Linux內存頁面分配

在Linux中，對物理內存的管理是怎麼實現的呢？對頁面的分配和回收是如何實現的呢？

Linux中對物理內存的管理是通過zone來管理的，以X86爲例，16MB以下的物理內存爲DMA zone；896MB以下的區域爲normal zone；然後896MB以上的區域統稱爲HighMemory zone；然後在分配內存的時候，可以指定區域來實現在某個特定的zone分配頁面；

那麼zone又是如何管理物理內存的呢？Linux同樣是使用夥伴系統來管理物理內存，來儘量避免碎片的產生；對夥伴系統的原理應該很清楚，就不說了

struct zone {
	/* Fields commonly accessed by the page allocator */

	/* zone watermarks, access with *_wmark_pages(zone) macros */
	unsigned long watermark[NR_WMARK];

	/*
	 * We don't know if the memory that we're going to allocate will be freeable
	 * or/and it will be released eventually, so to avoid totally wasting several
	 * GB of ram we must reserve some of the lower zone memory (otherwise we risk
	 * to run OOM on the lower zones despite there's tons of freeable ram
	 * on the higher zones). This array is recalculated at runtime if the
	 * sysctl_lowmem_reserve_ratio sysctl changes.
	 */
	unsigned long		lowmem_reserve[MAX_NR_ZONES];



	struct per_cpu_pageset __percpu *pageset;
	/*
	 * free areas of different sizes
	 */
	spinlock_t		lock;
        struct free_area	        free_area[MAX_ORDER];


	/* Fields commonly accessed by the page reclaim scanner */
	spinlock_t		lru_lock;
	struct lruvec		lruvec;

	unsigned long		pages_scanned;	   /* since last reclaim */
	unsigned long		flags;		   /* zone flags, see below */


	wait_queue_head_t	* wait_table;
	unsigned long		wait_table_hash_nr_entries;
	unsigned long		wait_table_bits;


	/*
	 * spanned_pages is the total pages spanned by the zone, including
	 * holes, which is calculated as:
	 * 	spanned_pages = zone_end_pfn - zone_start_pfn;
	 *
	 * present_pages is physical pages existing within the zone, which
	 * is calculated as:
	 *	present_pages = spanned_pages - absent_pages(pages in holes);
	 *
	 * managed_pages is present pages managed by the buddy system, which
	 * is calculated as (reserved_pages includes pages allocated by the
	 * bootmem allocator):
	 *	managed_pages = present_pages - reserved_pages;
	 *
	 * So present_pages may be used by memory hotplug or memory power
	 * management logic to figure out unmanaged pages by checking
	 * (present_pages - managed_pages). And managed_pages should be used
	 * by page allocator and vm scanner to calculate all kinds of watermarks
	 * and thresholds.
	  */

	unsigned long		spanned_pages;
	unsigned long		present_pages;
	unsigned long		managed_pages;

	/*
	 * rarely used fields:
	 */
	const char		*name;
};

其中和夥伴系統有關的是：          

struct free_area	        free_area[MAX_ORDER];   //MAX_ORDER=11

11個free_area 分別爲 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 和 1024個頁面，所以最大的爲4MB；

free_area的結構體爲

struct free_area {
	struct list_head	free_list[MIGRATE_TYPES];
	unsigned long		nr_free;
};

所有的空閒頁面都掛在free_list下；夥伴系統的回收和分配方式很容易理解，就不說了，至於代碼就不貼在這裏了

夥伴系統是物理內存頁分配的基礎，在分配頁面的時候，最主要的函數是_ _alloc_pages( )，而它的大體過程爲

for (i = 0; (z=zonelist->zones[i]) != NULL; i++) {
     if (zone_watermark_ok(z, order, ...)) {
        page = buffered_rmqueue(z, order, gfp_mask);
        if (page)
          return page;
     }
}

zone_watermark_ok(z, order, ...)函數，與zone的臨界值有關，用來檢測內存分配的安全性，以後還會詳細的討論。

buffered_rmqueue()函數是在給定的zone分配頁面，它繼續調用_ _rmqueue( )到夥伴系統中分配頁面；

struct free_area *area;
     unsigned int current_order;
     for (current_order=order; current_order<11; ++current_order) {
     area = zone->free_area + current_order;
     if (!list_empty(&area->free_list))
         goto block_found;
}
block_found:
   page = list_entry(area->free_list.next, struct page, lru);
   list_del(&page->lru);
   ClearPagePrivate(page);
   page->private = 0;
   area->nr_free--;
   zone->free_pages -= 1UL << order;
   size = 1 << curr_order;
   while (curr_order > order) {
     area--;
     curr_order--;
     size >>= 1;

    buddy = page + size;
    /* insert buddy as first element in the list */
    list_add(&buddy->lru, &area->free_list);
    area->nr_free++;
    buddy->private = curr_order;
    SetPagePrivate(buddy);
   }
return page;
return NULL;

這樣就可以完成對物理頁面的分配了，所有的物理頁面的分配，例如slab的分配，都是基於上面的過程實現的！