Linux內存管理之伙伴系統（內存釋放）

Linux內核伙伴系統中頁面釋放，主函數為free_pages()

一、上層操作

1. /*用虛擬地址進行釋放*/
2. void free_pages(unsigned long addr, unsigned int order)
3. {
4. if (addr != 0) {
5. VM_BUG_ON(!virt_addr_valid((void *)addr));
6. __free_pages(virt_to_page((void *)addr), order);/*具體的釋放函數*/
7. }
8. }

1. /*釋放頁面*/
2. void __free_pages(struct page *page, unsigned int order)
3. {
4. if (put_page_testzero(page)) {/*count值減一為0時釋放*/
5. /*調試*/
6. trace_mm_page_free_direct(page, order);
7. if (order == 0)
8. free_hot_page(page);/*釋放單個頁面*/
9. else
10. __free_pages_ok(page, order);
11. }
12. }

二、釋放單個頁面

釋放單個頁面free_hot_page()調用free_hot_cold_page()函數

1. static void free_hot_cold_page(struct page *page, int cold)
2. {
3. struct zone *zone = page_zone(page);
4. struct per_cpu_pages *pcp;
5. unsigned long flags;
6. int migratetype;
7. int wasMlocked = __TestClearPageMlocked(page);
8. /*調試代碼*/
9. kmemcheck_free_shadow(page, 0);
11. if (PageAnon(page))
12. page->mapping = NULL;
13. if (free_pages_check(page))
14. return;
16. if (!PageHighMem(page)) {
17. debug_check_no_locks_freed(page_address(page), PAGE_SIZE);
18. debug_check_no_obj_freed(page_address(page), PAGE_SIZE);
19. }
20. /*x86下為空*/
21. arch_free_page(page, 0);
22. /*調試用*/
23. kernel_map_pages(page, 1, 0);
24. /*獲得zone對應cpu的pcp*/
25. pcp = &zone_pcp(zone, get_cpu())->pcp;
26. /*獲得頁面的migratetype*/
27. migratetype = get_pageblock_migratetype(page);
28. set_page_private(page, migratetype);/*設置私有位為參數*/
29. local_irq_save(flags);/*保存中斷*/
30. if (unlikely(wasMlocked))
31. free_page_mlock(page);
32. __count_vm_event(PGFREE);
34. /*
35. * We only track unmovable, reclaimable and movable on pcp lists.
36. * Free ISOLATE pages back to the allocator because they are being
37. * offlined but treat RESERVE as movable pages so we can get those
38. * areas back if necessary. Otherwise, we may have to free
39. * excessively into the page allocator
40. */
41. if (migratetype >= MIGRATE_PCPTYPES) {
42. if (unlikely(migratetype == MIGRATE_ISOLATE)) {
43. /*釋放到伙伴系統 */
44. free_one_page(zone, page, 0, migratetype);
45. goto out;
46. }
47. migratetype = MIGRATE_MOVABLE;
48. }
50. if (cold)/*加入到pcp鏈表尾部*/
51. list_add_tail(&page->lru, &pcp->lists[migratetype]);
52. else/*加入到pcp鏈表頭部*/
53. list_add(&page->lru, &pcp->lists[migratetype]);
54. pcp->count++;/*pcp計數加一*/
55. if (pcp->count >= pcp->high) {/*當pcp中頁面數量超過他的最高值時，
56. 釋放pcp->batch個頁面到伙伴系統中*/
57. free_pcppages_bulk(zone, pcp->batch, pcp);
58. pcp->count -= pcp->batch;/*頁面數減去釋放的頁面數量*/
59. }
61. out:
62. local_irq_restore(flags);/*回復中斷*/
63. put_cpu();
64. }

從pcp中釋放頁面到伙伴系統中

free_pcppages_bulk()

1. are in same zone, and of same order.
2. * count is the number of pages to free.
3. *
4. * If the zone was previously in an "all pages pinned" state then look to
5. * see if this freeing clears that state.
6. *
7. * And clear the zone's pages_scanned counter, to hold off the "all pages are
8. * pinned" detection logic.
9. */
10. /*從PCP中釋放count個頁面到伙伴系統中*/
11. static void free_pcppages_bulk(struct zone *zone, int count,
12. struct per_cpu_pages *pcp)
13. {
14. int migratetype = 0;
15. int batch_free = 0;
16. /*
17. * 雖然管理區可以按照CPU節點分類，但是也可以跨CPU節點進行內存分配，
18. * 因此這裡需要用自旋鎖保護管理區
19. * 使用每CPU緩存的目的，也是為了減少使用這把鎖。
20. */
21. spin_lock(&zone->lock);
22. /* all_unreclaimable代表了內存緊張程度，釋放內存後，將此標志清除 */
23. zone_clear_flag(zone, ZONE_ALL_UNRECLAIMABLE);
24. zone->pages_scanned = 0;/* pages_scanned代表最後一次內存緊張以來，頁面回收過程已經掃描的頁數。
25. 目前正在釋放內存，將此清0，待回收過程隨後回收時重新計數 */
27. /*增加管理區空閒頁數*/
28. __mod_zone_page_state(zone, NR_FREE_PAGES, count);
29. while (count) {
30. struct page *page;
31. struct list_head *list;
33. /*
34. * Remove pages from lists in a round-robin fashion. A
35. * batch_free count is maintained that is incremented when an
36. * empty list is encountered. This is so more pages are freed
37. * off fuller lists instead of spinning excessively around empty
38. * lists
39. */
40. do {/*從pcp的三類鏈表中找出不空的一個，釋放*/
41. batch_free++;/*參看英文注釋*/
42. if (++migratetype == MIGRATE_PCPTYPES)
43. migratetype = 0;
44. list = &pcp->lists[migratetype];
45. } while (list_empty(list));
47. do {
48. page = list_entry(list->prev, struct page, lru);
49. /* must delete as __free_one_page list manipulates */
50. list_del(&page->lru);
51. /*釋放單個頁面到伙伴系統，注意這裡的分類回收*/
52. __free_one_page(page, zone, 0, migratetype);
53. trace_mm_page_pcpu_drain(page, 0, migratetype);
54. } while (--count && --batch_free && !list_empty(list));
55. }
56. spin_unlock(&zone->lock);
57. }