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()...
