Re: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for UBUF/IOVEC

[David Howells]

David Laight <David.Laight@xxxxxxxxxx> 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.  The same
> > for writing to /dev/zero.  Reading from /dev/zero does its own iteration 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 = iov_iter_count(from);
	iov_iter_advance(from, count);
	return count;
}

David