[系统运维]Linux内存子系统——分配物理页面（alloc_pages）

分配页面

??alloc_pages函数有两个参数，一个是分配掩码gfp_mask，另一个是分配阶数order。
[include/linux/gfp.h]

#define alloc_pages(gfp_mask, order) \
		alloc_pages_node(numa_node_id(), gfp_mask, order)
#define alloc_pages_vma(gfp_mask, order, vma, addr, node, false)\
	alloc_pages(gfp_mask, order)
#define alloc_hugepage_vma(gfp_mask, vma, addr, order)	\
	alloc_pages(gfp_mask, order)

??分配掩码，和kmalloc使用的是同一个掩码，同样定义在gfp.h中

/* Plain integer GFP bitmasks. Do not use this directly. */
#define ___GFP_DMA		0x01u
#define ___GFP_HIGHMEM		0x02u
#define ___GFP_DMA32		0x04u
#define ___GFP_MOVABLE		0x08u
#define ___GFP_WAIT		0x10u
#define ___GFP_HIGH		0x20u
#define ___GFP_IO		0x40u
#define ___GFP_FS		0x80u
#define ___GFP_COLD		0x100u
#define ___GFP_NOWARN		0x200u
#define ___GFP_REPEAT		0x400u
#define ___GFP_NOFAIL		0x800u
#define ___GFP_NORETRY		0x1000u
#define ___GFP_MEMALLOC		0x2000u
#define ___GFP_COMP		0x4000u
#define ___GFP_ZERO		0x8000u
#define ___GFP_NOMEMALLOC	0x10000u
#define ___GFP_HARDWALL		0x20000u
#define ___GFP_THISNODE		0x40000u
#define ___GFP_RECLAIMABLE	0x80000u
#define ___GFP_NOTRACK		0x200000u
#define ___GFP_NO_KSWAPD	0x400000u
#define ___GFP_OTHER_NODE	0x800000u
#define ___GFP_WRITE		0x1000000u

??分配掩码在内核中分成两类，一类叫zone modifiers，另一类叫action modifiers，zone指的是从哪个zone中分配所需要的页面。zone modifiers由分配掩码的最低4位组成。

/*
 * GFP bitmasks..
 *
 * Zone modifiers (see linux/mmzone.h - low three bits)
 *
 * Do not put any conditional on these. If necessary modify the definitions
 * without the underscores and use them consistently. The definitions here may
 * be used in bit comparisons.
 */
#define __GFP_DMA	((__force gfp_t)___GFP_DMA)
#define __GFP_HIGHMEM	((__force gfp_t)___GFP_HIGHMEM)
#define __GFP_DMA32	((__force gfp_t)___GFP_DMA32)
#define __GFP_MOVABLE	((__force gfp_t)___GFP_MOVABLE)  /* Page is movable */
#define GFP_ZONEMASK	(__GFP_DMA|__GFP_HIGHMEM|__GFP_DMA32|__GFP_MOVABLE)

