（转）linux内存管理之伙伴系统（内存分配）

 一、Linux伙伴系统分配器

伙伴系统分配器大体上分为两类。__get_free_pages()类函数返回分配的第一个页面的线性地址；alloc_pages()类函数返回页面描述符地址。不管以哪种函数进行分配，最终会调用alloc_pages()进行分配页面。

为清楚了解其分配制度，先给个伙伴系统数据的存储框图

也就是每个order对应一个free_area结构，free_area以不同的类型以链表的方式存储这些内存块。

二、主分配函数

下面我们来看这个函数（在UMA模式下）

#define alloc_pages(gfp_mask, order) \
        alloc_pages_node(numa_node_id(), gfp_mask, order)

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

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

static inline struct page *
__alloc_pages(gfp_t gfp_mask, unsigned int order,
        struct zonelist *zonelist)
{
    return __alloc_pages_nodemask(gfp_mask, order, zonelist, NULL);
}

上层分配函数__alloc_pages_nodemask()

/*
 * This is the 'heart' of the zoned buddy allocator.
 */
 /*上层分配器运用了各种方式进行*/
struct page *
__alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
            struct zonelist *zonelist, nodemask_t *nodemask)
{
    enum zone_type high_zoneidx = gfp_zone(gfp_mask);
    struct zone *preferred_zone;
    struct page *page;

    /* Convert GFP flags to their corresponding migrate type */
    int migratetype = allocflags_to_migratetype(gfp_mask);

    gfp_mask &= gfp_allowed_mask;
    /*调试用*/
    lockdep_trace_alloc(gfp_mask);
    /*如果__GFP_WAIT标志设置了，需要等待和重新调度*/
    might_sleep_if(gfp_mask & __GFP_WAIT);
    /*没有设置对应的宏*/
    if (should_fail_alloc_page(gfp_mask, order))
        return NULL;

    /*
     * Check the zones suitable for the gfp_mask contain at least one
     * valid zone. It's possible to have an empty zonelist as a result
     * of GFP_THISNODE and a memoryless node
     */
    if (unlikely(!zonelist->_zonerefs->zone))
        return NULL;

    /* The preferred zone is used for statistics later */
    /* 英文注释所说*/
    first_zones_zonelist(zonelist, high_zoneidx, nodemask, &preferred_zone);
    if (!preferred_zone)
        return NULL;

    /* First allocation attempt */
    /*从pcp和伙伴系统中正常的分配内存空间*/
    page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, nodemask, order,
            zonelist, high_zoneidx, ALLOC_WMARK_LOW|ALLOC_CPUSET,
            preferred_zone, migratetype);
    if (unlikely(!page))/*如果上面没有分配到空间，调用下面函数慢速分配，允许等待和回收*/
        page = __alloc_pages_slowpath(gfp_mask, order,
                zonelist, high_zoneidx, nodemask,
                preferred_zone, migratetype);
    /*调试用*/
    trace_mm_page_alloc(page, order, gfp_mask, migratetype);
    return page;
}

三、从pcp和伙伴系统中正常的分配内存空间

函数get_page_from_freelist()

/*
 * get_page_from_freelist goes through the zonelist trying to allocate
 * a page.
 */
/*为分配制定内存空间，遍历每个zone*/
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 migratetype)
{
    struct zoneref *z;
    struct page *page = NULL;
    int classzone_idx;
    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 */
    /*zone对应的下标*/
    classzone_idx = zone_idx(preferred_zone);
zonelist_scan:
    /*
     * Scan zonelist, looking for a zone with enough free.
     * See also cpuset_zone_allowed() comment in kernel/cpuset.c.
     */
     /*遍历每个zone，进行分配*/
    for_each_zone_zonelist_nodemask(zone, z, zonelist,
        /*在UMA模式下不成立*/              high_zoneidx, nodemask) {
        if (NUMA_BUILD && zlc_active &&
            !zlc_zone_worth_trying(zonelist, z, allowednodes))
                continue;
        if ((alloc_flags & ALLOC_CPUSET) &&
            !cpuset_zone_allowed_softwall(zone, gfp_mask))
                goto try_next_zone;

        BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
        /*需要关注水位*/
        if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
            unsigned long mark;
            int ret;
            /*从flags中取的mark*/
            mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
            /*如果水位正常，从本zone中分配*/
            if (zone_watermark_ok(zone, order, mark,
                    classzone_idx, alloc_flags))
                goto try_this_zone;

            if (zone_reclaim_mode == 0)/*如果上面检查的水位低于正常值，且没有设置页面回收值*/
                goto this_zone_full;
            /*在UMA模式下下面函数直接返回0*/
            ret = zone_reclaim(zone, gfp_mask, order);
            switch (ret) {
            case ZONE_RECLAIM_NOSCAN:
                /* did not scan */
                goto try_next_zone;
            case ZONE_RECLAIM_FULL:
                /* scanned but unreclaimable */
                goto this_zone_full;
            default:
                /* did we reclaim enough */
                if (!zone_watermark_ok(zone, order, mark,
                        classzone_idx, alloc_flags))
                    goto this_zone_full;
            }
        }

try_this_zone:/*本zone正常水位*/
    /*先从pcp中分配，然后不行的话再从伙伴系统中分配*/
        page = buffered_rmqueue(preferred_zone, zone, order,
                        gfp_mask, migratetype);
        if (page)
            break;
this_zone_full:
        if (NUMA_BUILD)/*UMA模式为0*/
            zlc_mark_zone_full(zonelist, z);
try_next_zone:
        if (NUMA_BUILD && !did_zlc_setup && nr_online_nodes > 1) {
            /*
             * we do zlc_setup after the first zone is tried but only
             * if there are multiple nodes make it worthwhile
             */
            allowednodes = zlc_setup(zonelist, alloc_flags);
            zlc_active = 1;
            did_zlc_setup = 1;
        }
    }

    if (unlikely(NUMA_BUILD && page == NULL && zlc_active)) {
        /* Disable zlc cache for second zonelist scan */
        zlc_active = 0;
        goto zonelist_scan;
    }
    return page;/*返回页面*/
}

主分配函数

/*
 * Really, prep_compound_page() should be called from __rmqueue_bulk().  But
 * we cheat by calling it from here, in the order > 0 path.  Saves a branch
 * or two.
 */
 /*先考虑从pcp中分配空间，当order大于0时再考虑从伙伴系统中分配*/
static inline
struct page *buffered_rmqueue(struct zone *preferred_zone,
            struct zone *zone, int order, gfp_t gfp_flags,
            int migratetype)
{
    unsigned long flags;
    struct page *page;
    int cold = !!(gfp_flags & __GFP_COLD);/*如果分配参数指定了__GFP_COLD标志，则设置cold标志*/
    int cpu;

again:
    cpu  = get_cpu();
    if (likely(order == 0)) {/*分配一个页面时，使用pcp*/
        struct per_cpu_pages *pcp;
        struct list_head *list;
        /*找到zone对应的pcp*/
        pcp = &zone_pcp(zone, cpu)->pcp;
        list = &pcp->lists[migratetype];/*pcp中对应类型的list*/

        /* 这里需要关中断，因为内存回收过程可能发送核间中断，强制每个核从每CPU
        缓存中释放页面。而且中断处理函数也会分配单页。 */
        local_irq_save(flags);
        if (list_empty(list)) {/*如果pcp中没有页面,需要补充*/
            /*从伙伴系统中获得batch个页面
            batch为一次分配的页面数*/
            pcp->count += rmqueue_bulk(zone, 0,
                    pcp->batch, list,
                    migratetype, cold);
            /*如果链表仍然为空，申请失败返回*/
            if (unlikely(list_empty(list)))
                goto failed;
        }
        /* 如果分配的页面不需要考虑硬件缓存(注意不是每CPU页面缓存)
        ，则取出链表的最后一个节点返回给上层*/
        if (cold)
            page = list_entry(list->prev, struct page, lru);
        else/* 如果要考虑硬件缓存，则取出链表的第一个页面，这个页面是最近刚释放到每CPU
            缓存的，缓存热度更高 */
            page = list_entry(list->next, struct page, lru);

