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1. int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
2. {
3. unsigned long free, allowed;
4. vm_acct_memory(pages);
5. /*
6. * Sometimes we want to use more memory than we have
7. */
8. if (sysctl_overcommit_memory == OVERCOMMIT_ALWAYS) //overcommit_memory=1，直接返回成功，不做任何限制。
9. return 0;
10. if (sysctl_overcommit_memory == OVERCOMMIT_GUESS) { //overcommit_memory=0，启发式方式，根据当前系统中空闲内存状况来决定是否可以分配内存。
11. unsigned long n;
12. free = global_page_state(NR_FILE_PAGES);
13. free += nr_swap_pages;
14. /*
15. * Any slabs which are created with the
16. * SLAB_RECLAIM_ACCOUNT flag claim to have contents
17. * which are reclaimable, under pressure. The dentry
18. * cache and most inode caches should fall into this
19. */
20. free += global_page_state(NR_SLAB_RECLAIMABLE);
21. /*
22. * Leave the last 3% for root
23. */
24. if (!cap_sys_admin)
25. free -= free / 32; //root用户可以在free更少(3%)的时候，分配内存。
26. if (free > pages) // pages为需要分配的内存大小，free为根据一定规则算出来的“空闲内存大小”，第一次free仅为NR_FILE_PAGES+NR_SLAB_RECLAIMABLE，由于直接或者系统中“实际空闲”内存代价比较大，所以进行分阶判断，提高效率。
27. return 0;
28. /*
29. * nr_free_pages() is very expensive on large systems,
30. * only call if we're about to fail.
31. */
32. n = nr_free_pages(); //当第一次判断不满足内存分配条件时，再进行“实际空闲”内存的获取操作。
33. /*
34. * Leave reserved pages. The pages are not for anonymous pages.
35. */
36. if (n <= totalreserve_pages)
37. goto error;
38. else
39. n -= totalreserve_pages;
40. /*
41. * Leave the last 3% for root
42. */
43. if (!cap_sys_admin)
44. n -= n / 32;
45. free += n;
46. if (free > pages)
47. return 0;
48. goto error;
49. }
50. allowed = (totalram_pages - hugetlb_total_pages()) //当overcommit_memory=2时，根据系统中虚拟地址空间的总量来进行限制。
51. * sysctl_overcommit_ratio / 100;
52. /*
53. * Leave the last 3% for root
54. */
55. if (!cap_sys_admin)
56. allowed -= allowed / 32;
57. allowed += total_swap_pages;
58. /* Don't let a single process grow too big:
59. leave 3% of the size of this process for other processes */
60. if (mm)
61. allowed -= mm->total_vm / 32;
62. if (percpu_counter_read_positive(&vm_committed_as) < allowed)
63. return 0;
64. error:
65. vm_unacct_memory(pages);
66. return -ENOMEM;
67. }