参考
- https://blog.csdn.net/qq_43573047/article/details/140734352
- https://zhuanlan.zhihu.com/p/479922331
在内存分配的时候，当指定的迁移类型的内存不足时，会向其他迁移类型借，内核通过下面的数组定义了迁移类型的借用顺序：
/** This array describes the order lists are fallen back to when* the free lists for the desirable migrate type are depleted** The other migratetypes do not have fallbacks.*/
static int fallbacks[MIGRATE_TYPES][3] = {[MIGRATE_UNMOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE, MIGRATE_TYPES },[MIGRATE_MOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_TYPES },[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE, MIGRATE_MOVABLE, MIGRATE_TYPES },
};
这部分的代码如下：
/** Try finding a free buddy page on the fallback list and put it on the free* list of requested migratetype, possibly along with other pages from the same* block, depending on fragmentation avoidance heuristics. Returns true if* fallback was found so that __rmqueue_smallest() can grab it.** The use of signed ints for order and current_order is a deliberate* deviation from the rest of this file, to make the for loop* condition simpler.*/
static __always_inline bool
__rmqueue_fallback(struct zone *zone, int order, int start_migratetype,unsigned int alloc_flags)
{struct 
