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From:   David Howells <dhowells@redhat.com>
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To:     David Laight <David.Laight@ACULAB.COM>
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        Linus Torvalds <torvalds@linux-foundation.org>,
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Subject: Re: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for UBUF/IOVEC
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Date:   Fri, 15 Sep 2023 12:23:00 +0100
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David Laight <David.Laight@ACULAB.COM> wrote:

> > > Some measurements can be made using readv() and writev()
> > > on /dev/zero and /dev/null.
> > =

> > Forget /dev/null; that doesn't actually engage any iteration code.  Th=
e same
> > for writing to /dev/zero.  Reading from /dev/zero does its own iterati=
on thing
> > rather than using iterate_and_advance(), presumably because it checks =
for
> > signals and resched.
> =

> Using /dev/null does exercise the 'copy iov from user' code.

Ummm....  Not really:

static ssize_t read_null(struct file *file, char __user *buf,
			 size_t count, loff_t *ppos)
{
	return 0;
}

static ssize_t write_null(struct file *file, const char __user *buf,
			  size_t count, loff_t *ppos)
{
	return count;
}

static ssize_t read_iter_null(struct kiocb *iocb, struct iov_iter *to)
{
	return 0;
}

static ssize_t write_iter_null(struct kiocb *iocb, struct iov_iter *from)
{
	size_t count =3D iov_iter_count(from);
	iov_iter_advance(from, count);
	return count;
}

David