/*
* Run address (or size) and various flags are stored together.  The bit
* layout looks like (assuming 32-bit system):
*
*   ???????? ???????? ???nnnnn nnndumla
*
* ? : Unallocated: Run address for first/last pages, unset for internal
*                  pages.
*     Small: Run page offset.
*     Large: Run page count for first page, unset for trailing pages.
* n : binind for small size class, BININD_INVALID for large size class.
* d : dirty?
* u : unzeroed?
* m : decommitted?
* l : large?
* a : allocated?
*
* Following are example bit patterns for the three types of runs.
*
* p : run page offset (这个page是run中第几个page (offset))
* s : run size (连续的空闲 page 个数)
* n : binind for size class; large objects set these to BININD_INVALID(该 page 对应的 bin Id)
* x : don't care
* - : 0
* + : 1
* [DUMLA] : bit set
* [dumla] : bit unset
*
*   Unallocated (clean):
*     ssssssss ssssssss sss+++++ +++dum-a
*     xxxxxxxx xxxxxxxx xxxxxxxx xxx-Uxxx
*     ssssssss ssssssss sss+++++ +++dUm-a
*
*   Unallocated (dirty):
*     ssssssss ssssssss sss+++++ +++D-m-a
*     xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
*     ssssssss ssssssss sss+++++ +++D-m-a
*
*   Small:
*     pppppppp pppppppp pppnnnnn nnnd---A
*     pppppppp pppppppp pppnnnnn nnn----A
*     pppppppp pppppppp pppnnnnn nnnd---A
*
*   Large:
*     ssssssss ssssssss sss+++++ +++D--LA
*     xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
*     -------- -------- ---+++++ +++D--LA
*
*   Large (sampled, size <= LARGE_MINCLASS):
*     ssssssss ssssssss sssnnnnn nnnD--LA
*     xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
*     -------- -------- ---+++++ +++D--LA
*
*   Large (not sampled, size == LARGE_MINCLASS):
*     ssssssss ssssssss sss+++++ +++D--LA
*     xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
*     -------- -------- ---+++++ +++D--LA
*/