Search for "Transaction based IO" in fs/libfs.c:

/*
 * Transaction based IO.
 * The file expects a single write which triggers the transaction, and then
 * possibly a read which collects the result - which is stored in a
 * file-local buffer.
 */

So the user is expected to do:

	open()
	write() #send argument
	read()  #get result
	close()

The read() is optional, but multiple writes without closing and reopening will
result in -EBUSY.  The size of the argument and response are limited by
SIMPLE_TRANSACTION_LIMIT, which is PAGE_SIZE (4k) minus a few bytes to store
the size of the argument or response.

To use from the kernel: call
	data = simple_transaction_set(file, buf, size)

from the filesystem write method.  This  returns pointer to the write data
copied from userspace.

After processing that write data, store the response in the returned data, then call

	simple_transaction_set(file, size)

All you need for the read method is simple_transaction_read().

Note nfsd uses this for the nfsd filesystem, but also allows

	open()
	read()
	write()

(So, e.g., you can just cat those files.), by doing a dummy zero-length write
when it gets the read.