??action modifiers和分配行为有关，其定义如下：

/*
 * Action modifiers - doesn't change the zoning
 *
 * __GFP_REPEAT: Try hard to allocate the memory, but the allocation attempt
 * _might_ fail.  This depends upon the particular VM implementation.
 *
 * __GFP_NOFAIL: The VM implementation _must_ retry infinitely: the caller
 * cannot handle allocation failures.  This modifier is deprecated and no new
 * users should be added.
 *
 * __GFP_NORETRY: The VM implementation must not retry indefinitely.
 *
 * __GFP_MOVABLE: Flag that this page will be movable by the page migration
 * mechanism or reclaimed
 */
#define __GFP_WAIT	((__force gfp_t)___GFP_WAIT)	/* Can wait and reschedule? */
#define __GFP_HIGH	((__force gfp_t)___GFP_HIGH)	/* Should access emergency pools? */
#define __GFP_IO	((__force gfp_t)___GFP_IO)	/* Can start physical IO? */
#define __GFP_FS	((__force gfp_t)___GFP_FS)	/* Can call down to low-level FS? */
#define __GFP_COLD	((__force gfp_t)___GFP_COLD)	/* Cache-cold page required */
#define __GFP_NOWARN	((__force gfp_t)___GFP_NOWARN)	/* Suppress page allocation failure warning */
#define __GFP_REPEAT	((__force gfp_t)___GFP_REPEAT)	/* See above */
#define __GFP_NOFAIL	((__force gfp_t)___GFP_NOFAIL)	/* See above */
#define __GFP_NORETRY	((__force gfp_t)___GFP_NORETRY) /* See above */
#define __GFP_MEMALLOC	((__force gfp_t)___GFP_MEMALLOC)/* Allow access to emergency reserves */
#define __GFP_COMP	((__force gfp_t)___GFP_COMP)	/* Add compound page metadata */
#define __GFP_ZERO	((__force gfp_t)___GFP_ZERO)	/* Return zeroed page on success */
#define __GFP_NOMEMALLOC ((__force gfp_t)___GFP_NOMEMALLOC) /* Don't use emergency reserves.
							 * This takes precedence over the
							 * __GFP_MEMALLOC flag if both are
							 * set
							 */
#define __GFP_HARDWALL   ((__force gfp_t)___GFP_HARDWALL) /* Enforce hardwall cpuset memory allocs */
#define __GFP_THISNODE	((__force gfp_t)___GFP_THISNODE)/* No fallback, no policies */
#define __GFP_RECLAIMABLE ((__force gfp_t)___GFP_RECLAIMABLE) /* Page is reclaimable */
#define __GFP_NOTRACK	((__force gfp_t)___GFP_NOTRACK)  /* Don't track with kmemcheck */

#define __GFP_NO_KSWAPD	((__force gfp_t)___GFP_NO_KSWAPD)
#define __GFP_OTHER_NODE ((__force gfp_t)___GFP_OTHER_NODE) /* On behalf of other node */
#define __GFP_WRITE	((__force gfp_t)___GFP_WRITE)	/* Allocator intends to dirty page */

??这些分配掩码看着就令人头大，不过正常情况下，不用特别关心这些掩码，正如kmalloc时，基本都直接填GFP_KERNEL。直接使用的各类掩码定义如下：

/* This equals 0, but use constants in case they ever change */
#define GFP_NOWAIT	(GFP_ATOMIC & ~__GFP_HIGH)
/* GFP_ATOMIC means both !wait (__GFP_WAIT not set) and use emergency pool */
#define GFP_ATOMIC	(__GFP_HIGH)
#define GFP_NOIO	(__GFP_WAIT)
#define GFP_NOFS	(__GFP_WAIT | __GFP_IO)
#define GFP_KERNEL	(__GFP_WAIT | __GFP_IO | __GFP_FS)
#define GFP_TEMPORARY	(__GFP_WAIT | __GFP_IO | __GFP_FS | \
			 __GFP_RECLAIMABLE)
#define GFP_USER	(__GFP_WAIT | __GFP_IO | __GFP_FS | __GFP_HARDWALL)
#define GFP_HIGHUSER	(GFP_USER | __GFP_HIGHMEM)
#define GFP_HIGHUSER_MOVABLE	(GFP_HIGHUSER | __GFP_MOVABLE)
#define GFP_IOFS	(__GFP_IO | __GFP_FS)
#define GFP_TRANSHUGE	(GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
			 __GFP_NOMEMALLOC | __GFP_NORETRY | __GFP_NOWARN | \
			 __GFP_NO_KSWAPD)

??alloc_pages调用关系如下：

alloc_pages(gfp_mask, order)
	alloc_pages_node(numa_node_id(), gfp_mask, order)
		__alloc_pages(gfp_mask, order, node_zonelist(nid, gfp_mask));
			__alloc_pages_nodemask(gfp_mask, order, zonelist, NULL);

/*
 * This is the 'heart' of the zoned buddy allocator.
 */
struct page *
__alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
			struct zonelist *zonelist, nodemask_t *nodemask)
{
	struct zoneref *preferred_zoneref;
	struct page *page = NULL;
	unsigned int cpuset_mems_cookie;
	int alloc_flags = ALLOC_WMARK_LOW|ALLOC_CPUSET|ALLOC_FAIR;
	gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
	struct alloc_context ac = {												(1)
		.high_zoneidx = gfp_zone(gfp_mask),
		.nodemask = nodemask,
		.migratetype = ``(gfp_mask),					(2)
	};
	......
}

