Linux migrate_type进一步探索

文章接着上回Linux migrate_type初步探索

1、物理页面添加到buddy系统

我们都知道物理内存一开始是由memblock进行分配管理，后面会切换到buddy系统管理。那么接下来我们看一下，memblock管理的物理页面是怎么添加到buddy系统中的。

start_kernel()
-> mm_init()
--> mem_init()
---> memblock_free_all()
----> free_low_memory_core_early()

1.1 free_low_memory_core_early()

static unsigned long __init free_low_memory_core_early(void)
{unsigned long count = 0;phys_addr_t start, end;u64 i;memblock_clear_hotplug(0, -1);// 处理预留内存for_each_reserved_mem_range(i, &start, &end)reserve_bootmem_region(start, end);/** We need to use NUMA_NO_NODE instead of NODE_DATA(0)->node_id*  because in some case like Node0 doesn't have RAM installed*  low ram will be on Node1*/// 遍历可释放物理内存区域，进行释放for_each_free_mem_range(i, NUMA_NO_NODE, MEMBLOCK_NONE, &start, &end,NULL)count += __free_memory_core(start, end);return count;
}static unsigned long __init __free_memory_core(phys_addr_t start,phys_addr_t end)
{unsigned long start_pfn = PFN_UP(start);unsigned long end_pfn = min_t(unsigned long,PFN_DOWN(end), max_low_pfn);if (start_pfn >= end_pfn)return 0;// 进行页面释放处理__free_pages_memory(start_pfn, end_pfn);return end_pfn - start_pfn;
}

1.2 __free_pages_memory()

static void __init __free_pages_memory(unsigned long start, unsigned long end)
{int order;while (start < end) {/*** 由于buddy系统最大能存放的页面order是MAX_ORDER - 1UL，所以这里要进行限制* __ffs()函数是用来根据start值计算出最合适的order值* __ffs()函数作用是求第start第一个位为1的位置，例如：start = 0x63300，* 说明该地址以0x100对齐，那么__ffs()返回值为8*/order = min(MAX_ORDER - 1UL, __ffs(start));// 如果发现order太大，实际没有那么多物理内存，则不断减小order，直至能包含为止while (start + (1UL << order) > end)order--;// 将页面释放到buddy系统memblock_free_pages(pfn_to_page(start), start, order);start += (1UL << order);}
}

1.3 memblock_free_pages()

void __init memblock_free_pages(struct page *page, unsigned long pfn,unsigned int order)
{if (early_page_uninitialised(pfn))return;// 调用内部接口释放页面__free_pages_core(page, order);
}void __free_pages_core(struct page *page, unsigned int order)
{unsigned int nr_pages = 1 << order;struct page *p = page;unsigned int loop;/** When initializing the memmap, __init_single_page() sets the refcount* of all pages to 1 ("allocated"/"not free"). We have to set the* refcount of all involved pages to 0.*/prefetchw(p);// 遍历当前order页面内所有page，并初始化for (loop = 0; loop < (nr_pages - 1); loop++, p++) {prefetchw(p + 1);// 清楚页面预留标记__ClearPageReserved(p);// 设置页面引用计数为0set_page_count(p, 0);}__ClearPageReserved(p);set_page_count(p, 0);atomic_long_add(nr_pages, &page_zone(page)->managed_pages);/** Bypass PCP and place fresh pages right to the tail, primarily* relevant for memory onlining.*/// 将页面释放到buddy系统中__free_pages_ok(page, order, FPI_TO_TAIL);
}static void __free_pages_ok(struct page *page, unsigned int order,fpi_t fpi_flags)
{unsigned long flags;int migratetype;unsigned long pfn = page_to_pfn(page);if (!free_pages_prepare(page, order, true))return;// 获取页面所在页块的迁移类型migratetype = get_pfnblock_migratetype(page, pfn);local_irq_save(flags);__count_vm_events(PGFREE, 1 << order);// 将页面放置在对应迁移类型对应order的管理链表上free_one_page(page_zone(page), page, pfn, order, migratetype,fpi_flags);local_irq_restore(flags);
}

这里就是物理内存从memblock转移到buddy系统的流程。

2、迁移类型fallback处理逻辑

接下来我们再来看看一个新问题：一开始页块的迁移类型都是MIGRATE_MOVABLE，那对于MIGRATE_UNMOVABLE迁移类型的内存分配应该怎么处理呢？

2.1 原理图

我们都知道Linux内核内存分配接口alloc_pages()，那么我们就跟踪这个接口，看看是如何分配出MIGRATE_UNMOVABLE迁移类型的内存。

alloc_pages()
-> alloc_pages_node()
--> __alloc_pages_node()
---> __alloc_pages()
----> __alloc_pages_nodemask()

