Re: [PATCHSET 0/2] Turn single segment imports into ITER_UBUF

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

On Mon, Mar 27, 2023 at 12:01:08PM -0600, Jens Axboe wrote:
> On 3/24/23 10:46 PM, Al Viro wrote:
> > On Fri, Mar 24, 2023 at 02:44:41PM -0600, Jens Axboe wrote:
> >> Hi,
> >>
> >> We've been doing a few conversions of ITER_IOVEC to ITER_UBUF in select
> >> spots, as the latter is cheaper to iterate and hence saves some cycles.
> >> I recently experimented [1] with io_uring converting single segment READV
> >> and WRITEV into non-vectored variants, as we can save some cycles through
> >> that as well.
> >>
> >> But there's really no reason why we can't just do this further down,
> >> enabling it for everyone. It's quite common to use vectored reads or
> >> writes even with a single segment, unfortunately, even for cases where
> >> there's no specific reason to do so. From a bit of non-scientific
> >> testing on a vm on my laptop, I see about 60% of the import_iovec()
> >> calls being for a single segment.
> >>
> >> I initially was worried that we'd have callers assuming an ITER_IOVEC
> >> iter after a call import_iovec() or import_single_range(), but an audit
> >> of the kernel code actually looks sane in that regard. Of the ones that
> >> do call it, I ran the ltp test cases and they all still pass.
> > 
> > Which tree was that audit on?  Mainline?  Some branch in block.git?
> 
> It was just master in -git. But looks like I did miss two spots, I've
> updated the series here and will send out a v2:
> 
> https://git.kernel.dk/cgit/linux-block/log/?h=iter-ubuf

Just to make sure - head's at 4d0ba2f0250d?

One obvious comment (just about the problems you've dealt with in that branch;
I'll go over that tree and look for other sources of trouble, will post tonight):
all 3 callers of iov_iter_iovec() in there are accompanied by the identical
chunks that deal with ITER_UBUF case; it would make more sense to teach
iov_iter_iovec() to handle that.  loop_rw_iter() would turn into
	if (!iov_iter_is_bvec(iter)) {
		iovec = iov_iter_iovec(iter);
	} else {
		iovec.iov_base = u64_to_user_ptr(rw->addr);
		iovec.iov_len = rw->len;
	}
and process_madvise() and do_loop_readv_writev() patches simply go away.

Again, I'm _not_ saying there's no other problems left, just that these are
better dealt with that way.

Something like

static inline struct iovec iov_iter_iovec(const struct iov_iter *iter)
{
	if (WARN_ON(!iter->user_backed))
		return (struct iovec) {
			.iov_base = NULL,
			.iov_len = 0
		};
	else if (iov_iter_is_ubuf(iter))
		return (struct iovec) {
			.iov_base = iter->ubuf + iter->iov_offset,
			.iov_len = iter->count
		}; 
	else
		return (struct iovec) {
			.iov_base = iter->iov->iov_base + iter->iov_offset,
			.iov_len = min(iter->count,
				       iter->iov->iov_len - iter->iov_offset),
		};
}

and no need to duplicate that logics in all callers.  Or get rid of those
elses, seeing that each alternative is a plain return - matter of taste...