进程需要的连续的页面，通过alloc_pages来完成
该函数在mm/numa.c和mm/page_alloc.h中都有定义
NUMA和UMA分配内存的函数是不并存的，根据CONFIG_DISCONTIGMEM的勾选与否选择其中一个
1. NUMA的分配内存函数：
选择CONFIG_DISCONTIGMEM选项
被编译的条件是“不连续的存储空间”，而不是CONFIG_NUMA，但是CONFIG_NUMA会对程序造成影响
alloc_pages（int gfp_mask,unsigned long order）
gfp_mask：表示采用哪一种分配策略
order：申请2^order个页面
numa的alloc_pages的代码：
如果定义了NUMA，就需要获取
pgdat_list，并且需要遍历所有的pg_data_t节点
分配时轮流从各个节点开始，并希望各节点负载均衡
在每个节点上使用
alloc_pages_pgdat函数
1
==================== mm/numa.c 43 43 ====================
2
43 #ifdef CONFIG_DISCONTIGMEM
3
==================== mm/numa.c 91 128 ====================
4
91 /*
5
92 * This can be refined. Currently, tries to do round robin, instead
6
93 * should do concentratic circle search, starting from current node.
7
94 */
8
95 struct page * alloc_pages(int gfp_mask, unsigned long order)
9
96 {
10
97 struct page *ret = 0;
11
98 pg_data_t *start, *temp;
12
99 #ifndef CONFIG_NUMA
13
100 unsigned long flags;
14
101 static pg_data_t *next = 0;
15
102 #endif
16
103
17
104 if (order >= MAX_ORDER)
18
105 return NULL;
19
106 #ifdef CONFIG_NUMA
20
107 temp = NODE_DATA(numa_node_id());
21
108 #else
22
109 spin_lock_irqsave(&node_lock, flags);
23
110 if (!next) next = pgdat_list;
24
111 temp = next;
25
112 next = next->node_next;
26
113 spin_unlock_irqrestore(&node_lock, flags);
27
114 #endif
28
115 start = temp;
29
116 while (temp) {
30
117 if ((ret = alloc_pages_pgdat(temp, gfp_mask, order)))
31
118 return(ret);
32
119 temp = temp->node_next;
33
120 }
34
121 temp = pgdat_list;
35
122 while (temp != start) {
36
123 if ((ret = alloc_pages_pgdat(temp, gfp_mask, order)))
37
124 return(ret);
38
125 temp = temp->node_next;
39
126 }
40
127 return(0);
41
128 }
1.2 alloc_pages_pgdat函数：
NUMA和UMA机制下都使用了相同的函数，在UMA处在做详细介绍
gfp_mask相当于node_zonelists数组的下标
1
85 static struct page * alloc_pages_pgdat(pg_data_t *pgdat, int gfp_mask,
2
86 unsigned long order)
3
87 {
4
88 return __alloc_pages(pgdat->node_zonelists + gfp_mask, order);
5
89 }
2. UMA的分配内存函数： 不选择CONFIG_DISCONTIGMEM选项
该函数只有在CONFIG_DISCONTIGMEM无定义时才编译
很明显在UMA结构下
只有一个pg_data_t节点，也就是
contig_page_data，所以无序遍历
具体的内存分配过程由__alloc_pages()完成
1
343 #ifndef CONFIG_DISCONTIGMEM
2
344 static inline struct page * alloc_pages(int gfp_mask, unsigned long order)
3
345 {
4
346 /*
5
347 * Gets optimized away by the compiler.