1）struct alloc_context数据结构是伙伴系统分配函数中用于保存相关参数的数据结构。gfp_zone函数用于从分配掩码中计算出zone的zoneidx，并存放在high_zoneidx成员中。

static inline enum zone_type gfp_zone(gfp_t flags)
{
	enum zone_type z;
	int bit = (__force int) (flags & GFP_ZONEMASK);

	z = (GFP_ZONE_TABLE >> (bit * ZONES_SHIFT)) &
					 ((1 << ZONES_SHIFT) - 1);
	VM_BUG_ON((GFP_ZONE_BAD >> bit) & 1);
	return z;
}

??GFP_KERNEL带入到gfp_zone接口中，则最终结果为0，即high_zoneidx为0。
2）gfpflags_to_migratetype把gfp_mask分配掩码转换成MIGRATE_TYPES，例如分配掩码为GFP_KERNEL，那么MIGRATE_TYPES类型是MIGRATE_UNMOVABLE；如果分配掩码为GFP_HIGHUSER_MOVABLE，那么MIGRATE_TYPES类型是MIGRATE_MOVABLE。

/* Convert GFP flags to their corresponding migrate type */
static inline int gfpflags_to_migratetype(const gfp_t gfp_flags)
{
	WARN_ON((gfp_flags & GFP_MOVABLE_MASK) == GFP_MOVABLE_MASK);

	if (unlikely(page_group_by_mobility_disabled))
		return MIGRATE_UNMOVABLE;

	/* Group based on mobility */
	return (((gfp_flags & __GFP_MOVABLE) != 0) << 1) |
		((gfp_flags & __GFP_RECLAIMABLE) != 0);
}

??继续回到__alloc_pages_nodemask中

/*
 * This is the 'heart' of the zoned buddy allocator.
 */
struct page *
__alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
			struct zonelist *zonelist, nodemask_t *nodemask)
{
	struct zoneref *preferred_zoneref;
	......
retry_cpuset:
	cpuset_mems_cookie = read_mems_allowed_begin();

	/* We set it here, as __alloc_pages_slowpath might have changed it */
	ac.zonelist = zonelist;
	/* The preferred zone is used for statistics later */
	preferred_zoneref = first_zones_zonelist(ac.zonelist, ac.high_zoneidx,
				ac.nodemask ? : &cpuset_current_mems_allowed,
				&ac.preferred_zone);
	if (!ac.preferred_zone)
		goto out;
	ac.classzone_idx = zonelist_zone_idx(preferred_zoneref);

	/* First allocation attempt */
	alloc_mask = gfp_mask|__GFP_HARDWALL;
	page = get_page_from_freelist(alloc_mask, order, alloc_flags, &ac);	
	if (unlikely(!page)) {
		/*
		 * Runtime PM, block IO and its error handling path
		 * can deadlock because I/O on the device might not
		 * complete.
		 */
		alloc_mask = memalloc_noio_flags(gfp_mask);

		page = __alloc_pages_slowpath(alloc_mask, order, &ac);
	}

	if (kmemcheck_enabled && page)
		kmemcheck_pagealloc_alloc(page, order, gfp_mask);

	trace_mm_page_alloc(page, order, alloc_mask, ac.migratetype);

out:
	/*
	 * When updating a task's mems_allowed, it is possible to race with
	 * parallel threads in such a way that an allocation can fail while
	 * the mask is being updated. If a page allocation is about to fail,
	 * check if the cpuset changed during allocation and if so, retry.
	 */
	if (unlikely(!page && read_mems_allowed_retry(cpuset_mems_cookie)))
		goto retry_cpuset;

	return page;
}

??首先get_page_from_freelist会去尝试分配物理页面，如果这里分配失败，就会调用到__alloc_pages_slowpath这个函数。本文主要介绍前者。

/*
 * get_page_from_freelist goes through the zonelist trying to allocate
 * a page.
 */
static struct page *
get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
						const struct alloc_context *ac)
{
	struct zonelist *zonelist = ac->zonelist;
	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() comment in kernel/cpuset.c.
	 */
	for_each_zone_zonelist_nodemask(zone, z, zonelist, ac->high_zoneidx,
								ac->nodemask) {

