How linux identify a particular file system to execute system call

Question

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.

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

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

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.

A simple flow chart involving actual data structures would be helpful

Please help.

Answer 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.

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. 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.
3. Back in the syscall handler, the file struct is passed to vfs_read, at fs/read_write.c:478.
4. vfs_read calls __vfs_read, which calls file->f_op->read. From here on, you are in filesystem-specific code.

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.