【摘要】
本文主要介绍linux系统中，伙伴管理系统是如何处理内存页的.
本文可以结合 linux内存管理之内存回收机制 一文观看.
【正文】伙伴系统基本信息
1 伙伴系统由来：linux内核支持申请连续的内存页，但由于内存碎片化，可能出现空闲内存足够，但连续页不足的情况,如此引入伙伴系统，在申请和释放内存过程，尽量保持页的连续性.
2 伙伴系统简介：linux分别把连续2的0次方页、1次方页 ...分别看作一组伙伴，最多有2的MAX_ORDER-1个伙伴组
本文以 MAX_ORDER=11为例进行介绍；
代码举例：
alloc_pages(gfp_mask,order)申请连续的2的order次方页;
#define alloc_page(gfp_mask,0)对于每个内存去struct zone来说(比如ZONE_NORMAL,值得一提的说，很多嵌入式系统只有这一个zone,可以参考
linux内存管理之内存回收机制 一文), zone->free_area用于描述zone的空闲内存信息，其中：
zone->free_area->nr_free 表示该zone内空闲内存数量;
zone->free_area->free_list[MIGRATE_TYPES] 表示相应MIGRATE_TYPES类型的空闲内存链表，alloc_page过程就是尝试从不同空闲链表上申请内存，见后文代码分析;
MIGRATE_TYPES类型如下：
enum {
MIGRATE_UNMOVABLE,
MIGRATE_RECLAIMABLE,
MIGRATE_MOVABLE,
MIGRATE_PCPTYPES, /* the number of types on the pcp lists */
MIGRATE_RESERVE = MIGRATE_PCPTYPES,
#ifdef CONFIG_CMA
/*
* MIGRATE_CMA migration type is designed to mimic the way
* ZONE_MOVABLE works. Only movable pages can be allocated
* from MIGRATE_CMA pageblocks and page allocator never
* implicitly change migration type of MIGRATE_CMA pageblock.
*
* The way to use it is to change migratetype of a range of
* pageblocks to MIGRATE_CMA which can be done by
* __free_pageblock_cma() function. What is important though
* is that a range of pageblocks must be aligned to
* MAX_ORDER_NR_PAGES should biggest page be bigger then
* a single pageblock.
*/
MIGRATE_CMA,
#endif
#ifdef CONFIG_MEMORY_ISOLATION
MIGRATE_ISOLATE, /* can't allocate from here */
#endif
MIGRATE_TYPES
};3 linux系统的伙伴系统信息查询：
1> cat /proc/buddyinfo
Node 0, zone Normal 29 17 8 2 5 3 3 1 1 1 1
Node:表示pgdata->node_id=0;Normal表示ZONE_NORMAL 可参考linux内存管理之内存回收机制一文;
29: 表示zone->free_area[order=0].nr_free;即连续2的0次方页的个数。依次类推：
1: 最后一个1表示zone->free_area[order=10].nr_free;即连续2的MAX_ORDER-1=10次方页的个数;
2>通过show_mem(0)接口查看系统内存分配情况. 使用举例：
oom_killer.c: show_mem(0)内存信息显示接口,列出系统内存规划;
MIGRATE_UNMOVABLE,MIGRATE_RECLAIMABLE,MIGRATE_MOVABLE等MIGRATE_TYPES类型在show_mem显示中被简化为U/R/M.
meminfo.c: cat /proc/meminfo显示系统内存信息;可以在meminfo的实现里加入show_mem(0);
【正文二】伙伴系统代码分析frea_area与free_list
1 zone内存区中free_area与free_list.
