【摘要】

本文主要介紹linux系統中，夥伴管理系統是如何處理記憶體頁的.

【正文】夥伴系統基本資訊

1  夥伴系統由來：linux核心支援申請連續的記憶體頁，但由於記憶體碎片化，可能出現空閒記憶體足夠，但連續頁不足的情況,如此引入夥伴系統，在申請和釋放記憶體過程，儘量保持頁的連續性.

2 夥伴系統簡介：linux分別把連續2的0次方頁、1次方頁 ...分別看作一組夥伴，最多有2的MAX_ORDER-1個夥伴組

本文以 MAX_ORDER=11為例進行介紹；

程式碼舉例：

alloc_pages(gfp_mask,order)申請連續的2的order次方頁;

#define alloc_page(gfp_mask,0)
 

對於每個記憶體去struct zone來說(比如ZONE_NORMAL,值得一提的說，很多嵌入式系統只有這一個zone,可以參考

linux記憶體管理之記憶體回收機制 一文), zone->free_area用於描述zone的空閒記憶體資訊，其中：

zone->free_area->nr_free  表示該zone內空閒記憶體數量;

zone->free_area->free_list[MIGRATE_TYPES]  表示相應MIGRATE_TYPES型別的空閒記憶體連結串列，alloc_page過程就是嘗試從不同空閒連結串列上申請記憶體，見後文程式碼分析; 

MIGRATE_TYPES型別

enum {
	MIGRATE_UNMOVABLE,
	MIGRATE_RECLAIMABLE,
	MIGRATE_MOVABLE,
	MIGRATE_PCPTYPES,	/* the number of types on the pcp lists */
	MIGRATE_RESERVE = MIGRATE_PCPTYPES,
#ifdef CONFIG_CMA
	/*
	 * MIGRATE_CMA migration type is designed to mimic the way
	 * ZONE_MOVABLE works.  Only movable pages can be allocated
	 * from MIGRATE_CMA pageblocks and page allocator never
	 * implicitly change migration type of MIGRATE_CMA pageblock.
	 *
	 * The way to use it is to change migratetype of a range of
	 * pageblocks to MIGRATE_CMA which can be done by
	 * __free_pageblock_cma() function.  What is important though
	 * is that a range of pageblocks must be aligned to
	 * MAX_ORDER_NR_PAGES should biggest page be bigger then
	 * a single pageblock.
	 */
	MIGRATE_CMA,
#endif
#ifdef CONFIG_MEMORY_ISOLATION
	MIGRATE_ISOLATE,	/* can't allocate from here */
#endif
	MIGRATE_TYPES
};
 

3  linux系統的夥伴系統資訊查詢：

1> cat /proc/buddyinfo

Node 0, zone Normal  29  17   8   2  5  3  3  1  1  1  1

Node:表示pgdata->node_id=0;Normal表示ZONE_NORMAL 可參考linux記憶體管理之記憶體回收機制一文;

29: 表示zone->free_area[order=0].nr_free;即連續2的0次方頁的個數。依次類推：

1: 最後一個1表示zone->free_area[order=10].nr_free;即連續2的MAX_ORDER-1=10次方頁的個數;

2>通過show_mem(0)介面檢視系統記憶體分配情況. 使用舉例：

oom_killer.c: show_mem(0)記憶體資訊顯示介面,列出系統記憶體規劃;

MIGRATE_UNMOVABLE,MIGRATE_RECLAIMABLE,MIGRATE_MOVABLE等MIGRATE_TYPES型別在show_mem顯示中被簡化為U/R/M.

meminfo.c: cat /proc/meminfo顯示系統記憶體資訊;可以在meminfo的實現里加入show_mem(0);

【正文二】夥伴系統程式碼分析frea_area與free_list

1  zone記憶體區中free_area與free_list.

