RE: [PATCH v3 2/2] iov_iter: Don't deal with iter->copy_mc in memcpy_from_iter_mc()

[David Laight <David.Laight@ACULAB.COM>]

From: Matthew Wilcox
> Sent: Friday, August 18, 2023 1:27 PM
> 
> On Fri, Aug 18, 2023 at 12:16:23PM +0000, David Laight wrote:
> > > > +	ITER_IOVEC = 1,
> > > > +	ITER_UBUF = 2,
> > > > +	ITER_KVEC = 4,
> > > > +	ITER_BVEC = 8,
> > > > +	ITER_XARRAY = 16,
> > > > +	ITER_DISCARD = 32,
> >
> > IIRC Linus had type:6 - that doesn't leave any headroom
> > for additional types (even though they shouldn't proliferate).
> 
> I have proposed an ITER_KBUF in the past (it is to KVEC as UBUF is
> to IOVEC).  I didn't care enough to keep pushing it, but it's clearly
> a common idiom.

Indeed, I didn't spot UBUF going in - I spot a lot of stuff.

I did wonder if you could optimise for a vector length of 1
(inside the KVEC conditional).
That would also pick up the cases where there only happens
to be a single buffer.

I also remember writing a patch that simplified import_iovec()
by combining the iov_iter with a struct iovec iovec[UIO_FASTIOV].
All got bogged down in io_uring which would need changing first.

	David

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