page _refcount和_mapcount字段

linux page有两个非常重要的引用计数字段_refcount和_mapcount，都是atomic_t类型，其中，_refcount表示内核中应用该page的次数。当_refcount = 0时，表示该page为空闲或者将要被释放。当_refcount > 0，表示该page页面已经被分配且内核正在使用，暂时不会被释放。

_refcount

内核中常用的加减_refcount应用计数的API：get_page()和put_page()

static inline void get_page(struct page *page)
{
    page = compound_head(page);
    /*
     * Getting a normal page or the head of a compound page
     * requires to already have an elevated page->_refcount.
     */
    VM_BUG_ON_PAGE(page_ref_count(page) <= 0, page);
    page_ref_inc(page);
}

static inline void put_page(struct page *page)
{
    page = compound_head(page);

    /*
     * For private device pages we need to catch refcount transition from
     * 2 to 1, when refcount reach one it means the private device page is
     * free and we need to inform the device driver through callback. See
     * include/linux/memremap.h and HMM for details.
     */
    if (IS_HMM_ENABLED && unlikely(is_device_private