rmqueue

static struct page * rmqueue(zone_t *zone, unsigned long order)
{
    free_area_t * area = zone->free_area + order;
    unsigned long curr_order = order;
    struct list_head *head, *curr;
    unsigned long flags;
    struct page *page;

    spin_lock_irqsave(&zone->lock, flags);
    do {
        head = &area->free_list;
        curr = memlist_next(head);

        if (curr != head) {//不为空
            unsigned int index;

            page = memlist_entry(curr, struct page, list);//取得第一页
            if (BAD_RANGE(zone,page))
                BUG();
            memlist_del(curr);//删除第一页
            index = (page - mem_map) - zone->offset;
            MARK_USED(index, curr_order, area);
            zone->free_pages -= 1 << order;//空闲页数减小

            page = expand(zone, page, index, order, curr_order, area);
            spin_unlock_irqrestore(&zone->lock, flags);

            set_page_count(page, 1);
            if (BAD_RANGE(zone,page))
                BUG();
            DEBUG_ADD_PAGE
            return page;
        }
        curr_order++;
        area++;
    } while (curr_order < MAX_ORDER);
    spin_unlock_irqrestore(&zone->lock, flags);

    return NULL;
}