linux如何识别特定文件系统以执行系统调用

Can please summarize the events/steps that happen when I try to execute a read()/write() system call. How does the kernel know which file system to issue these commands.

可以总结一下我尝试执行read（）/ write（）系统调用时发生的事件/步骤。内核如何知道发出这些命令的文件系统。

Lets say a process calls write(). Then It will call sys_write().

让我们说一个进程调用write（）。然后它将调用sys_write（）。

Now probably, since sys_write() is executed on behalf of the current process, it can access the struct task_struct and hence it can access the struct files_struct and struct fs_struct which contains file system information.

现在可能，因为sys_write（）是代表当前进程执行的，所以它可以访问struct task_struct，因此它可以访问包含文件系统信息的struct files_struct和struct fs_struct。

But after that I am not seeing, how this fs_struct is helping to identify the file system.

但之后我没有看到，这个fs_struct如何帮助识别文件系统。

Edit: Now that Alex has described the flow...I have still doubt how the read/write are getting routed to a FS, since the VFS does not do it, then it must be happening somewhere else, Also how is the underlying block device and then finally the hardware protocol PCI/USB getting attached.

编辑：既然Alex已经描述了流程......我仍然怀疑读/写如何被路由到FS，因为VFS没有这样做，那么它必须在其他地方发生，另外底层是怎么回事设备然后最终硬件协议PCI / USB附加。

A simple flow chart involving actual data structures would be helpful

涉及实际数据结构的简单流程图会很有帮助

Please help.

请帮忙。

2 个解决方案

#1

This answer is based on kernel version 4.0. I traced out some of the code which handles a read syscall. I recommend you clone the Linux source repo and follow along in the source code.

这个答案基于内核版本4.0。我找出了一些处理读取系统调用的代码。我建议您克隆Linux源代码库并在源代码中进行操作。

1. Syscall handler for read, at fs/read_write.c:620 is called. It receives a file descriptor (integer) as an argument, and calls fdget_pos to convert it to a struct fd.
2. 调用Syscall处理程序，在fs / read_write.c：620处调用。它接收文件描述符（整数）作为参数，并调用fdget_pos将其转换为struct fd。
3. fdget_pos calls __fdget_pos calls __fdget calls __fget_light. __fget_light uses current->files, the file descriptor table for the current process, to look up the struct file which corresponds to the passed file descriptor number.
4. fdget_pos调用__fdget_pos调用__fdget调用__fget_light。 __fget_light使用current-> files（当前进程的文件描述符表）来查找与传递的文件描述符号对应的struct文件。
5. Back in the syscall handler, the file struct is passed to vfs_read, at fs/read_write.c:478.
6. 回到syscall处理程序，文件结构传递给vfs_read，位于fs / read_write.c：478。
7. vfs_read calls __vfs_read, which calls file->f_op->read. From here on, you are in filesystem-specific code.
8. vfs_read调用__vfs_read，调用file-> f_op-> read。从此处开始，您将使用特定于文件系统的代码。

So the VFS doesn't really bother "identifying" the filesystem which a file lives on; it simply uses the table of "file operation" function pointers which is stored in its struct file. When that struct file is initialized, it is given the correct f_op function pointer table which implements all the filesystem-specific operations for its filesystem.

所以VFS并不真正打扰“识别”文件所在的文件系统;它只是使用存储在其struct文件中的“文件操作”函数指针表。初始化该结构文件时，会为其提供正确的f_op函数指针表，该表实现其文件系统的所有特定于文件系统的操作。

#2

Each filesystem registers itself to VFS. When a filesystem is mounted, its superblock is read and VFS superblock is populated with this information. Function pointer table for this filesystem is also populated at this time. when file->f_op->read call happens, registered function from the filesystem is actually called. You can refer to text in http://www.science.unitn.it/~fiorella/guidelinux/tlk/node102.html

每个文件系统都将自己注册到VFS。安装文件系统时，将读取其超级块，并使用此信息填充VFS超级块。此时还会填充此文件系统的函数指针表。当file-> f_op->读取调用发生时，实际调用文件系统中的注册函数。您可以参考http://www.science.unitn.it/~fiorella/guidelinux/tlk/node102.html中的文字。