我们接下来仔细研究一下__alloc_pages_nodemask()实现：

2.2 __alloc_pages_nodemask()

/** This is the 'heart' of the zoned buddy allocator.*/
struct page *
__alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,nodemask_t *nodemask)
{struct page *page;// 第一次分配时，我们会从zone的low水线以上分配内存unsigned int alloc_flags = ALLOC_WMARK_LOW;gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */struct alloc_context ac = { };/** There are several places where we assume that the order value is sane* so bail out early if the request is out of bound.*/// 如果order>=11时，buddy系统无法分配出内存，因此直接返回错误if (unlikely(order >= MAX_ORDER)) {WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));return NULL;}gfp_mask &= gfp_allowed_mask;alloc_mask = gfp_mask;// 这里是根据gfp设置分配标识，以及确定优先从哪个zone里进行内存分配if (!prepare_alloc_pages(gfp_mask, order, preferred_nid, nodemask, &ac, &alloc_mask, &alloc_flags))return NULL;/** Forbid the first pass from falling back to types that fragment* memory until all local zones are considered.*/alloc_flags |= alloc_flags_nofragment(ac.preferred_zoneref->zone, gfp_mask);// 内存分配快速路径，我们今天只需要跟踪该函数实现就好了/* First allocation attempt */page = get_page_from_freelist(alloc_mask, order, alloc_flags, &ac);if (likely(page))goto out;/** Apply scoped allocation constraints. This is mainly about GFP_NOFS* resp. GFP_NOIO which has to be inherited for all allocation requests* from a particular context which has been marked by* memalloc_no{fs,io}_{save,restore}.*/alloc_mask = current_gfp_context(gfp_mask);ac.spread_dirty_pages = false;/** Restore the original nodemask if it was potentially replaced with* &cpuset_current_mems_allowed to optimize the fast-path attempt.*/ac.nodemask = nodemask;// 当快速路径无法分配出内存时，就会调用该函数，走慢速路径内存分配，这里会异步唤醒kswapd线程回收内存等操作page = __alloc_pages_slowpath(alloc_mask, order, &ac);out:if (memcg_kmem_enabled() && (gfp_mask & __GFP_ACCOUNT) && page &&unlikely(__memcg_kmem_charge_page(page, gfp_mask, order) != 0)) {__free_pages(page, order);page = NULL;}trace_mm_page_alloc(page, order, alloc_mask, ac.migratetype);return page;
}
EXPORT_SYMBOL(__alloc_pages_nodemask);

2.3 get_page_from_freelist()