如：alloc_page()/alloc_pages()类型的函数从 zone->free_area[order]->free_list[MIGRATE_TYPES] 中申请page;
order:表示申请连续内存页个数，每个order对应一个free_area，每个free_area对应一组free_list链表，每个free_list都是连续order页的链表，order取值0到MAX_ORDER-1;
每个free_area上可以有MIGRATE_TYPES个类型的free_list，alloc_page等函数就是从MIGRATE_TYPES类型的free_list上申请内存的. MIGRATE_TYPES见如上定义;
free_area[order].nr_free表示该order对应的free_area上的空闲页个数;
1) 每个page都对应一个order，该order用于标记这个page属于哪个zone->frea_area[order];
set_page_order函数接口用于设置：page->private=order;
static inline void set_page_order(struct page *page, unsigned int order)
{
set_page_private(page, order);
__SetPageBuddy(page);
}
其中：
#define set_page_private(page, v) ((page)->private = (v))（1） 释放一个页时：__free_one_page()->set_page_order();
（2）申请一个页时：expand()->set_page_order();
2)任意page都属于一个伙伴（即zone->free_area[order]）,每个free_ara[order]中包含MIGRATE_TYPES个free_list[MIGRATE_TYPES] (即zone->free_area[order]->free_list[MIGRATE_TYPES]);
设置一个page属于哪个MIGRATE_TYPES(可以属于多个MIGRATE_TYPES):
通过set_freepage_migratetype接口设置，通常和get_pfnblock_migratetype同时使用；
get_pfnblock_migratetype和set_pageblock_migratetype函数是一对函数;
static void __free_pages_ok(struct page *page, unsigned int order)
{
unsigned long flags;
int migratetype;
unsigned long pfn = page_to_pfn(page);
/*先从pageblock获取migratetype，注意pageblock中的miggratetype是在
set_pageblock_migratetype中设置的，后文会有涉及*/
migratetype = get_pfnblock_migratetype(page, pfn);
/*设置page所属migratetype类型：page->index=migratetype*/
set_freepage_migratetype(page, migratetype);
free_one_page(page_zone(page), page, pfn, order, migratetype);
}free_hot_cold_page/__rmqueue_fallback/__rmqueue_smallest/move_freepages->set_freepage_migratetype
3) zone_watermark_ok与free_area ; zone_watermark_ok->__zone_watermark_ok
__zone_watermark_ok判断空闲内存释放足够，注意此处针对每个小于order所对应的伙伴进行判断free_area[order].nr_free
/* free_pages为所有空闲页个数，通过zone_page_state(zone,NR_FREE_PAGES)获取 */
static bool __zone_watermark_ok(struct zone *z, unsigned int order,
unsigned long mark, int classzone_idx, int alloc_flags,
long free_pages)
{
/* free_pages my go negative - that's OK; mark=zone->watermark[] */
long min = mark;
int o;
long free_cma = 0;
free_pages -= (1 << order) - 1;
if (alloc_flags & ALLOC_HIGH)
min -= min / 2;
if (alloc_flags & ALLOC_HARDER)
min -= min / 4;
#ifdef CONFIG_CMA
/* If allocation can't use CMA areas don't use free CMA pages */
if (!(alloc_flags & ALLOC_CMA))
free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
#endif
if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
return false;
/*alloc_pages申请连续order页，此处对每个小于order的frea_area[order]的空闲页个数进行校验*/
for (o = 0; o < order; o++) {
/* 此处减去order以下所有空闲页的原因：
At the next order, this order's pages become unavailable */
free_pages -= z->free_area[o].nr_free << o;
/* Require fewer higher order pages to be free */
min >>= min_free_order_shift;
/*此处free_pages为可申请的，即减去o<order,free_area[o].nr_free
注意此处是对每个free_area[o<order]进行判断
*/
if (free_pages <= min)
return false;
}
return true;
}【正文三】 free_list的添加和删除
page_alloc.c中搜索free_list[migrate]，可以确定migrate设置、使用、维护过程。
由上可知，任何一个页都属于某个伙伴系统,即zone->free_area[order];
并且struct page结构中保存了order（page->private=order）
我们在申请或释放某些页时，首先要根据连续页的个数(即order),找到连续order页所在伙伴系统zone->free_area[order];
然后，每个free_area对应MIGRATE_TYPES个free_list(即zone->free_area[order]->free_list[MIGRATE_TYPES])；
那么要理解如何从free_list链表上申请或者释放页，首先要弄明白page是何时挂载到free_list链表上的？
申请页过程，如何找到对应的free_list（每个free_area[order]对应MIGRATE_TYPES个free_list）？
1 free_list[MIGRATE_TYPES]添加过程：一般在释放page的过程完成.