6
348 */
7
349 if (order >= MAX_ORDER)
8
350 return NULL;
9
351 return __alloc_pages(contig_page_data.node_zonelists+(gfp_mask), order);
10
352 }
2.1 __alloc_pages函数：
该函数完成了内存的具体分配
zonelist是
contig_page_data节点的中的zone列表，要在这些zone里面找到合适物理页面进行分配
zonelist_t结构中存放了具体的内存分配策略，也就是gfp_task，其实是一些标志位
memory_pressure表示页面管理所受的压力，分配内存页面时增加，归还内存时减少
申请的页面数为1，而且允许等待完成、不用于管理的目的，则我们将
direct_reclaim设置为1，
表示可以从相应的页面管理区的“不活跃页面”中回收，一般而言，这些页面都不是连接成块的，
所以
提供给了单页面请求使用，而且这些页面的内容已经写出到了交换设备中（swap分区）
当发现页面短缺，则需要唤醒kswapd和bdflush线程，腾出空间
1
270 /*
2
271 * This is the 'heart' of the zoned buddy allocator:
3
272 */
4
273 struct page * __alloc_pages(zonelist_t *zonelist, unsigned long order)
5
274 {
6
275 zone_t **zone;
7
276 int direct_reclaim = 0;
8
277 unsigned int gfp_mask = zonelist->gfp_mask;
9
278 struct page * page;
10
279
11
280 /*
12
281 * Allocations put pressure on the VM subsystem.
13
282 */
14
283 memory_pressure++;
15
284
16
285 /*
17
286 * (If anyone calls gfp from interrupts nonatomically then it
18
287 * will sooner or later tripped up by a schedule().)
19
288 *
20
289 * We are falling back to lower-level zones if allocation
21
290 * in a higher zone fails.
22
291 */
23
292
24
293 /*
25
294 * Can we take pages directly from the inactive_clean
26
295 * list?
27
296 */
28
//申请的内存空间为1页，且允许等待
29
297 if (order == 0 && (gfp_mask & __GFP_WAIT) &&
30
298 !(current->flags & PF_MEMALLOC))
31
299 direct_reclaim = 1;
32
300
33
301 /*
34
302 * If we are about to get low on free pages and we also have
35
303 * an inactive page shortage, wake up kswapd.
36
84
37
304 */
38
305 if (inactive_shortage() > inactive_target / 2 && free_shortage())
39
306 wakeup_kswapd(0);
40
307 /*
41
308 * If we are about to get low on free pages and cleaning
42
309 * the inactive_dirty pages would fix the situation,
43
310 * wake up bdflush.
44
311 */
45
312 else if (free_shortage() && nr_inactive_dirty_pages > free_shortage()
46
313 && nr_inactive_dirty_pages >= freepages.high)
47
314 wakeup_bdflush(0);
48
315
对pgdata_t节点中的所有zone进行遍历（其实只有三个zone）
rmqueue从管理区中获取若干连续的内存页，当内存不足时，唤醒
kreclaimd（）
，让其帮助回收页面
1
316 try_again:
2
317 /*
3
318 * First, see if we have any zones with lots of free memory.
4
319 *
5
320 * We allocate free memory first because it doesn't contain
6
321 * any data ... DUH!
7
322 */
8
323 zone = zonelist->zones;
9
//死循环
10
324 for (;;) {
11
325 zone_t *z = *(zone++);
12
326 if (!z)
13
327 break;
14
328 if (!z->size)
15
329 BUG();
16
330
17
331 if (z->free_pages >= z->pages_low) {
18
332 page = rmqueue(z, order); //分配内存
19
333 if (page)
20
334 return page;
21
335 } else if (z->free_pages < z->pages_min &&
22
336 waitqueue_active(&kreclaimd_wait)) {
23
337 wake_up_interruptible(&kreclaimd_wait);
24
338 }
25
339
假如三个zone都失败，要考虑下面的事
1）降低页面管理区的中的“保持水位的
要求”
2）把缓存在管理区中的“不活跃干净页面”考虑进去
PAGES_LOW和
PAGES_HIGH其实表示了不同的free_list，然后使用
__alloc_pages_limit，
申请内存，再次失败，就说明内存真的短缺了
2.6之后的内核引用新的参数（migirate_type），用来表示迁移类型（数值越小说明内存与越紧张）
1
341 /*
2
342 * Try to allocate a page from a zone with a HIGH
3
343 * amount of free + inactive_clean pages.
