page结构和物理内存对应关系

page结构大致说明如下：

struct page {
    /*page相关标记     */
    page_flags_t flags;        /* Atomic flags, some possibly
                     * updated asynchronously */
    /*页框引用计数     */
    atomic_t _count;        /* Usage count, see below. */
    /*页框的映射此时，表示有多少个pte映射到此page;-1:表示没有页表项映射该页框；0:表示页非共享；>0:表示页共享*/
    atomic_t _mapcount;        /* Count of ptes mapped in mms,
                     * to show when page is mapped
                     * & limit reverse map searches.
                     */
    /*私有数据，不同场景作用不同，可用于伙伴系统*/
    unsigned long private;        /* Mapping-private opaque data:
                     * usually used for buffer_heads
                     * if PagePrivate set; used for
                     * swp_entry_t if PageSwapCache
                     * When page is free, this indicates
                     * order in the buddy system.
                     */
    /* 当页被用作page cache或者当页为匿名页时使用）。*/
    struct address_space *mapping;    /* If low bit clear, points to
                     * inode address_space, or NULL.
                     * If page mapped as anonymous
                     * memory, low bit is set, and
                     * it points to anon_vma object:
                     * see PAGE_MAPPING_ANON below.
                     */

    pgoff_t index;            /* Our offset within mapping. */
    /*lru链表     */
    struct list_head lru;        /* Pageout list, eg. active_list
                     * protected by zone->lru_lock !
                     */
    /*
     * 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(WANT_PAGE_VIRTUAL)
    /* 如果进行了内存映射，则表示虚拟地址。仅对高端内存有意义。 */
    void *virtual;            /* Kernel virtual address (NULL if
                     not kmapped, ie. highmem) */
#endif /* WANT_PAGE_VIRTUAL */
};

mem_mmap：内核全局变量，包含系统中所有的物理内存对应的page结构的数组，每个数组元素对应一个物理页，数组的索引对应于该物理页在物理内存中的位置，比如物理内存的第一个页对应于mem_map[0]，NUMA结构中有所差别(这里不描述~)

struct page *mem_map;

page和物理页帧转换

#define pfn_to_page(pfn) (mem_map + (pfn)) //pfn相当于该page在mem_map数组中的索引号，mem_map+pfn相当于mem_map[pfn]
#define page_to_pfn(page) ((unsigned long)((page) - mem_map))) //page-mem_map得到该page在mem_map数组中的索引，索引号就是物理页帧号pfn