首先要决定连续order页挂载到哪个free_list上，即选择合适的MIGRATE_TYPES,选择时机如下：
1> 内核启动过程会设置所有低端页挂载到free_lists[MIGRATE_MOVABLE]，过程如下：
第一步：set_pageblock_migratetype设置pageblock的migratetype=MIGRATE_MOVABLE;
注意pageblock_nr_pages为2的order=10次方个连续页;
setup_arch()->paging_init()->bootm_init()->arm_bootmem_free()->free_area_init_node()->free_area_init_core()
->memmap_init->memmap_init_zone-> set_pageblock_migratetype(page,MIGRATE_MOVABLE):第二步：通过get_pfnblock_migratetype获取migratetype，并通过set_freepage_migratetype设置page->index,且将page挂载到zone->free_area[order]->free_lists[migratetype]上.
mm_init->mem_init->free_all_bootmem->__free_memory_all->__free_pages_memory->__free_pages->__free_pages_ok
static void __free_pages_ok(struct page *page, unsigned int order)
{
unsigned long flags;
int migratetype;
unsigned long pfn = page_to_pfn(page);
if (!free_pages_prepare(page, order))
return;
/*先获取pageblock的migratetype;第一步时初始化为migratetype=MIGRATE_MOVABLE;*/
migratetype = get_pfnblock_migratetype(page, pfn);
local_irq_save(flags);
__count_vm_events(PGFREE, 1 << order);
/*设置page->index=migratetype=MIGRATE_MOVABLE*/
set_freepage_migratetype(page, migratetype);
free_one_page(page_zone(page), page, pfn, order, migratetype);
local_irq_restore(flags);
}__free_pages_ok-> __free_one_page:把要释放的page挂到zone->free_area[order]->free_list[migratetype]上,
且释放后该order对应的free_area[order]上空闲页++.
static inline void __free_one_page(struct page *page,unsigned long pfn,struct zone *zone,
unsigned int order,int migratetype)
{
unsigned long page_idx;
unsigned long combined_idx;
unsigned long uninitialized_var(buddy_idx);
struct page *buddy;
int max_order = MAX_ORDER;
VM_BUG_ON(migratetype == -1);
if (is_migrate_isolate(migratetype)) {
max_order = min(MAX_ORDER, pageblock_order + 1);
} else {
__mod_zone_freepage_state(zone, 1 << order, migratetype);
}
page_idx = pfn & ((1 << max_order) - 1);
VM_BUG_ON_PAGE(page_idx & ((1 << order) - 1), page);
VM_BUG_ON_PAGE(bad_range(zone, page), page);
while (order < max_order - 1) {
buddy_idx = __find_buddy_index(page_idx, order);
buddy = page + (buddy_idx - page_idx);
if (!page_is_buddy(page, buddy, order))
break;
/*
* Our buddy is free or it is CONFIG_DEBUG_PAGEALLOC guard page,
* merge with it and move up one order.