 如：alloc_page()/alloc_pages()型別的函式從 zone->free_area[order]->free_list[MIGRATE_TYPES] 中申請page;

order:表示申請連續記憶體頁個數，每個order對應一個free_area，每個free_area對應一組free_list連結串列，每個free_list都是連續order頁的連結串列，order取值0到MAX_ORDER-1;

每個free_area上可以有MIGRATE_TYPES個型別的free_list，alloc_page等函式就是從MIGRATE_TYPES型別的free_list上申請記憶體的.   MIGRATE_TYPES見如上定義;

free_area[order].nr_free表示該order對應的free_area上的空閒頁個數;

1) 每個page都對應一個order，該order用於標記這個page屬於哪個zone->frea_area[order];

set_page_order函式介面用於設定：page->private=order;

static inline void set_page_order(struct page *page, unsigned int order)
{
	set_page_private(page, order);
	__SetPageBuddy(page);
}
其中：
#define set_page_private(page, v)	((page)->private = (v))

（1） 釋放一個頁時：__free_one_page()->set_page_order();

（2）申請一個頁時：expand()->set_page_order();

2)任意page都屬於一個夥伴（即zone->free_area[order]）,每個free_ara[order]中包含MIGRATE_TYPES個free_list[MIGRATE_TYPES] (即zone->free_area[order]->free_list[MIGRATE_TYPES]);

設定一個page屬於哪個MIGRATE_TYPES(可以屬於多個MIGRATE_TYPES):

通過set_freepage_migratetype介面設定，通常和get_pfnblock_migratetype同時使用；

get_pfnblock_migratetype和set_pageblock_migratetype函式是一對函式;

static void __free_pages_ok(struct page *page, unsigned int order)
{
	unsigned long flags;
	int migratetype;
	unsigned long pfn = page_to_pfn(page);

        /*先從pageblock獲取migratetype，注意pageblock中的miggratetype是在

           set_pageblock_migratetype中設定的，後文會有涉及*/

	migratetype = get_pfnblock_migratetype(page, pfn);
        /*設定page所屬migratetype型別：page->index=migratetype*/
	set_freepage_migratetype(page, migratetype);
	free_one_page(page_zone(page), page, pfn, order, migratetype);
}

free_hot_cold_page/__rmqueue_fallback/__rmqueue_smallest/move_freepages->set_freepage_migratetype

3) zone_watermark_ok與free_area ;  zone_watermark_ok->__zone_watermark_ok

__zone_watermark_ok判斷空閒記憶體釋放足夠，注意此處針對每個小於order所對應的夥伴進行判斷free_area[order].nr_free

/* free_pages為所有空閒頁個數，通過zone_page_state(zone,NR_FREE_PAGES)獲取 */

static bool __zone_watermark_ok(struct zone *z, unsigned int order,

			unsigned long mark, int classzone_idx, int alloc_flags,
			long free_pages)
{
	/* free_pages my go negative - that's OK; mark=zone->watermark[] */

	long min = mark;
	int o;
	long free_cma = 0;

	free_pages -= (1 << order) - 1;
	if (alloc_flags & ALLOC_HIGH)
		min -= min / 2;
	if (alloc_flags & ALLOC_HARDER)
		min -= min / 4;
#ifdef CONFIG_CMA
	/* If allocation can't use CMA areas don't use free CMA pages */
	if (!(alloc_flags & ALLOC_CMA))
		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
#endif

	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
		return false;

        /*alloc_pages申請連續order頁，此處對每個小於order的frea_area[order]的空閒頁個數進行校驗*/

	for (o = 0; o < order; o++) {
		/* 此處減去order以下所有空閒頁的原因：

                    At the next order, this order's pages become unavailable */

		free_pages -= z->free_area[o].nr_free << o;

		/* Require fewer higher order pages to be free */
		min >>= min_free_order_shift;
                /*此處free_pages為可申請的，即減去o<order,free_area[o].nr_free

                   注意此處是對每個free_area[o<order]進行判斷

              */

		if (free_pages <= min)
			return false;
	}
	return true;
}

【正文三】 free_list的新增和刪除

page_alloc.c中搜索free_list[migrate]，可以確定migrate設定、使用、維護過程。

由上可知，任何一個頁都屬於某個夥伴系統,即zone->free_area[order];

並且struct page結構中儲存了order（page->private=order）

我們在申請或釋放某些頁時，首先要根據連續頁的個數(即order),找到連續order頁所在夥伴系統zone->free_area[order];

然後，每個free_area對應MIGRATE_TYPES個free_list(即zone->free_area[order]->free_list[MIGRATE_TYPES])；

那麼要理解如何從free_list連結串列上申請或者釋放頁，首先要弄明白page是何時掛載到free_list連結串列上的？

申請頁過程，如何找到對應的free_list（每個free_area[order]對應MIGRATE_TYPES個free_list）？

1 free_list[MIGRATE_TYPES]新增過程：一般在釋放page的過程完成.