/** get_page_from_freelist goes through the zonelist trying to allocate* a page.*/
static struct page *
get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,const struct alloc_context *ac)
{struct zoneref *z;struct zone *zone;struct pglist_data *last_pgdat_dirty_limit = NULL;bool no_fallback;retry:/** Scan zonelist, looking for a zone with enough free.* See also __cpuset_node_allowed() comment in kernel/cpuset.c.*/no_fallback = alloc_flags & ALLOC_NOFRAGMENT;z = ac->preferred_zoneref;// 优先从最优先的zone分配内存，若未分配到，降级到其他可分配内存的zonefor_next_zone_zonelist_nodemask(zone, z, ac->highest_zoneidx,ac->nodemask) {struct page *page;unsigned long mark;if (cpusets_enabled() &&(alloc_flags & ALLOC_CPUSET) &&!__cpuset_zone_allowed(zone, gfp_mask))continue;/** When allocating a page cache page for writing, we* want to get it from a node that is within its dirty* limit, such that no single node holds more than its* proportional share of globally allowed dirty pages.* The dirty limits take into account the node's* lowmem reserves and high watermark so that kswapd* should be able to balance it without having to* write pages from its LRU list.** XXX: For now, allow allocations to potentially* exceed the per-node dirty limit in the slowpath* (spread_dirty_pages unset) before going into reclaim,* which is important when on a NUMA setup the allowed* nodes are together not big enough to reach the* global limit.  The proper fix for these situations* will require awareness of nodes in the* dirty-throttling and the flusher threads.*/if (ac->spread_dirty_pages) {if (last_pgdat_dirty_limit == zone->zone_pgdat)continue;if (!node_dirty_ok(zone->zone_pgdat)) {last_pgdat_dirty_limit = zone->zone_pgdat;continue;}}if (no_fallback && nr_online_nodes > 1 &&zone != ac->preferred_zoneref->zone) {int local_nid;/** If moving to a remote node, retry but allow* fragmenting fallbacks. Locality is more important* than fragmentation avoidance.*/local_nid = zone_to_nid(ac->preferred_zoneref->zone);if (zone_to_nid(zone) != local_nid) {alloc_flags &= ~ALLOC_NOFRAGMENT;goto retry;}}mark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK);// 快速水线检查，查看当前zone是否可以分配出所需order大小的页面if (!zone_watermark_fast(zone, order, mark,ac->highest_zoneidx, alloc_flags,gfp_mask)) {int ret;#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT/** Watermark failed for this zone, but see if we can* grow this zone if it contains deferred pages.*/if (static_branch_unlikely(&deferred_pages)) {if (_deferred_grow_zone(zone, order))goto try_this_zone;}
#endif/* 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 (node_reclaim_mode == 0 ||!zone_allows_reclaim(ac->preferred_zoneref->zone, zone))continue;// 尝试内存回收ret = node_reclaim(zone->zone_pgdat, gfp_mask, order);switch (ret) {// 返回值为未进行回收扫描，则跳过该zonecase NODE_RECLAIM_NOSCAN:/* did not scan */continue;// 返回值为扫描但不能回收，则跳过该zonecase NODE_RECLAIM_FULL:/* scanned but unreclaimable */continue;default:/* did we reclaim enough */// 已经回收了一部分，检查水线是否满足，满足则使用该zone进行内存分配if (zone_watermark_ok(zone, order, mark,ac->highest_zoneidx, alloc_flags))// 使用该zone进行内存分配goto try_this_zone;continue;}}try_this_zone:// 通过rmqueue从zone的buddy系统中分配页面page = rmqueue(ac->preferred_zoneref->zone, zone, order,gfp_mask, alloc_flags, ac->migratetype);if (page) {// 分配到页面，进行一个处理prep_new_page(page, order, gfp_mask, alloc_flags);/** If this is a high-order atomic allocation then check* if the pageblock should be reserved for the future*/if (unlikely(order && (alloc_flags & ALLOC_HARDER)))reserve_highatomic_pageblock(page, zone, order);// 返回分配到的页面return page;} else {
#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT/* Try again if zone has deferred pages */if (static_branch_unlikely(&deferred_pages)) {if (_deferred_grow_zone(zone, order))goto try_this_zone;}
#endif}}/** It's possible on a UMA machine to get through all zones that are* fragmented. If avoiding fragmentation, reset and try again.*/if (no_fallback) {alloc_flags &= ~ALLOC_NOFRAGMENT;goto retry;}return NULL;
}

2.4 rmqueue()

/** Allocate a page from the given zone. Use pcplists for order-0 allocations.*/
static inline
struct page *rmqueue(struct zone *preferred_zone,struct zone *zone, unsigned int order,gfp_t gfp_flags, unsigned int alloc_flags,int migratetype)
{unsigned long flags;struct page *page;// 如果order为0，则从pcplist中分配，这样做是为了加快分配效率，这个在我之前的文章专门讲过，感兴趣的可以自己翻找一下if (likely(order == 0)) {/** MIGRATE_MOVABLE pcplist could have the pages on CMA area and* we need to skip it when CMA area isn't allowed.*/if (!IS_ENABLED(CONFIG_CMA) || alloc_flags & ALLOC_CMA ||migratetype != MIGRATE_MOVABLE) {page = rmqueue_pcplist(preferred_zone, zone, gfp_flags,migratetype, alloc_flags);goto out;}}/** We most definitely don't want callers attempting to* allocate greater than order-1 page units with __GFP_NOFAIL.*/WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));spin_lock_irqsave(&zone->lock, flags);do {page = NULL;/** order-0 request can reach here when the pcplist is skipped* due to non-CMA allocation context. HIGHATOMIC area is* reserved for high-order atomic allocation, so order-0* request should skip it.*/// 如果是中断上下文的内存分配，alloc_flags会带有ALLOC_HARDER标记，会走该分配路径if (order > 0 && alloc_flags & ALLOC_HARDER) {page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);if (page)trace_mm_page_alloc_zone_locked(page, order, migratetype);}// 一般的内存分配，会走该路径if (!page)page = __rmqueue(zone, order, migratetype, alloc_flags);} while (page && check_new_pages(page, order));spin_unlock(&zone->lock);if (!page)goto failed;__mod_zone_freepage_state(zone, -(1 << order),get_pcppage_migratetype(page));__count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);zone_statistics(preferred_zone, zone);local_irq_restore(flags);out:/* Separate test+clear to avoid unnecessary atomics */if (test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags)) {clear_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);wakeup_kswapd(zone, 0, 0, zone_idx(zone));}VM_BUG_ON_PAGE(page && bad_range(zone, page), page);return page;failed:local_irq_restore(flags);return NULL;
}

