Question

My question is how do we pass back the mmap’d area back to the module so that it can manipulate that memory.

What I am looking for is this:

1. My user space application runs using several hundred threads (so its hard to keep track of which buffer the kernel has given to which user-space thread).
2. The user thread calls mmap function of the module and after getting it, it fills the buffer with data.
3. Then it has to pass this buffer to the module using ioctl() to the module so that the module does some processing and modify the buffer.
4. User space thread reads the new data

I’m not sure as to how “step 3” can be implemented. Is there anyway to pass this user-buffer to the module so that it knows which address it is in the kernel space?

(I can’t use malloc or copy_from_user)

Answer 1

Check if virt_to_phys() works for you.

1. The userspace-thread calls mmap().
2. Obtains buffer address, fills the buffer with data.
3. Passes buffer address back to to the module using ioctl().
4. Module calls virt_to_phys(addr), does processing on buffer.
5. Later, userspace-thread reads the processed data.