第十六章 页高速缓存和页回写

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页高速缓存和页回写

页高速缓存为了减少对磁盘的I/O操作，将磁盘数据缓存在物理内存中，提高访问速度。

  • 访问内存的速度比访问硬盘速度快几个数量级
  • 局部性原理

1. 缓存方法

页高速缓存由内存中的物理页面组成，对应磁盘上的物理块。

写缓存策略：

  • Write-through: write is done synchronously both to the cache and to the backing store.
  • Write-back (also called write-behind): initially, writing is done only to the cache. The write to the backing store is postponed until the cache blocks containing the data are about to be modified/replaced by new content.

缓存回收策略

Page replacement algorithm

2. Linux页高速缓存

Linux页高速缓存的目标是缓存任何基于页的对象，包含各种类型的文件和各种类型的内存映射。为了普遍性，Linux使用一个新对象管理缓存项和页I/O操作——address_space结构体。

  • address_space结构体
    address_space定义在linux/fs.h中，对于一个文件，它可以对应多个vm_area_struct，但只有一个address_space。
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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 */
	atomic_t		i_mmap_writable;/* count VM_SHARED mappings */
	struct rb_root		i_mmap;		/* tree of private and shared mappings */
	struct rw_semaphore	i_mmap_rwsem;	/* protect tree, count, list */
	/* Protected by tree_lock together with the radix tree */
	unsigned long		nrpages;	/* number of total pages */
	/* number of shadow or DAX exceptional entries */
	unsigned long		nrexceptional;
	pgoff_t			writeback_index;/* writeback starts here */
	const struct address_space_operations *a_ops;	/* methods */
	unsigned long		flags;		/* error bits */
	spinlock_t		private_lock;	/* for use by the address_space */
	gfp_t			gfp_mask;	/* implicit gfp mask for allocations */
	struct list_head	private_list;	/* ditto */
	void			*private_data;	/* ditto */
} __attribute__((aligned(sizeof(long))));
	/*
	 * On most architectures that alignment is already the case; but
	 * must be enforced here for CRIS, to let the least significant bit
	 * of struct page's "mapping" pointer be used for PAGE_MAPPING_ANON.
	 */
  • address_space操作
    与vfs对象类似，a_ops域指向了address_space相关的操作函数，由结构体address_space_operations表示。
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struct address_space_operations {
	int (*writepage)(struct page *page, struct writeback_control *wbc);
	int (*readpage)(struct file *, 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 int, unsigned int);
	int (*releasepage) (struct page *, gfp_t);
	void (*freepage)(struct page *);
	ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
	/*
	 * migrate the contents of a page to the specified target. If
	 * migrate_mode is MIGRATE_ASYNC, it must not block.
	 */
	int (*migratepage) (struct address_space *,
			struct page *, struct page *, enum migrate_mode);
	bool (*isolate_page)(struct page *, isolate_mode_t);
	void (*putback_page)(struct page *);
	int (*launder_page) (struct page *);
	int (*is_partially_uptodate) (struct page *, unsigned long,
					unsigned long);
	void (*is_dirty_writeback) (struct page *, bool *, bool *);
	int (*error_remove_page)(struct address_space *, struct page *);
	/* swapfile support */
	int (*swap_activate)(struct swap_info_struct *sis, struct file *file,
				sector_t *span);
	void (*swap_deactivate)(struct file *file);
};

3. flush线程

由于页高速缓存的存在，写操作会被延迟，内存中的脏数据会在特定情况下写回磁盘：

  • 当空闲内存低于一个特定阈值，内核必须将脏页写回磁盘释放内存
  • 当脏页在内存中驻留时间超过一个特定的阈值，内核必须将超时脏页写回磁盘
  • 用户调用sync()和fsync()，内核会按要求执行回写操作

这些工作由内核的flusher线程执行。实现代码在mm/page_writeback.c和mm/backing-dev.c，回写机制实现在fs/fs-writeback.c

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