Linux内核伙伴系统中页面释放，主函数为free_pages()

一、上层操作

void free_pages(unsigned long addr, unsigned int order){if (addr != 0) {VM_BUG_ON(!virt_addr_valid((void *)addr));__free_pages(virt_to_page((void *)addr), order);}}

void __free_pages(struct page *page, unsigned int order){if (put_page_testzero(page)) {trace_mm_page_free_direct(page, order);if (order == 0)free_hot_page(page);else__free_pages_ok(page, order);}}

二、释放单个页面

释放单个页面free_hot_page()调用free_hot_cold_page()函数

static void free_hot_cold_page(struct page *page, int cold){struct zone *zone = page_zone(page);struct per_cpu_pages *pcp;unsigned long flags;int migratetype;int wasMlocked = __TestClearPageMlocked(page);kmemcheck_free_shadow(page, 0);if (PageAnon(page))page->mapping = NULL;if (free_pages_check(page))return;if (!PageHighMem(page)) {debug_check_no_locks_freed(page_address(page), PAGE_SIZE);debug_check_no_obj_freed(page_address(page), PAGE_SIZE);}arch_free_page(page, 0);kernel_map_pages(page, 1, 0);pcp = &zone_pcp(zone, get_cpu())->pcp;migratetype = get_pageblock_migratetype(page);set_page_private(page, migratetype);local_irq_save(flags);if (unlikely(wasMlocked))free_page_mlock(page);__count_vm_event(PGFREE);if (migratetype >= MIGRATE_PCPTYPES) {if (unlikely(migratetype == MIGRATE_ISOLATE)) {free_one_page(zone, page, 0, migratetype);goto out;}migratetype = MIGRATE_MOVABLE;}if (cold)list_add_tail(&page->lru, &pcp->lists[migratetype]);elselist_add(&page->lru, &pcp->lists[migratetype]);pcp->count++;if (pcp->count >= pcp->high) {free_pcppages_bulk(zone, pcp->batch, pcp);pcp->count -= pcp->batch;}out:local_irq_restore(flags);put_cpu();}

从pcp中释放页面到伙伴系统中

free_pcppages_bulk()

are in same zone, and of same order. * count is the number of pages to free. * * If the zone was previously in an "all pages pinned" state then look to * see if this freeing clears that state. * * And clear the zone's pages_scanned counter, to hold off the "all pages are * pinned" detection logic. */ static void free_pcppages_bulk(struct zone *zone, int count,struct per_cpu_pages *pcp){int migratetype = 0;int batch_free = 0;spin_lock(&zone->lock);zone_clear_flag(zone, ZONE_ALL_UNRECLAIMABLE);zone->pages_scanned = 0;__mod_zone_page_state(zone, NR_FREE_PAGES, count);while (count) {struct page *page;struct list_head *list;do {batch_free++;if (++migratetype == MIGRATE_PCPTYPES)migratetype = 0;list = &pcp->lists[migratetype];} while (list_empty(list));do {page = list_entry(list->prev, struct page, lru);list_del(&page->lru);__free_one_page(page, zone, 0, migratetype);trace_mm_page_pcpu_drain(page, 0, migratetype);} while (--count && --batch_free && !list_empty(list));}spin_unlock(&zone->lock);}

三、释放多个页面

释放多个页面__free_pages_ok()函数

int i;int bad = 0;int wasMlocked = __TestClearPageMlocked(page);kmemcheck_free_shadow(page, order);for (i = 0 ; i < (1 << order) ; ++i)bad += free_pages_check(page + i);if (bad)return;if (!PageHighMem(page)) {debug_check_no_locks_freed(page_address(page),PAGE_SIZE<

static void free_one_page(struct zone *zone, struct page *page, int order,int migratetype){spin_lock(&zone->lock);zone_clear_flag(zone, ZONE_ALL_UNRECLAIMABLE);zone->pages_scanned = 0;__mod_zone_page_state(zone, NR_FREE_PAGES, 1 << order);__free_one_page(page, zone, order, migratetype);spin_unlock(&zone->lock);}

etype){unsigned long page_idx;if (unlikely(PageCompound(page)))if (unlikely(destroy_compound_page(page, order)))return;VM_BUG_ON(migratetype == -1);page_idx = page_to_pfn(page) & ((1 << MAX_ORDER) - 1); VM_BUG_ON(page_idx & ((1 << order) - 1)); VM_BUG_ON(bad_range(zone, page));while (order < MAX_ORDER-1) {unsigned long combined_idx;struct page *buddy;buddy = __page_find_buddy(page, page_idx, order);if (!page_is_buddy(page, buddy, order))break;list_del(&buddy->lru);zone->free_area[order].nr_free--; rmv_page_order(buddy);combined_idx = __find_combined_index(page_idx, order);page = page + (combined_idx - page_idx);page_idx = combined_idx;order++;}set_page_order(page, order);list_add(&page->lru,&zone->free_area[order].free_list[migratetype]);zone->free_area[order].nr_free++;}

四、关于伙伴的操作

4.1，查找伙伴

static inline struct page *__page_find_buddy(struct page *page, unsigned long page_idx, unsigned int order){unsigned long buddy_idx = page_idx ^ (1 << order);return page + (buddy_idx - page_idx);}

4.2，检查是否为伙伴

static inline int page_is_buddy(struct page *page, struct page *buddy,int order){if (!pfn_valid_within(page_to_pfn(buddy)))return 0;if (page_zone_id(page) != page_zone_id(buddy))return 0;if (PageBuddy(buddy) && page_order(buddy) == order) {VM_BUG_ON(page_count(buddy) != 0);return 1;}return 0;}

总结：伙伴系统内存释放或称主要流程

1，如果释放的是单个页面，需要根据页面类型考虑是否释放到伙伴系统中，同时，将其加入到pcp链表中。如果pcp链表中内存过多，调用free_pcppages_bulk()函数将大块内存放回伙伴系统中；

2，如果释放的是多个页面，直接调用__free_one_page()释放到伙伴系统中。

3，释放到伙伴系统中时，需要考虑和伙伴的合并情况。