??函数首先需要判断可以从哪个zone来分配内存。for_each_zone_zonelist_nodemask宏扫描内存节点中的zonelist去查找合适分配内存的zone。

#define for_each_zone_zonelist_nodemask(zone, z, zlist, highidx, nodemask) \
	for (z = first_zones_zonelist(zlist, highidx, nodemask, &zone);	\
		zone;							\
		z = next_zones_zonelist(++z, highidx, nodemask),	\
			zone = zonelist_zone(z))	

??for_each_zone_zonelist_nodemask首先通过first_zones_zonelist从给定的zoneidx开始查找，这个给定的zoneidx就是highidx，之前通过gfp_zone函数转换得来的。

static inline struct zoneref *first_zones_zonelist(struct zonelist *zonelist,
					enum zone_type highest_zoneidx,
					nodemask_t *nodes,
					struct zone **zone)
{
	struct zoneref *z = next_zones_zonelist(zonelist->_zonerefs,
							highest_zoneidx, nodes);
	*zone = zonelist_zone(z);
	return z;
}

??first_zones_zonelist会调用next_zones_zonelist来计算zoneref，最后返回zone数据结构。

/* Returns the next zone at or below highest_zoneidx in a zonelist */
struct zoneref *next_zones_zonelist(struct zoneref *z,
					enum zone_type highest_zoneidx,
					nodemask_t *nodes)
{
	/*
	 * Find the next suitable zone to use for the allocation.
	 * Only filter based on nodemask if it's set
	 */
	if (likely(nodes == NULL))
		while (zonelist_zone_idx(z) > highest_zoneidx)
			z++;
	else
		while (zonelist_zone_idx(z) > highest_zoneidx ||
				(z->zone && !zref_in_nodemask(z, nodes)))
			z++;

	return z;
}

??计算zone的核心函数在next_zones_zonelist函数中，这里highest_zoneidx是gfp_zone函数计算分配掩码得来。zonelist有一个zoneref数组，zoneref数据结构里有一个成员zone指针会指向zone数据结构，还有一个zone_index成员指向zone的编号。zone在系统处理时会初始化这个数组。具体函数在build_zonelists_node中。在不考虑DMA情况下，zone类型、zoneref[]数组和zoneidx的关系如下

ZONE_HIGHMEM		_zonerefs[0]->zone_index=1
ZONE_NORMAL			_zonerefs[1]->zone_index=0

??zonerefs[0]表示ZONE_HIGHMEM，其zone的编号zone_index值为1；zonerefs[1]表示ZONE_NORMAL，其zone的编号zone_index为0。也就是说，内存分配时会优先考虑ZONE_HIGHMEM，因为ZONE_HIGHMEM在zonelist中排在ZONE_NORMAL前面。但是也要考虑分配掩码。
??当GFP_KERNEL时，gfp_zone(GFP_KERNEL)函数返回0，即highest_zoneidx为0，而这个内存节点的第一个zone是ZONE_HIGHMEM，其zone编号zone_index的值为1。因此在next_zone_zonelist中，z++，最终first_zones_zonelist函数会返回ZONE_NORMAL。在for_each_zone_zonelist_nodemask遍历中也只能遍历ZONE_NORMAL这一个zone了。
??当GFP_HIGHUSER_MOVABLE分配掩码，GFP_HIGHUSER_MOVABLE包含了__GFP_HIGHMEM，那么next_zone_zonelist函数。因为gfp_zone(GFP_HIGHUSER_MOVABLE0)函数等于2，即highest_zoneidx为2。而这个内存节点的第一个ZONE_HIGHMEM，其zone编号zone_index的值为1。所以返回第一个zone是HIGHMEM，其zone编号zone_index为1。
??随后会从HIGHMEM zone中尝试分配内存，大概不满足条件时，会执行for_each_zone_zonelist_nodemask查找下一个zone，即NORMAL zone。

要正确理解for_each_zone_zonelist_nodemask，需要理解以下两点。

• highest_zoneidx是如何计算来的，即如何解析分配掩码，这是gfp_zone函数的职责。
• 每个内存节点有一个struct pglist_data数据结构，对于非numa结构，只有一个内存节点。其成员node_zonelists是一个struct zonelist数据结构，zonelist中包含了struct zoneref _zonerefs[]数组来描述这些zone，其中ZONE_HIGHMEM排在前面，其_zonerefs[0]->zone_index=1，ZONE_NORMAL排在后面，且_zonerefs[1]->zone_index=0。

