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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 