zone 空闲页管理：
/* Free memory management - zoned buddy allocator. */
#ifndef CONFIG_FORCE_MAX_ZONEORDER
#define MAX_ORDER 11
#else
#define MAX_ORDER CONFIG_FORCE_MAX_ZONEORDER
#endif
#define MAX_ORDER_NR_PAGES (1 << (MAX_ORDER - 1))
/* free areas of different sizes */
struct free_area free_area[MAX_ORDER];
Order MIGRATE_TYPES
--------------- ----> ------------------
|free_area：0 | |MIGRATE_UNMOVABLE |
--------------- |MIGRATE_MOVABLE |
|MIGRATE_RECLAIMABLE|
|MIGRATE_PCPTYPES |
|MIGRATE_CMA |
|MIGRATE_ISOLATE |
| nr_free |
---------------------
---------------- ----> ------------------
|free_area：1 | |MIGRATE_UNMOVABLE |
--------------- |MIGRATE_MOVABLE |
|MIGRATE_RECLAIMABLE|
|MIGRATE_PCPTYPES |
|MIGRATE_CMA |
|MIGRATE_ISOLATE |
| nr_free |
---------------------
--------------- ----> ------------------
|free_area：2 | |MIGRATE_UNMOVABLE |
--------------- |MIGRATE_MOVABLE |
|MIGRATE_RECLAIMABLE|
|MIGRATE_PCPTYPES |
|MIGRATE_CMA |
|MIGRATE_ISOLATE |
| nr_free |
---------------------
---------------- ----> ------------------
|free_area：3 | |MIGRATE_UNMOVABLE |
--------------- |MIGRATE_MOVABLE |
|MIGRATE_RECLAIMABLE|
|MIGRATE_PCPTYPES |
|MIGRATE_CMA |
|MIGRATE_ISOLATE |
| nr_free |
---------------------
--------------- ----> ------------------
|free_area：4 | |MIGRATE_UNMOVABLE |
--------------- |MIGRATE_MOVABLE |
|MIGRATE_RECLAIMABLE|
|MIGRATE_PCPTYPES |
|MIGRATE_CMA |
|MIGRATE_ISOLATE |
| nr_free |
---------------------
---------------- ----> ------------------
|free_area：5 | |MIGRATE_UNMOVABLE |
--------------- |MIGRATE_MOVABLE |
|MIGRATE_RECLAIMABLE|
|MIGRATE_PCPTYPES |
|MIGRATE_CMA |
|MIGRATE_ISOLATE |
| nr_free |
---------------------
--------------- ----> ------------------
|free_area：6 | |MIGRATE_UNMOVABLE |
--------------- |MIGRATE_MOVABLE |
|MIGRATE_RECLAIMABLE|
|MIGRATE_PCPTYPES |
|MIGRATE_CMA |
|MIGRATE_ISOLATE |
| nr_free |
---------------------
---------------- ----> ------------------
|free_area：7 | |MIGRATE_UNMOVABLE |
--------------- |MIGRATE_MOVABLE |
|MIGRATE_RECLAIMABLE|
|MIGRATE_PCPTYPES |
|MIGRATE_CMA |
|MIGRATE_ISOLATE |
| nr_free |
---------------------
--------------- ----> ------------------
|free_area：8 | |MIGRATE_UNMOVABLE |
--------------- |MIGRATE_MOVABLE |
|MIGRATE_RECLAIMABLE|
|MIGRATE_PCPTYPES |
|MIGRATE_CMA |
|MIGRATE_ISOLATE |
| nr_free |
---------------------
---------------- ----> ------------------
|free_area：9 | |MIGRATE_UNMOVABLE |
--------------- |MIGRATE_MOVABLE |
|MIGRATE_RECLAIMABLE|
|MIGRATE_PCPTYPES |
|MIGRATE_CMA |
|MIGRATE_ISOLATE |
| nr_free |
---------------------
--------------- ----> ------------------
|free_area：10 | |MIGRATE_UNMOVABLE |
--------------- |MIGRATE_MOVABLE |
|MIGRATE_RECLAIMABLE|
|MIGRATE_PCPTYPES |
|MIGRATE_CMA |
|MIGRATE_ISOLATE |
| nr_free |
---------------------
