内存管理(三) 内存模型之page

接下来就是page,该结构的定义如下：

悲剧，总算找着page的源码了，在include/linux/Mm_types.h里面，貌似很多参考资料都说在include/linux/mm.h里，坑爹啊，废话不多说了，上代码

struct page {
    unsigned long flags;        /* Atomic flags, some possibly
                     * updated asynchronously */
    atomic_t _count;        /* Usage count, see below. */
    union {
        atomic_t _mapcount;    /* Count of ptes mapped in mms,
                     * to show when page is mapped
                     * & limit reverse map searches.
                     */
        struct {        /* SLUB */
            u16 inuse;
            u16 objects;
        };
    };
    union {
        struct {
        unsigned long private;        /* Mapping-private opaque data:
                          * usually used for buffer_heads
                         * if PagePrivate set; used for
                         * swp_entry_t if PageSwapCache;
                         * indicates order in the buddy
                         * system if PG_buddy is set.
                         */
        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.
                         */
        };
#if USE_SPLIT_PTLOCKS
        spinlock_t ptl;
#endif
        struct kmem_cache *slab;    /* SLUB: Pointer to slab */
        struct page *first_page;    /* Compound tail pages */
    };
    union {
        pgoff_t index;        /* Our offset within mapping. */
        void *freelist;        /* SLUB: freelist req. slab lock */
    };
    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)
    void *virtual;            /* Kernel virtual address (NULL if
                       not kmapped, ie. highmem) */
#endif /* WANT_PAGE_VIRTUAL */
#ifdef CONFIG_WANT_PAGE_DEBUG_FLAGS
    unsigned long debug_flags;    /* Use atomic bitops on this */
#endif

#ifdef CONFIG_KMEMCHECK
    /*
     * kmemcheck wants to track the status of each byte in a page; this
     * is a pointer to such a status block. NULL if not tracked.
     */
    void *shadow;
#endif
};

     page这个结构是比较值得仔细斟酌的,因为页帧是系统管理内存的最小单位，对于内存中的每个页都会创建struct page的一个实例，所以这个结构要尽量保持小，但是页帧在系统内部广泛使用，内核的很多部分都依赖页帧所提供的信息，但是这些信息也有可能对其他部分完全没有用。所以，用union来维护一些数据是比较合适的。另外，由于页帧的广泛使用，所以对其所作的任何修改都有可能导致保持所有page实例所需的内存暴涨。

    下面简单讲解一下各个字段的意义：

• flags：描叙了体系结构无关的一些标志，用于描叙页的属性,定义如下：

enum pageflags {
    PG_locked,        /* Page is locked. Don't touch. */
    PG_error,
    PG_referenced,
    PG_uptodate,
    PG_dirty,
    PG_lru,
    PG_active,
    PG_slab,
    PG_owner_priv_1,    /* Owner use. If pagecache, fs may use*/
    PG_arch_1,
    PG_reserved,
    PG_private,        /* If pagecache, has fs-private data */
    PG_private_2,        /* If pagecache, has fs aux data */
    PG_writeback,        /* Page is under writeback */
#ifdef CONFIG_PAGEFLAGS_EXTENDED
    PG_head,        /* A head page */
    PG_tail,        /* A tail page */
#else
    PG_compound,        /* A compound page */
#endif
    PG_swapcache,        /* Swap page: swp_entry_t in private */
    PG_mappedtodisk,    /* Has blocks allocated on-disk */
    PG_reclaim,        /* To be reclaimed asap */
    PG_swapbacked,        /* Page is backed by RAM/swap */
    PG_unevictable,        /* Page is "unevictable"  */
#ifdef CONFIG_MMU
    PG_mlocked,        /* Page is vma mlocked */
#endif
#ifdef CONFIG_ARCH_USES_PG_UNCACHED
    PG_uncached,        /* Page has been mapped as uncached */
#endif
#ifdef CONFIG_MEMORY_FAILURE
    PG_hwpoison,        /* hardware poisoned page. Don't touch */
#endif
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
    PG_compound_lock,
#endif
    __NR_PAGEFLAGS,

    /* Filesystems */
    PG_checked = PG_owner_priv_1,

    /* Two page bits are conscripted by FS-Cache to maintain local caching
     * state.  These bits are set on pages belonging to the netfs's inodes
     * when those inodes are being locally cached.
     */
    PG_fscache = PG_private_2,    /* page backed by cache */

    /* XEN */
    PG_pinned = PG_owner_priv_1,
    PG_savepinned = PG_dirty,

    /* SLOB */
    PG_slob_free = PG_private,

    /* SLUB */
    PG_slub_frozen = PG_active,
};

    

1. PG_locked指定了页是否锁定。如果该位置位，内核的其他部分不允许访问该页
2. PG_error如果在设计该页的I/O操作期间发生错误，则置该位
3. PG_referenced和PG_active控制了系统使用该页的活动程度
4. PG_update表示页的数据已经从块设备读取，没有出错
5. PG_dirty如果内存页上的数据有修改，但是没有同步到硬盘上，则置位
6. PG_lru内核用LRU算法来回收页帧，如果页在活动或不活动页链表其中一个，则设置该标志
7. PG_high表示页在高端内存中，无法持久映射到内核内存中
8. PG_writeback表示页的内容处于想块设备歇会的过程中
9. PG_slab如果页是slab分配器的一部分，则设置PG_slab位
10. PG_swapcache如果页处于交换缓存，则置位
11. PG_reclaim如果内存打算回收某页，会设置该标志
12. PG_buddy，若果空闲页包含在伙伴系统的列表中
13. PG_compound表示页属于一个更大的复合页

同时，内核也提供了一些标准宏，用于检查标志位，或者操作某个标志位

1. PageXXX(page)会检查是否设置了PG_XXX位，例如,PageDirty检查是否设置了PG_dirty位
2. SetPageXXX会在比特位没有设置的情况下设置PG_XXX位，并返回原值
3. ClearPageXXX无条件地清除某个特定的比特位
4. TestClearPageXXX清除某个设置的比特位，并返回原值

 

• atomic_t _count 引用计数器，表示内核中引用该页的次数
• atomic_t _mapcount 表示在页表中有多少个pte指向该页
• 
  

  struct {        /* SLUB */
              u16 inuse;
              u16 objects;
          };

  用于SLUB分配器，对象的数目
• lru是一个表头，用于在各种链表上维护该页，一遍将页按照不同类别分组，主要是按照活动页和不活动页分组
• mapping 指定了页帧所在的地址空间。
• private是一个指向私有数据的指针
• virtual用于高端内存域中的页，