On 3/27/23 12:52?PM, Jens Axboe wrote: > 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. Scratch that last one, user_backed should double as that as well. At least currently, where ITER_UBUF and ITER_IOVEC are the only two iterators that hold user backed memory. -- Jens Axboe
