Linux内核之物理内存管理

Linux内核对于内存的管理摒弃了i386复杂的段式管理，而是采用了页式管理。Linux把整个的物理内存空间化为成一个个单独的页面，每页占4K空间大小。Linux把所有页面链接到一个全局的数组mem_map[]中，mem_map的每一个元素都是一个指针指向page数据结构。系统中的每个物理页面都对应着一个page数据结构，并根据需要把这些页面划分为不同的管理区：ZONE_DMA，ZONE_NORMAL和ZONE_HIGHMEM(用于物理内存超过1G空间的地址)。

 

typedef struct page {
	struct list_head list;		/* ->mapping has some page lists. */
	struct address_space *mapping;	/* The inode (or ...) we belong to. */
	unsigned long index;		/* Our offset within mapping. */
	struct page *next_hash;		/* Next page sharing our hash bucket in
					   the pagecache hash table. */
	atomic_t count;			/* Usage count, see below. */
	unsigned long flags;		/* atomic flags, some possibly
					   updated asynchronously */
	struct list_head lru;		/* Pageout list, eg. active_list;
					   protected by pagemap_lru_lock !! */
	struct page **pprev_hash;	/* Complement to *next_hash. */
	struct buffer_head * buffers;	/* Buffer maps us to a disk block. */

	/*
	 * On machines where all RAM is mapped into kernel address space,
	 * we can simply calculate the virtual address. On machines with
	 * highmem some memory is mapped into kernel virtual memory
	 * dynamically, so we need a place to store that address.
	 * Note that this field could be 16 bits on x86 ... ;)
	 *
	 * Architectures with slow multiplication can define
	 * WANT_PAGE_VIRTUAL in asm/page.h
	 */
#if defined(CONFIG_HIGHMEM) || defined(WANT_PAGE_VIRTUAL)
	void *virtual;			/* Kernel virtual address (NULL if
					   not kmapped, ie. highmem) */
#endif /* CONFIG_HIGMEM || WANT_PAGE_VIRTUAL */
} mem_map_t;

 

 

DMA管理区是单独进行管理的，不经过MMU映射，而一般的外设都对地址空间有一定的限制。地址空间不能太大，而且要求地址连续。

 

每个管理区都对应着一个zone_struct结构，这个结构体中有一组空闲队列free_area_t。这些队列中要有一个队列保持一些离散的物理页面，另一个队列要保持长度为2的指数的连续物理页面。

 

 

typedef struct zone_struct {
	/*
	 * Commonly accessed fields:
	 */
	spinlock_t		lock;
	unsigned long		free_pages;
	unsigned long		pages_min, pages_low, pages_high;
	int			need_balance;

	/*
	 * free areas of different sizes
	 */
	free_area_t		free_area[MAX_ORDER];

	/*
	 * wait_table		-- the array holding the hash table
	 * wait_table_size	-- the size of the hash table array
	 * wait_table_shift	-- wait_table_size
	 * 				== BITS_PER_LONG (1 << wait_table_bits)
	 *
	 * The purpose of all these is to keep track of the people
	 * waiting for a page to become available and make them
	 * runnable again when possible. The trouble is that this
	 * consumes a lot of space, especially when so few things
	 * wait on pages at a given time. So instead of using
	 * per-page waitqueues, we use a waitqueue hash table.
	 *
	 * The bucket discipline is to sleep on the same queue when
	 * colliding and wake all in that wait queue when removing.
	 * When something wakes, it must check to be sure its page is
	 * truly available, a la thundering herd. The cost of a
	 * collision is great, but given the expected load of the
	 * table, they should be so rare as to be outweighed by the
	 * benefits from the saved space.
	 *
	 * __wait_on_page() and unlock_page() in mm/filemap.c, are the
	 * primary users of these fields, and in mm/page_alloc.c
	 * free_area_init_core() performs the initialization of them.
	 */
	wait_queue_head_t	* wait_table;
	unsigned long		wait_table_size;
	unsigned long		wait_table_shift;

	/*
	 * Discontig memory support fields.
	 */
	struct pglist_data	*zone_pgdat;
	struct page		*zone_mem_map;
	unsigned long		zone_start_paddr;
	unsigned long		zone_start_mapnr;

	/*
	 * rarely used fields:
	 */
	char			*name;
	unsigned long		size;
} zone_t;