2.5 __rmqueue()

/** Do the hard work of removing an element from the buddy allocator.* Call me with the zone->lock already held.*/
static __always_inline struct page *
__rmqueue(struct zone *zone, unsigned int order, int migratetype,unsigned int alloc_flags)
{struct page *page;#ifdef CONFIG_CMA/** Balance movable allocations between regular and CMA areas by* allocating from CMA when over half of the zone's free memory* is in the CMA area.*/// cma内存分配，这个我们暂时不进行讨论if (alloc_flags & ALLOC_CMA &&zone_page_state(zone, NR_FREE_CMA_PAGES) >zone_page_state(zone, NR_FREE_PAGES) / 2) {page = __rmqueue_cma_fallback(zone, order);if (page)return page;}
#endif
retry:// 根据迁移类型从buddy对应迁移链表中分配对应order页面的内存page = __rmqueue_smallest(zone, order, migratetype);if (unlikely(!page)) {if (alloc_flags & ALLOC_CMA)page = __rmqueue_cma_fallback(zone, order);// 未分配到，则转移到fallback流程，我们的迁移类型转换就在这里if (!page && __rmqueue_fallback(zone, order, migratetype,alloc_flags))goto retry;}trace_mm_page_alloc_zone_locked(page, order, migratetype);return page;
}

2.6 __rmqueue_smallest()

假设我们要分配的页面的迁移类型是MIGRATE_UNMOVABLE，但是一开始所有内存都是MIGRATE_MOVABLE，这样子肯定是分配不出来内存的。

/** Go through the free lists for the given migratetype and remove* the smallest available page from the freelists*/
static __always_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 */// 根据当前请求的order和migratetype进行分配，如果当前order无法满足，则向上找，一直找到最大的MAX_ORDER - 1为止for (current_order = order; current_order < MAX_ORDER; ++current_order) {// 先获取到对应的order数组链表area = &(zone->free_area[current_order]);// 以migratetype作为下标，确定存放page的链表page = get_page_from_free_area(area, migratetype);// 如果page没有找到，说明order无法满足分配，则尝试更大的orderif (!page)continue;// 从page链表中删除pagedel_page_from_free_list(page, zone, current_order);// 进行buddy调整expand(zone, page, order, current_order, migratetype);set_pcppage_migratetype(page, migratetype);// 返回已分配到的pagereturn page;}return NULL;
}static inline struct page *get_page_from_free_area(struct free_area *area,int migratetype)
{// 根据迁移类型作为下标return list_first_entry_or_null(&area->free_list[migratetype],struct page, lru);
}

2.7 __rmqueue_fallback() 核心处理

由于我们要分配的页面的迁移类型是MIGRATE_UNMOVABLE，但是一开始所有内存都是MIGRATE_MOVABLE，__rmqueue_smallest()无法分配出来内存，因此会走到__rmqueue_fallback()中进行处理。