*/
if (page_is_guard(buddy)) {
clear_page_guard_flag(buddy);
set_page_private(buddy, 0);
if (!is_migrate_isolate(migratetype)) {
__mod_zone_freepage_state(zone, 1 << order,
migratetype);
}
} else {
list_del(&buddy->lru);
zone->free_area[order].nr_free--;
rmv_page_order(buddy);
}
combined_idx = buddy_idx & page_idx;
page = page + (combined_idx - page_idx);
page_idx = combined_idx;
order++;
}
set_page_order(page, order);
/*
* If this is not the largest possible page, check if the buddy
* of the next-highest order is free. If it is, it's possible
* that pages are being freed that will coalesce soon. In case,
* that is happening, add the free page to the tail of the list
* so it's less likely to be used soon and more likely to be merged
* as a higher order page
*/
if ((order < MAX_ORDER-2) && pfn_valid_within(page_to_pfn(buddy))) {
struct page *higher_page, *higher_buddy;
combined_idx = buddy_idx & page_idx;
higher_page = page + (combined_idx - page_idx);
buddy_idx = __find_buddy_index(combined_idx, order + 1);
higher_buddy = higher_page + (buddy_idx - combined_idx);
if (page_is_buddy(higher_page, higher_buddy, order + 1)) {
list_add_tail(&page->lru,
&zone->free_area[order].free_list[migratetype]);
goto out;
}
}
/*释放的page挂载到free_list上*/
list_add(&page->lru, &zone->free_area[order].free_list[migratetype]);
out: /*释放后该order对应的free_area[order]上空闲页++*/
zone->free_area[order].nr_free++;
}
2> 设置page为MIGRATE_RESERVE：
第一步：set_pageblock_migratetype(page,MIGRATE_RESERVE);设置zone区page的migratetype
init_per_zone_wmark_min->setup_per_zone_wmarks->setup_zone_migrate_reserve->set_pageblock_migratetype:
static void setup_zone_migrate_reserve(struct zone *zone)
{
unsigned long start_pfn, pfn, end_pfn, block_end_pfn;
struct page *page;
unsigned long block_migratetype;
int reserve;
int old_reserve;
/*
* Get the start pfn, end pfn and the number of blocks to reserve
* We have to be careful to be aligned to pageblock_nr_pages to
* make sure that we always check pfn_valid for the first page in
* the block.
*/
start_pfn = zone->zone_start_pfn;
end_pfn = zone_end_pfn(zone);
start_pfn = roundup(start_pfn, pageblock_nr_pages);
reserve = roundup(min_wmark_pages(zone), pageblock_nr_pages) >>pageblock_order;
/*
* Reserve blocks are generally in place to help high-order atomic
* allocations that are short-lived. A min_free_kbytes value that
* would result in more than 2 reserve blocks for atomic allocations
* is assumed to be in place to help anti-fragmentation for the
* future allocation of hugepages at runtime.
*/
reserve = min(2, reserve);
old_reserve = zone->nr_migrate_reserve_block;
/* When memory hot-add, we almost always need to do nothing */
if (reserve == old_reserve)
return;
zone->nr_migrate_reserve_block = reserve;
/*遍历zone区内存页；注意pageblock_nr_pages为2的order=10次方个连续页*/
for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
if (!pfn_valid(pfn))
continue;
page = pfn_to_page(pfn);
/* Watch out for overlapping nodes */
if (page_to_nid(page) != zone_to_nid(zone))
continue;
/*注意上面第1种情况，设置block_migratetype=MIGRATE_MOVABLE*/
block_migratetype = get_pageblock_migratetype(page);
/* Only test what is necessary when the reserves are not met */
if (reserve > 0) {
/*
* Blocks with reserved pages will never free, skip
* them.
*/
block_end_pfn = min(pfn + pageblock_nr_pages, end_pfn);
if (pageblock_is_reserved(pfn, block_end_pfn))
continue;
/* If this block is reserved, account for it */
if (block_migratetype == MIGRATE_RESERVE) {
reserve--;
continue;
}
/*
Suitable for reserving if this block is movable
系统启动时mm_init初始化pageblock的migratetype为MIGRATE_MOVABLE
*/
if (block_migratetype == MIGRATE_MOVABLE) {
set_pageblock_migratetype(page,
MIGRATE_RESERVE);
/*setup_zone_migrate_reserve->move_freepages:
将page挂载到zone->free_area[order].free_list[migratetype]链表上
注意此处此page所在pageblock的order=10=pageblock_nr_pages；migratetype=MIGRATE_RESERVE
*/
move_freepages_block(zone, page,
MIGRATE_RESERVE);
reserve--;
continue;
}
} else if (!old_reserve) {
/*
* At boot time we don't need to scan the whole zone
* for turning off MIGRATE_RESERVE.
