COW 如何避免被写入磁盘？

zylthinking

如下：

1. We have always just done a COW if it's read-only - even if it's shared.
   
2. The point being that if a process mapped did a read-only mapping, and a
   
3. tracer wants to modify memory, the tracer is always allowed to do so, but
   
4. it's _not_ going to write anything back to the filesystem. Writing
   
5. something back to an executable just because the user happened to mmap it
   
6. with MAP_SHARED (but read-only) _and_ the user had the right to write to
   
7. that fd is _not_ ok.
   

复制代码

既然是 COW, 也基本上是修改只自己可见， 因此也不应该写入磁盘。
问题是，  COW 页面是如何避免自己被写入磁盘的？
看代码， 只不过是新分配一个页面， 修改页表属性而可写； 但这个页面仍然和 vma 相连接， vma 依然保存着 vm_file, vm_file 依然有 inode, inode 依然有 i_mapping, i_mapping 依然有个 a_ops, 这里面就有何 writepage 了。。。。。。而且， 似乎没有哪里有信息说明这个页面不应该被 writepage, 那么。。。。。。。。。。。。

zylthinking

已经搞明白了。。。。。。。