Re: [PATCH] fs: add RWF_DIRECT for preadv2/pwritev2/io_uring to set IOCB_DIRECT per io

[Christoph Hellwig <hch@lst.de>]

On Wed, Oct 21, 2026 at 01:04:06PM +0200, Christian Brauner wrote:
> On Mon, Oct 12, 2026 at 08:46:42PM +0200, Stefan Metzmacher wrote:
> > It's useful to control this per io request instead of using O_DIRECT
> > for the file descriptor.
> 
> @Christoph thoughts on this?

Puh.  This goes right into a big discussion right now.  We've
traditionally said mixing buffered and direct I/O is madness, and
that's certainly not untrue.  On the other hand people find this
useful - nfsd is now making heavy use of it, and a few year ago
there was a paper on how the lustre backend really gained by doing
that.  So while some folks like Dave Chinner still get really angry
at the idea of mixing the modes, I feel the ship has sailed and we
need to make it work as good as possible.  For some cases like
mmap'ed I/O it will always introduce data corruption, but we'd at
least make sure there's nothing exploitable.  And the direct user
exposed flag would open lots of these exploit vectors.

There's also issues where file systems still check O_DIRECT for
behavior and not IOCB_DIRECT, and this would increase the problems
with that compared to the existing in-kernel only users.
NFS for example has quite a few O_DIRECT checks, and some of them
feel wrong for this.