*/
break;
}
/*
* If the reserve is met and this is a previous reserved block,
* take it back
*/
if (block_migratetype == MIGRATE_RESERVE) {
set_pageblock_migratetype(page, MIGRATE_MOVABLE);
move_freepages_block(zone, page, MIGRATE_MOVABLE);
}
}
}init_per_zone_wmark_min->setup_per_zone_wmarks->setup_zone_migrate_reserve->set_pageblock_migratetype->
move_freepages:将page挂载到zone->free_area[order].free_list[migratetype]链表上.
int move_freepages(struct zone *zone, struct page *start_page, struct page *end_page, int migratetype)
{
struct page *page;
unsigned long order;
int pages_moved = 0;
for (page = start_page; page <= end_page;) {
/* Make sure we are not inadvertently changing nodes */
VM_BUG_ON_PAGE(page_to_nid(page) != zone_to_nid(zone), page);
if (!pfn_valid_within(page_to_pfn(page))) {
page++;
continue;
}
if (!PageBuddy(page)) {
page++;
continue;
}
order = page_order(page);
//order=10时；1024*4k=4096KB；migratetype=MIGRATE_RESERVE,此时设置reserve属性;
list_move(&page->lru,
&zone->free_area[order].free_list[migratetype]);
set_freepage_migratetype(page, migratetype);
page += 1 << order;
pages_moved += 1 << order;
}
return pages_moved;
}2 free_list[MIGRATE_TYPES]删除过程：一般在申请page的过程完成.
#define alloc_pages(gfp_mask,order)->alloc_pages_node(numa_node_id(),gfp_mask,order)
/*根据nid找到zone[ZONE_NORMAL];根据order找到zone->free_area[order];
根据MIGRATE_TYPES找到对应的free_list,此时__rmqueue_fallback会轮训migratetype,见后文分析*/
static inline struct page *alloc_pages_node(int nid, gfp_t gfp_mask,
unsigned int order)
{
if (nid < 0)
nid = numa_node_id();
return __alloc_pages(gfp_mask, order, node_zonelist(nid, gfp_mask));
}alloc_pages(gfp_mask,order)->get_page_from_freelist()从free_list上申请内存page，
free_list上page是挂载时机可以参看上文，该函数会通过zone_watermark_ok判断zone->free_area[order]上空闲内存是否超出wtatermark；如果zone->free_area[order]上nr_free充足，则通过buffered_rmqueue继续申请page：
注意migratetype是通过:__alloc_pages->__alloc_pages_nodemask()根据gfp_mask指定的,
如GFP_KERNEL对应migratetype=0：
static struct page *
get_page_from_freelist(gfp_t gfp_mask, nodemask_t *nodemask, unsigned int order,
struct zonelist *zonelist, int high_zoneidx, int alloc_flags,
struct zone *preferred_zone, int classzone_idx, int migratetype)
{
struct zoneref *z;
struct page *page = NULL;
struct zone *zone;
nodemask_t *allowednodes = NULL;/* zonelist_cache approximation */
int zlc_active = 0; /* set if using zonelist_cache */
int did_zlc_setup = 0; /* just call zlc_setup() one time */
bool consider_zone_dirty = (alloc_flags & ALLOC_WMARK_LOW) &&
(gfp_mask & __GFP_WRITE);
int nr_fair_skipped = 0;
bool zonelist_rescan;
zonelist_scan:
zonelist_rescan = false;
/*
* Scan zonelist, looking for a zone with enough free.
* See also __cpuset_node_allowed_softwall() comment in kernel/cpuset.c.
*/
for_each_zone_zonelist_nodemask(zone, z, zonelist, high_zoneidx, nodemask) {
unsigned long mark;
if (IS_ENABLED(CONFIG_NUMA) && zlc_active &&
!zlc_zone_worth_trying(zonelist, z, allowednodes))
continue;
if (cpusets_enabled() &&
(alloc_flags & ALLOC_CPUSET) &&
!cpuset_zone_allowed_softwall(zone, gfp_mask))
continue;
/*
* Distribute pages in proportion to the individual
* zone size to ensure fair page aging. The zone a
* page was allocated in should have no effect on the
* time the page has in memory before being reclaimed.