首先要決定連續order頁掛載到哪個free_list上，即選擇合適的MIGRATE_TYPES,選擇時機如下：

1> 核心啟動過程會設定所有低端頁掛載到free_lists[MIGRATE_MOVABLE]，過程如下：

第一步：set_pageblock_migratetype設定pageblock的migratetype=MIGRATE_MOVABLE;

 注意pageblock_nr_pages為2的order=10次方個連續頁;

setup_arch()->paging_init()->bootm_init()->arm_bootmem_free()->free_area_init_node()->free_area_init_core()

第二步：通過get_pfnblock_migratetype獲取migratetype，並通過set_freepage_migratetype設定page->index,且將page掛載到zone->free_area[order]->free_lists[migratetype]上.

mm_init->mem_init->free_all_bootmem->__free_memory_all->__free_pages_memory->__free_pages->__free_pages_ok

static void __free_pages_ok(struct page *page, unsigned int order)
{
	unsigned long flags;
	int migratetype;
	unsigned long pfn = page_to_pfn(page);

	if (!free_pages_prepare(page, order))
		return;

        /*先獲取pageblock的migratetype;第一步時初始化為migratetype=MIGRATE_MOVABLE;*/
	migratetype = get_pfnblock_migratetype(page, pfn);
	local_irq_save(flags);
	__count_vm_events(PGFREE, 1 << order);

        /*設定page->index=migratetype=MIGRATE_MOVABLE*/

	set_freepage_migratetype(page, migratetype);
	free_one_page(page_zone(page), page, pfn, order, migratetype);
	local_irq_restore(flags);
}

__free_pages_ok-> __free_one_page:把要釋放的page掛到zone->free_area[order]->free_list[migratetype]上,

且釋放後該order對應的free_area[order]上空閒頁++.

static inline void __free_one_page(struct page *page,unsigned long pfn,struct zone *zone, 

                                                        unsigned int order,int migratetype)

{
	unsigned long page_idx;
	unsigned long combined_idx;
	unsigned long uninitialized_var(buddy_idx);
	struct page *buddy;
	int max_order = MAX_ORDER;

	VM_BUG_ON(migratetype == -1);
	if (is_migrate_isolate(migratetype)) {
		max_order = min(MAX_ORDER, pageblock_order + 1);
	} else {
		__mod_zone_freepage_state(zone, 1 << order, migratetype);
	}

	page_idx = pfn & ((1 << max_order) - 1);

	VM_BUG_ON_PAGE(page_idx & ((1 << order) - 1), page);
	VM_BUG_ON_PAGE(bad_range(zone, page), page);

	while (order < max_order - 1) {
		buddy_idx = __find_buddy_index(page_idx, order);
		buddy = page + (buddy_idx - page_idx);
		if (!page_is_buddy(page, buddy, order))
			break;
		/*
		 * Our buddy is free or it is CONFIG_DEBUG_PAGEALLOC guard page,
		 * merge with it and move up one order.
		 */
		if (page_is_guard(buddy)) {
			clear_page_guard_flag(buddy);
			set_page_private(buddy, 0);
			if (!is_migrate_isolate(migratetype)) {
				__mod_zone_freepage_state(zone, 1 << order,
							  migratetype);
			}
		} else {
			list_del(&buddy->lru);
			zone->free_area[order].nr_free--;
			rmv_page_order(buddy);
		}
		combined_idx = buddy_idx & page_idx;
		page = page + (combined_idx - page_idx);
		page_idx = combined_idx;
		order++;
	}
	set_page_order(page, order);