/** Try finding a free buddy page on the fallback list and put it on the free* list of requested migratetype, possibly along with other pages from the same* block, depending on fragmentation avoidance heuristics. Returns true if* fallback was found so that __rmqueue_smallest() can grab it.** The use of signed ints for order and current_order is a deliberate* deviation from the rest of this file, to make the for loop* condition simpler.*/
static __always_inline bool
__rmqueue_fallback(struct zone *zone, int order, int start_migratetype,unsigned int alloc_flags)
{struct free_area *area;int current_order;int min_order = order;struct page *page;int fallback_mt;bool can_steal;/** Do not steal pages from freelists belonging to other pageblocks* i.e. orders < pageblock_order. If there are no local zones free,* the zonelists will be reiterated without ALLOC_NOFRAGMENT.*/// 一般的分配外面都会带有这个标记，为了减少页块内部碎片化if (alloc_flags & ALLOC_NOFRAGMENT)min_order = pageblock_order;/** Find the largest available free page in the other list. This roughly* approximates finding the pageblock with the most free pages, which* would be too costly to do exactly.*/// 我们从最大order开始分配，这样对于一个页块只需要改变迁移类型就好，否则会导致一个MIGRATE_MOVABLE迁移类型的页块部分内存用于了MIGRATE_MOVABLE迁移类型的内存分配，导致页块内部碎片化，无法进行回收for (current_order = MAX_ORDER - 1; current_order >= min_order;--current_order) {area = &(zone->free_area[current_order]);// 根据迁移类型查找是否可以从别的迁移类型中分配内存fallback_mt = find_suitable_fallback(area, current_order,start_migratetype, false, &can_steal);if (fallback_mt == -1)continue;/** We cannot steal all free pages from the pageblock and the* requested migratetype is movable. In that case it's better to* steal and split the smallest available page instead of the* largest available page, because even if the next movable* allocation falls back into a different pageblock than this* one, it won't cause permanent fragmentation.*/if (!can_steal && start_migratetype == MIGRATE_MOVABLE&& current_order > order)goto find_smallest;// 如果我们可以从别的迁移类型里偷到内存，则进行偷的处理goto do_steal;}return false;find_smallest:for (current_order = order; current_order < MAX_ORDER;current_order++) {area = &(zone->free_area[current_order]);fallback_mt = find_suitable_fallback(area, current_order,start_migratetype, false, &can_steal);if (fallback_mt != -1)break;}/** This should not happen - we already found a suitable fallback* when looking for the largest page.*/VM_BUG_ON(current_order == MAX_ORDER);do_steal:// 从可以偷的迁移类型里获取到内存page = get_page_from_free_area(area, fallback_mt);// 这里是将偷到的页块修改为分配请求需要的迁移类型steal_suitable_fallback(zone, page, alloc_flags, start_migratetype,can_steal);trace_mm_page_alloc_extfrag(page, order, current_order,start_migratetype, fallback_mt);return true;}

2.8 find_suitable_fallback()

查找一个合适的fallback迁移类型的页块。

/** Check whether there is a suitable fallback freepage with requested order.* If only_stealable is true, this function returns fallback_mt only if* we can steal other freepages all together. This would help to reduce* fragmentation due to mixed migratetype pages in one pageblock.*/
int find_suitable_fallback(struct free_area *area, unsigned int order,int migratetype, bool only_stealable, bool *can_steal)
{int i;int fallback_mt;if (area->nr_free == 0)return -1;// 先设置为不能偷*can_steal = false;for (i = 0;; i++) {// 从fallbacks数组里遍历fallback_mt = fallbacks[migratetype][i];if (fallback_mt == MIGRATE_TYPES)break;// 如果当前迁移类型没有内存，则换到下一个迁移类型if (free_area_empty(area, fallback_mt))continue;// 如果有内存，查看是否可以偷if (can_steal_fallback(order, migratetype))// 如果可以偷到话，设置为可偷*can_steal = true;if (!only_stealable)return fallback_mt;if (*can_steal)// 在能偷的前提下，将可偷的迁移类型返回return fallback_mt;}return -1;
}/** 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][3] = {[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,   MIGRATE_TYPES },[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_TYPES },[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,   MIGRATE_TYPES },
#ifdef CONFIG_CMA[MIGRATE_CMA]         = { MIGRATE_TYPES }, /* Never used */
#endif
#ifdef CONFIG_MEMORY_ISOLATION[MIGRATE_ISOLATE]     = { MIGRATE_TYPES }, /* Never used */
#endif
};

fallbacks是不同迁移类型内存不足时，可从哪个迁移类型中进行fallback操作，对于MIGRATE_UNMOVABLE迁移类型不足时，可以先MIGRATE_RECLAIMABLE迁移类型中偷内存，如果MIGRATE_RECLAIMABLE也没有内存的话，会进一步fallback到MIGRATE_MOVABLE迁移类型。因此对于我们一开始需要MIGRATE_UNMOVABLE类型的页面没有时，最终会fallback到MIGRATE_MOVABLE。