        list_del(&page->lru);/*从pcp中脱离*/
        pcp->count--;/*pcp计数减一*/
    }
    else {/*当order为大于1时，不从pcp中分配，直接考虑从伙伴系统中分配*/
        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);
        }
        /* 关中断，并获得管理区的锁*/
        spin_lock_irqsave(&zone->lock, flags);
        /*从伙伴系统中相应类型的相应链表中分配空间*/
        page = __rmqueue(zone, order, migratetype);
        /* 已经分配了1 << order个页面，这里进行管理区空闲页面统计计数*/
        __mod_zone_page_state(zone, NR_FREE_PAGES, -(1 << order));
        spin_unlock(&zone->lock);/* 这里仅仅打开自旋锁，待后面统计计数设置完毕后再开中断*/
        if (!page)
            goto failed;
    }
    /*事件统计计数，调试*/
    __count_zone_vm_events(PGALLOC, zone, 1 << order);
    zone_statistics(preferred_zone, zone);
    local_irq_restore(flags);/*恢复中断*/
    put_cpu();

    VM_BUG_ON(bad_range(zone, page));

     /* 这里进行安全性检查，并进行一些善后工作。
      如果页面标志破坏，返回的页面出现了问题，则返回试图分配其他页面*/
    if (prep_new_page(page, order, gfp_flags))
        goto again;
    return page;

failed:
    local_irq_restore(flags);
    put_cpu();
    return NULL;
}

3.1 pcp缓存补充

从伙伴系统中获得batch个页面，batch为一次分配的页面数rmqueue_bulk()函数。

/*
 * Obtain a specified number of elements from the buddy allocator, all under
 * a single hold of the lock, for efficiency.  Add them to the supplied list.
 * Returns the number of new pages which were placed at *list.
 */
 /*该函数返回的是1<
 处理中调用，其他地方没看到，order为0
  也就是说返回的是页面数，加入的链表为
  对应调用pcp的链表*/
static int rmqueue_bulk(struct zone *zone, unsigned int order,
            unsigned long count, struct list_head *list,
            int migratetype, int cold)
{
    int i;

    spin_lock(&zone->lock);/* 上层函数已经关了中断，这里需要操作管理区，获取管理区的自旋锁 */
    for (i = 0; i < count; ++i) {/* 重复指定的次数，从伙伴系统中分配页面*/
        /* 从伙伴系统中取出页面 */
        struct page *page = __rmqueue(zone, order, migratetype);
        if (unlikely(page == NULL))/*分配失败*/
            break;

        /*
         * Split buddy pages returned by expand() are received here
         * in physical page order. The page is added to the callers and
         * list and the list head then moves forward. From the callers
         * perspective, the linked list is ordered by page number in
         * some conditions. This is useful for IO devices that can
         * merge IO requests if the physical pages are ordered
         * properly.
         */
        if (likely(cold == 0))/*根据调用者的要求，将页面放到每CPU缓存链表的头部或者尾部*/
            list_add(&page->lru, list);
        else
            list_add_tail(&page->lru, list);
        set_page_private(page, migratetype);/*设置private属性为页面的迁移类型*/
        list = &page->lru;
    }
    /*递减管理区的空闲页面计数*/
    __mod_zone_page_state(zone, NR_FREE_PAGES, -(i << order));
    spin_unlock(&zone->lock);/*释放管理区的子璇锁*/
    return i;
}

3.2 从伙伴系统中取出页面

__rmqueue()函数

/*
 * Do the hard work of removing an element from the buddy allocator.
 * Call me with the zone->lock already held.
 */
 /*采用两种范式试着分配order个page*/
static struct page *__rmqueue(struct zone *zone, unsigned int order,
                        int migratetype)
{
    struct page *page;

retry_reserve:
    /*从指定order开始从小到达遍历,优先从指定的迁移类型链表中分配页面*/
    page = __rmqueue_smallest(zone, order, migratetype);