4
344 *
5
345 * If there is a lot of activity, inactive_target
6
346 * will be high and we'll have a good chance of
7
347 * finding a page using the HIGH limit.
8
348 */
9
349 page = __alloc_pages_limit(zonelist, order, PAGES_HIGH, direct_reclaim);
10
350 if (page)
11
351 return page;
12
352
13
353 /*
14
354 * Then try to allocate a page from a zone with more
15
355 * than zone->pages_low free + inactive_clean pages.
16
356 *
17
357 * When the working set is very large and VM activity
18
358 * is low, we're most likely to have our allocation
19
359 * succeed here.
20
360 */
21
361 page = (zonelist, order, PAGES_LOW, direct_reclaim);
22
362 if (page)
23
363 return page;
24
364
zone中的页面非常短缺，
1）唤醒内核线程
kswapd，让其设法换成一些页面，gfp_mask宁可等待也要申请内存，那就让系统进行一次调度
并让当前进程为其他进程让路，这样kswapd可能会立即执行
2）其他进程可能会释放一些页面，也可减缓了要求分配页面的速度，最后以
PAGES_MIN参数
再次执行
__alloc_pages_limit
当然还是可能会失败
1
365 /*
2
366 * OK, none of the zones on our zonelist has lots
3
367 * of pages free.
4
368 *
5
369 * We wake up kswapd, in the hope that kswapd will
6
370 * resolve this situation before memory gets tight.
7
371 *
8
372 * We also yield the CPU, because that:
9
373 * - gives kswapd a chance to do something
10
374 * - slows down allocations, in particular the
11
375 * allocations from the fast allocator that's
12
376 * causing the problems ...
13
377 * - ... which minimises the impact the "bad guys"
14
378 * have on the rest of the system
15
379 * - if we don't have __GFP_IO set, kswapd may be
16
380 * able to free some memory we can't free ourselves
17
381 */
18
382 wakeup_kswapd(0);
19
383 if (gfp_mask & __GFP_WAIT) {
20
384 __set_current_state(TASK_RUNNING);
21
385 current->policy |= SCHED_YIELD;
22
386 schedule();
23
387 }
24
388
25
389 /*
26
390 * After waking up kswapd, we try to allocate a page
27
391 * from any zone which isn't critical yet.
28
392 *
29
393 * Kswapd should, in most situations, bring the situation
30
394 * back to normal in no time.
31
395 */
32
396 page = __alloc_pages_limit(zonelist, order, PAGES_MIN, direct_reclaim);
33
397 if (page)
34
398 return page;
35
399
如果再次失败，需要查看是谁在有要求内核页面。如果是kswapd和kreclaimd，本身就是“
内存分配工作者”，要求分配内存页面的目的是执行公务，这
比一般进程更重要
这些进程task_struct结构中的flags字段的PF_MEMALLO标志位为1，一般进程为0
失败的原因：
1）可分配页面的数量太少；
2）页面总量不少，但是要求的页面块无法满足，此时往往有很多单个页面在管理区的
inactive_clean_pages中，回收的话，有可能拼装出较大的页面块
inactive_drity_pages队列中，把脏页面的内容写到交换设备上或文件中，可以使他们变成干净页面
加以回收
__free_page()释放页面时，会把空闲页面拼装起尽可能大的页面块，所以在回收每一个页面后都
调用一下rmqueue，看看是否满足要求
在调用
page_launder()期间把当前进程的PF_MEMALLOC标志位设为1，是其有了“执行公务”时的特权
这是因为page_la
uncher也会要求分配一些临时性的工作页面，不把PF_MEMALLOC标志位设为1
还是失败，唤醒kswapd，要求分配页面的进程等待，有kswapd完成一轮运行后，唤醒申请页面的进程
如果申请单个页面，通过goto语句转换__alloc_pages开头处的标号try_again处
另一种方法是直接调用try_to_free_pages，这个函数本来是kswaps调用的
如果是“执行公务”，或者想尽一切办法，只不过因为要求分配的是成块页面，所以才没有装回前面的
标号try_again处
1
400 /*
2
401 * Damn, we didn't succeed.
