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: page_chunk_mem.go
    path of file: ./repos/seaweedfs/weed/mount/page_writer
    
    the code of the file until where you have to start completion: package page_writer

import (
	"sync"
	"sync/atomic"

	"github.com/seaweedfs/seaweedfs/weed/util"
	"github.com
    
func (mc *MemChunk) WriteDataAt(src []byte, offset int64, tsNs int64) (n int) {
	mc.Lock()
	defer mc.Unlock()

	innerOffset := offset % mc.chunkSize
	n = copy(mc.buf[innerOffset:], src)
	mc.usage.MarkWritten(innerOffset, innerOffset+int64(n), tsNs)
	mc.activityScore.MarkWrite()

	return
}