Linux+page+cache+裡的幾個函數的源碼分析

page cache 在linux vfs 中是比較重要的一層，其功能就不詳細介紹了。主要介紹了幾個關鍵性函數，容易幫助了解page cache裡的整體邏輯和流程

先看一下page 的結構體

1. /*
2. * Each physical page in the system has a struct page associated with
3. * it to keep track of whatever it is we are using the page for at the
4. * moment. Note that we have no way to track which tasks are using
5. * a page.
6. */
7. struct page {
8. unsigned long flags; /* Atomic flags, some possibly
9. * updated asynchronously */
10. atomic_t _count; /* Usage count, see below. */
11. atomic_t _mapcount; /* Count of ptes mapped in mms,
12. * to show when page is mapped
13. * & limit reverse map searches.
14. */
15. union {
16. struct {
17. unsigned long private; /* Mapping-private opaque data:
18. * usually used for buffer_heads
19. * if PagePrivate set; used for
20. * swp_entry_t if PageSwapCache;
21. * indicates order in the buddy
22. * system if PG_buddy is set.
23. */
24. struct address_space *mapping; /* If low bit clear, points to
25. * inode address_space, or NULL.
26. * If page mapped as anonymous
27. * memory, low bit is set, and
28. * it points to anon_vma object:
29. * see PAGE_MAPPING_ANON below.
30. */
31. };
32. #if NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS
33. spinlock_t ptl;
34. #endif
35. };
36. pgoff_t index; /* Our offset within mapping. */
37. struct list_head lru; /* Pageout list, eg. active_list
38. * protected by zone->lru_lock !
39. */
40. /*
41. * On machines where all RAM is mapped into kernel address space,
42. * we can simply calculate the virtual address. On machines with
43. * highmem some memory is mapped into kernel virtual memory
44. * dynamically, so we need a place to store that address.
45. * Note that this field could be 16 bits on x86 ... ;)
46. *
47. * Architectures with slow multiplication can define
48. * WANT_PAGE_VIRTUAL in asm/page.h
49. */
50. #if defined(WANT_PAGE_VIRTUAL)
51. void *virtual; /* Kernel virtual address (NULL if
52. not kmapped, ie. highmem) */
53. #endif /* WANT_PAGE_VIRTUAL */
54. };

page_cache_get() 主要是調用函數get_page

1. static inline void get_page(struct page *page)
2. {
3. if (unlikely(PageCompound(page)))
4. page = (struct page *)page_private(page);
5. atomic_inc(&page->_count);
6. }

主要page裡的計數器+1，表示page引用的reference 次數

page_cache_release() 的核心函數 put_page_testzero

1. static inline int put_page_testzero(struct page *page)
2. {
3. BUG_ON(atomic_read(&page->_count) == 0);
4. return atomic_dec_and_test(&page->_count);
5. }

顯然是page的計數器-1, page的引用被釋放