	/*
	 * If this is not the largest possible page, check if the buddy
	 * of the next-highest order is free. If it is, it's possible
	 * that pages are being freed that will coalesce soon. In case,
	 * that is happening, add the free page to the tail of the list
	 * so it's less likely to be used soon and more likely to be merged
	 * as a higher order page
	 */
	if ((order < MAX_ORDER-2) && pfn_valid_within(page_to_pfn(buddy))) {
		struct page *higher_page, *higher_buddy;
		combined_idx = buddy_idx & page_idx;
		higher_page = page + (combined_idx - page_idx);
		buddy_idx = __find_buddy_index(combined_idx, order + 1);
		higher_buddy = higher_page + (buddy_idx - combined_idx);
		if (page_is_buddy(higher_page, higher_buddy, order + 1)) {
			list_add_tail(&page->lru,
				&zone->free_area[order].free_list[migratetype]);
			goto out;
		}
	}
       /*釋放的page掛載到free_list上*/
	list_add(&page->lru, &zone->free_area[order].free_list[migratetype]);
out:  /*釋放後該order對應的free_area[order]上空閒頁++*/
	zone->free_area[order].nr_free++;
}

2> 設定page為MIGRATE_RESERVE：

第一步：set_pageblock_migratetype(page,MIGRATE_RESERVE);設定zone區page的migratetype

init_per_zone_wmark_min->setup_per_zone_wmarks->setup_zone_migrate_reserve->set_pageblock_migratetype:

static void setup_zone_migrate_reserve(struct zone *zone)
{
	unsigned long start_pfn, pfn, end_pfn, block_end_pfn;
	struct page *page;
	unsigned long block_migratetype;
	int reserve;
	int old_reserve;

	/*
	 * Get the start pfn, end pfn and the number of blocks to reserve
	 * We have to be careful to be aligned to pageblock_nr_pages to
	 * make sure that we always check pfn_valid for the first page in
	 * the block.
	 */
	start_pfn = zone->zone_start_pfn;
	end_pfn = zone_end_pfn(zone);
	start_pfn = roundup(start_pfn, pageblock_nr_pages);
	reserve = roundup(min_wmark_pages(zone), pageblock_nr_pages) >>pageblock_order;

	/*
	 * Reserve blocks are generally in place to help high-order atomic
	 * allocations that are short-lived. A min_free_kbytes value that
	 * would result in more than 2 reserve blocks for atomic allocations
	 * is assumed to be in place to help anti-fragmentation for the
	 * future allocation of hugepages at runtime.
	 */
	reserve = min(2, reserve);
	old_reserve = zone->nr_migrate_reserve_block;

	/* When memory hot-add, we almost always need to do nothing */
	if (reserve == old_reserve)
		return;
	zone->nr_migrate_reserve_block = reserve;
        /*遍歷zone區記憶體頁；注意pageblock_nr_pages為2的order=10次方個連續頁*/
	for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
		if (!pfn_valid(pfn))
			continue;
		page = pfn_to_page(pfn);

		/* Watch out for overlapping nodes */
		if (page_to_nid(page) != zone_to_nid(zone))
			continue;
                /*注意上面第1種情況，設定block_migratetype=MIGRATE_MOVABLE*/
		block_migratetype = get_pageblock_migratetype(page);

		/* Only test what is necessary when the reserves are not met */
		if (reserve > 0) {
			/*
			 * Blocks with reserved pages will never free, skip
			 * them.
			 */
			block_end_pfn = min(pfn + pageblock_nr_pages, end_pfn);
			if (pageblock_is_reserved(pfn, block_end_pfn))
				continue;

			/* If this block is reserved, account for it */
			if (block_migratetype == MIGRATE_RESERVE) {
				reserve--;
				continue;
			}

			/* 

                           Suitable for reserving if this block is movable 

                           系統啟動時mm_init初始化pageblock的migratetype為MIGRATE_MOVABLE

                       */

			if (block_migratetype == MIGRATE_MOVABLE) {
				set_pageblock_migratetype(page,
							MIGRATE_RESERVE);

                              /*setup_zone_migrate_reserve->move_freepages:

                                將page掛載到zone->free_area[order].free_list[migratetype]連結串列上

                               注意此處此page所在pageblock的order=10=pageblock_nr_pages；migratetype=MIGRATE_RESERVE

                              */

				move_freepages_block(zone, page,
							MIGRATE_RESERVE);
				reserve--;
				continue;
			}
		} else if (!old_reserve) {
			/*
			 * At boot time we don't need to scan the whole zone
			 * for turning off MIGRATE_RESERVE.
			 */
			break;
		}

