/* 释放页框 */
void __free_pages(struct page *page, unsigned int order)
{/* 检查页框是否还有进程在使用，就是检查_count变量的值是否为0 */if (put_page_testzero(page)) {/* 如果是1个页框，则放回每CPU高速缓存中，如果是多个页框，则放回伙伴系统，放回CPU高速缓存中优先把其设置为热页，而不是冷页 */if (order == 0)free_hot_cold_page(page, false);else__free_pages_ok(page, order);}
}

void free_hot_cold_page(struct page *page, bool cold)
{/* 页框所处管理区 */struct zone *zone = page_zone(page);struct per_cpu_pages *pcp;unsigned long flags;/* 页框号 */unsigned long pfn = page_to_pfn(page);int migratetype;/* 检查 */if (!free_pages_prepare(page, 0))return;/* 获取页框所在pageblock的页框类型 */migratetype = get_pfnblock_migratetype(page, pfn);/* 设置页框类型为pageblock的页框类型，因为在页框使用过程中，这段pageblock可以移动到了其他类型(比如MIGRATE_MOVABLE -> MIGRATE_UNMOVABLE) */set_freepage_migratetype(page, migratetype);local_irq_save(flags);__count_vm_event(PGFREE);if (migratetype >= MIGRATE_PCPTYPES) {/* 如果不是高速缓存类型，则放回伙伴系统 */if (unlikely(is_migrate_isolate(migratetype))) {free_one_page(zone, page, pfn, 0, migratetype);goto out;}migratetype = MIGRATE_MOVABLE;}/* 放入当前CPU高速缓存中，要以migratetype区分开来 */pcp = &this_cpu_ptr(zone->pageset)->pcp;if (!cold)list_add(&page->lru, &pcp->lists[migratetype]);elselist_add_tail(&page->lru, &pcp->lists[migratetype]);pcp->count++;/* 当前CPU高速缓存中页框数量高于最大值，将pcp->batch数量的页框放回伙伴系统 */if (pcp->count >= pcp->high) {unsigned long batch = ACCESS_ONCE(pcp->batch);free_pcppages_bulk(zone, batch, pcp);pcp->count -= batch;}out:local_irq_restore(flags);
}

static void __free_pages_ok(struct page *page, unsigned int order)
{unsigned long flags;int migratetype;/* 获取页框号 */unsigned long pfn = page_to_pfn(page);/* 准备，各种检查 */if (!free_pages_prepare(page, order))return;/* 获取页框所在pageblock的页框类型 */migratetype = get_pfnblock_migratetype(page, pfn);/* 禁止中断 */local_irq_save(flags);/* 统计当前CPU一共释放的页框数 */__count_vm_events(PGFREE, 1 << order);/* 设置这块连续页框块的类型与所在pageblock类型一致，保存在page->index中 */set_freepage_migratetype(page, migratetype);/* 释放函数 */free_one_page(page_zone(page), page, pfn, order, migratetype);local_irq_restore(flags);
}

static void free_one_page(struct zone *zone,struct page *page, unsigned long pfn,unsigned int order,int migratetype)
{unsigned long nr_scanned;/* 管理区上锁 */spin_lock(&zone->lock);/* 数据更新 */nr_scanned = zone_page_state(zone, NR_PAGES_SCANNED);if (nr_scanned)__mod_zone_page_state(zone, NR_PAGES_SCANNED, -nr_scanned);/* 内存隔离使用 */if (unlikely(has_isolate_pageblock(zone) ||is_migrate_isolate(migratetype))) {migratetype = get_pfnblock_migratetype(page, pfn);}/* 释放page开始的order次方个页框到伙伴系统，这些页框的类型时migratetype */__free_one_page(page, pfn, zone, order, migratetype);/* 管理区解锁 */spin_unlock(&zone->lock);
}

static inline void __free_one_page(struct page *page,unsigned long pfn,struct zone *zone, unsigned int order,int migratetype)
{/* 保存块中第一个页框的下标，这个下标相对于管理区而言，而不是node */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(!zone_is_initialized(zone));if (unlikely(PageCompound(page)))if (unlikely(destroy_compound_page(page, order)))return;VM_BUG_ON(migratetype == -1);if (is_migrate_isolate(migratetype)) {/** We restrict max order of merging to prevent merge* between freepages on isolate pageblock and normal* pageblock. Without this, pageblock isolation* could cause incorrect freepage accounting.*//* 如果使用了内存隔离，则最大的order应该为MAX_ORDER与pageblock_order+1中最小那个，实际上在没有大页的情况下，这两个值相等，如果有大页的情况下，则不一定 */max_order = min(MAX_ORDER, pageblock_order + 1);} else {__mod_zone_freepage_state(zone, 1 << order, migratetype);}/* page的pfn号 */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);/* 主要，最多循环10次，每次都尽量把一个块和它的伙伴进行合并，以最小块开始 */while (order < max_order - 1) {/* buddy_idx = page_idx ^ (1 << order) *//* buddy_idx是page_idx的伙伴的页框号 *//* 伙伴的页框号就是page_idx的第(1 << order)位的相反数，比如(1<<order)是4，page_idx是01110，则buddy_idx是01010，由此可见伙伴并不一定是之后的区间 *//**      对于000000 ~ 001000这个页框号区间，假设order是3，左边是第一种情况，右边是另一种情况**                            -----------*                           |           |*                           |           |*                           |           |* page_idx = 000100 ------> |-----------|    计算后buddy_idx = 000100*                           |           |*                           |           |*                           |           |* 计算后buddy_idx = 000000   -----------     page_idx = 000000*/buddy_idx = __find_buddy_index(page_idx, order);/* 伙伴的页描述符，就是buddy_idx对应的页描述符 */buddy = page + (buddy_idx - page_idx);/* 检查buddy是否描述了大小为order的空闲页框块的第一个页 */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)) {/* 设置了PAGE_DEBUG_FLAG_GUARD */clear_page_guard_flag(buddy);set_page_private(buddy, 0);if (!is_migrate_isolate(migratetype)) {__mod_zone_freepage_state(zone, 1 << order,migratetype);}} else {/* 将伙伴从当前空闲链表中移除出来 */list_del(&buddy->lru);zone->free_area[order].nr_free--;rmv_page_order(buddy);}/* combined_idx 是 buddy_idx 与 page_idx 中最小的那个idx */combined_idx = buddy_idx & page_idx;page = page + (combined_idx - page_idx);page_idx = combined_idx;order++;}set_page_order(page, order);/* 循环结束，标记了释放的连续page已经和之后的连续页形成了一个2的order次方的连续页框块 *//* 检查能否再进一步合并 */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;}}/* 加入空闲块链表 */list_add(&page->lru, &zone->free_area[order].free_list[migratetype]);
out:/* 对应空闲链表中空闲块数量加1 */zone->free_area[order].nr_free++;
}