102 lines
2.8 KiB
Go
102 lines
2.8 KiB
Go
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/*
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* Copyright 2020 Dgraph Labs, Inc. and Contributors
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package z
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import (
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"fmt"
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"os"
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"reflect"
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"unsafe"
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"golang.org/x/sys/unix"
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)
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// mmap uses the mmap system call to memory-map a file. If writable is true,
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// memory protection of the pages is set so that they may be written to as well.
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func mmap(fd *os.File, writable bool, size int64) ([]byte, error) {
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mtype := unix.PROT_READ
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if writable {
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mtype |= unix.PROT_WRITE
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}
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return unix.Mmap(int(fd.Fd()), 0, int(size), mtype, unix.MAP_SHARED)
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}
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// mremap is a Linux-specific system call to remap pages in memory. This can be used in place of munmap + mmap.
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func mremap(data []byte, size int) ([]byte, error) {
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// taken from <https://github.com/torvalds/linux/blob/f8394f232b1eab649ce2df5c5f15b0e528c92091/include/uapi/linux/mman.h#L8>
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const MREMAP_MAYMOVE = 0x1
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header := (*reflect.SliceHeader)(unsafe.Pointer(&data))
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mmapAddr, mmapSize, errno := unix.Syscall6(
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unix.SYS_MREMAP,
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header.Data,
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uintptr(header.Len),
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uintptr(size),
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uintptr(MREMAP_MAYMOVE),
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0,
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0,
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)
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if errno != 0 {
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return nil, errno
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}
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if mmapSize != uintptr(size) {
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return nil, fmt.Errorf("mremap size mismatch: requested: %d got: %d", size, mmapSize)
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}
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header.Data = mmapAddr
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header.Cap = size
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header.Len = size
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return data, nil
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}
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// munmap unmaps a previously mapped slice.
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//
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// unix.Munmap maintains an internal list of mmapped addresses, and only calls munmap
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// if the address is present in that list. If we use mremap, this list is not updated.
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// To bypass this, we call munmap ourselves.
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func munmap(data []byte) error {
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if len(data) == 0 || len(data) != cap(data) {
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return unix.EINVAL
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}
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_, _, errno := unix.Syscall(
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unix.SYS_MUNMAP,
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uintptr(unsafe.Pointer(&data[0])),
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uintptr(len(data)),
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0,
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)
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if errno != 0 {
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return errno
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}
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return nil
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}
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// madvise uses the madvise system call to give advise about the use of memory
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// when using a slice that is memory-mapped to a file. Set the readahead flag to
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// false if page references are expected in random order.
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func madvise(b []byte, readahead bool) error {
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flags := unix.MADV_NORMAL
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if !readahead {
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flags = unix.MADV_RANDOM
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}
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return unix.Madvise(b, flags)
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}
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// msync writes any modified data to persistent storage.
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func msync(b []byte) error {
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return unix.Msync(b, unix.MS_SYNC)
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}
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