From: Matthew Wilcox

Sent: Friday, August 18, 2023 1:27 PM

On Fri, Aug 18, 2023 at 12:16:23PM +0000, David Laight wrote:

quoted

quoted

quoted

+	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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