On Wed, May 10, 2017 at 03:45:24AM +0100, Al Viro wrote:

FWIW, some parts of that queue are obviously sane; it's the conversions of
kernel_write() and friends to ->read_iter/->write_iter() that are non-starters.

And that part is the main point!

That stuff is used in too many situations; we can't guarantee that all of
them will be for files that have those.

That's why this series handles ITER_KVEC for this case, which is all
that's really needed for kernel_read/write.  If you insiste the bvec
and pipe cases are handled as well that couod be added fairly easily.

As for default_file_splice_read(), I seriously suspect that with your change
we could as well just make it return -EINVAL and be done with that; places
that have ->read_iter() tend to have explicit ->splice_read() and it looks
like the ones that do not should simply use generic_file_read_iter().
I hadn't checked that, but there's not a lot of those:

Making ->splice_read to default to the ->read_iter based implementation
and returning -EINVAL if neither that nor an explicit ->splice_read
is provided is useful, but wasn't the aim of this series.  Similar
on the write side.