Linux內存管理之slab機制（釋放對象）

代碼執行流程：

1，當本地CPU cache中空閒對象數小於規定上限時，只需將對象放入本地CPU cache中；

2，當local cache中對象過多（大於等於規定上限），需要釋放一批對象到slab三鏈中。由函數cache_flusharray()實現。

1）如果三鏈中存在共享本地cache，那麼首先選擇釋放到共享本地cache中，能釋放多少是多少；

2）如果沒有shared local cache，釋放對象到slab三鏈中，實現函數為free_block()。對於free_block()函數，當三鏈中的空閒對象數過多時，銷毀此cache。不然，添加此slab到空閒鏈表。因為在分配的時候我們看到將slab結構從cache鏈表中脫離了，在這裡，根據page描述符的lru找到slab並將它添加到三鏈的空閒鏈表中。

主實現

1. /*
2. * Release an obj back to its cache. If the obj has a constructed state, it must
3. * be in this state _before_ it is released. Called with disabled ints.
4. */
5. static inline void __cache_free(struct kmem_cache *cachep, void *objp)
6. {
7. /* 獲得本CPU的local cache */
8. struct array_cache *ac = cpu_cache_get(cachep);
10. check_irq_off();
11. kmemleak_free_recursive(objp, cachep->flags);
12. objp = cache_free_debugcheck(cachep, objp, __builtin_return_address(0));
14. kmemcheck_slab_free(cachep, objp, obj_size(cachep));
16. /*
17. * Skip calling cache_free_alien() when the platform is not numa.
18. * This will avoid cache misses that happen while accessing slabp (which
19. * is per page memory reference) to get nodeid. Instead use a global
20. * variable to skip the call, which is mostly likely to be present in
21. * the cache.
22. *//* NUMA相關 */
23. if (nr_online_nodes > 1 && cache_free_alien(cachep, objp))
24. return;
26. if (likely(ac->avail < ac->limit)) {
27. /* local cache中的空閒對象數小於上限時
28. ，只需將對象釋放回entry數組中 */
29. STATS_INC_FREEHIT(cachep);
30. ac->entry[ac->avail++] = objp;
31. return;
32. } else {
33. /* 大於等於上限時， */
34. STATS_INC_FREEMISS(cachep);
35. /* local cache中對象過多，需要釋放一批對象到slab三鏈中。*/
36. cache_flusharray(cachep, ac);
37. ac->entry[ac->avail++] = objp;
38. }
39. }

釋放對象到三鏈中

1. /*local cache中對象過多，需要釋放一批對象到slab三鏈中。*/
2. static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)
3. {
4. int batchcount;
5. struct kmem_list3 *l3;
6. int node = numa_node_id();
7. /* 每次���放多少個對象 */
8. batchcount = ac->batchcount;
9. #if DEBUG
10. BUG_ON(!batchcount || batchcount > ac->avail);
11. #endif
12. check_irq_off();
13. /* 獲得此cache的slab三鏈 */
14. l3 = cachep->nodelists[node];
15. spin_lock(&l3->list_lock);
16. if (l3->shared) {
17. /* 如果存在shared local cache，將對象釋放到其中 */
18. struct array_cache *shared_array = l3->shared;
19. /* 計算shared local cache中還有多少空位 */
20. int max = shared_array->limit - shared_array->avail;
21. if (max) {
22. /* 空位數小於要釋放的對象數時，釋放數等於空位數 */
23. if (batchcount > max)
24. batchcount = max;
25. /* 釋放local cache前面的幾個對象到shared local cache中
26. ，前面的是最早不用的 */
27. memcpy(&(shared_array->entry[shared_array->avail]),
28. ac->entry, sizeof(void *) * batchcount);
29. /* 增加shared local cache可用對象數 */
30. shared_array->avail += batchcount;
31. goto free_done;
32. }
33. }
34. /* 無shared local cache，釋放對象到slab三鏈中 */
35. free_block(cachep, ac->entry, batchcount, node);
36. free_done:
37. #if STATS
38. {
39. int i = 0;
40. struct list_head *p;
42. p = l3->slabs_free.next;
43. while (p != &(l3->slabs_free)) {
44. struct slab *slabp;
46. slabp = list_entry(p, struct slab, list);
47. BUG_ON(slabp->inuse);
49. i++;
50. p = p->next;
51. }
52. STATS_SET_FREEABLE(cachep, i);
53. }
54. #endif
55. spin_unlock(&l3->list_lock);
56. /* 減少local cache可用對象數*/
57. ac->avail -= batchcount;
58. /* local cache前面有batchcount個空位，將後面的對象依次前移batchcount位 */
59. memmove(ac->entry, &(ac->entry[batchcount]), sizeof(void *)*ac->avail);
60. }