You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: readfrom_linux.go
    path of file: ./repos/gccrs/libgo/go/os
    
    the code of the file until where you have to start completion: // Copyright 2020 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.

package os

import (
	"internal/poll"
    
func (f *File) readFrom(r io.Reader) (written int64, handled bool, err error) {
	// copy_file_range(2) does not support destinations opened with
	// O_APPEND, so don't even try.
	if f.appendMode {
		return 0, false, nil
	}

	remain := int64(1 << 62)

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

	src, ok := r.(*File)
	if !ok {
		return 0, false, nil
	}
	if src.checkValid("ReadFrom") != nil {
		// Avoid returning the error as we report handled as false,
		// leave further error handling as the responsibility of the caller.
		return 0, false, nil
	}

	written, handled, err = pollCopyFileRange(&f.pfd, &src.pfd, remain)
	if lr != nil {
		lr.N -= written
	}
	return written, handled, NewSyscallError("copy_file_range", err)
}