message: meshable-arena: basic freeSpan
diff: #include <sys/ioctl.h>
  +#include <algorithm>
  +
  #include "meshable-arena.h"
  #include "runtime.h"
  @@ -145,6 +147,9 @@ Span MeshableArena::reservePages(Length pageCount) {
  }
  void MeshableArena::freeSpan(Span span) {
  + d_assert(span.length > 0);
  + Length spanClass = std::min(span.length, static_cast<Length>(kSpanClassCount)) - 1;
  + _dirty[spanClass].push_back(span);
  }
  static void unmarkFreePages(const internal::vector<Span> freeSpans[kSpanClassCount], internal::Bitmap &bitmap) {
  