标签：linux kernel
/*
*Author : DavidLin
*Date : 2014-11-22pm
*Email : linpeng1577@163.com or linpeng1577@gmail.com
*world : the city of SZ, in China
*Ver : 000.000.001
*history : editor time do
* 1)LinPeng 2014-11-22 created this file!
* 2)
*/
/*
* Well, here is one of the most complicated functions in mm. It
* copies a range of linerar addresses by copying only the pages.
* Let‘s hope this is bug-free, ‘cause this one I don‘t want to debug :-)
*
* Note! We don‘t copy just any chunks of memory - addresses have to
* be divisible by 4Mb (one page-directory entry), as this makes the
* function easier. It‘s used only by fork anyway.
*
* NOTE 2!! When from==0 we are copying kernel space for the first
* fork(). Then we DONT want to copy a full page-directory entry, as
* that would lead to some serious memory waste - we just copy the
* first 160 pages - 640kB. Even that is more than we need, but it
* doesn‘t take any more memory - we don‘t copy-on-write in the low
* 1 Mb-range, so the pages can be shared with the kernel. Thus the
* special case for nr=xxxx.
*/
/* Linus认为下面copy_page_tables()函数是内存管理部分最难的之一
* copy_page_tables()函数只被fork函数调用
* 拷贝只是拷贝了一个页表，页表是管理4M地址的，所以按照4M对齐
* 不拷贝物理页内容，当发生写时拷贝才会拷贝页表所管理的物理页内容
* 对于进程0和1，只拷贝前160页共640Kb，出于效率考虑
* 0-1M作为内核驻留地址区域，禁止写覆盖
* 参数from，to是0-4G线性地址，size是字节为单位
*/
int copy_page_tables(unsigned long from,unsigned long to,long size)
{
unsigned long * from_page_table; //用于管理源页表
unsigned long * to_page_table; //用于管理目的页表
unsigned long this_page; //用于保存页表
unsigned long * from_dir, * to_dir; //用于管理源页目录项，目的页目录项
unsigned long nr; //用于保存页表项个数
if ((from&0x3fffff) || (to&0x3fffff)) //4M对齐检测，否则die
panic("copy_page_tables called with wrong alignment");
from_dir = (unsigned long *) ((from>>20) & 0xffc); /* _pg_dir = 0 */
//源页目录项
to_dir = (unsigned long *) ((to>>20) & 0xffc); //目的页目录项
size = ((unsigned) (size+0x3fffff)) >> 22; //页表项个数是字节数除以4M
for( ; size-->0 ; from_dir++,to_dir++) {
if (1 & *to_dir) //如果目的页目录项已经被使用，die
panic("copy_page_tables: already exist");
if (!(1 & *from_dir))
continue; //如果源页目录项未使用，跳过，不拷贝
from_page_table = (unsigned long *) (0xfffff000 & *from_dir);//取源页表
if (!(to_page_table = (unsigned long *) get_free_page()))
return -1; /* Out of memory, see freeing */ //取空闲物理页为to_page_table赋值
//如果没有空闲物理页，die
*to_dir = ((unsigned long) to_page_table) | 7; //将页表存进相应页目录项，
//7表示可读写
//想一下常用的chmod 777 anyfile
nr = (from==0)?0xA0:1024; //如果是0地址，只拷贝160页，否则拷贝1024页
//一个页目录表管理1024个页目录项
//一个页表管理1024个页表项
//一个页表项管理有4K物理地址
for ( ; nr-- > 0 ; from_page_table++,to_page_table++) {
this_page = *from_page_table; //从源页表中取源页表项
if (!(1 & this_page)) //如果源页表项未被使用，跳过
continue;
this_page &= ~2; //目的页表项读写位,
//设置为只读
*to_page_table = this_page; //将源页表项存进目的页表项
if (this_page > LOW_MEM) { //如果是主内存区
*from_page_table = this_page;//源页表项也要设置为只读
this_page -= LOW_MEM; //取相对主内存的偏移地址
this_page >>= 12; //取主内存管理数组索引
mem_map[this_page]++; //物理页引用次数加1
}
}
}
invalidate(); //刷新高速缓存
return 0; //返回0表示成功
}本文出自 “山下问童子” 博客，请务必保留此出处http://linpeng.blog.51cto.com/9779987/1596728
Linux0.11内核源码分析系列：内存管理copy_page_tables()函数分析
标签：linux kernel
原文地址：http://linpeng.blog.51cto.com/9779987/1596728
