if err := dstFD.pd.prepareWrite(dstFD.isFile); err != nil {
		return 0, err, false
	}

	dst := dstFD.Sysfd
	for remain > 0 {
		n := maxSendfileSize
		if int64(n) > remain {
			n = int(remain)
		}
		n, err = syscall.Sendfile(dst, src, nil, n)
		if n > 0 {
			written += int64(n)
			remain -= int64(n)
			continue
		} else if err != syscall.EAGAIN && err != syscall.EINTR {
			// This includes syscall.ENOSYS (no kernel

	}

	written, handled, err = pollSplice(&f.pfd, pfd, remain)

	if lr != nil {
		lr.N = remain - written
	}

	return written, handled, wrapSyscallError("splice", err)
}

func (f *File) copyFileRange(r io.Reader) (written int64, handled bool, err error) {
	var (
		remain int64
		lr     *io.LimitedReader
	)
	if lr, r, remain = tryLimitedReader(r); remain <= 0 {
		return 0, true, nil
	}

	var src *File

//
// if handled == false, sendFile performed no work.
func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
	var remain int64 = 1<<63 - 1 // by default, copy until EOF

	lr, ok := r.(*io.LimitedReader)
	if ok {
		remain, r = lr.N, lr.R
		if remain <= 0 {
			return 0, nil, true
		}
	}
	f, ok := r.(*os.File)
	if !ok {
		return 0, nil, false
	}

	sc, err := f.SyscallConn()

	// loop back to the beginning ad nauseam until it's sent
	// exactly the number of bytes told to. As such, we need to
	// know exactly how many bytes to send.
	var remain int64 = 0

	lr, ok := r.(*io.LimitedReader)
	if ok {
		remain, r = lr.N, lr.R
		if remain <= 0 {
			return 0, nil, true
		}
	}
	// r might be an *os.File or an os.fileWithoutWriteTo.

		return 0, err, false
	}

	dst := dstFD.Sysfd
	for remain > 0 {
		n := maxSendfileSize
		if int64(n) > remain {
			n = int(remain)
		}
		pos1 := pos
		n, err = syscall.Sendfile(dst, src, &pos1, n)
		if n > 0 {
			pos += int64(n)
			written += int64(n)
			remain -= int64(n)
		}
		if err == syscall.EINTR {
			continue
		}

	}

	dst := dstFD.Sysfd
	for remain > 0 {
		n := maxSendfileSize
		if int64(n) > remain {
			n = int(remain)
		}
		pos1 := pos
		n, err = syscall.Sendfile(dst, src, &pos1, n)
		if err == syscall.EAGAIN || err == syscall.EINTR {
			// partial write may have occurred
			n = int(pos1 - pos)
		}
		if n > 0 {
			pos += int64(n)
			written += int64(n)
			remain -= int64(n)
			continue

const maxCopyFileRangeRound = 1 << 30

// CopyFileRange copies at most remain bytes of data from src to dst, using
// the copy_file_range system call. dst and src must refer to regular files.
func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err error) {
	if !isKernelVersionGE53() {
		return 0, false, nil
	}

	for remain > 0 {
		max := remain
		if max > maxCopyFileRangeRound {
			max = maxCopyFileRangeRound
		}

//
// If spliceFrom returns handled == false, it has performed no work.
func spliceFrom(c *netFD, r io.Reader) (written int64, err error, handled bool) {
	var remain int64 = 1<<63 - 1 // by default, copy until EOF
	lr, ok := r.(*io.LimitedReader)
	if ok {
		remain, r = lr.N, lr.R
		if remain <= 0 {
			return 0, nil, true
		}
	}

	var s *netFD
	switch v := r.(type) {
	case *TCPConn:
		s = v.fd
	case tcpConnWithoutWriteTo:

	if err != nil {
		return 0, false, err
	}
	defer putPipe(p)
	var inPipe, n int
	for err == nil && remain > 0 {
		max := maxSpliceSize
		if int64(max) > remain {
			max = int(remain)
		}
		inPipe, err = spliceDrain(p.wfd, src, max)
		// The operation is considered handled if splice returns no
		// error, or an error other than EINVAL. An EINVAL means the
		// kernel does not support splice for the socket type of src.

// For each call, unfreezeServices must be called once.
func freezeServices() {
	// Use atomics for globalServiceFreeze, so we can read without locking.
	// We need a lock since we are need the 2 atomic values to remain in sync.
	globalServiceFreezeMu.Lock()
	// If multiple calls, first one creates channel.
	globalServiceFreezeCnt++
	if globalServiceFreezeCnt == 1 {
		globalServiceFreeze.Store(make(chan struct{}))
	}