		/*
		 * If the reserve is met and this is a previous reserved block,
		 * take it back
		 */
		if (block_migratetype == MIGRATE_RESERVE) {
			set_pageblock_migratetype(page, MIGRATE_MOVABLE);
			move_freepages_block(zone, page, MIGRATE_MOVABLE);
		}
	}
}

init_per_zone_wmark_min->setup_per_zone_wmarks->setup_zone_migrate_reserve->set_pageblock_migratetype->

move_freepages:將page掛載到zone->free_area[order].free_list[migratetype]連結串列上.

int move_freepages(struct zone *zone, struct page *start_page, struct page *end_page, int migratetype)
{
	struct page *page;
	unsigned long order;
	int pages_moved = 0;

	for (page = start_page; page <= end_page;) {
		/* Make sure we are not inadvertently changing nodes */
		VM_BUG_ON_PAGE(page_to_nid(page) != zone_to_nid(zone), page);

		if (!pfn_valid_within(page_to_pfn(page))) {
			page++;
			continue;
		}

		if (!PageBuddy(page)) {
			page++;
			continue;
		}

		order = page_order(page);
              //order=10時；1024*4k=4096KB；migratetype=MIGRATE_RESERVE,此時設定reserve屬性;
		list_move(&page->lru,
			  &zone->free_area[order].free_list[migratetype]);
		set_freepage_migratetype(page, migratetype);
		page += 1 << order;
		pages_moved += 1 << order;
	}

	return pages_moved;
}

2 free_list[MIGRATE_TYPES]刪除過程：一般在申請page的過程完成.

#define alloc_pages(gfp_mask,order)->alloc_pages_node(numa_node_id(),gfp_mask,order)

/*根據nid找到zone[ZONE_NORMAL];根據order找到zone->free_area[order];

根據MIGRATE_TYPES找到對應的free_list,此時__rmqueue_fallback會輪訓migratetype,見後文分析*/

static inline struct page *alloc_pages_node(int nid, gfp_t gfp_mask,
						unsigned int order)
{
	if (nid < 0)
		nid = numa_node_id();

	return __alloc_pages(gfp_mask, order, node_zonelist(nid, gfp_mask));
}

alloc_pages(gfp_mask,order)->get_page_from_freelist()從free_list上申請記憶體page，

free_list上page是掛載時機可以參看上文，該函式會通過zone_watermark_ok判斷zone->free_area[order]上空閒記憶體是否超出wtatermark；如果zone->free_area[order]上nr_free充足，則通過buffered_rmqueue繼續申請page：

注意migratetype是通過:__alloc_pages->__alloc_pages_nodemask()根據gfp_mask指定的,

如GFP_KERNEL對應migratetype=0：

static struct page *
get_page_from_freelist(gfp_t gfp_mask, nodemask_t *nodemask, unsigned int order,
		struct zonelist *zonelist, int high_zoneidx, int alloc_flags,
		struct zone *preferred_zone, int classzone_idx, int migratetype)
{
	struct zoneref *z;
	struct page *page = NULL;
	struct zone *zone;
	nodemask_t *allowednodes = NULL;/* zonelist_cache approximation */
	int zlc_active = 0;		/* set if using zonelist_cache */
	int did_zlc_setup = 0;		/* just call zlc_setup() one time */
	bool consider_zone_dirty = (alloc_flags & ALLOC_WMARK_LOW) &&
				(gfp_mask & __GFP_WRITE);
	int nr_fair_skipped = 0;
	bool zonelist_rescan;

zonelist_scan:
	zonelist_rescan = false;
	/*
	 * Scan zonelist, looking for a zone with enough free.
	 * See also __cpuset_node_allowed_softwall() comment in kernel/cpuset.c.
	 */
	for_each_zone_zonelist_nodemask(zone, z, zonelist, high_zoneidx, nodemask) {
		unsigned long mark;

		if (IS_ENABLED(CONFIG_NUMA) && zlc_active &&
			!zlc_zone_worth_trying(zonelist, z, allowednodes))
				continue;
		if (cpusets_enabled() &&
			(alloc_flags & ALLOC_CPUSET) &&
			!cpuset_zone_allowed_softwall(zone, gfp_mask))
				continue;
		/*
		 * Distribute pages in proportion to the individual
		 * zone size to ensure fair page aging.  The zone a
		 * page was allocated in should have no effect on the
		 * time the page has in memory before being reclaimed.
		 */
		if (alloc_flags & ALLOC_FAIR) {
			if (!zone_local(preferred_zone, zone))
				break;
			if (test_bit(ZONE_FAIR_DEPLETED, &zone->flags)) {
				nr_fair_skipped++;
				continue;
			}
		}
		
		if (consider_zone_dirty && !zone_dirty_ok(zone))
			continue;

		mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
		if (!zone_watermark_ok(zone, order, mark,
				       classzone_idx, alloc_flags)) {
			int ret;

