获得总页面数

5.2.2 获得总页面数

回到setup_arch()中，接下来，继续走，891行调用e820_end_of_ram_pfn()函数根据e820的数据来获得32位可用物理内存地址的最大值并右移PAGE_SHIFT，也就是12位，最后由函数e820_end_pfn返回这个20位的值，保存在内部变量max_pfn中，作为总的页面数量：

 

855static unsigned long __init e820_end_pfn(unsigned long limit_pfn, unsigned type)  856{  857        int i;  858        unsigned long last_pfn = 0;  859        unsigned long max_arch_pfn = MAX_ARCH_PFN;  860  861        for (i = 0; i < e820.nr_map; i++) {  862                struct e820entry *ei = &e820.map[i];  863                unsigned long start_pfn;  864                unsigned long end_pfn;  865  866                if (ei->type != type)  867                        continue;  868  869                start_pfn = ei->addr >> PAGE_SHIFT;  870                end_pfn = (ei->addr + ei->size) >> PAGE_SHIFT;  871  872                if (start_pfn >= limit_pfn)  873                        continue;  874                if (end_pfn > limit_pfn) {  875                        last_pfn = limit_pfn;  876                        break;  877                }  878                if (end_pfn > last_pfn)  879                        last_pfn = end_pfn;  880        }  881  882        if (last_pfn > max_arch_pfn)  883                last_pfn = max_arch_pfn;  884  885        printk(KERN_INFO "last_pfn = %#lx max_arch_pfn = %#lx/n",  886                         last_pfn, max_arch_pfn);  887        return last_pfn;  888}  889unsigned long __init e820_end_of_ram_pfn(void)  890{  891        return e820_end_pfn(MAX_ARCH_PFN, E820_RAM);  892}

 

紧接着，setup_arch的第902行调用find_low_pfn_range()，它根据max_pfn是否大于MAXMEM_PFN来判断是否启用了高端内存区。这个MAXMEM_PFN值等于多少，大家自己去查，不是很复杂，只是这个值跟max_pfn一样，也是除去低12位的20位的一个数值。如果启用了，就调用highmem_pfn_init()函数将全局变量max_low_pfn设置为MAXMEM_PFN；并设置全局变量highmem_pages为max_pfn - MAXMEM_PFN，作为高端页面的数量：

 

660void __init highmem_pfn_init(void)  661{  662        max_low_pfn = MAXMEM_PFN;  663  664        if (highmem_pages == -1)  665                highmem_pages = max_pfn - MAXMEM_PFN;  666  667        if (highmem_pages + MAXMEM_PFN < max_pfn)  668                max_pfn = MAXMEM_PFN + highmem_pages;  669  670        if (highmem_pages + MAXMEM_PFN > max_pfn) {  671                printk(KERN_WARNING MSG_HIGHMEM_TOO_SMALL,  672                        pages_to_mb(max_pfn - MAXMEM_PFN),  673                        pages_to_mb(highmem_pages));  674                highmem_pages = 0;  675        } ……  692}