private MappedByteBuffer map(@NotNull RegionAllocationAtomicityLock regionAllocationAtomicityLock,
                                 @NotNull FileChannel channel,
                                 int pageSize) throws IOException {
      //MAYBE RC: this could cause noticeable pauses, hence it may worth to enlarge file in advance, async.
      //          i.e. schedule enlargement as soon as last page is 50% full?
      //          It wouldn't work good for completely random-access storages, but most our use-cases are either append-only
      //          logs, or file-attributes, which indexed by fileId, which are growing quite monotonically, so it may work.
      //          ...But it is tricky to implement it on MMappedFileStorage level, since MMappedFileStorage has only info
      //          about page usage, but no info about page content usage -- i.e. MMappedFileStorage doesn't know which bytes
      //          on the page is already accessed by client(s), to forecast how soon the page will be 'exhausted' and the next
      //          page likely requested. 'Clients' on the other side, have this information, so it is much easier to implement
      //          forecasting on the client level -- but making it per-client is much less universal/more intrusive solution.

      long startedAtNs = System.nanoTime();
      try {
        ensureFileRegionAllocatedAndZeroed(regionAllocationAtomicityLock, channel, pageSize);
        //MAYBE RC: fill the page with 0 (via Unsafe.setMemory), _in addition_ to file region already filled with 0?
        //          It shouldn't be needed, but we have a lot of EA about non-zero values in not-yet-written mapped
        //          file regions, so maybe this helps?
        return channel.map(READ_WRITE, offsetInFile, pageSize);
      }
      finally {
        long timeSpentNs = System.nanoTime() - startedAtNs;
        totalTimeForPageMapNs.addAndGet(timeSpentNs);
      }
    }
private MappedByteBuffer map(@NotNull RegionAllocationAtomicityLock regionAllocationAtomicityLock,
                                 @NotNull FileChannel channel,
                                 int pageSize)