??__alloc_pages_nodemask中调用first_zones_zonelist，计算出preferred_zoneref并保存到ac.classzone_idx变量中，该变量在kswapd内核线程中还会用到。
??回到get_page_from_freelist函数，for_each_zone_zonelist_nodemask找到了用于分配的zone，下面开始做一下分配内存的准备。
get_page_from_freelist

		......
		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(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(ac->preferred_zone, zone))
				break;
			if (test_bit(ZONE_FAIR_DEPLETED, &zone->flags)) {
				nr_fair_skipped++;
				continue;
			}
		}

??然后检查zone的watermark水位是否充足。
get_page_from_freelist

		......
		mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
		if (!zone_watermark_ok(zone, order, mark,
				       ac->classzone_idx, alloc_flags)) {
			......
			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,
						ac->classzone_idx, alloc_flags))
					goto try_this_zone;

				/*
				 * Failed to reclaim enough to meet watermark.
				 * Only mark the zone full if checking the min
				 * watermark or if we failed to reclaim just
				 * 1<<order pages or else the page allocator
				 * fastpath will prematurely mark zones full
				 * when the watermark is between the low and
				 * min watermarks.
				 */
				if (((alloc_flags & ALLOC_WMARK_MASK) == ALLOC_WMARK_MIN) ||
				    ret == ZONE_RECLAIM_SOME)
					goto this_zone_full;

				continue;
			}
		}
try_this_zone:
		page = buffered_rmqueue(ac->preferred_zone, zone, order,
						gfp_mask, ac->migratetype);
		if (page) {
			if (prep_new_page(page, order, gfp_mask, alloc_flags))
				goto try_this_zone;
			return page;
		}

??zone数据结构中有一个成员watermark记录各种水位的情况。系统中定义了3中水位，分别是WMARK_MIN、WMARK_LOW和WMARK_HIGH。watermark水位计算在__setup_per_zone_wmarks()函数中。
??zone_watermark_ok会判断当前zone的空间空间是否满足WMARK_LOW水位。返回true代表空闲页面在某个水位之上，否则返回false。

??当判断当前zone的空闲页面低于WMARK_LOW水位，会调用zone_reclaim函数来回收页面。我们这里假设zone_watermark_ok判断页面充沛。就会调用buffered_rmqueue函数从伙伴系统中分配物理页面。
__alloc_pages_nodemask->get_page_from_freelist->buffered_rmqueue

/*
 * Allocate a page from the given zone. Use pcplists for order-0 allocations.
 */
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);

	if (likely(order == 0)) {
		struct per_cpu_pages *pcp;
		struct list_head *list;

		local_irq_save(flags);
		pcp = &this_cpu_ptr(zone->pageset)->pcp;
		list = &pcp->lists[migratetype];
		if (list_empty(list)) {
			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);

		list_del(&page->lru);
		pcp->count--;
	} else {
		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);
	return page;

failed:
	local_irq_restore(flags);
	return NULL;
}

??order等于0的情况和其他区别对待，只分配一个物理页面时，从zone->per_cpu_pageset列表中分配；另一路order大于0的情况，就从伙伴系统中分配。order大于0的情况，最终会调用__rmqueue_smallest函数
__alloc_pages_nodemask->get_page_from_freelist->buffered_rmqueue->__rmqueue->__rmqueue_smallest

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 */
	for (current_order = order; current_order < MAX_ORDER; ++current_order) {
		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);
		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;
}

