O_DIRECT

[Steve French <smfltc@us.ibm.com>]

Is there a precise defintion of the Linux O_DIRECT  semantics, in 
particularly addressing desired behavior in the cases of:
1) inodes opened more than once (some with and some without O_DIRECT) - 
do the other open file instances get disabled caching?
2) consistency of mmap data and sendfile data (how could this work 
without write through of the page cache?) when inode is opened O_DIRECT

If the goal of O_DIRECT is not only to bypass the local client's page 
cache, but also to bypass the 4K read/write page size (and allow larger 
read, writes), is it acceptable for an fs to handle files opened 
O_DIRECT to disable caching by turning off calls to generic_file_read 
and generic_file_write for reads and writes to that inode (after a 
sync)?  If not, is there a conventional "no cache" mount option used by 
other filesystems to do the equivalent on a mounted volume?