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

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

From: David Howells
> Sent: 15 September 2023 13:36
> 
> David Laight <David.Laight@ACULAB.COM> wrote:
> 
> > I was thinking of import_iovec() - or whatever its current
> > name is.
> 
> That doesn't actually access the buffer described by the iovec[].
> 
> > That really needs a single structure that contains the iov_iter
> > and the cache[] (which the caller pretty much always allocates
> > in the same place).
> 
> cache[]?

Ah it is usually called iovstack[].

That is the code that reads the iovec[] from user.
For small counts there is an on-stack cache[], for large
counts it has call kmalloc().
So when the io completes you have to free the allocated buffer.

A canonical example is:

static ssize_t vfs_readv(struct file *file, const struct iovec __user *vec,
		  unsigned long vlen, loff_t *pos, rwf_t flags)
{
	struct iovec iovstack[UIO_FASTIOV];
	struct iovec *iov = iovstack;
	struct iov_iter iter;
	ssize_t ret;

	ret = import_iovec(ITER_DEST, vec, vlen, ARRAY_SIZE(iovstack), &iov, &iter);
	if (ret >= 0) {
		ret = do_iter_read(file, &iter, pos, flags);
		kfree(iov);
	}

	return ret;
}

If 'iter' and 'iovstack' are put together in a structure the
calling sequence becomes much less annoying.
The kfree() can (probably) check iter.iovec != iovsatack (as an inline).

But io_uring manages to allocate the iov_iter and iovstack[] in
entirely different places - and then copies them about.

	David

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