        /*
         * 如果满足以下两个条件,就从备用链表中分配页面:
         *        快速流程没有分配到页面,需要从备用迁移链表中分配.
         *        当前不是从保留的链表中分配.因为保留的链表是最后可用的链表,
             *  不能从该链表分配的话,说明本管理区真的没有可用内存了.
         */
    if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
        /*order从大到小遍历，从备用链表中分配页面*/
        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) {/* 备用链表中没有分配到页面,从保留链表中分配页面了 */
            migratetype = MIGRATE_RESERVE;
            goto retry_reserve;/* 跳转到retry_reserve,从保留的链表中分配页面*/
        }
    }
    /*调试代码*/
    trace_mm_page_alloc_zone_locked(page, order, migratetype);
    return page;
}

3.2.1 从指定的迁移类型链表中分配页面

从指定order开始从小到达遍历,优先从指定的迁移类型链表中分配页面__rmqueue_smallest(zone, order, migratetype);

/*
 * Go through the free lists for the given migratetype and remove
 * the smallest available page from the freelists
 */
 /*从给定的order开始，从小到大遍历；
  找到后返回页面基址，合并分割后的空间*/
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]);/*得到指定order的area*/
        /*如果area指定类型的伙伴系统链表为空*/
        if (list_empty(&area->free_list[migratetype]))
            continue;/*查找下一个order*/
        /*对应的链表不空，得到链表中数据*/
        page = list_entry(area->free_list[migratetype].next,
                            struct page, lru);
        list_del(&page->lru);/*从伙伴系统中删除；*/
        rmv_page_order(page);/*移除page中order的变量*/
        area->nr_free--;/*空闲块数减一*/
        /*拆分、合并*/
        expand(zone, page, order, current_order, area, migratetype);
        return page;
    }

    return NULL;
}

伙伴系统内存块拆分和合并

看一个辅助函数，用于伙伴系统中内存块的拆分、合并

/*
 * The order of subdivision here is critical for the IO subsystem.
 * Please do not alter this order without good reasons and regression
 * testing. Specifically, as large blocks of memory are subdivided,
 * the order in which smaller blocks are delivered depends on the order
 * they're subdivided in this function. This is the primary factor
 * influencing the order in which pages are delivered to the IO
 * subsystem according to empirical testing, and this is also justified
 * by considering the behavior of a buddy system containing a single
 * large block of memory acted on by a series of small allocations.
 * This behavior is a critical factor in sglist merging's success.
 *
 * -- wli
 */
 /*此函数主要用于下面这种情况:
  分配函数从high中分割出去了low大小的内存；
  然后要将high留下的内存块合并放到伙伴系统中；*/
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) {/*因为去掉了low的大小，所以最后肯定剩下的
     是low的大小(2的指数运算)*/
        area--;/*减一到order减一的area*/
        high--;/*order减一*/
        size >>= 1;/*大小除以2*/
        VM_BUG_ON(bad_range(zone, &page[size]));
        /*加到指定的伙伴系统中*/
        list_add(&page[size].lru, &area->free_list[migratetype]);
        area->nr_free++;/*空闲块加一*/
        set_page_order(&page[size], high);/*设置相关order*/
    }
}

3.2.2 从备用链表中分配页面

/* Remove an element from the buddy allocator from the fallback list */
static inline struct page *
__rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
{
    struct free_area * area;
    int current_order;
    struct page *page;
    int migratetype, i;

    /* Find the largest possible block of pages in the other list */

    /* 从最高阶搜索,这样可以尽量的将其他迁移列表中的大块分割,避免形成过多的碎片 */
    for (current_order = MAX_ORDER-1; current_order >= order;
                        --current_order) {
        for (i = 0; i < MIGRATE_TYPES - 1; i++) {
            /*回调到下一个migratetype*/
            migratetype = fallbacks[start_migratetype][i];

            /* MIGRATE_RESERVE handled later if necessary */

              /* 本函数不处理MIGRATE_RESERVE类型的迁移链表,如果本函数返回NULL,
            则上层函数直接从MIGRATE_RESERVE中分配 */
            if (migratetype == MIGRATE_RESERVE)
                continue;/*访问下一个类型*/

            area = &(zone->free_area[current_order]);
            /*如果指定order和类型的链表为空*/
            if (list_empty(&area->free_list[migratetype]))
                continue;/*访问下一个类型*/
            /*得到指定类型和order的页面基址*/
            page = list_entry(area->free_list[migratetype].next,
                    struct page, lru);
            area->nr_free--;/*空闲块数减一*/