*/
if (alloc_flags & ALLOC_FAIR) {
if (!zone_local(preferred_zone, zone))
break;
if (test_bit(ZONE_FAIR_DEPLETED, &zone->flags)) {
nr_fair_skipped++;
continue;
}
}
if (consider_zone_dirty && !zone_dirty_ok(zone))
continue;
mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
if (!zone_watermark_ok(zone, order, mark,
classzone_idx, alloc_flags)) {
int ret;
/* Checked here to keep the fast path fast */
BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
if (alloc_flags & ALLOC_NO_WATERMARKS)
goto try_this_zone;
if (IS_ENABLED(CONFIG_NUMA) &&
!did_zlc_setup && nr_online_nodes > 1) {
/*
* we do zlc_setup if there are multiple nodes
* and before considering the first zone allowed
* by the cpuset.
*/
allowednodes = zlc_setup(zonelist, alloc_flags);
zlc_active = 1;
did_zlc_setup = 1;
}
if (zone_reclaim_mode == 0 ||
!zone_allows_reclaim(preferred_zone, zone))
goto this_zone_full;
/*
* As we may have just activated ZLC, check if the first
* eligible zone has failed zone_reclaim recently.
*/
if (IS_ENABLED(CONFIG_NUMA) && zlc_active &&
!zlc_zone_worth_trying(zonelist, z, allowednodes))
continue;
ret = zone_reclaim(zone, gfp_mask, order);
switch (ret) {
case ZONE_RECLAIM_NOSCAN:
/* did not scan */
continue;
case ZONE_RECLAIM_FULL:
/* scanned but unreclaimable */
continue;
default:
/* did we reclaim enough */
if (zone_watermark_ok(zone, order, mark,
classzone_idx, alloc_flags))
goto try_this_zone;
if (((alloc_flags & ALLOC_WMARK_MASK) == ALLOC_WMARK_MIN) ||
ret == ZONE_RECLAIM_SOME)
goto this_zone_full;
continue;
}
}
try_this_zone:
page = buffered_rmqueue(preferred_zone, zone, order,
gfp_mask, migratetype);
if (page)
break;
this_zone_full:
if (IS_ENABLED(CONFIG_NUMA) && zlc_active)
zlc_mark_zone_full(zonelist, z);
}
if (page) {
/*
* page->pfmemalloc is set when ALLOC_NO_WATERMARKS was
* necessary to allocate the page. The expectation is
* that the caller is taking steps that will free more
* memory. The caller should avoid the page being used
* for !PFMEMALLOC purposes.
*/
page->pfmemalloc = !!(alloc_flags & ALLOC_NO_WATERMARKS);
return page;
}
/*
* The first pass makes sure allocations are spread fairly within the
* local node. However, the local node might have free pages left
* after the fairness batches are exhausted, and remote zones haven't
* even been considered yet. Try once more without fairness, and
* include remote zones now, before entering the slowpath and waking
* kswapd: prefer spilling to a remote zone over swapping locally.
