What's the entire process by which data gets written?  I'd like to understand
this.

nfs4_proc_write_setup() is called using the write_setup pointer in
nfs_write_rpcsetup(), which is called by nfs_flush_one(), called by
nfs_flush_list(), called by do_nfs_direct_IO() and nfs_flush_file().

nfs4_proc_write_setup() gives the rpc layer nfs4_write_done() as a callback,
which updates times and such and calls nfs_writeback_done(), which does a
number of mysterious things.

Similarly: nfs4_proc_read_setup() called by read_setup pointer in
nfs_read_rpcsetup().  The callback is nfs4_read_done(), which calls
nfs_readpage_result().