            /*
             * If breaking a large block of pages, move all free
             * pages to the preferred allocation list. If falling
             * back for a reclaimable kernel allocation, be more
             * agressive about taking ownership of free pages
             */
            if (unlikely(current_order >= (pageblock_order >> 1)) ||/* 要分割的页面是一个大页面,则将整个页面全部迁移到当前迁移类型的链表中,
                这样可以避免过多的碎片 */
                    start_migratetype == MIGRATE_RECLAIMABLE ||/* 目前分配的是可回收页面,这类页面有突发的特点,将页面全部迁移到可回收链表中,
                可以避免将其他迁移链表分割成太多的碎片 */
                page_group_by_mobility_disabled) {/* 指定了迁移策略,总是将被分割的页面迁移 */

                unsigned long pages;
                /*移动到先前类型的伙伴系统中*/
                pages = move_freepages_block(zone, page,
                                start_migratetype);

                /* Claim the whole block if over half of it is free */

                 /* pages是移动的页面数,如果可移动的页面数量较多,
                则将整个大内存块的迁移类型修改 */
                if (pages >= (1 << (pageblock_order-1)) ||
                        page_group_by_mobility_disabled)
                    /*设置页面标示*/
                    set_pageblock_migratetype(page,
                                start_migratetype);

                migratetype = start_migratetype;
            }

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

            /* Take ownership for orders >= pageblock_order */
            if (current_order >= pageblock_order)//大于pageblock_order的部分设置相应标示
            /*这个不太可能，因为pageblock_order为10*/
                change_pageblock_range(page, current_order,
                            start_migratetype);
            /*拆分和合并*/
            expand(zone, page, order, current_order, area, migratetype);

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

            return page;
        }
    }

    return NULL;
}

备用链表

/*
 * This array describes the order lists are fallen back to when
 * the free lists for the desirable migrate type are depleted
 */
 /*指定类型的链表为空时，这个数组规定
  回调的到那个类型的链表*/
static int fallbacks[MIGRATE_TYPES][MIGRATE_TYPES-1] = {
    [MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,   MIGRATE_RESERVE },
    [MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,   MIGRATE_RESERVE },
    [MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_RESERVE },
    [MIGRATE_RESERVE]     = { MIGRATE_RESERVE,     MIGRATE_RESERVE,   MIGRATE_RESERVE }, /* Never used */
};

移动到指定类型的伙伴系统中

/*将指定区域段的页面移动到指定类型的
  伙伴系统中，其实就是将页面的类型做了
  更改，但是是采用移动的方式

 功能和上面函数类似，但是要求以
 页面块方式对其*/
static int move_freepages_block(struct zone *zone, struct page *page,
                int migratetype)
{
    unsigned long start_pfn, end_pfn;
    struct page *start_page, *end_page;

/*如下是对齐操作，其中变量pageblock_nr_pages为MAX_ORDER-1*/
    start_pfn = page_to_pfn(page);
    start_pfn = start_pfn & ~(pageblock_nr_pages-1);
    start_page = pfn_to_page(start_pfn);
    end_page = start_page + pageblock_nr_pages - 1;
    end_pfn = start_pfn + pageblock_nr_pages - 1;

    /* Do not cross zone boundaries */
    if (start_pfn < zone->zone_start_pfn)
        start_page = page;
    /*结束边界检查*/
    if (end_pfn >= zone->zone_start_pfn + zone->spanned_pages)
        return 0;
/*调用上面函数*/
    return move_freepages(zone, start_page, end_page, migratetype);
}

/*
 * Move the free pages in a range to the free lists of the requested type.
 * Note that start_page and end_pages are not aligned on a pageblock
 * boundary. If alignment is required, use move_freepages_block()
 */
 /*将指定区域段的页面移动到指定类型的
  伙伴系统中，其实就是将页面的类型做了 更改，但是是采用移动的方式*/
static 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;

