github.com/etecs-ru/ristretto@v0.9.1/z/mmap_linux.go (about)

     1  /*
     2   * Copyright 2020 Dgraph Labs, Inc. and Contributors
     3   *
     4   * Licensed under the Apache License, Version 2.0 (the "License");
     5   * you may not use this file except in compliance with the License.
     6   * You may obtain a copy of the License at
     7   *
     8   *     http://www.apache.org/licenses/LICENSE-2.0
     9   *
    10   * Unless required by applicable law or agreed to in writing, software
    11   * distributed under the License is distributed on an "AS IS" BASIS,
    12   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13   * See the License for the specific language governing permissions and
    14   * limitations under the License.
    15   */
    16  
    17  package z
    18  
    19  import (
    20  	"os"
    21  	"reflect"
    22  	"unsafe"
    23  
    24  	"golang.org/x/sys/unix"
    25  )
    26  
    27  // mmap uses the mmap system call to memory-map a file. If writable is true,
    28  // memory protection of the pages is set so that they may be written to as well.
    29  func mmap(fd *os.File, writable bool, size int64) ([]byte, error) {
    30  	mtype := unix.PROT_READ
    31  	if writable {
    32  		mtype |= unix.PROT_WRITE
    33  	}
    34  	return unix.Mmap(int(fd.Fd()), 0, int(size), mtype, unix.MAP_SHARED)
    35  }
    36  
    37  // mremap is a Linux-specific system call to remap pages in memory. This can be used in place of munmap + mmap.
    38  func mremap(data []byte, size int) ([]byte, error) {
    39  	// taken from <https://github.com/torvalds/linux/blob/f8394f232b1eab649ce2df5c5f15b0e528c92091/include/uapi/linux/mman.h#L8>
    40  	const MREMAP_MAYMOVE = 0x1
    41  
    42  	header := (*reflect.SliceHeader)(unsafe.Pointer(&data))
    43  	mmapAddr, _, errno := unix.Syscall6(
    44  		unix.SYS_MREMAP,
    45  		header.Data,
    46  		uintptr(header.Len),
    47  		uintptr(size),
    48  		uintptr(MREMAP_MAYMOVE),
    49  		0,
    50  		0,
    51  	)
    52  	if errno != 0 {
    53  		return nil, errno
    54  	}
    55  
    56  	header.Data = mmapAddr
    57  	header.Cap = size
    58  	header.Len = size
    59  	return data, nil
    60  }
    61  
    62  // munmap unmaps a previously mapped slice.
    63  //
    64  // unix.Munmap maintains an internal list of mmapped addresses, and only calls munmap
    65  // if the address is present in that list. If we use mremap, this list is not updated.
    66  // To bypass this, we call munmap ourselves.
    67  func munmap(data []byte) error {
    68  	if len(data) == 0 || len(data) != cap(data) {
    69  		return unix.EINVAL
    70  	}
    71  	_, _, errno := unix.Syscall(
    72  		unix.SYS_MUNMAP,
    73  		uintptr(unsafe.Pointer(&data[0])),
    74  		uintptr(len(data)),
    75  		0,
    76  	)
    77  	if errno != 0 {
    78  		return errno
    79  	}
    80  	return nil
    81  }
    82  
    83  // madvise uses the madvise system call to give advise about the use of memory
    84  // when using a slice that is memory-mapped to a file. Set the readahead flag to
    85  // false if page references are expected in random order.
    86  func madvise(b []byte, readahead bool) error {
    87  	flags := unix.MADV_NORMAL
    88  	if !readahead {
    89  		flags = unix.MADV_RANDOM
    90  	}
    91  	return unix.Madvise(b, flags)
    92  }
    93  
    94  // msync writes any modified data to persistent storage.
    95  func msync(b []byte) error {
    96  	return unix.Msync(b, unix.MS_SYNC)
    97  }