[PATCHSET v2 0/4] Turn single segment imports into ITER_UBUF

[Jens Axboe <axboe@kernel.dk>]

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.

Ran some unscientific performanc tests [2] as well.

Changes since v1:

- Add two prep patches converting ITER_IOVEC assumptions
- Add missing import_ubuf() return value check in __import_iovec_ubuf()

[1] https://lore.kernel.org/io-uring/43cb1fb7-b30b-8df1-bba6-e50797d680c6@kernel.dk/
[2] https://lore.kernel.org/all/45c46ee9-8479-4114-6ce9-ae3082335cb8@kernel.dk/