#ifndef CONFIG_HOLES_IN_ZONE
    /*
     * page_zone is not safe to call in this context when
     * CONFIG_HOLES_IN_ZONE is set. This bug check is probably redundant
     * anyway as we check zone boundaries in move_freepages_block().
     * Remove at a later date when no bug reports exist related to
     * grouping pages by mobility
     */
    BUG_ON(page_zone(start_page) != page_zone(end_page));
#endif

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

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

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

        order = page_order(page);
        list_del(&page->lru);/*将页面块从原来的伙伴系统链表*/
        /*中删除，注意，这里不是一个页面
        *而是以该页面的伙伴块*/
        list_add(&page->lru,/*添加到指定order和类型下的伙伴系统链表*/
            &zone->free_area[order].free_list[migratetype]);
        page += 1 << order;/*移动页面数往上定位*/
        pages_moved += 1 << order;/*移动的页面数*/
    }

    return pages_moved;
}

四、慢速分配，允许等待和回收

/**
 * 当无法快速分配页面时，如果调用者允许等待
 ，则通过本函数进行慢速分配。
 * 此时允许进行内存回收。
 */
static inline struct page *
__alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
    struct zonelist *zonelist, enum zone_type high_zoneidx,
    nodemask_t *nodemask, struct zone *preferred_zone,
    int migratetype)
{
    const gfp_t wait = gfp_mask & __GFP_WAIT;
    struct page *page = NULL;
    int alloc_flags;
    unsigned long pages_reclaimed = 0;
    unsigned long did_some_progress;
    struct task_struct *p = current;

    /*
     * In the slowpath, we sanity check order to avoid ever trying to
     * reclaim >= MAX_ORDER areas which will never succeed. Callers may
     * be using allocators in order of preference for an area that is
     * too large.
     *//*参数合法性检查*/
    if (order >= MAX_ORDER) {
        WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
        return NULL;
    }

    /*
     * GFP_THISNODE (meaning __GFP_THISNODE, __GFP_NORETRY and
     * __GFP_NOWARN set) should not cause reclaim since the subsystem
     * (f.e. slab) using GFP_THISNODE may choose to trigger reclaim
     * using a larger set of nodes after it has established that the
     * allowed per node queues are empty and that nodes are
     * over allocated.
     */
         /**
          * 调用者指定了GFP_THISNODE标志，表示不能进行内存回收。
          * 上层调用者应当在指定了GFP_THISNODE失败后，使用其他标志进行分配。
          */
    if (NUMA_BUILD && (gfp_mask & GFP_THISNODE) == GFP_THISNODE)
        goto nopage;

restart:/*如果调用者没有禁止kswapd，则唤醒该线程进行内存回收。*/
    wake_all_kswapd(order, zonelist, high_zoneidx);

    /*
     * OK, we're below the kswapd watermark and have kicked background
     * reclaim. Now things get more complex, so set up alloc_flags according
     * to how we want to proceed.
     */
     /*根据分配标志确定内部标志，主要是用于水线 */
    alloc_flags = gfp_to_alloc_flags(gfp_mask);

        /**
          * 与快速分配流程相比，这里的分配标志使用了低的水线。
          * 在进行内存回收操作前，我们使用低水线再尝试分配一下。
          * 当然，不管是否允许ALLOC_NO_WATERMARKS标志，我们都将它清除。
          */
    /* This is the last chance, in general, before the goto nopage. */
    page = get_page_from_freelist(gfp_mask, nodemask, order, zonelist,
            high_zoneidx, alloc_flags & ~ALLOC_NO_WATERMARKS,
            preferred_zone, migratetype);
    if (page)/*分配成功，找到页面*/
        goto got_pg;

rebalance:
    /* Allocate without watermarks if the context allows */
/* 某些上下文，如内存回收进程及被杀死的任务，都允许它完全突破水线的限制分配内存。 */
    if (alloc_flags & ALLOC_NO_WATERMARKS) {
        page = __alloc_pages_high_priority(gfp_mask, order,
                zonelist, high_zoneidx, nodemask,
                preferred_zone, migratetype);
        if (page))/* 在不考虑水线的情况下，分配到了内存 */
            goto got_pg;
    }

