O_DIRECT as a hint, was: Re: [PATCH] ext4: refuse O_DIRECT opens for mode where DIO doesn't work

On Tue, Apr 26, 2016 at 09:49:46AM +1000, Dave Chinner wrote:

Why not just transparently fall back to buffered IO if direct IO
cannot be done? Saves people from wondering why applications fail
on one ext4 filesystem and not another....

I've been doing an audit of our direct I/O implementations, and most
of them does some form of transparent fallback, including some that
only pretend to support O_DIRECT, but do anything special for it at all,
while at the same time we go through greast efforts to check a file
system actualy supports direct I/O, leading to nasty no-op ->direct_IO
implementations as we even got that abstraction wrong.

At this point I wonder if we should simply treat O_DIRECT as a hint
and always allow it, and just let the file system optimize for it
(skip buffering, require alignment, relaxed Posix atomicy requirements)
if it is set.