3
402 *
4
403 * This can be due to 2 reasons:
5
404 * - we're doing a higher-order allocation
6
405 * --> move pages to the free list until we succeed
7
406 * - we're /really/ tight on memory
8
407 * --> wait on the kswapd waitqueue until memory is freed
9
408 */
10
409 if (!(current->flags & PF_MEMALLOC)) {
11
410 /*
12
411 * Are we dealing with a higher order allocation?
13
412 *
14
413 * Move pages from the inactive_clean to the free list
15
414 * in the hope of creating a large, physically contiguous
16
415 * piece of free memory.
17
416 */
18
417 if (order > 0 && (gfp_mask & __GFP_WAIT)) {
19
418 zone = zonelist->zones;
20
419 /* First, clean some dirty pages. */
21
420 current->flags |= PF_MEMALLOC;
22
421 page_launder(gfp_mask, 1);
23
422 current->flags &= ~PF_MEMALLOC;
24
423 for (;;) {
25
424 zone_t *z = *(zone++);
26
425 if (!z)
27
426 break;
28
427 if (!z->size)
29
428 continue;
30
429 while (z->inactive_clean_pages) {
31
430 struct page * page;
32
431 /* Move one page to the free list. */
33
432 page = reclaim_page(z);
34
433 if (!page)
35
434 break;
36
435 __free_page(page);
37
436 /* Try if the allocation succeeds. */
38
437 page = rmqueue(z, order);
39
438 if (page)
40
439 return page;
41
440 }
42
441 }
43
442 }
44
443 /*
45
444 * When we arrive here, we are really tight on memory.
46
445 *
47
446 * We wake up kswapd and sleep until kswapd wakes us
48
447 * up again. After that we loop back to the start.
49
448 *
50
449 * We have to do this because something else might eat
51
450 * the memory kswapd frees for us and we need to be
52
451 * reliable. Note that we don't loop back for higher
53
452 * order allocations since it is possible that kswapd
54
453 * simply cannot free a large enough contiguous area
55
454 * of memory *ever*.
56
455 */
57
456 if ((gfp_mask & (__GFP_WAIT|__GFP_IO)) == (__GFP_WAIT|__GFP_IO)) {
58
457 wakeup_kswapd(1);
59
458 memory_pressure++;
60
459 if (!order)
61
460 goto try_again;
62
461 /*
63
462 * If __GFP_IO isn't set, we can't wait on kswapd because
64
463 * kswapd just might need some IO locks /we/ are holding ...
65
464 *
66
465 * SUBTLE: The scheduling point above makes sure that
67
466 * kswapd does get the chance to free memory we can't
68
467 * free ourselves...
69
468 */
70
469 } else if (gfp_mask & __GFP_WAIT) {
71
470 try_to_free_pages(gfp_mask);
72
471 memory_pressure++;
73
472 if (!order)
74
473 goto try_again;
75
474 }
76
475
77
476 }
78
477
前面使用
__alloc_pages_limit()，其实还有所保留
我们使用
PAGES_MIN为参数，此时判断是否可以分配的准则是管理区中可分配页面的“水位”高于
z->pages_min，所以还留着一些“老本
为了应付紧急情况，已经到了“不惜血本”的时候了，继续下面处理
1
478 /*
2
479 * Final phase: allocate anything we can!
3
480 *
4
481 * Higher order allocations, GFP_ATOMIC allocations and
5
482 * recursive allocations (PF_MEMALLOC) end up here.
6
483 *
7
484 * Only recursive allocations can use the very last pages
8
485 * in the system, otherwise it would be just too easy to
9
486 * deadlock the system...