			/* Checked here to keep the fast path fast */
			BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
			if (alloc_flags & ALLOC_NO_WATERMARKS)
				goto try_this_zone;

			if (IS_ENABLED(CONFIG_NUMA) &&
					!did_zlc_setup && nr_online_nodes > 1) {
				/*
				 * we do zlc_setup if there are multiple nodes
				 * and before considering the first zone allowed
				 * by the cpuset.
				 */
				allowednodes = zlc_setup(zonelist, alloc_flags);
				zlc_active = 1;
				did_zlc_setup = 1;
			}

			if (zone_reclaim_mode == 0 ||
			    !zone_allows_reclaim(preferred_zone, zone))
				goto this_zone_full;

			/*
			 * As we may have just activated ZLC, check if the first
			 * eligible zone has failed zone_reclaim recently.
			 */
			if (IS_ENABLED(CONFIG_NUMA) && zlc_active &&
				!zlc_zone_worth_trying(zonelist, z, allowednodes))
				continue;

			ret = zone_reclaim(zone, gfp_mask, order);
			switch (ret) {
			case ZONE_RECLAIM_NOSCAN:
				/* did not scan */
				continue;
			case ZONE_RECLAIM_FULL:
				/* scanned but unreclaimable */
				continue;
			default:
				/* did we reclaim enough */
				if (zone_watermark_ok(zone, order, mark,
						classzone_idx, alloc_flags))
					goto try_this_zone;

				if (((alloc_flags & ALLOC_WMARK_MASK) == ALLOC_WMARK_MIN) ||
				    ret == ZONE_RECLAIM_SOME)
					goto this_zone_full;

				continue;
			}
		}
try_this_zone:
		page = buffered_rmqueue(preferred_zone, zone, order,
						gfp_mask, migratetype);
		if (page)
			break;
this_zone_full:
		if (IS_ENABLED(CONFIG_NUMA) && zlc_active)
			zlc_mark_zone_full(zonelist, z);
	}

	if (page) {
		/*
		 * page->pfmemalloc is set when ALLOC_NO_WATERMARKS was
		 * necessary to allocate the page. The expectation is
		 * that the caller is taking steps that will free more
		 * memory. The caller should avoid the page being used
		 * for !PFMEMALLOC purposes.
		 */
		page->pfmemalloc = !!(alloc_flags & ALLOC_NO_WATERMARKS);
		return page;
	}
	/*
	 * The first pass makes sure allocations are spread fairly within the
	 * local node.  However, the local node might have free pages left
	 * after the fairness batches are exhausted, and remote zones haven't
	 * even been considered yet.  Try once more without fairness, and
	 * include remote zones now, before entering the slowpath and waking
	 * kswapd: prefer spilling to a remote zone over swapping locally.
	 */
	if (alloc_flags & ALLOC_FAIR) {
		alloc_flags &= ~ALLOC_FAIR;
		if (nr_fair_skipped) {
			zonelist_rescan = true;
			reset_alloc_batches(preferred_zone);
		}
		if (nr_online_nodes > 1)
			zonelist_rescan = true;
	}

	if (unlikely(IS_ENABLED(CONFIG_NUMA) && zlc_active)) {
		/* Disable zlc cache for second zonelist scan */
		zlc_active = 0;
		zonelist_rescan = true;
	}

	if (zonelist_rescan)
		goto zonelist_scan;

	return NULL;
}

3 夥伴系統處理

alloc_pages(gfp_mask,order)->get_page_from_freelist()->buffered_rmqueue()

static inline
struct page *buffered_rmqueue(struct zone *preferred_zone,
			struct zone *zone, unsigned int order,
			gfp_t gfp_flags, int migratetype)
{
	unsigned long flags;
	struct page *page;
	bool cold = ((gfp_flags & __GFP_COLD) != 0);

again://只申請1個頁
	if (likely(order == 0)) {
		struct per_cpu_pages *pcp;
		struct list_head *list;

		local_irq_save(flags);
		pcp = &this_cpu_ptr(zone->pageset)->pcp;
               /*先在pcp->lists上查詢有沒有對應型別的空閒頁*/
		list = &pcp->lists[migratetype];

