linux-kernel - Re: [PATCH 1/4] iov_iter: Provide copy_iomem_to|from

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Message-ID: <Zy0fg6E7Fbmi6lsP@casper.infradead.org>
Date: Thu, 7 Nov 2024 20:13:55 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Michal Wajdeczko <michal.wajdeczko@...el.com>
Cc: intel-xe@...ts.freedesktop.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Matthew Brost <matthew.brost@...el.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/4] iov_iter: Provide copy_iomem_to|from_iter()

On Thu, Nov 07, 2024 at 05:34:45PM +0100, Michal Wajdeczko wrote:
> +size_t copy_iomem_to_iter(const void __iomem *from, size_t offset,
> +			  size_t bytes, struct iov_iter *i)
> +{
> +	unsigned char buf[SMP_CACHE_BYTES];
> +	size_t progress = 0, copied, len;
> +
> +	from += offset;
> +	while (bytes) {
> +		len = min(bytes, sizeof(buf));
> +		memcpy_fromio(buf, from + progress, len);
> +		copied = _copy_to_iter(buf, len, i);
> +		if (!copied)
> +			break;
> +		bytes -= copied;
> +		progress += copied;
> +	}

This seems like a rather sad implementation.  Why not:

	if (WARN_ON_ONCE(i->data_source))
		return 0;
	if (user_backed_iter(i))
		might_fault();
	return iterate_and_advance(i, bytes, (void *)addr,
			copy_iomem_to_iter, memcpy_iomem_to_iter);

along with

size_t memcpy_iomem_to_iter()
{
	memcpy_fromio(iter_to, from + progress, len);
	return 0;
}`