深入理解 Linux 内核（二）（下）

// include/linux/mm_types.h
/*
 * 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.
 */
struct page {
  /* 一组标志（参见表 8-2）。也对页框所在的管理区进行编号 */
  unsigned long flags;    /* Atomic flags, some possibly
                 * updated asynchronously */
  /* 页框的引用计数器 */
  atomic_t _count;    /* Usage count, see below. */
  union {
    /* 页框中的页表项数目（如果没有则为 -1） */
    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 {
      /* 可用于正在使用页的内核成分（例如，在缓冲页的情况下它是一个缓冲器
       * 头指针；参见第15章的 “块缓冲区和缓冲区首部” 一节）。如果页是
       * 空闲的，则该字段由伙伴系统使用 */
    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.
             */
    /* 当页被插入页高速缓存中时使用（参见第15章 “页高速缓存” 一节），
     * 或者当页属于匿名区时使用（参见第17章的 “匿名页的反向映射” 
     * 一节） */        
    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 {
    /* 作为不同的含义被几种内核成分使用。例如，它在页磁盘映像
     * 或匿名区中标识存放在页框中的数据的位置（参见第15章），
     * 或者它存放一个换出页标识符（第17章） */
    pgoff_t index;    /* Our offset within mapping. */
    void *freelist;   /* SLUB: freelist req. slab lock */
  };
  /* 包含页的最近最少使用（LRU）双向链表的指针 */
  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
};

// include/linux/mmzone.h
typedef struct pglist_data {
  /* 节点中管理区描述符的数组 */
  struct zone node_zones[MAX_NR_ZONES];
  /* 页分配器使用的 zonelist 数据结构的数组
   * （参见后面 “内存管理区” 一节） */
  struct zonelist node_zonelists[MAX_ZONELISTS];
  /* 节点中管理区的个数 */
  int nr_zones;
#ifdef CONFIG_FLAT_NODE_MEM_MAP /* means !SPARSEMEM */
  /* 节点中页描述符的数组 */
  struct page *node_mem_map;
#ifdef CONFIG_CGROUP_MEM_RES_CTLR
  struct page_cgroup *node_page_cgroup;
#endif
#endif
#ifndef CONFIG_NO_BOOTMEM
  /* 用在内核初始化阶段 */
  struct bootmem_data *bdata;
#endif
#ifdef CONFIG_MEMORY_HOTPLUG
  /*
   * Must be held any time you expect node_start_pfn, node_present_pages
   * or node_spanned_pages stay constant.  Holding this will also
   * guarantee that any pfn_valid() stays that way.
   *
   * Nests above zone->lock and zone->size_seqlock.
   */
  spinlock_t node_size_lock;
#endif
  /* 节点中第一个页框的下标 */
  unsigned long node_start_pfn;
  /* 内存节点的大小，不包括洞（以页框为单位） */
  unsigned long node_present_pages; /* total number of physical pages */
  /* 节点的大小，包括洞（以页框为单位） */
  unsigned long node_spanned_pages; /* total size of physical page
               range, including holes */
  /* 节点标识符 */            
  int node_id;
  /* kswapd 页换出守护进程使用的等待队列
   * （参加第17章的 “周期回收” 一节） */
  wait_queue_head_t kswapd_wait;
  /* 指针指向 kswapd 内核线程的进程描述符  */
  struct task_struct *kswapd;
  /* kswapd 将要创建的空闲块大小取对数的值 */
  int kswapd_max_order;
} pg_data_t;