Re: [PATCH 3/3] iov_iter: import single vector iovecs as ITER_UBUF

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Mar 28, 2023 at 01:08:11AM +0100, Al Viro wrote:
> On Mon, Mar 27, 2023 at 05:27:13PM -0600, Jens Axboe wrote:
> > Add a special case to __import_iovec(), which imports a single segment
> > iovec as an ITER_UBUF rather than an ITER_IOVEC. ITER_UBUF is cheaper
> > to iterate than ITER_IOVEC, and for a single segment iovec, there's no
> > point in using a segmented iterator.
> 
> Won't that enforce the "single-segment readv() is always identical to
> read()"?  We'd been through that before - some of infinibarf drvivers
> have two different command sets, one reached via read(), another - via
> readv().  It's a userland ABI.  Misdesigned one, but that's infinibarf
> for you.
> 
> Does anyone really need this particular microoptimization?

static ssize_t qib_write_iter(struct kiocb *iocb, struct iov_iter *from)
{
        struct qib_filedata *fp = iocb->ki_filp->private_data;
	struct qib_ctxtdata *rcd = ctxt_fp(iocb->ki_filp);
	struct qib_user_sdma_queue *pq = fp->pq;

	if (!iter_is_iovec(from) || !from->nr_segs || !pq)
		return -EINVAL;

	return qib_user_sdma_writev(rcd, pq, from->iov, from->nr_segs);
}

Hit this with single-segment writev() and you've got yourself -EINVAL.
Sure, that could be adjusted for (check for user_backed_iter(), then
if it's ubuf form an iovec and pass that to qib_user_sdma_writev()),
but that's a clear regression.

Found by simple grepping for iter_is_iovec()...