??在__rmqueue_smallest函数中，首先从order开始查找zone中空闲链表。如果zone的当前order对应的空闲区free_area中相应migratetype类型的链表里没有空闲对象，那么就会查找下一级order。
??系统启动时，memblock后期，会释放空闲页面到buddy子系统中，空闲页面会尽可能的分配到MAX_ORDER-1的链表中，这个可以在文件系统中，通过cat /proc/pagetypeinfo命令看出端倪。当找到某一个order空闲区中对应的migratetype类型的空闲链表中有空闲内存块时，就会从中把一个内存块摘下来，然后调用expand函数来“切蛋糕”。因为通常摘下来的内存块要比需要的内存大，切完之后需要把剩下的内存块重新放进伙伴系统中。
??expadn函数就是实现切蛋糕功能，这里参数high就是current_order，通常current_order要比需求的order大。每比较一次，area减1，相当于退了一级order，最后通过list_add把剩下的内存块添加进低一级的空闲链表中。
__alloc_pages_nodemask->get_page_from_freelist->buffered_rmqueue->__rmqueue->__rmqueue_smallest->expand

static inline void expand(struct zone *zone, struct page *page,
	int low, int high, struct free_area *area,
	int migratetype)
{
	unsigned long size = 1 << high;

	while (high > low) {
		area--;
		high--;
		size >>= 1;
		VM_BUG_ON_PAGE(bad_range(zone, &page[size]), &page[size]);

		if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC) &&
			debug_guardpage_enabled() &&
			high < debug_guardpage_minorder()) {
			/*
			 * Mark as guard pages (or page), that will allow to
			 * merge back to allocator when buddy will be freed.
			 * Corresponding page table entries will not be touched,
			 * pages will stay not present in virtual address space
			 */
			set_page_guard(zone, &page[size], high, migratetype);
			continue;
		}
		list_add(&page[size].lru, &area->free_list[migratetype]);
		area->nr_free++;
		set_page_order(&page[size], high);
	}
}

??所需要的页面分配成功后，__rmqueue函数返回这个内存块的起始页面的struct page数据结构。回到buffered_rmqueue函数，最后还需要利用zone_statistics函数做一些统计数据的计算。
??回到get_page_from_freelist中，最后还要通过prep_new_page()函数做一些检查。
__alloc_pages_nodemask->get_page_from_freelist->prep_new_page

static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
								int alloc_flags)
{
	int i;

	for (i = 0; i < (1 << order); i++) {
		struct page *p = page + i;
		if (unlikely(check_new_page(p)))
			return 1;
	}

	set_page_private(page, 0);
	set_page_refcounted(page);

	arch_alloc_page(page, order);
	kernel_map_pages(page, 1 << order, 1);
	kasan_alloc_pages(page, order);

	if (gfp_flags & __GFP_ZERO)
		prep_zero_page(page, order, gfp_flags);

	if (order && (gfp_flags & __GFP_COMP))
		prep_compound_page(page, order);

	set_page_owner(page, order, gfp_flags);

	/*
	 * 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 0;
}

??首先通过check_new_page检查struct page成员属性

static inline int check_new_page(struct page *page)
{
	const char *bad_reason = NULL;
	unsigned long bad_flags = 0;

	if (unlikely(page_mapcount(page)))
		bad_reason = "nonzero mapcount";
	if (unlikely(page->mapping != NULL))
		bad_reason = "non-NULL mapping";
	if (unlikely(atomic_read(&page->_count) != 0))
		bad_reason = "nonzero _count";
	if (unlikely(page->flags & PAGE_FLAGS_CHECK_AT_PREP)) {
		bad_reason = "PAGE_FLAGS_CHECK_AT_PREP flag set";
		bad_flags = PAGE_FLAGS_CHECK_AT_PREP;
	}
#ifdef CONFIG_MEMCG
	if (unlikely(page->mem_cgroup))
		bad_reason = "page still charged to cgroup";
#endif
	if (unlikely(bad_reason)) {
		bad_page(page, bad_reason, bad_flags);
		return 1;
	}
	return 0;
}

• 刚分配的struct page的_mapcount计数应该为0。
• page->mapping应该为NULL。
• 判断page->_count是否为0。注意alloc_pages分配的page的_count应该为1，但是这里为0，这个函数之后，在prep_new_page中还会调用set_page_refcounted->set_page_count增加_count计数值，把_count设置为1。
• 检查page->flags，PAGE_FLAGS_CHECK_AT_PREP通常来说代表flags低21位，flag这部分，在分配页面时，应该是被清除状态。如果这些bit被置位，则页面分配存在问题。大概率是页面被使用过，且释放时没有清标志位。

??随后prep_new_page再设置一些参数后，分配的页面就合格了，可以出厂提供各个模块使用了。当然，还是需要建立页表后才能访问对应的内存。