struct page的解析

内存管理

struct page是内存管理中重要的数据结构，它代表系统内存的最小单位。
其数据结构如下所示：

/*
 * Each physical page in the system has a struct page associated with
 * it to keep track of whatever it is we are using the page for at the
 * moment. Note that we have no way to track which tasks are using
 * a page, though if it is a pagecache page, rmap structures can tell us
 * who is mapping it.
 *
 * The objects in struct page are organized in double word blocks in
 * order to allows us to use atomic double word operations on portions
 * of struct page. That is currently only used by slub but the arrangement
 * allows the use of atomic double word operations on the flags/mapping
 * and lru list pointers also.
 */
struct page {
    /* First double word block */
    unsigned long flags;        /* Atomic flags, some possibly
                     * updated asynchronously */
    union {
        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.
                         */
        void *s_mem;            /* slab first object */
        atomic_t compound_mapcount; /* first tail page */
        /* page_deferred_list().next     -- second tail page */
    };
    /* Second double word */
    struct {
        union {
            pgoff_t index;      /* Our offset within mapping. */
            void *freelist;     /* sl[aou]b first free object */
            /* page_deferred_list().prev    -- second tail page */
        };
        union {
#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
    defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
            /* Used for cmpxchg_double in slub */
            unsigned long counters;
#else
            /*
             * Keep _count separate from slub cmpxchg_double data.
             * As the rest of the double word is protected by
             * slab_lock but _count is not.
             */
            unsigned counters;
#endif
            struct {
                union {
                    /*
                     * Count of ptes mapped in mms, to show
                     * when page is mapped & limit reverse
                     * map searches.
                     */
                    atomic_t _mapcount;
                    struct { /* SLUB */
                        unsigned inuse:16;
                        unsigned objects:15;
                        unsigned frozen:1;
                    };
                    int units;  /* SLOB */
                };
                atomic_t _count;        /* Usage count, see below. */
            };
            unsigned int active;    /* SLAB */
        };
    };
    /*
     * Third double word block
     *
     * WARNING: bit 0 of the first word encode PageTail(). That means
     * the rest users of the storage space MUST NOT use the bit to
     * avoid collision and false-positive PageTail().
     */
    union {
        struct list_head lru;   /* Pageout list, eg. active_list
                     * protected by zone->lru_lock !
                     * Can be used as a generic list
                     * by the page owner.
                     */
        struct dev_pagemap *pgmap; /* ZONE_DEVICE pages are never on an
                        * lru or handled by a slab
                        * allocator, this points to the
                        * hosting device page map.
                        */
        struct {        /* slub per cpu partial pages */
            struct page *next;  /* Next partial slab */
#ifdef CONFIG_64BIT
            int pages;  /* Nr of partial slabs left */
            int pobjects;   /* Approximate # of objects */
#else
            short int pages;
            short int pobjects;
#endif
        };
        struct rcu_head rcu_head;   /* Used by SLAB
                         * when destroying via RCU
                         */
        /* Tail pages of compound page */
        struct {
            unsigned long compound_head; /* If bit zero is set */
            /* First tail page only */
#ifdef CONFIG_64BIT
            /*
             * On 64 bit system we have enough space in struct page
             * to encode compound_dtor and compound_order with
             * unsigned int. It can help compiler generate better or
             * smaller code on some archtectures.
             */
            unsigned int compound_dtor;
            unsigned int compound_order;
#else
            unsigned short int compound_dtor;
            unsigned short int compound_order;
#endif
        };
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS
        struct {
            unsigned long __pad;    /* do not overlay pmd_huge_pte
                         * with compound_head to avoid
                         * possible bit 0 collision.
                         */
            pgtable_t pmd_huge_pte; /* protected by page->ptl */
        };
#endif
    };
    /* Remainder is not double word aligned */
    union {
        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.
                         */
#if USE_SPLIT_PTE_PTLOCKS
#if ALLOC_SPLIT_PTLOCKS
        spinlock_t *ptl;
#else
        spinlock_t ptl;
#endif
#endif
        struct kmem_cache *slab_cache;  /* SL[AU]B: Pointer to slab */
    };
#ifdef CONFIG_MEMCG
    struct mem_cgroup *mem_cgroup;
#endif
    /*
     * 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_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
#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
    int _last_cpupid;
#endif
}

因为内核会为每一个物理页帧创建一个struct page的结构体，因此要保证page结构体足够的小，否则仅struct page就要占用大量的内存。出于节省内存的考虑，struct page中使用了大量的联合体union。下面仅对常用的一些的字段做说明：

• flags：描述page的状态和其他信息。
• _count：引用计数，表示内核中引用该page的次数，如果要操作该page，引用计数会+1，操作完成-1。当该值为0时，表示没有引用该page的位置，所以该page可以被解除映射，这往往在内存回收时是有用的。
• _mapcount：被页表映射的次数，也就是说该page同时被多少个进程共享。初始值为-1，如果只被一个进程的页表映射了，该值为0 。

注意区分_count和_mapcount，_mapcount表示的是映射次数，而_count表示的是使用次数；被映射了不一定在使用，但要使用必须先映射。

• mapping：有三种含义
  a: 如果mapping = 0，说明该page属于交换缓存（swap cache）；当需要使用地址空间时会指定交换分区的地址空间swap_space。
  b: 如果mapping != 0，bit[0] = 0，说明该page属于页缓存或文件映射，mapping指向文件的地址空间address_space。
  c: 如果mapping != 0，bit[0] != 0，说明该page为匿名映射，mapping指向struct anon_vma对象。

参考资料:
1. chinaunix 夕阳下的孤影