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memory: alloc_page
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gatieme
@@ -1000,4 +1000,177 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
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### 3.4.4 `__rmqueue_fallback`
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-------
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如果前面的流程都分配失败了, 那么说明当前 ZONG 区域指定 MIGRATE_TYPE 中没有足够的空闲页来完成本次分配了. 那么内核将通过 [`__rmqueue_fallback`](https://elixir.bootlin.com/linux/v5.10/source/mm/page_alloc.c#L2759) 尝试从当前 ZONE 的其他 MIGRATE_TYPE 的空闲链表中挪用内存,
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如果前面的流程都分配失败了, 那么说明当前 ZONG 区域指定 MIGRATE_TYPE 中没有足够的空闲页来完成本次分配了. 那么内核将通过 [`__rmqueue_fallback`](https://elixir.bootlin.com/linux/v5.10/source/mm/page_alloc.c#L2759) 尝试从当前 ZONE 的其他 MIGRATE_TYPE 的空闲链表中挪用内存.
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#### 3.4.4.1 窃取的顺序
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-------
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对于某个指定的 MIGRATE_TYPES, 内核 fallback 去其他 MIGRATE_TYPE 的空闲内存中窃取内存时, 有什么优先级顺序么?
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肯定有的, 内核定义了一个 fallbacks 数组, 用来指定每个迁移类型 fallback 时备选 MIGRATE_TYPE 的顺序.
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比如 fallbacks[MIGRATE_UNMOVABLE] 行值为 : { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE, MIGRATE_TYPES }
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就意味着, 当分配 UNMOVABLE 的内存页失败的时候, 先去 RECLAIMABLE 类型的页面中分配, 再去 MOVABLE 类型的页面中分配. MIGRATE_TYPES 标记了 fallback 的终止, 如果前两个类型都没有找到可分配的内存, 那么 fallback 就失败了.
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CMA 和 ISOLATE 不会进行 fallback 操作, 因此其 fallback 数组只有一项 { MIGRATE_TYPES }.
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```cpp
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/*
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* This array describes the order lists are fallen back to when
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* the free lists for the desirable migrate type are depleted
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*/
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static int fallbacks[MIGRATE_TYPES][3] = {
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[MIGRATE_UNMOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE, MIGRATE_TYPES },
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[MIGRATE_MOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_TYPES },
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[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE, MIGRATE_MOVABLE, MIGRATE_TYPES },
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#ifdef CONFIG_CMA
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[MIGRATE_CMA] = { MIGRATE_TYPES }, /* Never used */
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#endif
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#ifdef CONFIG_MEMORY_ISOLATION
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[MIGRATE_ISOLATE] = { MIGRATE_TYPES }, /* Never used */
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#endif
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};
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```
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| 迁移类别 | 窃取顺序 |
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|:-------:|:-------:|
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| 不可移动(MIGRATE_UNMOVABLE) | MIGRATE_RECLAIMABLE > MIGRATE_MOVABLE |
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| 可移动(MIGRATE_MOVABLE) | MIGRATE_RECLAIMABLE > MIGRATE_UNMOVABLE |
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| 可回收(MIGRATE_RECLAIMABLE) | MIGRATE_UNMOVABLE > MIGRATE_MOVABLE |
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#### 3.4.4.2 简易流程(4.13 之前的内核版本)
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-------
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我们继续回到 [`__rmqueue_fallback`](https://elixir.bootlin.com/linux/v5.10/source/mm/page_alloc.c#L2759), 看着流程有点复杂, 前后两次循环, 一次从 (MAX_ORDER, order] 方向遍历, 此外还有 find_smallest 流程从 [order, MAX_ORDER) 遍历, 咋一看两个流程并没有过于明显的区别, 最后是 do_steal 真正完成页面的窃取.
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find_smallest 流程其实是内核 4.13 合入的优化, 那么我们先来看这个优化合入之前的版本, 接着再结合优化来看, 当前这个版本的实现.
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我们先来看 v4.12 时 [`__rmqueue_fallback`](https://elixir.bootlin.com/linux/v4.12/source/mm/page_alloc.c#L2599) 的实现
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```cpp
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/*
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* Try finding a free buddy page on the fallback list and put it on the free
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* list of requested migratetype, possibly along with other pages from the same
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* block, depending on fragmentation avoidance heuristics. Returns true if
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* fallback was found so that __rmqueue_smallest() can grab it.
