Linux+page+cache+里的几个函数的源码分析

page cache在linuxvfs 中是比较重要的一层，其功能就不详细介绍了。主要介绍了几个关键性函数，容易帮助了解page cache里的整体逻辑和流程

先看一下page 的结构体

/*
 * 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.
 */
struct page {
	unsigned long flags;		/* Atomic flags, some possibly
					 * updated asynchronously */
	atomic_t _count;		/* Usage count, see below. */
	atomic_t _mapcount;		/* Count of ptes mapped in mms,
					 * to show when page is mapped
					 * & limit reverse map searches.
					 */
	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 NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS
	    spinlock_t ptl;
#endif
	};
	pgoff_t index;			/* Our offset within mapping. */
	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 */
};

page_cache_get() 主要是调用函数get_page

static inline void get_page(struct page *page)
{
	if (unlikely(PageCompound(page)))
		page = (struct page *)page_private(page);
	atomic_inc(&page->_count);
}

主要page里的计数器+1，表示page引用的reference 次数

page_cache_release() 的核心函数 put_page_testzero

static inline int put_page_testzero(struct page *page)
{
	BUG_ON(atomic_read(&page->_count) == 0);
	return atomic_dec_and_test(&page->_count);
}

显然是page的计数器-1, page的引用被释放

page 的flags 参数， 在page 的结构体里定义了flags参数，用bit位来标识page的状态，定义在page-flags.h文件里

这是在32位机 和 64位 系统的关于flags 定义

32 bit -------------------------------| FIELDS | FLAGS |
64 bit | FIELDS | ?????? FLAGS |
63 32 0

从bit0-bit19是常用的，其他位保留给了mapping zone, node and SPARSEMEM

#define PG_locked	 	 0	/* Page is locked. Don't touch. */
#define PG_error		 1
#define PG_referenced		 2
#define PG_uptodate		 3

#define PG_dirty	 	 4
#define PG_lru			 5
#define PG_active		 6
#define PG_slab			 7	/* slab debug (Suparna wants this) */

#define PG_checked		 8	/* kill me in 2.5.<early>. */
#define PG_arch_1		 9
#define PG_reserved		10
#define PG_private		11	/* Has something at ->private */

#define PG_writeback		12	/* Page is under writeback */
#define PG_nosave		13	/* Used for system suspend/resume */
#define PG_compound		14	/* Part of a compound page */
#define PG_swapcache		15	/* Swap page: swp_entry_t in private */

#define PG_mappedtodisk		16	/* Has blocks allocated on-disk */
#define PG_reclaim		17	/* To be reclaimed asap */
#define PG_nosave_free		18	/* Free, should not be written */
#define PG_buddy		19	/* Page is free, on buddy lists */

SetPageUptodate 原子设置bit PG_uptodate 状态为1，表示改页被更新

#define SetPageUptodate(page)set_bit(PG_uptodate, &(page)->flags)

ClearPageUptodate 原子设置bit PG_uptodate 状态为0，表示页没有被更新

#define ClearPageUptodate(page)clear_bit(PG_uptodate, &(page)->flags)

TestSetPageLocked 设置原子设置page locked状态，并返回改变前的原来状态

#define TestSetPageLocked(page)		\
		test_and_set_bit(PG_locked, &(page)->flags)

__lock_page 函数

void fastcall __lock_page(struct page *page)
{
	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);

	__wait_on_bit_lock(page_waitqueue(page), &wait, sync_page,
							TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(__lock_page);

将当前进程设置成Task_uninterruptible状态，并将进程挂载到 wait对队列中，如果PG_Locked的状态为1时，触发sync_page的方法，只有在sync_page方法中才会调用schedule()调度当前进程，直到PG_locked的状态为0，注意当执行完__wait_on_bit_lock 的时候PG_locked仍然是1，因为__wait_on_bit_lock是用test_and_set_bit来进行while条件判断的,最后将进程设置成TASK_RUNNING 状态，把该进程从wait 队列中移除。

unlock_page 函数

void fastcall unlock_page(struct page *page)
{
	smp_mb__before_clear_bit();
	if (!TestClearPageLocked(page))
		BUG();
	smp_mb__after_clear_bit(); 
	wake_up_page(page, PG_locked);
}
EXPORT_SYMBOL(unlock_page);

设置PG_Locked 的状态是0，遍历等待队列，执行唤醒函数

static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
			     int nr_exclusive, int sync, void *key)
{
	struct list_head *tmp, *next;

	list_for_each_safe(tmp, next, &q->task_list) {
		wait_queue_t *curr = list_entry(tmp, wait_queue_t, task_list);
		unsigned flags = curr->flags;

		if (curr->func(curr, mode, sync, key) &&
				(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
			break;
	}
}

其中func的定义是

.func		= autoremove_wake_function,

在autoremove_wake_function里，调用sched.c 的default_wake_function -> try_to_wake_up

将等待队列里的线程状态置为 TASK_RUNNING 并放置到运行队列中去