private void swapIn() {
        if (this.swappedOut) {
            this.writeLock.lock();
            try {
                if (!this.swappedOut) {
                    return;
                }
                long startMs = Utils.monotonicMs();
                mmapFile(false);
                this.swappedOut = false;
                LOG.info("Swapped in segment file {} cost {} ms.", this.path, Utils.monotonicMs() - startMs);
            } finally {
                this.writeLock.unlock();
            }
        }
    }
return true when this segment file is blank that we don't write any data into it.
@return
jraft-core/src/main/java/com/alipay/sofa/jraft/storage/log/SegmentFile.java