一、 查看当前系统当前碎片化的情况
cat /proc/buddyinfo
This file is used primarily for diagnosing memory fragmentation issues. **cat
/proc/pagetypeinfo
The pagetypinfo begins with information on the size of a page block. It
then gives the same type of information as buddyinfo except broken down
by migrate-type and finishes with details on how many page blocks of each
type exist.
二、内核将已分配页的3种不同类型
分别介绍三种页的实现
用户态使用malloc。在访问的时候会触发缺页终端，
/*
* We enter with non-exclusive mmap_sem (to exclude vma changes,
* but allow concurrent faults), and pte mapped but not yet locked.
* We return with mmap_sem still held, but pte unmapped and unlocked.
*/
static int do_anonymous_page(struct fault_env *fe)
{
struct vm_area_struct *vma = fe->vma;
struct mem_cgroup *memcg;
struct page *page;
pte_t entry;
page = alloc_zeroed_user_highpage_movable(vma, fe->address);
}
/**
* alloc_zeroed_user_highpage_movable - Allocate a zeroed HIGHMEM page for a VMA that the caller knows can move
* @vma: The VMA the page is to be allocated for
* @vaddr: The virtual address the page will be inserted into
*
* This function will allocate a page for a VMA that the caller knows will
* be able to migrate in the future using move_pages() or reclaimed
*/
static inline struct page *
alloc_zeroed_user_highpage_movable(struct vm_area_struct *vma,
unsigned long vaddr)
{
return __alloc_zeroed_user_highpage(__GFP_MOVABLE, vma, vaddr);
}
可以发现，通过匿名页处理后得到的是 __GFP_MOVABLE ，是可以移动的页。
三、fallback机制 （内核申请的页也未必都是不可移动的）
/*
* This array describes the order lists are fallen back to when
* the free lists for the desirable migrate type are depleted
*/
static int fallbacks[MIGRATE_TYPES][4] = {
// 分配不可移动页失败的备用列表
[MIGRATE_UNMOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE, MIGRATE_TYPES },
// 分配可回收页失败时的备用列表
[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE, MIGRATE_MOVABLE, MIGRATE_TYPES },
// 分配可移动页失败时的备用列表
[MIGRATE_MOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_TYPES },
#ifdef CONFIG_CMA
[MIGRATE_CMA] = { MIGRATE_TYPES }, /* Never used */
#endif
#ifdef CONFIG_MEMORY_ISOLATION
[MIGRATE_ISOLATE] = { MIGRATE_TYPES }, /* Never used */
#endif
};
如果内核申请不可移动的页申请不到，就从可回收链表里拿，拿不到再从可移动链表里去拿。
用户空间申请的都是可移动的，直接从可移动的里面去拿，（有CMA的话，如果可移动页如果没有了，先从CMA里找，）如果找不到就从fallback链表找：先找可回收的，再找不可移动的。
四、 pageblock 作用
pageblock -migrate 是一起作用的。
/*
* Isolate all pages that can be migrated from the first suitable block,
* starting at the block pointed to by the migrate scanner pfn within
* compact_control.
*/
static isolate_migrate_t isolate_migratepages(struct zone *zone,
struct compact_control *cc)
{
unsigned long block_start_pfn;
unsigned long block_end_pfn;
unsigned long low_pfn;
struct page *page;
const isolate_mode_t isolate_mode =
(sysctl_compact_unevictable_allowed ? ISOLATE_UNEVICTABLE : 0) |
(cc->mode != MIGRATE_SYNC ? ISOLATE_ASYNC_MIGRATE : 0);
/*
* Start at where we last stopped, or beginning of the zone as
* initialized by compact_zone()
*/
low_pfn = cc->migrate_pfn;
block_start_pfn = pageblock_start_pfn(low_pfn);
if (block_start_pfn < zone->zone_start_pfn)
block_start_pfn = zone->zone_start_pfn;
/* Only scan within a pageblock boundary */
block_end_pfn = pageblock_end_pfn(low_pfn);
/*
* Iterate over whole pageblocks until we find the first suitable.
* Do not cross the free scanner.
*/
for (; block_end_pfn <= cc->free_pfn;
low_pfn = block_end_pfn,
block_start_pfn = block_end_pfn,
block_end_pfn += pageblock_nr_pages) {
/*
* This can potentially iterate a massively long zone with
* many pageblocks unsuitable, so periodically check if we
* need to schedule, or even abort async compaction.
*/
if (!(low_pfn % (SWAP_CLUSTER_MAX * pageblock_nr_pages))
&& compact_should_abort(cc))
break;
page = pageblock_pfn_to_page(block_start_pfn, block_end_pfn,
zone);
if (!page)
continue;
/* If isolation recently failed, do not retry */
if (!isolation_suitable(cc, page))
continue;
/*
* For async compaction, also only scan in MOVABLE blocks.
* Async compaction is optimistic to see if the minimum amount
* of work satisfies the allocation.
*/
if (!suitable_migration_source(cc, page))
continue;
/* Perform the isolation */
low_pfn = isolate_migratepages_block(cc, low_pfn,
block_end_pfn, isolate_mode);
if (!low_pfn || cc->contended)
return ISOLATE_ABORT;
/*
* Either we isolated something and proceed with migration. Or
* we failed and compact_zone should decide if we should
* continue or not.
*/
break;
}
/* Record where migration scanner will be restarted. */
cc->migrate_pfn = low_pfn;
return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
}
for (; block_end_pfn <= cc->free_pfn;
low_pfn = block_end_pfn,
block_start_pfn = block_end_pfn,
block_end_pfn += pageblock_nr_pages)
可以发现以 pageblock 为单位，扫描可移动页，最终把可移动的页添加到struct compact_control结构中的migratepages链表中。
/* If huge pages are not used, group by MAX_ORDER_NR_PAGES */
#define pageblock_order (MAX_ORDER-1)
#define pageblock_nr_pages (1UL << pageblock_order)
由于每个迁移链表都应该有适当数量的内存，内核需要定义”适当”的概念. 这是通过两个全局变量pageblock_order和pageblock_nr_pages提供的. pageblock 就是为了这个而存在。
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