*
	 * Unused dirty memory this arena manages.  Dirty memory is conceptually
	 * tracked as an arbitrarily interleaved LRU of dirty runs and cached
	 * chunks, but the list linkage is actually semi-duplicated in order to
	 * avoid extra arena_chunk_map_misc_t space overhead.
	 *
	 *   LRU-----------------------------------------------------------MRU
	 *
	 *        /-- arena ---\
	 *        |            |
	 *        |            |
	 *        |------------|                             /- chunk -\
	 *   ...->|chunks_cache|<--------------------------->|  /----\ |<--...
	 *        |------------|                             |  |node| |
	 *        |            |                             |  |    | |
	 *        |            |    /- run -\    /- run -\   |  |    | |
	 *        |            |    |       |    |       |   |  |    | |
	 *        |            |    |       |    |       |   |  |    | |
	 *        |------------|    |-------|    |-------|   |  |----| |
	 *   ...->|runs_dirty  |<-->|rd     |<-->|rd     |<---->|rd  |<----...
	 *        |------------|    |-------|    |-------|   |  |----| |
	 *        |            |    |       |    |       |   |  |    | |
	 *        |            |    |       |    |       |   |  \----/ |
	 *        |            |    \-------/    \-------/   |         |
	 *        |            |                             |         |
	 *        |            |                             |         |
	 *        \------------/                             \---------/
	 *