写时复制期间父进程是否会失去写能力? [英] Does parent process lose write ability during copy on write?
问题描述
假设我们有一个特定的父进程，内存中存储了一些任意数量的数据，我们使用 fork 来生成一个子进程.我了解，为了让操作系统在写入时执行复制，内存中包含我们正在修改的数据的特定页面将设置其只读位，并且操作系统将使用孩子尝试时将导致的异常修改数据以将整个页面复制到内存中的另一个区域，以便孩子获得自己的副本.我不明白的是，如果内存中的特定部分被标记为只读，那么数据最初所属的父进程也无法修改数据.那么这整个方案如何运作呢?父级是否会失去其数据的所有权，并且即使父级本身尝试修改数据也必须执行写入时复制?
Say we have a certain parent process with some arbitrary amount of data stored in memory and we use fork to spawn a child process. I understand that in order for the OS to perform copy on write, the certain page in memory that contains the data that we are modifying will have its Read-only bit set, and the OS will use the exception that will result when the child tries to modify the data to copy the entire page into another area in memory so that the child gets it's own copy. What I don't understand is, if that specific section in memory is marked as Read-only, then the parent process, to whom the data originally belonged, would not be able to modify the data neither. So how can this whole scheme work? Does the parent lose ownership of its data and copy on write will have to be performed even when the parent itself tries to modify the data?
推荐答案
对了，如果有一个进程写了一个COW页面，就会触发页面错误.
Right, if either process writes a COW page, it triggers a page fault.
在页面错误处理程序中，如果页面假定是可写的，它会分配一个新的物理页面并执行 memcpy(newpage, shared_page, pagesize)，然后更新出现故障的进程的页表，以将新页映射到该虚拟地址.然后返回用户空间重新运行 store 指令.
In the page fault handler, if the page is supposed to be writeable, it allocates a new physical page and does a memcpy(newpage, shared_page, pagesize), then updates the page table of whichever process faulted to map the newpage to that virtual address. Then returns to user-space for the store instruction to re-run.
这对于 fork 之类的东西来说是一个胜利，因为一个进程通常会在通常触摸一页(堆栈内存)之后立即进行 execve 系统调用.execve 销毁该进程的所有内存映射，有效地将其替换为一个新进程.父级再次拥有每个页面的唯一副本.(除了已经写时复制的页面，例如用 mmap 分配的内存通常被 COW 映射到单个零物理页面，因此读取可以在 L1d 中命中缓存).
This is a win for something like fork, because one process typically makes an execve system call right away, after touching typically one page (of stack memory). execve destroys all memory mappings for that process, effectively replacing it with a new process. The parent once again has the only copy of every page. (Except pages that were already copy-on-write, e.g. memory allocated with mmap is typically COW-mapped to a single physical page of zeros, so reads can hit in L1d cache).
一个聪明的优化是 fork 实际复制包含堆栈顶部的页面，但仍然对所有其他页面执行惰性 COW，假设子进程通常会execve 立即删除它对所有其他页面的引用.但是，将所有页面临时翻转为只读并返回仍然需要在父级中使 TLB 失效.
A smart optimization would be for fork to actually copy the page containing the top of the stack, but still do lazy COW for all the other pages, on the assumption that the child process will normally execve right away and thus drop its references to all the other pages. It still costs a TLB invalidation in the parent to temporarily flip all the pages to read-only and back, though.
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