               /*如果pcp->lists上沒有對應型別的空閒頁，則從夥伴系統中查詢，並掛載的pcp->list上面*/
		if (list_empty(list)) {
                       /*rmqueue_bulk將申請到的page->list掛載到pcp->lists[migratetype]上*/
			pcp->count += rmqueue_bulk(zone, 0,
					pcp->batch, list,
					migratetype, cold);
			if (unlikely(list_empty(list)))
				goto failed;
		}

		if (cold)
			page = list_entry(list->prev, struct page, lru);
		else
			page = list_entry(list->next, struct page, lru);
               /*從pcp->lists[migratetype]刪除page->list，因為該頁被申請了*/
		list_del(&page->lru);
		pcp->count--;
	} else { //直接申請連續多頁,沒有從pcp->list連結串列上申請
		if (unlikely(gfp_flags & __GFP_NOFAIL)) {
			/*
			 * __GFP_NOFAIL is not to be used in new code.
			 *
			 * All __GFP_NOFAIL callers should be fixed so that they
			 * properly detect and handle allocation failures.
			 *
			 * We most definitely don't want callers attempting to
			 * allocate greater than order-1 page units with
			 * __GFP_NOFAIL.
			 */
			WARN_ON_ONCE(order > 1);
		}
		spin_lock_irqsave(&zone->lock, flags);
		page = __rmqueue(zone, order, migratetype);
		spin_unlock(&zone->lock);
		if (!page)
			goto failed;
		__mod_zone_freepage_state(zone, -(1 << order),
					  get_freepage_migratetype(page));
	}

	__mod_zone_page_state(zone, NR_ALLOC_BATCH, -(1 << order));
	if (atomic_long_read(&zone->vm_stat[NR_ALLOC_BATCH]) <= 0 &&
	    !test_bit(ZONE_FAIR_DEPLETED, &zone->flags))
		set_bit(ZONE_FAIR_DEPLETED, &zone->flags);

	__count_zone_vm_events(PGALLOC, zone, 1 << order);
	zone_statistics(preferred_zone, zone, gfp_flags);
	local_irq_restore(flags);

	VM_BUG_ON_PAGE(bad_range(zone, page), page);
	if (prep_new_page(page, order, gfp_flags))
		goto again;
	return page;

failed:
	local_irq_restore(flags);
	return NULL;
}

無論申請連續一頁還是多頁都會呼叫到buffered_rmqueue()->__rmqueue():

static struct page *__rmqueue(struct zone *zone, unsigned int order,
						int migratetype)
{
	struct page *page;

retry_reserve:
        /*
         第一次執行到此：

         系統最開始將page掛載到MIGRATE_MOVABLE和MIGRATETYPE_RESERVE對應的free_list[migratetype]上；

         alloc_pages()過程，當gfp_mask=GFP_KERNEL時，migratetype=0對應的free_list[migratetype]上沒有page，

         所以此處返回空,繼續向下分析程式碼*/

	page = __rmqueue_smallest(zone, order, migratetype);
       /*第一次執行到此：page為空migratetype != MIGRATE_RESERVE，

          注意此時所有page都在MIGRATE_MOVABLE和MIGRATETYPE_RESERVE對應的free_list[migratetype]*/

	if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
		page = __rmqueue_fallback(zone, order, migratetype);
		/*
		 * Use MIGRATE_RESERVE rather than fail an allocation. goto
		 * is used because __rmqueue_smallest is an inline function
		 * and we want just one call site
		 */
		if (!page) {
			migratetype = MIGRATE_RESERVE;
			goto retry_reserve;
		}
	}

	trace_mm_page_alloc_zone_locked(page, order, migratetype);
	return page;
}

static inline struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
						 int migratetype)
{
	unsigned int current_order;
	struct free_area *area;
	struct page *page;