*/
if (alloc_flags & ALLOC_FAIR) {
alloc_flags &= ~ALLOC_FAIR;
if (nr_fair_skipped) {
zonelist_rescan = true;
reset_alloc_batches(preferred_zone);
}
if (nr_online_nodes > 1)
zonelist_rescan = true;
}
if (unlikely(IS_ENABLED(CONFIG_NUMA) && zlc_active)) {
/* Disable zlc cache for second zonelist scan */
zlc_active = 0;
zonelist_rescan = true;
}
if (zonelist_rescan)
goto zonelist_scan;
return NULL;
}3 伙伴系统处理
alloc_pages(gfp_mask,order)->get_page_from_freelist()->buffered_rmqueue()
static inline
struct page *buffered_rmqueue(struct zone *preferred_zone,
struct zone *zone, unsigned int order,
gfp_t gfp_flags, int migratetype)
{
unsigned long flags;
struct page *page;
bool cold = ((gfp_flags & __GFP_COLD) != 0);
again://只申请1个页
if (likely(order == 0)) {
struct per_cpu_pages *pcp;
struct list_head *list;
local_irq_save(flags);
pcp = &this_cpu_ptr(zone->pageset)->pcp;
/*先在pcp->lists上查找有没有对应类型的空闲页*/
list = &pcp->lists[migratetype];
/*如果pcp->lists上没有对应类型的空闲页，则从伙伴系统中查找，并挂载的pcp->list上面*/
if (list_empty(list)) {
/*rmqueue_bulk将申请到的page->list挂载到pcp->lists[migratetype]上*/
pcp->count += rmqueue_bulk(zone, 0,
pcp->batch, list,
migratetype, cold);
if (unlikely(list_empty(list)))
goto failed;
}
if (cold)
page = list_entry(list->prev, struct page, lru);
else
page = list_entry(list->next, struct page, lru);
/*从pcp->lists[migratetype]删除page->list，因为该页被申请了*/
list_del(&page->lru);
pcp->count--;
} else { //直接申请连续多页,没有从pcp->list链表上申请
if (unlikely(gfp_flags & __GFP_NOFAIL)) {
/*
* __GFP_NOFAIL is not to be used in new code.
*
* All __GFP_NOFAIL callers should be fixed so that they
* properly detect and handle allocation failures.
*
* We most definitely don't want callers attempting to
* allocate greater than order-1 page units with
* __GFP_NOFAIL.
*/
WARN_ON_ONCE(order > 1);
}
spin_lock_irqsave(&zone->lock, flags);
page = __rmqueue(zone, order, migratetype);
spin_unlock(&zone->lock);
if (!page)
goto failed;
__mod_zone_freepage_state(zone, -(1 << order),
get_freepage_migratetype(page));
}
__mod_zone_page_state(zone, NR_ALLOC_BATCH, -(1 << order));
if (atomic_long_read(&zone->vm_stat[NR_ALLOC_BATCH]) <= 0 &&
!test_bit(ZONE_FAIR_DEPLETED, &zone->flags))
set_bit(ZONE_FAIR_DEPLETED, &zone->flags);
__count_zone_vm_events(PGALLOC, zone, 1 << order);
zone_statistics(preferred_zone, zone, gfp_flags);
local_irq_restore(flags);
VM_BUG_ON_PAGE(bad_range(zone, page), page);
if (prep_new_page(page, order, gfp_flags))
goto again;
return page;
failed:
local_irq_restore(flags);
return NULL;
}无论申请连续一页还是多页都会调用到buffered_rmqueue()->__rmqueue():
static struct page *__rmqueue(struct zone *zone, unsigned int order,
int migratetype)
{
struct page *page;
retry_reserve:
/*
第一次执行到此：
系统最开始将page挂载到MIGRATE_MOVABLE和MIGRATETYPE_RESERVE对应的free_list[migratetype]上；
alloc_pages()过程，当gfp_mask=GFP_KERNEL时，migratetype=0对应的free_list[migratetype]上没有page，
所以此处返回空,继续向下分析代码*/
page = __rmqueue_smallest(zone, order, migratetype);
/*第一次执行到此：page为空migratetype != MIGRATE_RESERVE，
注意此时所有page都在MIGRATE_MOVABLE和MIGRATETYPE_RESERVE对应的free_list[migratetype]*/
if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
page = __rmqueue_fallback(zone, order, migratetype);
/*
* Use MIGRATE_RESERVE rather than fail an allocation. goto
* is used because __rmqueue_smallest is an inline function
* and we want just one call site
*/
if (!page) {
migratetype = MIGRATE_RESERVE;
goto retry_reserve;
}
}
trace_mm_page_alloc_zone_locked(page, order, migratetype);
return page;
}static inline struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
int migratetype)
{
