Re: O_DIRECT fails in some kernel and FS

[Jeff Garzik <garzik@havoc.gtf.org>]

On Sat, Feb 02, 2002 at 02:16:41PM -0600, Stephen Lord wrote:
> Can't you fall back to buffered I/O for the tail? OK it complicates the
> code, probably a lot, but it keeps things sane from the user's point of
> view.

For O_DIRECT, IMHO you should fail not fallback.  You're simply lying
to the underlying program otherwise.

In the ibu fs I am hacking on, the idea for O_DIRECT is to fail a read
if the file is small enough to fit in the inode.  If the O_DIRECT
action is a write, then I will invalidate the data in the inode,
then follow the standard path (which eventually calls get_block()).

For file tails (a different case from small-file-in-inode), I
imagine it would be prudent to support O_DIRECT for all actions
except reading the file tail.  If you want to be complicated, you
could provide userspace with a way to say "this is a dense file"
and/or simply not create a tail at all...

	Jeff