page cache 在linux vfs 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
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#define PG_locked0 /* Page is locked. Don't touch. */
#define PG_error 1
#define PG_referenced2
#define PG_uptodate 3
#define PG_dirty 4
#define PG_lru 5
#define PG_active6
#define PG_slab 7 /* slab debug (Suparna wants this) */
#define PG_checked 8 /* kill me in 2.5.<early>. */
#define PG_arch_19
#define PG_reserved 10
#define PG_private 11 /* Has something at ->private */
#define PG_writeback12 /* Page is under writeback */
#define PG_nosave 13 /* Used for system suspend/resume */
#define PG_compound 14 /* Part of a compound page */
#define PG_swapcache15 /* 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_buddy19 /* 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。