10
487 */
11
488 zone = zonelist->zones;
12
489 for (;;) {
13
490 zone_t *z = *(zone++);
14
491 struct page * page = NULL;
15
492 if (!z)
16
493 break;
17
494 if (!z->size)
18
495 BUG();
19
496
20
497 /*
21
498 * SUBTLE: direct_reclaim is only possible if the task
22
499 * becomes PF_MEMALLOC while looping above. This will
23
500 * happen when the OOM killer selects this task for
24
501 * instant execution...
25
502 */
26
503 if (direct_reclaim) {
27
504 page = reclaim_page(z);
28
505 if (page)
29
506 return page;
30
507 }
31
508
32
509 /* XXX: is pages_min/4 a good amount to reserve for this? */
33
510 if (z->free_pages < z->pages_min / 4 &&
34
511 !(current->flags & PF_MEMALLOC))
35
512 continue;
36
513 page = rmqueue(z, order);
37
514 if (page)
38
515 return page;
39
516 }
40
517
41
518 /* No luck.. */
42
519 printk(KERN_ERR "__alloc_pages: %lu-order allocation failed.\n", order);
43
520 return NULL;
44
521 }
如果再次失败，那就是系统有问题
2.2 rmqueue函数：
从指定的zone中，获取2^order数量的页面
zone中的free_area是按照order建立的数组，每个free_area里面有多个free_list，
也可能为空,
也就是当前order中没有空闲空间，为空时从order更大的free_area中申请
我们要在order对应的free_area中申请内存页面，并将其从page链中摘出，
摘链的过程不能被打断，需要
spin_lock_irqsave加锁
rmqueue函数代码（2.4版本），2.6以及之后的版本发生了修改（看page_alloc.c文件）
memlist_entry提供了需要的free_list的头page结构，
memlist_del帮我们把free_list从free_area中删除
1
172 static struct page * rmqueue(zone_t *zone, unsigned long order)
2
173 {
3
174 free_area_t * area = zone->free_area + order;
4
175 unsigned long curr_order = order;
5
176 struct list_head *head, *curr;
6
177 unsigned long flags;
7
178 struct page *page;
8
179
9
180 spin_lock_irqsave(&zone->lock, flags);
10
181 do {
11
182 head = &area->free_list;
12
183 curr = memlist_next(head);
13
184
14
185 if (curr != head) {
15
186 unsigned int index;
16
187
17
188 page = memlist_entry(curr, struct page, list);
18
189 if (BAD_RANGE(zone,page))
19
190 BUG();
20
191 memlist_del(curr);
21
192 index = (page - mem_map) - zone->offset;
22
193 MARK_USED(index, curr_order, area);
23
194 zone->free_pages -= 1 << order;
24
195 //用来分解大块内存
25
196 page = expand(zone, page, index, order, curr_order, area);
26
197 spin_unlock_irqrestore(&zone->lock, flags);
27
198
28
199 set_page_count(page, 1);
29
200 if (BAD_RANGE(zone,page))
30
201 BUG();
31
202 DEBUG_ADD_PAGE
32
203 return page;
33
204 }
34
205 curr_order++;
35
206 area++;
36
207 } while (curr_order < MAX_ORDER);
37
208 spin_unlock_irqrestore(&zone->lock, flags);
38
209
39
210 return NULL;
40
211 }
2.3 expand函数
用来将order更大的free_area中free_list进行分割，并存入order更低的free_area
high为更大的order，low申请的order，当将high中一个free_list分割到low的大小时，就停止
1
150 static inline struct page * expand (zone_t *zone, struct page *page,
2
151 unsigned long index, int low, int high, free_area_t * area)
3
152 {
4
153 unsigned long size = 1 << high;
5
154
6
155 while (high > low) {
7
156 if (BAD_RANGE(zone,page))
8
157 BUG();
9
158 area--;
10
159 high--;
11
160 size >>= 1;
12
161 memlist_add_head(&(page)->list, &(area)->free_list);
13
162 MARK_USED(index, high, area);
14
163 index += size;
15
164 page += size;
16
165 }
17
166 if (BAD_RANGE(zone,page))
18
167 BUG();
19
168 return page;
20
169 }