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*/
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static inline bool
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__rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
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{
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struct free_area *area;
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unsigned int current_order;
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struct page *page;
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int fallback_mt;
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bool can_steal;
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/* Find the largest possible block of pages in the other list */
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for (current_order = MAX_ORDER-1;
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current_order >= order && current_order <= MAX_ORDER-1;
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--current_order) {
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area = &(zone->free_area[current_order]);
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fallback_mt = find_suitable_fallback(area, current_order,
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start_migratetype, false, &can_steal);
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if (fallback_mt == -1)
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continue;
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page = list_first_entry(&area->free_list[fallback_mt],
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struct page, lru);
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steal_suitable_fallback(zone, page, start_migratetype,
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can_steal);
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trace_mm_page_alloc_extfrag(page, order, current_order,
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start_migratetype, fallback_mt);
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return true;
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}
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return false;
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}
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```
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#### 3.4.4.3 find_suitable_fallback 查找适合被窃取的 MIGRATE_TYPE
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-------
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[`find_suitable_fallback`](https://elixir.bootlin.com/linux/v5.10/source/mm/page_alloc.c#L2599) 遍历当前
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migratetype 的 fallbacks 数组, 为此次 fallback 查找合适 MIGRATE_TYPE 的备选空闲链表.
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```cpp
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// https://elixir.bootlin.com/linux/v5.10/source/mm/page_alloc.c#L2599
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/*
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* Check whether there is a suitable fallback freepage with requested order.
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* If only_stealable is true, this function returns fallback_mt only if
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* we can steal other freepages all together. This would help to reduce
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* fragmentation due to mixed migratetype pages in one pageblock.
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*/
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int find_suitable_fallback(struct free_area *area, unsigned int order,
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int migratetype, bool only_stealable, bool *can_steal)
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{
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int i;
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int fallback_mt;
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if (area->nr_free == 0)
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return -1;
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*can_steal = false;
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for (i = 0;; i++) {
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fallback_mt = fallbacks[migratetype][i];
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if (fallback_mt == MIGRATE_TYPES)
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break;
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if (free_area_empty(area, fallback_mt))
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continue;
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if (can_steal_fallback(order, migratetype))
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*can_steal = true;
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if (!only_stealable)
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return fallback_mt;
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if (*can_steal)
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return fallback_mt;
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}
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return -1;
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}
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```
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find_suitable_fallback 用于找到合适的迁移类型
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1. 根据当前的迁移类型[获取到一个备份的迁移类型](https://elixir.bootlin.com/linux/v5.10/source/mm/page_alloc.c#L2610), 如果[迁移类型 MIGRATE_TYPES](https://elixir.bootlin.com/linux/v5.10/source/mm/page_alloc.c#L2611), 说明已经没有合适的备选 MIGRATE_TYPE 可选, 则 break;
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2. 如果当前的[迁移类型的 freelist 的链表为空](https://elixir.bootlin.com/linux/v5.10/source/mm/page_alloc.c#L2614), 说明备份的迁移类型没有可用的页, 则去下一优先级获取页;
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3. 接着使用 [can_steal_fallback](https://elixir.bootlin.com/linux/v5.10/source/mm/page_alloc.c#L2617) 来判断此迁移类型是否可以作为盗用迁移类型, 如果是返回 true 即可.
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[commit 2149cdaef6c0 ("mm/compaction: enhance compaction finish condition")](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2149cdaef6c0eb59a9edf3b152027392cd66b41f)
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| 时间 | 作者 | 特性 | 描述 | 是否合入主线 | 链接 |
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|:----:|:----:|:---:|:----:|:---------:|:----:|
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| 2015/02/12 | Joonsoo Kim <iamjoonsoo.kim@lge.com> | [mm/compaction: enhance compaction finish condition](https://lore.kernel.org/patchwork/patch/542063) | 同样的, 之前 NULL 指针和错误指针的输出也很混乱, 进行了归一化. | v1 ☑ 4.1-rc1 | [PatchWork](https://lore.kernel.org/patchwork/cover/542063)<br>*-*-*-*-*-*-*-* <br>[关键 commit 2149cdaef6c0](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2149cdaef6c0eb59a9edf3b152027392cd66b41f) |
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#### 3.4.4.4 steal_suitable_fallback 从窃取其他 MIGRATE_TYPE 的页面
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-------
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[`steal_suitable_fallback`](https://elixir.bootlin.com/linux/v5.10/source/mm/page_alloc.c#L2516)
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```cpp
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static void steal_suitable_fallback(struct zone *zone, struct page *page,
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unsigned int alloc_flags, int start_type, bool whole_block)
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```
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[7a8f58f39188 ("mm, page_alloc: fallback to smallest page when not stealing whole pageblock")](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7a8f58f3918869dda0d71b2e9245baedbbe7bc5e), 4.13-rc1 合入
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