rw whether to READ or WRITE or maybe READA (readahead)
nrnumber of &struct buffer_heads in the array
bhsarray of pointers to &struct buffer_head
ll_rw_block takes an array of pointers to &struct buffer_heads,
and requests an I/O operation on them, either a READ or a WRITE.
The third READA option is described in the documentation for
generic_make_request which ll_rw_block calls.
This function provides extra functionality that is not in
generic_make_request that is relevant to buffers in the buffer
cache or page cache. In particular it drops any buffer that it
cannot get a lock on (with the BH_Lock state bit), any buffer that
appears to be clean when doing a write request, and any buffer that
appears to be up-to-date when doing read request. Further it marks
as clean buffers that are processed for writing (the buffer cache
wont assume that they are actually clean until the buffer gets
unlocked).
ll_rw_block sets b_end_io to simple completion handler that marks
the buffer up-to-date (if approriate), unlocks the buffer and wakes
any waiters. As client that needs a more interesting completion
routine should call submit_bh (or generic_make_request)
directly.