Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list from an iterator

[Al Viro]

On Thu, Jan 26, 2023 at 02:16:20PM +0000, David Howells wrote:

> +/**
> + * iov_iter_extract_will_pin - Indicate how pages from the iterator will be retained
> + * @iter: The iterator
> + *
> + * Examine the iterator and indicate by returning true or false as to how, if
> + * at all, pages extracted from the iterator will be retained by the extraction
> + * function.
> + *
> + * %true indicates that the pages will have a pin placed in them that the
> + * caller must unpin.  This is must be done for DMA/async DIO to force fork()
> + * to forcibly copy a page for the child (the parent must retain the original
> + * page).
> + *
> + * %false indicates that no measures are taken and that it's up to the caller
> + * to retain the pages.
> + */
> +static inline bool iov_iter_extract_will_pin(const struct iov_iter *iter)
> +{
> +	return user_backed_iter(iter);
> +}
> +

Wait a sec; why would we want a pin for pages we won't be modifying?
A reference - sure, but...