unsigned int current_order;
struct free_area *area;
struct page *page;
/* Find a page of the appropriate size in the preferred list
遍历所有大于等于要申请order的free_area[order]区；
*/
for (current_order = order; current_order < MAX_ORDER; ++current_order) {
area = &(zone->free_area[current_order]);
if (list_empty(&area->free_list[migratetype]))
continue;
/*如果free_list上有page则申请*/
page = list_entry(area->free_list[migratetype].next,
struct page, lru);
list_del(&page->lru);
rmv_page_order(page);
area->nr_free--;
expand(zone, page, order, current_order, area, migratetype);
set_freepage_migratetype(page, migratetype);
return page;
}
return NULL;
}buffered_rmqueue()->__rmqueue()->__rmqueue_fallback中遍历fallbacks:fallbacks指明了free_list[migratetype]链表的申请顺序，如下：zone->free_area[order]->free_list[migratetype]；
static int fallbacks[MIGRATE_TYPES][4] = {
[MIGRATE_UNMOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE, MIGRATE_RESERVE },
[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE, MIGRATE_MOVABLE, MIGRATE_RESERVE },
#ifdef CONFIG_CMA
[MIGRATE_MOVABLE] = { MIGRATE_CMA, MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_RESERVE },
[MIGRATE_CMA] = { MIGRATE_RESERVE }, /* Never used */
#else
[MIGRATE_MOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_RESERVE },
#endif
[MIGRATE_RESERVE] = { MIGRATE_RESERVE }, /* Never used */
#ifdef CONFIG_MEMORY_ISOLATION
[MIGRATE_ISOLATE] = { MIGRATE_RESERVE }, /* Never used */
#endif
};buffered_rmqueue()->__rmqueue()->__rmqueue_fallback:
static inline struct page *__rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
{
struct free_area *area;
unsigned int current_order;
struct page *page;
int migratetype, new_type, i;
/* Find the largest possible block of pages in the other list */
for (current_order = MAX_ORDER-1;
current_order >= order && current_order <= MAX_ORDER-1;
--current_order) {
for (i = 0;; i++) {
migratetype = fallbacks[start_migratetype][i];
/* MIGRATE_RESERVE handled later if necessary */
if (migratetype == MIGRATE_RESERVE)
break;
area = &(zone->free_area[current_order]);
if (list_empty(&area->free_list[migratetype]))
continue;
page = list_entry(area->free_list[migratetype].next,
struct page, lru);
area->nr_free--;
new_type = try_to_steal_freepages(zone, page,
start_migratetype,
migratetype);
/* Remove the page from the freelists */
list_del(&page->lru);
rmv_page_order(page);
expand(zone, page, order, current_order, area,
new_type);
/* The freepage_migratetype may differ from pageblock's
* migratetype depending on the decisions in
* try_to_steal_freepages. This is OK as long as it does
* not differ for MIGRATE_CMA type.
*/
set_freepage_migratetype(page, new_type);
trace_mm_page_alloc_extfrag(page, order, current_order,
start_migratetype, migratetype);
return page;
}
}
return NULL;
}【总结】
本文简要介绍了伙伴系统的维护方法，需要注意__rmqueue()/zone_watermark_ok()重点函数的分析,同时要厘清
zone->free_area[order]->free_list[migratetype] 各项的含义；一般zone指ZONE_NORMAL区。
1>zone的获取：通过遍历zone_list，zone_list通过node_zonelist()获取
/*对应node_id=numa_node_id();其中ZONE_NORMAL对应nid=0*/
static inline struct zonelist *node_zonelist(int nid, gfp_t flags)
{
return NODE_DATA(nid)->node_zonelists + gfp_zonelist(flags);
}
2>free_area[order]:order表示连续2的order次方的连续页，其中初始化过程会是否所有野到free_area[order=10]空闲区的free_list链表，其他释放过程同样是否page到对应order的free_area[order]空闲区的free_list链表上;
3>free_area[order]->free_list[migratetype]:初始化过程会挂载page页到free_list[migratetype];migratetype=MIGRATE_MOVABLE和MIGRATE_RESERVE，此处注意page同时挂载到了两个free_list上。申请过程中从哪个free_list[migratetype]上申请，优先级取决于 fallbacks[MIGRATE_TYPES][4]定义：可以参考上文代码.