Windows 上的写时复制文件映射 [英] Copy-on-write file mapping on windows

问题描述

我有 3 个进程通过命名管道进行通信:服务器、写入器、读取器.基本思想是 writer 可以在 server 上存储巨大的 (~GB) 二进制 blob，并且 reader(s) 可以检索它.但不是在命名管道上发送数据，而是使用内存映射.

I have 3 processes communicating over named pipes: server, writer, reader. The basic idea is that the writer can store huge (~GB) binary blobs on the server, and the reader(s) can retrieve it. But instead of sending data on the named pipe, memory mapping is used.

服务器使用CreateFileMapping和PAGE_READWRITE保护创建一个未命名的文件支持映射，然后将句柄复制到writer.writer 完成其工作后，句柄被复制到任意数量的感兴趣的读者.

The server creates an unnamed file-backed mapping with CreateFileMapping with PAGE_READWRITE protection, then duplicates the handle into the writer. After the writer has done its job, the handle is duplicated into any number of interested readers.

writer 在 FILE_MAP_WRITE 模式下用 MapViewOfFile 映射句柄.

The writer maps the handle with MapViewOfFile in FILE_MAP_WRITE mode.

reader 在 FILE_MAP_READ|FILE_MAP_COPY 模式下用 MapViewOfFile 映射句柄.

The reader maps the handle with MapViewOfFile in FILE_MAP_READ|FILE_MAP_COPY mode.

在阅读器上我想要写时复制语义，所以只要映射是只读的，它就会在所有阅读器实例之间共享.但是，如果读者想要写入其中(例如，就地解析或图像处理)，则影响应仅限于使用尽可能少的复制页面的修改过程.

On the reader I want copy-on-write semantics, so as long the mapping is only read it is shared between all reader instances. But if a reader wants to write into it (eg. in-place parsing or image processing), the impacts should be limited to the modifying process with the least number of copied pages possible.

问题
当 reader 尝试写入映射时，它会因分段错误而死亡，就好像没有考虑 FILE_MAP_COPY 一样.上述方法有什么问题?根据 MSDN 这应该可以工作...

The problem
When the reader tries to write into the mapping it dies with segmentation fault as if FILE_MAP_COPY was not considered. What's wrong with the above described method? According to MSDN this should work...

我们也在 linux 上实现了相同的机制(使用 mmap 和 fd 传入 AF_UNIX 辅助缓冲区)并且它按预期工作.

We have the same mechanism implemented on linux as well (with mmap and fd passing in AF_UNIX ancillary buffers) and it works as expected.

推荐答案

MapViewOfFile 设计或/和记录不当.这是ZwMapViewOfSection. 的 dwDesiredAccess 参数MapViewOfFile 转换为 ZwMapViewOfSection.

problem here that MapViewOfFile bad designed or/and documented. this is shell (with restricted functionality) over ZwMapViewOfSection. the dwDesiredAccess parameter of MapViewOfFile converted to Win32Protect parameter of ZwMapViewOfSection.

FILE_MAP_READ|FILE_MAP_COPY 组合转换为 PAGE_READONLY 页面保护，因为这样你会在写入时出现页面错误.

the FILE_MAP_READ|FILE_MAP_COPY combination converted to PAGE_READONLY page protection, because this you and get page fault on write.

您只需要使用 FILE_MAP_COPY 标志 - 它转换为 PAGE_WRITECOPY 页面保护，在这种情况下，一切都将起作用.

you need use FILE_MAP_COPY only flag - it converted to PAGE_WRITECOPY page protection and in this case all will be work.

最好的解决方案当然是直接使用ZwMapViewOfSection 带有 PAGE_WRITECOPY 页面保护

the best solution of course direct use ZwMapViewOfSection with PAGE_WRITECOPY page protection

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