 func (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