/** When we are falling back to another migratetype during allocation, try to* steal extra free pages from the same pageblocks to satisfy further* allocations, instead of polluting multiple pageblocks.** If we are stealing a relatively large buddy page, it is likely there will* be more free pages in the pageblock, so try to steal them all. For* reclaimable and unmovable allocations, we steal regardless of page size,* as fragmentation caused by those allocations polluting movable pageblocks* is worse than movable allocations stealing from unmovable and reclaimable* pageblocks.*/
static bool can_steal_fallback(unsigned int order, int start_mt)
{/** Leaving this order check is intended, although there is* relaxed order check in next check. The reason is that* we can actually steal whole pageblock if this condition met,* but, below check doesn't guarantee it and that is just heuristic* so could be changed anytime.*/// 如果我们偷的是一整个页块的大小，是允许的if (order >= pageblock_order)return true;if (order >= pageblock_order / 2 ||start_mt == MIGRATE_RECLAIMABLE ||start_mt == MIGRATE_UNMOVABLE ||page_group_by_mobility_disabled)return true;return false;
}

can_steal_fallback()函数用来检查当前order大小页面是否可以从fallback的迁移类型中偷取，当我们偷取的内存是一整个页块时，页面偷取是可以的。

2.9 steal_suitable_fallback()

/** This function implements actual steal behaviour. If order is large enough,* we can steal whole pageblock. If not, we first move freepages in this* pageblock to our migratetype and determine how many already-allocated pages* are there in the pageblock with a compatible migratetype. If at least half* of pages are free or compatible, we can change migratetype of the pageblock* itself, so pages freed in the future will be put on the correct free list.*/
static void steal_suitable_fallback(struct zone *zone, struct page *page,unsigned int alloc_flags, int start_type, bool whole_block)
{unsigned int current_order = buddy_order(page);int free_pages, movable_pages, alike_pages;int old_block_type;// 获取当前page的迁移类型old_block_type = get_pageblock_migratetype(page);/** This can happen due to races and we want to prevent broken* highatomic accounting.*/if (is_migrate_highatomic(old_block_type))goto single_page;/* Take ownership for orders >= pageblock_order */// 如果我们是偷的一整个页块的话，进入该函数处理if (current_order >= pageblock_order) {// 修改当前page的所在页块的迁移类型，也就是从MIGRATE_MOVABLE变为MIGRATE_UNMOVABLEchange_pageblock_range(page, current_order, start_type);goto single_page;}/** Boost watermarks to increase reclaim pressure to reduce the* likelihood of future fallbacks. Wake kswapd now as the node* may be balanced overall and kswapd will not wake naturally.*/boost_watermark(zone);if (alloc_flags & ALLOC_KSWAPD)set_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);/* We are not allowed to try stealing from the whole block */if (!whole_block)goto single_page;free_pages = move_freepages_block(zone, page, start_type,&movable_pages);/** Determine how many pages are compatible with our allocation.* For movable allocation, it's the number of movable pages which* we just obtained. For other types it's a bit more tricky.*/if (start_type == MIGRATE_MOVABLE) {alike_pages = movable_pages;} else {/** If we are falling back a RECLAIMABLE or UNMOVABLE allocation* to MOVABLE pageblock, consider all non-movable pages as* compatible. If it's UNMOVABLE falling back to RECLAIMABLE or* vice versa, be conservative since we can't distinguish the* exact migratetype of non-movable pages.*/if (old_block_type == MIGRATE_MOVABLE)alike_pages = pageblock_nr_pages- (free_pages + movable_pages);elsealike_pages = 0;}/* moving whole block can fail due to zone boundary conditions */if (!free_pages)goto single_page;/** If a sufficient number of pages in the block are either free or of* comparable migratability as our allocation, claim the whole block.*/if (free_pages + alike_pages >= (1 << (pageblock_order-1)) ||page_group_by_mobility_disabled)set_pageblock_migratetype(page, start_type);return;single_page:// 并将页面迁移到对应的新的迁移类型所在的链表中move_to_free_list(page, zone, current_order, start_type);
}static void change_pageblock_range(struct page *pageblock_page,int start_order, int migratetype)
{int nr_pageblocks = 1 << (start_order - pageblock_order);while (nr_pageblocks--) {// 设置迁移类型set_pageblock_migratetype(pageblock_page, migratetype);pageblock_page += pageblock_nr_pages;}
}

将从fallback的迁移类型中获取到页面，并将页面放置到所需求的迁移类型后，会重新retry进行内存分配。

好了，这里对于migrate_type进一步探索就到这里了，感谢各位读者浏览！！！
预知后续如何，请看下个博文的分析。