    /* Atomic allocations - we can't balance anything */
    /* 调用者希望原子分配内存，此时不能等待内存回收，返回NULL */
    if (!wait)
        goto nopage;

    /* Avoid recursion of direct reclaim */
/* 调用者本身就是内存回收进程，不能进入后面的内存回收处理流程，否则死锁 */
    if (p->flags & PF_MEMALLOC)
        goto nopage;

    /* Avoid allocations with no watermarks from looping endlessly */
    /**
    * 当前线程正在被杀死，它可以完全突破水线分配内存。这里向上层返回NULL，是为了避免系统进入死循环。
    * 当然，如果上层调用不允许失败，则死循环继续分配，等待其他线程释放一点点内存。
    */
    if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
        goto nopage;

    /* Try direct reclaim and then allocating */
    /**
    * 直接在内存分配上下文中进行内存回收操作。
    */
    page = __alloc_pages_direct_reclaim(gfp_mask, order,
                    zonelist, high_zoneidx,
                    nodemask,
                    alloc_flags, preferred_zone,
                    migratetype, &did_some_progress);
    if (page))/* 庆幸，回收了一些内存后，满足了上层分配需求 */
        goto got_pg;

    /*
     * If we failed to make any progress reclaiming, then we are
     * running out of options and have to consider going OOM
     */
    /* 内存回收过程没有回收到内存，系统真的内存不足了 */
    if (!did_some_progress) {
        /**
         * 调用者不是文件系统的代码，允许进行文件系统操作，并且允许重试。
         * 这里需要__GFP_FS标志可能是进入OOM流程后会杀进程或进入panic，需要文件操作。
         */
        if ((gfp_mask & __GFP_FS) && !(gfp_mask & __GFP_NORETRY)) {
            if (oom_killer_disabled)/* 系统禁止了OOM，向上层返回NULL */
                goto nopage;
            /**
             * 杀死其他进程后再尝试分配内存
             */
            page = __alloc_pages_may_oom(gfp_mask, order,
                    zonelist, high_zoneidx,
                    nodemask, preferred_zone,
                    migratetype);
            if (page)
                goto got_pg;

            /*
             * The OOM killer does not trigger for high-order
             * ~__GFP_NOFAIL allocations so if no progress is being
             * made, there are no other options and retrying is
             * unlikely to help.
             */)/* 要求的页面数量较多，再试意义不大 */
            if (order > PAGE_ALLOC_COSTLY_ORDER &&
                        !(gfp_mask & __GFP_NOFAIL))
                goto nopage;

            goto restart;
        }
    }

    /* Check if we should retry the allocation */
 /* 内存回收过程回收了一些内存，接下来判断是否有必要继续重试 */
    pages_reclaimed += did_some_progress;
    if (should_alloc_retry(gfp_mask, order, pages_reclaimed)) {
        /* Wait for some write requests to complete then retry */
        congestion_wait(BLK_RW_ASYNC, HZ/50);
        goto rebalance;
    }

nopage:
/* 内存分配失败了，打印内存分配失败的警告 */
    if (!(gfp_mask & __GFP_NOWARN) && printk_ratelimit()) {
        printk(KERN_WARNING "%s: page allocation failure."
            " order:%d, mode:0x%x\n",
            p->comm, order, gfp_mask);
        dump_stack();
        show_mem();
    }
    return page;
got_pg:
    /* 运行到这里，说明成功分配了内存，这里进行内存检测调试 */
    if (kmemcheck_enabled)
        kmemcheck_pagealloc_alloc(page, order, gfp_mask);
    return page;

}

总结：Linux伙伴系统主要分配流程为

正常非配（或叫快速分配）流程：

1，如果分配的是单个页面，考虑从per CPU缓存中分配空间，如果缓存中没有页面，从伙伴系统中提取页面做补充。

2，分配多个页面时，从指定类型中分配，如果指定类型中没有足够的页面，从备用类型链表中分配。最后会试探保留类型链表。

慢速（允许等待和页面回收）分配：

3，当上面两种分配方案都不能满足要求时，考虑页面回收、杀死进程等操作后在试。