1 files changed, 62 insertions, 0 deletions
diff --git a/src/cmd/internal/bio/buf_mmap.go b/src/cmd/internal/bio/buf_mmap.go
new file mode 100644
index 0000000..4b43d74
--- /dev/null
+++ b/src/cmd/internal/bio/buf_mmap.go
@@ -0,0 +1,62 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
+package bio
+
+import (
+	"runtime"
+	"sync/atomic"
+	"syscall"
+)
+
+// mmapLimit is the maximum number of mmaped regions to create before
+// falling back to reading into a heap-allocated slice. This exists
+// because some operating systems place a limit on the number of
+// distinct mapped regions per process. As of this writing:
+//
+//  Darwin    unlimited
+//  DragonFly   1000000 (vm.max_proc_mmap)
+//  FreeBSD   unlimited
+//  Linux         65530 (vm.max_map_count) // TODO: query /proc/sys/vm/max_map_count?
+//  NetBSD    unlimited
+//  OpenBSD   unlimited
+var mmapLimit int32 = 1<<31 - 1
+
+func init() {
+	// Linux is the only practically concerning OS.
+	if runtime.GOOS == "linux" {
+		mmapLimit = 30000
+	}
+}
+
+func (r *Reader) sliceOS(length uint64) ([]byte, bool) {
+	// For small slices, don't bother with the overhead of a
+	// mapping, especially since we have no way to unmap it.
+	const threshold = 16 << 10
+	if length < threshold {
+		return nil, false
+	}
+
+	// Have we reached the mmap limit?
+	if atomic.AddInt32(&mmapLimit, -1) < 0 {
+		atomic.AddInt32(&mmapLimit, 1)
+		return nil, false
+	}
+
+	// Page-align the offset.
+	off := r.Offset()
+	align := syscall.Getpagesize()
+	aoff := off &^ int64(align-1)
+
+	data, err := syscall.Mmap(int(r.f.Fd()), aoff, int(length+uint64(off-aoff)), syscall.PROT_READ, syscall.MAP_SHARED|syscall.MAP_FILE)
+	if err != nil {
+		return nil, false
+	}
+
+	data = data[off-aoff:]
+	r.MustSeek(int64(length), 1)
+	return data, true
+}