Re: [PATCH 0/4] io_uring: use ITER_UBUF

[Keith Busch <kbusch@kernel.org>]

On Mon, Nov 07, 2022 at 10:54:06PM -0800, Christoph Hellwig wrote:
> On Mon, Nov 07, 2022 at 09:56:06AM -0800, Keith Busch wrote:
> >   1. io_uring will always prefer using the _iter versions of read/write
> >      callbacks if file_operations implement both, where as the generic
> >      syscalls will use .read/.write (if implemented) for non-vectored IO.
> 
> There are very few file operations that have both, and for those
> the difference matters, e.g. the strange vectors semantics for the
> sound code. 
 
Yes, thankfully there are not many. Other than the two mentioned
file_operations, the only other fops I find implementing both are
'null_ops' and 'zero_ops'; those are fine. And one other implements
just .write/.write_iter: trace_events_user.c, which is also fine.

> I would strongly suggest to mirror what the normal
> read/write path does here.

I don't think we can change that now. io_uring has always used the
.{read,write}_iter callbacks if available ever since it introduced
non-vectored read/write (3a6820f2bb8a0). Altering the io_uring op's ABI
to align with the read/write syscalls seems risky.

But I don't think there are any real use cases affected by this series
anyway.

> >   2. io_uring will use the ITER_UBUF representation for single vector
> >      readv/writev, but the generic syscalls currently uses ITER_IOVEC for
> >      these.
> 
> Same here.  It might be woth to use ITER_UBUF for single vector
> readv/writev, but this should be the same for all interfaces.  I'd
> suggest to drop this for now and do a separate series with careful
> review from Al for this.

I feel like that's a worthy longer term goal, but I'll start looking
into it now.