页高速缓存
页高速缓存是Linux内核实现磁盘缓存，它的主要作用是减少对磁盘的IO操作。
页高速缓存是由内存中的物理页组成的，其内容对应磁盘上的物理块。
当内核开始一个读操作时，会先检查需要的数据是否在页高速缓存中。
写缓存有几种策略：
- 不缓存；
- 同时更新内存缓存和磁盘文件；
- 回写：数据直接写到内存缓存，而磁盘写放到回写进程中稍后处理（由flusher线程完成，用户进程中可以通过sync()和fsync()系统调用来完成）；
页高速缓存能够动态调整。
当需要腾出空间给其它地方使用是，会采用的是缓存回收策略回收内存缓存。
Linux采用双链策略。
页高速缓存缓存任何基于页的对象，包括各种类型的文件和各种类型的内存映射。（前面有提到主要是为了磁盘缓存，但是实际上可以缓存的有很多）
address_space对象
Linux页高速缓存使用address_space结构体管理缓存项和页IO操作。
address_space结构体如下：
struct address_space {
struct inode *host; /* owner: inode, block_device */
struct radix_tree_root page_tree; /* radix tree of all pages */
spinlock_t tree_lock; /* and lock protecting it */
unsigned int i_mmap_writable;/* count VM_SHARED mappings */
struct prio_tree_root i_mmap; /* tree of private and shared mappings */
struct list_head i_mmap_nonlinear;/*list VM_NONLINEAR mappings */
spinlock_t i_mmap_lock; /* protect tree, count, list */
unsigned int truncate_count; /* Cover race condition with truncate */
unsigned long nrpages; /* number of total pages */
pgoff_t writeback_index;/* writeback starts here */
const struct address_space_operations *a_ops; /* methods */
unsigned long flags; /* error bits/gfp mask */
struct backing_dev_info *backing_dev_info; /* device readahead, etc */
spinlock_t private_lock; /* for use by the address_space */
struct list_head private_list; /* ditto */
struct address_space *assoc_mapping; /* ditto */
} __attribute__((aligned(sizeof(long))));其中a_ops指向地址空间对象中的操作函数表：
struct address_space_operations {
int (*writepage)(struct page *page, struct writeback_control *wbc);
int (*readpage)(struct file *, struct page *);
void (*sync_page)(struct page *);
/* Write back some dirty pages from this mapping. */
int (*writepages)(struct address_space *, struct writeback_control *);
/* Set a page dirty. Return true if this dirtied it */
int (*set_page_dirty)(struct page *page);
int (*readpages)(struct file *filp, struct address_space *mapping,
struct list_head *pages, unsigned nr_pages);
int (*write_begin)(struct file *, struct address_space *mapping,
loff_t pos, unsigned len, unsigned flags,
struct page **pagep, void **fsdata);
int (*write_end)(struct file *, struct address_space *mapping,
loff_t pos, unsigned len, unsigned copied,
struct page *page, void *fsdata);
/* Unfortunately this kludge is needed for FIBMAP. Don't use it */
sector_t (*bmap)(struct address_space *, sector_t);
void (*invalidatepage) (struct page *, unsigned long);
int (*releasepage) (struct page *, gfp_t);
ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
loff_t offset, unsigned long nr_segs);
int (*get_xip_mem)(struct address_space *, pgoff_t, int,
void **, unsigned long *);
/* migrate the contents of a page to the specified target */
int (*migratepage) (struct address_space *,
struct page *, struct page *);
int (*launder_page) (struct page *);
int (*is_partially_uptodate) (struct page *, read_descriptor_t *,
unsigned long);
int (*error_remove_page)(struct address_space *, struct page *);
};