	/* Find a page of the appropriate size in the preferred list 
            遍歷所有大於等於要申請order的free_area[order]區；
        */
	for (current_order = order; current_order < MAX_ORDER; ++current_order) {
		area = &(zone->free_area[current_order]);
		if (list_empty(&area->free_list[migratetype]))
			continue;
                /*如果free_list上有page則申請*/
		page = list_entry(area->free_list[migratetype].next,
							struct page, lru);
		list_del(&page->lru);
		rmv_page_order(page);
		area->nr_free--;
		expand(zone, page, order, current_order, area, migratetype);
		set_freepage_migratetype(page, migratetype);
		return page;
	}
	return NULL;
}

buffered_rmqueue()->__rmqueue()->__rmqueue_fallback中遍歷fallbacks:fallbacks指明瞭free_list[migratetype]連結串列的申請順序，如下：zone->free_area[order]->free_list[migratetype]；

static int fallbacks[MIGRATE_TYPES][4] = {
	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,     MIGRATE_RESERVE },
	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,     MIGRATE_RESERVE },
#ifdef CONFIG_CMA
	[MIGRATE_MOVABLE]     = { MIGRATE_CMA,         MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_RESERVE },
	[MIGRATE_CMA]         = { MIGRATE_RESERVE }, /* Never used */
#else
	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE,   MIGRATE_RESERVE },
#endif
	[MIGRATE_RESERVE]     = { MIGRATE_RESERVE }, /* Never used */
#ifdef CONFIG_MEMORY_ISOLATION
	[MIGRATE_ISOLATE]     = { MIGRATE_RESERVE }, /* Never used */
#endif
};

buffered_rmqueue()->__rmqueue()->__rmqueue_fallback:

static inline struct page *__rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
{
	struct free_area *area;
	unsigned int current_order;
	struct page *page;
	int migratetype, new_type, i;

	/* Find the largest possible block of pages in the other list */
	for (current_order = MAX_ORDER-1;
				current_order >= order && current_order <= MAX_ORDER-1;
				--current_order) {
		for (i = 0;; i++) {
			migratetype = fallbacks[start_migratetype][i];

			/* MIGRATE_RESERVE handled later if necessary */
			if (migratetype == MIGRATE_RESERVE)
				break;

			area = &(zone->free_area[current_order]);
			if (list_empty(&area->free_list[migratetype]))
				continue;

			page = list_entry(area->free_list[migratetype].next,
					struct page, lru);
			area->nr_free--;

			new_type = try_to_steal_freepages(zone, page,
							  start_migratetype,
							  migratetype);

			/* Remove the page from the freelists */
			list_del(&page->lru);
			rmv_page_order(page);

			expand(zone, page, order, current_order, area,
			       new_type);
			/* The freepage_migratetype may differ from pageblock's
			 * migratetype depending on the decisions in
			 * try_to_steal_freepages. This is OK as long as it does
			 * not differ for MIGRATE_CMA type.
			 */
			set_freepage_migratetype(page, new_type);

			trace_mm_page_alloc_extfrag(page, order, current_order,
				start_migratetype, migratetype);

			return page;
		}
	}

	return NULL;
}

【總結】

本文簡要介紹了夥伴系統的維護方法，需要注意__rmqueue()/zone_watermark_ok()重點函式的分析,同時要釐清

zone->free_area[order]->free_list[migratetype] 各項的含義；一般zone指ZONE_NORMAL區。

1>zone的獲取：通過遍歷zone_list，zone_list通過node_zonelist()獲取

/*對應node_id=numa_node_id();其中ZONE_NORMAL對應nid=0*/

static inline struct zonelist *node_zonelist(int nid, gfp_t flags)
{
return NODE_DATA(nid)->node_zonelists + gfp_zonelist(flags);

}

2>free_area[order]:order表示連續2的order次方的連續頁，其中初始化過程會是否所有野到free_area[order=10]空閒區的free_list連結串列，其他釋放過程同樣是否page到對應order的free_area[order]空閒區的free_list連結串列上;

3>free_area[order]->free_list[migratetype]:初始化過程會掛載page頁到free_list[migratetype];migratetype=MIGRATE_MOVABLE和MIGRATE_RESERVE，此處注意page同時掛載到了兩個free_list上。申請過程中從哪個free_list[migratetype]上申請，優先順序取決於 fallbacks[MIGRATE_TYPES][4]定義：可以參考上文程式碼.