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

[Jens Axboe <axboe@kernel.dk>]

On 3/27/23 12:42?PM, Al Viro wrote:
> 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?

Correct!

> 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...

That's a great idea. Two questions - do we want to make that
WARN_ON_ONCE()? And then do we want to include a WARN_ON_ONCE for a
non-supported type? Doesn't seem like high risk as they've all been used
with ITER_IOVEC until now, though.

-- 
Jens Axboe